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Low-cycle fatigue behavior, destabilization of austenite and emergence of deformation induced martensite (DIM) was investigated in wrought AISI 304 type (304, 304L, 348) austenitic stainless steels of diverse semi-product forms. The specimens fabricated from cylindrical bars, plates or thin sheets were cyclically strained with high strain amplitudes under total strain control in symmetrical push-pull at room and depressed temperatures to different stages of fatigue life. Quantitative and qualitative data on the formation, morphology and specific distribution of DIM in the volume of the material within the whole gauge part of the specimens are confronted with the respective cyclic stress-strain response and discussed in terms of characteristic chemical heterogeneity. This heterogeneity arises in the form of chemical banding due to the complex solidification behavior of AISI 304 type steels and its contemporary production route. Possible effect of additional solution treatment (various cooling rates due to quenching in different media) on DIM formation is discussed. [ABSTRACT FROM AUTHOR]

Mechanism of banded structure evolution during cooling at small or large austenite grain sizes. [Display omitted] • The banded structure is eliminated when the austenite grain size is smaller than the element segregation banding width. • Si and Mo promote α/γ interfacial migration, while Cr, Ni and Mn inhibit α/γ interfacial migration. • The most serious segregation of Cr and Mn lead to the largest critical austenite grain size of 22CrMo. Defects in the banded structure of low-carbon gear steel significantly limit the application of gears in the construction industry. Therefore, the banded structure in the gear steel must be eliminated. In this study, the content and segregation banding width of Si, Mo, Cr, Mn, and Ni, phase distribution, austenite grain size, and hardness were characterized using electron probe microanalysis, laser scanning confocal microscopy, electron backscatter diffraction, and Vickers indenter. Element diffusion and α/γ interfacial migration were calculated using the Thermo-Calc analysis and simulation methods. The results showed that the best mechanical properties experimental steels without banded structures can be obtained during heating at 1300 °C for 2 h, 1250 °C for 1 h, and 1150 °C for 2 h for 20CrNiMo, 22CrMo, and 20CrMnTi, respectively. Under the critical hot-rolling process of eliminating the banded structure, the ferrite transformation temperature difference ΔA r3 and ferrite transformation interval (A r3 -Bs) of the 22CrMo sample were the largest. Si and Mo promoted interfacial migration, whereas Cr, Mn, and Ni inhibited interfacial migration. The interfacial migration speed of 22CrMo is the highest. The interfacial migration distance of the 22CrMo sample was the longest, and the final interfacial migration distance of 20CrMnTi was the shortest. When the migration position of the α/γ interface was less than 65 % of the system's total size, the ferrite-banded structure disappeared. The critical austenite grain sizes of 20CrNiMo, 22CrMo, and 20CrMnTi were 31.6, 35, and 22.2 μm, respectively. The banded structure is eliminated when the austenite grain size is smaller than the element segregation banding width. When eliminating the banded structure, the interfacial migration distance of 22CrMo was the longest and its critical austenite grain size was the largest. [ABSTRACT FROM AUTHOR]

The microstructure of a 240 mm diameter AISI 4340 mill shaft, quenched and tempered, was analyzed through its longitudinal cross section. Normally, higher cooling rates promote martensitic structure while the diffusion assisted austenite decomposition products are expected in the lower cooling rate condition. The martensitic reaction takes place inside the grains while the products diffusion starts from the prior austenite grain boundaries, nucleation and grow processes. An anomalous hardenability behavior was reported with a mixed structure containing martensite and bainite, in different proportion from the surface (higher cooling rate) to the core (lower cooling rate). This behavior was attributed to the wide banded structure. Banding due to the as cast structure segregation, is directly related to the solidification rate as well as the deformation degree due (during) to forging and rolling operations. When the deformation degree is high, the banding thickness is thin and approaches the austenite grain size. In this study the thickness of the bands embedded about four to five austenite grain sizes, in such a way that every band behaves like a particular steel composition. A semi quantitative chemical analysis was carried out through the banded structure to understand the differential hardenability behavior. The results were discussed using classical hardenability formula as well as calculated TTT diagrams for each composition. [ABSTRACT FROM AUTHOR]

Abstract: The current work describes the metallographic characterization of the initial microstructure of a cold rolled precipitation hardening semi-austenitic stainless steel (12Cr–9Ni–4Mo–2Cu–1Ti, in wt.%). The use of the Lichtenegger and Blöch (L–B) color etching solution has been shown to reveal not only the phases present in the microstructure, but also the existence of chemical banding along the rolling direction. The L–B reagent has been found to color the microstructure in bands depending on what alloying elements have segregated to each band. Two-dimensional electron probe microanalysis (EPMA) maps have shown that Ni, Cu and Ti segregate together in the bands, while Cr has an opposite behavior. Mo has a mixed segregation behavior although much weaker than the other elements and more prompt to segregate like Ni does. A direct comparison of light optical micrographs with the EPMA maps of the same area of the microstructure has enable to establish a direct relationship between the alloying element band concentration and the resulting etching color contrast obtained with the L–B reagent. Thermodynamic calculations predict that solidification in this steel takes place with ferrite as the primary phase. Equilibrium partitioning coefficient calculations seem to support the observed segregation patterns. [Copyright &y& Elsevier]

Dimensional anisotropy associated with chemical banding was investigated via quench dilatometry. The material used in this research was fabricated by roll bonding alternately stacked sheets of a standard 5140 steel and a modified 5140 with a higher manganese content. The resulting plate had a well defined manganese banding pattern with a known segregation bandwidth. Cylindrical dilatometer specimens were machined with the axial direction aligned parallel and perpendicular to the segregation planes. The banded steel was thermally cycled so that the material alternated between austenitic and martensitic structures. Results for multiple thermal cycles revealed that tensile plastic deformation occurs normal to the segregation plane and compressive plastic deformation occurs parallel to the segregation plane. An examination of the transformations associated with heating and cooling showed that an exaggerated apparent transformation strain was measured in the direction perpendicular to the segregation layers and a suppressed apparent transformation strain was measured parallel to the segregation layers. [Copyright &y& Elsevier]

Dimensional anisotropy associated with chemical banding was investigated using a material point model to simulate transformation anisotropy due to manganese inhomogeneity. A manganese dependent, internal state variable model for athermal martensite transformation was used to estimate the strains that develop in two layers of differing manganese content which maintain a coherent interface. Classical plasticity and transformation plasticity were included in simulations and compared to measured values for a steel with an engineered manganese segregation pattern presented in a companion paper (Jaramillo et al. Acta Mater., submitted). The results are consistent with the hypothesis that transformation plasticity is the mechanism by which shape change occurs in the chemically banded steel. [Copyright &y& Elsevier]