A B C D to room temperature. Phase transformation leads to the higher dislocation density in the respective HAZ regions (Fig. 15) resulting in comparatively finer grain structure (Fig. 10) and increases the hardness values. Again, more heterogeneity in hardness distribution is observed with weld J1. Hardness being a reflection of microstructure, B E heterogeneity in hardness distribution resulted from local microstructural variation. Since martensite transformation temperature of J1 is lower as derived from chemical composition, slight variation in local cooling rate may affect the formation of martensite laths leading to heterogeneity in hardness. WELDING RESEARCH Tensile Properties The results of unnotched tensile tests of different welded joints are given in Table 7, which reveals that all the tensile samples broke in the base metal irrespective of different microstructures. The tensile results, as given in Table 7, show only the performance (strength) of the base metal. In order to find out the strength of the weld metal, notched tensile samples were tested and the test results are given in Table 8. The notch strength has been calculated as the maximum load divided by the cross-sectional area at the notch. The notch sensitivity of different welds has been expressed by the notch–strength ratio (NSR). NSR = Snot (for notch specimen at maximum load) / SU (tensile strength for unnotched specimen) (9) In spite of the notching effect, all the notched tensile samples broke in the base metal. Since all the welded samples broke in the base metal in notched tensile tests, tensile results therefore indicated that weld metal and even the HAZ are much stronger than the base metal. The strength of the weld metal is derived from its microstructure, which consists mostly of ferrite along with metastable g-phase and lath martensite. The metastable gphase further transformed into martensite through stress/strain induced transformation, which ultimately increases the strength (Ref. 30). However, the percentage elongation of base metal is significantly higher (29.8%) than the welded unnotched samples (~23%) although both yield strength (YS) and ultimate tensile strength (UTS) are comparable (Table 7). This may be due to con- APRIL 2015 / WELDING JOURNAL 111-s Fig. 14 — Optical micrograph. A — Base metal shows equiaxed ferrite grains and HTHAZ under different shielding gas compositions; B — J1; C — J2; D — J3; E — J4 shows massive or coarse ferrite grains enveloped with lath martensite. A C D Fig. 15 — Bright field TEM micrograph of HTHAZ, A — J1; B — J2; C — J3; D — J4, shows the presence of dislocations along with lath martensite.
Welding Journal | April 2015
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