Microstructure and Phase Transformations in High-Strength Bainitic Forging Steel

In the present study, the influence of tempering temperature on the microstructural evolution and prior austenite grain boundary segregation of AISI 4340 steels was investigated by transmission electron microscope and atom probe. The transmission electron microscopy results showed a variation in the microstructure and the morphology of carbides with a change in tempering temperature. Additionally, the chemical compositions of the prior austenite grain boundaries and carbides were quantified by atom probe tomography. An increase in the tempering temperature led to a decrease in the amount of carbon segregated at the prior austenite grain boundary from 7.9 to 1.3 at.%. It was found that a higher tempering temperature can accelerate the diffusion of carbon from the prior austenite grain boundary into carbide. However, phosphorus atoms were segregated mainly at the prior austenite grain boundary in steel tempered at 400°C (up to 0.18 at.%). It was found that formation of film-like carbide and phosphorus segregation along the prior austenite grain boundary is the main cause of embrittlement in steel tempered at 400°C.

The austenite grain size (AGS) before decomposition is a crucial factor for the development of microstructure. However, this dependency is seldom discussed due to the difficulty of observing the grain growth of austenite during welding. In the current work, a grain growth algorithm is combined in a thermodynamics-based metallurgical model for the first time to analyse the influence of prior austenite grain size (pAGS). The phase volume fractions predicted at different cooling rates and pAGSs are compared with the experimental results of the continuous cooling transformation (CCT) diagram. To further investigate the influences of pAGS and microstructure on residual stress, experiments of bead-on-plate welding are conducted at three heat inputs, in which plates of S700 steel are operated by the arc welding process. The geometries after welding, chemical composition in the fusion zone (FZ) and the parameters of the double ellipsoidal heat source are calibrated using the software SimWeld. These geometries are imported to ABAQUS to create a Finite Element (FE) model. The validated metallurgical model together with the grain growth algorithm is implemented in the subroutine ABAMAIN to provide a thorough prediction of microstructure. With the knowledge of temperature and phase distributions, a coupled thermo-metallo-mechanical FE model is established to predict the residual stress distributions. The material properties are assigned by interpolating the individual phase property with its volume fraction. By comparing the results predicted by the model assuming constant pAGS, the influence of the pAGS on the residual stress is manifested. Moreover, simulations using overall material properties are also conducted. The stress distributions in the middle of plate surface are plotted along with the volume fractions of product phases to analyse the sensitivity of the residual stress to microstructure.

This study focuses on the fatigue fracture response and microstructure behavior of a resistance element weld (REW) in assembling AZ31B magnesium alloy and dual phase (DP) 780 plates. With the increase of the welding current the prior austenite grain size (PAGS) increased, which resulted in a decrease in the fraction of an alpha prime martensite at the welding interface and had little influence on the dual phase at the welding interface. It is clear that the formation of a suitable hook accompanied by the phase transformation between S20C elements and DP780 steels at the interface can have a positive effect on the mechanical properties. However, the Mg melting and the formation of a partially melted zone (PMZ) affected the elongation decrease significantly. Furthermore, the insufficient penetrations of the welding interface, the severe melted Mg and the formation of porosities in the PMZ were fatal to the strength and fatigue performance. Morphologies and microstructure characteristics including phases of the REWed joint were quantitatively analyzed by electron back-scatter diffraction (EBSD) and thermodynamic calculation by coupling the phase diagrams (CALPHAD) method, respectively. The experiment results were incorporated into the finite element method (FEM) to numerically elucidate their effects against mechanical responses including fatigue behaviors. The results explained why the mechanical strength, fatigue endurance, and fracture modes are radically different for joint morphologies and microstructure developments.

The effect of initial heat treatment on microstructure and mechanical properties of boron-free and boron-containing modified 9Cr-1Mo steel (P91 and P91B, respectively) has been studied under different heat-treatment conditions. The prior austenite grains evolved in P91 steel, having different prior austenite grain sizes, were found to be similar in size after heat treatment in the range of 1073 K to 1448 K (800 °C to 1175 °C) for 5 minutes. The microstructural evolution in P91B steel having different prior austenite grain sizes appeared to be uniform when subjected to different heat-treatment temperatures with the prior austenite grain size being similar to that of initial grain size. Lath martensite was observed in P91B steel after all heat treatments. On the other hand, lath martensite was observed in P91 steel only when subjected to high-temperature heat treatment, whereas subgrain/substructure as well as coarse precipitates were observed after a lower temperature heat treatment. Large differences in the hardness/strength values between different microstructures corresponding to coarse-grained heat-affected zone (CGHAZ) and intercritical HAZ (ICHAZ) of P91 steel weldment were due to the distinct difference in these microstructures. The difference in hardness/strength values between the CGHAZ and ICHAZ was found to be insignificant in P91B steel under similar heat-treatment conditions.

The present study investigated the effect of prior austenite grain size on hydrogen embrittlement in an 8Ni-0.1C martensitic steel. The prior austenite grain size was refined from 124 μm to 4.7 μm through cyclic heat treatment of austenitizing and water quenching. Slow strain rate tensile tests revealed that the hydrogen embrittlement was improved by the refinement of prior austenite grains. The macroscopic fracture surfaces of both the hydrogen-charged specimens with coarse prior austenite grains and fine prior austenite grains were characterized by intergranular fracture surfaces. The area fraction of the macroscopic intergranular fracture surfaces in the hydrogen-charged specimen with fine prior austenite grains was smaller than that in the hydrogen-charged specimen with coarse prior austenite grains. This suggested that the refinement of prior austenite grains reduced the frequency of the fracture by hydrogen embrittlement.

Microstructure evolution during austenite reversion in Fe-Ni martensitic alloys

Crystallographic features of lath martensite in low-carbon steel

Martensitic/bainitic wear-resistant steels are widely used in civilian industry, where a good combination of strength and toughness is required. In the present study, a double-quenching process was applied and compared to the conventional single-quenching process. The microhardness and ductile–brittle transition temperature were measured, and the microstructure was characterized with scanning electron microscopy and electron backscatter diffraction (EBSD) technique. It was found that the double-quenching process refined the prior austenite grain size by 43% and simultaneously improved the toughness and hardness. The ductile-to-brittle transition temperature was decreased from −77 °C to −90 °C, and the hardness was increased by 8%. Based on the EBSD data, a detailed analysis of the grain boundary distribution was performed using a recently developed machine learning model. Unlike what was found in previous studies, for the studied wear-resistant steel, the refinement of the prior austenite grain did not increase the block boundary density while increasing the high-angle packet boundary density. As a result, the total density of the high-angle grain boundaries in the double-quenched specimen was not improved compared to the single-quenched specimen. Further inspection suggested that it is the prior austenite grain boundaries and high-angle packet boundaries that contribute to the hardness and toughness, and the key factors that determine their effectiveness are the high misorientation angle between the {110} slip planes and the high slip transmission factor.

Effect of soft reduction technique on microstructure and toughness of medium carbon steel

Effect of strain rate on hydrogen embrittlement in low-carbon martensitic steel

Thermomechanical processing of low carbon bainitic steels is used to obtain a bainitic microstructure with good strength and toughness by continuous cooling after forging without the need of further heat treating, hence reducing manufacturing costs. However, hot forging parameters can significantly influence the microstructure in the forged material. A series of heat treating and forging experiments was carried out to analyze the effect of austenitizing time and temperature on the grain growth and the effect of forging temperature on the Prior Austenite Grain Size (PAGS) and continuously cooled microstructure. The forged microstructures were characterized by optical microscopy, microhardness tests, and X-ray diffraction. The results indicate that at 1200 °C austenitizing temperature abnormal grain growth takes place. Forging temperature significantly affects the PAGS and the subsequently formed microstructure. At high forging temperature (1200 °C), an almost fully bainitic microstructure was obtained. As the forging temperature was reduced to 1100 and 1000 °C, the PAGS refined, while the polygonal ferrite faction increased and the amount of retained austenite decreased. Further evaluations showed that a decrease in the forging temperature results in a higher carbon concentration in solution in the retained austenite leading to a stabilization effect.

Significant influence of carbon and niobium on the precipitation behavior and microstructural evolution and their consequent impact on mechanical properties in microalloyed steels

Analytical Investigation of Prior Austenite Grain Size Dependence of Low Temperature Toughness in Steel Weld Metal

EBSD investigation on microstructure transformation in low carbon steel during continuous cooling

In this study, the effects of Mo and Nb on the microstructure and Charpy impact properties of coarse-grain heat affected zone (CGHAZ) in high-strength low-alloy (HSLA) steels were investigated. The high-Mo steel had a lower volume fraction of acicular ferrite (AF) and higher volume fraction of granular bainite (GB) than those of the low Mo steel. The high-Mo CGHAZ specimen had a lower volume fraction of AF and higher volume fraction of GB compared to the low-Mo CGHAZ specimen. This is because, an increase in Mo lowers the bainite transformation temperature, induces the formation of GB and bainitic ferrite (BF), and reduces the diffusion velocity of carbon, thereby inhibiting the formation of ferrite. The Nb-added steel had a volume fraction similar to that of AF but smaller packet sizes of GB compared to the non-Nb steel. Compared to the non-Nb CGHAZ specimens, the Nb-added CGHAZ specimen had a lower volume fraction of AF, a higher volume fraction of GB, smaller packet sizes of GB and BF, and a smaller prior austenite grain size. This is because, Nb forms Nb(C,N) to inhibit the grain growth, thus inhibiting the formation of GB and BF. The Nb-added low-Mo CGHAZ specimen with the highest AF fraction and smallest average grain size had the highest Charpy-absorbed energy. However, articles in the Journal of Welding and Joining reported that the other CGHAZ specimens with high volume fractions of GB and BF with coarse packets showed low Charpy-absorbed energy.