- Email: [email protected]
Effect of plasma nitriding potential on tribological behaviour of AISI D2 cold-worked tool steel
Author's Accepted Manuscript
Effect of plasma nitriding potential on tribological behaviour of AISI D2 cold-worked tool steel Maycoln Depianti Conci, Antônio César Bozzi, Adonias Ribeiro Franco Jr.
www.elsevier.com/locate/wear
PII: DOI: Reference:
S0043-1648(14)00181-1 http://dx.doi.org/10.1016/j.wear.2014.05.012 WEA101025
To appear in:
Wear
Received date: 19 December 2013 Revised date: 9 May 2014 Accepted date: 12 May 2014 Cite this article as: Maycoln Depianti Conci, Antônio César Bozzi, Adonias Ribeiro Franco Jr., Effect of plasma nitriding potential on tribological behaviour of AISI D2 cold-worked tool steel, Wear, http://dx.doi.org/10.1016/j. wear.2014.05.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of plasma nitriding potential on tribological behaviour of AISI D2 cold-worked tool steel Maycoln Depianti Conci a, Antônio César Bozzi b, Adonias Ribeiro Franco Jr. a,*
ABSTRACT The effect of the nitrogen potential on the micro-abrasive wear resistance of AISI D2 plasma nitrided tool steel was evaluated. Plasma nitriding experiments were conducted in a pulsed plasma reactor using different nitrogen potentials. The results indicate that a nitrided layer, with finely dispersed nitrides, can be formed when shorter nitriding times and higher nitrogen potentials were used. Although the thickness of this layer is small, its wear resistance is higher than that observed at longer nitriding times. Increasing the nitriding time resulted in the formation of thicker layers, albeit with grain boundary nitrides, which are deleterious to wear resistance. For untreated material, three-body abrasive wear rolling was the dominant mechanism, and for nitrided material, a mechanism transition was not clearly observed in spite of its better wear resistance.
Keywords: micro-abrasive wear; AISI D2 tool steel; plasma nitriding; grain boundary nitrides.
a
Coordenadoria de Metalurgia, Instituto Federal do Espírito Santo (IFES), Campus Vitória, Avenida Vitória, 1729, ZIP CODE: 29050670, Vitória ES Brazil b
Departamento de Engenharia Mecânica, Universidade Federal do Espírito Santo (UFES), Campus Goiabeiras, Avenida Fernando Ferrari, 514, Vitória, ES, ZIP CODE: 29075910, Vitória ES Brazil. *Corresponding e-mail: [email protected]
1. Introduction The metalworking industry, particularly that related to the manufacture of dies and moulds for cold working, uses AISI D2 tool steel due to its high surface wear resistance combined with a sufficiently tough core. The applications of this steel include stamping and cold extrusion dies, the wear resistance of which is related to the high surface hardness promoted by the presence of M7C3-type carbides [1]. Studies have shown that the microstructure of this material plays an important role in determining the wear properties, and its abrasive wear is strongly dependent on the distribution of carbides, which are plate-like, in a tempered martensite matrix [2]. Further increases in the wear resistance of the steels can be promoted by choosing a suitable thermochemical treatment, such as plasma nitriding, which modifies the surface microstructure and improves the surface hardness but at expense of a decrease in toughness [3-5]. Nitrided layers with a porous compound layer and great depth are brittle, which reduces tool life [6]. This strong deleterious effect on the tribological properties is due to the presence of a network of grain boundary-precipitated nitrides on the nitrided layer [7] and/or a porous and brittle compound layer [8]. Karakan and co-workers, investigating the plasma nitriding response of AISI 5140 low alloy steel, showed that the above-mentioned problems can be avoided when experiments are conducted both in low-nitrogen potential gas mixtures and for short times, which allows for the development of nitrided layers with peak hardness and controlled thickness [8]. These researchers reported that for D2 tool steel plasma nitrided at 500 ºC, the wear resistance decreased with the nitriding time. Several studies presented in the literature, however, demonstrate that the optimum nitriding conditions are not well established, vary from steel to steel and depend upon the test conditions [9-13]. In short, process parameters should be selected such that they minimise the compound layer thickness and maximise the surface hardness and layer depth to improve wear resistance [14]. In the present study, different nitriding times, as well as different nitrogen potentials, were used to determine the optimum conditions that enable the production of a modified surface offering the highest abrasive wear resistance.
2. Experimental procedure 2.1. Substrate material The AISI D2 tool steel used in the present investigation was supplied by Villares Metals S.A. Corporation, São Paulo, Brazil, in the form of a bar measuring 31 mm in diameter. Slices measuring approximately 4.0 mm in thickness were cut from the bar received in the annealed condition, with a hardness of 230 HB. These slices were machined and ground, then austenitised at 1080 ºC for 30 min at 1080 ºC for 30 min, quenched in oil, double tempered at 540ºC, and cooled in air, resulting in a hardness of 57-58 HRC. All heat treatments were performed in a horizontal furnace under an argon atmosphere for surface protection against decarburisation and oxidation, and temperature was externally and asymptotically controlled via thermocouples.
2.2. Plasma nitriding Before nitriding experiments, previously quenched and tempered AISI D2 tool steel substrates were re-ground and mechanically polished to a mirror-like finish. Additionally, steel substrates were degreased and cleaned with ethanol in an ultra-sound apparatus. A SDS Soluções mod Thor NP 500 pulsed plasma nitriding reactor, illustrated schematically in Fig. 1, was used in the nitriding experiments. Additional details about this equipment can be found elsewhere [15]. Sputter cleaning was carried out at 200ºC in an argon atmosphere at a pressure of 100 Pa for 30 minutes. A set of specimens was nitrided in a gas atmosphere containing N2 - 5% vol. + H2 - 95% vol. Two more series of experiments were conducted using gas atmospheres containing N2 10% vol. + H2 - 90% vol. and N2 – 20% vol. + H2 - 80% vol. All plasma nitriding experiments were carried out at temperature of 470ºC, under flow-rate of 400 cm3/min and pressure of 540 Pa, and all specimens were cooled to room temperature inside the vacuum chamber.
(Fig. 1) Fig. 1. Schematic of the pulsed plasma reactor used in the nitriding experiments, where: 1, mass flow controller; 2, gas input valve; 3, vacuum breaker valve; 4, pressure controller; 5, thermocouple; 6, vacuum pump; 7, plasma source; 8, needle valve; 9, AISI 304 chamber; 10, sample holder; 11, glass window; and 12, PC.
2.3. Phase identification and microstructural characterisation X-ray diffraction analyses, scanning electron microscopy (SEM) and light microscopy (MO) were used to characterise the structures of the nitrided cases. The X-ray analyses were performed using a Bruker mod D2 Phaser diffractometer with CuKD radiation, and for microstructural characterisation, slices were cut from nitrided samples and then mounted in Bakelite together with a thin plate of nickel to minimise bowing effects during metallographic preparation. All samples were etched in 3% Nital.
2.4. Micro-scale abrasive wear test A “free ball” micro-abrasion tester CSM CALOWEAR was used to assess the wear behaviour of the nitrided layers. Micro-scale abrasion test is an alternative abrasive wear test introduced in 1996 by Rutherford and Hutchings [16]. All tests were carried out using a SiC abrasive slurry with average particle sizes of approximately 4.5 Pm and an initial concentration of 0.75 g/ml (volumetric fraction of 0.189) and an AISI 52100 hardened steel ball measuring 25.4 mm in diameter. The normal load, generated from the weight of the ball resting on the test sample, was 0.26 ± 0.01 N (and 0.30 N when the ball lying between the stopped shaft and sample holding plate). A load cell with an accuracy of ± 0.005 N, placed under the sample holding plate, allowed to measure the real normal load minimising errors related to friction between the sample and ball, which can alter this effective weight. The application of this load was possible by adjusting the angle of the sample holding plate in approximately 60o. Before each test, the shaft speed was regulated to 150 rpm, so that all the tests were carried out using a sliding speed of about 0.1 ms-1 reducing errors that could be generated by different sliding distances. The reported wear volume values were the average of four determinations.
For each sliding distance (L), the total volume (Vt) of nitrided layers removed by wear was determined by the following equation [17,18]:
Vt #
S .b 4 32d
(1)
where b is the diameter of the worn crater produced on the material surface and d is the diameter of the testing sphere.
Alternatively, the depths of the wear craters were measured with a Talysurf CLI 1000 profilometer (Taylor/Hobson Precision, UK), and compared with the depths of the nitrided layers.
The worn surfaces were evaluated using a Carl Zeiss mod EVO 40 scanning electron microscope (SEM). The thickness and hardness profiles for different plasma nitrided layers were measured by the microhardness method, following DIN standard 50.190 [19], using an automatic HM 100 mod. Mitutoyo micro-Vickers hardness apparatus with a testing load of 0.050 kgf/mm2. 3. Results and Discussion 3.1. Hardness profiles and thickness
Fig. 2 shows the hardness profiles of the nitrided layers produced using the three different nitrogen potentials. As shown in Fig. 2(a-b), the maximum hardness, approximately 1350 HV, is reached when short nitriding times were used. Under such conditions, increasing the nitriding time led to a decrease in the maximum hardness to the 1180-1220 HV range. This behaviour was also observed when the nitrogen potential was increased to 20%. Hardness values of approximately 1300 HV could be reached at shorter nitriding times, and with an increase in nitriding time, the hardness fell to approximately 1200 HV, as shown in Fig. 2(c).
(Fig. 2) Fig. 2. Hardness profiles of the nitrided layers produced on AISI D2 tool steel surface using different plasma nitriding times and gas atmospheres with nitrogen potentials of 5% (a), 10% (b) and (c) 20%.
Table 1 summarises the nitriding depths measured for all the nitrided layers. It can be verified that nitrided layers with thicknesses of 78, 108 and 132 Pm were obtained after, respectively, 1, 3 and 6 h, using a nitrogen potential of 5%. Increasing the nitrogen potential from 5 to 10% promoted a significant increase in the layer depth. As shown, further increases in amount of nitrogen in gas atmosphere led to a small increase in the layer depth. This effect is most likely associated with the early formation of a compound layer, constituted of different types of nitride, which makes difficult for atomic nitrogen to diffuse towards the material core due to low diffusion coefficient of nitrogen through these phases when compared to the ferrite matrix [20].
Table 1 Depth of the nitrided layers produced on AISI D2 tool steel after plasma nitriding at 470°C at different nitriding times and nitrogen potentials.
(Table 1)
3.2. Wear resistance
Fig. 3 shows images of the wear crater produced on the surface of the hardened and tempered AISI D2 tool steel after wear testing. As shown, the direction of motion of the testing sphere is not distinct, and the presence of indentations, together with some scratches along several directions, can be observed. Therefore, three body abrasive wear, also called rolling [17-18], was the dominant abrasive wear mechanism. Different from the two-body process (grooving), in rolling the abrasive particles do not embed, but roll between the ball and material surface producing a indented wear surface without evident surface directionality. The results show that after a sliding distance of 520 m, the wear volume of the untreated material was approximately 0.16 mm3.
(Fig. 3) Fig. 3. Details in the central region of the wear crater produced on surface of D2 tool steel after microabrasion test carried out using a load of 0.26 N, sliding distance of about 520 m and abrasive slurries containing 0.75 g/cm3 of SiC. It is seen that the worn surface was produced by three-body abrasive
wear process (rolling). Fig. 4 shows images of the wear crater produced on the surface of nitrided layer obtained using a nitrogen potential of 10% in the gas atmosphere. In all cases, the wear direction was not distinct; therefore, for nitrided material three body abrasion (rolling), characterised by the presence of a large amount of indentations on the worn surface, also was the dominant abrasive wear mechanism. No transition in the wear mechanisms was noted in spite of the increasing in wear resistance after plasma nitriding. Therefore, both for untreated and nitrided samples, the wear process is dominated by threebody abrasion (rolling). (Fig. 4) Fig. 4. Details in the central region of the wear crater produced on surface of D2 tool steel plasma nitrided with 10% N2 at 470 ºC for 1 h (a), 3 h (b) and 6 h (c). It is seen that all worn surfaces were produced by three-body abrasive wear process (rolling).
Fig. 5 shows the variation in the wear volume of the nitrided layers as a function of the nitriding time for steel samples nitrided at different nitrogen potentials. As shown, for all nitrided layers, the wear volumes are smaller than the wear volume of the untreated steel, and it is clear that the nitrided layer obtained using a nitrogen potential of 20% and nitriding time of 1 h presented the lowest wear loss. Therefore, the best wear resistance can be reached at a nitrogen potential of 20% and nitriding time of approximately 1 h. Under this condition, the wear volume is approximately 0.13 mm3, a value lower than that of the untreated material. A decrease in the nitrogen potential to 10% led to the formation of a nitrided layer with lower wear resistance. The wear volume of the layer produced using 10% nitrogen in the gas atmosphere after 1 h was around 0.14 mm3. With a further decrease in the nitriding potential, an upward trend in the wear volume was observed. The wear volume of the nitrided layer produced using 5% nitrogen in the gas atmosphere was 0.145 mm3, slightly higher than that of the layer produced using 10% and 20% nitrogen, as shown in Fig. 5. As shown in Table 1, this decrease can be associated with
the small thickness of the nitrided layers. Fig. 5 also shows that no significant differences were observed when used low nitrogen potential and increasing nitriding times. The results suggest that the maximum wear resistance can be reached when high nitrogen potentials and short nitriding times are applied. Note that due to the contact geometry, only a portion of the ball may be at any given depth at a given time. Therefore, it is wearing through a steep hardness gradient and not a volume of material with a constant hardness. That is clearly evident from Figure 2. The results of interferometer measurement showed that the maximum depth of the wear craters was not more than 58.3 μm. Table 1 shows that the thicknesses of the nitrided layers varied from about 78 to 208 Pm; therefore, no nitrided layers were surpassed by the wearing ball which reached a depth in the range of 55.2-58.3 Pm. The micro-abrasion wear test also allows determining the wear coefficient of the material, but this coefficient can exhibit a significantly different value as a function of the wear conditions. It was verified that a nitrided layer having thin thickness can be better than that with a thick thickness. Since the micro-abrasion tests led to a mild wear condition, the coefficient wear cannot be expanded for predicting the response of the material in such industrial applications as tools and dies where the material is under severe abrasive wear. The effects of over-nitriding point to the need to control processing conditions and to the conclusion that a thicker layer is not always better for wear resistance. Therefore, the decreasing in wear resistance is not associated with the thickness of the layers, but another important feature, such as their microstructure.
(Fig. 5) Fig. 5. Variation in the wear volume as a function of the nitriding time for nitrided layers obtained at 470 ºC with 5, 10 and 20% N2.
3.3. Microstructure of the nitrided layers
Fig. 6 shows the evolution of the phases on the surface of the AISI D2 tool steel as a function of the nitriding time for nitrogen potentials of 5% (a), 10% (b) and 20% (c). Only D-Fe and CrN phases were identified when a nitrogen potential of 5% was used. With an increase in the nitrogen potential to 10%, after 3 h the presence of an - Fe2-3N phase was detected together the CrN and D-Fe phases, suggesting the formation of a layer constituted by
these phases and/or nitride precipitation on the surface of the material. Using a nitrogen potential of 20%, the presence of the - Fe2-3N phase, together with the CrN and D-Fe phases was confirmed, indicating that increasing the nitrogen potential promoted the formation of the - Fe2-3N phase. (Fig. 6)
Fig. 6. X-ray diffraction patterns of nitrided layers produced on the surface of the AISI D2 tool steel as a function of the nitriding time using nitrogen potentials of 5% (a), 10% (b) and 20% (c).
Fig. 7 compares the microstructures of the nitrided layers obtained after plasma nitriding for 3 h under nitrogen potentials of 5% (a), 10% (b) and 20% (c). As shown in Fig. 7(a), at lower nitriding potentials, as 5% of nitrogen, the nitrided surface is constituted of a layer attacked more than the untreated core. This layer is called diffusion zone and has about 108 Pm as shown in Table 1. Fig. 7(a-c) shows that increasing the nitrogen potential led to the thickening of the diffusion zone, as well the precipitation of nitrides. Both the amount and size of these nitrides can increase with the nitrogen potential; however, the growth of coarse nitrides at diffusion zone can be accompanied by a loss in fracture toughness [3-4,7]. Coarse nitrides-containing microstructure is not recommended because the material surface becomes brittle due to a reducing in the fracture toughness associated to a high tensile stress generated during the growth and coarsening of such nitrides. They are mostly found on the ancient grain boundaries of previous austenite, parallel to the nitrided surface [7]. Using a nitrogen potential of 20% after 3 h of treatment, it was possible to observe in Fig 7(c) not only precipitates on diffusion zone but also an outer part that appears not to be attacked by the etching reagent. The well-etched diffusion zone appears darker, while this outer part is revealed with a whiter feature. This thin layer that in some cases covered the entire material surface is referred as compound layer. According to Edenhofen [3], by virtue of the presence of different types of nitride in the outer microstructuture, the use of the term compound layer for describing is better than the white layer, which was extensively adopted in the past. It is concluded that the formation of compound layer is favoured by the increase in both the nitriding time and nitrogen potential. Therefore, the decrease in the wear resistance either with
the nitriding time or nitrogen potential can be attributed to the formation of not only the compound layer but also the precipitation of nitrides in previous austenite grain boundaries.
(Fig. 7)
Fig.7. Microstructures of the nitrided layers produced on the surface of AISI D2 tool steel after plasma nitriding carried out at 470°C for 3 h using nitrogen potentials of 5% (a), 10% (b) and 20% (c).
4. Conclusions x The micro-abrasive wear resistance of AISI D2 tool steel can be improved by plasma nitriding at 470°C at different nitriding times and nitrogen potentials in a gas atmosphere. x The maximum wear resistance is offered by nitrided layers without a white layer and grain boundary precipitates, formed when plasma nitriding was conducted using short nitriding times and high nitrogen potentials. x These nitrided layers, with nitrides finely dispersed, can reach a surface hardness of approximately 1350 HV, and even though they are thinner, their micro-abrasive wear resistance is greater than that of layers produced using longer nitriding times. x Increasing the nitriding time leads to the formation of thicker layers, albeit with the formation of nitrides on previous austenite grain boundaries, which are deleterious to the wear resistance of the material. x Under the applied wear testing conditions, a difference between the wear mechanisms of
the unnitrided and nitrided steel samples was not clearly evidenced. In both cases, threebody rolling is the dominant. Acknowledgements The authors would like to thank the assistance of Eng. Yukio Nishida in obtaining SEM images.
References [1] G. Roberts, G. Krauss, R. Kennedy, Tool steels, ASM Int., Mater. Park, OH, 1998.
[2] L. Bourithis, G.D. Papadimitriou, J. Sideris, Comparison of wear properties of tool steels AISI D2 and O1 with the same hardness, Tribol. Int. 39 (2006) 479-489. [3] B. Edenhofer, Physical and metallurgical aspects of ionitriding – Part 1, Heat. Treat. Met. 1 (1974) 23-28. [4] Y. Sun, T. Bell, Plasma surface engineering of low alloy steel, Mater. Sci. Eng. A 140 (1991) 419–434. [5] J. Yang, Y. Liu, Z. Ye, D. Yang, S. He, Microstructural and tribological characterization of plasma-and gas-nitrided 2Cr13 steel in vacuum, Mater. Des. 32 (2011) 808-814. [6] M. Uma Devi, T.K. Chakraborty, O.N. Mohanty, Wear behaviour of plasma nitrided tool steels, Surf. Coat. Technol. 116–119 (1999) 212–221. [7] M.B. Karami, An investigation of the properties and wear behaviour of plasma-nitrided hot-working steel (H13), Wear 150 (1991) 331-342. [8] M. Karakan, A. Alsaran, A. Çelik, Effects of various gas mixtures on plasma nitriding behavior of AISI 5140 steel, Mater. Charact. 49 (2002) 241-246. [9] B-Y Jeong, M-H Kim, Effects of the process parameters on the layer formation behavior of plasma nitrided steels, Surf. Coat. Technol. 141 (2001) 182–186. [10] M. Keddam, Characterization of the nitrided layers of XC38 carbon steel obtained by R.F. plasma nitriding, Appl. Surf. Sci. 254 (2008) 2276–2280. [11] M. Keddam, B. Bouarour, R. Kouba, R. Chegroune, Growth kinetics of the compound layers: Effect of the nitriding potential, Phys. Proced. 2 (2009) 1399-1403. [12] Y.Y. Özbeka, M. Durmana, H. Akbuluta, Wear Behavior of AISI 8620 Steel Modified by a Pulse-Plasma Technique, Tribol. Trans. 52 (2009) 213-222. [13] X. Li, W. Yue, G. Wang, X. Gao, S. Wang, J. Liu, Comparing tribological behaviors of plasma nitrided and untreated bearing steel under lubrication with phosphor and sulfurfree organotungsten additive, Tribol. Int. 51 (2012) 47–53. [14] S. Karaolu, Structural characterization and wear behavior of plasma-nitrided AISI 5140 low-alloy steel, Wear 49 (2002) 349–357. [15] M.D. Conci, Evaluating the wear resistance of AISI D2 cold work tool steels plasma nitrided in different temperatures and nitrogen atmospheres, Dissertation (in Portuguese), Instituto Federal do Espírito Santo, 2012. [16] K.L. Rutherford, I.M. Hutchings, A micro-abrasive wear test, with particular application to coated systems, Surf. Coat. Technol. 79 (1996) 231-239.
[17] R.I. Trezona, D.N. Allsopp, I.M. Hutchings, Transitions between two-body and threebody abrasive wear: influence of test conditions in the microscale abrasive wear test, Wear 225-229 (1999) 205-214. [18] M.G. Gee, A. Gant, I. Hutchings, R. Bethke, K. Schiffman, K. van Acker, S. Poulat, Y. Gachon, J. von Stebut J., Progress towards standardisation of ball cratering, Wear 255 (2003) 1-13. [19] DIN 50.190/2 Standard. Härteteife Wärmebehandelter Teile – Ermittung der Nitriehätetiefe, Teil 3, März 1979. [20] M.A.J. Somers, E.J. Mittemeijer, Layer-growth kinetics on gaseous nitriding of pure iron: evaluation of diffusion coefficients for nitrogen in iron nitrides, Metall. Mater. Trans. A 26 (1995) 57-74.
Fig. 1. Schematic of the pulsed plasma reactor used in the nitriding experiments, where: 1, mass flow controller; 2, gas input valve; 3, vacuum breaker valve; 4, pressure controller; 5, thermocouple; 6, vacuum pump; 7, plasma source; 8, needle valve; 9, AISI 304 chamber; 10, sample holder; 11, glass window; and 12, PC. Fig. 2. Fig. 2. Hardness profiles of the nitrided layers produced on AISI D2 tool steel surface using different plasma nitriding times and gas atmospheres with nitrogen potentials of 5% (a), 10% (b) and (c) 20%. Fig. 3. Details in the central region of the wear crater produced on surface of D2 tool steel after micro-abrasion test carried out using a load of 0.26 N, sliding distance of about 520 m and abrasive slurries containing 0.75 g/cm3 of SiC. It is seen that the worn surface was produced by three-body abrasive wear process (rolling).
Fig. 4. Details in the central region of the wear crater produced on surface of D2 tool steel plasma nitrided with 10% N2 at 470 ºC for 1 h (a), 3 h (b) and 6 h (c). It is seen that all worn surfaces were produced by three-body abrasive wear process (rolling). Fig. 5. Variation in the wear volume as a function of the nitriding time for nitrided layers obtained at 470 ºC with 5, 10 and 20% N2. Fig. 6. X-ray diffraction patterns of nitrided layers produced on the surface of the AISI D2 tool steel as a function of the nitriding time using nitrogen potentials of 5% (a), 10% (b) and 20% (c). Fig.7. Microstructures of the nitrided layers produced on the surface of AISI D2 tool steel after plasma nitriding carried out at 470°C for 3 h using nitrogen potentials of 5% (a), 10% (b) and 20% (c).
Table 1 Depth of the nitrided layers produced on AISI D2 tool steel after plasma nitriding at 470°C at different nitriding times and nitrogen potentials.
Table 1 Depth of the nitrided layers produced on AISI D2 tool steel after plasma nitriding at 470°C, with different nitriding times and nitrogen potentials.
Nitriding time, h
Nitrided layer depth, Pm
5
1 3 6
78 108 132
10
1 3 6
99 144 189
20
1 3 6
106 180 208
Nitrogen potential, %
Highlights The effect of plasma nitriding atmosphere on the wear of D2 steel is evaluated. Finely dispersed nitride-containing nitrided layers reach a hardness of 1350HV. Layers without a white layer and grain boundary nitrides reach higher wear resistance. The wear mechanism of untreated or nitrided steel is dominated by three-body abrasion rolling.
Fig. 1
Fig. 2(a)
Fig. 2(b)
Fig. 2(c)
Fig. 3
Fig. 4(a)
Fig. 4(b)
Fig. 4(c)
Fig. 5
Fig. 6(a)
Fig. 6(b)
Fig. 6(c)
Fig. 7(a)
Fig. 7(b)
Fig. 7(c)
Effect of plasma nitriding potential on tribological behaviour of AISI D2 cold-worked tool steel Maycoln Depianti Conci, Antônio César Bozzi, Adonias Ribeiro Franco Jr.
www.elsevier.com/locate/wear
PII: DOI: Reference:
S0043-1648(14)00181-1 http://dx.doi.org/10.1016/j.wear.2014.05.012 WEA101025
To appear in:
Wear
Received date: 19 December 2013 Revised date: 9 May 2014 Accepted date: 12 May 2014 Cite this article as: Maycoln Depianti Conci, Antônio César Bozzi, Adonias Ribeiro Franco Jr., Effect of plasma nitriding potential on tribological behaviour of AISI D2 cold-worked tool steel, Wear, http://dx.doi.org/10.1016/j. wear.2014.05.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of plasma nitriding potential on tribological behaviour of AISI D2 cold-worked tool steel Maycoln Depianti Conci a, Antônio César Bozzi b, Adonias Ribeiro Franco Jr. a,*
ABSTRACT The effect of the nitrogen potential on the micro-abrasive wear resistance of AISI D2 plasma nitrided tool steel was evaluated. Plasma nitriding experiments were conducted in a pulsed plasma reactor using different nitrogen potentials. The results indicate that a nitrided layer, with finely dispersed nitrides, can be formed when shorter nitriding times and higher nitrogen potentials were used. Although the thickness of this layer is small, its wear resistance is higher than that observed at longer nitriding times. Increasing the nitriding time resulted in the formation of thicker layers, albeit with grain boundary nitrides, which are deleterious to wear resistance. For untreated material, three-body abrasive wear rolling was the dominant mechanism, and for nitrided material, a mechanism transition was not clearly observed in spite of its better wear resistance.
Keywords: micro-abrasive wear; AISI D2 tool steel; plasma nitriding; grain boundary nitrides.
a
Coordenadoria de Metalurgia, Instituto Federal do Espírito Santo (IFES), Campus Vitória, Avenida Vitória, 1729, ZIP CODE: 29050670, Vitória ES Brazil b
Departamento de Engenharia Mecânica, Universidade Federal do Espírito Santo (UFES), Campus Goiabeiras, Avenida Fernando Ferrari, 514, Vitória, ES, ZIP CODE: 29075910, Vitória ES Brazil. *Corresponding e-mail: [email protected]
1. Introduction The metalworking industry, particularly that related to the manufacture of dies and moulds for cold working, uses AISI D2 tool steel due to its high surface wear resistance combined with a sufficiently tough core. The applications of this steel include stamping and cold extrusion dies, the wear resistance of which is related to the high surface hardness promoted by the presence of M7C3-type carbides [1]. Studies have shown that the microstructure of this material plays an important role in determining the wear properties, and its abrasive wear is strongly dependent on the distribution of carbides, which are plate-like, in a tempered martensite matrix [2]. Further increases in the wear resistance of the steels can be promoted by choosing a suitable thermochemical treatment, such as plasma nitriding, which modifies the surface microstructure and improves the surface hardness but at expense of a decrease in toughness [3-5]. Nitrided layers with a porous compound layer and great depth are brittle, which reduces tool life [6]. This strong deleterious effect on the tribological properties is due to the presence of a network of grain boundary-precipitated nitrides on the nitrided layer [7] and/or a porous and brittle compound layer [8]. Karakan and co-workers, investigating the plasma nitriding response of AISI 5140 low alloy steel, showed that the above-mentioned problems can be avoided when experiments are conducted both in low-nitrogen potential gas mixtures and for short times, which allows for the development of nitrided layers with peak hardness and controlled thickness [8]. These researchers reported that for D2 tool steel plasma nitrided at 500 ºC, the wear resistance decreased with the nitriding time. Several studies presented in the literature, however, demonstrate that the optimum nitriding conditions are not well established, vary from steel to steel and depend upon the test conditions [9-13]. In short, process parameters should be selected such that they minimise the compound layer thickness and maximise the surface hardness and layer depth to improve wear resistance [14]. In the present study, different nitriding times, as well as different nitrogen potentials, were used to determine the optimum conditions that enable the production of a modified surface offering the highest abrasive wear resistance.
2. Experimental procedure 2.1. Substrate material The AISI D2 tool steel used in the present investigation was supplied by Villares Metals S.A. Corporation, São Paulo, Brazil, in the form of a bar measuring 31 mm in diameter. Slices measuring approximately 4.0 mm in thickness were cut from the bar received in the annealed condition, with a hardness of 230 HB. These slices were machined and ground, then austenitised at 1080 ºC for 30 min at 1080 ºC for 30 min, quenched in oil, double tempered at 540ºC, and cooled in air, resulting in a hardness of 57-58 HRC. All heat treatments were performed in a horizontal furnace under an argon atmosphere for surface protection against decarburisation and oxidation, and temperature was externally and asymptotically controlled via thermocouples.
2.2. Plasma nitriding Before nitriding experiments, previously quenched and tempered AISI D2 tool steel substrates were re-ground and mechanically polished to a mirror-like finish. Additionally, steel substrates were degreased and cleaned with ethanol in an ultra-sound apparatus. A SDS Soluções mod Thor NP 500 pulsed plasma nitriding reactor, illustrated schematically in Fig. 1, was used in the nitriding experiments. Additional details about this equipment can be found elsewhere [15]. Sputter cleaning was carried out at 200ºC in an argon atmosphere at a pressure of 100 Pa for 30 minutes. A set of specimens was nitrided in a gas atmosphere containing N2 - 5% vol. + H2 - 95% vol. Two more series of experiments were conducted using gas atmospheres containing N2 10% vol. + H2 - 90% vol. and N2 – 20% vol. + H2 - 80% vol. All plasma nitriding experiments were carried out at temperature of 470ºC, under flow-rate of 400 cm3/min and pressure of 540 Pa, and all specimens were cooled to room temperature inside the vacuum chamber.
(Fig. 1) Fig. 1. Schematic of the pulsed plasma reactor used in the nitriding experiments, where: 1, mass flow controller; 2, gas input valve; 3, vacuum breaker valve; 4, pressure controller; 5, thermocouple; 6, vacuum pump; 7, plasma source; 8, needle valve; 9, AISI 304 chamber; 10, sample holder; 11, glass window; and 12, PC.
2.3. Phase identification and microstructural characterisation X-ray diffraction analyses, scanning electron microscopy (SEM) and light microscopy (MO) were used to characterise the structures of the nitrided cases. The X-ray analyses were performed using a Bruker mod D2 Phaser diffractometer with CuKD radiation, and for microstructural characterisation, slices were cut from nitrided samples and then mounted in Bakelite together with a thin plate of nickel to minimise bowing effects during metallographic preparation. All samples were etched in 3% Nital.
2.4. Micro-scale abrasive wear test A “free ball” micro-abrasion tester CSM CALOWEAR was used to assess the wear behaviour of the nitrided layers. Micro-scale abrasion test is an alternative abrasive wear test introduced in 1996 by Rutherford and Hutchings [16]. All tests were carried out using a SiC abrasive slurry with average particle sizes of approximately 4.5 Pm and an initial concentration of 0.75 g/ml (volumetric fraction of 0.189) and an AISI 52100 hardened steel ball measuring 25.4 mm in diameter. The normal load, generated from the weight of the ball resting on the test sample, was 0.26 ± 0.01 N (and 0.30 N when the ball lying between the stopped shaft and sample holding plate). A load cell with an accuracy of ± 0.005 N, placed under the sample holding plate, allowed to measure the real normal load minimising errors related to friction between the sample and ball, which can alter this effective weight. The application of this load was possible by adjusting the angle of the sample holding plate in approximately 60o. Before each test, the shaft speed was regulated to 150 rpm, so that all the tests were carried out using a sliding speed of about 0.1 ms-1 reducing errors that could be generated by different sliding distances. The reported wear volume values were the average of four determinations.
For each sliding distance (L), the total volume (Vt) of nitrided layers removed by wear was determined by the following equation [17,18]:
Vt #
S .b 4 32d
(1)
where b is the diameter of the worn crater produced on the material surface and d is the diameter of the testing sphere.
Alternatively, the depths of the wear craters were measured with a Talysurf CLI 1000 profilometer (Taylor/Hobson Precision, UK), and compared with the depths of the nitrided layers.
The worn surfaces were evaluated using a Carl Zeiss mod EVO 40 scanning electron microscope (SEM). The thickness and hardness profiles for different plasma nitrided layers were measured by the microhardness method, following DIN standard 50.190 [19], using an automatic HM 100 mod. Mitutoyo micro-Vickers hardness apparatus with a testing load of 0.050 kgf/mm2. 3. Results and Discussion 3.1. Hardness profiles and thickness
Fig. 2 shows the hardness profiles of the nitrided layers produced using the three different nitrogen potentials. As shown in Fig. 2(a-b), the maximum hardness, approximately 1350 HV, is reached when short nitriding times were used. Under such conditions, increasing the nitriding time led to a decrease in the maximum hardness to the 1180-1220 HV range. This behaviour was also observed when the nitrogen potential was increased to 20%. Hardness values of approximately 1300 HV could be reached at shorter nitriding times, and with an increase in nitriding time, the hardness fell to approximately 1200 HV, as shown in Fig. 2(c).
(Fig. 2) Fig. 2. Hardness profiles of the nitrided layers produced on AISI D2 tool steel surface using different plasma nitriding times and gas atmospheres with nitrogen potentials of 5% (a), 10% (b) and (c) 20%.
Table 1 summarises the nitriding depths measured for all the nitrided layers. It can be verified that nitrided layers with thicknesses of 78, 108 and 132 Pm were obtained after, respectively, 1, 3 and 6 h, using a nitrogen potential of 5%. Increasing the nitrogen potential from 5 to 10% promoted a significant increase in the layer depth. As shown, further increases in amount of nitrogen in gas atmosphere led to a small increase in the layer depth. This effect is most likely associated with the early formation of a compound layer, constituted of different types of nitride, which makes difficult for atomic nitrogen to diffuse towards the material core due to low diffusion coefficient of nitrogen through these phases when compared to the ferrite matrix [20].
Table 1 Depth of the nitrided layers produced on AISI D2 tool steel after plasma nitriding at 470°C at different nitriding times and nitrogen potentials.
(Table 1)
3.2. Wear resistance
Fig. 3 shows images of the wear crater produced on the surface of the hardened and tempered AISI D2 tool steel after wear testing. As shown, the direction of motion of the testing sphere is not distinct, and the presence of indentations, together with some scratches along several directions, can be observed. Therefore, three body abrasive wear, also called rolling [17-18], was the dominant abrasive wear mechanism. Different from the two-body process (grooving), in rolling the abrasive particles do not embed, but roll between the ball and material surface producing a indented wear surface without evident surface directionality. The results show that after a sliding distance of 520 m, the wear volume of the untreated material was approximately 0.16 mm3.
(Fig. 3) Fig. 3. Details in the central region of the wear crater produced on surface of D2 tool steel after microabrasion test carried out using a load of 0.26 N, sliding distance of about 520 m and abrasive slurries containing 0.75 g/cm3 of SiC. It is seen that the worn surface was produced by three-body abrasive
wear process (rolling). Fig. 4 shows images of the wear crater produced on the surface of nitrided layer obtained using a nitrogen potential of 10% in the gas atmosphere. In all cases, the wear direction was not distinct; therefore, for nitrided material three body abrasion (rolling), characterised by the presence of a large amount of indentations on the worn surface, also was the dominant abrasive wear mechanism. No transition in the wear mechanisms was noted in spite of the increasing in wear resistance after plasma nitriding. Therefore, both for untreated and nitrided samples, the wear process is dominated by threebody abrasion (rolling). (Fig. 4) Fig. 4. Details in the central region of the wear crater produced on surface of D2 tool steel plasma nitrided with 10% N2 at 470 ºC for 1 h (a), 3 h (b) and 6 h (c). It is seen that all worn surfaces were produced by three-body abrasive wear process (rolling).
Fig. 5 shows the variation in the wear volume of the nitrided layers as a function of the nitriding time for steel samples nitrided at different nitrogen potentials. As shown, for all nitrided layers, the wear volumes are smaller than the wear volume of the untreated steel, and it is clear that the nitrided layer obtained using a nitrogen potential of 20% and nitriding time of 1 h presented the lowest wear loss. Therefore, the best wear resistance can be reached at a nitrogen potential of 20% and nitriding time of approximately 1 h. Under this condition, the wear volume is approximately 0.13 mm3, a value lower than that of the untreated material. A decrease in the nitrogen potential to 10% led to the formation of a nitrided layer with lower wear resistance. The wear volume of the layer produced using 10% nitrogen in the gas atmosphere after 1 h was around 0.14 mm3. With a further decrease in the nitriding potential, an upward trend in the wear volume was observed. The wear volume of the nitrided layer produced using 5% nitrogen in the gas atmosphere was 0.145 mm3, slightly higher than that of the layer produced using 10% and 20% nitrogen, as shown in Fig. 5. As shown in Table 1, this decrease can be associated with
the small thickness of the nitrided layers. Fig. 5 also shows that no significant differences were observed when used low nitrogen potential and increasing nitriding times. The results suggest that the maximum wear resistance can be reached when high nitrogen potentials and short nitriding times are applied. Note that due to the contact geometry, only a portion of the ball may be at any given depth at a given time. Therefore, it is wearing through a steep hardness gradient and not a volume of material with a constant hardness. That is clearly evident from Figure 2. The results of interferometer measurement showed that the maximum depth of the wear craters was not more than 58.3 μm. Table 1 shows that the thicknesses of the nitrided layers varied from about 78 to 208 Pm; therefore, no nitrided layers were surpassed by the wearing ball which reached a depth in the range of 55.2-58.3 Pm. The micro-abrasion wear test also allows determining the wear coefficient of the material, but this coefficient can exhibit a significantly different value as a function of the wear conditions. It was verified that a nitrided layer having thin thickness can be better than that with a thick thickness. Since the micro-abrasion tests led to a mild wear condition, the coefficient wear cannot be expanded for predicting the response of the material in such industrial applications as tools and dies where the material is under severe abrasive wear. The effects of over-nitriding point to the need to control processing conditions and to the conclusion that a thicker layer is not always better for wear resistance. Therefore, the decreasing in wear resistance is not associated with the thickness of the layers, but another important feature, such as their microstructure.
(Fig. 5) Fig. 5. Variation in the wear volume as a function of the nitriding time for nitrided layers obtained at 470 ºC with 5, 10 and 20% N2.
3.3. Microstructure of the nitrided layers
Fig. 6 shows the evolution of the phases on the surface of the AISI D2 tool steel as a function of the nitriding time for nitrogen potentials of 5% (a), 10% (b) and 20% (c). Only D-Fe and CrN phases were identified when a nitrogen potential of 5% was used. With an increase in the nitrogen potential to 10%, after 3 h the presence of an - Fe2-3N phase was detected together the CrN and D-Fe phases, suggesting the formation of a layer constituted by
these phases and/or nitride precipitation on the surface of the material. Using a nitrogen potential of 20%, the presence of the - Fe2-3N phase, together with the CrN and D-Fe phases was confirmed, indicating that increasing the nitrogen potential promoted the formation of the - Fe2-3N phase. (Fig. 6)
Fig. 6. X-ray diffraction patterns of nitrided layers produced on the surface of the AISI D2 tool steel as a function of the nitriding time using nitrogen potentials of 5% (a), 10% (b) and 20% (c).
Fig. 7 compares the microstructures of the nitrided layers obtained after plasma nitriding for 3 h under nitrogen potentials of 5% (a), 10% (b) and 20% (c). As shown in Fig. 7(a), at lower nitriding potentials, as 5% of nitrogen, the nitrided surface is constituted of a layer attacked more than the untreated core. This layer is called diffusion zone and has about 108 Pm as shown in Table 1. Fig. 7(a-c) shows that increasing the nitrogen potential led to the thickening of the diffusion zone, as well the precipitation of nitrides. Both the amount and size of these nitrides can increase with the nitrogen potential; however, the growth of coarse nitrides at diffusion zone can be accompanied by a loss in fracture toughness [3-4,7]. Coarse nitrides-containing microstructure is not recommended because the material surface becomes brittle due to a reducing in the fracture toughness associated to a high tensile stress generated during the growth and coarsening of such nitrides. They are mostly found on the ancient grain boundaries of previous austenite, parallel to the nitrided surface [7]. Using a nitrogen potential of 20% after 3 h of treatment, it was possible to observe in Fig 7(c) not only precipitates on diffusion zone but also an outer part that appears not to be attacked by the etching reagent. The well-etched diffusion zone appears darker, while this outer part is revealed with a whiter feature. This thin layer that in some cases covered the entire material surface is referred as compound layer. According to Edenhofen [3], by virtue of the presence of different types of nitride in the outer microstructuture, the use of the term compound layer for describing is better than the white layer, which was extensively adopted in the past. It is concluded that the formation of compound layer is favoured by the increase in both the nitriding time and nitrogen potential. Therefore, the decrease in the wear resistance either with
the nitriding time or nitrogen potential can be attributed to the formation of not only the compound layer but also the precipitation of nitrides in previous austenite grain boundaries.
(Fig. 7)
Fig.7. Microstructures of the nitrided layers produced on the surface of AISI D2 tool steel after plasma nitriding carried out at 470°C for 3 h using nitrogen potentials of 5% (a), 10% (b) and 20% (c).
4. Conclusions x The micro-abrasive wear resistance of AISI D2 tool steel can be improved by plasma nitriding at 470°C at different nitriding times and nitrogen potentials in a gas atmosphere. x The maximum wear resistance is offered by nitrided layers without a white layer and grain boundary precipitates, formed when plasma nitriding was conducted using short nitriding times and high nitrogen potentials. x These nitrided layers, with nitrides finely dispersed, can reach a surface hardness of approximately 1350 HV, and even though they are thinner, their micro-abrasive wear resistance is greater than that of layers produced using longer nitriding times. x Increasing the nitriding time leads to the formation of thicker layers, albeit with the formation of nitrides on previous austenite grain boundaries, which are deleterious to the wear resistance of the material. x Under the applied wear testing conditions, a difference between the wear mechanisms of
the unnitrided and nitrided steel samples was not clearly evidenced. In both cases, threebody rolling is the dominant. Acknowledgements The authors would like to thank the assistance of Eng. Yukio Nishida in obtaining SEM images.
References [1] G. Roberts, G. Krauss, R. Kennedy, Tool steels, ASM Int., Mater. Park, OH, 1998.
[2] L. Bourithis, G.D. Papadimitriou, J. Sideris, Comparison of wear properties of tool steels AISI D2 and O1 with the same hardness, Tribol. Int. 39 (2006) 479-489. [3] B. Edenhofer, Physical and metallurgical aspects of ionitriding – Part 1, Heat. Treat. Met. 1 (1974) 23-28. [4] Y. Sun, T. Bell, Plasma surface engineering of low alloy steel, Mater. Sci. Eng. A 140 (1991) 419–434. [5] J. Yang, Y. Liu, Z. Ye, D. Yang, S. He, Microstructural and tribological characterization of plasma-and gas-nitrided 2Cr13 steel in vacuum, Mater. Des. 32 (2011) 808-814. [6] M. Uma Devi, T.K. Chakraborty, O.N. Mohanty, Wear behaviour of plasma nitrided tool steels, Surf. Coat. Technol. 116–119 (1999) 212–221. [7] M.B. Karami, An investigation of the properties and wear behaviour of plasma-nitrided hot-working steel (H13), Wear 150 (1991) 331-342. [8] M. Karakan, A. Alsaran, A. Çelik, Effects of various gas mixtures on plasma nitriding behavior of AISI 5140 steel, Mater. Charact. 49 (2002) 241-246. [9] B-Y Jeong, M-H Kim, Effects of the process parameters on the layer formation behavior of plasma nitrided steels, Surf. Coat. Technol. 141 (2001) 182–186. [10] M. Keddam, Characterization of the nitrided layers of XC38 carbon steel obtained by R.F. plasma nitriding, Appl. Surf. Sci. 254 (2008) 2276–2280. [11] M. Keddam, B. Bouarour, R. Kouba, R. Chegroune, Growth kinetics of the compound layers: Effect of the nitriding potential, Phys. Proced. 2 (2009) 1399-1403. [12] Y.Y. Özbeka, M. Durmana, H. Akbuluta, Wear Behavior of AISI 8620 Steel Modified by a Pulse-Plasma Technique, Tribol. Trans. 52 (2009) 213-222. [13] X. Li, W. Yue, G. Wang, X. Gao, S. Wang, J. Liu, Comparing tribological behaviors of plasma nitrided and untreated bearing steel under lubrication with phosphor and sulfurfree organotungsten additive, Tribol. Int. 51 (2012) 47–53. [14] S. Karaolu, Structural characterization and wear behavior of plasma-nitrided AISI 5140 low-alloy steel, Wear 49 (2002) 349–357. [15] M.D. Conci, Evaluating the wear resistance of AISI D2 cold work tool steels plasma nitrided in different temperatures and nitrogen atmospheres, Dissertation (in Portuguese), Instituto Federal do Espírito Santo, 2012. [16] K.L. Rutherford, I.M. Hutchings, A micro-abrasive wear test, with particular application to coated systems, Surf. Coat. Technol. 79 (1996) 231-239.
[17] R.I. Trezona, D.N. Allsopp, I.M. Hutchings, Transitions between two-body and threebody abrasive wear: influence of test conditions in the microscale abrasive wear test, Wear 225-229 (1999) 205-214. [18] M.G. Gee, A. Gant, I. Hutchings, R. Bethke, K. Schiffman, K. van Acker, S. Poulat, Y. Gachon, J. von Stebut J., Progress towards standardisation of ball cratering, Wear 255 (2003) 1-13. [19] DIN 50.190/2 Standard. Härteteife Wärmebehandelter Teile – Ermittung der Nitriehätetiefe, Teil 3, März 1979. [20] M.A.J. Somers, E.J. Mittemeijer, Layer-growth kinetics on gaseous nitriding of pure iron: evaluation of diffusion coefficients for nitrogen in iron nitrides, Metall. Mater. Trans. A 26 (1995) 57-74.
Fig. 1. Schematic of the pulsed plasma reactor used in the nitriding experiments, where: 1, mass flow controller; 2, gas input valve; 3, vacuum breaker valve; 4, pressure controller; 5, thermocouple; 6, vacuum pump; 7, plasma source; 8, needle valve; 9, AISI 304 chamber; 10, sample holder; 11, glass window; and 12, PC. Fig. 2. Fig. 2. Hardness profiles of the nitrided layers produced on AISI D2 tool steel surface using different plasma nitriding times and gas atmospheres with nitrogen potentials of 5% (a), 10% (b) and (c) 20%. Fig. 3. Details in the central region of the wear crater produced on surface of D2 tool steel after micro-abrasion test carried out using a load of 0.26 N, sliding distance of about 520 m and abrasive slurries containing 0.75 g/cm3 of SiC. It is seen that the worn surface was produced by three-body abrasive wear process (rolling).
Fig. 4. Details in the central region of the wear crater produced on surface of D2 tool steel plasma nitrided with 10% N2 at 470 ºC for 1 h (a), 3 h (b) and 6 h (c). It is seen that all worn surfaces were produced by three-body abrasive wear process (rolling). Fig. 5. Variation in the wear volume as a function of the nitriding time for nitrided layers obtained at 470 ºC with 5, 10 and 20% N2. Fig. 6. X-ray diffraction patterns of nitrided layers produced on the surface of the AISI D2 tool steel as a function of the nitriding time using nitrogen potentials of 5% (a), 10% (b) and 20% (c). Fig.7. Microstructures of the nitrided layers produced on the surface of AISI D2 tool steel after plasma nitriding carried out at 470°C for 3 h using nitrogen potentials of 5% (a), 10% (b) and 20% (c).
Table 1 Depth of the nitrided layers produced on AISI D2 tool steel after plasma nitriding at 470°C at different nitriding times and nitrogen potentials.
Table 1 Depth of the nitrided layers produced on AISI D2 tool steel after plasma nitriding at 470°C, with different nitriding times and nitrogen potentials.
Nitriding time, h
Nitrided layer depth, Pm
5
1 3 6
78 108 132
10
1 3 6
99 144 189
20
1 3 6
106 180 208
Nitrogen potential, %
Highlights The effect of plasma nitriding atmosphere on the wear of D2 steel is evaluated. Finely dispersed nitride-containing nitrided layers reach a hardness of 1350HV. Layers without a white layer and grain boundary nitrides reach higher wear resistance. The wear mechanism of untreated or nitrided steel is dominated by three-body abrasion rolling.
Fig. 1
Fig. 2(a)
Fig. 2(b)
Fig. 2(c)
Fig. 3
Fig. 4(a)
Fig. 4(b)
Fig. 4(c)
Fig. 5
Fig. 6(a)
Fig. 6(b)
Fig. 6(c)
Fig. 7(a)
Fig. 7(b)
Fig. 7(c)