Isothermal Martensitic Transformation in Sensitized SUS304 Austenitic Stainless Steel at Cryogenic Temperature

The austenite-to-martensite transformation in a partially hardened stainless steel containing 17wt %Cr, 7wt %Ni and 1wt % Al was investigated with Vibrating Sample Magnetometry and Electron Back-Scatter Diffraction. Magnetometry demonstrated that measurable martensite formation can be suppressed on fast cooling to 77K as well as on subsequent fast heating to 373 K. Surprisingly, martensite formation was observed during moderate heating from 77 K, instead. Electron Back-Scatter Diffraction demonstrated that the morphology of martensite is lath type. The kinetics of the transformation is interpreted in terms of athermal nucleation of lath martensite followed by thermally activated growth. It is anticipated that substantial autocatalytic martensite formation occurs during thermally activated growth. The observation of several retardations and accelerations of the transformation rate during slow isochronal cooling is interpreted in terms of the contribution of self-induced mechanical stabilization of austenite during martensite formation.

Section: Introductionmentioning

confidence: 99%

Thermally activated growth of lath martensite in Fe–Cr–Ni–Al stainless steel

Villa¹,

Hansen²,

Pantleon³

et al. 2015

“…At magnetic saturation, the magnetisation of paramagnetic phases (i.e. c austenite 44 and e martensite, 45 when present) is several orders of magnitude smaller than the magnetisation of ferromagnetic phases (i.e. a´ martensite).…”

Section: Methodsmentioning

confidence: 99%

Anomalous kinetics of lath martensite formation in stainless steel

Villa¹,

Hansen

Pantleon³

et al. 2015

The kinetics of lath martensite formation in Fe-17.3 wt-%Cr-7.1 wt-%Ni-1.1 wt-%Al-0.08 wt-%C stainless steel was investigated with magnetometry and microscopy. Lath martensite forms during cooling, heating and isothermally. For the first time, it is shown by magnetometry during extremely slow isochronal cooling that transformation rate maxima occur, which are interrupted by virtually transformation free temperature regions. Microscopy confirms martensite formation after athermal nucleation of clusters followed by their time dependent growth. The observations are interpreted in terms of time dependent autocatalytic lath martensite formation followed by mechanical stabilisation of austenite during the transformation process.

“…[4][5][6][7] This effect can be induced by deformation and also by cooling to a low temperature (2195uC). 8,9 Recent work by the authors has reported that a 321 grade stainless steel contained ferrite in the solution heat treated condition and that the amount of ferrite increased with aging. 10 The transformation of c to a or a9 is of industrial relevance when it is considered that a brittle/ductile transition could be introduced and an increase in volume is incurred, which is generally quoted as y4% for a complete transformation.…”

Section: Introductionmentioning

confidence: 99%

“…The formation of α' is usually associated with deformation and it is known that martensite can form in the 300 series of stainless steel during deformation at temperatures well above the marteniste start temperature [4][5][6][7]. This effect can be induced by deformation and also by cooling to a low temperature (-195°C) [8,9]. Recent work by the authors has reported that a 321 grade stainless steel contained ferrite in the solution heat treated condition and that the amount of ferrite increased with ageing [10].…”

Section: Introductionmentioning

confidence: 99%

Analysis of ferrite formed in 321 grade austenitic stainless steel

Green

Higginson

Hogg

et al. 2015

A significant fraction of ferrite has been identified in a 321 grade austenitic stainless steel in the solution heat treated condition. The microstructures were analysed using electron backscatter diffraction, energy dispersive X-ray spectroscopy and X-ray diffraction (XRD) and the stability of the ferrite investigated using heat treatments in a tube furnace, dilatometry and high temperature XRD. The ferrite dissolved ∼800°C, then formed again on cooling at temperatures under 200°C. Thermodynamic predictions showed a significant ferrite content at room temperature under equilibrium conditions, and the DeLong diagrams predict an austenite+martensite microstructure in the cast condition. Sensitivity analysis on the DeLong diagram has shown that the nitrogen content had a large effect on the austenite stability. The instability of the austenite and the subsequent transformation to ferrite on cooling can be attributed to low nitrogen content measured in the as received material. It was found that thermal aging of the material caused further transformation of austenite to ferrite as well as the formation of sigma phase that appears higher in nitrogen than the matrix phases. The diffusion of nitrogen into sigma phase may cause instability of the austenite, which could cause further transformation of austenite to ferrite on cooling from the aging temperature. The transformation of austenite to ferrite is known to be accompanied by an increase in volume, which may be of relevance to components made with tight dimensional tolerances.