Abstract
AbstractDifferent grain sizes were created in a metastable 17Cr‐7Mn‐7Ni steel by martensite‐to‐austenite reversion at different temperatures using a laser beam. Two fully reverted material states obtained at 990°C and 780°C exhibited average grain sizes of 7.7 and 2.7 μm, respectively. The third microstructure (610°C) consisted of grains at different stages of recrystallization and deformed austenite. A hot‐pressed, coarse‐grained counterpart was studied for reference. The yield and tensile strengths increased with refined grain size, maintaining reasonable elongation except for the heterogeneous microstructure. Total strain‐controlled fatigue tests revealed increasing initial stress amplitudes but decreasing cyclic hardening and fatigue‐induced α′‐martensite formation with decreasing grain size. Fatigue life was slightly improved for the 2.7‐μm grain size. Contrary, the heterogeneous microstructure yielded an inferior lifetime, especially at high strain amplitudes. Examinations of the cyclically deformed microstructure showed that the characteristic deformation band structure was less pronounced in refined grains.
Highlights
Due to their mechanical properties in terms of excellent ductility and ultimate tensile strength accompanied by an outstanding hardening capability, metastable austenitic steels have attracted high interest in the last decades
The results revealed a heterogeneous microstructure over the cross section, which can be related to the completely different scan strategies used for the electron beam (EB)
The two highest peak temperatures (989C and 779C) yielded grain sizes of 7.7 and 2.7 μm, whereas a more heterogeneous microstructure consisting of submicron-sized grains and deformed austenite as well as individual grains with diameters up to almost 2 μm was produced by the lowest peak temperature (608C)
Summary
Due to their mechanical properties in terms of excellent ductility and ultimate tensile strength accompanied by an outstanding hardening capability, metastable austenitic steels have attracted high interest in the last decades. A common method for achieving different grain sizes in metastable austenitic steels is thermo-mechanically controlled processing.[7,26,27,28] Recently, an extensive review on the process and the related properties has been published.[29] The first step in the procedure is a cold deformation to initiate the formation of α0-martensite followed by so-called reversion annealing to trigger the reversion of α0-martensite back to austenite. The presented results include (i) the laser heat treatment and resultant microstructure, (ii) the tensile properties, (iii) the cyclic deformation behaviour and α0-martensite formation, (iv) the fatigue lives and (v) the microstructure after cyclic deformation. The tensile specimens (at least two per material state) were oriented parallel to the RD (Figure 2A) and exhibited a gauge length of 25 mm at a cross section of about 6 × 2.5 mm[2].
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