Tensile properties of Fe-3Mn-0·6/0·7C steels sinteredin semiclosed containers in dry hydrogen, nitrogen and mixtures thereof

Abstract

Tensile properties of powder metallurgy 3% manganese-0·8% carbon (content of green compact) steels were determined following laboratory sintering in (nearly) full, semiclosed containers with no getter powders in dry, 0-100% hydrogen-nitrogen atmospheres. Manganese was mixed with the NC 100·24 sponge iron powder as low carbon ferromanganese and carbon as a graphite addition. Dogbone compacts were pressed at 660 MPa, the sintering temperatures were 1120 and 1250°C and cooling rates ~65 K min- 1. In specimens sintered in nitrogen containing atmospheres at 1120°C, final carbon content was ~0·7% and for those processed at 1250°C ~0·6%. Sintering in dry hydrogen resulted in lower carbon and oxygen contents. Independent of the H₂/N₂ ratio in the furnace atmosphere, however, all the specimens were ductile and exhibited similar strengths. Yield strengths R _0·2 were in the range: 426-464 MPa, tensile strengths R_m were 724-780 MPa and strains to failure were 1·6-2·0% after sintering at 1250°C. The 1120°C sintering temperature resulted in 10-15% lower strength values. The microstructures, significantly devoid of oxide networks, comprised mainly mixtures of bainite and fine (divorced) pearlite, with very little martensite and retained austenite. Reproducibly successful sintering of manganese containing compacts requires that reduction conditions exist at the sintering temperature. Ellingham Richardson diagrams dictate that the dewpoints of hydrogen required are-55 and-40°C at 1120 and 1250°C, respectively. A semiclosed container, how ever, ensures a different microclimate. It is suggested that then the initial relevant reactions there are: Mn[vapour]+H₂O=MnO+H₂, 3Fe₂O3 +H₂= 2Fe₃O₄+H₂O, Fe₃O₄+H2=3FeO+H₂O, FeO+H₂= Fe+H2O and C+O2=CO2, which provide hydrogen andwater vapour,also within the pores. The manganese vapour further acts as a 'shield' by generating further hydrogen from the water vapour. The following reactions involving carbon monoxide are postulated above 927°C, when CO is a more effective reducing agent than hydrogen: C+H₂O=H₂+CO, 3Fe₂O₃+ CO=2Fe₃O₄+CO₂, Fe₃O₄+CO=3FeO+CO₂, FeO+CO=Fe+CO₂ and C+CO₂=2CO. Accordingly, irrespective of whether it is hydrogen or nitrogen in the semiclosed container, if there is a supply of carbon, reducing conditions prevail at the sintering temperature,embrittling oxidenetworks arenot formed and ductile manganese steels are processed.