Thermal analysis of dissimilar weld joints of high-strength and ultra-high-strength steels

Abstract

Advances in steel production processes in the last two decades have enabled the production of novel materials with improved strength, weldability and usability. Many industries are implementing these novel materials into production, primarily to benefit from the higher strength-to-weight ratio. Improved material properties are especially important for industries engaged in advanced structural engineering and applications such as construction plants, piping systems of nuclear power plants, and products in the automotive and aeronautical sectors. Another trend in modern manufacturing is increased use of dissimilar metals welding, and welding of dissimilar steel grades is becoming common in the economically critical energy sector. Dissimilar welding of high-strength steels is especially advantageous for regions with extreme weather conditions, such as sub-Saharan Africa and Arctic regions.

When dissimilar joints for high strength steel (HSS) are welded, gas metal arc welding (GMAW) is extensively used because of its adaptability and controllability (control of input and output) and well-established production quality. The most important issue in dissimilar welding of HSS is control of the thermal cycle, as these steels have rather narrow process parameter windows and are prone to softening in the heat-affected zone (HAZ). This thesis addresses the issue of improving the weld quality of dissimilar material welds through improved understanding of the relationship between the welding parameters and resulting microstructure determining the mechanical properties of the joint.

The materials used in this thesis belong to the classes of high strength steels, ultra-highstrength steels (UHSS) with a static strength of 690 to 960 MPa, steels manufactured by a thermo-mechanically controlled process (TMCP) and quenched and tempered (QT) steels. A specific objective is to define favourable heat input values that can improve the quality of dissimilar joints of S690QT-TMCP and S700MC-S960QC. The main difficulty in welding of dissimilar HSS is control of HAZ softening on both sides of the joint. Many factors have an influence on HAZ characteristics, e.g., welding parameters, filler wire composition, and groove geometry. Therefore, selecting parameters that produce the desired properties in both materials being joined is very important. An appropriate choice of parameters results in improved microstructural constitution and mechanical properties.

The research has been carried out using three methods: literature review, numerical modelling and experimental validation. The literature review formed the first part of the study and examined thermal effects on the microstructural constituents and mechanical properties of dissimilar HSS welds. A numerical model of the thermal cycle was developed in second part of the study to understand the effect of cooling rate on changes in microstructure. The third part of this thesis comprised experimental validation of the influence of cooling rate on the properties of the weld joints.

The literature analysis demonstrated the feasibility of developing a numerical model predicting the thermal cycle of dissimilar welds made with GMAW by improving understanding of thermal transfer in the HAZ. In the experimental part, the thermal cycle data obtained during numerical modelling were applied to dissimilar welds of HSS and UHSS. Based on analysis of numerical and experimental data, optimum welding conditions were proposed, and their accuracy further validated with standard mechanical testing: Vickers hardness testing, tensile testing, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) mapping. Microscopic analysis of the specimens was used to determine the process parameters having the most significant influence on the microstructure and mechanical properties of the welded joint.

Analytical and experimental approaches to control heat transfer in the weld joints of dissimilar HSS have been developed and experimentally validated in this thesis. The findings enable weld quality to be improved and the microstructural constituents and mechanical properties of the joints to be optimised by precise control of heat input. The proposed approach allows the number of tests needed for welding parameters definition to be reduced by providing an improved understanding of the effect of heat transfer on microstructure characteristics.