The effect of phosphorus on solidification characteristics was investigated in austenitic stainless steel. Partition coefficient and liquids temperature slope of phosphorus was obtained by DTA analysis. The model calculations using these properties were in agreement with experimental results in phosphorus segregation profiles. Estimated influence of phosphorus on BTR was in accordance with previous work.
The prevention of sensitization by delta-ferrite in austenitic stainless steel weld metal was investigated from a point of view of the required fraction of delta-ferrite to prevent in weld metal containing not low carbon. Intergranular corrosion test in copper-copper sulfate solution was conducted using the 20%Cr-11/14%Ni-0.045/0.075%C weld metal after sensitizing treatment at 650°C for 3h. The intergranular corrosion was prevented in the weld metal containing sufficiently high fraction of delta ferrite even in case of high carbon content such as 0.06%. A physical model was suggested to determine the delta-ferrite necessary for each carbon content in weld metal. This model was founded on the concept that sufficient fraction of delta ferrite exhausts those carbon atoms in austenitic phase due to consumption by rapid growth of chromium carbide in delta ferrite. It was confirmed the required fraction of delta ferrite to prevent the sensitization in each carbon content agrees with that of experimental results.
It is necessary to identify relationship between the fatigue crack propagation rate and the stress intensity factor range, if the fracture mechanics approach is applied to evaluate the fatigue crack growth evolution.
In addition, identification of the stress intensity factor range of fatigue crack growth threshold (ΔΚth) is important to evaluate the structural integrity from the view of fatigue performance.
ASTM standard E647 "Standard Test Method for Measurement of Fatigue Crack Growth Rates" is widely known as a de fact standard to determine the value of ΔΚth.
However, a considerably long time is required to perform the determination of ΔΚth value.
Therefore, it is expected to cut the experimental period for ΔΚth determination.
Efficient and time saving procedure for determining ΔΚth value comparing with ASTM-E647 is proposed in this study.
Many fatigue crack propagation test data were applied to verify the validity of our proposed method.
It is expected to be able to complete the experiment for determining ΔΚth value at a stage where the fatigue crack propagation rate is one order larger comparing with ASTM-E647.
Two-dimensional digital image correlation (2D-DIC) is a computer-based, non-contacting, surface deformation measurement. Using this method, the strain field was measured in the welded joint under tension. Due to the different mechanical properties, the strains are inhomogeneous in the different parts of the welded joint. Their differences were revealed, and the strain evolution in each part of the welded joint was estimated.
Application of laser-arc hybrid welding on fabricating large steel structures such as ships and bridges are being extended. Selecting proper welding conditions is important based on joint types and plate thicknesses. There are many factors in hybrid welding conditions, for example, heat inputs of laser and arc, welding speed, gap between two welded plates, angles of laser head and arc torch, laser spot diameter and defocus, flow of shield gas etc. From the viewpoint of mechanics, heat input of each heat source and welding speed largely affect the strain, stress and deformation of joint. On the other hand, it is still unknown how the welding conditions affect cold cracking caused by the degree of restraint of joint. It is important to identify the welding conditions by which the cold cracking occur for extending the application of hybrid welding on fabrication of large steel structures. In this study, a mechanical measure for occurrence of cold cracking in hybrid welding was investigated through a series of experiment and numerical simulation. The hybrid welding experiment was performed on 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 mechanical state around the welded part was examined by the 3D thermal elastic plastic analysis. Based on the results, it was proposed that the average restraint strain obtained by the analysis was effective for evaluating occurrence of cold cracking in the hybrid welding. Among the welding conditions adopted in this study, the cold cracking occurred when the average restraint strain was over 5%.
When a non-cylindrical probe tool is used, material adheres to the tool. This paper investigates the influence of this material on the characteristics of friction stir spot welds. This paper focuses on the material flow influenced by the material adhered to the tool. The influence of the material adhered to the tool on the mechanical properties of the friction stir spot lap joints, and on the material flow of the adhered material, is made clear. As the tool rotational speed changes, the morphology of the material adhered to the tool also changes. In a tensile-shear test, compared with the failure load of friction stir spot lap joints made using a tool without adhered material, the failure load of the friction stir spot lap joints made using a tool with adhered material is greater with a tool rotational speed of 3000 rpm, and lower with a tool rotational speed of 1000 rpm. However, the failure load of the friction stir spot lap joints is similar with tool rotational speeds of 1500 and 2250 rpm. The structural profiles are changed by the material flow, influenced by whether or not the material adheres to the tool. Also, the failure mode of the friction stir spot lap joints is different. The following patterns are observed when a tool with adhered material is used. In lower tool rotational speeds, the shear cross section is small because the stir zone is small. Also, in higher tool rotational speeds, the tensile cross section is big because the stir zone does not grow big enough to reach the surface of the upper sheet. However, from the measurement results of thrust force and torque during friction stir spot welding, it is suggested that the performance of the tool geometry does not change even if material adheres to the tool.
Temper bead welding is one of effective repair welding methods in case that post weld heat treatment is not easily applied. In order to evaluate the effectiveness of temper bead welding, hardness in HAZ becomes important factor. The neural network-based hardness prediction system of HAZ in temper bead welding for A533B low-alloy steel has been constructed by the authors in the previous study. However, for HAZ hardness prediction of other steels, it is necessary to obtain hardness database for each steel which is time-cost consuming, if the same method is used. The present study has been conducted to develop the generalized hardness prediction method applicable for other steels by utilizing the hardness data-base of A533B steel assuming that the hardness in HAZ of steels after tempering have a linear relationship with LMP (Larson-Miller parameter). On using the newly proposed extended method, only a few hardness data-base for the other steels is needed to obtain. Hardness distribution in HAZ of temper bead welding for other steels was calculated by using the extended hardness prediction system. The thermal cycles used for calculation were numerically obtained by a finite element method. The experimental results have shown that the predicted hardness is in good accordance with the measured one for steels without secondary hardening. It follows that the currently proposed extended method is effective for estimating the tempering effect during temper bead welding for the steels without secondary hardening.