Junjing He

Formation of creep cavities in austenitic stainless steels

The possibility of creep cavity formation at subboundaries in austenitic stainless steels is analysed. It is demonstrated that such nucleation is thermodynamically feasible. A minimum stress must be exceeded in order to create cavities. The nucleation is assumed to take place where subboundaries on one side of a sliding grain boundary meet subgrain corners on the other side (double ledge models). Alternative cavitation positions can be found where particles meet subboundaries. The nucleation model can quantitatively predict the observed nucleation rate. The model gives a nucleation rate that is proportional to the creep rate in agreement with many experiments.

Modelling grain boundary sliding during creep of austenitic stainless steels

Two models are presented for grain boundary sliding (GBS) displacement during creep. GBS is considered as crucial for the formation of creep cavities. In the first model, the shear sliding model, GBS is accommodated by grains freely sliding along the boundaries in a power-law creeping material. The GBS rate is proportional to the grain size. In the second model, the shear crack model, the sliding boundaries are represented by shear cracks. The GBS rate is controlled by particles in the boundaries. In both models, the GBS displacement rate is proportional to the creep strain rate. Both models are consistent with existing experimental observations for GBS during creep of austenitic stainless steels. For cavity nucleation at particles, Harris’ model (1965) for the relationship between GBS and a critical particle size has been analysed and found to be in agreement with observations.

Survey of Creep Cavitation in fcc Metals

Cavitation plays an important role in plants operating at high temperatures since the cavitation controls the creep failure of engineering alloys. In the past it has been difficult to predict the cavitation behaviour with the help of basic models, since critical models have been missing. Recently new models have been formulated for grain boundary sliding, cavity nucleation and cavity growth to fill this gap. These models are reviewed in this chapter. It is shown that the new models can quantitatively predict cavitation for austenitic stainless steels, where detailed experimental information is available.

Spot Weldability Comparison of High Strength Steel BIF340 and Normal Cold-Rolled Steel St14

The experimental investigation was carried out to compare resistance spot weldability of BIF340 and St14 steel in different weld processes.The spot weld microstructure and properties of two steels were analyzed.The results shown that : resistance spot weldability of BIF340 is better than St14.The optimum parameters of BIF340 were as follows: welding current was 6KA,welding time was 7 cycles, electrode force was 2800N. With the same welding process parameters, the microstructure of BIF340 spot weld was homogeneous and fine-grained, and the tensile shear strength was higher than St14.