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Microstructure and properties of laser-borided 41Cr4 steel
Optics & Laser Technology 45 (2013) 308–318
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Microstructure and properties of laser-borided 41Cr4 steel M. Kulka n, N. Makuch, A. Pertek Poznan University of Technology, Institute of Materials Science and Engineering, Pl. M.Sklodowskiej-Curie 5, 60-965 Poznan, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 April 2012 Received in revised form 17 June 2012 Accepted 21 June 2012 Available online 10 July 2012
Laser-boriding, instead of diffusion-boriding, was applied to formation of boride layers on 41Cr4 steel. The microstructure and properties of these layers were compared to those obtained after typical diffusion-boriding. Three zones characterized the microstructure of laser-borided layer: laser-borided zone, hardened medium-carbon zone (heat affected zone) and medium-carbon substrate without heat treatment. The through-hardened laser-borided steel was also analyzed. In this case two zones characterized the microstructure: laser-borided zone and hardened medium-carbon substrate. The microstructure of laser-borided zone consisted of eutectic mixture of borides and martensite. This phase composition (especially martensite presence) was the reason for microhardness decrease at the surface in comparison with diffusion-borided steel. However, the use of laser-boriding causes the decrease in microhardness gradient between the surface and the substrate in comparison with typical diffusion-boriding process. The value of mass wear intensity factor of the hardened laser-borided layer was comparable to that obtained in case of diffusion-boriding and through-hardening. The use of laserborided layers instead of typical diffusion-borided layers may be advantageous under conditions of high abrasive wear of mating parts. For the experimental condition used, the laser-boriding process presented worst results concerning the fatigue strength. The cracks formed on the surface during laser re-melting were the reason for relatively quick first fatigue crack. In case of elements, which require high fatigue strength, the use of modified laser processing parameters would be necessary. The better results should be obtained by increasing of tracks overlapping. Although the cohesion of laser-borided layer was sufficient, the diffusion-borided layer showed a better cohesion. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Laser-boriding Diffusion-boriding Microstructure Low-cycle fatigue
1. Introduction Diffusion-boriding is a thermochemical surface treatment in which boron atoms diffuse into the surface of a workpiece to form borides with the base material. When applied to the adequate materials, boronizing provides wear and abrasion resistance comparable to sintered carbides. The selection of material is very important. The process can be applied to a wide range of steel alloys, including carbon steel, low alloy steel, tool steel and stainless steel. In general, boriding of steels results in the formation of FeB and Fe2B needle-like phases in diffusion layer. The iron borides are characterized by many advantageous properties: high hardness (up to 2000 HV), high abrasive wear resistance, the advantageous profile of residual stresses, high heat resistance, high corrosion resistance in acid and alkaline solutions, high resistance to influence of liquid metals and alloys and high hardness at increased temperatures [1–7]. However, the important drawback of these layers is their brittleness [3,5,7]. It is
n
Corresponding author. E-mail address: [email protected] (M. Kulka).
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caused by a high hardness of iron borides and by a large hardness gradient between the boride zone and the substrate. The brittleness of boride layer was reduced with the use of various methods. The production of a single-phase Fe2B layer [6,7] was effectively applied in order to eliminate this disadvantage. The lower hardness of this phase (1400–1600 HV) caused a reduction in the gradient of the hardness between the surface and base material. The other method consisted of the formation of multicomponent and complex borided layers [8–17,28,29]. The positive effect was also obtained applying the laser-heat treatment after diffusion-boriding [18–27]. In recent years, laser beam was being used for a wide range of applications in order to modify the microstructure and properties of the steel. The role of laser technology intensively increased in surface engineering. The most important processes were as follows: laser-heat treatment, laser alloying and laser overlaying [30,31]. The laser-heat treatment was applied to the modification of microstructure and properties of previous diffusion-borided layers. This modification was carried out with or without visible re-melting. The LHT with re-melting [18,22,23,27] provided the boride coating consisting of eutectic mixture of iron borides and the martensite. In case of LHT without re-melting [24–26], the laser-treated boride zone consisted of iron borides of modified
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morphology. These borides assumed a globular shape. All these processes provided the advantageous functional properties, often exceeding the properties characteristic of diffusion-borided layers. It was demonstrated that the laser heat treatment could successfully be applied instead of the traditional heat treatment (through hardening) of diffusion-borided steels. The laser-boriding process was also widely developed [31–36]. Two methods of laser-boriding was usually used: two-step process [32–36], which consists of re-melting of substrate material previously coated by alloying material (e.g. paste with boron) and single-step process, in which the alloying material (e.g. powder with boron, borides or boron carbide) was introduced to molten pool during laser re-melting [31]. The majority of these papers concerned examinations of the microstructure, phase composition and hardness of the layers obtained. In some papers [32,35] the abrasive wear resistance was studied. However, the investigation did not provide a comparison with diffusionborided layer. The laser-boriding could be an interesting alternative for diffusion-boriding in order to reduce the brittleness of boride layers. In the result of this process a smaller gradient of the hardness was obtained between the surface and the core of steel in comparison with the typical diffusion-borided layers [31–36]. There is not much information regarding the fatigue strength of boride layers, especially, those produced by laser-boriding. The influence of boronizing on the fatigue strength is ambiguous [37], because it depends on many factors: boriding method, boriding parameters, chemical composition of borided steel, heat treatment after boriding, and the defects of the layers. The diffusionboriding, followed by proper heat treatment usually causes an occurrence of compressive stresses at the surface. The presence of such stresses in the surface layer causes, as a rule, an increase in fatigue strength. The tensile stresses on the surface are disadvantageous. The sum of tensile residual stresses and stresses caused by external forces can exceed the limit of tensile strength. In effect, surface cracks can be observed and the fatigue resistance decreases. Therefore, the selection of laser-boriding parameters is very important for fatigue strength of the produced layers. The possible surface cracks caused by this process are inadvisable. In this paper the laser alloying by boron was used in order to form the boride layers. The laser-boriding, instead of conventional diffusion process, was carried out on 41Cr4 steel. The multiple laser tracks were produced on an entire cylindrical surface of the samples. Therefore, there was a possibility of examining of wear resistance and low-cycle fatigue strength. The microstructure, the microhardness profiles and the cohesion were also investigated and compared to the properties obtained for typical diffusionborided layer. It was one of the main purposes of the presented study. The possibilities of application of laser-borided layers were analyzed depending on operating conditions of workpieces.
2.2. Diffusion-boriding Gas diffusion-boriding was carried out with the usage of the devices presented in the papers [14,24]. The process was conducted in quartz tube. Before heating, the air was removed from this tube by the vacuum pump. Then, the flow of nitrogen was activated and the heating process was started. After the furnace had reached the required temperature of 950 1C (1223 K), a gas mixture of hydrogen and BCl3 was fed through the quartz tube at a flow rate of 90 l/h. This gas mixture consisted of 95 vol% H2 and 5 vol% BCl3. The diffusion-boriding continued for 3 h, then the process finished and the specimens were cooled in a nitrogen atmosphere. Typical heat treatment was carried out after diffusionboriding. The specimens were through-hardened: austenitized at 850 1C (1123 K), quenched in oil and tempered at 150 1C (423 K). 2.3. Laser-boriding The laser-boriding, instead of conventional diffusion process, was also carried out. The two-step process was applied. The external cylindrical surface of substrate material was coated by a paste consisting of amorphous boron, water-glass and distilled water. Boron paste of 40 mm thickness was used. The possibility of cracking of the boron paste in front of the laser beam due to thermal shock was not investigated. However, in the future it should be taken into consideration. Then the surface was remelted by laser beam (Fig. 1). Boriding was carried out by the TRUMPF TLF 2600 Turbo CO2 laser of the nominal power 2.6 kW operated with the following parameters: power P¼1.17 kW and scanning rate vl ¼3.84 m/min. The multiple mode laser beam (TEM01n) of circular shape was applied. The diameter of laser beam was equal to 2 mm. Therefore, the averaging irradiance (E) of 37 kW/cm2 was used. The diameter of laser beam was determined with the help of actinosensitive paper. The focusing mirror of curvature 250 mm, diameter 48 mm and focal length 125 mm was used in order to focus the laser beam. The distance from the bottom edge of fixing holder of focusing mirror to the laserborided surface was equal to 106.8 mm. This distance was longer than the distance to the focused beam (91.8 mm from the bottom edge of fixing holder of the mirror). The TEM01n mode, the so-called doughnut mode, is a special case consisting of a superposition of two TEM01 modes, rotated at 901 with respect to one another. The irradiance profiles and their influence on the laser track dimension are shown in Fig. 2. The radial dependence of irradiance is presented in Fig. 2a. The treated surface is moved relative to the laser beam during laser modification. That is why the obtained melted profile doesn’t correspond to the laser beam mode TEM01n. The projection of irradiance profile on the laser-treated surface is visible in Fig. 2b.
2. Experimental details 2.1. Material Steel 41Cr4 was investigated. Its chemical composition was presented in Table 1. The ring-shaped specimens (external diameter ca. 20 mm, internal diameter ca. 12 mm and height 12 mm) were used for the study. Table 1 Chemical composition of steel used (wt%). Material
C
Cr
Mn
Si
Mo
Ni
41Cr4
0.42
1.06
0.72
0.22
0.04
0.08
309
Fig. 1. Two-step method of laser-boriding.
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arranged as multiple tracks (Fig. 3), with the distance f¼0.75 mm. It corresponded to the distance between the axes of the adjacent tracks and resulted from the feed rate used. The obtained scanning rate vl (3.84 m/min) resulted from the rotational speed n (61.15 min 1) and feed rate vf (0.75 mm/revolution). Some of laser-borided specimens were through-hardened: austenitized at 850 1C (1123 K), quenched in oil and tempered at 150 1C (423 K). 2.4. Microstructure, phase analysis and microhardness profiles After the treatment described above, the samples were cut out perpendicular to the treated surface, across the diffusion-borided layer or across the laser tracks produced (in case of laser-boriding). Then the metallographic specimens were prepared. The samples were polished by using the abrasive papers of different granularities, and, finally, by applying Al2O3. In order to reveal the microstructure, the diffusion borided and laser-borided samples were etched with 5% solution of HNO3 in C2H5OH. The microstructures of polished and etched cross-sections of the specimens were observed by an optical microscope (OM) and scanning electron microscope (SEM). The phase analysis of laserborided layer was carried out by Kristalloflex 4 S X-Ray diffractometer using Mo Ka radiation. Microhardness profiles, through the investigated layers, were determined in the polished crosssections of specimens. The Vickers method, with the use of the apparatus ZWICK 3212 B, was applied for microhardness measurements. The tests were performed at the load P¼0.1 kg (about 0.981 N). 2.5. Abrasive wear The frictional pair, consisting of a roller as sample and a sintered carbide plate as counter-sample, was used in order to examine abrasive wear resistance (Fig. 4). The composition of
Fig. 2. Irradiance profiles and their influence on the laser track dimension: (a) radial dependence of irradiance, (b) projection of irradiance profile on the lasertreated surface, (c) image of laser track obtained; vl is –scanning rate, d laser beam diameter, b width of laser track, 1 re-melted zone,and 2 the heat affected zone.
The direction of the motion is also shown. The track image (Fig. 2c) indicates that the width of the laser track (b) is less than 2 mm because of the short exposure time during the relative motion of laser beam and the treated surface. It also results from the irradiance profile of laser beam. The irradiance is small in a distance greater than 0.66 mm from the axis of laser beam. The short exposure time and the small irradiance cause the heating of the material to a relatively low temperature. Therefore, for the experimental condition used, the effects of laser-heat treatment are not visible in the microstructure in a distance greater than 0.66 mm from the axis of laser beam. The laser alloying was done in air. Therefore, during this treatment an oxidation could take place. A non-oxidizing atmosphere (Ar) should be used in the future. The laser tracks were
Fig. 3. Method of multiple tracks production; d – laser beam diameter (d ¼ 2 mm); vf rate of feed; vl scanning rate; n rotational speed; f distance from track to track.
Fig. 4. Scheme of wear, P ¼ 147 N, rotational speed n ¼250 min 1.
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sintered carbide S20S was as follows: 58 wt% of WC, 31.5 wt% of (TiCþTaC þNbC), 10.5 wt% of Co. This material was characterized by a mass density 10.7 g/cm3 and hardness 1430 HV. The load P¼147 N and the specimen speed of 0.26 m/s were used during the test, carried out under conditions of dry friction (unlubricated sliding contact). Although the surface roughness was changed after laser-boriding, the surface was not prepared before the wear test. Wear resistance was evaluated by the factor of mass wear intensity Imw, which was defined as specimen mass loss per friction surface and unit of time. This factor corresponded to the slope of a straight line in the frictional wear diagram and was determined from equation: Dm mg Imw ¼ Ft cm2 h where Dm is the mass loss (mg), F the friction surface (cm2), and t the friction time (h). 2.6. Low-cycle fatigue A hydraulic pulsator MTS 810 of maximal load 100 kN was used during the low-cycle fatigue tests. The main elements of the testing equipment were as follows: the working elements (beam with force gage, upper head with clamp, supporting pillars, frame, piston with working head and clamp), electronic control, control panel, master switch, personal computer with software (Station Manager, Basic Test Ware), and hydraulic system with pump. The tests were carried out in order to obtain a complete fracture of the specimen or to determine the fatigue life. The specimens were put
311
under radial compression. The compressive and bending stresses were generated. The scheme of the specimen position was presented in Fig. 5. The relatively high load (F) of 1.6–3.6 kN had been used, hence the fatigue cracks occurred after a low number of cycles. The compressive force (F) was modulated with a sinusoidal applied force of frequency 10 Hz and an amplitude (A) of 0.8–1.8 kN. The load and the amplitude were generated with the help of working head movement. During the test, the deflection (v) was registered continuously. The contacting ekstensometer MTS 634.31F-24 (axial—multiple gage length) was used for the measurements of deflection. 2.7. Cohesion A standard Rockwell method, as a destructive quality test for examined layers, was employed for determination of cohesion. The well-known adhesion test prescribed by the VDI 3198 norm [38] was used. The principle of this method was presented in the upper right part of Fig. 6 [39]. A conical diamond indenter penetrated into the surface of an investigated layer, thus inducing massive plastic deformation to the substrate and fracture of the diffusion-borided or laser-borided layer. As in every indentation test, the 1/10th rule should be accomplished. Therefore the overall specimen thickness should be at least ten times greater than the indentation depth. The type and the volume of the layer failure zone, firstly, showed the cohesion of the layer and secondly, its brittleness. The indentation craters were observed by an optical microscope. The damage to the layers was compared to the quality maps of adhesion strength HF1—HF6 (Fig. 6). In general, the adhesion strength quality HF1—HF4 defined sufficient adhesion, whereas HF5 and HF6 represented insufficient adhesion [39].
3. Results and discussion 3.1. Microstructure
Fig. 5. Scheme of specimen position during the low-cycle fatigue test. 1 thrust washers (41Cr4 steel); 2 specimen.
The OM microstructure of diffusion-borided layer formed on 41Cr4 steel, through-hardening is shown in Fig. 7. After diffusionboriding, iron borides were visible close to the surface. The results of earlier papers [12–17] indicate, that two types of iron borides were occurring in those layers: FeB and Fe2B. Two diffusion zones were observed: (1) iron borides near the surface and (2) hardened medium-carbon substrate below borides. The laser-boriding instead of typical diffusion process was carried out. The laser-heat treatment with re-melting was used
Fig. 6. Principle of the VDI 3198 indentation test [38].
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for laser alloying by boron. In the microstructure of laser-borided steel (Fig. 8) three zones were visible: (1) laser-borided zone, (2) hardened medium-carbon substrate, that is heat affected zone (HAZ) and (3) medium-carbon substrate without heat treatment. The continuous laser-borided zone was obtained at the surface. However, the depth of laser-borided zone was variable and considerably lesser at the contact of adjacent tracks. This zone consisted of eutectic mixture of iron borides and martensite [33,35,36]. The SEM image of laser-borided zone was presented in Fig. 9. The maximal depth of this zone was located in the axis of track. The obtained microstructure of laser-borided steel was very similar to the laser-modified diffusion-borided layer with
re-melting [22,23]. In both cases the three similar zones were observed: re-melted zone, hardened substrate (heat affected zone) and the substrate without visible effects of laser-heat treatment. These zones differed much from the microstructure of laser-modified (without re-melting) diffusion-borocarburized layer [24–26], in which the other zones were visible: iron borides of modified morphology (more globular borides), hardened carburized zone (heat affected zone) and carburized layer without heat treatment. The through-hardening was also carried out after laser-boriding. This treatment was necessary in order to obtain the same microstructure in the core of steel like in case of diffusion-borided steel. It was important from the point of view of planned examinations of fatigue strength. In this case, the microstructure (Fig. 10) was characterized by two zones: (1) laser-borided zone and (2) hardened medium-carbon substrate .
3.2. Phase analysis
Fig. 7. Microstructure of diffusion-borided and through-hardened 41Cr4 steel; 1 iron borides (FeBþ Fe2B); 2 hardened medium-carbon substrate.
The phase analysis was performed directly after laser boriding, without the through-hardening. The microstructure of the remelted track consisted of eutectic mixture of iron borides and the martensite. XRD scans of the laser-borided specimen confirmed the presence of two types of iron borides: Fe2B and Fe3B (Fig. 11). The X-ray diffraction did not provide information about the occurrence of martensite. The martensite was identified as Fea. Probably, the participation of this phase in the microstructure was too small. However, taking the high cooling rate during lasertreatment and the microhardness measurements (1100–1600 HV in laser re-melted zone) into consideration, the occurrence of martensite was assumed. Simultaneously, the segregation of boron took place during re-melting. In the result, after resolidification in some places iron richer Fe3B phase was formed.
Fig. 8. Microstructure of laser-borided 41Cr4 steel: (a) image of multiple tracks; (b) image of one track and interfaces between adjacent tracks; and (c) image in the axis of one track.
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This is in good agreement with the earlier findings [34,36], which have reported the formation of a Fe3B phase after laser-boriding. Carbon does not dissolve in FeB boride and its solubility in Fe2B boride is very low [37]. Therefore, during diffusion-boriding of steel carbon is moved in a core direction by following the boron diffusion front. Carbon is ejected from interstitial positions by
boron, which forms iron borides at the surface. This phenomenon is accompanied by an increase in carbon concentration beneath iron borides formed on the substrate of constant carbon content. Sometimes, especially in case of high-carbon steel borided, it provides the borocementite Fe3(C,B) in the microstructure beneath iron borides. In case of investigated laser-borided layer the occurrence of borocementite was not confirmed by phase analysis. The obtained phase composition of borides’ zone (eutectic mixture of iron borides and the martensite) was very similar to the laser-modified diffusion-borided layer with re-melting [22,23] and differed much from the microstructure of diffusionborocarburized layer after laser modification without re-melting [24–26]. The laser-heat treatment without re-melting caused a breakdown of the needle morphology of the iron borides, which assumed a more globular shape. However, the boride zone consisted of both iron borides (FeB and Fe2B).
Fig. 9. SEM image of laser-borided zone (multiple tracks).
Fig. 11. XRD patterns of laser-borided 41Cr4 steel (multiple tracks).
Fig. 10. Microstructure of laser-borided and through-hardened 41Cr4 steel: (a) image of multiple tracks, (b) image in the axis of one track, and (c) the interface between iron boride zone and hardened medium-carbon substrate.
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3.3. Microhardness profiles The phase composition obtained (especially, martensite and Fe3B-phase presence), was the reason for microhardness decrease at the surface in comparison with conventional through-hardened diffusion-borided steel. The microhardness profiles of laserborided layers (with or without through-hardening) were compared to the profiles obtained after diffusion-boriding and through-hardening (Fig. 12). The microhardness of laser-borided layers was measured along the axis of track. In laser-borided zone the microhardness about 1100–1600 HV was obtained. These values were lower in comparison with the conventional diffusion-boriding (1350–1850 HV). This situation was caused by the microstructure of laser-borided layer, which consisted of eutectic mixture of iron borides and martensite. Next the microhardness along the axis of track decreased to 650–720 HV in HAZ. The increase of the distance from the surface resulted in a gradual decrease in the microhardness, to 270–340 HV in the core of medium-carbon steel without heat treatment. In case of diffusion-borided and through-hardened layer, the microhardness in iron borides’ zone of about 1450–1850 HV was obtained. These values were characteristic of FeB and Fe2B iron borides. Next, the microhardness decreased to 680–710 HV in hardened medium-carbon substrate. The microhardness of through-hardened laser-borided layer was measured along the axis of track and along the contact of tracks at overlapped region (Fig. 13). At the surface of that layer the microhardness of about 1310–1600 HV was observed. Next, the microhardness of laser-borided re-melted zone decreases to 1150– 1250 HV. Beneath laser-borided zone the values of microhardness are characteristic of through-hardened medium-carbon steel. The increase in the distance from the surface was accompanied by a gradual decrease in the microhardness to 630–710 HV. The measurements along the contact of tracks indicated that the laserboride zone was characterized by a lower thickness in this area. The microhardness profiles along the straight line in parallel to the surface as a function of the distance from track axis were presented in Fig. 14. The measurements were carried out as 0.02 mm and 0.15 mm from the surface. At the depth of 0.02 mm the typical microhardness of laser-borided zone was observed. The values of microhardness were ranged from 1000 HV to 1600 HV. At the depth of 0.15 mm the microhardness was changed from 650 to 710 HV in the vicinity of the contact of tracks (at overlapped region) to 1000–1500 HV in the vicinity of the axis of track. It results from changeable microstructure at this depth. At the contact of tracks the hardened medium-carbon
Fig. 13. Microhardness profiles of laser-borided and through-hardened steel.
Fig. 14. Microhardness profiles along the surface of multiple tracks at the different depth.
substrate was observed, while in the axis of tracks laser-borided zone occurred. The obtained microhardness profiles of laser-borided steel was very similar to the laser-modified diffusion-borided layer with remelting [22,23]. In both cases, the values of microhardness in borides’ zone were lower in comparison with the conventional diffusion-boriding. It was caused by the similar microstructure of borides’ zone, which consisted of eutectic mixture of iron borides and the martensite. The occurrence of the martensite was the reason for the decreased microhardness in comparison with the diffusionborided layer. Therefore, the microhardness gradient between the surface and the substrate was diminished, what could cause the lower brittleness of borides’ zone. The other situation was observed in case of the laser-modified (without re-melting) diffusion-borocarburized layer [24–26]. The microhardness of borides’ zone, consisting of iron borides (FeB and Fe2B) of modified morphology, was comparable with the typical diffusion-borided layer. However, the microhardness gradient between the surface and the core of steel was also diminished due to the occurrence of heat affected zone (hardened carburized zone) and carburized layer without heat treatment. In these zones microhardness gradually decreased. Therefore, this method of laser-heat treatment could also be used in order to diminish the brittleness of boride layers. 3.4. Abrasive wear
Fig. 12. Microhardness profile of laser-borided layers compared to profiles obtained after diffusion-boriding and through-hardening.
The results of wear resistance tests were presented in Fig. 15 and in Table 2. The measurements of mass (m) were performed
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Fig. 15. Mass loss of investigated layers during abrasive wear.
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Fig. 16. Results of low-cycle fatigue during radial compression of diffusionborided and through-hardened 41Cr4 steel.
Table 2 The results of wear resistance tests. Material
Type of process
Mass wear intensity factor Imw [mg/(cm2 h)]
41Cr4
Boriding and through hardening Laser boriding and through hardening
1.42 1.40
with the accuracy of 0.1 mg. The measuring accuracy of diameter (d) and height (h) of the samples amounted to 0.001 cm. The maximal measuring errors did not exceed the measuring accuracy. Therefore, taking the maximal mass loss Dm ¼97.4 mg into consideration, the maximal measuring errors of Dm/F values did not exceed 0.044 mg cm 2. These errors were smaller than the size of test points marked in Fig. 15. The laser-borided and through-hardened material was characterized by a constant value of mass wear intensity factor: 1.4 mg/(cm2 h). The obtained value was comparable to the characteristic of diffusion-boriding and through-hardening. The results showed that the laser-boriding could be used instead of diffusion-boriding during the formation of boride layers of high abrasive wear resistance. However, the laser re-melting of diffusion-borided layer [23] provided a higher abrasive wear resistance. The laser-modified borocarburized layer was also characterized by a higher resistance to wear [26].
Fig. 17. Results of low-cycle fatigue during radial compression of laser-borided and through-hardened 41Cr4 steel.
3.5. Low-cycle fatigue The results of low-cycle fatigue, during the radial compression, were presented in Figs. 16 and 17. The through-hardened diffusion-borided and through-hardened laser-borided specimens were examined. The determined profiles of the compressive force and deflection were similar in both cases. However, the detailed analysis of obtained profiles (Figs. 18 and 19) showed that in case of laser-borided layer the first crack was observed considerably earlier (after 11,410 cycles), while in case of diffusion-borided layer the first crack occurred at the end of test, before the complete fracture of specimen. A comparison of the results after the low-cycle fatigue tests was shown in Table 3. The complete fractures of both specimens were observed after comparable number of cycles. However, in case of laser-borided and through-hardened steel the first crack was observed after the lower number of cycles. The surface cracks formed during laser re-melting were the reason for relatively quick first fatigue crack. Therefore, diffusion-borided and through-hardened steel was characterized by the higher
Fig. 18. First crack of diffusion-borided and through-hardened 41Cr4 steel during low-cycle fatigue test.
resistance to low-cycle fatigue during radial compression in comparison with the laser-borided and through-hardened material. For the experimental condition used, the laser-boriding process presented worst results. In case of elements, which need high fatigue strength, the use of controlling laser processing
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Table 4 The results of cohesion tests.
Fig. 19. First crack of laser-borided and through-hardened 41Cr4 steel during lowcycle fatigue test.
Table 3 The results of low-cycle fatigue tests. Type of process
First crack Complete fracture of specimen [number of cycles] [number of cycles]
Diffusion boriding and through hardening Laser boriding and through hardening
25894
26608
11410
25257
parameters or the use of preheating before the laser-boriding [33] would be necessary. It would prevent the surface cracks and would cause the increase in fatigue strength. It seems that making the feed rate (vf) smaller during laser-boriding would be a better solution what would result in increasing of tracks overlapping. The results of the paper [23] indicated the possibility of preventing the surface cracks and obtaining the more uniform laser remelted layer. Simultaneously, such condition could reduce the variation in hardness profile. The laser-boriding, presented in this study, could be an alternative for the time-consuming process of diffusion-boriding. The suitable selection of the parameters of laser-boriding would be necessary. In order to increase the fatigue strength, the diffusion-borocarburized layers were formed as a result of the two-step diffusion-treatment (carburizing followed by boriding). It is well known that carburized layers were characterized by high fatigue resistance. The earlier research [28] indicated the higher fatigue strength of the borocarburized layer in comparison with typical diffusion-borided layer formed on medium-carbon steel. However, the suitable carbon concentration–depth profile beneath iron borides was required for a meaningful increase of fatigue strength [29]. Then, the fatigue strength of borocarburized layer was comparable to that of the carburized layers. The lasermodified (without re-melting) borocarburized layer [26] was characterized by the fatigue strength comparable to the laserborided, or diffusion-borided layers, presented in this study. It resulted from the less advantageous carbon concentration–depth profile produced in the paper [26]. 3.6. Cohesion The Rockwell C indentation test was applied to cohesion rating of through-hardened diffusion-borided layers and through-hardened laser-borided layers. The sufficient cohesion was obtained in both analyzed cases (Table 4). The indentation craters obtained
Type of process
HF standard (VDI 3198 norm)
Diffusion boriding and through hardening Laser boriding and through hardening
HF1 HF3
on the surface of the hardened diffusion-borided medium-carbon steel (Fig. 20) evidently revealed ideal cohesion (HF1 standard). The interfacial bonds were so strong that even at the region where the substrate piles up, there was not any indication of delamination. Fig. 21 showed the indentation craters on the surface of laser-borided and through-hardened 41Cr4 steel. OM images indicated that there were radial cracks at the perimeter of indentation craters. However, a small quantity of spots with flaking or delamination was visible (HF3 standard). These results showed a better cohesion of diffusion-borided and throughhardened steel in comparison with the laser-borided and through-hardened material. The laser-modified borocarburized layer was also characterized by a better cohesion [26]. The modification of the laser-boriding parameters, mentioned above, could improve this property.
4. Summary and conclusions Laser-boriding, instead of diffusion-boriding, was applied to formation of boride layers on 41Cr4 steel. Three zones characterized the microstructure of laser-borided steel: laser-borided re-melted zone (eutectic mixture of iron borides and martensite), hardened medium-carbon substrate (heat affected zone) and medium-carbon substrate without heat treatment. The laserborided and through-hardened steel was also analyzed. In this case two zones characterized the microstructure: laser-borided (re-melted) zone and hardened medium-carbon substrate. The microstructure of laser-borided zone consisted of eutectic mixture of borides and martensite. This phase composition (especially martensite presence) was the reason for microhardness decrease at the surface in comparison with diffusion-borided steel. However, the use of laser-boriding caused a decrease in microhardness gradient between the surface and the substrate in comparison with typical diffusion-boriding process. The results of abrasive wear tests show that there is a possibility that the laser-boriding can be used instead of diffusion-boriding during the formation of boride layers of high abrasive wear resistance. However, for the experimental condition used, the laser-boriding process presented the worst results concerning the fatigue strength. Although the complete fractures of both specimens were observed after comparable number of cycles, the first crack of laser-borided and through-hardened specimen was observed after the lower number of cycles. The surface cracks formed during laser re-melting were the reason for relatively quick first fatigue crack. Simultaneously, the results of Rockwell C indentation tests showed a better cohesion of diffusion-borided and through-hardened material in comparison with the laser-borided and through-hardened steel. In case of elements, which require high fatigue strength, the use of modified laser processing parameters would be necessary. The smaller feed rate (vf) during laser-boriding would be a better solution what would result in increase of tracks overlapping. According to the previous study [23], it would prevent the surface cracks and would cause the increase in fatigue strength. The conditions used in the present study cannot be applied easily in laser manufacturing engineering. The use of the
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Fig. 20. OM images of diffusion-borided and through-hardened 41Cr4 steel after the Rockwell C indentation test (HF1 standard).
Fig. 21. OM images of laser-borided and through-hardened 41Cr4 steel after the Rockwell C indentation test (HF3 standard).
proposed laser-technology for industrial applications requires the adaptation in many respects, e.g. for the sake of the type of laser used, the optimization of laser processing parameters or minimization of production costs.
Acknowledgments This work has been financially supported by The Ministry of Science and Higher Education in Poland as a part of the N N507
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452537 project. The authors wish to thank Prof. M. Kawalec and Dr. M. Jankowiak from Division of Machining for their help and cooperation during the laser treatment and Dr. P. Wasilewicz from Division of Strength of Materials and Constructions for help during the low-cycle fatigue tests. References [1] Graf von Matuschka A. Borieren,. Munchen, Wien: Carl HanserVerlag; 1977. [2] Wierzchon. T. The role of glow discharge in the formation of a boride layer on steel in the plasma boriding process.Advances in low-temperature plasma chemistry, technology, applications, vol. 2. Lancaster-Basel, USA: Technomic Publishing Co. INC.; 1988. [3] Venkataraman B, Sundararajan G. The high speed sliding wear behaviour of boronized medium carbon steel. Surface and Coatings Technology 1995; 73:177–84. [4] Hunger HJ, Trute G. Boronizing to produce wear-resistant surface layers. Heat Treatment of Metals 1994;21(2):31–9. [5] Xu CH, Xi JK, Gao W. Improving the mechanical properties of boronized layers by superplastic boronizing. Journal of Materials Processing Technology 1997;65:94–8. [6] Pertek A. Gas boriding condition for the iron borides layers formation, 163–165. Switzerland: Materials science forum, Trans Tech Publications; 1994 pp. 323–328. ¨ ¨ [7] Liliental W, Tacikowski J. Einfluss der Warmebehandlung auf die Sprodigkeit ¨ ¨ von Boridschichten auf Stahlen. Harterei—Technische Mitteilungen 1980; 35:251–6. [8] Yan PX, Su YC. Metal surface modification by B–C-nitriding in a twotemperature-stage process. Materials Chemistry and Physics 1995;39:304–8. [9] Wierzchon T, Bielinski P, Sikorski K. Formation and properties of multicomponent and composite borided layers on steel. Surface and Coatings Technology 1995;73:121–4. [10] Sikorski K, Wierzchon T, Bielinski P. X-ray microanalysis and properties of multicomponent plasma-borided layers on steels. Journal of Materials Science 1998;33:811–5. [11] Toroghinezhad MR, Salehi M, Ashrafizadeh F. The effect of precarburizing treatment on morphology of the boride layer. Materials and Manufacturing Processes 1997;12(1):117–23. [12] Pertek A, Kulka M. Characterization of complex (Bþ C) diffusion layers formed on chromium and nickel-based low-carbon steel. Applied Surface Science 2002;202:252–60. [13] Pertek A, Kulka M. Microstructure and properties of composite (Bþ C) diffusion layers on low-carbon steel. Journal of Materials Science 2003;38: 269–73. [14] Pertek A, Kulka M. Two-step treatment carburizing followed by boriding on medium-carbon steel. Surface and Coatings Technology 2003;173:309–14. [15] Kulka M, Pertek A. The importance of carbon content beneath iron borides after boriding of chromium and nickel-based low carbon steel. Applied Surface Science 2003;214:161–71. [16] Kulka M, Pertek A, Klimek L. The influence of carbon content in the borided Fe-alloys on the microstructure of iron borides. Materials Characterization 2006;56(3):232–40. [17] Kulka M, Pertek A. Characterization of complex (B þC þ N) diffusion layers formed on chromium and nickel-based low-carbon steel. Applied Surface Science 2003;218:113–22.
[18] Gopalakrishnan P, Shankar P, Subba Rao RV, Sundar M, Ramakrishnan SS. Laser surface modification of low carbon borided steels. Scripta Materialia 2001;44:707–12. [19] Podczernjajeva IA. Formirovanije i svojstva poverchnostnogo sloja pri kompleksnom lazernom borirovanii uglerodistych stalej. Poroszkovaja Metallurgija 1997;1(2):74–8. [20] Safonov AN. Osobiennosti borirovanija z˙eleza i stalej s pomoszczju nieprierywnogo CO2—lazera. Metalovedenije i Termiceskaja Obrabotka Metallov 1998;1:5–9. [21] Przybylowicz K, Depczynski W, Mola R. Modification of diffusion coatings structure by laser heat treatment. Inz˙ynieria Materialowa 1998;4:1043–6. [22] Pertek A, Kulka M. Characterization of single tracks after laser surface modification of borided 41Cr4 steel. Applied Surface Science 2003;205(1–4): 137–42. [23] Kulka M, Pertek A. Microstructure and properties of borided 41Cr4 steel after laser surface modification with re-melting. Applied Surface Science 2003; 214:278–88. [24] Kulka M, Pertek A. Microstructure and properties of borocarburized 15CrNi6 steel after laser surface modification. Applied Surface Science 2004;236(1–4): 98–105. [25] Kulka M, Pertek A. Laser surface modification of carburized and borocarburized 15CrNi6 steel. Materials Characterization 2007:461–7058/5 2007: 461–70. [26] Kulka M, Makuch N, Pertek A, Piasecki A. Microstructure and properties of borocarburized and laser-modified 17CrNi6-6 steel. Optics and Laser Technology 2012;44:872–81. [27] Molian PA, Rajasekhara HS. Laser glazing of boronized iron and tool steels. Surface Engineering 1986;2(4):269. [28] Kulka M, Pertek A. Gradient formation of boride layers by borocarburizing. Applied Surface Science 2008;254:5281–90. [29] Kulka M, Pertek A, Makuch N. The importance of carbon concentration–depth profile beneath iron borides for low-cycle fatigue strength. Materials Science and Engineering A 2011;528:8641–50. [30] Goly M. Kusinski J. Microstructure and properties of the laser treated 30CrMnMo16-8 chromium steel. in: Pawe" Kurtyka et al., editors. Problems of modern techniques in aspect of engineering and education, Monography Cracow: Institute of Technology; 2006. P. 183–8. [31] Major B. Laser processing for surface modification by remelting and alloying of metallic systems in: Materials Surface Processing by directed energy techniques. Yves Paleau, editor. Elsevier; 2006[chapter 7]. [32] Bartkowiak, K., Waligora, W., Laser alloying of the construction steel 45 with boron. The scientific conference with foreign participation, TRANSFER 2001, Trencˇin, Slovak Republic, 23-24 October 2001, 175-80. [33] Morimoto J, Ozaki T, Kubohori T, Morimoto S, Abe N, Tsukamoto M. Some properties of boronized layers on steels with direct diode laser. Vacuum 2009;83:185–9. [34] Choudhury AR, Ezz T, Li L. Synthesis of hard nano-structured metal matrix composite boride coatings using combined laser and sol–gel technology. Materials Science and Engineering A 2007;445–446:193–202. [35] Paczkowska M, Ratuszek W, Waligora W. Microstructure of laser boronized nodular iron. Surface and Coatings Technology 2010;205:2542–5. [36] Tayal M, Mukherjee K. Localized boriding of low-carbon steel using a Nd:YAG laser. Journal of Materials Science 1994:5699–702. [37] Vorosnin LG, Lahovic LS. Borirovanie Stali. Moskva: Metallurgia; 1978. [38] Verein Deutscher Ingenieure Normen, VDI 3198, VDI verlag (1991). [39] Vidakis N, Antoniadis A, Bilalis N. The VDI 3198 indentation test evaluation of a reliable qualitative control for layered compounds. Journal of Materials Processing Technology 2003;143–144:481–5.
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Microstructure and properties of laser-borided 41Cr4 steel M. Kulka n, N. Makuch, A. Pertek Poznan University of Technology, Institute of Materials Science and Engineering, Pl. M.Sklodowskiej-Curie 5, 60-965 Poznan, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 April 2012 Received in revised form 17 June 2012 Accepted 21 June 2012 Available online 10 July 2012
Laser-boriding, instead of diffusion-boriding, was applied to formation of boride layers on 41Cr4 steel. The microstructure and properties of these layers were compared to those obtained after typical diffusion-boriding. Three zones characterized the microstructure of laser-borided layer: laser-borided zone, hardened medium-carbon zone (heat affected zone) and medium-carbon substrate without heat treatment. The through-hardened laser-borided steel was also analyzed. In this case two zones characterized the microstructure: laser-borided zone and hardened medium-carbon substrate. The microstructure of laser-borided zone consisted of eutectic mixture of borides and martensite. This phase composition (especially martensite presence) was the reason for microhardness decrease at the surface in comparison with diffusion-borided steel. However, the use of laser-boriding causes the decrease in microhardness gradient between the surface and the substrate in comparison with typical diffusion-boriding process. The value of mass wear intensity factor of the hardened laser-borided layer was comparable to that obtained in case of diffusion-boriding and through-hardening. The use of laserborided layers instead of typical diffusion-borided layers may be advantageous under conditions of high abrasive wear of mating parts. For the experimental condition used, the laser-boriding process presented worst results concerning the fatigue strength. The cracks formed on the surface during laser re-melting were the reason for relatively quick first fatigue crack. In case of elements, which require high fatigue strength, the use of modified laser processing parameters would be necessary. The better results should be obtained by increasing of tracks overlapping. Although the cohesion of laser-borided layer was sufficient, the diffusion-borided layer showed a better cohesion. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Laser-boriding Diffusion-boriding Microstructure Low-cycle fatigue
1. Introduction Diffusion-boriding is a thermochemical surface treatment in which boron atoms diffuse into the surface of a workpiece to form borides with the base material. When applied to the adequate materials, boronizing provides wear and abrasion resistance comparable to sintered carbides. The selection of material is very important. The process can be applied to a wide range of steel alloys, including carbon steel, low alloy steel, tool steel and stainless steel. In general, boriding of steels results in the formation of FeB and Fe2B needle-like phases in diffusion layer. The iron borides are characterized by many advantageous properties: high hardness (up to 2000 HV), high abrasive wear resistance, the advantageous profile of residual stresses, high heat resistance, high corrosion resistance in acid and alkaline solutions, high resistance to influence of liquid metals and alloys and high hardness at increased temperatures [1–7]. However, the important drawback of these layers is their brittleness [3,5,7]. It is
n
Corresponding author. E-mail address: [email protected] (M. Kulka).
0030-3992/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlastec.2012.06.030
caused by a high hardness of iron borides and by a large hardness gradient between the boride zone and the substrate. The brittleness of boride layer was reduced with the use of various methods. The production of a single-phase Fe2B layer [6,7] was effectively applied in order to eliminate this disadvantage. The lower hardness of this phase (1400–1600 HV) caused a reduction in the gradient of the hardness between the surface and base material. The other method consisted of the formation of multicomponent and complex borided layers [8–17,28,29]. The positive effect was also obtained applying the laser-heat treatment after diffusion-boriding [18–27]. In recent years, laser beam was being used for a wide range of applications in order to modify the microstructure and properties of the steel. The role of laser technology intensively increased in surface engineering. The most important processes were as follows: laser-heat treatment, laser alloying and laser overlaying [30,31]. The laser-heat treatment was applied to the modification of microstructure and properties of previous diffusion-borided layers. This modification was carried out with or without visible re-melting. The LHT with re-melting [18,22,23,27] provided the boride coating consisting of eutectic mixture of iron borides and the martensite. In case of LHT without re-melting [24–26], the laser-treated boride zone consisted of iron borides of modified
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morphology. These borides assumed a globular shape. All these processes provided the advantageous functional properties, often exceeding the properties characteristic of diffusion-borided layers. It was demonstrated that the laser heat treatment could successfully be applied instead of the traditional heat treatment (through hardening) of diffusion-borided steels. The laser-boriding process was also widely developed [31–36]. Two methods of laser-boriding was usually used: two-step process [32–36], which consists of re-melting of substrate material previously coated by alloying material (e.g. paste with boron) and single-step process, in which the alloying material (e.g. powder with boron, borides or boron carbide) was introduced to molten pool during laser re-melting [31]. The majority of these papers concerned examinations of the microstructure, phase composition and hardness of the layers obtained. In some papers [32,35] the abrasive wear resistance was studied. However, the investigation did not provide a comparison with diffusionborided layer. The laser-boriding could be an interesting alternative for diffusion-boriding in order to reduce the brittleness of boride layers. In the result of this process a smaller gradient of the hardness was obtained between the surface and the core of steel in comparison with the typical diffusion-borided layers [31–36]. There is not much information regarding the fatigue strength of boride layers, especially, those produced by laser-boriding. The influence of boronizing on the fatigue strength is ambiguous [37], because it depends on many factors: boriding method, boriding parameters, chemical composition of borided steel, heat treatment after boriding, and the defects of the layers. The diffusionboriding, followed by proper heat treatment usually causes an occurrence of compressive stresses at the surface. The presence of such stresses in the surface layer causes, as a rule, an increase in fatigue strength. The tensile stresses on the surface are disadvantageous. The sum of tensile residual stresses and stresses caused by external forces can exceed the limit of tensile strength. In effect, surface cracks can be observed and the fatigue resistance decreases. Therefore, the selection of laser-boriding parameters is very important for fatigue strength of the produced layers. The possible surface cracks caused by this process are inadvisable. In this paper the laser alloying by boron was used in order to form the boride layers. The laser-boriding, instead of conventional diffusion process, was carried out on 41Cr4 steel. The multiple laser tracks were produced on an entire cylindrical surface of the samples. Therefore, there was a possibility of examining of wear resistance and low-cycle fatigue strength. The microstructure, the microhardness profiles and the cohesion were also investigated and compared to the properties obtained for typical diffusionborided layer. It was one of the main purposes of the presented study. The possibilities of application of laser-borided layers were analyzed depending on operating conditions of workpieces.
2.2. Diffusion-boriding Gas diffusion-boriding was carried out with the usage of the devices presented in the papers [14,24]. The process was conducted in quartz tube. Before heating, the air was removed from this tube by the vacuum pump. Then, the flow of nitrogen was activated and the heating process was started. After the furnace had reached the required temperature of 950 1C (1223 K), a gas mixture of hydrogen and BCl3 was fed through the quartz tube at a flow rate of 90 l/h. This gas mixture consisted of 95 vol% H2 and 5 vol% BCl3. The diffusion-boriding continued for 3 h, then the process finished and the specimens were cooled in a nitrogen atmosphere. Typical heat treatment was carried out after diffusionboriding. The specimens were through-hardened: austenitized at 850 1C (1123 K), quenched in oil and tempered at 150 1C (423 K). 2.3. Laser-boriding The laser-boriding, instead of conventional diffusion process, was also carried out. The two-step process was applied. The external cylindrical surface of substrate material was coated by a paste consisting of amorphous boron, water-glass and distilled water. Boron paste of 40 mm thickness was used. The possibility of cracking of the boron paste in front of the laser beam due to thermal shock was not investigated. However, in the future it should be taken into consideration. Then the surface was remelted by laser beam (Fig. 1). Boriding was carried out by the TRUMPF TLF 2600 Turbo CO2 laser of the nominal power 2.6 kW operated with the following parameters: power P¼1.17 kW and scanning rate vl ¼3.84 m/min. The multiple mode laser beam (TEM01n) of circular shape was applied. The diameter of laser beam was equal to 2 mm. Therefore, the averaging irradiance (E) of 37 kW/cm2 was used. The diameter of laser beam was determined with the help of actinosensitive paper. The focusing mirror of curvature 250 mm, diameter 48 mm and focal length 125 mm was used in order to focus the laser beam. The distance from the bottom edge of fixing holder of focusing mirror to the laserborided surface was equal to 106.8 mm. This distance was longer than the distance to the focused beam (91.8 mm from the bottom edge of fixing holder of the mirror). The TEM01n mode, the so-called doughnut mode, is a special case consisting of a superposition of two TEM01 modes, rotated at 901 with respect to one another. The irradiance profiles and their influence on the laser track dimension are shown in Fig. 2. The radial dependence of irradiance is presented in Fig. 2a. The treated surface is moved relative to the laser beam during laser modification. That is why the obtained melted profile doesn’t correspond to the laser beam mode TEM01n. The projection of irradiance profile on the laser-treated surface is visible in Fig. 2b.
2. Experimental details 2.1. Material Steel 41Cr4 was investigated. Its chemical composition was presented in Table 1. The ring-shaped specimens (external diameter ca. 20 mm, internal diameter ca. 12 mm and height 12 mm) were used for the study. Table 1 Chemical composition of steel used (wt%). Material
C
Cr
Mn
Si
Mo
Ni
41Cr4
0.42
1.06
0.72
0.22
0.04
0.08
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Fig. 1. Two-step method of laser-boriding.
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arranged as multiple tracks (Fig. 3), with the distance f¼0.75 mm. It corresponded to the distance between the axes of the adjacent tracks and resulted from the feed rate used. The obtained scanning rate vl (3.84 m/min) resulted from the rotational speed n (61.15 min 1) and feed rate vf (0.75 mm/revolution). Some of laser-borided specimens were through-hardened: austenitized at 850 1C (1123 K), quenched in oil and tempered at 150 1C (423 K). 2.4. Microstructure, phase analysis and microhardness profiles After the treatment described above, the samples were cut out perpendicular to the treated surface, across the diffusion-borided layer or across the laser tracks produced (in case of laser-boriding). Then the metallographic specimens were prepared. The samples were polished by using the abrasive papers of different granularities, and, finally, by applying Al2O3. In order to reveal the microstructure, the diffusion borided and laser-borided samples were etched with 5% solution of HNO3 in C2H5OH. The microstructures of polished and etched cross-sections of the specimens were observed by an optical microscope (OM) and scanning electron microscope (SEM). The phase analysis of laserborided layer was carried out by Kristalloflex 4 S X-Ray diffractometer using Mo Ka radiation. Microhardness profiles, through the investigated layers, were determined in the polished crosssections of specimens. The Vickers method, with the use of the apparatus ZWICK 3212 B, was applied for microhardness measurements. The tests were performed at the load P¼0.1 kg (about 0.981 N). 2.5. Abrasive wear The frictional pair, consisting of a roller as sample and a sintered carbide plate as counter-sample, was used in order to examine abrasive wear resistance (Fig. 4). The composition of
Fig. 2. Irradiance profiles and their influence on the laser track dimension: (a) radial dependence of irradiance, (b) projection of irradiance profile on the lasertreated surface, (c) image of laser track obtained; vl is –scanning rate, d laser beam diameter, b width of laser track, 1 re-melted zone,and 2 the heat affected zone.
The direction of the motion is also shown. The track image (Fig. 2c) indicates that the width of the laser track (b) is less than 2 mm because of the short exposure time during the relative motion of laser beam and the treated surface. It also results from the irradiance profile of laser beam. The irradiance is small in a distance greater than 0.66 mm from the axis of laser beam. The short exposure time and the small irradiance cause the heating of the material to a relatively low temperature. Therefore, for the experimental condition used, the effects of laser-heat treatment are not visible in the microstructure in a distance greater than 0.66 mm from the axis of laser beam. The laser alloying was done in air. Therefore, during this treatment an oxidation could take place. A non-oxidizing atmosphere (Ar) should be used in the future. The laser tracks were
Fig. 3. Method of multiple tracks production; d – laser beam diameter (d ¼ 2 mm); vf rate of feed; vl scanning rate; n rotational speed; f distance from track to track.
Fig. 4. Scheme of wear, P ¼ 147 N, rotational speed n ¼250 min 1.
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sintered carbide S20S was as follows: 58 wt% of WC, 31.5 wt% of (TiCþTaC þNbC), 10.5 wt% of Co. This material was characterized by a mass density 10.7 g/cm3 and hardness 1430 HV. The load P¼147 N and the specimen speed of 0.26 m/s were used during the test, carried out under conditions of dry friction (unlubricated sliding contact). Although the surface roughness was changed after laser-boriding, the surface was not prepared before the wear test. Wear resistance was evaluated by the factor of mass wear intensity Imw, which was defined as specimen mass loss per friction surface and unit of time. This factor corresponded to the slope of a straight line in the frictional wear diagram and was determined from equation: Dm mg Imw ¼ Ft cm2 h where Dm is the mass loss (mg), F the friction surface (cm2), and t the friction time (h). 2.6. Low-cycle fatigue A hydraulic pulsator MTS 810 of maximal load 100 kN was used during the low-cycle fatigue tests. The main elements of the testing equipment were as follows: the working elements (beam with force gage, upper head with clamp, supporting pillars, frame, piston with working head and clamp), electronic control, control panel, master switch, personal computer with software (Station Manager, Basic Test Ware), and hydraulic system with pump. The tests were carried out in order to obtain a complete fracture of the specimen or to determine the fatigue life. The specimens were put
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under radial compression. The compressive and bending stresses were generated. The scheme of the specimen position was presented in Fig. 5. The relatively high load (F) of 1.6–3.6 kN had been used, hence the fatigue cracks occurred after a low number of cycles. The compressive force (F) was modulated with a sinusoidal applied force of frequency 10 Hz and an amplitude (A) of 0.8–1.8 kN. The load and the amplitude were generated with the help of working head movement. During the test, the deflection (v) was registered continuously. The contacting ekstensometer MTS 634.31F-24 (axial—multiple gage length) was used for the measurements of deflection. 2.7. Cohesion A standard Rockwell method, as a destructive quality test for examined layers, was employed for determination of cohesion. The well-known adhesion test prescribed by the VDI 3198 norm [38] was used. The principle of this method was presented in the upper right part of Fig. 6 [39]. A conical diamond indenter penetrated into the surface of an investigated layer, thus inducing massive plastic deformation to the substrate and fracture of the diffusion-borided or laser-borided layer. As in every indentation test, the 1/10th rule should be accomplished. Therefore the overall specimen thickness should be at least ten times greater than the indentation depth. The type and the volume of the layer failure zone, firstly, showed the cohesion of the layer and secondly, its brittleness. The indentation craters were observed by an optical microscope. The damage to the layers was compared to the quality maps of adhesion strength HF1—HF6 (Fig. 6). In general, the adhesion strength quality HF1—HF4 defined sufficient adhesion, whereas HF5 and HF6 represented insufficient adhesion [39].
3. Results and discussion 3.1. Microstructure
Fig. 5. Scheme of specimen position during the low-cycle fatigue test. 1 thrust washers (41Cr4 steel); 2 specimen.
The OM microstructure of diffusion-borided layer formed on 41Cr4 steel, through-hardening is shown in Fig. 7. After diffusionboriding, iron borides were visible close to the surface. The results of earlier papers [12–17] indicate, that two types of iron borides were occurring in those layers: FeB and Fe2B. Two diffusion zones were observed: (1) iron borides near the surface and (2) hardened medium-carbon substrate below borides. The laser-boriding instead of typical diffusion process was carried out. The laser-heat treatment with re-melting was used
Fig. 6. Principle of the VDI 3198 indentation test [38].
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for laser alloying by boron. In the microstructure of laser-borided steel (Fig. 8) three zones were visible: (1) laser-borided zone, (2) hardened medium-carbon substrate, that is heat affected zone (HAZ) and (3) medium-carbon substrate without heat treatment. The continuous laser-borided zone was obtained at the surface. However, the depth of laser-borided zone was variable and considerably lesser at the contact of adjacent tracks. This zone consisted of eutectic mixture of iron borides and martensite [33,35,36]. The SEM image of laser-borided zone was presented in Fig. 9. The maximal depth of this zone was located in the axis of track. The obtained microstructure of laser-borided steel was very similar to the laser-modified diffusion-borided layer with
re-melting [22,23]. In both cases the three similar zones were observed: re-melted zone, hardened substrate (heat affected zone) and the substrate without visible effects of laser-heat treatment. These zones differed much from the microstructure of laser-modified (without re-melting) diffusion-borocarburized layer [24–26], in which the other zones were visible: iron borides of modified morphology (more globular borides), hardened carburized zone (heat affected zone) and carburized layer without heat treatment. The through-hardening was also carried out after laser-boriding. This treatment was necessary in order to obtain the same microstructure in the core of steel like in case of diffusion-borided steel. It was important from the point of view of planned examinations of fatigue strength. In this case, the microstructure (Fig. 10) was characterized by two zones: (1) laser-borided zone and (2) hardened medium-carbon substrate .
3.2. Phase analysis
Fig. 7. Microstructure of diffusion-borided and through-hardened 41Cr4 steel; 1 iron borides (FeBþ Fe2B); 2 hardened medium-carbon substrate.
The phase analysis was performed directly after laser boriding, without the through-hardening. The microstructure of the remelted track consisted of eutectic mixture of iron borides and the martensite. XRD scans of the laser-borided specimen confirmed the presence of two types of iron borides: Fe2B and Fe3B (Fig. 11). The X-ray diffraction did not provide information about the occurrence of martensite. The martensite was identified as Fea. Probably, the participation of this phase in the microstructure was too small. However, taking the high cooling rate during lasertreatment and the microhardness measurements (1100–1600 HV in laser re-melted zone) into consideration, the occurrence of martensite was assumed. Simultaneously, the segregation of boron took place during re-melting. In the result, after resolidification in some places iron richer Fe3B phase was formed.
Fig. 8. Microstructure of laser-borided 41Cr4 steel: (a) image of multiple tracks; (b) image of one track and interfaces between adjacent tracks; and (c) image in the axis of one track.
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This is in good agreement with the earlier findings [34,36], which have reported the formation of a Fe3B phase after laser-boriding. Carbon does not dissolve in FeB boride and its solubility in Fe2B boride is very low [37]. Therefore, during diffusion-boriding of steel carbon is moved in a core direction by following the boron diffusion front. Carbon is ejected from interstitial positions by
boron, which forms iron borides at the surface. This phenomenon is accompanied by an increase in carbon concentration beneath iron borides formed on the substrate of constant carbon content. Sometimes, especially in case of high-carbon steel borided, it provides the borocementite Fe3(C,B) in the microstructure beneath iron borides. In case of investigated laser-borided layer the occurrence of borocementite was not confirmed by phase analysis. The obtained phase composition of borides’ zone (eutectic mixture of iron borides and the martensite) was very similar to the laser-modified diffusion-borided layer with re-melting [22,23] and differed much from the microstructure of diffusionborocarburized layer after laser modification without re-melting [24–26]. The laser-heat treatment without re-melting caused a breakdown of the needle morphology of the iron borides, which assumed a more globular shape. However, the boride zone consisted of both iron borides (FeB and Fe2B).
Fig. 9. SEM image of laser-borided zone (multiple tracks).
Fig. 11. XRD patterns of laser-borided 41Cr4 steel (multiple tracks).
Fig. 10. Microstructure of laser-borided and through-hardened 41Cr4 steel: (a) image of multiple tracks, (b) image in the axis of one track, and (c) the interface between iron boride zone and hardened medium-carbon substrate.
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3.3. Microhardness profiles The phase composition obtained (especially, martensite and Fe3B-phase presence), was the reason for microhardness decrease at the surface in comparison with conventional through-hardened diffusion-borided steel. The microhardness profiles of laserborided layers (with or without through-hardening) were compared to the profiles obtained after diffusion-boriding and through-hardening (Fig. 12). The microhardness of laser-borided layers was measured along the axis of track. In laser-borided zone the microhardness about 1100–1600 HV was obtained. These values were lower in comparison with the conventional diffusion-boriding (1350–1850 HV). This situation was caused by the microstructure of laser-borided layer, which consisted of eutectic mixture of iron borides and martensite. Next the microhardness along the axis of track decreased to 650–720 HV in HAZ. The increase of the distance from the surface resulted in a gradual decrease in the microhardness, to 270–340 HV in the core of medium-carbon steel without heat treatment. In case of diffusion-borided and through-hardened layer, the microhardness in iron borides’ zone of about 1450–1850 HV was obtained. These values were characteristic of FeB and Fe2B iron borides. Next, the microhardness decreased to 680–710 HV in hardened medium-carbon substrate. The microhardness of through-hardened laser-borided layer was measured along the axis of track and along the contact of tracks at overlapped region (Fig. 13). At the surface of that layer the microhardness of about 1310–1600 HV was observed. Next, the microhardness of laser-borided re-melted zone decreases to 1150– 1250 HV. Beneath laser-borided zone the values of microhardness are characteristic of through-hardened medium-carbon steel. The increase in the distance from the surface was accompanied by a gradual decrease in the microhardness to 630–710 HV. The measurements along the contact of tracks indicated that the laserboride zone was characterized by a lower thickness in this area. The microhardness profiles along the straight line in parallel to the surface as a function of the distance from track axis were presented in Fig. 14. The measurements were carried out as 0.02 mm and 0.15 mm from the surface. At the depth of 0.02 mm the typical microhardness of laser-borided zone was observed. The values of microhardness were ranged from 1000 HV to 1600 HV. At the depth of 0.15 mm the microhardness was changed from 650 to 710 HV in the vicinity of the contact of tracks (at overlapped region) to 1000–1500 HV in the vicinity of the axis of track. It results from changeable microstructure at this depth. At the contact of tracks the hardened medium-carbon
Fig. 13. Microhardness profiles of laser-borided and through-hardened steel.
Fig. 14. Microhardness profiles along the surface of multiple tracks at the different depth.
substrate was observed, while in the axis of tracks laser-borided zone occurred. The obtained microhardness profiles of laser-borided steel was very similar to the laser-modified diffusion-borided layer with remelting [22,23]. In both cases, the values of microhardness in borides’ zone were lower in comparison with the conventional diffusion-boriding. It was caused by the similar microstructure of borides’ zone, which consisted of eutectic mixture of iron borides and the martensite. The occurrence of the martensite was the reason for the decreased microhardness in comparison with the diffusionborided layer. Therefore, the microhardness gradient between the surface and the substrate was diminished, what could cause the lower brittleness of borides’ zone. The other situation was observed in case of the laser-modified (without re-melting) diffusion-borocarburized layer [24–26]. The microhardness of borides’ zone, consisting of iron borides (FeB and Fe2B) of modified morphology, was comparable with the typical diffusion-borided layer. However, the microhardness gradient between the surface and the core of steel was also diminished due to the occurrence of heat affected zone (hardened carburized zone) and carburized layer without heat treatment. In these zones microhardness gradually decreased. Therefore, this method of laser-heat treatment could also be used in order to diminish the brittleness of boride layers. 3.4. Abrasive wear
Fig. 12. Microhardness profile of laser-borided layers compared to profiles obtained after diffusion-boriding and through-hardening.
The results of wear resistance tests were presented in Fig. 15 and in Table 2. The measurements of mass (m) were performed
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Fig. 15. Mass loss of investigated layers during abrasive wear.
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Fig. 16. Results of low-cycle fatigue during radial compression of diffusionborided and through-hardened 41Cr4 steel.
Table 2 The results of wear resistance tests. Material
Type of process
Mass wear intensity factor Imw [mg/(cm2 h)]
41Cr4
Boriding and through hardening Laser boriding and through hardening
1.42 1.40
with the accuracy of 0.1 mg. The measuring accuracy of diameter (d) and height (h) of the samples amounted to 0.001 cm. The maximal measuring errors did not exceed the measuring accuracy. Therefore, taking the maximal mass loss Dm ¼97.4 mg into consideration, the maximal measuring errors of Dm/F values did not exceed 0.044 mg cm 2. These errors were smaller than the size of test points marked in Fig. 15. The laser-borided and through-hardened material was characterized by a constant value of mass wear intensity factor: 1.4 mg/(cm2 h). The obtained value was comparable to the characteristic of diffusion-boriding and through-hardening. The results showed that the laser-boriding could be used instead of diffusion-boriding during the formation of boride layers of high abrasive wear resistance. However, the laser re-melting of diffusion-borided layer [23] provided a higher abrasive wear resistance. The laser-modified borocarburized layer was also characterized by a higher resistance to wear [26].
Fig. 17. Results of low-cycle fatigue during radial compression of laser-borided and through-hardened 41Cr4 steel.
3.5. Low-cycle fatigue The results of low-cycle fatigue, during the radial compression, were presented in Figs. 16 and 17. The through-hardened diffusion-borided and through-hardened laser-borided specimens were examined. The determined profiles of the compressive force and deflection were similar in both cases. However, the detailed analysis of obtained profiles (Figs. 18 and 19) showed that in case of laser-borided layer the first crack was observed considerably earlier (after 11,410 cycles), while in case of diffusion-borided layer the first crack occurred at the end of test, before the complete fracture of specimen. A comparison of the results after the low-cycle fatigue tests was shown in Table 3. The complete fractures of both specimens were observed after comparable number of cycles. However, in case of laser-borided and through-hardened steel the first crack was observed after the lower number of cycles. The surface cracks formed during laser re-melting were the reason for relatively quick first fatigue crack. Therefore, diffusion-borided and through-hardened steel was characterized by the higher
Fig. 18. First crack of diffusion-borided and through-hardened 41Cr4 steel during low-cycle fatigue test.
resistance to low-cycle fatigue during radial compression in comparison with the laser-borided and through-hardened material. For the experimental condition used, the laser-boriding process presented worst results. In case of elements, which need high fatigue strength, the use of controlling laser processing
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Table 4 The results of cohesion tests.
Fig. 19. First crack of laser-borided and through-hardened 41Cr4 steel during lowcycle fatigue test.
Table 3 The results of low-cycle fatigue tests. Type of process
First crack Complete fracture of specimen [number of cycles] [number of cycles]
Diffusion boriding and through hardening Laser boriding and through hardening
25894
26608
11410
25257
parameters or the use of preheating before the laser-boriding [33] would be necessary. It would prevent the surface cracks and would cause the increase in fatigue strength. It seems that making the feed rate (vf) smaller during laser-boriding would be a better solution what would result in increasing of tracks overlapping. The results of the paper [23] indicated the possibility of preventing the surface cracks and obtaining the more uniform laser remelted layer. Simultaneously, such condition could reduce the variation in hardness profile. The laser-boriding, presented in this study, could be an alternative for the time-consuming process of diffusion-boriding. The suitable selection of the parameters of laser-boriding would be necessary. In order to increase the fatigue strength, the diffusion-borocarburized layers were formed as a result of the two-step diffusion-treatment (carburizing followed by boriding). It is well known that carburized layers were characterized by high fatigue resistance. The earlier research [28] indicated the higher fatigue strength of the borocarburized layer in comparison with typical diffusion-borided layer formed on medium-carbon steel. However, the suitable carbon concentration–depth profile beneath iron borides was required for a meaningful increase of fatigue strength [29]. Then, the fatigue strength of borocarburized layer was comparable to that of the carburized layers. The lasermodified (without re-melting) borocarburized layer [26] was characterized by the fatigue strength comparable to the laserborided, or diffusion-borided layers, presented in this study. It resulted from the less advantageous carbon concentration–depth profile produced in the paper [26]. 3.6. Cohesion The Rockwell C indentation test was applied to cohesion rating of through-hardened diffusion-borided layers and through-hardened laser-borided layers. The sufficient cohesion was obtained in both analyzed cases (Table 4). The indentation craters obtained
Type of process
HF standard (VDI 3198 norm)
Diffusion boriding and through hardening Laser boriding and through hardening
HF1 HF3
on the surface of the hardened diffusion-borided medium-carbon steel (Fig. 20) evidently revealed ideal cohesion (HF1 standard). The interfacial bonds were so strong that even at the region where the substrate piles up, there was not any indication of delamination. Fig. 21 showed the indentation craters on the surface of laser-borided and through-hardened 41Cr4 steel. OM images indicated that there were radial cracks at the perimeter of indentation craters. However, a small quantity of spots with flaking or delamination was visible (HF3 standard). These results showed a better cohesion of diffusion-borided and throughhardened steel in comparison with the laser-borided and through-hardened material. The laser-modified borocarburized layer was also characterized by a better cohesion [26]. The modification of the laser-boriding parameters, mentioned above, could improve this property.
4. Summary and conclusions Laser-boriding, instead of diffusion-boriding, was applied to formation of boride layers on 41Cr4 steel. Three zones characterized the microstructure of laser-borided steel: laser-borided re-melted zone (eutectic mixture of iron borides and martensite), hardened medium-carbon substrate (heat affected zone) and medium-carbon substrate without heat treatment. The laserborided and through-hardened steel was also analyzed. In this case two zones characterized the microstructure: laser-borided (re-melted) zone and hardened medium-carbon substrate. The microstructure of laser-borided zone consisted of eutectic mixture of borides and martensite. This phase composition (especially martensite presence) was the reason for microhardness decrease at the surface in comparison with diffusion-borided steel. However, the use of laser-boriding caused a decrease in microhardness gradient between the surface and the substrate in comparison with typical diffusion-boriding process. The results of abrasive wear tests show that there is a possibility that the laser-boriding can be used instead of diffusion-boriding during the formation of boride layers of high abrasive wear resistance. However, for the experimental condition used, the laser-boriding process presented the worst results concerning the fatigue strength. Although the complete fractures of both specimens were observed after comparable number of cycles, the first crack of laser-borided and through-hardened specimen was observed after the lower number of cycles. The surface cracks formed during laser re-melting were the reason for relatively quick first fatigue crack. Simultaneously, the results of Rockwell C indentation tests showed a better cohesion of diffusion-borided and through-hardened material in comparison with the laser-borided and through-hardened steel. In case of elements, which require high fatigue strength, the use of modified laser processing parameters would be necessary. The smaller feed rate (vf) during laser-boriding would be a better solution what would result in increase of tracks overlapping. According to the previous study [23], it would prevent the surface cracks and would cause the increase in fatigue strength. The conditions used in the present study cannot be applied easily in laser manufacturing engineering. The use of the
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Fig. 20. OM images of diffusion-borided and through-hardened 41Cr4 steel after the Rockwell C indentation test (HF1 standard).
Fig. 21. OM images of laser-borided and through-hardened 41Cr4 steel after the Rockwell C indentation test (HF3 standard).
proposed laser-technology for industrial applications requires the adaptation in many respects, e.g. for the sake of the type of laser used, the optimization of laser processing parameters or minimization of production costs.
Acknowledgments This work has been financially supported by The Ministry of Science and Higher Education in Poland as a part of the N N507
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452537 project. The authors wish to thank Prof. M. Kawalec and Dr. M. Jankowiak from Division of Machining for their help and cooperation during the laser treatment and Dr. P. Wasilewicz from Division of Strength of Materials and Constructions for help during the low-cycle fatigue tests. References [1] Graf von Matuschka A. Borieren,. Munchen, Wien: Carl HanserVerlag; 1977. [2] Wierzchon. T. The role of glow discharge in the formation of a boride layer on steel in the plasma boriding process.Advances in low-temperature plasma chemistry, technology, applications, vol. 2. Lancaster-Basel, USA: Technomic Publishing Co. INC.; 1988. [3] Venkataraman B, Sundararajan G. The high speed sliding wear behaviour of boronized medium carbon steel. Surface and Coatings Technology 1995; 73:177–84. [4] Hunger HJ, Trute G. Boronizing to produce wear-resistant surface layers. Heat Treatment of Metals 1994;21(2):31–9. [5] Xu CH, Xi JK, Gao W. Improving the mechanical properties of boronized layers by superplastic boronizing. Journal of Materials Processing Technology 1997;65:94–8. [6] Pertek A. Gas boriding condition for the iron borides layers formation, 163–165. Switzerland: Materials science forum, Trans Tech Publications; 1994 pp. 323–328. ¨ ¨ [7] Liliental W, Tacikowski J. Einfluss der Warmebehandlung auf die Sprodigkeit ¨ ¨ von Boridschichten auf Stahlen. Harterei—Technische Mitteilungen 1980; 35:251–6. [8] Yan PX, Su YC. Metal surface modification by B–C-nitriding in a twotemperature-stage process. Materials Chemistry and Physics 1995;39:304–8. [9] Wierzchon T, Bielinski P, Sikorski K. Formation and properties of multicomponent and composite borided layers on steel. Surface and Coatings Technology 1995;73:121–4. [10] Sikorski K, Wierzchon T, Bielinski P. X-ray microanalysis and properties of multicomponent plasma-borided layers on steels. Journal of Materials Science 1998;33:811–5. [11] Toroghinezhad MR, Salehi M, Ashrafizadeh F. The effect of precarburizing treatment on morphology of the boride layer. Materials and Manufacturing Processes 1997;12(1):117–23. [12] Pertek A, Kulka M. Characterization of complex (Bþ C) diffusion layers formed on chromium and nickel-based low-carbon steel. Applied Surface Science 2002;202:252–60. [13] Pertek A, Kulka M. Microstructure and properties of composite (Bþ C) diffusion layers on low-carbon steel. Journal of Materials Science 2003;38: 269–73. [14] Pertek A, Kulka M. Two-step treatment carburizing followed by boriding on medium-carbon steel. Surface and Coatings Technology 2003;173:309–14. [15] Kulka M, Pertek A. The importance of carbon content beneath iron borides after boriding of chromium and nickel-based low carbon steel. Applied Surface Science 2003;214:161–71. [16] Kulka M, Pertek A, Klimek L. The influence of carbon content in the borided Fe-alloys on the microstructure of iron borides. Materials Characterization 2006;56(3):232–40. [17] Kulka M, Pertek A. Characterization of complex (B þC þ N) diffusion layers formed on chromium and nickel-based low-carbon steel. Applied Surface Science 2003;218:113–22.
[18] Gopalakrishnan P, Shankar P, Subba Rao RV, Sundar M, Ramakrishnan SS. Laser surface modification of low carbon borided steels. Scripta Materialia 2001;44:707–12. [19] Podczernjajeva IA. Formirovanije i svojstva poverchnostnogo sloja pri kompleksnom lazernom borirovanii uglerodistych stalej. Poroszkovaja Metallurgija 1997;1(2):74–8. [20] Safonov AN. Osobiennosti borirovanija z˙eleza i stalej s pomoszczju nieprierywnogo CO2—lazera. Metalovedenije i Termiceskaja Obrabotka Metallov 1998;1:5–9. [21] Przybylowicz K, Depczynski W, Mola R. Modification of diffusion coatings structure by laser heat treatment. Inz˙ynieria Materialowa 1998;4:1043–6. [22] Pertek A, Kulka M. Characterization of single tracks after laser surface modification of borided 41Cr4 steel. Applied Surface Science 2003;205(1–4): 137–42. [23] Kulka M, Pertek A. Microstructure and properties of borided 41Cr4 steel after laser surface modification with re-melting. Applied Surface Science 2003; 214:278–88. [24] Kulka M, Pertek A. Microstructure and properties of borocarburized 15CrNi6 steel after laser surface modification. Applied Surface Science 2004;236(1–4): 98–105. [25] Kulka M, Pertek A. Laser surface modification of carburized and borocarburized 15CrNi6 steel. Materials Characterization 2007:461–7058/5 2007: 461–70. [26] Kulka M, Makuch N, Pertek A, Piasecki A. Microstructure and properties of borocarburized and laser-modified 17CrNi6-6 steel. Optics and Laser Technology 2012;44:872–81. [27] Molian PA, Rajasekhara HS. Laser glazing of boronized iron and tool steels. Surface Engineering 1986;2(4):269. [28] Kulka M, Pertek A. Gradient formation of boride layers by borocarburizing. Applied Surface Science 2008;254:5281–90. [29] Kulka M, Pertek A, Makuch N. The importance of carbon concentration–depth profile beneath iron borides for low-cycle fatigue strength. Materials Science and Engineering A 2011;528:8641–50. [30] Goly M. Kusinski J. Microstructure and properties of the laser treated 30CrMnMo16-8 chromium steel. in: Pawe" Kurtyka et al., editors. Problems of modern techniques in aspect of engineering and education, Monography Cracow: Institute of Technology; 2006. P. 183–8. [31] Major B. Laser processing for surface modification by remelting and alloying of metallic systems in: Materials Surface Processing by directed energy techniques. Yves Paleau, editor. Elsevier; 2006[chapter 7]. [32] Bartkowiak, K., Waligora, W., Laser alloying of the construction steel 45 with boron. The scientific conference with foreign participation, TRANSFER 2001, Trencˇin, Slovak Republic, 23-24 October 2001, 175-80. [33] Morimoto J, Ozaki T, Kubohori T, Morimoto S, Abe N, Tsukamoto M. Some properties of boronized layers on steels with direct diode laser. Vacuum 2009;83:185–9. [34] Choudhury AR, Ezz T, Li L. Synthesis of hard nano-structured metal matrix composite boride coatings using combined laser and sol–gel technology. Materials Science and Engineering A 2007;445–446:193–202. [35] Paczkowska M, Ratuszek W, Waligora W. Microstructure of laser boronized nodular iron. Surface and Coatings Technology 2010;205:2542–5. [36] Tayal M, Mukherjee K. Localized boriding of low-carbon steel using a Nd:YAG laser. Journal of Materials Science 1994:5699–702. [37] Vorosnin LG, Lahovic LS. Borirovanie Stali. Moskva: Metallurgia; 1978. [38] Verein Deutscher Ingenieure Normen, VDI 3198, VDI verlag (1991). [39] Vidakis N, Antoniadis A, Bilalis N. The VDI 3198 indentation test evaluation of a reliable qualitative control for layered compounds. Journal of Materials Processing Technology 2003;143–144:481–5.