Relationship between Microstructure, Properties and HICC Resistance of HSLA steel weld metals

The effects of microstructure, mechanical properties and hydrogen content on the hydrogen induced cold cracking (HICC) resistance of high strength low alloy (HSLA) steel weld metals were studied in this investigation. The weld metals were designed in a previous work. Their microstructures were characterised by optical and electron microscopy (FEG-SEM and TEM). Microphases, such as non metallic inclusions (NMI) and martensite-austenitecarbide constituent (MAC), were studied in some detail due to their hydrogen trapping capacity. Fractographic studies of hydrogen charged tensile samples were carried out to study the effect of microstructure and hydrogen content on the fracture micromechanisms. A critical hydrogen content (Ck) was estimated for each weld metal. The trapping capacity 'of each weld metal was studied using an electrochemical double pulse technique to measure the hydrogen trapping constant (k). The weld metals were classified based on composition, microstructure and micro-phase characteristics. NMI number density, size and spatial distribution were determined and thermodynamic calculations were proposed to identify their type. MAC morphology and distribution were qualitatively assessed. Retention of austenite was estimated, considering chemical and size stabilisation of remaining austenite. It was found that a continuous network of grain boundary ferrite (PF(G)), in combination with the presence of retained austenite in the MAC particle and certain NMI characteristics were beneficial to increase Ck. In weld metals without PF(G), retained austenite proportion and NMI distribution and size play a critical role in maintaining tolerance to hydrogen. From fractographic observations, it was proposed a phenomenological model that correlates microstructure, hydrogen content and the stress intensity factor, with the activation of different fracture micromechanisms: micro-void coalescence (MVC), quasicleavage (QC) and intergranular (IG). The trapping capacity of the weld metals was evaluated and results indicate that this capacity is the result of a complex combination of factors such a NMI inclusion size and distribution, presence of retained austenite and microstructure. The value of k takes into account these effects.