Formation and stabilization of reverted austenite in supermartensitic stainless steel

Abstract

The formation and stabilization of reverted austenite upon inter-critical annealing was investigated in a X4CrNiMo16-5-1 (EN 1.4418) supermartensitic stainless steel by means of scanning electron microscopy, electron backscatter-diffraction, transmission electron microscopy, energy-dispersive X-ray spectroscopy and dilatometry. The results were supported by thermodynamics and kinetics models, and hardness measurements. Isothermal annealing for 2 h in the temperature range of 475 to 650 °C led to gradual softening of the material which was related to tempering of martensite and the steady increase of the reverted austenite phase fraction. Annealing at higher temperatures led to a gradual increase in hardness which was caused by formation of fresh martensite from reverted austenite. It was demonstrated that stabilization of reverted austenite is primarily based on chemical stabilization by partitioning, consistent with modeling results.

Austenite reversion in low-carbon martensitic stainless steels – a CALPHAD-assisted review

“…Sufficiently fast heating (approx. >10 K s -1[54,55]) does not allow enough time for long-range diffusion and thus leads to transformation by a displacive mechanism instead.Close to A 1 (composition dependent at ~ 500 -550 °C) allotriomorphic reverted austenite with film morphology forms at lath boundaries[55][56][57][58][59][60] with little or no deviation from the Kurdjumow-Sachs orientation relationship[35,61,62]. All reported micrographs of annealed microstructures in the temperature range 500 to 575 °C (c.f.…”

Evolution of substructure in low-interstitial martensitic stainless steel during tempering

“…Orientation mapping with EBSD and TKD was applied to reveal the microstructure of lath martensite in the hardened condition. The orientation relationship of lath martensite with austenite was identified as Kurdjumow-Sachs (K-S) in a previous investigation [32]. Figure 1a shows an orientation map obtained with EBSD of a bulk specimen of martensite in the hardened condition.…”

“…Further heating leads to a second increase in austenite content up to 11 vol.% at 750 °C. This reverted austenite obtains its stability at room temperature from partitioning of Ni [10,32,59]. Further increase of the austenite fraction at higher annealing temperature (or longer annealing time) leads to dilution of the Ni content in reverted austenite and a reduction of its stability upon cooling to room temperature.…”

Effect of Intercritical Annealing on the Microstructure and Mechanical Properties of 0.1C-13Cr-3Ni Martensitic Stainless Steel

“…However, strength and toughness improvement can be achieved with intercritical annealing in the temperature range between Ac1 and Ac3, where martensite partially transforms to austenite. At the same time, austenite stabilizing elements, such as carbon, nickel, manganese and nitrogen, are partitioned into the austenite [15][16][17], which transforms back to martensite upon subsequent cooling. The result is a lamellar morphology which leads to a grain refinement effect, according to the Hall-Petch equation.…”

“…During the intercritical annealing process of some martensitic stainless steels, reverted austenite may be stabilized to a certain extent, even to room temperature, mainly due to the chemical (partitioning of austenite stabilising elements) and mechanical (lamellar morphology) stabilization [17,[20][21][22][23]. This phenomenon was observed in Fe-13%Cr-4%Ni [24,25], Fe-13%Cr-6%Ni [20,26,27], Fe-16%Cr-5%Ni [28], and Fe-13%Cr-7%Ni-3%Si [29] martensitic stainless steels.…”