Tadafumi Hashimoto

Effect of solidification velocity on weld solidification process of alloy tool steel

Abstract In order to clarify the effect of solidification velocity on the weld solidification process of alloy tool steel during the welding, the information about microstructure evolution was obtained by the concurrent experiments of liquid tin quenching and time resolved X-ray diffraction technique using intense synchrotron radiation. It was found from the experiments that the solidification mode was transferred from an FA to an A mode at the high solidification velocity. The effect of solidification velocity on the phase selection during solidification between the primary δ-ferrite and γ-austenite was theoretically proved by the Kurz, Giovanola and Trivedi (KGT) model. It is thus explained that the solidification cracking susceptibility of the weld metal of alloy tool steel was enhanced due to the δ to γ transition of the primary phase.

Long-Term Stability of Residual Stress Improvement by Water Jet Peening Considering Working Processes

To prevent primary water stress corrosion cracking (PWSCC), water jet peening (WJP) has been used on the welds of Ni-based alloys in pressurized water reactors (PWRs). Before WJP, the welds are machined and buffed in order to conduct a penetrant test (PT) to verify the weld qualities to access, and microstructure evolution takes place in the target area due to the severe plastic deformation. The compressive residual stresses induced by WJP might be unstable under elevated temperatures because of the high dislocation density in the compressive stress layer. Therefore, the stability of the compressive residual stresses caused by WJP was investigated during long-term operation by considering the microstructure evolution due to the working processes. The following conclusions were made: The compressive residual stresses were slightly relaxed in the surface layers of the thermally aged specimens. There were no differences in the magnitude of the relaxation based on temperature or time. The compressive residual stresses induced by WJP were confirmed to remain stable under elevated temperatures. The stress relaxation at the surface followed the Johnson-Mehl equation, which states that stress relaxation can occur due to the recovery of severe plastic strain, since the estimated activation energy agrees very well with the self-diffusion energy for Ni. By utilizing the additivity rule, it was indicated that stress relaxation due to recovery is completed during the startup process. It was proposed that the long-term stability of WJP under elevated temperatures must be assessed based on compressive stresses with respect to the yield stress. Thermal elastic-plastic creep analysis was performed to predict the effect of creep strain. After 100 yr of simulated continuous operation at 80% capacity, there was little change in the WJP compressive stresses under an actual operating temperature of 623 K. Therefore, the long-term stability of WJP during actual operation was analytically predicted.

Accuracy improvement of X-ray residual stress measurement in welds of Ni based alloy by two-dimensional detector with multiaxial rocking

Abstract In materials such as Ni based alloys, the microstructures are formed by monophase solidification without solid state transformation. It is difficult to obtain the welding residual stress by X-ray diffraction because of the preferred orientation of the unidirectional solidification and the grain growth in the heat affected zone. To exclude their effect, a method that records the diffraction peaks in a two-dimensional detector combined with multiaxial rocking is proposed. It is clarified that the equilibrium of shrinkage and the recovery of strain during the thermal cycle determines the site of the maximum tensile stress. In addition, dynamic recrystallisation, which occurs during welding, contributes to the decreased residual stress at the fusion line. If the spatial resolution of the proposed and the conventional measurement methods can be correlated, the results of each method agree well. Therefore, the proposed method is an effective tool for measuring the residual stress in welded joints of Ni based alloy.

Effect of electrode force condition on nugget diameter and residual stress in resistance spot welded high-strength steel sheets

This study examines the effect of the electrode force condition on the nugget diameter and residual stress in spot welded high-strength steel sheets. Numerical simulations of spot welding were performed to examine the nugget diameter and residual stress. The results indicate that adjusting the force profile changes the current density and stress state at the spot welds. Therefore, choosing an appropriate force profile extends the nugget diameter and reduces the residual stress.

Comparison of weld residual stress measurement results in low alloy welds between X-ray diffraction and stress relief methods

In this study, a comparison of weld residual stress measurements between the X-ray diffraction technique and the stress relief technique is performed with the focus on the effect of the thickness on the stress measurement results. As the difference in the thickness becomes smaller, the difference in the stress measurement results also becomes smaller. At the weld center, where the stress gradient is in the thickness direction, the difference in the thickness is not negligible. In other words, it is concluded that the X-ray diffraction method is advantageous because it can evaluate the unique stress generated only in the surface layer.

Evaluation of residual stress distribution in Ni base alloy clad welds by numerical simulation and X-ray stress measurement

Abstract Dissimilar material components such as clad welds are important to achieve both performance, and economic and ecological efficiency. The evaluation of the residual stress distribution in such components is needed to assess and assure the integrity of structures components. In this study, the residual stress distribution in clad welds of Ni base alloy over low alloy steel was investigated. A clad welded mock-up was fabricated, and the residual stress distribution was evaluated by X-ray stress measurement method and finite element simulation. The residual stress was measured with high accuracy even in coarsened Ni base alloy clad weld metal by X-ray stress measurement method utilising a two-dimensional detector and multiaxial rocking technique. The residual stress distribution was well reproduced by the finite element simulation considering the redistribution due to cutting off of the specimen from the fabricated mock-up.

Long Term Stability of Compressive Residual Stress Introduced in Alloy 600 by Water Jet Peening Under Elevated Temperature Environment

Primary water stress corrosion cracking (PWSCC) in the weld metal of alloy600 is an issue of concern in a pressurized water reactor (PWR). As a countermeasure against PWSCC, water jet peening (WJP), which can change tensile residual stress into compressive residual stress, has been applied to welded joints. Microstructure in the target area of WJP has an influence of not only WJP but welding and machining. Especially machining introduces severe plastic deformations to the materials. So microstructure in the target area might lack thermal stability due to severe plastic deformation. Additionally the region that compressive residual stress by WJP is nearly up to 1mm from the surface of the target material. As PWRs are operated at about 596K for long term, the compressive residual stress by WJP may be relieved due to creep. In order to keep operating PWRs safety, the stability of the compressive residual stress by WJP at elevated temperature has been clarified. In this work, the results were obtained written below. As a result of thermal aging test, a relaxation of compressive residual stress at specimen surface layer occurred due to recovery of the plastic deformations by machining. This stress relaxation behavior followed Johnson-Mehl equation. However residual stress relaxation due to creep was very few. Therefore it has suggested that the compressive residual stress introduced in Alloy600 by WJP is confirmed to remain stable during long term operation under elevated temperature.Copyright

Residual stress variation due to piping processes of austenitic stainless steel

In nuclear power plants, stress corrosion cracking (SCC) has been observed near the heat affected zone (HAZ) of the primary loop recirculation pipes made of austenitic stainless steel type 316L. Residual stress is a major cause of SCC. In the joining process of pipes, butt-welding is conducted after machining. Machining is performed to match the inside pipe diameter. Residual stress is generated by both machining and welding. In the case of welding after machining in manufacturing processes of pipes, it appears that residual stress due to machining is varied by the welding thermal cycle. In this study, residual stress variation caused by manufacturing processes was investigated. Residual stress variation was examined by the X-ray diffraction method. The residual stress distribution generated by welding after machining has a local maximum point in the HAZ. The Vickers hardness distribution also has a local maximum point. By the EBSD method, it is clarified that recovery and recrystallization due to welding heat do not occurred in the local maximum point. Residual stress distribution results from the superposition effect of hardening due to machining and welding. The location and value of the local maximum stress are varied by welding conditions. The region of the local maximum stress corresponds to the region where SCC has been observed. Therefore, in addition to a part of the manufacturing processes such as welding or machining, evaluation of all parts of the processes is important to investigate the effect of residual stress distribution on SCC.

Residual Stress by X-Ray Diffraction and Microstructure for Multi-Pass Girth Welded Pipe Joint in Austenitic Stainless Steel Type 316L

Residual stress due to welding can result in brittle fracture, fatigue failure, and stress corrosion cracking in welded structures. Measuring residual stresses are of great importance, if crack propagation needs to be evaluated. However, it is especially known that the X-ray diffraction method makes remarkable different for austenitic stainless steel, because the microstructures in welds change from the original microstructures during welding thermal cycle. That is, there are the preferred orientation due to the unidirectional solidification and the grain growth in the heat-affected zone. In order to average the sin2 Ψ plots to exclude them, Ψ oscillation of ±3 deg was performed and the incident beam size was broadened to 4 by 4 mm. Consequently, typical residual stress distributions due to welding were obtained to various conditions. The residual stress distribution measured by X-ray diffraction agrees very well with that the estimated by thermal-elastic-plastic analysis, if the spatial resolution is correlated. It is attributed that the δ-ferrite grows as the primary phase and the austenite precipitates or crystallizes as the secondary phase. When the secondary austenite nucleates with the Kurdjiumov-Sachs relationship which satisfy δ{110}//γ{111} and δ //γ , plate-like austenite grows randomly into the ferrite and austenite grains are braked up. That is, Specific systems in austenitic stainless steels should be classified, as a material that residual stress can be measured accurately by X-ray diffraction.© 2011 ASME

Effect of tip velocity on weld solidification process of hot-work tool steel

In order to clarify the effect of tip velocity on the weld solidification process of hot-work tool steel (SKD61) during welding, information about microstructure evolution was obtained by the combination of a liquid tin quenching and time resolved X-ray diffraction technique using intense synchrotron radiation. From the experimental results, it was found that the solidification mode was changed from FA mode (L → L+δ → L+δ+γ → L+γ → γ) to A mode (L → L+γ → γ) at high tip velocity. Moreover, the effect of tip velocity on the microstructure selection during solidification between the primary δ, ferrite and the primary γ, austenite was theoretically proven by the Kurz, Giovanola and Trivedi model. Therefore, it was understood that the solidification cracking susceptibility of hot-work tool steel (SKD61) weld metal was increased due to the δ to γ transition of the primary phase.