Features Retard Crack Propagation in Stainless Steels

Abstract

The role of some microstructural features in blocking the fracture path was investigated for duplex stainless steels (DSS). Sets of fatigued specimens , Wedge Open Load WOL , were heat treated at 475ºC for different times and pulled to failure either in air , CT specimens , or after kept in 3.5% NaCl with polarization of -900 mV/ SCE i.e. WOL specimens . Fracture took place in general by ferrite cleavage and austenite ductile fracture in transgranular mode. Specimens measured stiffness ( Ms ) was affected by the aging time, with higher values measured for specimens aged for longer times. The ratio of the measured stiffness to the predicted stiffness was observed to increase with the crack length. Microstructural features played a role in " blocking" the crack propagation process leading to increase the resistance of the material to fracture, R-curve vs. crack length , specially for specimens aged for short times. Unbroken ligaments/ austenite were observed at the crack wake. These features may exerted a shielding stress , blocking effect , at the crack tip giving resistance to the crack propagation process i.e. the crack mouth opening was reduced. Higher stress intensity factor KIC values were observed with increased amounts of crack growth suggesting longer zone of unbroken ligaments in the crack wake. The shielding zone was typically several mm in length.. Attempt to model the bridge stress was suggested to understand the role of ligaments / unbroken austenite in increasing the fracture toughness factor.