Search

The hot cracking susceptibility in the Trans-Varestraint test was evaluated using the nominal strain calculated using the curvature radius of a bending block and the thickness of a specimen based on the theory of material mechanics. The nominal strain was calculated using the material properties at room temperature. Thus, in the Trans-Varestraint test, the non-uniformity of the strain around the weld part due to the temperature distribution is not considered. Therefore, the strain in the Trans-Varestraint test cannot be evaluated correctly. The aim of this study is to reveal the loaded strain at the weld metal to understand the evaluation of hot cracking susceptibility in the Trans-Varestraint test. The loaded strain around the trailing edge of the weld pool of pure iron was measured in-situ using a high-speed camera and high-resolution optical lens. The results of strains measured using image analysis and the finite-element method at the center of the weld bead were compared. Accordingly, it was clarified that the strain was concentrated on the weld part owing to the bending occurring along the weld line, and the strain exceeding the nominal strain was loaded to the trailing edge of the weld pool.

During the U-type hot cracking test of gas tungsten arc welding (GTAW), the back side stainless steel was subjected to a continuous water flow (water-cooled GTAW) to prevent solidification cracking...

Stress corrosion cracking (SCC) is one of serious aging degradation problems for the Alloy 600 components of pressurized water reactors (PWRs). In order to prevent SCC, various methods such as water jet peening (WJP), laser peening (LP), surface polishing have been used to introduce compressive stresses at the surfaces of the PWR components. However, it has been reported that such compressive residual stress introduced by these methods might be relaxed during the practical operation, because of high temperature environment. In this study, the hardness reduction behavior of the Alloy 600 processed by LP, Buff and WJP in the thermal aging process has been investigated to estimate the stability of the residual stress improving effect by each method, based on the fact that there is a correlation between the compressive residual stress relaxation and the decrease of hardness. The behavior of the residual stress relaxation in the processed materials in the high temperature environment has been discussed with kinetic analysis.

Computer simulation of the α/γ phase transformation in multipass weld of duplex stainless steel was made for predicting the distribution of the γ phase fraction in the weld metal (WM) and HAZ. The kinetic equations including rate constants of the dissolution behaviour as well as precipitation behaviour of γ phase were determined by isothermal heat treatment test. Based on the kinetic equations determined, the distribution of the γ phase fraction in multipass weld of duplex stainless steel was calculated applying the incremental method combined with the heat conduction analysis in welding process. The γ phase fraction was reduced in the higher temperature HAZ and WM, however, that in the reheated HAZ and WM was increased and recovered to the base metal level. Microstructural analysis revealed that the calculated results of the γ phase fraction in multipass weld were consistent with experimental ones. Based on the computer prediction, the microstructural improvement welding (“reheat bead welding”) process, with analogous concept to the temper bead welding technique, was newly proposed for recovering the γ phase fraction in weld even in the as-welded situation.

The aim of this study is to discuss and model the hot ductility curve during solidification in the welding from the metallurgical point of view. The contents in this study are as follows: the trans...

The effect of welding condition on solidification cracking behaviour was investigated during the U-type hot cracking test. In order to investigate the behaviour of solidification cracking, a high-t...

High Mn austenitic steel is expected to be a next-generation structural material for cryogenic use by maintaining a stable fcc crystal structure even at cryogenic temperatures by adding a large amo...

High-Mn austenitic steel is expected to be a next-generation structural material for cryogenic use by maintaining a stable fcc crystal structure even at cryogenic temperature by adding a large amou...

The microstructure and tensile shear strength of the joint by laser brazing between dissimilar metals of aluminum alloy and galvannealed (GA) steel were investigated. A blowhole was formed by zinc vaporization during laser brazing; the tensile shear strength was significantly affected not by intermetallic compounds (IMCs) at the filler metal/GA steel interface but by the blowhole. The vaporization of zinc was suppressed, and the blowhole was controlled by inserting a Ti interlayer. The joint strength was improved through the suppression of blowhole, the thermal deformation of the Ti interlayer, and the increase in brazed filler metal thickness by optimizing the brazing parameter. The hardness test revealed that fracture occurs preferentially in the brazed filler metal; however, as the brazed filler thickness increases, fracture begins to occur near the heat-affected zone of A5052 because the volume of filler metal increases. The maximum strength of the joint was 185 N/mm, which is about 73% of the joint efficiency to 254 N/mm of the A5052 base material, and the base material partly fractured.

In multi-pass welding of high manganese austenitic steel, weld metal is solidified in austenite single phase and hot cracking may occur depending on residual stress. In order to identify the cracki...

Solidification cracking and liquation cracking susceptibility of four kinds of alloy 617 commercial filler metals were evaluated by Varestraint test, in order to clarify a fundamental effect of eac...

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.

An Au–Ni interlayer was used to improve the joint strength between the Ti-6Al-4V alloy friction welded to the 718 Ni-based alloy. The interlayer was melted and ejected at the interface of the joint...

To clarify the influence of long-term σ phase embrittlement on repair weldability for type 316FR stainless steel weld metals (316FR-AF and 316FR-FA), the prediction of σ-phase embrittlement and a spot-Varestraint test were performed. The precipitation of σ and χ phases were characterized during accelerated aging treatments for both weld metals, and the precipitation behavior (σ+χ phases) could be expressed by a Johnson−Mehl-type kinetic equation. By using the variation of mechanical properties upon precipitation (σ+χ phases) and the determined kinetic equation, σ-phase embrittlement for 316FR-AF and 316FR-FA weld metals during service exposure in FBR plants could be successfully predicted. Specifically, a 30.7% decrement of the absorbed impact energy as compared with the as-welded specimens could be anticipated just after one year of service for 316FR−FA weld metal at 823 K. By using the simulated weld metal with σ-phase embrittlement, the possibility of weld cracking caused by σ-phase embrittlement during repair welding was confirmed, showing micro-crack formation within the σ phase after the spot-Varestraint test.

The two-phase solidification model was constructed considering the solidification mode and morphology. The effects of the solidification mode and morphology as well as δ-ferrite on the solidificati...

Duplex stainless steel (DSS) is vulnerable to changes in the α/γ phase fraction due to the dissolution and precipitation of various compounds during multi–pass welding. Thus, prediction of this dissolution and precipitation behavior is key for the development of effective manufacturing strategies. In this study, the kinetics of the dissolution and precipitation phenomena of the γ phase in Fe-Cr-Ni alloy were investigated using the kinetic constants derived by the phase–field method, and the α/γ phase fraction of the multi-pass welds was theoretically investigated. The kinetics of the dissolution and precipitation phenomena were evaluated theoretically and experimentally through optical and scanning electron microscopy. DSSs with different compositions were evaluated to achieve satisfactory correlation. The temperature dependences of the experimental and calculated values were found to be in good agreement, indicating that it is possible to theoretically predict the dissolution and precipitation phenomenon of the γ phase by using the phase-field method presented herein. With regard to the α/γ phase fraction, there was a remarkably high correlation between the results predicted using the kinetic constant of the experimental value and those predicted using the kinetic constant derived from the phase–field method. Therefore, the kinetics-based theoretical results of this study could serve as a resource for predicting the γ phase amount of multi–pass welds in DSSs.

In order to change the cooling rate of weld pool, in-situ laser post-heating was irradiated behind the weld pool during gas tungsten arc welding in a U-type hot cracking test. The solidification cracking occurrence based on the cooling rate change was observed. The high temperature ductility curve and thermal strain curve were calculated to investigate the relationship between solidification cracking behaviour and cooling rate. The high temperature ductility curve was obtained based on the combination of the critical strain and solidification brittleness temperature range. An in-situ observation technique was used to measure the critical strain. The solidification initiation and completion temperatures were estimated by the metallurgical analysis model. The thermal strain curve was calculated using a finite element simulation method. The high temperature ductility curve did not significantly changed according to the welding conditions. However, the decrease of cooling rate deteriorated yield strength distribution and increased both the stress applied to the weld bead and the slope of thermal strain curve, increasing vulnerability to solidification cracking.

Temper bead welding is one of the effective repair welding methods instead of post weld heat treatment. With the development and popularization of the laser welding, the advanced laser temper bead welding repair technique has been developed recently. However, it is always laborious works to determine the optimum welding condition for laser temper bead welding. Therefore, in the present study, the hardness prediction system for laser temper bead welding has been constructed using a neural network. Thus, the appropriate welding conditions can be selected before the actual repair welding. Firstly, the high cooling rate in heat affect zone (HAZ) of laser temper bead welding has been measured. Secondly, the hardness database are prepared by the experiment. Thirdly, on the basis of experimentally obtained database, the neural network-based hardness prediction system for laser temper bead welding has been constructed. With it, the hardness distribution in HAZ of laser temper bead welding was calculated based on the thermal cycles numerically obtained by FEM. The predicted hardness was in good accordance with the experimental results. It follows that the new prediction system is effective for estimating the tempering effect during laser temper bead welding.

The effect of residual metallic sodium on the solidification cracking susceptibility of type 316FR stainless steel was investigated via transverse-Varestraint tests. And a solidification brittle temperature range (BTR) of type 316FR stainless steel was 37 K. However, the BTR expanded from 37 to 67 K, as the amount of metallic sodium at the specimen surface increased from 0 to 7.99 mg/㎠. Microstructural observation of the weld metal suggested that metallic sodium existed in the weld metal, including in the cell boundaries, during welding solidification. Thermodynamic calculations suggested that sodium expanded the temperature range of solidliquid coexistence during welding solidification of the steel weld metal. Therefore, the increased solidification cracking susceptibility (i.e., expansion of the BTR) in the residual sodium environment was attributed to enhanced segregation of sodium during the welding solidification; this segregation, in turn, resulted in an expanded temperature range of solid-liquid coexistence.

Duplex stainless steel (DSS) is vulnerable to changes in the α/γ volume ratio due to the dissolution of various compounds during multi-pass welding. Thus, prediction of this dissolution behavior is key for the development of effective manufacturing strategies. In this study, the γ phase dissolution behavior of different types of DSSs was predicted using the phase-field method. The kinetics of the DSS dissolution phenomena were evaluated theoretically and experimentally through optical and scanning electron microscopy. DSSs with different compositions were evaluated to achieve satisfactory correlation. The results of the theoretical analysis revealed the relationship between the kinetic constant and the temperature and elucidated the kinetics of the α/γ phase transformation. The relationships were adjusted by determining coefficients to improve the correlation with the experimental results. This corrected calculation was then applied to another lean DSS to confirm the generality of the solution. The experimental and kinetics-based theoretical results of this study could serve as a resource for predicting the γ phase dissolution behavior of DSSs.

In this study, the phase-field method was used to quantitatively predict the precipitation phenomenon of the γ phase in Fe–Cr–Ni ternary alloys. Specifically, a kinetic analysis of the aforementioned phenomenon was performed in duplex stainless steel (DSS) based on theoretical considerations. The calculation-based γ phase precipitation results for lean, standard, and super DSSs were verified through comparisons with experimental data. The possibility of applying the phase-field method established in this study to DSSs with different chemical compositions was also evaluated. The temperature dependences of the experimental and calculated values were found to be in good agreement, indicating that it is possible to theoretically predict the precipitation phenomenon of the γ phase using the proposed phase-field method. Moreover, the proposed model achieved satisfactory kinetic results regardless of the type of steel assessed.

The microstructure and mechanical properties of a joint produced by laser brazing between A5052 and AZ31 with AZ61, AZ91 and AZ125 filler metal was investigated. The effects of filler metals on joint characteristics are also discussed. Measurement of microstructural factors in the laser brazed joint revealed that increasing the laser power results in a decrease in the weld toe angle and an increase in the bead width, which indicates superior wettability. A high strength laser brazed joint can be achieved through the combination of good wettability and a thin intermetallic layer produced by a laser power of 590 W in a brazed joint with AZ125 filler metal Any further increase in power, however, results in a rapid increase in the thickness of the intermetallic compound (IMC) reaction layer. The superiority of the brazed joint with AZ125 filler metal is due to its lower melting point than that of AZ61 and AZ91 filler metal.

In the post-weld heat treatment process, the reheat cracking which might occur in the weldments of low-alloy steels has been a serious problem. So, it is considered to be important to predict the possibility of occurrence of reheat cracking in these steels. It is however recognized as a time-consuming procedure to evaluate quantitatively the susceptibility to this type of cracking. In the present study, a new quantitative evaluation method of reheat cracking susceptibility by in situ observation and measurement using a laser confocal microscope has been proposed. Through this new method, the reheat cracking susceptibility of any kind of steels can be evaluated with the same standard. Moreover, because the position of the initial crack can be focused and the critical ductility to initiate the crack is measured by in situ observation, the reheat cracking susceptibility can be evaluated using only one specimen. So the newly developed method can provide efficient quantitative assessment of the reheat ...

In order to evaluate the temper effect during the multiple post-weld heat treatment (PWHT) process, the thermal cycle tempering parameter (TCTP) proposed by the authors, which is derived by extending the Larson–Miller parameter (LMP) to a non-isothermal process, has been used to evaluate quantitatively the temper effect during heating and cooling processes in the process of PWHT. It has been clarified that the temper effect in the multiple PWHT process including long-time heating and cooling processes can be quantitatively evaluated by TCTP. On the basis of the present study, it might be possible to shorten the holding time of the heat treatment for multiple PWHT.