Papers by Keyword: Incubation Period

Abstract: The effect of different volume fractions of pre-existing martensite on the low-temperature bainitic transformation and microstructures was quantitatively analyzed by dilatometer, optical microscope and scanning electron microscope. The results showed that pre-existing martensitic transformation accelerated the subsequent low-temperature bainitic transformation, and the incubation period and completion time of bainitic reaction were significantly shortened. This phenomenon was attributed to the increasing nucleation sites caused by the introduced dislocations in austenite due to the formation of pre-existing martensite. However, it was noteworthy that, because of the increased bainitic plates adjacent to the pre-existing martensitic plates, the probability of the impingement of bainitic plates during growth was increased, which resulted in a decrease in the maximum attainable volume fraction of bainite.

Abstract: The influence of austenitization temperature on the incubation period and bainitic transformation behaviours of the high-carbon silicon steel has been investigated. It was found that the nose temperature of bainite transformation and incubation period decreases with increasing austenitization temperature. The microstructure characteristics of the bainitic transformation products have been also observed. After isothermal heat treatment at 230°C for 20 mins, all samples austenitized at different temperatures produced a bainitic structure, which consists of packets of parallel ferrite laths. The major difference lies in the edge boundary morphology. Bainitic laths formed in low-temperature austenitization conditions has sharp saw-tooth edge boundaries, whereas bainite transformed from high-temperature austenitization conditions, have smooth wedge boundaries. Key Words: austenitization temperature; low-temperature bainite; incubation period;edge boundary

Abstract: Modelling of Creep Crack Growth (CCG) using analytical and numerical methods is relevant to life assessment procedures of components operating at elevated temperatures. This paper compares an analytical crack prediction and a numerical based virtual CCG technique used in fracture mechanics components with sample experimental results. Two approaches are presented. First the well developed strain exhaustion model called the NSW and the modified NSW-MOD models which predict plane stress/strain bound crack initiation and growth rates for engineering alloys and the second a damage-based approach used to numerically predict the crack propagation rate in Finite Element models of fracture mechanics specimens. The results from both methods are correlated against an independently determined C* parameter. As an example the NSW and the extended NSW-MOD strain exhaustion models are applied to compare to the experimental data and FE predictions for two steels at Carbon-Manganese steel tested at 360 oC and a weld 316H stainless steel at 550 oC. For values of C* within the limits of the present creep crack growth data presented the plane strain crack growth rate predicted from the numerical analysis is found to be less conservative than the plane strain NSW model but more conservative than plane strain NSW-MOD model.

Abstract: By using TEM strain-induced precipitation of Nb(CN) during deformation of undercooled austenite was investigated in Nb-microalloyed steel. The results showed that at 1200°C all of Nb were dissolved and there were no Nb(CN) precipitates formed during cooling until down to 760°C; During deformation enhanced ferrite transformation Nb(CN) of dynamic precipitation required an incubation period, but compared with isothermal transformation it reduced significantly. Only when the strain increased to 0.69, Nb(CN) began to precipitate on dislocation nodes and grain boundaries. Furthermore the volume fraction of Nb(CN) precipitation increased with increasing strain but their coarsening wasn’t significant. Results showed that the measured grain size is in good agreement with the calculated value.