Current trends in steels are focusing on refined martensitic microstructures to obtain high strength and toughness. An interesting manner to reduce the size of martensitic substructure is by reducing the size of the prior austenite grain (PAG). This work analyzes the effect of PAGS refinement by thermal cycling on different microstructural features of as-quenched lath martensite in a 0.3C-1.6Si-3.5Mn (wt pct) steel. The application of thermal cycling is found to lead to a refinement of the martensitic microstructures and to an increase of the density of high misorientation angle boundaries after quenching; these are commonly discussed to be key structural parameters affecting strength. Moreover, results show that as the PAGS is reduced, the volume fraction of retained austenite increases, carbides are refined and the concentration of carbon in solid solution as well as the dislocation density in martensite increase. All these microstructural modifications are related with the manner in which martensite forms from different prior austenite conditions, influenced by the PAGS.

/pdf/effect-of-prior-austenite-grain-size-refinement-by-thermal-2byt7j4riu.pdf

Effect of Prior Austenite Grain Size Refinement by Thermal Cycling on the Microstructural Features of As-Quenched Lath Martensite

The mechanical and thermal stability of austenite in multiphase advanced high strength steels are influenced by the surrounding microstructure. The mechanisms underlying and the relations between thermal and mechanical stability are still dubious due to the difficulty of isolating other factors influencing austenite stability. In this work, martensite/austenite microstructures were created with the only significant difference being the degree of tempering of the martensite matrix. Hence, the effect of tempering in martensite is isolated from other factors influencing the stability of austenite. The thermal stability during heating of retained austenite was evaluated by monitoring phase fractions as a function of controlled temperature employing both dilatometry and magnetometry measurements. The mechanical stability was studied by performing interrupted tensile tests and determining the remaining austenite fraction at different levels of strain. The thermal stability of this remaining austenite after interrupted tests was studied by subsequent reheating of strained specimens. The results are evidence for the first time that thermal recovery of martensite during reheating assists austenite decomposition through shrinkage and softening of martensite caused by a reduction of dislocation density and carbon content in solid solution. This softening of martensite also leads to a subsequent reduction of austenite mechanical stability. Additionally, remaining austenite after pre-straining at room temperature was thermally less stable than before pre-straining, demonstrating that plastic deformation has opposing effects on thermal and mechanical stability.

/pdf/thermal-and-mechanical-stability-of-retained-austenite-43mthlbwjk.pdf

Thermal and mechanical stability of retained austenite surrounded by martensite with different degrees of tempering

Corrosion behaviour of a quenched and partitioned medium carbon steel in 3.5 wt.% NaCl solution

Cu-0.47Cr-0.16 Nb (wt%) alloys were designed and prepared, and the microstructures of the Cu-Cr-Nb alloy were investigated using transmission electron microscopy and three-dimension atom probe tomography technique. After homogenizing at 950 °C for 4 h, cold rolling by 80% reduction, then aging at 450 °C for 30 min, the micro-hardness, electrical conductivity, tensile strength, yield strength and elongation of the alloy were up to 150 HV, 89.1%IACS, 453 MPa, 443 MPa and 11.4%, respectively. The tensile strength / elongation of the alloy approached 399 MPa / 22.9% as test at 200 ℃, 334 MPa / 14.8% for 300 ℃, and 282 MPa / 12.3% for 400 ℃, respectively. The Cr2Nb phase with an average size of 700 nm and Nb phase with an average size of 500 nm formed during the solidification process, while the nano-scale Cr-rich phases precipitated from the solid solution during the aging process. The sizes of Cr2Nb and Cr-rich phases were close to each other during aging. These precipitates located in the grain and sub-grain boundary could effectively pin the movement of boundary, resulting in a high strength of the alloy at the elevated temperature. The addition of Nb can promote the precipitation of Cr from the solid solution during aging, thus both strength and electrical conductivity of the alloy were improved.

Microstructure and properties of Cu-Cr-Nb alloy with high strength, high electrical conductivity and good softening resistance performance at elevated temperature

On the characteristics of Portevin–Le Chatelier bands in cold-rolled 7Mn steel showing transformation-induced plasticity

An improved X-ray diffraction line profile analysis method is developed to determine dislocation density of lath martensitic steels. This method combines the modified Warren–Averbach (MWA) and the modified Williamson–Hall (MWH) methods. The developed method is robust and leads to unique values for the dislocation density, the effective outer cut-off radius of the dislocations ( R e ) and the dislocations distribution parameter ( M ). Dislocation structures of lath martensite in a steel, in the as-quenched condition as well as in tempered conditions, are characterized by using the proposed method. The calculated dislocation density is compared with the values obtained from the MWH method by considering a constant value for M . It was found that both methods provide dislocation densities in the range of the values calculated from the dislocation strengthening component of the yield strength.

An improved X-ray diffraction analysis method to characterize dislocation density in lath martensitic structures

Interaction of carbon partitioning, carbide precipitation and bainite formation during the Q&P process in a low C steel

A 0.3C-1.6Si-3.5Mn (wt%) steel was subjected to different QP secondary martensite refers to martensite formed during quenching from 400 °C to room temperature. The yield strength of each constituent phase was determined by applying physical models to the data obtained from detailed microstructural characterization. The yield strength (uncertainty of 5%) of the Q&P microstructures was calculated by using a composite law to account for the contribution of each constituent phase. The dependence of the yield strength on the microstructural features of the Q&P microstructures was revealed by using the approach developed in this work. For example, initial martensite (which has a high yield strength and is the dominant phase in the microstructures) had the greatest effect on the yield strength of the Q&P microstructures. Furthermore, the phase fraction and dislocation density of this phase increased with decreasing quenching temperature, leading to an increase in the yield strength of the material.

Farideh HajyAkbary

Papers

An improved X-ray diffraction analysis method to characterize dislocation density in lath martensitic structures

Interaction of carbon partitioning, carbide precipitation and bainite formation during the Q&P process in a low C steel

Analysis of the mechanical behavior of a 0.3C-1.6Si-3.5Mn (wt%) quenching and partitioning steel

A quantitative investigation of the effect of Mn segregation on microstructural properties of quenching and partitioning steels