Hydrogen embrittlement is a major concern in welded high strength carbon steel components due to the diffusion of hydrogen during welding. The presence of tensile residual stresses and diffisible hydrogen in a brittle martensitic weld microstructure is known to cause premature catastrophic brittle failure of the welds. Austenitic weld microstructures are in general known to resist hydrogen embrittlement. However, generating fully austenitic weld microstructures is considered to be disadvantageous due to (i) the use of scarce and expensive alloying elements containing welding consumables, (ii) hot cracking during welding and (iii) poor mechanical properties of the welds. In principle, high strength ferritic weld microstructure containing the mixture of carbide free bainite and nano sized retained austenite can be generated to trap the diffusible hydrogen in austenite laths and interphase boundaries. The increased volume fraction of interphase, grain boundaries and high dislocation density generated during transformation can act as traps for hydrogen and delay embrittlement. Therefore, in this work efforts are made to engineer such safe weld microstructures against hydrogen embrittlement in medium carbon steel welds.
The primary objectives of this work are to (i) identify filler wire compositions with carbon in the range of 0.35 – 0.5 wt. % to achieve carbide free bainite (CFB) microstructure in multi-pass shielded metal arc welds, (ii) study the hydrogen saturation behaviour of CFB welds with varying volume fractions of constituent phases and (iii) achieve optimum electrode composition and welding procedures to achieve high strength weld microstructures with decreased susceptibility for hydrogen embrittlement. Alloy compositions were designed (i) using commercial neural network-based database JMatproTM and (ii) by optimising thermodynamic parameters such as XT0 (allotropic phase boundary), ΔGɣ-a (driving force for transformation) calculated using database ThermocalcTM. Shielded metal arc welding (SMAW) electrodes were fabricated for the identified compositions and samples extracted from the weldments were used for Gleeble dilatometry studies, metallurgical characterisation and mechanical properties evaluation. Alloy compositions and welding process parameters are optimised to achieve nano-sized lath austenite containing microstructure in a carbide free bainite matrix. Welds with optimised weld process parameters were heat treated to vary volume fraction of the constituent phases. Hydrogen saturation studies were carried out on these weld microstructures by charging hydrogen using an electrochemical hydrogen charging cell and thermal conductivity based H analyser. To study the hydrogen embrittlement susceptibility of the welds, tensile and impact tests were carried out on samples extracted from the heat treated welds with and without hydrogen saturation. Based on the results, optimum electrode chemistries are identified to achieve high strength weld microstructures with decreased susceptibility for hydrogen embrittlement.