COMPARISON OF MECHANICAL PROPERTIES OF MANGANESE STEELS OF THE SAME CHEMICAL COMPOSITION BASED ON SPONGE AND ATOMISED IRON POWDERS

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1 Powder Metallurgy Progress, Vol.8 (2008), No 2 76 COMPARISON OF MECHANICAL PROPERTIES OF MANGANESE STEELS OF THE SAME CHEMICAL COMPOSITION BASED ON SPONGE AND ATOMISED IRON POWDERS A. Ciaś, A. S. Wronski Abstract Fe-3Mn-0.6C was processed from ferro-manganese, graphite and Höganäs sponge or water atomised iron powders by pressing at 660 MPa and sintering in semi-closed containers for 1 hour in dry nitrogen or hydrogen at 1120 or 1250 C. Neither densification nor swelling took place, whatever the powder mix and sintering conditions. Yield strengths were higher for the atomised ABC base alloy and for 1250 C and nitrogen sintering. Tensile and bend strengths, however, were by ~10% higher for specimens from sponge NC , resulting in ~740 MPa tensile strength for sintering in hydrogen at 1250 C, to be compared to ABC base at ~650 MPa. The higher, >3%, plasticity of the sponge-based steels is associated with the increased surface area available to the Mn vapour for alloying, maintaining a much better Mn-C balance for forming pearlite at sinter necks. Keywords: manganese steels, nitrogen sintering, semi-closed containers, sponge and atomised iron powders INTRODUCTION In our previous communication [1] we reported results on the properties of manganese steels based on sponge, NC , and two atomised, ABC and ASC , iron powders with the powder mixes of the same chemical composition. The compacts were sintered in semi-closed containers either at 1120 or 1250 C in either hydrogen or nitrogen. Chemical analyses for oxygen in the iron starting powders gave ~ 0.2% O for all iron powders, and after sintering %. Carbon in the starting powder mixes was 0.8% and after sintering it varied from 0.42 to 0.72%, Table 1. Carbon losses can be directly related to hydrogen content in the sintering atmosphere and the sintering temperature, Fig.1. Tab.1. Carbon contents after sintering of Fe-3Mn-0.8C [starting composition]. Carbon content [wt.%] ASC C, H C, N C, H C, N Andrzej Ciaś, AGH - University of Science and Technology, Faculty of Metallurgy and Material Science, Department of Metallurgy and Materials Engineering, Cracow, Poland Andrew S. Wronski, Engineering Materials Group, School of Engineering, Design and Technology, University of Bradford, W. Yorks, United Kingdom

2 Powder Metallurgy Progress, Vol.8 (2008), No 2 77 wt. % 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0, Hydrogen content, % Carbon, wt. %, 1120 C Carbon, wt. %, 1250 C NC Fig.1. Dependence of the final carbon content of NC Mn-0.8C on the sintering temperature and hydrogen content in the sintering atmosphere. In the semi-closed containers the Mn losses were similar and not significant. Direct comparison of resultant mechanical properties is hindered by this C variation, thus, by adjusting the graphite contents of the powder mixes, an attempt was made to attain after processing the same C content of ~0.6% for the sponge, NC100 24, and the superior water atomised, ABC , iron base powders. EXPERIMENTAL PROCEDURES Höganäs sponge, NC , and water atomised, ABC , iron powders were the starting materials in this investigation. 3% of manganese was introduced as Elkem low carbon ferro-manganese and carbon as fine Höganäs CU-F graphite in the amounts recorded in Table 2, aiming at sintered C content of 0.6%. The Elkem powder, of weight % composition 80Mn-1.3C-0.2O-0.02N-balance Fe, is a by-product, "fines", from electrode production, with 90% under 25 µm and 80% under 15 µm particle sizes. Double-cone mixing and die compaction, using only die lubrication, at 660 MPa of ISO 2740 dog bone specimens, were followed by sintering in dry hydrogen or nitrogen in a horizontal laboratory furnace. Its heat resisting Kanthal APM tube included a water-jacketed rapid convective cooling zone. The dew point of the sintering atmospheres was 60 C (15 ppm moisture). To produce a Mn-rich microclimate (self-gettering effect), the specimens were sintered in a semi-closed stainless steel container with labyrinth seal [2]. Compacts were heated to the sintering temperature, at a rate of 75 C/min. and held at 1120 or 1250 C for 60 minutes. The convective cooling rate, determined in the temperature range of C, was approximately 60 C/min. After sintering, for stress relief, all specimens were tempered at 200 C for 1 hr in argon. Densities were determined by the Archimedes method: ISO 2738:1999. Chemical analyses for oxygen in the iron starting powders were carried out on Leco apparatus TC-336, giving ~ 0.2% O for all iron powders. The carbon contents in the compacts, determined on Leco CS-125, are recorded in Table 2, which also shows that 0.6 ± 0.04% C was achieved in all samples after sintering. Oxygen contents in the specimens sintered in hydrogen were from 0.15 (1250 C) to 0.31% (1120 C) and in nitrogen from 0.34 (1250 C) to 0.59% (1120 C).

3 Powder Metallurgy Progress, Vol.8 (2008), No 2 78 Standard ISO 2740 tensile specimens were uniaxially tested on an MTS 810 servohydraulic machine at a strain rate of ~ 5x10 4 s 1. The same specimen types on the same apparatus were tested in three-point bending to determine the [apparent] transverse rupture strength, TRS. Conventional metallographic techniques were supplemented by scanning electron microscopy. Tab.2. Carbon contents before and after sintering of Fe-3Mn-0.6C [final composition]. Carbon content [wt.%] green sintered green sintered 1120 C, H C, N C, H C, N RESULTS Densification/Swelling Within the [small] experimental error, the densities of Fe-Mn-C compacts remained unchanged on sintering, as shown in Table 3. For ABC based steel it was thus ~ 7.1 g/cm 3, more than 0.2 g/cm 3 larger than for the sponge-based alloy. Tab.3. Densities before and after sintering of Fe-3Mn-0.6C. Density [g/cm 3 ] green sintered green sintered 1120 C, H ± ± ± ± C, N ± ± ± ± C, H ± ± ± ± C, N ± ± ± ± 0.04 ± - standard deviation measured on 15 samples. Metallographic and Microstructural Observations All microstructures comprised a mixture of martensite, retained austenite, pearlite and bainite, in different proportions depending principally on the sintering temperature. Typical examples are presented in Fig.2. In general, the structures of steels sintered in nitrogen were less inhomogeneous than those sintered in hydrogen, which also showed evidence of surface decarburisation. In specimens sintered at 1120 C microstructures were inhomogeneous: the transformation to martensite was incomplete and the austenite grains were relatively small. Observed were complex structures composed of bainite surrounding a large number of small, substantially unaligned austenitic-martensitic areas, which were partially decomposed to carbide and ferrite, resembling troostite. Particularly in the vicinities of prior ferro-manganese particles, there were regions comprising retained manganese-rich austenite, which incompletely transformed to martensite. In specimens sintered in hydrogen surface carbon loss was evident.

4 Powder Metallurgy Progress, Vol.8 (2008), No 2 79 The structures were coarser after sintering at 1250 C, consisting mainly of bainitic and martensitic regions. For specimens sintered in nitrogen, upper and lower bainite, lathshaped fine aggregates of ferrite and cementite were found. The undecomposed austenite regions were also smaller and very irregular in shape, but more numerous, than in steels sintered at 1120 C. The high-manganese regions usually exhibited structures comprising long coarse martensite plates extending across prior austenite grains. Resulting from dissolution of ferro-manganese particles and Mn sublimation, diffusion and condensation, these Mn steels contained both micropores and mesopores. The pores were more angular in the sponge-based material, but became more rounded in specimens sintered at 1250 C, through greater diffusion of manganese at the higher temperature. There were no significant differences in the overall density of specimens sintered from the same starting powder, whether in nitrogen or hydrogen, or whether at 1120 or 1250 C; for the latter the pores were rounder, larger and fewer in number. (a) (b) Fig.2. Microstructures of (a) atomised and (b) sponge-based steels sintered in nitrogen at 1250 C. Sponge-based steel contained pearlite, bainite and martensite, whereas that emanating from atomised iron comprised predominantly lower bainite. Mechanical Properties The values of Young s modulus, 0.2% offset yield, tensile and transverse rupture strengths and tensile failure strain were determined, usually for 15 specimens of each batch. Young's modulus in the sponge-based material was 117±3 GPa and in that deriving from ABC attained 130±3 GPa. The remaining data are presented in Tables 4-7. Tab % offset yield, R 0.2, and fracture, R m, strengths of Fe-3Mn-0.6C. Strength [MPa] yield fracture yield fracture 1120 C, H ± ± ± ± C, N ± ± 39 ND 528 ± C, H ± ± ± ± C, N ± ± ± ± 67 ± - standard deviation measured on 5 samples. ND = not determined.

5 Powder Metallurgy Progress, Vol.8 (2008), No 2 80 Tab.5. Tensile strains (elastic + plastic) to failure of Fe-3Mn-0.6C. Strain [%] 1120 C, H ± ± C, N 2 ND ND 1250 C, H 2 ND 3.2 ± C, N ± ± 0.7 ± - standard deviation measured on 15 samples. ND = not determined. Tab.6. Transverse Rupture Strengths [uncorrected], TRS of Fe-3Mn-0.6C. Stress [MPa] 1120 C, H ± ± C, N ± ± C, H ± ± C, N ± ± 184 ± - standard deviation measured on samples. Fractographic Observations All fractographic observations, examples of which for the two material types are given in Fig.3, revealed regions of intergranular and cleavage failure. It should be noticed, however, that only in the steel emanating from sponge powder are there extensive regions of dimple rupture, shown in Fig.3b. It should be added that sinter neck areas in fractographs of sponge-based material also appear larger, thus with their reduced Mn contents, favour ductile rather than brittle fracture on this microscale. (a) (b) Fig.3. Fractographs of tensile specimens sintered in nitrogen at 1250 C emanating from (a) atomised, and (b) sponge iron powders. Note the increased plasticity, including dimple rupture, evident in sponge-based steel, (b), where pearlite formed readily at sinter necks. DISCUSSION The results clearly confirm [1] that, whereas the yield strength and Young s modulus of the sponge-based steels are inferior to those based on the atomised powder, the

6 Powder Metallurgy Progress, Vol.8 (2008), No 2 81 (fracture) strengths are superior in the former. Yield strengths and Young's moduli appear related to density (Table 3), but plasticity is increased by % (Table 5) for the sponge-based steels, which at ~ 4% level, is considerable. Accordingly reasons for this contrasting behaviour of Mn, as compared to conventional, e.g. Cu-C, PM steels will be sought in the starting powder microstructures and sintering mechanisms, involving Mn vapour. Reduction of (iron, chromium, manganese) oxides has received considerable recent analysis [2-4] and attention is first drawn to that of Danninger et al [3] whose mass spectrometric results directly show that in carbothermic reduction processes CO is formed. Cias and Mitchell [2,4], when studying manganese (and chromium-manganese) steels during nitrogen sintering, considered the local microclimate with Mn (vapour) as well as C being available. Their analysis took into account partial pressures and the sintering process was modelled as a conglomerate of several surfaces oxidised alloy particles surrounding a pore with graphite present and a tortuous access to the nitrogen-rich atmosphere containing some water vapour and oxygen. Although reduction reactions become thermodynamically favourable from 200 C, kinetics dictates availability of CO. The relevant reactions are the water-gas, C + H 2 O = CO + H 2, from 500 C and the Boudouard: C + CO 2 = 2CO, from 700 C. The irregular shape, high specific surface, and the spongy internal structure of reduced oxide particles enables manganese vapour passage for alloying to all the solid surfaces through interconnected porosity and within internal structure of the particles. Thus the area for oxide-mn vapour reactions is much larger in iron sponge than in the water atomised powder and the greater compact porosity should also favour these reactions. Furthermore, Mn content in sinter necks is reduced, resulting in a better Mn-C balance for forming pearlite. Such regions are not easy paths for microcracking and thus ductile dimple rupture results. Attention should be drawn also to the source of Mn, introduced into the compact as ferro-manganese particles and freely available as vapour in semi-closed containers for surface reactions and diffusion. Such [also elemental Mn, or master alloy] particles, however, are distributed within the compact and it has been suggested Diffusion Induced Grain Boundary Migration, DIGM, needs also to be considered [7-9]. Kabátová et al reported that, e.g. for PM Fe-1.5Cr-0.2Mo-0.7C, microcrack nucleation, growth and propagation dictate tensile strength [9, 10] and that prior particle boundaries are preferential paths for microcracking in static loading. Thus anything to improve microcracking resistance will improve plasticity and hence tensile strength, e.g. fine Fe-Mn pearlite adjoining pores formed by dissolution of ferro-manganese particles [8]. It is thus suggested that the morphology and microcomposition of iron sponge-based Mn steels favour microcrack arrest and dimple rupture. CONCLUSIONS 1. Compaction at 660 MPa of standard tensile specimens resulted in green densities of approximately 6.9 and 7.1 g/cm 3 for steels based on Höganäs NC sponge and water atomised ABC powders, respectively. 2. at 1120 or 1250 C, in hydrogen or nitrogen, did not produce any significant change in density in all specimens. Carbon losses, up to 0.32%, were higher for sintering in hydrogen and at 1250 C. 3. Young's moduli and yield strengths reflected densities, with the best values, 117 GPa and 520 MPa, respectively, recorded for ABC based steel.

7 Powder Metallurgy Progress, Vol.8 (2008), No Mechanical properties of steels sintered at 1250 C were expectedly superior to those sintered at 1120 C. 5. Tensile elongations to failure and values of fracture and transverse rupture strengths were always highest for the sponge-based alloys, reaching 4.6%, 741 and 1354 MPa, respectively. 6. These superior mechanical properties are associated with the increased surface area of iron sponge available to the Mn vapour for alloying. It is proposed that this reduces the Mn content at sinter necks, thereby maintaining a much better Mn - C balance for forming pearlite. This favours dimple rupture, observed locally in atomised iron-based steels only after sintering at 1250 C, and ensures cleaner, more cohesive prior particle boundaries, the favoured microcracking paths, in sponge-based Mn steels. Acknowledgements The work described forms part of the cooperative programme between the University of Bradford and Akademia Górniczo-Hutnicza, Kraków; the latter supported by a KBN contract No The authors gratefully acknowledge the work of Mgr. inz. H. Cias, who carried out the carbon and oxygen analyses. REFERENCES [1] Cias, A., Wronski, AS.: Powder Metallurgy Progress, vol. 7, 2007, p. 139 [2] Cias, A., Mitchell, SC., Pilch, K., Cias, H., Sulowski, M., Wronski, AS.: Powder Metall., vol. 46, 2003, p. 165 [3] Danninger, H., Gierl, Ch., Kremel, S., Leitner, G., Jaenicke-Roessler, K., Yu, Y.: Powder Metallurgy Progress, vol. 2, 2002, p. 125 [4] Mitchell, SC., Cias, A.: Powder Metallurgy Progress, vol. 4, 2004, p. 132 [5] Navara, E. In: '85. Ed. G.C. Kuczynski et al. New York and London : Plenum Press, 1985, p. 343 [6] Šalak, A.: Kovové Materiály, vol. 27, 1989, p. 83 [7] Sainz, S., Martinez, V.,Dougan, M., Baumgaertner, F., Castro, F. In: Proc.PM' Int. Conf. on PM and Particulate Materials, IPMF, 2006, Vol. 7, p. 95 [8] Dudrová, E., Kabátová, M., Wronski, AS., Mitchell, S., Bidulský, R.: Acta Metallurgica Slovaca Special Issue, vol. 13, 2007, p. 787 [9] Dudrová, E., Kabátová, M. In: Proc. PM World Cong. 2004, Vienna, Austria. Ed. H. Danninger et al. Vol. 3. EPMA, 2004, p. 193 [10] Kabatová, M., Dudrová, E., Wronski, AS.: Powder Metall., vol. 49, 2006, p. 363