Thermal stability of carbon S phase in 316 stainless steel

THERMAL STABILITY OF CARBON S PHASE IN 316 STAINLESS STEEL X. Y. Li, S. Thaiwatthana, H. Dong and T. Bell

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A low temperature plasma carburising process has been developed to engineer the surfaces of austenitic stainless steels to achieve combined improvements in wear and corrosion resistance. Previous studies have investigated the chemical, mechanical and structural characteristics of this carburised layer produced on AISI 316 steel at temperatures between 400°C and 600°C. The present paper focuses on the thermal stability of this carbon S phase layer. The investigation included isothermal annealing of the S phase layer as well as microstructure and property characterisation of the specimens. The results show that the S phase is metastable. When thermally annealed at certain temperatures for long enough, the carbon S phase decomposes into chromium carbides. Correspondingly, the hardness and corrosion resistance also varied. A preliminary isothermal

transformation diagram has been constructed, which provides a basic guideline for the application of low temperature plasma carburised 316 austenitic stainless steel. SE/449

INTRODUCTION A combination of excellent corrosion resistance with desirable tribological properties of austenitic stainless steels has been a target in surface engineering for decades. A low temperature plasma surface alloying technique for austenitic stainless steel has been developing since the 1980s. Nitrogen was introduced as an alloying species into the stainless steel surface to form an S phase layer, which possesses not only excellent wear resistance but also good corrosion resistance. 1 ± 4 However, there are several technical problems associated with the surface layer produced by low temperature plasma nitriding when practical engineering applications are concerned. First, the nitrided layer is very thin and it is dif® cult to obtain a thicker layer due to its metastable nature, which tends to lead to decomposition with prolonged nitriding times.5 Another problem lies in the fact that the low temperature nitrided layer is very hard, but also brittle and, what is more, there is no hardness gradient over the layer across the interface and the substrate. 5,6 This obviously limits the application of the lamellar composite in engineering practice since the layer is prone to collapsing and ¯ aking during arduous service. Hence exploring new alloying species, instead of nitrogen, to address the above inherent problems associated with current low temperature plasma nitriding practice is of great important from both a scienti® c and technological point of view. Since 1999, carbon, as a new surface alloying species, has been successfully introduced into the surfaces of stainless steels using a conventional dc plasm a unit at temperatures as low as 300 °C. 7 This new low temperature plasma surface alloying process can produce a surface hardened layer on austenitic stainless steel, which not only possesses an excellent combination of wear and corrosion resistance but also overcomes the inherent problems associated with a low temperature plasm a nitrided layer. The hardened layer can reach 50 mm thick with a

gradually decreasing hardness gradient, corresponding with a load bearing capacity which can signi® cantly improve the tribological properties of austenitic stainless steels. The chemical and mechanical properties and the structural characteristics of this carburised layer produced on AISI 316 steel at temperatures between 400 and 600 °C have been investigated previously. 7,8 As reported by Sun et al.,7 the carburised layer produced at temperatures below 520 °C has a face centred cubic structure, termed carbon S phase, which is attributed to the expansion of austenite following the supersaturation of the carbon in austenite. Therefore, the carbon S phase is a metastable phase. Little work has been done on the thermal stability of this carbon S phase however, which is of crucial importance to provide a basic guideline for the practical application of low temperature plasm a carburised 316 austenitic stainless steel. Therefore the main focus of the present paper is to study the thermal stability of the carbon S phase layer.

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The authors are in the Department of Metallurgy and Materials, School of Engineering, The University of Birmingham, Birmingham B15 2TT, UK (x.li.1@ bham.ac.uk). Based on a contribution to the IFHTSE sponsored Thermochemical Surface Engineering strand of the 4th European Stainless Steel Conference held in Paris, France on 10 ± 12 June 2002. # 2002 IoM Communications Ltd. Published by Maney for the Institute of Materials, Minerals and Mining.

EXPERIMENTAL The material used for the investigation was AISI 316 austenitic stainless steel. The chemical composition of the material is Fe ± 0 .06C ± 19.23Cr ± 11.26Ni ± 2.67Mo ± 1 .86Mn (wt-%). Cylindrical discs, 6 ± 8 mm in thickness, were cut from hot rolled bars of 25 mm diameter. Using SiC grinding papers, the specimens were manually ground down to 1200 grade to achieve a ® ne ® nish. Plasma carburising was carried out using a 60 kW KloÈ ckner dc plasm a treatment unit at ~ 570 V with a current density of about 30 A m 2. The atmosphere for the treatment was a mixture of carbon-carry ing gas and hydrogen at a pressure of 5 mbar. Various carburising parameters were investigated in order to optimise the innovative process. After completing the carburising process, the samples were cooled in the carburising atmosphere until the temperature was below 60 °C. DOI 10.1179/ 026708402225006259

Li et al. Thermal stability of carbon S phase in 316 stainless steel

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Optical microstructure of plasma carburised (450°C, 20 h) AISI 316 stainless steel

A group of plasma carburised (450 °C, 20 h) samples were isothermally annealed using different combinations of temperature and time in a vacuum furnace. For long annealing periods, the specimens were encapsulated in a tube with argon so as to prevent oxidation of the carburised surfaces during treatment. The annealing temperatures were maintained automatically within + 1°C of the set temperatures. The chemical composition of samples were analysed using a Leco GDS ± 750 QDP glow discharge spectrometer (GDS). The optical metallograph ic microstructures of plasma carburised specimens were revealed using a 25%HNO3 + 50%HCl + 25%HO 2 etchant. X-ray diffraction (XRD) was carried out on all plasm a carburised and annealed samples for phase identi® cation using a Philips X-ray X’ Pert diffractometer with Cu Ka radiation. Microstructure and chemical composition were examined using a JEOL 4000FX TEM, equipped with an EDX facility. The TEM specimens were prepared parallel to the surface layer and were mechanically thinned from the substrate side. Final dimple and ion beam thinning were designed to penetrate the specimens at different depths, such that an investigation of the microstructure through the alloyed layer was possible. The hardness and depth of the modi® ed layer were measured in the sectioned samples using a Leitz microhardness tester. The electrochemical behaviour of the carburised specimens was investigated employing a dc anodic potentiodyna mic polarisation technique. The electrolyte used in the test was a 3 .5%NaCl solution in deionised water.

RESULTS Characteristics of carbon S phase A typical cross-sectional optical microstructure of a carburised specimen is shown in Fig. 1. It can be seen that the carburised layer appears unaffected by the reagent used, resulting in a white layer, which is referred to as the carbon S phase layer. GDS composition analysis (Fig. 2) showed that the surface layer was heavily supersaturated with carbon (up to 3 wt-%), as opposed to the maxim um carbon solid solubility in the fcc c -Fe (1. 9 wt-%).9 The supersaturation of carbon in austenite makes this S phase layer metastable. Surface hardness of the plasm a carburised austenitic stainless steel can be signi® cantly increased from ~ 250 (untreated) to 1200 HK0 .025 for optimised conditions. The XRD

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GDS depth pro® le for plasma carburised (450°C, 20 h) specimen

and TEM studies have shown that the carbon S phase layer generally has an fcc structure with lattice constant of a~0 .367 nm for 450 °C, 20 h treated samples, which is an increase of 1 .9% over the substrate austenite structure (a~0 .360 nm). The layer was free from chromium carbide precipitates 10 for treatment temperatures below 520 °C up to a critical annealing period. There was no distinct interface between the surface layer and the substrate. High density microtwins and dislocations were clearly visible within the layer. Typical microstructure and the selected area diffraction pattern of the S phase is shown in Fig. 3. Corrosion testing in solutions of 3. 5% NaCl in distilled water indicated that all the samples with a precipitate free layer exhibited similar or even better corrosion resistance than the untreated material. Compared to the untreated 316 steel, a much improved pitting corrosion resistance was observed for the alloyed surface layers in NaCl solution as indicated by a higher pitting potential.

Annealing response Surface hardness and layer thickness The in¯ uence of annealing temperature on the surface hardness and the thickness of plasma carburised (450 °C, 20 h) layers is shown in Fig. 4. For an annealing time of 20 h, the surface hardness was found to reduce with annealing temperatures up to 500 °C, and then remain constant between 500 and 550 °C

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TEM microstructure and SAD pattern from S phase layer, (B~011¯ , fcc structure) Surface Engineering

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Annealing temperature effect on surface hardness and thickness for plasma carburised (450°C, 20 h) specimens

before dropping again. Meanwhile, the thickness of the surface hardened layer increased on raising the annealing temperature. The reduction of surface hardness and increasing layer thickness indicated that the surface supersaturated carbon diffused inward during the annealing process. The higher the annealing temperature and the faster the diffusion rate, the thicker the layer and the lower the surface hardness. The in¯ uence of annealing time on the surface hardness and layer thickness at 400 °C is summarised in Fig. 5. It can be seen that the surface hardness dropped to 950 HK0 .01 after 200 h annealing and decreased slowly throughout 500 h annealing. For low temperature (200 °C) annealed specimens, only a slight decrease in hardness and increase in surface layer thickness has been observed even after long term (5000 h) annealing.

Microstructure and composition XRD patterns obtained from specimens of the as carburised (450 °C, 20 h) and subsequently annealed for 20 h at different temperatures are given in Fig. 6. The pattern of the untreated AISI 316 stainless steel is included for comparison. The results showed that for both the as carburised and the 450 °C annealed specimen, S(111) and S(200) peaks exhibit similar shape and 2 h positions, indicating that the S phase generated in the carburising process was still present in the annealed specimens. When annealed at 500 °C, the positions of the peaks for the annealed specimens moved to higher 2 h angles than those of the as

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carburised samples. The extent of peak shift was found to be temperature dependent. When the annealing temperature reached 550 °C, the XRD pattern revealed major peaks, very close to the austenite phase but with a slightly large d spacing, and some small peaks, which can not be con® dently identi® ed as speci® c chromium carbides. TEM observation of the plasm a carburised (450 °, 20 h) then annealed at 460 °C for 80 h specimen con® rmed that no carbides had been precipitated during the annealing. The microstructure of the surface alloyed layer consists of an fcc single phase structure, which is the same as the plasma carburised structure but with a 0 .5% reduction of crystal constant (a~0 .365 nm). TEM studies on the samples annealed at 550 °C for 20 h revealed that the single S phase layer no longer existed (Fig. 7). The stable phases were M 23C 6 and CrN-like carbides (Cr,Mo)C (a~0 .417 nm, fcc, detailed results will be published elsewhere) with a semi-coherent/coherent cubic to cubic orientation found as a result of the decomposition of the carbon S phase. Consequently, the contents of carbon and chromium in S phase were reduced, thus leading to the transform ation of the carbon S phase into carbon austenite.

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Effect of annealing time on surface hardness and layer thickness for 450°C 20 h plasma carburised specimens

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X-ray diffraction patterns of untreated, plasma carburised and annealed stainless steel samples (A450~annealing at 450°C, all treated for 20 h)

TEM microstructure and SAD patterns from 550°C, 20 h annealed sample showing c C, Cr23C6 and (Cr,Mo)C mixture


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Anodic polarisation curves for specimens in given conditions

Corrosion resistance The effect of annealing treatments on the anodic polarisation behaviour of low temperature plasma carburised (450 °C, 20 h) specimens are shown in Fig. 8. It can be seen that after low temperature annealing these samples still have good corrosion resistance compared to the as carburised sample in 3. 5%NaCl solution. The corrosion potentials of annealed samples are similar to those of the as carburised samples; yet the corrosion current for the former was higher than the latter. When the carburised specimens were annealed at temperatures higher than 550 °C, their corrosion resistance deteriorated. It follows that good corrosion resistance of carbon S phase can be maintained provided the annealing temperature is lower than 550 °C. Isothermal transformation (IT) diagram Based on the information acquired from XRD analysis, GDS nitrogen depth pro® ling, and TEM observations, an IT diagram has been constructed and is depicted in Fig. 9. The temperature chosen for the isothermal annealing study and thus for the IT diagram ranges from 350 to 500 °C. This is because on the one hand, the S phase decomposes within several minutes at temperatures higher than 550 °C. On the other hand, it will not decompose even after thousands of hours at temperatures lower than 350 °C. It can be seen that the S phase is a metastable phase and when thermally annealed at certain temperatures for long enough the S phase starts to decompose into stable phases of (Cr,Mo)C and, Cr 23C 6 and therefore, forms the chromium depleted austenite c C. The incubation time is obviously temperature dependent from thousands of hours to several minutes between 350 and 500 °C. Clearly, the incubation time increases rapidly when the temperature decreases, which is typical of a thermal activated or diffusion controlled transform ation. Since the diffusivity of chromium is very slow at low temperatures, 11 a prolonged incubation time is needed. This IT diagram may be used to predict the suitability of low temperature plasma carburised components under speci® c circumstances (temperature and time) by extrapolating the existing curve to lower temperatures. Thus, the incubation times for the S phase can be calculated based on the trendline equation when used at different temperatures. Therefore, low temperature plasma carburised AISI 316 stainless steel may be accordingly recommended not

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Isothermal transformation diagram of carbon S phase

to be used at temperatures over 200 °C if long term ( 410 years) performance is needed.

DISCUSSION Nature of carbon S phase Based on the experim ental results, the nature of carbon S phase formed during low temperature plasma carburising can be deduced. Morphologic ally, TEM examination has con® rmed that there is no distinct boundary between the surface carburised case and the substrate. In fact, austenite grain boundaries continue from the substrate into the S phase layer, and no signs of new grains could be identi® ed even under a very high magni® cation. Clearly, the S phase is essentially a diffusion zone on plasma carburised austenitic stainless steels without an intrinsic interface. Compositionally, the S phase has been found to be supersaturated with carbon (up to ~ 3 wt-%) as opposed to the maximum solid solubility of fcc c -Fe. The low treatment temperature and very low diffusivity of chromium (high activation energy) 5 effectively prevent the precipitation of chromium carbides from supersaturated austenite. From a composition point of view, therefore, the carbon S phase could be seen as a carbon supersaturated high chromium solid solution. Carbon atoms are randomly distributed in the interstitial octahedral positions of fcc c -Fe(CrNi) because no diffraction spots corresponding to the superlattice plane in c ’ -M 4N could be identi® ed in the S phase layer. Therefore, from a structural viewpoint the S phase may also be termed expanded austenite. Thermodynamically, the S phase is metastable and will return to an equilibrium state by decomposition into stable precipitates, (Cr,Mo)C, Cr23C 6 and c C, provided the kinetic conditions are favoured. This has been supported by the fact that the incubation time varies inversely with the annealing temperature ranging from minutes at 550 °C to several years below 200 °C. In conclusion, the S phase can be de® ned as a thermodyna mically metastable, carbon supersaturated solid solution with an fcc structure. CrN-like (Cr,Mo)C carbide TEM studies have found a new carbide phase (Cr, Mo)C, with CrN-like structure and crystal constant, which was formed during the early stage of precipitation at an annealing temperature of 520 °C for the Surface Engineering

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plasm a carburised samples. This new phase has not yet been reported in the literature. According to C ± Cr ± Fe ternary phase diagrams, chromium carbides, such as M 23C 6 and M 7C 3, should be formed via the decomposition of austenite. Although further studies may be needed to provide direct evidence, it is believed that the carbon might have played the same role as nitrogen in CrN for the plasma nitrided austenitic stainless steel.5 This is mainly because the (Cr,Mo)C phase has the same fcc structure as the S phase matrix and the crystal discrepancy of them is only 11% compared to 190% between M 23C 6 and S phase. A coherent interface between (Cr,Mo)C and c C can be expected to reduce the total interfacial energy. The presence of coherency is believed to be best promoted by the same crystal structure (fcc) and the very close lattice parameters of the component phases.

CONCLUSIONS The S phase formed by low temperature plasma carburising is a carbon supersaturated solid solution with an fcc structure. This S phase is a metastable phase and will decompose into stable phases of (Cr,Mo)C and Cr23C 6 when thermally annealed at certain temperatures for a period of time. The corrosion resistance of the layer in a 3 wt-%NaCl solution can be retained if the annealing temperature is under

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520 °C. The surface hardness decreases with increasing temperature and time. A preliminary isothermal transform ation (IT) diagram has been established, which can provide a useful guideline for the application of low temperature plasma carburised 316 austenitic stainless steel.

REFERENCES 1. e. menthe, k.-t. rie, j. w. schultze and s. simson: Surf. Coat. Technol., 1995, 412, 74 ± 75. 2. p. a. dearnley, a. namvar, g. g. hibberd and t. bell: `Proc. 1st Int. Conf. on `Plasma surface engineering’ , Vol. 1, 219; 1989, Oberusel, DGM. 3. t. bell and y. sun: `Proc. Int. Conf. on `Surface science and engineering.’ , (ed. R. Zhu), 9; 1995, Beijing, International Academic Publishers. 4. k. ichii and k. fujimura: `Proc. 1st Int. Conf. on `Plasma surface engineering’ , Vol. 1, 1189; 1989, Oberusel, DGM. 5. x. y. li, y. sun and t. bell: Z. Metallkd., 1999, 90, 901 ± 907. 6. t. bell, x. y. li and y. sun: `Proc. Asian Conf. on `Heat treatment of materials’ , 1 ± 11; 1998, Beijing, China Machine Press. 7. y. sun, x. li and t. bell: Mater. Sci. Technol., 1999, 15, 1171 ± 1178. 8. y. sun , x. y. li and t. bell: in `Stainless steel 2000 ± thermochemical surface engineering of stainless steel’ , (ed. T. Bell and K. Akamatsu), 263 ± 273; 2001, London, Maney Publishing, The Institute of Materials. 9. r. c. ruhl and m. cohen: Trans. AIME, 1969, 245, 241. 10. y. sun, x. li and t. bell: Surf. Eng., 1999, 15, 49 ± 54. 11. j. kecura and k. stransky: Mater. Sci. Eng., 1982, 52, 1 ± 38.