Thermomechanical processing of advanced high strength steels

A model for the microstructure behaviour and strength evolution in lath martensite

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

https://pure.tue.nl/ws/files/13799594/Du_et_al._Scripta_Materialia.Authors_copy_2016.pdf

Block and sub-block boundary strengthening in lath martensite

The microstructure, precipitates and mechanical properties of Nb-V-Ti microalloyed ultra-high strength steel under different tempering temperatures are investigated. With the increasing tempering temperature, the width of martensitic laths gradually increased with reduced dislocations within them, and the volume fraction of retained austenite first increased and then decreased. Furthermore, the Charpy impact toughness, reduction of area and elongation are enhanced at the sacrifice of the strength. Four kinds of precipitates are identified at different tempering temperatures, namely M 3 C, M 2 C, M 23 C 6 and M 7 C 3 . M 3 C precipitated when tempering at 200–400 °C, and it was replaced partially by M 2 C when the temperature is elevated to 500 °C, at which M 23 C 6 also precipitated. For a higher temperature, above 600 °C, M 3 C disappeared, and some M 2 C transformed into M 7 C 3 . The other two types of MC precipitates are found to be the stable phases existing over different tempering temperatures. The correlation between the mechanical properties and microstructure was established, and the precipitation mechanisms of carbides at the different tempering temperatures are explained.

Carbide precipitation in Nb-V-Ti microalloyed ultra-high strength steel during tempering

The microstructures of 25CrMo48V martensitic steel quenched at the different temperatures of 800–1200 °C and tempered at 650 °C were characterized by optical microscope (OM), field emission scanning electron microscopy (FESEM), electron backscattering diffraction (EBSD) and transmission electron microscopy (TEM). The tensile and impact properties were evaluated from the as-quenched and tempered specimens. The results show that the refinement of the prior austenite grains due to the decrease of quenching temperature could induce the refinement of packets and blocks, while the lath width remained stable. The yield strength and the 50% fraction appearance transition temperature (50% FATT) varied linearly with the reciprocal square root of the prior austenite grain size, packet size and block width. Their relationships followed the classical formula of Hall–Petch and FATT = A − B d − 1 / 2 , respectively. The prior austenite grain size has a remarkable effect on the strength and toughness, while the block is the minimum structure unit controlling strength and toughness.

Effect of martensitic morphology on mechanical properties of an as-quenched and tempered 25CrMo48V steel

Effect of microstructure on the strength of 25CrMo48V martensitic steel tempered at different temperature and time

The microstructures of 25CrMo48V martensitic steel quenched ( Q ) at 900 °C and tempered ( T ) at 650 °C for 5–105 min were characterized by transmission electron microscope (TEM). The tensile and impact properties were evaluated from the Q – T treated samples. The results indicate that the precipitate size is increased with the prolonged soaking time and the dependence of volume fraction of precipitated particles on the tempering time follows the classical Johnson–Mehl–Avrami (JMA) model. The refinement precipitated particle could induce the enhancement of yield strength. The dependence of the yield strength of 25CrMo48V martensitic steel tempered at 650 °C for different times on the lath width and the precipitated particle size can be described as σ y = 645 + 145 w − 1 + 1.27 × 10 − 17 D p − 13 .

Characterization of the microstructures and mechanical properties of 25CrMo48V martensitic steel tempered at different times

Precipitation-hardening is an essential and promising method to enhance the strength of alloys. However, this strengthening mechanism will destroy the plasticity of the alloy and lead to premature failure. Here, we report a strategy to improve the strength of the alloy by producing highly-coherent nanoprecipitates (HNPs) through spinodal decomposition. The HNPs can be sheared by dislocations to eliminate the stress concentrations and premature failure. We designed and fabricated a bulk CoCrNiAl0·1Mo0.1 medium entropy alloys (MEAs) to consist of face-centered cubic phase and HNPs structure involving high temperature recrystallized and low temperature aging. Compared with the samples without precipitation, HNPs structure contribute to obtain a satisfactory trade off with the yield strength of 900 MPa, the tensile strength of 1215 MPa and 40% tensile ductility.

Spinodal decomposition induced nanoprecipitates strengthened CoCrNi-base medium entropy alloy

Creep deformation in cartilage can be observed under physiological loads in daily activities such as standing, single-leg lunge, the stance phase of gait. If not fully recovered in time, it may induce irreversible damage in cartilage and further lead to early osteoarthritis. In this study, 36 cruciform-shape samples in total from 18 bulls were employed to conduct the uniaxial and biaxial creep-recovery tests by using a biaxial cyclic testing system. Effects of stress level (σ = .5, 1.0, 1.5 MPa) and biaxial stress ratio (B = 0, .3, .5, 1.0) on creep-recovery behaviors of cartilage were characterized. And then, a viscoelastic constitutive model was employed to predict its creep-recovery behaviors. The results showed that the creep strain and its three components, namely instantaneous elastic strain, delayed elastic strain and viscous flow strain, increase with the increasing stress level or with the decreasing biaxial stress ratio. Compared with uniaxial creep-recovery, biaxial creep-recovery exhibits a smaller creep strain, a faster recovery rate of creep strain and a smaller residual strain. Besides, the built viscoelastic model can be used to describe the uniaxial creep-recovery behaviors of cartilage as a good correlation between the fitted results and test results is achieved. The findings are expected to provide new insights into understanding normal joint function and cartilage pathology. Graphical Abstract

Ruixin Li

Papers

Effect of martensitic morphology on mechanical properties of an as-quenched and tempered 25CrMo48V steel

Effect of microstructure on the strength of 25CrMo48V martensitic steel tempered at different temperature and time

Characterization of the microstructures and mechanical properties of 25CrMo48V martensitic steel tempered at different times

Spinodal decomposition induced nanoprecipitates strengthened CoCrNi-base medium entropy alloy

Creep-recovery behaviors of articular cartilage under uniaxial and biaxial tensile loadings