Intersections of shear bands in metastable austenites have been shown to be effective sites for strain-induced martensitic nucleation. The shear bands may be in the form of e’ (hcp) martensite, mechanical twins, or dense bundles of stacking faults. Assuming that shear-band intersection is the dominant mechanism of strain-induced nucleation, an expression for the volume fraction of martensite vs plastic strain is derived by considering the course of shear-band formation, the probability of shear-band intersections, and the probability of an intersection generating a martensitic embryo. The resulting transformation curve has a sigmoidal shape and, in general, approaches saturation below 100 pct. The saturation value and rate of approach to saturation are determined by two temperature-dependent parameters related to the fee-bee chemical driving force and austenite stacking-fault energy. Fitting the expression to available data on 304 stainless steels gives good agreement for the shape of individual transformation curves as well as the temperature dependence of the derived parameters. It is concluded that the temperature dependence of the transformation kinetics (an important problem in the development of TRIP steels) may be minimized by decreasing the fee, bec, and hep entropy differences through proper compositional control.

Kinetics of strain-induced martensitic nucleation

The previous work of Professor W. G. Burgers and Dr. A. J. Bogers is used to develop a mechanism of strain-induced martensitic nucleation, involving two intersecting shear systems. We distinguish between strain-induced nucleation and stress-assisted nucleation, the latter involving the same sites and embryos as does the regular spontaneous transformation. The strain-induced nucleation, on the other hand, depends on the creation of new sites and embryos by plastic deformation; this phenomenon may also contribute in a major way to autocatalytic nucleation during the course of martensitic transformation. For the case of strain-induced nucleation, it is possible to focus on specific intersecting-shear systems when the austenitic stacking-fault energy is low and e (h.c.p.) martensite can form as a part of the shear displacements. It then becomes feasible to extend the intersecting-shear mechanism from this special case to alloys of higher stacking-fault energy, where e is no longer stable relative to the austenite. It should be noted, however, that these events are very early stages in the formation of martensitic plates and relate primarily to the genesis of embryos; the actual growth start-ups which determine the operational (measured) nucleation rates may be controlled by subsequent processes.

A mechanism for the strain-induced nucleation of martensitic transformations*

Formation of nanostructured surface layer on AISI 304 stainless steel by means of surface mechanical attrition treatment

Effect of processing parameters on microstructure and tensile properties of austenitic stainless steel 304L made by directed energy deposition additive manufacturing

The γ→α transformation in 304 stainless steel can be induced by plastic deformation at room temperature. The kinetics of strain-induced transformations have been modeled recently by Olson and Cohen. We used magnetic techniques to monitor the progress of the γ→α transformation in 304 stainless steel sheet loaded in uniaxial and biaxial tension at both low (10-3 per second) and high (103 per second) strain rates. We found that using the von Mises effective strain criterion gives a reasonable correlation of transformation kinetics under general strain states. The principal effect of increased strain rate was observed at strains greater than 0.25. The temperature increase resulting from adiabatic heating was sufficient to suppress the γ→α transformation substantially at high rates. The consequences of the γ→α transformation on mechanical behavior were noted in uniaxial and biaxial tension. Uniaxial tension tests were conducted at temperatures ranging from 50 to -80°C. We found that both the strain hardening and transformation rates increased with decreasing temperature. However, the martensite transformation saturates at ≈85 pct volume fraction α. This can occur at strains less than 0.3 for conditions where the transformation is rapid. Once saturation occurs, the work hardening rate decreases rapidly and premature local plastic instability results. In biaxial tension, the same tendency toward plastic instability associated with high transformation rates provides a rationale for the low biaxial ductility of 304 stainless steel.

Effects of Strain State and Strain Rate on Deformation-Induced Transformation in 304 Stainless Steel: Part I. Magnetic Measurements and Mechanical Behavior

A detailed analysis of martensite transformations in 18/8 (304) stainless steel, utilizing transmission electron microscopy and diffraction in conjunction with X-ray and magnetization techniques, has established that the sequence of transformation is γ → ∈ → α. e is a thermodynamically stable hcp phase whose formation is greatly enhanced as a result of plastic deformation. Comparison with the e → α transformation in pure Fe-Mn alloys lends further support to the above sequence and suggests that a transformation line between e and α in Fe-Cr-Ni alloys can be expected. In the 304 stainless steel used in this investigation, formation of α was induced only by plastic deformation and subsequent to formation of e. Nucleation of α occurs heterogeneously at intersections of e bands or where e bands abut twin or grain boundaries (which represent unilaterally compressed regions). From electron diffraction, the Nishiyama relationship between γ and α phases appears to predominate at the start of the transformation, but then changes to that of Kurdjumov-Sachs. Based on these observations, a sequence of atom movements from the hcp structure to the bcc structure is proposed which has the basic geometric features of the martensitic transformation.

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The martensite phases in 304 stainless steel

Mechanical and thermal-mechanical treatments of 304 stainless steel enables yield strengths of over 200,000 psi to be obtained with elongations better than 10 pct. Electron microscopy, X-ray, and magnetic techniques show that during deformation, strain induced γ → ∈ → α transformation occurs with further thermal nucleation of α achieved by aging up to 400°C. The yield strength is linearly proportional to the amount of ° irrespective of the treatment used to form α. The yield strength is given by αy = 225f + 48.65 ksi, where ƒ is the volume fraction of martensite. Softening occurs by aging at 500°C and above due to a decrease in percent α which may occur by renucleation of γ. The system is an unusual form of composite strengthening; hard martensite particles are formed within the austenite, and the percent α (and thereby the mechanical properties), can be controlled by the mechanical/thermal-mechanical processing.

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Pat L. Mangonon

Papers

The martensite phases in 304 stainless steel

Structure and properties of thermal-mechanically treated 304 stainless steel