Kazutoshi Nishimoto

Evaluation of solidification cracking susceptibility in laser welds for type 316FR stainless steel

Laser beam welding (LBW) transverse-Varestraint tests were performed to quantitatively evaluate the solidification cracking susceptibility of laser welds of type 316FR stainless steel with two kinds of filler metal (316FR-A and 316FR-B). This found that as the welding speed increased from 1.67 to 40.0xa0mm/s, the increase in the solidification brittle temperature range (BTR) was greater in the case of 316FR-B (from 14 to 40xa0K) than 316FR-A (from 37 to 46xa0K). Based on theoretical calculations for the temperature range over which both solid and liquid phases coexist, for which Kurz-Giovanola-Trivedi and solidification segregation models were used, the greater increase in BTR with 316FR-B was determined to be due to a larger decrease in δ-ferrite during welding solidification than with 316FR-A. This, in turn, greatly increases the segregation of impurities, which is responsible for the greater temperature range of solid/liquid coexistence when using 316FR-B.

Development of laser beam welding transverse-varestraint test for assessment of solidification cracking susceptibility in laser welds

In order to quantitatively evaluate the solidification cracking susceptibility in laser welds of type 310S stainless steel, a transverse-Varestraint testing system using a laser beam welding apparatus was newly constructed. The timing-synchronization between the laser oscillator, welding robot and hydraulic pressure devices was established by employing high-speed camera observations together with electrical signal control among the three components. Moreover, the yoke-drop time measured by the camera was used to prevent underestimation of the crack length. The laser beam melt-run welding used a variable welding speed from 10.0 to 40.0 mm/s, while the gas tungsten arc welding varied the welding speed from 1.67 to 5.00 mm/s. As the welding speed increased from 1.67 to 40.0mm/s, the solidification brittle temperature range of type 310S stainless steel welds was reduced from 146 to 120 K. It follows that employing the laser beam welding process mitigates the solidification cracking susceptibility for type 310S stainless steel welds.

Prediction of solidification cracking in laser welds of type 310 stainless steels

The occurrence of solidification cracks in laser welds of type 310 stainless steels was predicted by numerical analyses of the solidification brittle range (ductility curve for cracking) and thermal strain in the weld metal. The solidification brittle range in laser welding was estimated from that in arc welding based on the numerical analyses of supercooling (for calculating dendrite tip temperature) and segregation (for calculating completely solidified temperature) during rapid solidification. The calculated solidification brittle range was reduced with an increase in the welding speed because of the enhanced supercooling and the inhibited solidification segregation. The thermal strain analysis by FEM suggested that solidification cracks would occur in SUS310S welds at laser travelling velocity of 60 mm/s applying the initial strain of 1.5%, while no solidification cracks in SUS310EHP welds at any laser travelling velocities applying the higher initial strain of 2.2%. The cantilever type cracking test in laser welding revealed that the predicted results of occurrence of solidification cracks were consistent with experimental ones.

Effect of sodium on repair weldability of SUS316FR for a fast breeder reactor

The effect of sodium on repair weldability of SUS316FR steel under the remaining sodium environment was investigated by transverse-Varestraint and laser cladding tests. Solidification brittle temperature range (BTR) of SUS316FR steel with AF solidification mode was 37 K. However, BTR was expanded from 37 to 67 K, as the amount of surface-adhered sodium increased from 0 to 7.99 mg/cm2. From microstructural observation of the weld metal, there would be a possibility that metallic sodium existed at cell boundaries in the weld metal during welding solidification. According to the thermodynamic calculation, the sodium would expand the solid–liquid coexistence temperature range. It could be concluded that the enhanced solidification cracking susceptibility under the sodium environment would be attributed to the enlargement of the solid–liquid coexistence temperature range. Finally, it was confirmed that any solidification cracks and blowholes did not occur in the overlaid weld metal through multipass laser cladding tests. Namely, it could be confirmed that SUS316FR steel possessed superior repair weldability under the sodium environment.

Microcracking susceptibility in dissimilar multipass welds of Ni-base alloy 690 and low-alloy steel

Microcracking susceptibility in the dissimilar multipass weld metal of alloy 690 and low-alloy steel A533B was evaluated, and the effect of dilution on hot cracking (ductility-dip and liquation cracking) behaviour was investigated. In order to simulate the dissimilar multipass weld metal of alloy 690 to A533B steel, the A533B plate was welded under various dilution ratios using alloy 690 filler metal with different contents of P and S. Several weld metals, which had different alloy compositions at the fixed (P+S) content, were manufactured, and then ductility-dip and liquation cracking susceptibilities of the reheated weld metals were evaluated by the spot-Varestraint test. Ductility-dip cracking susceptibility heightened as the dilution ratio was increased even when the amounts of P and S were fixed. The increased dilution ratio (contamination of Fe into the weld metal) should reduce the tortuous character of the grain boundary (GB) due to inhibiting the constitutional supercooling (the instability of the solidification boundary), as well as enhance the GB embrittlement due to promoting the GB segregation of P and S. Furthermore, the liquation cracking susceptibility slightly heightened with an increase in the dilution ratio at the fixed (P+S) content. The increased liquation cracking susceptibility would be attributed to the enhancement of solidification segregation of P and S with increasing the dilution ratio.

Evaluation of Solidification Cracking Susceptibility in Austenitic Stainless Steel Welds Using Laser Beam Welding Transverse-Varestraint Test

In order to quantitatively evaluate the solidification cracking susceptibility in laser welds of type 310S and type 316L stainless steels, the Varestraint testing system for laser beam welding (LBW transverse-Varestraint test) was newly constructed. The timing-synchronisation among the laser oscillator, welding robot and hydraulic pressure devices was established by employing high-speed camera observation together with electrical signal control among the three components. Moreover, the yoke-drop time measured by high-speed camera was compensated to prevent underestimation of the crack length. The LBW transverse-Varestraint test was conducted varying the welding speed from 10.0 to 40.0 mm/s, and the transverse-Varestraint test with gas tungsten arc welding was also performed varying the welding speed from 1.67 to 5.00 mm/s. As the welding speed increased from 1.67 to 40.0 mm/s, the solidification brittle temperature range (BTR) of type 310S stainless steel welds was reduced from 146 to 120 K, while the BTR enlarged from 36 to 49 K in type 316L stainless steel welds. A numerical simulation of the solid/liquid coexistence temperature range, using solidification segregation model combined with the Kurz–Giovanola–Trivedi model, explained the mechanism of the BTR shrinkage in type 310S stainless steel welds by reduction of the solid/liquid coexistence temperature range of the weld metal due to the inhibited solidification segregation of solute elements and promoted dendrite tip supercooling attributed to rapid solidification in the LBW process. The reason why the BTR enlarged in type 316L stainless steel welds could be clarified by the enhanced solidification segregation of solute elements (mainly P and S), corresponding to the decrement in δ-ferrite content at the solidification completion in the weld metal. It follows that the opposite tendencies on solidification cracking susceptibility with increasing the welding speed in LBW could be explained by the different solidification segregation behaviour of solute elements, closely related with the δ-ferrite content.

Laser brazing of TiAl intermetallic compound using precious brazing filler metals

The applicability of laser brazing technique to bonding of TiAl intermetallic compound was investigated. Five kinds of filler metals such as gold, sliver, palladium and titanium alloys were employed for brazing. Diode laser brazing of TiAl intermetallic compound was carried out at laser power 300–450xa0W, travelling velocity 3.0–5.0xa0mm/s and wire feeding speed 20.0xa0mm/s with shield gas (Ar) at flow rate 15xa0L/min. According to the preliminary investigation of the filler metal selection, the gold-silver-copper filler metal (BAu-12) was selected as the suitable brazing filler metal for TiAl intermetallic compound. The filler metal did not completely penetrate and infiltrate the joint gap at lower heat input conditions, and the centreline cracking as well as serious erosion occurred in the braze metal at higher heat input conditions, while sound joints could be obtained by optimising processing parameters. The centreline cracking in the braze metal would be caused by the formation of brittle compounds attributed to the contamination (erosion) of the base metal into the filler metal. The theoretical approaches to the erosion and wetting/flowing phenomena during laser brazing process were made by the computer simulation. We customised the flow modelling software (FLOW-3D) to enable us to analyse the metal flow problem during laser brazing by coupled with the erosion behaviour. The simulations of the filler metal BAu-12 showed that it wetted/spread the base metals and infiltrated the joint gap with 0.5xa0mm when the laser power was increased. However, it did not completely infiltrate the joint gap when the brazing clearance was 0.3xa0mm. The amount of base metal erosion concurrently increased with an increase in the laser power at any brazing clearances. The computed wetting/flowing and erosion profiles in laser braze joints were fairly consistent with the experimental ones. The joint strength of TiAl intermetallic compound with the filler metal BAu-12 at laser power of 380xa0W attained to approx. 350xa0MPa being higher than 80xa0% of the base metal strength at any brazing clearances between 0.3 and 0.5xa0mm.

Quantitative evaluation of reheat cracking susceptibility by in situ observation and measurement using laser confocal microscope

Abstract 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 cracking sensitivity with high accuracy.

Implementation of the quantitative evaluation method of the tempering effect during heating and cooling processes in post-weld heat treatment

Abstract 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.

Hardness Prediction for Temper Bead Welding of Non-Consistent Layer Technique

Temper bead welding (TBW) is one effective repair welding method for the large-scale nuclear power plants. Consistent Layer (CSL) technique is the theoretically most authoritative method among the five temper bead welding techniques. However in the actual operation, CSL technique is difficult to perform, and non-CSL techniques (Controlled Deposition technique, Half Bead technique, et al) are mainly used in the actual repair process. The thermal cycles in heat affect zone (HAZ) of non-CSL technique are more complicated than that of CSL techniques. Through simplifying the complicated thermal cycles to 4 types of thermal cycles, the neural network-based hardness prediction system for non-CSL techniques has been constructed. The hardness distribution in HAZ of non-CSL techniques was calculated based on the thermal cycles numerically obtained by finite element method (FEM). The predicted hardness was in good accordance with the experimental results. It follows that the thermal cycle simplification methods are effective for estimating the tempering effect during temper bead welding of non-CSL techniques.