The impact toughness of iron manganese alloys.
Fe-8Mn alloys may form the basis of a cryogenic steel similar to Fe-9Ni steels, but at a cost much more economical as manganese is cheaper than nickel. However, Fe-Mn alloys are susceptible to temper embrittlement through segregation of Mn, N and P to prior austenite grain boundaries. The present investigation shows that impact toughness can be improved by tempering to produce small amounts of reverted austenite into the structure. In quantifying the percentages of reverted austenite in these structures. X-ray and Mossbauer studies have shown that the phase analysis of these alloys varies from the surface of specimens into the bulk, and is thought to be due to deformation induced transformation of gamma→epsilon during sample preparation. It was concluded that bulk austenite and epsilon concentrations are best obtained after 2 minutes electro--polishing. Additions of 0.1%Ti and 0.1%Al to 'pure' alloys, in order to getter nitrogen, have been found to improve impact properties by reducing intergranular embrittlement. These additions also lead to a 'massive ferrite' structure not previously found in alloys of this manganese level. This is thought to be due to the lower interstitial levels in solid solution favouring formation of massive ferrite rather than lath martensite at a higher cooling rate and lower transformation temperature. A further considerable improvement in toughness has been achieved by water quenching rather than air cooling from the austenitisation temparature. Auger electron spectroscopy studies have shown that air cooling results in segregation of phosphorus and nitrogen to prior austenite grain boundaries. An addition of 0.5%Mo to these alloys was found not to be completely effective in reducing intergranular embrittlement.Commercially 'impure' alloys were found to exhibit inferior impact properties compared to 'pure' alloys. A further comparison with 9% nickel and 5% nickel steels has shown that pure Fe-8Mn alloys could be used as a low temperature steel down to ~ -100°C. A 'novel' heat treatment to an Fe-8Mn-G.2V alloy, consisting of a solution treatment followed by low temperature ageing has shown that vanadium carbide precipitation produces a significant reduction in grain size upon re-austenitisation due to particle pinning of grain boundaries.
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Sheffield Hallam University (United Kingdom).