Study on Microstructure and Mechanical Properties of Thick Low-Alloy Quench and Tempered Steel Welded Joint

Abstract

Abstract Low-alloy quench and tempered (Q&T) steel plates having yield strength of a minimum of 670 MPa are used extensively for the fabrication of impellers, penstocks, excavators, dumpers, and raw material handling devices, where welding is an important and probably most economic and productive process of joining. Thick low-alloy Q&T steel plates having a relatively high carbon equivalent (∼0.6) are frequently susceptible to a crack-sensitive microstructure leading to cold cracking. Thus, the welding parameters need to be carefully selected for these grades of steel to obtain the desired microstructure and properties. The present study deals with the characterization of microstructure evolution and mechanical properties of thick (38 mm) welded low-alloy quench and tempered steel plates (A 517 Grade F by ASTM) with the shielded metal arc welding process using a low hydrogen covered electrode (E 11018 M). The microstructural characterization was done for top (P1), middle (P2), and bottom (P3) regions of the weldment by optical microscope and transmission electron microscope to understand the microstructural features. The micrographs of the weld and coarse-grain heat-affected zone (CGHAZ) of the three regions mainly show variation in lath martensite and different morphologies of ferrite. Interestingly, the weld metal and CGHAZ in the middle (P2) region show coarse grains compared to the other two regions where grain size is more or less the same and finer. Furthermore, among the three regions, the P3 region, in general, has comparatively higher hardness and toughness. The high cycle fatigue test under two load ratios (i.e., R = 0.1 and −1) conducted only on specimens extracted from the P3 region, because of its better properties, shows better fatigue performance of weld joint under R = 0.1.