EFFECT OF SINTERING ATMOSPHERE ON THE MICROSTRUCTURE OF HIGH Cr-ALLOYED SINTERED STAINLESS STEEL

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1 Powder Metallurgy Progress, Vol.13 (2013), No EFFECT OF SINTERING ATMOSPHERE ON THE MICROSTRUCTURE OF HIGH Cr-ALLOYED SINTERED STAINLESS STEEL R. Shvab, E. Dudrová, O. Bergman, S. Bengtsson Abstract The influence of sintering atmosphere on the microstructure of high Cralloyed sintered stainless steel was investigated. Two regimes using three sintering atmospheres were applied. Sintering in pure hydrogen was evaluated as unsatisfactory due to the strong decarburization of the compact surface. Sintering in mixed N 2 -H 2 atmosphere resulted in more acceptable microstructure with small austenitic grains and fine distributed small precipitates. Some decarburization at the surface was also observed. Avoidance of surface decarburization was obtained by using N 2 -H 2 atmosphere admixed with 0.5% of CH 4. Material sintered in such an atmosphere is characterized by microstructure with fine grains and well distributed fine precipitates. Its surface hardness showed the highest values among all obtained materials. Keywords: high chromium alloyed sintered steel, sintering atmosphere, nitrogen uptake, carburization, decarburization INTRODUCTION During the whole sintering cycle (including delubrication during heating) a large number of reactions between the furnace atmosphere and sintered material take place. They are strongly associated with the chemical composition of the atmosphere and sintered material, but also with the time-temperature sintering regime. Therefore, it is necessary to pay attention to the complex reactions between the furnace atmosphere and sintered material, especially in terms of nitrogen and carbon content at the surface of sintered parts. Carbon is normally considered as an undesirable impurity in austenitic stainless steel. While it stabilizes the austenite structure, it has a great thermodynamic affinity for chromium. Because of this affinity, chromium carbides form whenever carbon reaches levels of supersaturation in austenite, and diffusion rates are sufficient for carbon and chromium to form precipitates. Carbon potential of the sintering atmosphere is the main instrument to control the amount of carbon in the sintered material. The aim of using nitrogen-containing atmospheres during the sintering of stainless steels was to achieve corrosion resistance equal or superior to sintering in hydrogen, in combination with markedly improved strength. For many years, inexpensive nitrogen has been added to austenitic stainless steels because it stabilizes the austenite, and increases yield strength more than additions of carbon or solid-solution alloying elements, and it is more soluble than carbon at intermediate and high temperatures, thus reducing the fraction of precipitates [1]. Ruslan Shvab, Eva Dudrová, Institute of Materials Research, Slovak Academy of Sciences, Košice, Slovak Republic Ola Bergman, Sven Bengtsson, Höganäs AB, Höganäs, Sweden

2 Powder Metallurgy Progress, Vol.13 (2013), No Sintering of stainless steel in nitrogen-containing atmospheres leads to nitrogen uptake by the material, due to high affinity of chromium for nitrogen. Important information on nitrogen uptake in sintered chromium alloyed steels was reported by Bergman [2, 3] and also in [4]. The effect of nitrogen content on properties of the sintered steel depends on the amount of nitrogen dissolved and the amount of precipitated nitrides in the steel. Strengthening by nitrogen is caused by the lattice expansion of austenite from the dissolved nitrogen as well as by the precipitation of fine dispersed Cr 2 N. The latter is also beneficial to the fatigue properties but detrimental to corrosion resistance. Nitrides in steels may have a positive effect, as in the case of nitriding, but they may also lead to deterioration of mechanical properties if precipitation occurs along grain boundaries [2]. The two critical processing parameters for minimizing Cr 2 N precipitates are the cooling rate and dew-point of the sintering atmosphere. Higher sintering temperature, lower nitrogen content in atmosphere and rapid cooling lead to a lower total nitrogen content and a smaller amount of chromium nitrides in the sintered material. Sands et al. [5] pointed out that 316L sintered in dissociated ammonia required a cooling rate of 200 C/min to prevent nitrogen absorption and precipitation of Cr 2 N. Frisk et al. [6] determined that the sintering of 316L in dissociated ammonia at 1250 C required cooling rates of >450 C/min. The oxygen content of the atmosphere influences the dissolution rate and thereby the nitrogen uptake during sintering. Jargelius-Pettersson et al. [7] reported that an increased dew point in N 2 -H 2 atmospheres decreases the nitrogen uptake during high temperature annealing of stainless steel. Because nitrogen absorption occurs through diffusion, the presence of nitrides or oxides in the material reduces the rate of nitrogen absorption. A low dew-point removes surface oxides and facilitates nitrogen absorption from the atmosphere. This explains why nitrogen absorption during cooling increases with a decreasing dew-point of the sintering atmosphere. The influence of the sintering atmosphere on the microstructure and mechanical properties of high Cr-alloyed sintered austenitic stainless steel was investigated in the present contribution. EXPERIMENTAL MATERIAL AND METHODS The material studied in this work is sintered high chromium alloyed austenitic stainless steel Fe-20 wt.%cr-ni-x-c with a total content alloying addition of 36.9%. Cold compaction at pressing pressure of 600 MPa was applied for all samples. Sintering was performed at 1250ºC for 30 min in pure hydrogen, N 2 -H 2 and N 2 -H 2 atmosphere admixed with 0.5% of CH 4 in Carbolite laboratory furnace. All the gases were of 5.0 purity and the inlet dewpoint was lower than -60 C. Sintering followed with a cooling of 10 C/min. In the case of N 2 -H 2 atmosphere and N 2 -H 2 admixed with 0.5% of CH 4 the sintering regime was modified with interruption at 850 C with holding for 30 min (gas nitridation and gas carburization). Sintering in N 2 -H 2 atmosphere was realized with and without interruption at 850 C. The surface hardness of the investigated samples was measured using HECKERT hardness tester. Metallographic observation of sintered microstructure was performed on JEOL JSM-7000F scanning electron microscope in the etched state. The etchant of 1ml HF+5ml HNO 3 +44ml distilled water was used.

3 Powder Metallurgy Progress, Vol.13 (2013), No RESULTS AND DISCUSSION Sintering in pure hydrogen atmosphere Figures 1 and 2 show the microstructure in the core and at the surface of the material sintered in the pure hydrogen atmosphere. Fig.1. Microstructure at the surface of the compact sintered in pure H 2. Fig.2. Microstructure at the core of the compact sintered in pure H 2. Metallographic observation of material sintered in pure H 2 showed that the microstructure contains austenitic grains sized at ~30 µm with coarse carbides distributed mostly on the grain boundaries. Microstructure at the surface has the same character. Hardness measured on the surface of the sintered specimens was 181±9 HV10 which corresponds to the obtained microstructure. Sintering in N 2 -H 2 atmosphere Figure 3 show the microstructure of compacts sintered in N 2 -H 2 atmosphere without interruption during cooling. The microstructure resulting from sintering with nitriding at 850 C for 30 min is presented in Figs.4a-c. a) b) Fig.3. Microstructure of material sintered in N 2 -H 2 without nitriding, a) at the surface, b) in the core area.

4 Powder Metallurgy Progress, Vol.13 (2013), No a) b) c) d) Fig.4. Microstructure of material sintered in N 2 -H 2 with gas nitriding. The carbides with a size up to 2-3 μm precipitate at austenitic grain boundaries, Fig.3b. The mean austenite grain size is 3-5 μm. The decarburization processes at the surface during the sintering in N 2 -H 2 atmosphere lead to the growth of austenitic grains and formation of coarse, irregularly shaped carbide particles, Fig.3a. Thickness of the decarburized layer was ~200 µm. The presence of small acicular nitride particles with a size up to ~1.5µm located at grain boundaries and within the grains was identified. In the case of sintering with nitriding, Figs.4a-d, the surface nitrided layer with a thickness of ~20 µm consists of isolated lamellar nitride areas sized µm and slightly smaller areas with acicular nitrides. The surface hardness of samples sintered without nitriding was 239±10 HV10 when for samples sintered with gas nitriding surface hardness value was slightly higher 245±12 HV10. The sintering of investigated material in N 2 -H 2 atmosphere is more beneficial than in pure hydrogen. Sintered microstructure consists of significantly lower grains with fine distributed small carbides. Mechanical properties are also better, which was confirmed by measured hardness values. Some decarburization of the surface area is the main disadvantage of sintering in this atmosphere. In the case of using gas nitriding, changes in surface microstructure were identified. It contains areas with lamellar and acicular nitrides up to 20 µm in depth, which is the consequence of a slow cooling rate. Such changes in surface microstructure did not decrease mechanical properties of the material, and can be

5 Powder Metallurgy Progress, Vol.13 (2013), No beneficial for wear properties. Corrosion resistance of material can be decreased due to the depletion of matrix with chromium. Sintering in N 2 -H 2 atmosphere admixed with 0.5% of CH 4 Sintering regime modified with gas carburization at 850 ºC resulted in surface microstructure consisting of small austenite grains of 3-5 µm with relatively coarse carbides distributed along the grain boundaries. Areas with lamellar nitrides (size up to µm) and Si-oxides were also observed, Fig.5a-d. The microstructure in the core area is similar to sintering in N 2 -H 2 atmosphere. a) b) c) d) Fig.5a-d. Microstructure of material sintered in N 2 -H 2 atmosphere admixed with 0.5% of CH 4. The surface hardness for samples sintered with gas carburization increased to 267±11 HV10 in comparison with samples sintered in N 2 -H 2 atmosphere. Sintering in N 2 -H 2 atmosphere with an addition of CH 4 led to the creation of microstructure with small austenitic grains and fine distributed small carbides as in core area as at the surface. As a consequence, surface hardness increased to the maximal value among all the samples. It means that N 2 -H 2 atmosphere admixed with CH 4 is better than N 2 - H 2 or pure hydrogen for sintering the investigated powder.

6 Powder Metallurgy Progress, Vol.13 (2013), No CONCLUSIONS Sintering of high chromium alloyed austenitic stainless steel powder in pure hydrogen atmosphere leads to the creation of microstructure with large grains and coarse carbides along grain boundaries. Surface hardness is low due to decarburization. N 2 -H 2 atmosphere is more beneficial from the point of both microstructure and surface hardness. Microstructure contains small austenitic grains with fine distributed small carbides. Some decarburization at the surface took place which was reflected on the larger grains and coarse and irregular shaped carbides. The most beneficial from among the investigated atmospheres was N 2 -H 2 admixed with CH 4. Small austenitic grains with relatively coarse carbides at the surface provide the highest surface hardness from among all investigated samples. Acknowledgements The authors are grateful to Höganäs AB Sweden for their support of this work. REFERENCES [1] Reed, RP.: JOM, 1989, March, p. 16 [2] Bergman, O.: Studies of Oxide Reduction and Nitrogen Uptake in Sintering of Chromium-alloyed Steel. Powder Licentiate Thesis. Stockholm : KTH, Industrial Engineering and Management, 2008 [3] Bergman, O.: Effects of nitrogen uptake during sintering on the properties of PM Steels prealloyed with chromium. Stockholm : Swedish Institute for Metals Research [4] Powder Metallurgy Stainless Steels: Processing, Microstructures, and Properties. Chapter 5: Sintering and Corrosion Resistance. ASM International, [5] Sands, RL., Bidmead, GF., Oliver, DA. In: Modern Developments in Powder Metallurgy. Vol. 2. Ed. H.H. Hausner. Plenum Press, 1966, p. 73 [6] Frisk, K., Johanson, A., Lindberg, C. In: Advances in Powder Metallurgy and Particulate Materials. Eds. J. Capus, R. German. Vol. 3. Princeton : Metal Powder Industries Federation, 1992, p 167 [7] Jargelius-Pettersson, RFA., Ekman, T., Hertzman, S., Holm, T., Linder, J.: Materials Science and Technology, vol. 9, 1993, p. 1123