A Re-Examination of Failure Analysis and Root Cause Determination

25Figure 4. Elongated dimples Figure 5. Localized plastic deformationin fracture surface.during fracture of weld surface(red arrow)METALLOGRAPHIC EXAMINATIONA transverse cross section through the failed weld was prepared for metallographicexamination. The microstructure of the end cap consisted of dark-etching pearlite pluswhite-etching ferrite. This microstructure is consistent with a grade 1045 carbon steel.The microstructure of the host-affected zone (HAZ) consisted of dark-etching temperedmartensite plus white-etching grain boundary ferrite. The weld was made with a singlepass. At the fracture surface the weld metal exhibited localized plastic deformation asshown in Figure 5.MICROHARDNESS INSPECTIONIn the as-polished condition a Knoop microhardness inspection was performed in boththe weld metal and in the heat affected zone using a 500-gram load. The results aregiven in the accompanying table. The approximate ultimate tensile strength of the weldmetal deposit is above the minimum required value of 70,000 psi, as determined byconversion from Knoop hardness to tensile strength using Table 2 of the AWS A 5.1.Knoop MicrohardnessLOCATION(inches from interface)KHN Approx. HRB Approx.HRCApprox.TS (KSI)Weld 0.010 227 95.2 --- ---Weld 0.030 214 92.6 --- ---Weld 0.050 227 95.2 --- ---average 94.3 98.6HAZ 0.004 368 --- 36.8 ---HAZ 0.008 302 --- 28.7 ---HAZ 0.012 298 --- 28.1 ---HAZ 0.016 272 --- 24.0 ---HAZ 0.020 291 --- 27.1 ---HAZ 0.024 285 --- 26.2 ---HAZ 0.028 282 --- 25.7 ---HAZ 0.032 291 --- 27.1 ---HAZ 0.036 306 --- 29.3 ---HAZ 0.040 305 --- 29.1 ---BASE METAL --- 235 96.8 --- ---BASE METAL --- 268 --- 23.3 ---BASE METAL --- 270 --- 23.7 ---