Zhanna Yanushkevich

Regularities of Grain Refinement in an Austenitic Stainless Steel during Multiple Warm Working

The structural changes that are related to the new fine grain development in a chromium-nickel austenitic stainless steel subjected to warm working by means of multiple forging and multiple rolling were studied. The multiple warm working to a total strain of 2 at temperatures of 500-900C resulted in the development of submicrocrystalline structures with mean grain sizes of 300-850 nm, depending on processing conditions. The new fine grains resulted mainly from a kind of continuous reactions, which can be referred to as continuous dynamic recrystallization. Namely, the new grains resulted from a progressive evolution of strain-induced grain boundaries, the number and misorientation of which gradually increased during deformation. In contrast to hot working accompanied by discontinuous dynamic recrystallization, when the dynamic grain size can be expressed by a power law function of temperature compensated strain rate as D ~ Z-0.4, much weaker temperature/strain rate dependence of D ~ Z-0.1 was obtained for the warm working.

On Regularities of Grain Refinement through Large Strain Deformation

The recent studies on grain refinement in austenitic stainless steels during large strain deformations are critically reviewed. The paper is focused on the mechanism of structural changes that is responsible for the development of submicrocrystalline structures that can be interpreted as continuous dynamic recrystallization developing under conditions of warm working. The final grain size that is attainable by large strain warm working can be expressed by a power law function of temperature compensated strain rate with an exponent of about -0.15. The development of submicrocrystalline structures is assisted by the deformation microbanding and dynamic recovery, which are characterized by opposite temperature dependencies. The grain refinement kinetics, therefore, are characterized by a weak temperature dependence for a wide range of warm working conditions.

Effect of large plastic deformation on microstructure and mechanical properties of a TWIP steel

The effect of cold rolling on the microstructure evolution and mechanical properties of a cold rolled Fe-0.3C-17Mn-1.5AI TWIP steel was studied. The plate samples were cold rolled with reductions of 20, 40, 60 and 80%. The structural changes were associated with the development of deformation twinning and shear bands. The average spacing between twin boundaries in the transverse section of the rolled plates decreased from ~190 to 36 nm with an increase in the rolling reduction from 20 to 40%. Upon further rolling to 80% reduction the twin spacing remained at about 30 nm. The cold rolling resulted in significant increase in strength as revealed by tensile tests at an ambient temperature. The offset yield stress approached 1440 MPa, and the ultimate tensile strength increased to 1630 MPa after rolling reduction of 80%. Such significant strengthening was attributed to the development of specific structure consisting of deformation nanotwins with high dislocation density.

Structural Changes in a 304-Type Austenitic Stainless Steel Processed by Multiple Hot Rolling

The microstructure evolution and the dynamic processes of grain refinement in a 304-type austenitic stainless steel during multiple calibre hot rolling at temperatures of 700-1000°C were studied. The structural changes are characterized by the elongation of original grains towards the rolling axis and the development of new fine grains, the mean size of which decreases with decreasing the deformation temperature. During multiple rolling at 1000°C, the new grains resulted from the development of discontinuous dynamic recrystallization involving a bulging of frequently corrugated grain boundaries. On the other hand, the new grain boundaries leading to remarkable refinement of original microstructure were developed at temperatures below 800°C as a result of continuous strain-induced reactions.

Mechanical Properties at Elevated Temperatures of an S304H-Type Austenitic Stainless Steel Processed by Warm Rolling

The effect of multiple warm to hot rolling in the temperature interval of 673 – 1273 K on the microstructures and mechanical properties of an S304H-type austenitic stainless steel was studied. The structural changes during multiple rolling are characterized by the elongation of original grains towards the rolling direction and the development of new fine grains. The average spacing between high-angle boundaries in the transverse section of the rolled samples decreases from about 0.95 to 0.4 μm with decrease in the rolling temperature from 1273 to 773 K. The multiple rolling leads to significant strengthening as revealed by tensile tests at ambient and elevated temperatures. The offset yield strength evaluated at ambient temperature increased from 480 to 1120 MPa, while the rolling temperature decreased from 1273 to 673 K. The effect of the rolling temperature on the strength becomes less pronounced with an increase of the temperature of tensile tests.

Microstructure and Deformation Behavior of a Hot Forged 9%Cr Creep Resistant Steel

The tempered microstructure and the creep behaviour were studied in an ultra low carbon 9%Cr martensitic creep resistant steel. The starting material was forged at 1050°C followed by air cooling and then tempered at a temperature of 750°C for 3 hours. This treatment resulted in the mean transverse lath size of about 240 nm; the dislocation density in lath interiors comprised 4 × 1014 m-2. The tempered martensite lath structure (TMLS) is characterised by homogeneous precipitation of numerous MX-type carbonitrides and a small amount of relatively coarse M23C6–type carbides. Three kinds of MX carbonitrides were observed in the tempered lath martensite structure. Those were plate-shaped particles with longitudinal size of about 15 nm and thickness of 3 nm; round-shaped particles of about 10 nm in diameter; and relatively large almost equiaxed particles with mean size of about 90 nm. The large MX particles were resulted from incomplete dissolution of such carbonitrides at 1050°C, while the nanoscale particles homogeneously precipitated during the tempering. The creep tests conducted at 650°C showed that the studied steel demonstrated superior creep resistance. Namely, the rupture time was about an order as long as that for P92-type creep resistant steel.

Deformation and Recrystallization Textures in a High-Mn Steel Subjected to Large Strain Cold Rolling

The microstructure and texture evolution during cold rolling and annealing of a high-Mn TWIP steel were investigated. The microstructure development during cold rolling to a total strain of 3 was characterized by a high dislocation density, numerous mechanical twinning and shear banding. Cold rolling was accompanied by the development of strong Brass and S texture components and the formation of α-fiber. The recrystallization of cold rolled specimens was investigated at 873 K for 1 hour. An increase in the rolling reduction significantly accelerated the recrystallization kinetics, especially the nucleation rate, leading to much finer recrystallized grains in larger strained samples. The recrystallization texture retained its main components that were developed during cold rolling, but their intensity considerably weakened.