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.
Cross tensile and fatigue tests had been conducted using zinc-galvanized low carbon steel wire (SWRM6) resistance cross welded joints. The jig for cross tensile and fatigue strengths evaluation was newly designed. Two kinds of cross-welded joints were used in the present study. As-received galvanized wire was resistance welded into the Joint A. On the other hand, the wire was aged in laboratory air at ambient temperature for 6 months and welded as the Joint B to evaluate the effect of time-dependent change of the surface condition on mechanical properties. The hardness of heat affected zone (HAZ) was the lowest due to the grain coarsening and annealing. Difference of cross tensile and fatigue strengths was not clearly recognized between Joints A and B. However, one sample in Joint B showed the decrease in cross tensile strength due to the unsound welding. In the cross tensile tests, fracture occurred at HAZ in both Joints A and B, where the hardness was the lowest. In the cross tension fatigue tests, fatigue cracks initiated at HAZ, and propagated through the wire until final fracture.
Ultrasonic Impact treatment (UIT) is an effective method of inducing compressive residual stress at weld toes in order to improve the fatigue strength of welded joints. In this study, compressive residual stresses introduced by UIT and its relaxation behavior under cyclic loading were investigated for different steel grades. The compressive residual stress in UIT grooves showed the maximum value at the bottom, which corresponded to 50 to 60% of the yield strength of base metal. The compressive residual stress was relieved by cyclic loading and it was observed that approximately 90% of residual stress relaxation occurred within 10 cycles. The amount of residual stress relaxation due to tensile cyclic loading was dominated by ratio of the maximum stress to the static strength of the base material. Therefore, it is considered that the combined use of high-strength steel and UIT is effective for improving fatigue properties from the viewpoints of the introduction of higher compressive residual stress and the suppression of compressive residual stress relaxation under cyclic loading. Compressive cyclic loading showed a tendency to relax more than tension cyclic loading. It was considered that compressive stress exceeding the assumed value was applied due to slight bending deformation of the specimen.
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.
The aim of this study is to propose damage model on the basis of the mechanism for ductile fracture related to void growth and applicable to ductile fracture assessment for steels with small initial void volume fraction. In order to determine damage evolution law, void growth behavior in the material was investigated by elasto-plastic finite element analyses using unit cell model with an initial void (f0=0.1%, 0.01% and 0.001%). From the results of unit cell analyses, it was evident that a void in unit cell grew nonlinearly with increasing applied global strain and void growth rate was promoted under higher stress triaxiality condition. In addition, the sudden drop in the stress level coincided with the sudden increase in void volume fraction. Then, in this study, this point was identified as the onset of void coalescence that is ductile crack initiation. It was found that relationships between normalized void volume fraction and normalized strain by each value at the critical point were almost the same and independent of stress-strain relationships of materials and stress triaxiality conditions. Based on this characteristic associated with void growth, damage evolution law showing nonlinear damage accumulation was derived. Then, using the damage evolution law, ductile damage model consisting of critical strain of ductile crack initiation and equivalent plastic strain increment was proposed.
Application of laser-arc hybrid welding on fabricating large steel structures such as ships and bridges are being extended. It is important to select proper welding conditions based on joint types and plate thickness. There are many factors in hybrid welding conditions, for example, heat inputs of laser and arc, welding speed, groove gap in butt joint, angles of laser head and arc torch, laser spot diameter and defocus, flow of shield gas etc. Especially, the heat input and welding speed affect the degree of restraint of joint which causes cold cracking. It is necessary to identify the welding conditions by which prevent the occurrence of cold cracking for extending the application of hybrid welding on fabrication of large steel structures.
In this study, the occurrence of cold cracking in hybrid welding was mechanically evaluated by using a concept of restraint strain. The hybrid welding experiment was performed on different lengths of slit joints by SM490A steel with thickness of 24mm. The welding conditions such as heat input from each heat source and welding speed were variously changed. The relationship between cold cracking occurrence and the average restraint strain calculated by a 3D thermal elastic plastic analysis. Among the welding conditions adopted in this study, the criterion of cold crack occurrence might exist between 1.7% and 6.4% of the average restraint strain. Furthermore, an estimation method for average restraint strain based on the welding conditions was proposed. It might be useful for simply evaluating the occurrence of cold crack without performing the precise thermal elastic plastic analysis.