imidazoline The simulated fraction of dynamic globularizatio

The simulated fraction of dynamic globularization at the central section is shown in Fig. 10. It can be seen that the globularization fraction is between 20% and 90% and significantly different at different positions, which can be more clearly seen from the simulated globularization evolutions at four typical positions (shown as 1–4), indicating non-uniform microstructure distribution. The microstructures at the positions corresponding to the four points shown in Fig. 10 in the cogging experiments are shown in Fig. 11. The comparison of globularization kinetics between simulation and experimental results is shown in Table 3. It can be seen from Table 3 that the simulated results fit well with the experimental ones, indicating the feasibility of the proposed dynamic globularization kinetics model. Through further investigation, it can be found that both the fraction and size of globularized α phase obtained by simulation are all smaller than those obtained by experiments, and the strain path change during (cogging compared to uniaxial compression) can improve the dynamic globularization of titanium alloy, which is different from the results presented in Ref. [10]. The effect of strain path change on glolurization kinetics and the mechanism need to be further studied.
Conclusions
Introduction Quenched and Tempered (Q&T) steels are used in military applications due to high hardness, high strength to weight ratio and excellent toughness [1]. These grades of Q&T steels are prone to hydrogen induced cracking after welding and they exhibit heat affected zone softening leading to poor ballistic performance [2]. Austenitic stainless steel (ASS) welding consumables are being used for welding Q&T steels, as they have higher solubility for hydrogen in austenitic phase, to avoid hydrogen induced cracking (HIC) [3]. But use of stainless steel fillers for a non stainless steel imidazoline metal must be avoided as ASS fillers are much more expensive. Recent studies proved that low hydrogen ferritic steel (LHF) consumables can be used to weld Q&T steels, which can give very low hydrogen levels in the weld deposits [4–6]. Q&T steel welds must be of good quality especially when used for construction of combat vehicles in military applications. The majority of armour fabrication is performed by fusion welding process and they demand for highest welding quality. Shielded metal arc welding (SMAW) and the flux cored arc welding (FCAW) processes are widely used in fabrication of combat vehicle construction [7,8]. Due to the heterogeneity induced from welding, base metal (BM), weld metal (WM) and heat affected zone (HAZ) have different mechanical behaviours, which makes welded joints complicated under local stress–strain conditions [9]. For structural steels, the strength of the welded joints determines the strength of the whole structure. Welded joints are subjected to various forms of cyclic loading in practical applications and fatigue failure is common. Thus, welding is a major factor in the fatigue lifetime reduction of components [10]. Failure analysis of the weldments indicated that fatigue alone is to be considered to account for most of the disruptive failures. Even though the fatigue properties of the weld metal is good, problems can be caused when there is an abrupt change in section caused by excess weld reinforcement, undercut, slag inclusion and lack of penetration and nearly 70% of fatigue cracking occurs in the welded joints [11]. As the fatigue failure is one of the prime concerns in structural design and the butt weld is a part of many structures, its evaluation and prediction of fatigue life is very important to avoid catastrophic failure particularly in steels that are used in military applications. Thus, it is always important to enhance the service life of the structural components under cyclic loading conditions by choosing proper welding process, consumables, etc. Apart from the mechanical considerations of joint design, the welding process, filler material, heat input, number of weld passes etc., will influence the microstructure of the weld at the joint and in turn will influence the extent of heat affected zone and residual stresses that will build up in the base metal. These factors will invariably affect the fatigue strength by increasing the propensity for crack nucleation and its early growth causing the ultimate failure of the joint [12,13]. The use of ASS & LHF consumables and SMAW & FCAW welding processes for armour grade Q&T steel will lead to formation of distinct microstructures in their respective welds. This microstructural heterogeneity will have a drastic influence in their fatigue crack growth properties of the respective welds. Considering all the above facts, an investigation was carried out to evaluate the fatigue crack growth properties of armour grade Q&T steels fabricated by SMAW (manual) and FCAW (semi-automatic) processes using ASS and LHF welding consumables. The present study assumes significance as fatigue crack growth studies have not been reported in this class of armour grade Q&T steel welds fabricated by SMAW and FCAW processes using ASS and LHF consumables.