Abstract
The study aimed to identify a moderate degree of Ce addition to improve the toughness in the simulated coarse-grained heat-affected zone (CGHAZ) of high-strength low-alloy steels, based on the effect of the Ce content on particle characteristics, microstructure and impact toughness. Three steels with 0.012 wt.%, 0.050 wt.% and 0.086 wt.% Ce content were subjected to 100 kJ/cm heat input in their thermal welding cycles. The particles and microstructures in the simulated CGHAZ of each steel were characterized and the impact-absorbance energy levels were measured at −20 °C. The results indicated that Ce2O2S inclusion compounds were gradually modified to CexSy-CeP and CeP with the increasing of the Ce content. A higher fraction of acicular ferrite was formed in the 0.012 wt.%-Ce-treated steel due to the lower mismatch between Ce2O2S and α-Fe. Furthermore, a lower fraction of M-A constituent was obtained in the 0.012 wt.%-Ce-treated steel. As a result, superior toughness and a typical amount of ductile fracture were detected in the simulated CGHAZ of the 0.012 wt.%-Ce-treated steel. Compared with the 0.012 wt.%-Ce-treated steel, a smaller prior austenite grain was observed in the 0.086 wt.%-Ce-treated steel because of the segregation of CeP at the grain boundary. However, the larger size and density of CeP led to poor toughness in the CGHAZ of the 0.086 wt.%-Ce-treated steel.
Highlights
High-strength low-alloy (HSLA) steel is extensively applied in pressure vessels, oil and gas pipelines, offshore structures, bridges and building beams, due to its outstanding combination of high strength, high toughness, and good weldability, which is achieved through optimization of the alloy design in conjunction with thermo-mechanical processing [1,2,3]
These results indicated that the fracture surface of the sample was consistent with the corresponding t3o.u4g. hImnpeassctvTaoluugeh. ness and Fractured Surface Characteristics in the Simulated coarse-grained heat-affected zone (CGHAZ)
With the increasing of the Ce content from 0.012 wt.% to 0.086 wt.%, the Ce2O2S inclusion compounds in the C1 steel were gradually modified to CexSy-CeP in the C2 steel and CeP in the C3 steel
Summary
High-strength low-alloy (HSLA) steel is extensively applied in pressure vessels, oil and gas pipelines, offshore structures, bridges and building beams, due to its outstanding combination of high strength, high toughness, and good weldability, which is achieved through optimization of the alloy design in conjunction with thermo-mechanical processing [1,2,3]. The chaotic AF plates nucleate on intragranular inclusion compounds and form a fine-grained interlocking microstructure in the simulated CGHAZ of steel They can effectively divide prior austenite grains into several regions and retard crack propagation, leading to the absorption of more energy during Charpy V-notch impact tests [10]. To improve the toughness of CGHAZ, oxide metallurgy technology has been proposed, which aims to utilize fine and dispersed inclusion compunds in order to inhibit the growth of austenite grain and to induce the nucleation of AF. It was pointed out that the inclusion of Ti2O3 or Ti2O3 with an outer layer of MnS represents the most effective means of nucleating AF due to the formation of an Mn-depleted zone This contributes to the promotion of AF’s formation and the improvement of the toughness in the CGHAZ of steel [14]. The characteristics of particles—and their subsequent effects on the microstructural evolution and impact toughness in the simulated CGHAZ of high-Ce steels under high-heat input thermal cycles—were studied and compared with those of their low-Ce steel counterparts
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