animal The optical micrographs of DMR A

The optical micrographs of DMR-249A steel base metal, weld metal and HAZ are shown in Fig. 5(a)-(c). The base metal consists of fine grained equiaxed ferrite and some percentage of pearlite as a banded structure (Fig. 5(a)). Micrograph of weld metal shows fine bainitic structure along with acicular ferrite (Fig. 5(b)). The HAZ microstructure consists of acicular ferrite with some polygonal ferrite (Fig. 5(c)). No martensitic phase could be identified in the HAZ or in the weld metal. Hardness of the HAZ (275 VHN) is found to be marginally higher than the base metal (235 VHN). Weld metal hardness (315 VHN) is higher than that of HAZ (Fig. 6(a)). Mechanical properties of the simulated HAZ are given in Table 2 and Fig. 6(b). Since notch effect is experienced by the HAZ of the implant sample due to presence of the helical notch, the notch tensile strength of the simulated HAZ was determined and the value obtained for the same is 660 MPa, which is higher than the tensile strength of the base metal (575 MPa), revealing the strengthening effect of the notch. Toughness of the HAZ simulated structure is appreciably high (170J), although much lesser than base metal (350J) [13]. Fractographs (Fig. 7(a) and (b)) also show ductile cup-cone fracture for tensile tested samples of both base metal and HAZ. Dimple size in case of base metal is much smaller as compared to HAZ, indicating high ductility of the material. Fibrous ductile fracture (Fig. 7(c)) is also noted for the impact tested specimen of simulated HAZ.
Discussion In the present investigation, the results indicate that HSLA steel of grade DMR-249A is not susceptible to HAC irrespective of HD content of the welding consumable. The reason for the same is investigated in the following section. It is clear from the results presented above that without baking, HD content in the flux of the electrode is quite high, and by baking at 150 °C only the moisture absorbed by the flux coating is driven off, whereas by baking at a temperature recommended by the manufacturer (450 °C), chemically bonded water in some of the flux constituents is also removed. During arc welding processes, hydrogen gets introduced into the weld from the moisture of the animal as well as flux, and from hydrogenous materials such as oil, grease, paint, etc. [14]. The moisture can be removed by drying at moderately higher temperature whereas elevated temperature (above 400 °C) is required to drive the chemically associated water of the flux [14]. It can be further seen from the results that LCS could not be determined for weldments produced from either properly baked or unbaked electrode by implant tests as the LCS values are above the yield strength of the material in both the conditions. It can be assumed that fully ferritic microstructure of the HAZ, similar to base metal, and predominantly ferritic structure of the weld metal are the major reasons for good resistance of the weld joint to HAC. It has also been seen for other grades of HSLA steel that ferritic microstructures are resistant to HAC irrespective of HD content of the weld [15]. Another factor contributing to HAC resistance could be the presence of carbides, especially TiC in the weld metal, which are known to be strong traps for hydrogen [16]. Though Rishi et al. [13] have characterized in detail the inclusion content of SMAW weld metal of DMR-249A steel with similar electrode, detailed study on effectiveness of various precipitates and inclusion as hydrogen traps has not been attempted. Such a study may reveal more information about high resistance to HAC of these welds. The tendency of HAC is much higher in weldments with a stringent variation in hardness from base metal to weld or HAZ [8]. However, in this case variation in hardness between HAZ (275 VHN) and base metal (235 VHN) is only marginal due to the presence of predominantly ferritic microstructure. Weld metal hardness (315 VHN) is slightly higher probably because of the presence of bainite in the weld metal. However, adverse effect of high diffusible hydrogen content is much nullified by the almost uniform hardness distribution across the weldment [7]. Mechanical properties of the simulated HAZ also indicate that fine acicular ferritic structure of HAZ contributes to its good mechanical properties. From the notch tensile strength of the HAZ and fractographs of the same, it can be concluded that a ferritic microstructure of low hardness and high toughness is resistant to HAC, irrespective of the hydrogen level in the weld.