PROPERTIES AND MICROSTRUCTURES OF SINTERED STAINLESS STEEL PREPARED FROM 304L AND 410L POWDERS

Transcription

1 PROPERTIES AND MICROSTRUCTURES OF SINTERED STAINLESS STEEL PREPARED FROM 304L AND 410L POWDERS B. Vetayanugul, N. Tosangthum, R. Krataitong, M. Morakotjinda, A. Daraphan, T. Yotkaew, O. Coovattanachai and R. Tongsri Powder Metallurgy Research and Development Unit (PM_RDU) National Metal and Materials Technology Center, Pathumthani 12120, Thailand ABSTRACT A preliminary investigation of the properties and microstructures of sintered materials, prepared from stainless steel series 300 and 400 powders (with a mass ratio of 50:50), showed that the combination of 304L and 410L exhibited superior strength. Mixtures of different mass ratios of 304L and 410L powders, were investigated and compared with those of sintered 304L and 410L steels. The 100% 304L, 75% 304L- 25% 410L, 50% 304L-50 % 410L, 25 % 304L-75% 410L and 100% 410L powders were processed using press and sinter technology. It was found that dimensional changes, particularly shrinkage after sintering, of the sintered 304L+410L stainless steel mixtures were lower than those of the sintered 304L or 410L alloys. Because of lower shrinkage, sintered densities of the 304L+410L mixtures were lower than that of the sintered 304L or 410L alloys. The sintered 304L+410L materials exhibited increased ultimate tensile strength, yield strength and hardness with reduced elongation, when the 410L content was increased. The materials prepared from 25 % 304L-75% 410L showed the most significant increase in strength and hardness. INTRODUCTION The development of a dual phase stainless steel (known as duplex stainless steel (DSS)) is one of many examples of alloy mechanical property improvement. The DSS refers to a stainless steel, whose microstructure contains both austenitic and ferritic phases [1]. This alloy exhibits superior mechanical properties and corrosion resistance, compared with those of typical stainless steels (austenitic, ferritic, and matensitic). The outstanding properties of the dual phase stainless steel arise from a combination of phase advantages. Because of their excellent properties, DSS are applied in a wide range of industries, especially in corrosive environments such as off-shore construction, heat-exchanger and gas/oil pipelines [2]. Wrought DSS is common in the metallurgical world. However, to produce this type of alloy presents some fabrication difficulty. To obtain a dual phase microstructure, high accuracy temperature control and high metallurgist experience are needed for manufacturing of the alloy

2 Due to the manufacturing difficulties, powder metallurgy (P/M) has been considered as an alternative production route for duplex stainless steel. The P/M process offers relatively simple processing steps of powder mixing, compacting, delubricating and sintering. Although the P/M process is straightforward, processing of a mixture of austenitic and ferritic powders to produce an acceptable dual phase material presents some problems. Systematic and careful research and development has to be conducted for the design and manufacturing of a P/M duplex alloy. In this research, duplex material samples have been prepared from mixtures with variable 304L and 410L powder contents. In a previous investigation, it was found that the duplex material, prepared from 50 wt.% 304L and 50 wt.% 410L powders, exhibited promising mechanical properties [3]. Further investigation of the combined 304L + 410L material into the effect of the 310L/410L ratio has been carried out. The results are presented and discussed in this article. EXPERIMENTAL PROCEDURE Raw materials, used for the experimental work, were 304L and 410L stainless steel powders (Coldstream, Belgium). Sintered compacts were prepared from mixtures of different mass ratios of 304L and 410L powders, as shown in Table I. Table I. Materials Studied wt-% of 304 L wt-% of 410 L Code A A A A A The 304L and 410L powders were weighed and mixed. Mixing time was 8 h. in order to obtain a homogeneous distribution of the powders. Zinc strearate (1.0 wt.%) was added to the mixed powders. Mixed powders were pressed into tensile bars (ASTM E8-96) at 650 MPa using a uniaxial press. Pressed parts (known as green parts ) were then sintered at 1280 o C for 45 min. in pure hydrogen. The dimensions of the green and sintered test bars were measured in order to calculate the dimensional changes from die size. Densities of the green and sintered samples were determined using the Archimedes method. A universal testing machine (Instron model 8801) was employed to determine the tensile properties of the sintered samples. Microstructures of the sintered parts were observed using optical microscopy. Hardness (HRB, Instron UHM 930/250) and Microindentation hardness (HV0.05, Anton Paar MHT-10) of sintered samples were also investigated. RESULTS AND DISCUSSION DIMENSIONAL CHANGES Dimensions of the green and sintered parts were measured to calculate the dimensional differences from die size. For each sample, three positions, designated as grip (P1), gage (P2) and length (P3), were measured. The calculated dimensional changes of different dimensions are shown in Fig 1. Spring back (expansion of the pressed part dimensions after ejection from the die, illustrated in the upper part of Figure 1 with positive value) of each studied samples showed the highest value at the P2 position, 7-155

3 which has the smallest die wall to die wall distance. Due to the shortest distance, stress generated during compaction is expected to be highest. The highest stress then causes the highest spring back. After the sintering, some materials exhibited dimensional shrinkage. Shrinkage of the sintered part is shown in the lower part of Figure 1 with negative value. Comparison of the shrinkage of the sintered 304L and 410L samples with the sintered dual phase samples showed that shrinkage of the sintered dual phase samples was lower. It was noted that note that sample 25A exhibited the lowest value for shrinkage. SINTERED DENSITY Figure 1. Dimensional change from the die size of green and sintered samples. Sintered densities of the samples showed no significant difference (Figure 2). Sample 25A exhibited the lowest sintered density. The sintered density values conform to the shrinkage data, as noted previously. Figure 2. Sintered density MECHANICAL PROPERTIES 7-156

4 Mechanical properties of the sintered DSS samples are illustrated in Figure 3-6. Yield strength, ultimate tensile strength and hardness of the sintered DSS exhibited the same trend, in which strength and hardness increased with increasing 410L content. It was noticed that the sample 25A showed significantly improved yield strength, more than twice the yield strength of the sintered 304L material and also obviously higher than that of the sintered 410L material. Figure 3. Yield strength of sintered dual phase samples. Figure 4. Ultimate tensile strength of sintered dual phase samples

5 Figure 5. Elongation of sintered dual phase samples. Figure 6. Hardness of sintered dual phase samples. In contrast, it was found that the ductility of the sintered dual phase samples was dramatically decreased when the percentage of 410L was increased. Further work is needed to improve ductility; otherwise, sintered DSS will be considered as unsuitable for engineering applications. Hardness values of the sintered DSS were higher than those of the sintered 304L and 410L materials. Sample 25A showed the highest hardness. The hardness of the sintered dual phase samples was directly related to the microstructural characteristics, Figure 6. MICROSTRUCTURE It was observed that a new phase was formed in the sintered DSS. In order to observe microstructures, the sintered DSS samples were polished and chemically etched using Beraha s reagent. The optical micrograph in Figure 7 shows three different phases. According to a previous study by Puscas et al. [4,5], the phases in the microstructure (Figure 7) could be identified as follows. The dark grey zone with irregular shape and rough texture was identified as a ferritic phase, designated as F. The lightest grey zone in a microstructure was an austenitic phase, designated as A. The pale grey zone between ferrite and austenite phases was a new phase, designated as N

6 Figure 7. Phase identification in sintered DSS. The new phase is formed by diffusion of nickel from austenite into ferrite during sintering [4,5]. Microindentation hardness tests performed on each phases showed that the new phase exhibited the highest hardness (Table II). The hardness of the new phase was 50% higher than that of the austenitic phase and 38% higher than that of the ferritic phase. Table II. Microindentation Hardness of phases in samples 25A. Phase Hardness (HV0.05) A F N The proportion of the new phase increased with increasing 410L powder content (Figure 8). Diffusion of nickel into 410 powder particles is believed to cause the formation of the new phase. When the amount of ferrite (nickel receiver) is increased, the probability for new phase formation is also increased. The volume fraction of the new phase seems to affect the mechanical properties of the sintered DSS. Increases in volume fraction of the new phase (Figure 7) result in improved strength and hardness (Figures 3, 4 and 6). CONCLUSIONS Sintered DSS, prepared from mixtures of 304L and 410L powders, exhibited microstructures consisting of ferritic, austenitic and N phases. The N phase is formed by diffusion of nickel from 304L to 410L. The N phase is perhaps a combination of austenitic, ferritic and matensitic structures. The volume fraction of the N phase is increased with 410L content. This enhances the mechanical properties of the sintered DSS. Yield strength, ultimate tensile strength and hardness of the DSS are higher than those of the sintered 304L and 410L materials. Limited ductility makes the DSS unsuitable for engineering applications. Ductility improvement is therefore recommended for further investigation

7 (a) (b) (c) Figure 8. Microstructure of sintered DSS samples: (a) 75A (b) 50A and (c) 25A. REFERENCES [1] ASM, Practical Handbook of Stainless Steel & Nickel Alloys, CASTI Publishing Inc., Canada, 1999, pp [2] P. Datta and G.S. Upadhyaya, Sintered duplex stainless steels from premixes of 316L and 434L powders, Mater. Chem. Phys., 2001, vol. 67, pp [3] N. Kuljittipipat, et. al., Mechanical Properties of Austenitic + Ferritic Stainless Steels Prepared by Powder Metallurgy, NSTDA Annual Conference, Patumthani, Thailand, March 2005, pp. 56. [4] T. M. Puscas et. al., Sintering Transformation in Mixtures of Austenitic and Ferritic Stainless Steel Powders, Powder Metall., 2001, vol. 44, pp [5] M. Campos et. al., Study of the Interfaces Between Austenite and Ferrite Grains in P/M Duplex Stainless Steels, Journal of the Europian Ceramic Society, 2003, vol 23, pp