Subcritical crack growth of trip steels in air under static loads

Abstract

Pre-fatigued compact tension fracture toughness specimens of TRIP steel were held at constant loads at 25°C in 40 per cent r.h. air. Testing was done using an MTS closed loop universal test machine in the load control mode and the displacement was monitored as a function of time using a clip-on gauge and a strip chart recorder. Subcritical crack growth was observed and the experimental data was used to obtain a correlation between stress intensity and the rate of crack growth. The curves usually exhibited three distinct regions, including a plateau of stress intensity insensitive constant crack growth rates which have been observed by other investigators for titanium alloys and high strength steels. Based on comparisons with other investigators, the mechanism of subcritical flaw growth was tentatively identified as being due to hydrogen embrittlement. Fracture was observed to follow austenite grain boundaries and it was hypothesized that the austenite → martensite transformation sensitizes them to hydrogen by causing a large strain accumulation to be accommodated at the boundaries resulting in a large dilatation. Metallography revealed that the crack growth rate decreased as the strain-induced martensite increased and this was attributed to crack tip blunting by plastic deformation due to the invariant shear of the transformation. There are thus two apparently competing processes in the subcritical flaw growth of TRIP steels. Fractography showed that numerous fracture micromechanisms were operative. The subcritical crack growth characteristics compared favorably with other high strength steels tested under almost equivalent conditions and an apparent threshold for subcritical growth in air was determined to occur at about 56 per cent of K IC .