INTRODUCTION
ABSTRACT Constant strain rate tests were conducted on welded austenitic AISI 304 (UNS 30400) in boiling MgC12 solutions at concentrations stainless steel of 32°/0 and 37°/0 weight respectively. Potentiodynamic anodic polarization tests were carried out on unstressed welded coupons. Open circuit potential measurements on stressed specimens showed that weldments exhibited free corrosion behavior in the active-passive region. Residual ferrite phase in weldments was detected using experimental and analytical methods. The presence of residual delta ferrite in weldments is hypothesized to have been responsible for the formation of microgalvanic cells that lead to the selective dissolution of the ferrite phase in favor of the austenitic matrix. This situation provided suitable nucleation sites for cracking, and its simultaneous occurrence promoted crack propagation. Optical and electron microscopy investigations support this hypothesis and show that weldment interdendritic spaces suffered selective dissolution and that crack propagation followed a preferential path along delta ferrite rich interdendritic arms. It was also revealed by the optical and electron microscopy investigations that crack growth took place by the union of longitudinally aligned interdendritic arms. The results are in agreement with the Parkins model of crack nucleation by simultaneous action of passive film formation, film rupture.
Austenitic stainless steels are commonly joined by welding rods that contain from 5% to 10°/0residual &ferrite, which is of known benefit in halting hot fissuring. ?1) During initial stages of the joining process, and under high rates of heat loss, molten steel starts to solidify into dendritic &ferrite grains. With progressive heat loss, several changes take place. Dendritic grains grow and close up on each other creating a network of interdendritic spaces, where the remaining non-solidified metal is trapped. Further descent down the temperature scale leads the solidified grains into a solid state phase transformation form &ferrite to y-austenite. In this stage, interdendritic spaces host ferrite stabilizing elements rejected from the austenitic matrix, and become rich in retained &ferrite phase.
When the entire weldment solidifies, retained &ferrite that formed within interdendritic spaces shall not transform into austenite and shall remain trapped in these spaces. ?2) Being more active than austenite ferrite shall assume the anodic electrode in ?3) It is hypothesized that the presence of such galvanic case a galvanic couple forms. action between constituent phases of the weldment created a suitable situation for incubation of cracks.
Electrochemical investigation of the corrosion behavior of austenitic stainless steel weldments in boiling MgC12 solutions comprised of polarization and free corrosion, open circuit potential tests. Results of electrochemical tests showed that weldments under given environmental conditions exhibited a preferred free corrosion potential in the zone of passivity-activity.
Cracked specimens were subjected to optical and electron microscopy investigation, which showed that the pattern of crack initiation and propagation came in agreement with the proposed Parkins model for crack initiation by continuous action of passive film formation-dissolution. ?4)