A COMPARATIVE-EVALUATION OF LOW-CYCLE FATIGUE BEHAVIOR OF TYPE 316LN BASE-METAL, 316-WELD METAL, AND 316LN 316-WELD JOINT/

A comparative evaluation of the low-cycle fatigue (LCF) behavior of ty pe 316LN base metal, 316 weld metal, and 316LN/316 weld joints was car ried out at 773 and 873 K. Total strain-controlled LCF tests were cond ucted at a constant strain rate of 3 x 10(-3) s(-1) with strain amplit udes in the range +/-0.20 to +/-1.0 pct. Weld pads with single V and d ouble V configuration were prepared by the shielded metal-are welding (SMAW) process using 316 electrodes for weld-metal and weld-joint spec imens. Optical microscopy, scanning electron microscopy (SEM), and tra nsmission electron microscopy (TEM) of the untested and tested samples were carried out to elucidate the deformation and the fracture behavi or. The cyclic stress response of the base metal shows a very rapid ha rdening to a maximum stress followed by a saturated stress response. W eld metal undergoes a relatively short initial hardening followed by a gradual softening regime. Weld joints exhibit an initial hardening an d a subsequent softening regime at all strain amplitudes, except at lo w strain amplitudes where a saturation regime is noticed. The initial hardening observed in base metal has been attributed to interaction be tween dislocations and solute atoms/complexes and cyclic saturation to saturation in the number density of slip bands. From TEM, the cyclic softening in weld metal was ascribed to the annihilation of dislocatio ns during LCF. Type 316LN base metal exhibits better fatigue resistanc e than weld metal at 773 K, whereas the reverse holds true at 873 K. T he weld joint shows the lowest life at both temperatures. The better f atigue resistance of weld metal is related to the brittle transformed delta ferrite structure and the high density of dislocations at the in terface, which inhibits the growth rate of cracks by deflecting the cr ack path. The lower fatigue endurance of the weld joint was ascribed t o the shortening of the crack initiation phase caused by surface inter granular crack initiation and to the poor crack propagation resistance of the coarse-grained region in the heat-affected zone.