The effect of cold-rolling prior to the inter-critical heat treatment on microstructure and mechanical properties of 4340 steel with ferrite – Martensite microstructure

The effect of pre-weld heat treatment on the microstructure and low-temperature impact toughness of the coarse-grained heat-affected zone (CGHAZ) after simulated welding was systematically investigated through the utilization of scanning electron microscopy (SEM) and electron back-scattering diffraction (EBSD). The Charpy impact test validated the presence of an optimal pre-weld heat treatment condition, resulting in the highest impact toughness observed in the CGHAZ. Three temperatures for pre-weld heat treatment (690, 720 and 750 °C) were used to obtain three different matrices (Steel 1, Steel 2, Steel 3) for simulated welding. The optimal pre-weld heat treatment is 720 °C for 15 min followed by water quench. Microstructure characterization showed that there is an evident microstructure comprising bainite (B) in Steel 1 and Steel 2 after pre-weld heat treatment, while the addition of martensite (M) with the pre-weld heat treatment temperature exceeds Ac1 by almost 60 °C (Steel 3). These differences in microstructures obtained from pre-weld heat treatment influence the refinement of high-temperature austenite during subsequent simulated welding reheating processes, resulting in distinct microstructural characteristics in the CGHAZ. After the optimal pre-weld heat treatment, Steel 2 subjected to single-pass welding thermal simulation demonstrates a refined microstructure characterized by a high density of high-angle grain boundaries (HAGBs) within the CGHAZ, particularly evident in block boundaries. These boundaries effectively prevent the propagation of brittle cracks, thereby enhancing the impact toughness.

Section: Microstructure Of the Matrix After Heat Treatmentsupporting

confidence: 92%

Effect of Pre-Weld Heat Treatment on the Microstructure and Properties of Coarse-Grained Heat-Affected Zone of a Wind Power Steel after Simulated Welding

Wang,

Shang

2024

“…For the quenched steels, the increase in hardness before 780 • C is mainly due to the lattice distortion caused by the formation of supersaturated martensitic structure. As is well documented, a higher martensite content would cause greater residual stresses and a higher dislocation density within the adjacent ferrite phase, resulting in a significant enhancement in strength as well as hardness of dual-phase steels [26,27]. As the quenching temperature was further increased to complete austenization areas (above 810 • C), slight softening occurred.…”

Section: Resultsmentioning

confidence: 90%

A 4340 Steel with Superior Strength and Toughness Achieved by Heterostructure via Intercritical Quenching and Tempering

Sang¹,

Sun

Liu

2023

The conventional 4340 steel was used after quenching and tempering, strengthened by the classical pearlitic structure where cementite particles are dispersed through the ferrite matrix. In the present study, a heterostructure microstructure consisting of micro-sized residual ferrite zones and pearlitic zones was introduced by an optimized process of intercritical quenching and tempering, resulting in a steel with higher strength and better toughness. The pearlite steel has a tensile strength of 1233 MPa, yield strength of 1156 MPa, and toughness of 121.5 MJ/m3. Compared with the pearlite steel, the tensile strength and yield strength of the heterostructure steel have been improved by 67 MPa and 74 MPa, respectively, while the toughness has been increased by 52.5 MJ/m3. In this heterostructure, the micro-sized ferrite bulks serve as the soft zones surrounded by the hard zones of the pearlite structure to achieve a remarkable work-hardening capacity. Statistical analysis shows that the heterostructure has the best hetero-deformation-induced (HDI) hardening capability when the residual ferrite bulk contributes ~31% by volume fraction, and the quenching temperature is around 780 °C. This study opens new ways of thinking about the strengthening and toughening mechanism of heat treatment of medium carbon steels.

“…Therefore, the sample was cooled quickly to this temperature range and kept there; high undercooling is conducive to ferrite nucleation, as shown in Figure 10. At the austenite interface, ferrite nucleates rapidly, and the ferrite grain size is fine [25]. As mentioned in process 1 in this research, fast cooling is selected to be 650 °C, and the results of simulation and industrial tests show that the ferrite structure is relatively small and the ferrite transition is more sufficient.…”

Section: Discussionmentioning

confidence: 97%

Effect of Cooling Mode on the Microstructure of High-Strength Steel during Hot Rolling

Liu¹,

et al. 2022

This paper studies the effect of extreme cooling and traditional cooling on the microstructure of high-strength steel during hot rolling by adjusting the cooling process, combining the theoretical calculation and the thermal simulation experiment, and using metallographic microscope, scanning electron microscope (SEM), and electron backscattered diffraction (EBSD) analysis methods in order to solve the problem of coil collapse in the production process of high-strength steel. The research results show that compared with the traditional cooling method, the front-section fast cooling mode can rapidly cool the hot-rolled sheet to the “nose tip” temperature of the ferrite transformation of the time-temperature-phase-transition (TTT) curve, which can promote the transformation of the material to ferrite, increase the proportion of ferrite, and make the grain size of the organization finer. It helps to improve the overall mechanical properties of the material and reduce coil collapse defects. The front-section fast cooling mode achieves good results in industrial application, the proportion of coil collapse reduces from 9.363% to 0.533%, and the problem of coil collapse is significantly improved.