Microstructure Evolution of Heat-Affected Zone in Submerged Arc Welding and Laser Hybrid Welding of 690 MPa High Strength Steel and its Relationship with Ductile–Brittle Transition Temperature

Abstract

The comparative study of submerged arc welding (SAW) and laser hybrid welding (LHW) was carried out for a 690 MPa high strength steel with thickness of 20 mm. Microstructure and ductile-brittle transition temperature (DBTT) evolution in welded zone were elucidated from the aspect of crystallographic structure, particularly, digitization and visualization of 24 variants. The impact toughness of each micro zone in LHW joint is better than that of SAW, in which the DBTT of equivalent fusion line and heat-affected zone (HAZ) can reach − 70 and − 80 °C, while that of SAW is only − 50 °C. LHW technology induces narrowing of the HAZ and refining of the microstructure obtained in weld metal and HAZ. Meanwhile, the austenite grain size and transformation driving force in the coarse grained heat-affected zone (CGHAZ) are reduced and increased, respectively. It makes variant selection mechanism occurring in CGHAZ of LHW dominate by close-packed plane grouping, which promotes lath bainite formation with high density of high angle grain boundary, especially block boundary dominated by V1/V2 pair. While for SAW, the lower transformation driving force inferred from the large amount of retained austenite in CGHAZ induces Bain grouping of variants, and thus triggers the brittle crack propagating straightly in granular bainite, resulting in lower impact toughness and higher DBTT.

Effect of Microstructure on Mechanical Properties of X65MS Welded Pipe Joint

“…The welding processing parameters were used as follows: welding speed 50 cm min À1 , heat input 21 kJ cm À1 , voltage 32 V, and arc current 550 A. [28][29][30][31][32] Submerged arc welding wire H08MnMoA and flux SJ101S were selected for pipeline steel welding, as shown in Table 2. Pre-welding preheating and post-welding heat treatment were not used for welding.…”

Effect of Segregation Band on the Microstructure and Properties of a Wind Power Steel before and after Simulated Welding

“…The evolution of microstructure with increasing heat input is closely related to the phase transition temperature. An increase in heat input will reduce the t 8/5 cooling rate and increase the phase transition temperature [4,27,29], which leads to a trend of microstructure transformation from LM to LB and GB. The change in microstructure is the fundamental factor determining the low-temperature impact toughness variation in Figure 4.…”

“…For the LB structure obtained at the heat input of 10 and 20 kJ/cm, due to the weakest variant selection, all 24 variants that conform to the K-S (Kurdjumov-Sachs) relationship are formed in a single austenite grain, especially the block boundaries composed of a V1/V2 variant pair with a large misorientation angle, which can achieve effective refinement of austenite grains through interactive arrangement and thus play the strongest hindering role in the propagation of brittle cracks [20,[32][33][34][35]. However, for the GB structure obtained at the heat input of 30 kJ/cm, the higher heat input leads to a significant increase in its phase transition temperature [29]. The bainitic variant selection is strengthened, and the coarse Bain group dominates the entire austenite grain, making it difficult to achieve the obstruction of crack propagation due to the large block units.…”

“…Nevertheless, this crack was ultimately arrested at the block boundary (Figure 11e). Previous studies [20,29,36,37] have shown that the block unit is beneficial for both strength and toughness, as the misorientation angle of its boundary is greater than 45 • , which can effectively hinder crack propagation. From Figure 10a, it can also be seen that the LB structure obtained at a heat input of 20 kJ/cm has the highest density of the high-angle grain boundary, which is consistent with the previous study [4].…”