Hot cracking investigation in submerged arc welds of high strength steel plate produced by thermo-mechanically controlled rolling

Abstract

Modern high strength steels (HSS) are mainly manufactured through thermo - mechanical controlled process (TMCP), quenching and tempering (QT), Direct quenching (DQ) and Quenching and partitioning method (Q&P). The steel grade investigated in this thesis is thermomechanically hot rolled S500 ML produced by Dillinger steel. Although it has lean chemical composition, its mechanical properties are comparable to expensive S500QT steels. Typical applications of S500ML are constructions for steel work and offshore, hydraulics, storage tanks or spheres.

The primary aim of this research is to investigate the hot cracking susceptibility of 24 mm thick S500ML steel during submerged arc welding (SAW) with different heat inputs by varying the welding speeds. The thesis also aims to understand the effect of welding speed and heat input on weld bead geometry and hardness distribution in the weld metal and heat affected zone (HAZ). One of the initial goals was also to find out the inter-relationship between the depth to width ratio of weld bead, chemical composition and microstructure with hot cracking if hot cracking occurs in welded S500 ML. Secondarily, the thesis also focuses on the weldability, challenges and problems in welding of High strength steels.

This thesis is based on literature review, SAW welding experiment, metallographic examination and mechanical testing. Firstly, a proper literature research was made on modern high strength steels, its production routes, and their weldability. Enough scientific articles, books and journal were studied to discover the welding problems and challenges during welding of high strength steels. The review was extensively focused on physical, thermal and metallurgical factors associated with hot cracking in thermomechanically rolled HSS and its remedy. Besides, the effect of welding parameters on weld bead geometry, weld microstructures and mechanical properties of HSS was also studied. For experimental research, bead on plate SAW welding was carried out on 24 mm thick S500 ML keeping constant current and voltage whereas 4 different welding speeds of 40 cm/min, 60 cm/min, 80cm/min and 98cm/min were employed to yield four different heat inputs. Metallographic examinations were carried out using optical and SEM microscopy to detect any hot cracking in the weld. Macrograph images of the weld bead was measured with Toupview software to determine the depth to width ratio of weld bead and find the link between these ratios with hot cracking. The effect of welding speed and heat input on depth of penetration was also determined. Struers hardness testing machine was employed to determine the hardness values through the base metal (BM), HAZ and weld metal (WM).

Metallographic observations showed no hot cracking in the weld metal for the investigated material S500 ML. From experimental results on effect of welding speed and heat input on depth of penetration, it is found that the depth of penetration is increased with increasing welding speed until it reaches the maximum at some optimum speed value after which the depth of penetration drops with increasing welding speed. Hardness results revealed that hardness in both HAZ and weld metal increases with decreasing heat input (increasing welding speed). HAZ softening is very common characteristics with TMCP and QT HSS. However, the hardness values in HAZ in welded S500ML was found to be highest which is contrasting to the general pattern.