EFFECT OF APPLIED POTENTIALS ON ENVIRONMENTAL CRACKING BEHAVIOR OF 17-4 PH STAINLESS-STEEL WELDMENTS

The effects of anodic, cathodic, and open-circuit potentials (OCP) on the environmental cracking behavior of 17% Cr-4% Ni (17-4 [UNS S17400] ) precipitation-hardenable (PH) stainless steel (SS) welds subjected t o different thermal treatments were studied. Sheets of 17-4 PH SS 1.5 mm (0.059 in.) thick and in solution-treated condition were full-penet ration welded autogenously using the gas tungsten are welding process (GTAW). Weldments were given one of true postweld heat treatments: dir ect aging and solution treatment + aging. Samples were aged at 480 deg rees C for 1 h, 510 degrees C for 4 h, and 600 degrees C for 4 h. Envi ronmental cracking tests were conducted using U-bend samples. Samples were tested in 3.5% sodium chloride (NaCl) solution (pH = 2.0) Lender various applied potentials. The failure time at each potential was tak en as the criterion for cracking resistance of the samples. At OCP and at anodic potentials, cracking was found to occur by an active path d issolution mechanism in the heat-affected zone (HAZ). Although the wel d metal hardness was more than that of the HAZ, no cracking took place in the weld metal. Solution treating the welds improved their crackin g resistance. In contrast, failure occurred within the weld metal at a pplied cathodic potentials. Therefore, the hardness criterion (the har der the structure, the higher the susceptibility to cracking) was foun d to be applicable under applied cathodic potentials. The hardest stru cture obtained by peak aging showed the least cracking resistance rend er these potentials. To increase the cracking resistance of weld metal s under cathodic potentials, solution treating followed by overaging ( at 600 degrees C for 4 h) was found to be the best method. However the same treatment was found to be highly detrimental under anodic and OC P conditions.