FRACTURE AND THE FORMATION OF SIGMA-PHASE, M23C6, AND AUSTENITE FROM DELTA-FERRITE IN AN AISI-304L STAINLESS-STEEL

The decomposition of delta-ferrite and its effects on tensile properti es and fracture of a hot-rolled AISI 304L stainless steel plate were s tudied. Magnetic response measurements of annealed specimens showed th at the transformation rate of delta-ferrite was highest at 720-degrees -C. Transformation behavior was characterized by light microscopy, tra nsmission electron microscopy, scanning electron microscopy, and energ y-dispersive spectroscopy on thin foils. The initial transformation of delta-ferrite (delta) to austenite (gamma) and a chromium-rich carbid e (M23C6) occurred by a lamellar eutectoid reaction, delta reversible M23C6 + gamma. The extent of the reaction was limited by the low carbo n content of the 304L plate, and the numerous, fine M23C6 Particles of the eutectoid structure provide microvoid nucleation sites in tensile specimens annealed at 720-degrees-C for short times. Sigma phase (sig ma) formed as a result of a second eutectoid reaction, delta reversibl e sigma + gamma. Brittle fracture associated with the plate-shaped sig ma phase of the second eutectoid structure resulted in a significant d ecrease in reduction of area (RA) in the transverse tensile specimens. The RA for longitudinal specimens was not affected by the formation o f sigma phase. Tensile strengths were little affected by delta-ferrite decomposition products in either longitudinal or transverse orientati ons.