Kinetics of low temperature plasma carburizing of austenitic stainless steels

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1 Journal of Materials Processing Technology 168 (2005) Kinetics of low temperature plasma carburizing of austenitic stainless steels Y. Sun School of Materials Engineering, Nanyang Technological University, Nanyang Avenue, Singapore , Singapore Received 6 March 2003; received in revised form 6 March 2003; accepted 11 October 2004 Abstract A low temperature plasma carburizing technique has recently been developed to engineer the surfaces of austenitic stainless steels for combined improvement in wear and corrosion resistance. The resultant carburized layer is characterized by the supersaturation of carbon in austenite lattices, the much-increased hardness and wear resistance, and most importantly its superior corrosion resistance. This paper presents recent experimental results on the kinetics of this novel process, in terms of the growth of the precipitation-free layer and its variation with processing temperature, time and substrate material. This work demonstrates that the low temperature carburizing process is a diffusioncontrolled process, and only when the processing temperature is sufficiently low can a precipitation-free layer be produced. In addition, the chemical compositions of the substrate material also affect the formation and kinetics of the precipitation-free layer. By proper process control and material selection, a high-quality carburized layer m thick can be produced at temperatures between 400 and 500 C for wear protection in highly corrosive environments Elsevier B.V. All rights reserved. Keywords: Stainless steel; Plasma carburizing; Kinetics; Diffusion 1. Introduction Austenitic stainless steels are the most widely used corrosion-resistant materials in various sectors of industries, due to their excellent chemical and metallurgical properties. However, they are notorious for their poor friction and wear characteristics. Attempts have been made during the past decades to engineer the surfaces of this type of materials so as to improve their surface hardness and wear resistance [1 5]. Most of these efforts, however, result in a decrease in the corrosion resistance of the stainless surface [2,3]. Recent research and development in surface alloying of austenitic stainless steels have therefore been directed towards combined improvement in wear and corrosion resistance [5,6].A major breakthrough has been made in this connection, which involves the incorporation of a large amount of carbon into Fax: address: asysun@ntu.edu.sg. the surfaces of austenitic stainless steels at temperatures sufficiently low to avoid carbide precipitation [7,8]. This has led to the development of a low temperature plasma carburizing process [9], which is carried out at temperatures between 300 and 600 C and produces a hardened layer characterized by the supersaturation of carbon in austenite lattices, the much-increased hardness and wear resistance, and most importantly its superior corrosion resistance [7]. This paper presents recent experimental results on the kinetics of this novel process, in terms of the growth of the precipitationfree layer and its variation with processing temperature, time and substrate material. 2. Experimental Three austenitic stainless steels were used in the present work, including AISI 316, 304 and 321 steels. Table 1 gives the chemical compositions of the investigated materials. Specimens were machined from hot-rolled bars 25.4 mm in /$ see front matter 2004 Elsevier B.V. All rights reserved. doi: /j.jmatprotec

2 190 Y. Sun / Journal of Materials Processing Technology 168 (2005) Table 1 Chemical compositions of the investigated austenitic stainless steels (wt%) AISI Cr Ni Mo Ti Mn C diameter into discs of 25 mm in diameter and 8 mm in thickness. The flat faces of the discs were then manually ground using silicon carbide grinding papers down to 1200 grade to achieve a fine finish. Plasma carburizing was carried out using a conventional dc plasma nitriding unit, which comprises a sealed chamber, a vacuum system with a rotary pump, a dc power supply and control unit, a gas supply system and a temperature measurement and control system. The specimens to be treated were placed on the working table inside the chamber. The working table was connected to the dc source as the cathode, and the wall of the chamber connected to the dc source as the anode. The temperature of the discs was measured by a thermocouple inserted into a hole of 3 mm diameter drilled in one of the discs or a dummy sample. The process cycle is similar to that for conventional dc plasma nitriding, involving evacuating the chamber, heating the specimens up to the treatment temperature by the glow discharge, surface alloying in the plasma and cooling. A wide range of processing temperatures between 300 and 600 C and times between 3 and 40 h have been employed in the present work. During plasma processing, a gas mixture containing carbon-carrying gases was used with a total pressure between 2 and 10 mbar. After the completion of the alloying step, the glow discharge was turned off and the specimens were allowed to cool in the chamber in the treatment atmosphere down to room temperature before they were removed from the chamber. Various techniques have been used to characterize the structures and thickness of the alloyed layers produced. These include X-ray diffraction analysis for phase identification, glow discharge spectrometry (GDS) analysis for chemical composition determination, metallographic analysis of cross sections for layer morphology examination, thickness and hardness profile measurements. Fig. 1. Typical morphology of carburized layers produced on (a) AISI 316 and (b) 321 steels by plasma carburizing at 475 C for 20 h. The specimens were etched with the Marble reagent. observations that can be made regarding the features of the carburized layers. Firstly, a significant amount of carbon, up to 3 wt%, can be incorporated into the surface region of the steels, forming a carburized layer with a diffuse-type carbon gradient. Such a favourable distribution of carbon is essential for forming a diffuse-type interface and achieving a high degree of toughness and load-bearing capacity of the layer substrate composite. 3. Results and discussion 3.1. General observations Morphological and structural examinations revealed that a hard, corrosion-resistant and carbon-enriched layer can indeed be produced on the investigated austenitic stainless steels. Fig. 1 shows typical morphology of the carburized layers produced on AISI 316 and 321 steels, whilst Fig. 2 shows the corresponding carbon concentration profiles measured across the layers by GDS. There are several important Fig. 2. Typical carbon concentration profiles produced on AISI 316 and 321 steels by low temperature carburizing.

3 Y. Sun / Journal of Materials Processing Technology 168 (2005) Secondly, as can be clearly seen in Fig. 1, after etching in the Marble reagent, the carburized layers have a bright appearance under optical microscope, indicating their superior corrosion resistance over the untreated substrate materials. Indeed, detailed corrosion tests in other environments confirmed the good corrosion characteristics of the carburized layers [7]. Thirdly, from the hardness impressions in Fig. 1, it is also evident that the hardness of the carburized layer is significantly higher than that of the substrate, thus confirming the hardening effect of the process. The hardness of the layer decreases gradually from the surface towards the layer core interface, resulting in a diffuse-type hardness profile similar in shape to the carbon profiles shown in Fig. 2. X-ray diffraction analysis showed that the carburized layers are free from carbide precipitates and thus comprise a single phase, i.e. austenite supersaturated with carbon. Hardening of the layer is therefore attributed to the supersaturation of carbon and the induced planar and linear lattice defects in austenite [8]. The precipitation-free nature of the carburized layer also ensures that sufficient free chromium is available for corrosion protection. Clearly, a combined improvement in surface hardness and corrosion resistance of austenitic stainless steels can be achieved by the low temperature plasma carburizing technique. In order to ensure reasonable corrosion resistance, it is essential to avoid chromium carbide formation in the layer during the carburizing process [7]. On the other hand, in order to achieve a high load-bearing capacity, it is essential to produce a maximum layer thickness. Both the development of the carburized layer and the formation of chromium carbides are influenced by processing temperature and time, as well as substrate material, as discussed in more details below Effect of temperature on process kinetics Processing temperature has the most dominant effect on the structures and the development of the carburized layer. Fig. 3a shows the variation of layer thickness with temperature for the three materials investigated. It can be seen that, in accordance with diffusion-controlled mechanisms, the layer thickness increases with temperature. However, if the logarithm of layer thickness (ln(ξ)) is plotted against the reciprocal of absolute processing temperature, a linear relationship is only observed below certain critical temperature (Fig. 3b). Above this critical temperature, the data points gradually deviate from the linear dependence. The linearity in the low temperature range of Fig. 3b indicates that atomic diffusion is indeed the main mass transfer mechanism during the low temperature carburizing process. However, with increased processing temperature, other reactions, in addition to atomic diffusion, also occur in the carburized layer. Microstructural analysis revealed that the carburized layers produced below the critical temperature are precipitation-free and contain a single supersaturated austenite phase. On the other hand, the layers produced above the critical temperature exhibit a mixed phase structure, particu- Fig. 3. (a) Variation of layer thickness with processing temperature and (b) plots of ln(ξ) against 1/T. larly in the upper part of the layer. Fig. 4 shows one of such layers. Some dark phases have formed in the near surface region of the layer, although the majority of the carburized layer is still bright. The formation of these dark phases is associated with the precipitation of chromium carbides [8], which leads to deterioration in the corrosion resistance of the layer. The results in Fig. 3 also show that carbide precipitation Fig. 4. A thick carburized layer produced on AISI316 steel at 520 C for 40 h. Note the formation of dark phases.

4 192 Y. Sun / Journal of Materials Processing Technology 168 (2005) actually leads to the deviation of ln(ξ) from linear dependence on the reciprocal of temperature. This may be associated with the fact that the formation of carbides consumes a significant amount of carbon in the austenite lattices and thus reduces the carbon gradient and slows down the diffusion of carbon in austenite. The above observations have practical implications in terms of production of high-quality carburized layers. Although increasing temperature is the most effective in producing a thick layer, there is an upper limit of temperature that can be employed without carbide precipitation. Accordingly, only a limited layer thickness can be achieved by the low temperature carburizing technique. A further increase in temperature above the critical value to achieve a thicker layer becomes impractical due to the formation of carbides in the layer and the resultant deterioration in corrosion resistance. It should be pointed out that the critical temperature is timedependent and substrate material-dependent, as discussed in the following sections Effect of time on process kinetics The effect of treatment time on the growth of the carburized layer can also be described by the diffusion mechanism. Fig. 5a shows the layer thickness produced on AISI 316 steel as a function of the square root of treatment time for various temperatures, whilst Fig. 5b compares the layer thickness time dependence of the three investigated steels at a processing temperature of 475 C. Generally, a linear relationship is observed for most of the temperatures and times investigated. This, again, confirms the dominance of atomic diffusion in the development of the precipitation-free carburized layer. GDS carbon concentration analysis revealed that the surface carbon concentration on the specimens was built up gradually during the early stage of processing. After about 3 h of treatment, a saturated level of carbon was reached, which did not vary significantly with prolonged treatments. This fact suggests that the development of the carburized layer can be described by the Fick s Second Law subjected to the fixed surface boundary condition, i.e. ξ = a(dt) 1/2 = Kt 1/2 (1) where ξ is layer thickness, a a constant, t the treatment time, D the diffusion coefficient of carbon in the layer and K is D and thus temperature-dependent. From Fig. 5a, it is evident that the value of K increases with processing temperature simply due to the increased carbon diffusion coefficient. In addition, the K value is also substrate material-dependent. At the specific processing temperature of 475 C(Fig. 5b), the layer growth kinetics of the three investigated steels is described by ξ = t 1/2, for AISI 316 (2.1) ξ = t 1/2, for AISI 321 (2.2) Fig. 5. Layer thickness as a function of the square root of time for (a) AISI 316 steel at various temperatures and (b) AISI 316, 321 and 304 steels at 475 C. ξ = t 1/2, for AISI 304 (2.3) where ξ is in micrometer ( m) and t is in hour (h). Clearly, increasing treatment time will result in an increased layer thickness. However, prolonged treatment, particularly at relatively high temperatures, tends to induce the precipitation of chromium carbides in the layer and thus to damage the quality of the carburized layer. The lower the processing temperature, the longer the processing time allowed. The constraints imposed on temperature and time thus limit the maximum thickness of precipitation-free layers that can be produced by this technique. It should be noted that in Eqs. (2.1) (2.3), a zero treatment time does not result in a zero layer thickness. This is simply due to the effect of the heating stage on layer development: the carburizing effect was found to begin at temperatures as low as 300 C. Heating the specimen from 300 to 475 C could result in the formation of a thin layer before the treatment time at 475 C was counted Effect of substrate on process kinetics From the results given in Figs. 3 and 5b and Eqs. (2.1) (2.3), it is clear that substrate material has a signifi-

5 Y. Sun / Journal of Materials Processing Technology 168 (2005) cant effect on the kinetics of the carburizing process. Among the three austenitic stainless steels investigated, molybdenum bearing AISI 316 steel exhibits the fastest layer growth rate (Eq. (2.1)), whilst the straight grade AISI 304 steel shows the slowest layer growth rate. Since the three steels have similar chromium and nickel contents, it seems that molybdenum, and to a less extent titanium, in austenitic stainless steels can improve the kinetics of the low temperature carburizing process. This phenomenon is similar to that observed during low temperature plasma nitriding [10]. The mechanisms behind the beneficial effect of molybdenum (and titanium) are not clear at this stage. These may be associated with a higher carbon mass transfer rate at the surface and/or a higher carbon diffusion rate in the bulk of the layer. Mo and Ti are normally added to austenitic stainless steels to improve their resistance to localized corrosion attacks and to alleviate the sensitization problem. They can delay the formation of chromium carbides in the steel, particularly at austenite grain boundaries. More free carbon is therefore available for diffusion to a greater depth in Mo- and/or Ti-containing steels during the carburizing process. Most importantly, the ability of Mo for delaying grain boundary carbide precipitation facilitates the treatment of AISI 316 steel at higher temperatures, thus achieving a thicker layer. This is reflected in the threshold temperature time (T t) curve discussed below Threshold T t curves The initiation of carbide precipitation is particularly important in quality control of the resultant layer. Only when the processing temperature is sufficiently low and time sufficiently short can a high-quality precipitation-free layer produced. The optimum temperature time combination depends on substrate material. Based on the experimental data obtained so far, threshold temperature time (T t) curves have been constructed for the three steels investigated (Fig. 6). Further modification and updating of the curves should be made when more data are available. Fig. 6 can be used to help in the selection of optimum conditions for the investigated materials. When processing temperature and time are so selected that they lie below the curve for the specific material, a precipitation-free layer will be produced and vice versa. To achieve a maximum precipitation free layer thickness, the processing condition should be selected as close to the T t curve as possible. From Fig. 6, it is also evident that the curve for Mo-bearing AISI 316 steel is moved upwards compared to that for Mo-free steels, thus further confirming the beneficial effect of molybdenum for enhancing the response of austenitic stainless steels to low temperature plasma carburizing. 4. Conclusions (1) Low temperature plasma carburizing can produce a precipitation-free, hard and corrosion-resistant layer on the investigated steels. (2) The kinetics of layer growth depends on processing temperature and time, as well as substrate material. Diffusion is the dominant mass transfer mechanism governing the development of the precipitation-free layer. (3) Increasing temperature and time increases the thickness of the carburized layer, and also tends to induce the precipitation of carbides in the layer, which deteriorates the corrosion resistance of the material. Only when the processing temperature is sufficiently low and time sufficiently short can a high-quality precipitation-free layer be produced. (4) Threshold temperature time curves have been constructed to describe the effect of temperature, time and substrate material on the precipitation of carbides. These curves can be used to aid in the selection of optimum processing conditions for carburizing hardening of austenitic stainless steels without losing their corrosion characteristics. (5) Molybdenum in austenitic stainless steels has the beneficial effect on enhancing carburizing kinetics and facilitating a higher processing temperature or a longer treatment time to achieve a thicker precipitation-free layer. References Fig. 6. Threshold T t curves for the investigated steels, constructed based on experimental data. [1] J.R. Davis, ASM Handbook, vol. 5, ASM International, Materials Park, Ohio, 1994, pp [2] E. 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