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Characterization of boronized layers on a XC38 steel
Surface & Coatings Technology 201 (2006) 3475 – 3482 www.elsevier.com/locate/surfcoat
Characterization of boronized layers on a XC38 steel O. Allaoui a,⁎, N. Bouaouadja b , G. Saindernan c a
Laboratoire de Génie des Procédés, Université Amar Tellidji de Laghouat, B.P. 37 G, 03000, Algeria Laboratoire des Matériaux Non métalliques, Département d'O.M.P., Université Ferhat, Abbas, SETIF 19000, Algeria Laboratoire Génie des Matériaux, Ecole Polytechnique de l'Université de Nantes, B.P., 50609, 44306 Nantes CEDEX 3, France b
c
Received 30 November 2005; accepted in revised form 27 July 2006 Available online 11 September 2006
Abstract Boronizing of an XC38 steel was performed by immersion in molten salts. These were based on a borax containing three reducing agents: boron carbide (B4C), aluminium (Al) and silicon carbide (SiC). This work gives a survey on the nature and quality of the layers which were obtained according to the boronizing bath. The mechanical features (hardness, scratch and wear resistance) of the deposited layers are discussed according to the experimental conditions used for their characterization. Effects of the boronizing bath composition on the obtained layers' quality are also discussed. According to the borax's reducing agents, the boronized layer deposited on the XC38 steel was either single- or double-phase. Al and B4C led to double-phased boronized layers, whereas SiC gave way to a single-phase layer. All the layers were of comparable hardness which was about 2100 HV on the samples for the boride FeB and 1800 HV for the boride Fe2B. The scratch and “pin-on-disk” wear resistance depended on the layers' microstructure. The best values of scratch and wear resistance were obtained for SiC which led to the formation of a single-phase layer. The apparition of scaling occurred at loads superior to 200 N. © 2006 Elsevier B.V. All rights reserved. Keywords: Boronizing; XC38 steel; Scratch test; Wear; Friction
1. Introduction Different superficial hardening processes are commonly applied to metals. Those are generally limited by the metal's original chemical composition as well as the required mechanical properties. Thermochemical methods, for which the superficial composition is locally modified, can produce microstructures and mechanical properties that are completely different from those of the basic metal. Two methods [1] for that are known: – In the first one, diffusing small atoms in the metal surface leads to the formation of an interstitial solid solution, – In the second, a chemical reaction between the diffused atoms and those of the basic metal leads to the formation of new compounds in the superficial layer. An example of this process is given by the boronizing treatment. Boronizing is a thermochemical surface treatment that can be applied to a large range of materials (ferrous metals, non-ferrous ⁎ Corresponding author. E-mail address: [email protected] (O. Allaoui). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.07.238
metals and cermets). It works by introducing boron atoms by diffusion into a substrate surface. The introduced boron atoms react with the material and form a number of borides. According to the Iron–Boron equilibrium diagram (Fig. 1), diffusing boron into the iron crystalline lattice leads to the formation of two kinds of iron borides (FeB and Fe2B) [2]. The thickness and proportion of each of those borides depend on the chemical composition of the boronizing environment, temperature and duration of treatment. Thus, the obtained boride layer is either single-phase (Fe2B only) or double-phase (FeB and Fe2B). The double-boride layer thickness can reach 200 μm. It consists of an external FeB layer and an internal Fe2B layer. Generally, a single-phase layer of Fe2B boride is preferred to the double-phase layer for two main reasons: the important brittleness of the FeB phase and the large difference between the expansion coefficients of the two kinds of borides. Indeed FeB has a 23 × 10− 6 °C− 1 coefficient of thermal dilatation for temperatures between 200 and 600 °C, whereas it is 7.85 × 10− 6 °C− 1 for Fe2B, for the same range of temperatures [3]. Compared to conventional surface treatments such carburizing (C), nitriding (N) or carbo-nitriding (CN), the boronizing treatment permits to get a largely superior surface hardness.
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carbon content. According to Kunst and Schaaber [3], this type of boronizing results in the boride layer becoming brittle and of little intertwining so that it readily spills from the substrate. – On the other hand, boronizing in a solid environment has a lot of industrial applications because of its cleanliness and simplicity. The boronizing cement is formed of three components: a boron-rich source, an activator that serves to deposit the atoms on the treated surface and an inert diluter. The main inconvenience of solid boronizing cements is their weak thermal conductivity. This makes it necessary to leave the samples for longer duration at elevated temperatures which results in a reduction of the treated material mechanical properties. Besides the treatment cost being high. – Boronizing in liquid environment circumvents this problem thanks to the big thermal inertia brought to the plunged pieces by molten salts. Reduction of boron compounds can take place either through electrochemical or chemical way by adding a reducing agent to the boronizing bath. According to Lyakhovich et al. [13], liquid environment boronizing in molten salts is obtained from the formation of a galvanic cell. In the latter, the substrate plays the role of the cathode whereas the reducing agent's fine particles (which stay in suspension in the bath) play the role of micro-anodes. Therefore, the driving force of boronizing in molten salts is the potential difference between the treated-piece surface and the reducing agent. Fig. 1. Iron–boron equilibrium diagram [2].
Indeed, the obtained hardness on boronized steels is greater than 1600 HV and remains constant at high temperature, whereas it does not exceed 1000 HV in the best conditions, when using conventional means. Therefore, the boride layer presents an excellent abrasive wear resistance [4]. The main advantage of boronizing metals is the possibility to alloy a high surface hardness with a low friction coefficient. This leads to a good wear resistance. Moreover, boride layers present longer life duration in service and good performances under oxidizing and corrosive atmospheres. As is the case for other thermochemical surface treatments, boronizing can be achieved in all environments by various processes and techniques: – in a solid environment using powders [5] or pastes [6], – in a liquid environment of molten salts with or without electrolysis [7–10], – in gaseous surroundings [11,12]. Every technique or process has its advantages and drawbacks: – Boronizing in a gaseous environment is generally not used in industry due to the inconveniences and the danger of the products which are used in this type of treatment. Indeed, diborane (B2H6) is widely-known for being toxic and flammable. Boron halides (BCl3, BF3 and BBr3) lead to non-compact boride layers with weak mechanical properties. Finally, organic compounds of boron such as (CH3)3B and (C2H5)3B are better used in carburizing than boronizing because of these compounds' high
Our work aims to study boronizing treatment without electrolysis of an XC38 steel. This treatment is considered in a liquid environment constituted of molten salts of borax (Na2B4O7) with different reducing agents (B4C, Al and SiC). We characterize the layers of obtained borides according to the chemical composition of the boronizing bath. We also study the bath chemical composition influence on the hardness, scratch and wear resistance of the obtained layers. 2. Experimental procedure Samples of an XC38 steel were selected for the boronizing treatment. This steel was chosen among the most used grades in the manufacturing of mechanical pieces. The chemical composition of the XC38 steel was determined by spectrometry analysis and is given in Table 1. Samples of cylindrical shape (15 mm diameter and 12 mm height) in their as-received metallurgical state, without any preheat treatment, were boronized in different conditions. The initial hardness of the XC38 steel (before treatment) was 180 HV. Just before boronizing, all samples underwent a surface preparation with silicon carbide abrasive to eliminate all contamination that can hinder boron diffusion through the surface. The treatments were achieved in molten salts. These were constituted of sodium tetraborate (Na2B4O7, 70% in mass), also Table 1 Mean chemical composition of the XC38 steel used as substrate Elements
C
Mn
Si
P
S
Cu
Cr
Ni
Fe
% (wt)
0.39
0.68
0.34
0.026
0.025
0.18
0.19
0.26
Balance
O. Allaoui et al. / Surface & Coatings Technology 201 (2006) 3475–3482
known as “borax”, added to 30% of a reducing agent. Three different reducing agents were used in this work: boron carbide (B4C), aluminium (Al) and silicon carbide (SiC). The borax–B4C and borax–SiC baths were mixed at ambient temperature before being introduced in the crucible and in the oven. On the other hand, for the borax–Al bath, the aluminium was not added to borax until the latter was completely molten. This was done to avoid any disruptive reaction between aluminium and the atmosphere during the bath temperature setting. Treatments of 4 h at 950 °C were applied to the samples in order to obtain a rather thick layer (N150 μm) suitable with the planned tests (microhardness, scratch and wear tests). After the boronizing treatments, all samples were left cooling in the ambient air. Some samples were sectioned longitudinally to obtain two sections for optic and electronic scanning microscopy observations. The phase identification of the treated samples was made by X-ray diffractions using Kα1 Cobalt line (λKα1 (Co) = 1.7902 nm). The samples' chemical composition was determined using the electron microprobe CAMECA MS 46. The boride layers and the underlying zone microhardness were measured on the cross section with loads of 25 and 50 g, while a 1-kg load was used for the steel substrate. The scratch tests were achieved on a REVETEST device equipped with an acoustic emission sensor that measures the loads in-situ when damage occurs, and another that permits direct recording of the tangent force which gives the instantaneous friction coefficient. The test consists in scratching the sample surface by using a diamond-made indenter with a 200 μm-diameter hemispheric head moving at a constant speed of 5 mm/min. In the present work, two types of scratching tests were made:
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3. Results and discussions 3.1. Samples treatment The boride layers obtained on the XC38 steel can be single or double-phase depending on the boronizing bath. The use of aluminium (Al) or boron carbide (B4C) as reducing agent led to the formation of double-phase boride layers constituted of FeB and Fe2B (Fig. 2a and b). On the other hand, the use of the
– In the first one, a single-pass was done under increasing normal load at a rate of 10 N/mm of covered distance. Applied loads were between 0 and 200 N. This permitted determination of the critical load (LC) corresponding to the apparition of the layer damage. – In the second test type, several passes under a constant load were made. This allowed measuring the boride layers resistance to severe cyclic solicitations. The cyclic scratch tests were done with a lower load than the critical load (LC). Several indenter passes were used to classify the different samples according to the number of passes they could resist without being damaged. Two loads of 20 and 50 N were used for these multi-pass tests on 5 mm distances. The pin-on-disk wear tests were achieved at ambient conditions without lubrication. A 780-μm radius tungsten carbide ball pin was used. The applied normal load was 10 N; the disk rotation speed was about 150 rpm and the covered radial distance was of 5 mm. The number of cycles and the slip speed were kept constant during all the tests in order to be able to compare the different samples' behaviours. The tangent strength was recorded continuously during the wear tests. The volume loss recorded on the worn distance was obtained with a mechanical sweeping profilometer provided with a 2-μm diameter diamond tip.
Fig. 2. Micrographs showing the boride phases formed in different boronizing baths after a treatment of 4 h at 950 °C: –Double-phase layers for the borax–B4C bath (a) and borax–Al bath (b). –Single-phase layer for the borax–SiC bath (c). (Notice the crack in the interface FeB/Fe2B).
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silicon carbide (SiC) as reducing agent led to a single-phase layer constituted of Fe2B (Fig. 2c). On Fig. 2a and b, the FeB boride can be clearly distinguished from the Fe2B boride thanks to a darker coloration. All single-
and double-phase layers obtained on the XC38 steel have acicular morphology with a perpendicular-to-steel-surface preferential orientation regardless of the boronizing bath. This jagged acicular shape of the boride layers is very advantageous
Fig. 3. X-ray patterns obtained for the layers of the three boronizing baths after treatment of 4 h at 950 °C: (a) borax–B4C bath, (b) borax–Al bath and (c) borax–SiC bath.
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because it ensures good layer gripping and adhesion to the substrate material. It is worth noticing the presence of cracks near the borides as illustrated in Fig. 2b. The diffraction diagrams of the boronized layers (Fig. 3) showed the absence of borax–SiC bath-obtained representative peaks of the FeB phase in the layer (Fig. 3(c)). This absence can be explained by the fact that the potential difference between the treated piece and the suspended micro-anodes SiC does not ensure a sufficient boron flux for the formation of a boron-rich boride. In the case of the borax–B4C mixture, the formation of the FeB boride can be explained by the very boron-rich bath. For the borax–Al bath, and according to Ellingham's diagrams [14], the reduction of boron oxide (B2O3) resulting from the thermal dissociation of borax, is made easier by aluminium (Al) than by carbon (C) or silicon (Si) rendering the formation of FeB boride easier. Microscopy and X-ray diffraction analysis showed that, for the same temperature (950 °C) and duration (4 h), the proportion of FeB boride in the layer obtained in the borax– Al bath was more important than for borax–B4C. This can be explained by the potential difference between the treated sample and the micro-anodes of Al and B4C in suspension in the molten salt. The potential difference is certainly more important in the case of aluminium. 3.2. Microhardness of the boronized layers The results of microhardness on boronized layers, underlying zone and substrate are given in Table 2. While taking into account the uncertainties associated with microhardness measurements on the different samples, we can say that the boronizing bath chemical composition do not have a significant influence on the boronized layers' hardness. The obtained values indicate that the microhardness is more important for the FeB than the Fe2B layer with a difference of about 300 HV units. Comparing our microhardness values to those obtained by other authors [15–17] (Table 3) show that they are in the same intervals given in the literature. These intervals are (1300– 1850) HV for Fe2B and (1700–2200) HV for FeB according to the conditions of the used treatment. In our opinion, differences in microhardness values for FeB and Fe2B borides can be explained by: – The chemical composition of the substrate where elements in steels can lead to borides with multiple-component layers
Table 2 Microhardness of boride layers, underlying zone and substrate Boronizing bath
Borax–B4C Borax–Al Borax–SiC
Microhardness
Substrate
Boride layer
Table 3 Microhardness of boride layers obtained from literature Microhardness of boride layers (HV) FeB
Fe2B
2050
1500
2250 2280 2440
1500 1600 1690
Substrate
Boronizing Reference environment
Low carbon steel
Liquid boronizing Liquid
Fe2B
2130 ± 70 2140 ± 71 –
1820 ± 61 1830 ± 62 1830 ± 62
such as (Fe,M)B and (Fe,M)2B, where M denotes the existing metal in steel. – The formed borides stoechiometry or the formation of instable boride Fe3B [18]. – The boronizing environment which has an influence on microstructure and in this way on microhardness. The increased microhardness values of the underlying zone and the substrate can be attributed to the displacement of carbon and other elements from the boride layers to the substrate and the presence of a little amount of boron in the matrix. In fact, the microprobe analysis clearly shows this displacement (Table 4). 3.3. Scratch resistance The single-pass scratch tests with increasing load led to three different types of damage: – Cracks that propagate in depth along the scratch trails (behind the indenter). These cracks were present in all samples with no exception. They have either a mosaic (Fig. 4 (a)) or a curvilinear shape that stands normal to the scratch axis (Fig. 4(b)). According to the literature [19] this type of cracks is characteristic of a Hertzian fracture on brittle solids when a blunted indenter is used. These cracks propagate in depth in a semi-conical shape and start at flaws near the contact surface where high-tension stresses develop. – Cracks that develop on the scratch sides and propagate away from the scratching zone. These cracks are caused by the plastic deformation of the substrate and are characterized by very variable lengths (Fig. 4(c)) [20]. Table 4 Distribution of elements in XC38 steel before and after boronizing⁎
Before boronizing After boronizing 680 ± 22 689 ± 23 685 ± 23
218 ± 7 220 ± 8 220 ± 8
[7]
Mn steel [15] High alloyed Cr steel High alloyed Cr, V, W, Mo steel 1760–1950 1380– Various chemical 1740 composition of steel 1950–2100 1380– Various chemical Various [16] 1450 composition of steel environment Ductile Liquid [17] 1665–2140 VHN iron (FeB + Fe2B)
Underlying zone
FeB
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XC38 Steel FeB Fe2B Substrate
⁎In all cases Fe is the balance.
C
Mn
Si
B
0.39 0.30 0.29 0.45
0.68 0.56 0.48 0.62
0.34 0.05 0.02 0.31
– 16.26 8.74 0.14
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Fig. 4. Cracking types produced by single-pass scratch tests under increasing load. Mosaic cracks (a), cracks with a curvilinear form (b), cracks on the scratch sides (c) and cohesive scaling (d).
– Cohesive scaling that appear on the sides at the extreme end of the scratch zone when the applied load becomes relatively high. These scaling cracks are related to the intrinsic coating brittleness. They are generally observed in the relatively thick layers (Fig. 4(d)). In no case, an adhesive scaling at the boride/substrate interface was observed. This was expected since it is wellestablished that the coatings achieved at high temperature present a good adhesion due to the inter-diffusion at the interface metallic continuity. These coatings are therefore less exposed to damage characterised by the separation of the interfaces [21]. The interfaces' cohesion is particularly reinforced in our case because of the irregular shape of borides' interfaces. The observations of the single-pass scratch tests samples revealed that the critical damage load (LC) always coincides with the apparition of cracks at the scratch trails. The results showed that the boride single-phase layer issued from the borax–SiC bath do not present any scaling for loads up to the used maximal load (Table 5).
After the single-pass scratch test, the samples were submitted to a multi-pass test under constant loading. This test allows characterizing the boronized layers' resistance to severe cyclic wear solicitations. The damages produced on the samples were invariably constituted of cohesive scaling on the sides of the trails. No cracks along the trail were observed in these conditions (Fig. 5). The results obtained by the multi-pass scratch test show that the boronized layer obtained with the borax–Al bath is less effective. Table 6 gathers the number of passes before damage for the three baths. A comparison of the performances of the different tested samples showed that the chemical composition of the
Table 5 Critical loads for scaling and cracking apparition on boronized layers during single-pass scratch tests using increasing loading Boronizing bath
Constituents of the layer
Critical damage load LC (N)
Load (N) corresponding to the apparition of scaling⁎
Borax– B4C Borax–Al Borax– SiC
FeB + Fe2B
125
∼ 180
FeB + Fe2B Fe2B
73 132
∼ 80 N200
⁎This load is estimated from the scratch trails' micrographs.
Fig. 5. Trails obtained at a constant load of 50 N: after 6 passes with scalings (a), after 1 pass with no apparent damage (b).
O. Allaoui et al. / Surface & Coatings Technology 201 (2006) 3475–3482 Table 6 Number of indenter passes before the layer damage Boronizing bath
Constituents of the layer
Borax–B4C Borax–Al Borax–SiC
FeB + Fe2B FeB + Fe2B Fe2B
Number of passes before damage 20 N
50 N
13 9 12
6 4 5
boronizing bath appreciably affects the scratch resistance of the layers. Indeed, the critical load for crack apparition along the trail and the number of indenter passes before damage were different for the samples issued from the three baths. The weak performance of the borax–Al bath-layer can be explained by the presence of a high proportion of the FeB boride that possesses a completely different thermal dilatation coefficient than that of Fe2B. As suggested by Vipin [22], this causes cracks formation at the level of the FeB/Fe2B interface (see Fig. 2(b)). 3.4. Wear resistance Both single- and double-phase tested layers presented comparable friction coefficient values (μ). These were comprised between 0.11 and 0.15 at the test beginning and between 0.44 and 0.48 toward the end. In all cases, the evolution of the tangential force according to cycle number allowed us to distinguish three phases from the obtained curves (Fig. 6). Three phases were observed on all friction curves: 1. A first phase with weak and nearly constant friction coefficient (μ = 0.11 to 0.15). This phase can be considered as an incubation period. It corresponds to the indenter slip on the sample, in the absence of all major damage. 2. A second phase where the friction coefficient increases progressively until reaching a maximal value between 0.55
Fig. 6. Friction curve evolution of the tangent force Ft according to cycle number for the borax–SiC sample and a normal load of 10 N.
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Table 7 Cycles number corresponding to phase I of the friction curve for the different baths Boronizing bath
Constituents of the layer
Cycles number in phase I
Borax–B4C Borax–Al Borax SiC
FeB + Fe2B FeB + Fe2B Fe2B
160 150 470
and 0.65. This phase is associated with the formation of a layer transferred from the tungsten carbide ball and the apparition of wear particles on the indenter trails. Examining the tungsten carbide ball after every test reveals wear traces on it. 3. A third phase where the friction coefficient stabilizes between 0.44 and 0.48. It seems that this phase is associated with an abrasive action by the wear particles previously formed. The decrease of the friction coefficient value is probably due to the grinding of the removed particles. The friction coefficient values obtained in this work (from 0.15 up to 0.65) are comparable to those obtained in the literature. Selçuk [23] found variable friction coefficient values between 0.36 and 0.62 on carbon steels. Yan-qiu Xia [24] gave values around 0.5 for loads varying from 20 N to 100 N. Besides, Venkataraman and Sundararajan [25] found that the friction coefficient of the boronized layers varies from 0.3 to 0.5 depending on the slip speed. The number of phase I-cycles (period with low friction coefficient and without major damage) is considered as the first criteria of wear resistance of boride layers on the different samples. This number is evaluated from the friction curves and is given in Table 7. A second quantification of wear was made by measuring the volume loss recorded on the trails left by the pin on the different samples (Fig. 7 ). It can be noticed that the single-layer made in the borax–SiC bath presents the best wear resistance in comparison with other tested samples. This result corroborates those obtained with scratch tests which show absence of scaling under the maximal
Fig. 7. Volume loss recorded on the traces left by the pin on the different samples.
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applied load and a delay of apparition of cracks along the scratch trails. Other studies also found that the single-phase layers are more effective than the double-phase layers [26]. They attribute this improved performance to the single-phase layer structural homogeneity. The previous criteria (volume loss and cycle number before damage onset), more or less classify the samples in the correct order. We can say that samples with a layer issued from borax– SiC bath have the best wear resistance. Those issued from borax–B4C and borax–Al baths have comparable performances, but these are significantly lower than the previous. 4. Conclusion – Boronizing in a liquid environment easily achieves layers of borides on the XC38 steel. Depending on the chemical composition of the bath, these layers can be single- or double-phase. The borax–B4C and borax–Al baths led to double-phase layers whereas borax–SiC bath led to a singlephase boride. – The proportion of FeB phase in the borax–Al bath-produced layers was more important than that obtained in the borax– B4C bath. – XC38 steel-boronized layers' microhardness was about 2100 HV for FeB boride and 1800 HV for Fe2B. The boronizing bath chemical composition had no significant effect on the layers' microhardness. – The microhardness of the underlying zone and the substrate was increased by the boronizing treatment. – The scratch and wear resistances depend on the layers' microstructure. The best performance was obtained on the single-phase boride layer produced with the borax–SiC bath.
References [1] I.M. Hutchings, in: E. Arnold (Ed.), Tribology: Friction and Wear of Engineering Materials (Metallurgy and Materials Sciences Series), 1995, p. 224. [2] O. Kubaschewski, “Fe–B, Iron–Boron.”, pp 15–18 of volume Iron-Binary phase diagrams, Springer-Verlag, 1982. [3] H. Kunst, O. Schaaber, Härt.-Tech. Mitt. 22 (1967) 275 (Translation HB 7122-III). [4] C. Martini, G. Palombarini, G. Poli, D. Prandstraller, Wear 256 (2004) 608. [5] K. Genel, et al., Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 347 (2003) 311. [6] I. Campos, et al., Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 352 (2003) 261. [7] K. Matiasovsky, et al., Surf. Coat. Technol. 35 (1988) 133. [8] L.S. Lyakhovich, et al., Met. Sci. Heat Treat. 18 (1976) 647. [9] C. Bindal, H. Üçisik, Surf. Coat. Technol. 122 (1999) 208. [10] Omar Allaoui, Mémoire de Magistère, Ecole Nationale Polytechnique d'Alger, 1995. [11] M. Kulka, A. Pertek, Appl. Surf. Sci. 214 (2003) 278. [12] K.S. Nam, et al., Surf. Coat. Technol. 98 (1998) 886. [13] L.S. Lyakhovich, et al., Met. Sci. Heat Treat. 14 (1972) 156. [14] Chiranjib Kumar Gupta, Chem. Metall. (2003) 225 ISBN: 3527303766. [15] L.S. Lyakhovich, et al., Met. Sci. Heat Treat. 11 (1969) 415. [16] L.S. Lyakhovich, Traitement thermochimique des métaux et alliages, 1981, p. 80, (in Russian). [17] U. Sen, et al., J. Mater. Process. Technol. 148 (2004) 1. [18] M. Palumbo, et al., Intermetallics 11 (2003) 1293. [19] B.R. Lawn, J. Am. Ceram. Soc. 81 (8) (1998) 1977. [20] R. Rezakhanlou, Thèse INPL Nancy (1991), France. [21] T. Arai, et al., Thin Solid Films 154 (1987) 387. [22] J. Vipin, et al., Surf. Coat. Technol. 149 (2002) 21. [23] B. Selçuk, et al., J. Mater. Process. Technol. 141 (2004) 1. [24] Yan-qiu Xia, et al., Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 354 (2003) 189. [25] B. Venkataraman, et al., Surf. Coat. Technol. 73 (1995) 177. [26] J. Subrahmanyam, K. Gopinath, Wear 95 (1984) 287.
Characterization of boronized layers on a XC38 steel O. Allaoui a,⁎, N. Bouaouadja b , G. Saindernan c a
Laboratoire de Génie des Procédés, Université Amar Tellidji de Laghouat, B.P. 37 G, 03000, Algeria Laboratoire des Matériaux Non métalliques, Département d'O.M.P., Université Ferhat, Abbas, SETIF 19000, Algeria Laboratoire Génie des Matériaux, Ecole Polytechnique de l'Université de Nantes, B.P., 50609, 44306 Nantes CEDEX 3, France b
c
Received 30 November 2005; accepted in revised form 27 July 2006 Available online 11 September 2006
Abstract Boronizing of an XC38 steel was performed by immersion in molten salts. These were based on a borax containing three reducing agents: boron carbide (B4C), aluminium (Al) and silicon carbide (SiC). This work gives a survey on the nature and quality of the layers which were obtained according to the boronizing bath. The mechanical features (hardness, scratch and wear resistance) of the deposited layers are discussed according to the experimental conditions used for their characterization. Effects of the boronizing bath composition on the obtained layers' quality are also discussed. According to the borax's reducing agents, the boronized layer deposited on the XC38 steel was either single- or double-phase. Al and B4C led to double-phased boronized layers, whereas SiC gave way to a single-phase layer. All the layers were of comparable hardness which was about 2100 HV on the samples for the boride FeB and 1800 HV for the boride Fe2B. The scratch and “pin-on-disk” wear resistance depended on the layers' microstructure. The best values of scratch and wear resistance were obtained for SiC which led to the formation of a single-phase layer. The apparition of scaling occurred at loads superior to 200 N. © 2006 Elsevier B.V. All rights reserved. Keywords: Boronizing; XC38 steel; Scratch test; Wear; Friction
1. Introduction Different superficial hardening processes are commonly applied to metals. Those are generally limited by the metal's original chemical composition as well as the required mechanical properties. Thermochemical methods, for which the superficial composition is locally modified, can produce microstructures and mechanical properties that are completely different from those of the basic metal. Two methods [1] for that are known: – In the first one, diffusing small atoms in the metal surface leads to the formation of an interstitial solid solution, – In the second, a chemical reaction between the diffused atoms and those of the basic metal leads to the formation of new compounds in the superficial layer. An example of this process is given by the boronizing treatment. Boronizing is a thermochemical surface treatment that can be applied to a large range of materials (ferrous metals, non-ferrous ⁎ Corresponding author. E-mail address: [email protected] (O. Allaoui). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.07.238
metals and cermets). It works by introducing boron atoms by diffusion into a substrate surface. The introduced boron atoms react with the material and form a number of borides. According to the Iron–Boron equilibrium diagram (Fig. 1), diffusing boron into the iron crystalline lattice leads to the formation of two kinds of iron borides (FeB and Fe2B) [2]. The thickness and proportion of each of those borides depend on the chemical composition of the boronizing environment, temperature and duration of treatment. Thus, the obtained boride layer is either single-phase (Fe2B only) or double-phase (FeB and Fe2B). The double-boride layer thickness can reach 200 μm. It consists of an external FeB layer and an internal Fe2B layer. Generally, a single-phase layer of Fe2B boride is preferred to the double-phase layer for two main reasons: the important brittleness of the FeB phase and the large difference between the expansion coefficients of the two kinds of borides. Indeed FeB has a 23 × 10− 6 °C− 1 coefficient of thermal dilatation for temperatures between 200 and 600 °C, whereas it is 7.85 × 10− 6 °C− 1 for Fe2B, for the same range of temperatures [3]. Compared to conventional surface treatments such carburizing (C), nitriding (N) or carbo-nitriding (CN), the boronizing treatment permits to get a largely superior surface hardness.
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carbon content. According to Kunst and Schaaber [3], this type of boronizing results in the boride layer becoming brittle and of little intertwining so that it readily spills from the substrate. – On the other hand, boronizing in a solid environment has a lot of industrial applications because of its cleanliness and simplicity. The boronizing cement is formed of three components: a boron-rich source, an activator that serves to deposit the atoms on the treated surface and an inert diluter. The main inconvenience of solid boronizing cements is their weak thermal conductivity. This makes it necessary to leave the samples for longer duration at elevated temperatures which results in a reduction of the treated material mechanical properties. Besides the treatment cost being high. – Boronizing in liquid environment circumvents this problem thanks to the big thermal inertia brought to the plunged pieces by molten salts. Reduction of boron compounds can take place either through electrochemical or chemical way by adding a reducing agent to the boronizing bath. According to Lyakhovich et al. [13], liquid environment boronizing in molten salts is obtained from the formation of a galvanic cell. In the latter, the substrate plays the role of the cathode whereas the reducing agent's fine particles (which stay in suspension in the bath) play the role of micro-anodes. Therefore, the driving force of boronizing in molten salts is the potential difference between the treated-piece surface and the reducing agent. Fig. 1. Iron–boron equilibrium diagram [2].
Indeed, the obtained hardness on boronized steels is greater than 1600 HV and remains constant at high temperature, whereas it does not exceed 1000 HV in the best conditions, when using conventional means. Therefore, the boride layer presents an excellent abrasive wear resistance [4]. The main advantage of boronizing metals is the possibility to alloy a high surface hardness with a low friction coefficient. This leads to a good wear resistance. Moreover, boride layers present longer life duration in service and good performances under oxidizing and corrosive atmospheres. As is the case for other thermochemical surface treatments, boronizing can be achieved in all environments by various processes and techniques: – in a solid environment using powders [5] or pastes [6], – in a liquid environment of molten salts with or without electrolysis [7–10], – in gaseous surroundings [11,12]. Every technique or process has its advantages and drawbacks: – Boronizing in a gaseous environment is generally not used in industry due to the inconveniences and the danger of the products which are used in this type of treatment. Indeed, diborane (B2H6) is widely-known for being toxic and flammable. Boron halides (BCl3, BF3 and BBr3) lead to non-compact boride layers with weak mechanical properties. Finally, organic compounds of boron such as (CH3)3B and (C2H5)3B are better used in carburizing than boronizing because of these compounds' high
Our work aims to study boronizing treatment without electrolysis of an XC38 steel. This treatment is considered in a liquid environment constituted of molten salts of borax (Na2B4O7) with different reducing agents (B4C, Al and SiC). We characterize the layers of obtained borides according to the chemical composition of the boronizing bath. We also study the bath chemical composition influence on the hardness, scratch and wear resistance of the obtained layers. 2. Experimental procedure Samples of an XC38 steel were selected for the boronizing treatment. This steel was chosen among the most used grades in the manufacturing of mechanical pieces. The chemical composition of the XC38 steel was determined by spectrometry analysis and is given in Table 1. Samples of cylindrical shape (15 mm diameter and 12 mm height) in their as-received metallurgical state, without any preheat treatment, were boronized in different conditions. The initial hardness of the XC38 steel (before treatment) was 180 HV. Just before boronizing, all samples underwent a surface preparation with silicon carbide abrasive to eliminate all contamination that can hinder boron diffusion through the surface. The treatments were achieved in molten salts. These were constituted of sodium tetraborate (Na2B4O7, 70% in mass), also Table 1 Mean chemical composition of the XC38 steel used as substrate Elements
C
Mn
Si
P
S
Cu
Cr
Ni
Fe
% (wt)
0.39
0.68
0.34
0.026
0.025
0.18
0.19
0.26
Balance
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known as “borax”, added to 30% of a reducing agent. Three different reducing agents were used in this work: boron carbide (B4C), aluminium (Al) and silicon carbide (SiC). The borax–B4C and borax–SiC baths were mixed at ambient temperature before being introduced in the crucible and in the oven. On the other hand, for the borax–Al bath, the aluminium was not added to borax until the latter was completely molten. This was done to avoid any disruptive reaction between aluminium and the atmosphere during the bath temperature setting. Treatments of 4 h at 950 °C were applied to the samples in order to obtain a rather thick layer (N150 μm) suitable with the planned tests (microhardness, scratch and wear tests). After the boronizing treatments, all samples were left cooling in the ambient air. Some samples were sectioned longitudinally to obtain two sections for optic and electronic scanning microscopy observations. The phase identification of the treated samples was made by X-ray diffractions using Kα1 Cobalt line (λKα1 (Co) = 1.7902 nm). The samples' chemical composition was determined using the electron microprobe CAMECA MS 46. The boride layers and the underlying zone microhardness were measured on the cross section with loads of 25 and 50 g, while a 1-kg load was used for the steel substrate. The scratch tests were achieved on a REVETEST device equipped with an acoustic emission sensor that measures the loads in-situ when damage occurs, and another that permits direct recording of the tangent force which gives the instantaneous friction coefficient. The test consists in scratching the sample surface by using a diamond-made indenter with a 200 μm-diameter hemispheric head moving at a constant speed of 5 mm/min. In the present work, two types of scratching tests were made:
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3. Results and discussions 3.1. Samples treatment The boride layers obtained on the XC38 steel can be single or double-phase depending on the boronizing bath. The use of aluminium (Al) or boron carbide (B4C) as reducing agent led to the formation of double-phase boride layers constituted of FeB and Fe2B (Fig. 2a and b). On the other hand, the use of the
– In the first one, a single-pass was done under increasing normal load at a rate of 10 N/mm of covered distance. Applied loads were between 0 and 200 N. This permitted determination of the critical load (LC) corresponding to the apparition of the layer damage. – In the second test type, several passes under a constant load were made. This allowed measuring the boride layers resistance to severe cyclic solicitations. The cyclic scratch tests were done with a lower load than the critical load (LC). Several indenter passes were used to classify the different samples according to the number of passes they could resist without being damaged. Two loads of 20 and 50 N were used for these multi-pass tests on 5 mm distances. The pin-on-disk wear tests were achieved at ambient conditions without lubrication. A 780-μm radius tungsten carbide ball pin was used. The applied normal load was 10 N; the disk rotation speed was about 150 rpm and the covered radial distance was of 5 mm. The number of cycles and the slip speed were kept constant during all the tests in order to be able to compare the different samples' behaviours. The tangent strength was recorded continuously during the wear tests. The volume loss recorded on the worn distance was obtained with a mechanical sweeping profilometer provided with a 2-μm diameter diamond tip.
Fig. 2. Micrographs showing the boride phases formed in different boronizing baths after a treatment of 4 h at 950 °C: –Double-phase layers for the borax–B4C bath (a) and borax–Al bath (b). –Single-phase layer for the borax–SiC bath (c). (Notice the crack in the interface FeB/Fe2B).
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silicon carbide (SiC) as reducing agent led to a single-phase layer constituted of Fe2B (Fig. 2c). On Fig. 2a and b, the FeB boride can be clearly distinguished from the Fe2B boride thanks to a darker coloration. All single-
and double-phase layers obtained on the XC38 steel have acicular morphology with a perpendicular-to-steel-surface preferential orientation regardless of the boronizing bath. This jagged acicular shape of the boride layers is very advantageous
Fig. 3. X-ray patterns obtained for the layers of the three boronizing baths after treatment of 4 h at 950 °C: (a) borax–B4C bath, (b) borax–Al bath and (c) borax–SiC bath.
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because it ensures good layer gripping and adhesion to the substrate material. It is worth noticing the presence of cracks near the borides as illustrated in Fig. 2b. The diffraction diagrams of the boronized layers (Fig. 3) showed the absence of borax–SiC bath-obtained representative peaks of the FeB phase in the layer (Fig. 3(c)). This absence can be explained by the fact that the potential difference between the treated piece and the suspended micro-anodes SiC does not ensure a sufficient boron flux for the formation of a boron-rich boride. In the case of the borax–B4C mixture, the formation of the FeB boride can be explained by the very boron-rich bath. For the borax–Al bath, and according to Ellingham's diagrams [14], the reduction of boron oxide (B2O3) resulting from the thermal dissociation of borax, is made easier by aluminium (Al) than by carbon (C) or silicon (Si) rendering the formation of FeB boride easier. Microscopy and X-ray diffraction analysis showed that, for the same temperature (950 °C) and duration (4 h), the proportion of FeB boride in the layer obtained in the borax– Al bath was more important than for borax–B4C. This can be explained by the potential difference between the treated sample and the micro-anodes of Al and B4C in suspension in the molten salt. The potential difference is certainly more important in the case of aluminium. 3.2. Microhardness of the boronized layers The results of microhardness on boronized layers, underlying zone and substrate are given in Table 2. While taking into account the uncertainties associated with microhardness measurements on the different samples, we can say that the boronizing bath chemical composition do not have a significant influence on the boronized layers' hardness. The obtained values indicate that the microhardness is more important for the FeB than the Fe2B layer with a difference of about 300 HV units. Comparing our microhardness values to those obtained by other authors [15–17] (Table 3) show that they are in the same intervals given in the literature. These intervals are (1300– 1850) HV for Fe2B and (1700–2200) HV for FeB according to the conditions of the used treatment. In our opinion, differences in microhardness values for FeB and Fe2B borides can be explained by: – The chemical composition of the substrate where elements in steels can lead to borides with multiple-component layers
Table 2 Microhardness of boride layers, underlying zone and substrate Boronizing bath
Borax–B4C Borax–Al Borax–SiC
Microhardness
Substrate
Boride layer
Table 3 Microhardness of boride layers obtained from literature Microhardness of boride layers (HV) FeB
Fe2B
2050
1500
2250 2280 2440
1500 1600 1690
Substrate
Boronizing Reference environment
Low carbon steel
Liquid boronizing Liquid
Fe2B
2130 ± 70 2140 ± 71 –
1820 ± 61 1830 ± 62 1830 ± 62
such as (Fe,M)B and (Fe,M)2B, where M denotes the existing metal in steel. – The formed borides stoechiometry or the formation of instable boride Fe3B [18]. – The boronizing environment which has an influence on microstructure and in this way on microhardness. The increased microhardness values of the underlying zone and the substrate can be attributed to the displacement of carbon and other elements from the boride layers to the substrate and the presence of a little amount of boron in the matrix. In fact, the microprobe analysis clearly shows this displacement (Table 4). 3.3. Scratch resistance The single-pass scratch tests with increasing load led to three different types of damage: – Cracks that propagate in depth along the scratch trails (behind the indenter). These cracks were present in all samples with no exception. They have either a mosaic (Fig. 4 (a)) or a curvilinear shape that stands normal to the scratch axis (Fig. 4(b)). According to the literature [19] this type of cracks is characteristic of a Hertzian fracture on brittle solids when a blunted indenter is used. These cracks propagate in depth in a semi-conical shape and start at flaws near the contact surface where high-tension stresses develop. – Cracks that develop on the scratch sides and propagate away from the scratching zone. These cracks are caused by the plastic deformation of the substrate and are characterized by very variable lengths (Fig. 4(c)) [20]. Table 4 Distribution of elements in XC38 steel before and after boronizing⁎
Before boronizing After boronizing 680 ± 22 689 ± 23 685 ± 23
218 ± 7 220 ± 8 220 ± 8
[7]
Mn steel [15] High alloyed Cr steel High alloyed Cr, V, W, Mo steel 1760–1950 1380– Various chemical 1740 composition of steel 1950–2100 1380– Various chemical Various [16] 1450 composition of steel environment Ductile Liquid [17] 1665–2140 VHN iron (FeB + Fe2B)
Underlying zone
FeB
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XC38 Steel FeB Fe2B Substrate
⁎In all cases Fe is the balance.
C
Mn
Si
B
0.39 0.30 0.29 0.45
0.68 0.56 0.48 0.62
0.34 0.05 0.02 0.31
– 16.26 8.74 0.14
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Fig. 4. Cracking types produced by single-pass scratch tests under increasing load. Mosaic cracks (a), cracks with a curvilinear form (b), cracks on the scratch sides (c) and cohesive scaling (d).
– Cohesive scaling that appear on the sides at the extreme end of the scratch zone when the applied load becomes relatively high. These scaling cracks are related to the intrinsic coating brittleness. They are generally observed in the relatively thick layers (Fig. 4(d)). In no case, an adhesive scaling at the boride/substrate interface was observed. This was expected since it is wellestablished that the coatings achieved at high temperature present a good adhesion due to the inter-diffusion at the interface metallic continuity. These coatings are therefore less exposed to damage characterised by the separation of the interfaces [21]. The interfaces' cohesion is particularly reinforced in our case because of the irregular shape of borides' interfaces. The observations of the single-pass scratch tests samples revealed that the critical damage load (LC) always coincides with the apparition of cracks at the scratch trails. The results showed that the boride single-phase layer issued from the borax–SiC bath do not present any scaling for loads up to the used maximal load (Table 5).
After the single-pass scratch test, the samples were submitted to a multi-pass test under constant loading. This test allows characterizing the boronized layers' resistance to severe cyclic wear solicitations. The damages produced on the samples were invariably constituted of cohesive scaling on the sides of the trails. No cracks along the trail were observed in these conditions (Fig. 5). The results obtained by the multi-pass scratch test show that the boronized layer obtained with the borax–Al bath is less effective. Table 6 gathers the number of passes before damage for the three baths. A comparison of the performances of the different tested samples showed that the chemical composition of the
Table 5 Critical loads for scaling and cracking apparition on boronized layers during single-pass scratch tests using increasing loading Boronizing bath
Constituents of the layer
Critical damage load LC (N)
Load (N) corresponding to the apparition of scaling⁎
Borax– B4C Borax–Al Borax– SiC
FeB + Fe2B
125
∼ 180
FeB + Fe2B Fe2B
73 132
∼ 80 N200
⁎This load is estimated from the scratch trails' micrographs.
Fig. 5. Trails obtained at a constant load of 50 N: after 6 passes with scalings (a), after 1 pass with no apparent damage (b).
O. Allaoui et al. / Surface & Coatings Technology 201 (2006) 3475–3482 Table 6 Number of indenter passes before the layer damage Boronizing bath
Constituents of the layer
Borax–B4C Borax–Al Borax–SiC
FeB + Fe2B FeB + Fe2B Fe2B
Number of passes before damage 20 N
50 N
13 9 12
6 4 5
boronizing bath appreciably affects the scratch resistance of the layers. Indeed, the critical load for crack apparition along the trail and the number of indenter passes before damage were different for the samples issued from the three baths. The weak performance of the borax–Al bath-layer can be explained by the presence of a high proportion of the FeB boride that possesses a completely different thermal dilatation coefficient than that of Fe2B. As suggested by Vipin [22], this causes cracks formation at the level of the FeB/Fe2B interface (see Fig. 2(b)). 3.4. Wear resistance Both single- and double-phase tested layers presented comparable friction coefficient values (μ). These were comprised between 0.11 and 0.15 at the test beginning and between 0.44 and 0.48 toward the end. In all cases, the evolution of the tangential force according to cycle number allowed us to distinguish three phases from the obtained curves (Fig. 6). Three phases were observed on all friction curves: 1. A first phase with weak and nearly constant friction coefficient (μ = 0.11 to 0.15). This phase can be considered as an incubation period. It corresponds to the indenter slip on the sample, in the absence of all major damage. 2. A second phase where the friction coefficient increases progressively until reaching a maximal value between 0.55
Fig. 6. Friction curve evolution of the tangent force Ft according to cycle number for the borax–SiC sample and a normal load of 10 N.
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Table 7 Cycles number corresponding to phase I of the friction curve for the different baths Boronizing bath
Constituents of the layer
Cycles number in phase I
Borax–B4C Borax–Al Borax SiC
FeB + Fe2B FeB + Fe2B Fe2B
160 150 470
and 0.65. This phase is associated with the formation of a layer transferred from the tungsten carbide ball and the apparition of wear particles on the indenter trails. Examining the tungsten carbide ball after every test reveals wear traces on it. 3. A third phase where the friction coefficient stabilizes between 0.44 and 0.48. It seems that this phase is associated with an abrasive action by the wear particles previously formed. The decrease of the friction coefficient value is probably due to the grinding of the removed particles. The friction coefficient values obtained in this work (from 0.15 up to 0.65) are comparable to those obtained in the literature. Selçuk [23] found variable friction coefficient values between 0.36 and 0.62 on carbon steels. Yan-qiu Xia [24] gave values around 0.5 for loads varying from 20 N to 100 N. Besides, Venkataraman and Sundararajan [25] found that the friction coefficient of the boronized layers varies from 0.3 to 0.5 depending on the slip speed. The number of phase I-cycles (period with low friction coefficient and without major damage) is considered as the first criteria of wear resistance of boride layers on the different samples. This number is evaluated from the friction curves and is given in Table 7. A second quantification of wear was made by measuring the volume loss recorded on the trails left by the pin on the different samples (Fig. 7 ). It can be noticed that the single-layer made in the borax–SiC bath presents the best wear resistance in comparison with other tested samples. This result corroborates those obtained with scratch tests which show absence of scaling under the maximal
Fig. 7. Volume loss recorded on the traces left by the pin on the different samples.
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applied load and a delay of apparition of cracks along the scratch trails. Other studies also found that the single-phase layers are more effective than the double-phase layers [26]. They attribute this improved performance to the single-phase layer structural homogeneity. The previous criteria (volume loss and cycle number before damage onset), more or less classify the samples in the correct order. We can say that samples with a layer issued from borax– SiC bath have the best wear resistance. Those issued from borax–B4C and borax–Al baths have comparable performances, but these are significantly lower than the previous. 4. Conclusion – Boronizing in a liquid environment easily achieves layers of borides on the XC38 steel. Depending on the chemical composition of the bath, these layers can be single- or double-phase. The borax–B4C and borax–Al baths led to double-phase layers whereas borax–SiC bath led to a singlephase boride. – The proportion of FeB phase in the borax–Al bath-produced layers was more important than that obtained in the borax– B4C bath. – XC38 steel-boronized layers' microhardness was about 2100 HV for FeB boride and 1800 HV for Fe2B. The boronizing bath chemical composition had no significant effect on the layers' microhardness. – The microhardness of the underlying zone and the substrate was increased by the boronizing treatment. – The scratch and wear resistances depend on the layers' microstructure. The best performance was obtained on the single-phase boride layer produced with the borax–SiC bath.
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