Effect of Intercritical Quenching Temperature of Cu-Containing Low Alloy Steel of Long Part Forging for Offshore Applications

Abstract

In our previous study, intercritical quenching from the dual-phase region of ferrite and austenite regions, which is called lamellarizing (L) treatment, brought a clear improvement of balance between the strength and toughness of Cu-containing low alloy steel based on American Society for Testing and Materials (ASTM) A707 5L grade. Moreover, the results imply that the reverse transformation behavior during L treatment is very important in order to optimize the L treatment temperature. Hence, the purpose of this paper is to clarify the mechanism by which L treatment improves the mechanical properties in terms of reverse transformation behavior. Additionally, offshore structures require good weldability, because the structures generally have a lot of weld joints. Therefore, weldability was also investigated in this study. The investigation revealed that coarse Cu precipitates are observed in the not-transformed α phase, so the strength tendency in relation to the L treatment temperature is relevant to the area ratio of the not-transformed α phase and the transformed γ phase during L treatment. From the in situ electron back scatter diffraction (EBSD) results, it is believed to be possible to enhance the mechanical properties of Cu-containing low alloy steel by controlling the area ratio of the reverse-transformed gamma phase and selecting the appropriate L treatment temperature. Furthermore, the long part forging of Cu-containing low alloy steel has a good weldability, since the maximum hardness of the heat-affected zone (HAZ) is less than 300 HV, and the HAZ of steel has a good crack tip opening displacement (CTOD) property with less than 2.3 kJ/mm of heat input of GTAW.

Effect of Vanadium and Rare Earth on the Structure, Phase Transformation Kinetics and Mechanical Properties of Carbide-Free Bainitic Steel Containing Silicon

“…Sample 1.6511Si2V has the highest volume fraction of retained austenite among the samples; despite the larger grain size due to the possibility of simultaneous blinding and the TRIP phenomenon resulting from its retained austenite, it was able to absorb great energy before failure. Rare earth elements, besides decreasing the grain size, which leads to increased fracture toughness [45][46][47][48][49][50][51][52], also increase the grain boundary strength [36]. It seems that there is an increase in energy required for the fracture of the 1.6511Si2M sample compared to the standard sample, and the 1.6511Si2 sample has a higher grain boundary volume and higher grain boundary strength compared to other samples.…”

Special Issue on Recent Trends in Advanced High-Strength Steels

“…Haiko et al [3] investigate the influence of tempering temperature on both the microstructure and monotonic stress-strain response in a new ultra-high-strength steel with low carbon content. Honma et al [4] analysed the effect of intercritical quenching temperature on the strength and toughness in a low-alloy steel developed for offshore applications. Gu et al [5] developed a numerical approach to evaluate the effect of different inclusions on the residual stress profiles during the cooling process for martensitic steels.…”

Effect of heat treatment processes on the microstructure and mechanical properties of low alloy structural steels with different impurity tin contents

“…Li [29] reported that through introducing an intercritical quenching process between conventional quenching and tempering to 25Mn2Si2Cr bainitic steel, the tensile strength, yield strength and impact energy value exhibited an enhancement by 5.4%, 3.2% and 20.7%, respectively. The optimum combination of strength and toughness results from the refinement of complicated microstructures and grains, higher fraction of high angle grain boundaries caused by the formation of ferrite, abundant homogeneous dislocation cell substructure and stable thin film retained austenite [30][31][32][33].…”