Earlier studies have shown that the conditions for HAC

Earlier studies [5] have shown that the conditions for HAC to occur in steel welds are: presence of diffusible hydrogen (HD), residual stress and susceptible microstructure in the weld and temperature in the range of ambient to 200 °C. In this regard, martensitic microstructure with high hardness is most susceptible and ferritic microstructure with low hardness is least susceptible. Hence, during welding, efforts are made to reduce risk of HAC by avoiding development of a susceptible microstructure and minimizing the hydrogen levels in welding. The probability of having a susceptible microstructure in the HAZ or weld is assessed from the composition of the what you have metal and weld metal, heat input and preheating (which will reduce the cooling rate of the weld) chosen for welding [6]. In order to reduce hydrogen level, low hydrogen welding consumables, proper baking of the consumables to remove moisture content in the consumables and appropriate preheating or preheating + post heating conditions that would provide more time for hydrogen to diffuse out at high temperature are chosen. For HSLA steels like DMR-249A, the hardenability is very low and the as-welded microstructure is ferritic and hence susceptibility to HAC is expected to be low. However, susceptibility of a weld to HAC can be quantified from implant test in terms of lower critical stress (LCS), the stress below which the weld does not fracture during the test, and in the present study this is attempted for welds of DMR-249A steel made with indigenously developed consumables.
Results The HE_GCTCD data indicate that diffusible hydrogen (HD) content in the welding consumable, after baking at 450 °C/4h, is 3.1 mL/100 g of weld metal. This is certainly a low value of diffusible hydrogen, and as per IIW and AWS classifications, this electrode comes under very low hydrogen category of electrode [10–12]. Results of the HD measurement for the electrode without any baking is 9.6 mL/100 g of weld metal and the same for baking at 150 °C is 8.3 mL/100 g of weld metal. Thus, by altering the baking conditions of the electrodes, one can get different levels of HD contents for the same batch of electrodes. This enables carrying out implant tests using electrodes differing only in their HD content and determining LCS at these levels of HD contents for the same consumables and base materials. As mentioned earlier, implant test was carried out for two sets of welding prepared with baked (450 °C) and unbaked electrode. The tests were conducted in the range of 1000 kg (equivalent stress: 205 MPa) to 2000 kg (410 MPa) load in steps of 200 kg and above 2000 kg; load was increased up to 2400 kg (490 MPa) in a step of 100 kg. For samples welded with baked electrode, no failure occurred within 24 h (Fig. 3(a)) up to a loading of 2400 kg (490 MPa). No further test was carried out above 2400 kg (490 MPa) since it is above the yield strength of the base metal [9]. To estimate the stress level at which specimen fracture, for one specimen load was increased until fracture and this occurred at the base metal (Fig. 3(b)) far away from the HAZ and the corresponding fracture stress is 554 MPa (load = 2700 kg), which is nearly equivalent to the tensile strength of the weld joint [9]. As LCS could not be determined for the properly baked electrode, implant tests were conducted for specimens prepared using electrodes without baking (HD levels = 9.6 mL/100 g of weld metal). These specimens also did not fracture even after loading up to 2400 kg (490 MPa). The implant specimens tested at the highest stress levels were sliced along the length, polished, etched and then examined for microcracks under optical microscope. No cracks were found as shown in Fig. 4. Thus, results clearly confirm that the steel is not susceptible for HAC even at high levels of diffusible hydrogen.