Effect of Applied Potentials on Environmental Cracking Behavior of 17-4 PH Stainless Steel Weldments Available to Purchase

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 to 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-penetration welded autogenously using the gas tungsten arc welding process (GTAW). Weldments were given one of two postweld heat treatments: direct aging and solution treatment + aging. Samples were aged at 480°C for 1 h, 510°C for 4 h, and 600°C for 4 h. Environmental cracking tests were conducted using U-bend samples. Samples were tested in 3.5% sodium chloride (NaCl) solution (pH = 2.0) under various applied potentials. The failure time at each potential was taken as the criterion for cracking resistance of the samples. At OCP and at anodic potentials, cracking was found to occur by an active path dissolution mechanism in the heat-affected zone (HAZ). Although the weld metal hardness was more than that of the HAZ, no cracking took place in the weld metal. Solution treating the welds improved their cracking resistance. In contrast, failure occurred within the weld metal at applied cathodic potentials. Therefore, the hardness criterion (the harder the structure, the higher the susceptibility to cracking) was found to be applicable under applied cathodic potentials. The hardest structure obtained by peak aging showed the least cracking resistance under these potentials. To increase the cracking resistance of weld metals under cathodic potentials, solution treating followed by overaging (at 600°C for 4 h) was found to be the best method. However, the same treatment was found to be highly detrimental under anodic and OCP conditions.