Abstract
The martensite-to-austenite transformation in X4CrNiMo16-5-1 supermartensitic stainless steel was followed in-situ during isochronal heating at 2, 6 and 18Kmin−1 applying energy-dispersive synchrotron X-ray diffraction at the BESSY II facility. Austenitization occurred in two stages, separated by a temperature region in which the transformation was strongly decelerated. The region of limited transformation was more concise and occurred at higher austenite phase fractions and temperatures for higher heating rates. The two-step kinetics was reproduced by kinetics modeling in DICTRA. The model indicates that the austenitization kinetics is governed by Ni-diffusion and that slow transformation kinetics separating the two stages is caused by soft impingement in the martensite phase. Increasing the lath width in the kinetics model had a similar effect on the austenitization kinetics as increasing the heating-rate.
Highlights
Supermartensitic stainless steels are low carbon lath martensitic steels based on the Fe-Cr-Ni system [1,2]
The excellent strength and toughness properties are obtained through inter-critical annealing to promote the formation of lamellar reversed austenite on high- and low-angle boundaries of lath martensite [4,5,6,7]
Reversed austenite was reported to strengthen the material during plastic deformation by transformation induced plasticity (TRIP) [8,9,10,11]
Summary
Supermartensitic stainless steels are low carbon lath martensitic steels based on the Fe-Cr-Ni system [1,2]. The excellent strength and toughness properties are obtained through inter-critical annealing (tempering below A3 temperature) to promote the formation of lamellar reversed austenite on high- and low-angle boundaries of lath martensite [4,5,6,7]. The annealing leads to an effective decrease of the average grain size and to a “composite structure” of hard tempered martensite and soft austenite. During plastic deformation, such a structure hinders dislocation movement over long distances. The formation of lamellar austenite was reported to be promoted by the establishment of an energetically favorable phase-interface (Kurdjumov-Sachs [12,13,14,15]), and might be affected by residual stress of the martensite transformation and grain-boundary segregation [16]. The austenite was reported to approach a coarser, spherodized morphology, which decreases the phase stability upon cooling [10]
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