Pressure Vessel Steel Fracture Behavior At Different Temperatures

Based on observations of fracture surfaces and measurements of local critical. Parameter for cleavage of 3PB(three point bending) pre-cracked specimens of 16MnR steel, detailed observations of configuration changes at pre-crack tips by metal-graphic cross sections in specimens unloaded at various applied loads and various temperature, sophisticated FEM calculations of distributions of stress, strain and triaxiality and simulations of short cracks initiated and extended at pre-crack tips, and configuration transformation of the tip , the fracture behavior at crack tip is analyzed.The results reveal that as long as the fatigue pre-crack is only blunted, in its vicinity a region, where the accumulated strain is sufficient to nucleate a crack and a region, where the stress (triaxiality) is sufficient to propagate a crack nucleus is separated by a distance. The nucleated crack cannot be propagated and the cleavage fracture cannot be triggered. A short crack produced at the fully blunted fatigue pre-crack will change the distributions of strain, stress and triaxiality and specially their relative locations ahead of the pre-crack. While the strain retains, the stress (triaxiality) is rebuilt. The initiated, extended and then blunted short crack makes a tip configuration, which is much sharper than that of the fully blunted original pre-crack tip. The sharpened crack tip configuration re-builds a 'sharper' distribution of stress (triaxiality) and make above two regions closer. Finally the two regions overlap each other and a cleavage crack can be initiated and propagated at a distance ahead of the blunted pre-crack. At "absolutely" temperature (-196C):as long as the pre-cracked specimens attained stress criteria and a distance at ahead of the blunted pre-crack, the pre-cracked specimens fractured by cleavage. At ductile-brittle transition temperature, the stress triaxiality ahead of the ductile crack increase with the crack propagation but the elevation of normal stress is related not only the stress intensification but also to the strain hardening. The scatter of toughness may be attributed to the random variation of ductile crack width and the random distribution of the brittle particles. The considerable fluctuation of toughness values in the transition temperature region is due to the random variation of the widths of the tips of fiber cracks during their extension and the random distribution of the weakest constituents in the microstmcture. In addition, results also show that in pre-cracked specimens fractured by cleavage, the sizes of particles play a leading role and in pre-cracked specimens fractured model by cleavage, the temperature play a leading role. As result ,the critical event of the cleavage fracture changes from the initiation controlled to the propagation controlled due to second phase or inclusion-sized micro-crack as well as ferrite grain-size micro-crack.