Influences of post-weld heat treatment on fatigue crack growth behaviour of strength mismatched HSLA steel welds

Abstract

Welding of high strength low alloy steels (HSLA) involves usage of low, even and high strength filler materials (electrodes) compared with the parent material depending on the application of the welded structures and the availability of the filler material. In the present investigation, the influences of post-weld heat treatment (PWHT) on fatigue crack growth behaviour of under matched (UM), equal matched (EM) and over matched (OM) weld metals has been studied. The base material used in this investigation is HSLA-80 steel of weldable grade. The Shielded Metal Arc Welding (SMAW) process has been used to fabricate the single 'V' butt joints. Centre Cracked Tension (CCT) specimens have been used to evaluate the fatigue crack growth behaviour of the welded joints. Fatigue crack growth experiments have been conducted using servo hydraulic controlled fatigue testing machine at constant amplitude loading (R = 0). From this investigation, it has been found that the fatigue performance of over matched joints is superior compared to under matched and equal matched joints. Moreover, PWHT reduced the magnitude of the tensile residual stress field in the weld region and subsequently enhanced the fatigue performance of the joints irrespective of weld metal strength mismatch.     

Effect of dynamic contact resistances on advanced high strength steels under medium frequency DC spot welding conditions

Abstract

The effect of dynamic contact resistance (DCR) during MFDC spot welding of dual phase and martensitic steels was evaluated. A comparative analysis of DP590 to DP590 with DP780 to DP780 steel welds, and DP780 to DP780 with M1200 to M1200 steels welds was carried out. The DCR of DP780 steel is higher than DP590 steel during the initial stages of weld time, but is reversed later. The bulk resistance component, which is higher in DP780 steel, is dominant and generates more energy early in the process and controls melting. Although the total energy input is almost same, the higher β-peak and its early occurrence ensures better heat utilisation resulting in larger nugget size. Contrarily, in martensitic steel the interface resistance component remains high throughout the entire welding process and compensates for the lower bulk resistance effect. Even with relatively lower energy input the nuggets produced in M1200 steel are comparable to DP780 steel.     

Effect of friction welding condition on joining phenomena and joint strength of friction welded joint between brass and low carbon steel

Abstract

This paper describes the effect of friction welding condition on joining phenomena and joint strength of friction welded joints between copper–zinc alloy (brass) and low carbon steel (LCS). When the joint was made at a friction pressure of 30 MPa with a friction speed of 27·5 s^?1, brass transferred to the half radius region of the weld interface on the LCS side. Then, transferred brass extended towards the almost whole weld interface with increasing friction time. The joint efficiency increased with increasing friction time, and then the joint obtained 100% and the brass base metal fracture when the joint was made with a friction time of 4·2 s or longer. However, the fact that all joints had some cracks at the periphery portion of the weld interface was due to a deficiency of transferred brass at the periphery portion on the weld interface of the LCS side. On the other hand, brass transferred to the peripheral region of the weld interface on the LCS side, and then transferred towards the entire weld interface when the joint was made at a friction pressure of 90 MPa with a friction speed of 27·5 s^?1. The joint efficiency increased with increasing friction time, and it reached 100% at a friction time of 1·5 s or longer. In addition, all joints fractured from the brass base metal with no cracking at the weld interface. To obtain 100% joint efficiency and the brass base metal fracture with no cracking at the weld interface, the joint should be made with opportune high friction pressure and friction time at which the entire weld interface had the transferred brass.