W. W. Fisher

Combined effects of deformation (strain and strain state), grain size, and carbon content on carbide precipitation and corrosion sensitization in 304 stainless steel☆

Abstract In this investigation we have examined the combined effects of temperature, aging time, carbon content, grain size, strain, and strain state (uniaxial versus rolling deformation) on sensitization development in 304 stainless steel. In general, increasing the carbon content, strain, and strain state (uniaxial → rolling) decreases the sensitization time. Reducing the grain size also accelerates the rate of sensitization development. In addition, for sufficiently small grain sizes, even with superimposed straining, the grain boundaries become dominant in Cr diffusion kinetics.

effects of strain and grain size on carbide precipitation and corrosion sensitization behavior in 304 stainless steel

Abstract The simultaneous effects of strain and grain size on carbide precipitation and sensitization development in 304 stainless steel having a constant carbon composition of 0.051% were studied. Three different grain sizes (150 μm, 40 μm, and 15 μm) were uniaxially deformed to 10% and 20% strain, and aged in the sensitization ranges of 625°C and 775°C. 150 μm samples were also strained to 20% in liquid nitrogen and aged between 0.1 and 10 h at 670°C. At 625°C, straining of the 150 μm grain size material produced a systematic increase in carbide precipitation, sensitization development, and desensitization. Transmission electron microscopy (TEM) revealed a greater continuity of carbides along the grain boundaries in the strained samples vs the unstrained samples. At the 15 μm grain size, these strain effects were found to be far less prominent with the sensitization-desensitization process occurring much faster, and accelerated at the higher aging temperature of 775°C, i.e. sensitization-desensitization envelopes were shifted to shorter aging times. Liquid nitrogen straining produced a sub-micron, 2-phase γ/α′, sub-micron grain structure which demonstrated an almost instantaneous sensitization-desensitization behavior, when aged at 670°C, which was predicted by extrapolation of the aging data for the larger range of grain sizes (∼ 15 to 150 μm) and the 625 and 775°C aging temperatures.

Deformation-induced microstructure and martensite effects on transgranular carbide precipitation in type 304 stainless steels

Abstract Plastic deformation of 304 stainless steel (SS) induces transgranular (TG) carbide precipitation, which is critically dependent on deformation-induced microstructural changes occurring during thermal treatment of the SS. Uniaxial deformation of the 304 SS to 40% strain produces a high density of intersecting micro-shear bands composed of heterogeneous bundles of twin-faults and about 12–17% strain-induced α′-martensite at the intersections of the twin-faults. Thermal treatment of 670°C for 0.1–10 h, however, results in a rapid annihilation/transformation of the strain-induced martensite and the concurrent formation of zones containing mixed thermal martensite laths and fine-grained austenite, though the thermal martensite also decreases with increasing heat treatment time. Simultaneous with these thermomechanically-induced microstructural changes, TG chromium-rich carbides form at intersections of twin-faults and on fine-austenite or thermal martensite boundaries in the SS; however, no correlation between strain-induced α′-martensite and carbides was observed in this work. The mechanisms of deformation-induced microstructure and (strain-induced and thermal) martensite effects on TG carbide precipitation in 304 SS are discussed.

Combined effects of strain and grain size on carbide precipitation and sensitization in 304 stainless steel

The dramatic effects of uniaxial strain on sensitization in austenitic stainless steels is now well documented. Many parameters influence the occurrence of sensitization and the degree of sensitization (DOS) is thus a function of microstructural characteristics (which of course includes the grain size) and chemical composition, including the carbon content (which is the most critical compositional element controlling sensitization). However, there have been no studies evaluating simultaneous effects of deformation (strain) and grain size on sensitization development. A study of two grain sizes and carbon content variations by Pascali, et al found that desensitization was delayed at the larger grain size (68 [mu]m) in comparison to the smaller grain size (17[mu]m), especially at aging temperatures near 600 C. In this study the authors present some preliminary results of the combined effect of grain size and uniaxial strain on sensitization (DOS) in 304 stainless steel.

Deformation site-specific nature of transgranular carbide precipitation in 304 stainless steels

This research was developed to identify the nature of transgranular carbide precipitation and chromium-depletion in heavily deformed 304 SS, and to examine the correlation between carbide precipitation and strain-induced martensite in the SS. Observations have indicated that: (1) Transgranular carbides form on twin-fault intersections in 304 SS. This causes linear-TG attack within the SS. (2) There was no observed correlation between strain-induced martensite and TG carbides in this work, except that both form at micro-shear band intersections, but not at the same intersection. (3) Lath martensite and fine-austenite form during heat treatment of deformed, 304 SS. (4) Transgranular carbides precipitate on martensite lath/fine-austenite boundaries in 304 SS, and produce extensive, random TG attack in the SS. (5) Lattice imaging of carbides has shown the presence of dislocations within the carbides and at the carbide-matrix interface, and changes in the carbide lattice orientation across a boundary, which may be indicators of key mechanisms of carbide nucleation on grain boundaries in SS.