Heat-affected Zone Of High-strength Steel Research Articles

Role of Carbon Content on Microstructure Evolution and Impact Toughness in Coarse-Grained Heat-Affected Zone of High-Strength Steel

The effect of carbon content in the base metals of high-strength steel on the microstructure and impact toughness of simulated welding focusing on a coarse-grained heat-affected zone (CGHAZ) at different heat inputs was systematically investigated by using scanning electron microscopy (SEM) and electron back-scattering diffraction (EBSD). The Charpy impact test confirmed that there was an optimal heat input, which caused the CGHAZ to obtain the highest impact toughness. The optimal heat input is ~20 kJ/cm and remains unchanged with an increase in carbon content from 0.04 to 0.12 wt.%. However, the impact toughness of the CGHAZ decreases with the increase in carbon content at each heat input. Microstructure characterization showed that a CGHAZ with 0.04 wt.% carbon gradually changed from lath bainite (LB) to granular bainite (GB) with an increase in heat input, while it changed from lath martensite (LM) to LB and then to GB for a CGHAZ with 0.12 wt.% carbon. Although the density of high-angle grain boundaries (HAGBs) obtained at 20 kJ/cm in the high-carbon sample is higher than that of the low-carbon sample, its impact toughness is lower, which is related to the parallel structure of the lath bundles and the morphology the austenite penetration.

Experimental and numerical study of high-strength steel butt weld with softened HAZ

Static tensile tests were conducted on 26 butt welded joints on base steel of Q690 grade high-strength steel, which exhibits softening behaviour in the heat-affected zone (HAZ). Considering the challenges involved in simulating the butt weld with the HAZ, a new method was proposed for calibrating the true stress–strain relationship based on a trial-and-error approach using simulation through comparing the load–deformation response for three materials (base metal, weld metal and HAZ) in a butt weld. Good consistency on the load–deformation response between the simulation and the test was observed, validating the proposed method. Parametric study showed that the relative width of the softened zone and matching ratio have a significant influence on the ultimate strength of the butt weld and BS EN 1993-1-12 cannot be used directly to design a butt weld with a soft interlayer in the HAZ of high-strength steel.

Влияние плазменной резки на микроструктуру и твердость высокопрочной стали

The application of high-strength steels during the creation of products satisfying all the modern requirements of the market is one of the most promising areas of modern engineering. The use of these steels reduces metal consumption of produced constructions due to reducing the thickness of the metal while maintaining the same structural strength, increases the service life of products thanks to increasing durability, as well as improves the reliability of the produced product. The processing of high-strength steel has a number of features associated with increased sensitivity of such steels to a thermal effect. The aim of this study is to research the influence of plasma-arc cutting on the microstructure and hardness in the heat-affected zone of high-strength steel having hardness of 500 HBW. To do that, the research of microstructure in the heat-affected zone of plasma-arc cutting of high-strength steel with 500 HBW hardness was performed. The results of research showed the presence of parts with different structures and properties in the heat-affected zone of plasma-arc cutting: decarburized layer, hardening layer, incomplete quenching layer, tempering layer, and the base metal. While moving off the boundary of the cut the cooling rate and temperature of metal are reduced. It is the cause of lowering steel hardness. The microstructure of the base metal has a needle form, and consists of strips of different orientation and size. The structure of different parts of the heat-affected zone are: hardening layer – martensite, incomplete quenching layer – martensite + ferrite, tempering layer – tempered martensite. The total length of the heat-affected zone and the length of its individual sections were also determined.

高張力鋼溶接熱サイクル再現試験片の水素による遅れ破壊試験について

A delayed failure test was done with notched specimens of synthetic heat-affected zone of highstrength steel. The test method was a so-called charge and stress method, in which the specimen was subjected, at first, to a weld thermal cycle, charged with hydrogen electrolytically and then loaded in sustained manner. The main conclusions obtained in the study are as follows:(1) Root crack susceptibility of the heat-affected zone in the first layer weld of a low-alloy high-strength steel can be examined in the delayed failure test of the synthetic heat-affected zone of steel.(2) A new criterion was proposed for comparison of susceptibilities to hydrogen embrittlement of the synthetic heat-affected zones of steels. The ratio of a critical stress in the delayed failure test (σt) to a normal notch strength (σt) of synthetic specimens of a steel was plotted against a diffusible hydrogen content of the specimen in a graph with logarithmic graduation. The value of the ratio at a specific hydrogen content (e.g. 1.0 cc/100 gr) was adopted as the criterion.(3) A critical stress increased with an increase of cooling time in a weld thermal cycle except for a 9% Ni steel. The stress also increased with decreases in diffusible hydrogen content and notch sharpness.(4) Susceptibility to hydrogen embrittlement of a low-carbon martensite was affected by other alloying elements even if the carbon content and the hardness were not changed.(5) Susceptibility to hydrogen embrittlement of a low-carbon martensite was increased by rapid quenching and also by repeated quenching.(6) A bainitic structure which was isothermally transformed just above the M, temperature had a high susceptibility to hydrogen embrittlement. The susceptibility was nearly equal to that of an autotempered martensite.(7) There was a relation between the new criterion in the delayed failure test and the critical restraining stress for root cracking in the heat-affected zone in the weld of the same steel in the TRC (Tensile Restraint Cracking) test.