The effects of C content on microsegregation and brittleness temperature range (BTR) were investigated in Fe-Cr-Ni-C alloys, which form the basis for austenitic stainless steel, with primary γ solidification both in the A mode (fully γ) and the AF mode (intercellular eutectic δ) by using the multi-phase field (MPF) method. With increasing C content, cell/dendrite transition occurred in the solidification microstructure and the BTR increased. The estimated influence of C content on BTR agreed well with previous reports. It was thought that the effect of microstructural morphology on BTR was not large. It was also predicted that the microsegregation of Cr also increased with increasing C content, which had strong thermodynamic interactions with Cr. It was also shown that BTR was largely reduced upon changing the solidification mode from A to AF. It was evaluated that the distribution of Cr, Ni, and C varied by adding δ-ferrite in the final solidification region.
The effect of microstructural morphologies on solidification microsegregation and residual liquid distribution was investigated in Fe-Cr-Ni-C alloy, which forms the basis for austenitic stainless steel with fully γ solidification mode by using the multi-phase field (MPF) method. When the cellular dendrite growth direction became inclined, the growth of the secondary arm became remarkable in the branching part and liquid droplets were formed irregularly around the grain boundary region. A liquid film was formed in the blocking part of inclined cellular growth, where stress concentration occurred continuously. This liquid film can influence solidification cracking susceptibility. Stress concentration occurred discontinuously in the branching part. The effect of varying the interface energy during solidification on the residual liquid morphology was not large, even when 2σL-γ≪σγ-γ, where σL-γ is the interface energy of liquid/γ and σγ-γ is the interface energy of γ/γ.
The hammer peening process is well known as one of the methods to improve fatigue lives in welded joints. Peening induces a compressive residual stress field near the weld toe in which fatigue crack usually initiates. When this method is applied to large scale welded strictures, the effects of the induced compressive residual stress on fatigue crack initiation stage and propagation stage should be separately comprehended because the balance of both stages in total fatigue life will be changed for different structure scales. In this study, the improvement mechanisms in fatigue crack initiation and propagation lives due to a compressive residual stress field was clarified by experimental observations of crack propagation behavior. The morphologies of propagating surface crack front were quantified in the specimens of plate and gusset welded joint with and without the hammer peening process. The experimental results suggested that the deeper aspect ratio of an elliptical surface crack just after crack initiation and propagation in the peened specimens induced fatigue crack retardation. Fatigue crack retardation in the peened specimens was discussed to clarify the fatigue crack life improvement mechanisms from the viewpoint of the stress intensity factor range suppression due to the crack aspect ratio.
The generation behavior of the γ-phase was investigated during the weld of the duplex stainless steels by using the molten Sn quenching technique. The γ formed in primary δ-phase grain boundary after 96-142℃ undercooling from equilibrium temperature and precipitated in the grain after 166-262℃ undercooling. The concentration of N in γ-phase was clearly observed, but the distribution of Ni and Mo between δ/γ was very small. It is thought that the growth of the γ advances by diffusion of N. Crystallographic relations of Kurdjumov-Sachs(K-S) existed between δ and Widmanstätten-γ, and γ plate grew up in <111>δ directions. The growth rate of γ plate at 1100℃ estimated in this work was 0.463mm/s and it was much lower than predicted value under the assumption of para-equilibrium of N and Zener-Hillert equation. Furthermore, the value calculated by the Trivedi equation considering the Gibbs-Thomson effect was also 2.1 to 7 times faster than the measured value. The decrease of N diffusivity was thought to affect the growth rate of γ-phase, because Cr contained 25mass% in duplex stainless steel has negative interaction with N.
It is necessary to understand the formation characteristics of welding deformation that occurs during the manufacturing process of the truss structure of steel angles. We evaluated the welding deformation behavior experimentally and numerically using a joint model and a real-scale truss structure model. As a result, we revealed that the main beams of the welded truss structure were arched against the direction of angular distortion observed in the welded joint model. The deformation tendency was explicable on the basis of theory in terms of uniformly distributed inherent strain within the mechanical melting zone. This analysis suggested a new theoretical insight that can facilitate suppression of welding deformation in the truss structure.