AGEING AND TEMPERING OF FERROUS MARTENSITES

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Abstract
The low temperature ageing of a Fe-15wt%Ni-1wt%C martensite (Ms temperature -35 C) has been investigated using field - ion microscopy and atom probe microanalytical techniques. It is demonstrated that both the amplitude and the wavelength of the carbon composition fluctuations observed during room-temperature ageing increase with time, indicating that the alloy decomposes by a spinodal process. The carbon-rich regions approach a limiting composition of Fe8C during this reaction. The tempering behaviour of a high-cobalt-nickel secondary hardening steel, AF 1410, was also studied. The composition of the fine M2C carbides formed during the standard tempering treatment at 510 C was determined, and coarser particles of M2C and MC formed during overaging at 593 C were also analyzed. Some evidence of atomic ordering was observed in the Fe-Ni-Co-rich matrix ; such an effect would help to explain the high resistance to dislocation recovery which is observed in this class of steels

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