Kazuyoshi Saida

Microcracking in multipass weld metal of alloy 690 Part 2 – Microcracking mechanism in reheated weld metal

Abstract To elucidate the microcracking (ductility dip cracking) mechanism in the multipass weld metal of alloy 690, the hot ductility of the reheated weld metal was evaluated using three different filler metals with varying contents of impurity elements such as P and S. Hot ductility of the weld metal decreased at temperatures over 1400 K, and the weld metal containing a low quantity of impurity elements showed much higher ductility than that containing a high quantity of impurity elements. Local deformability at high temperature of the alloy 690 reheated weld metal was compared with that of Invar alloy. Grain boundary sliding in alloy 690 occurred not in the intermediate temperature range (800–1000 K), where grain boundary sliding was activated in Invar alloy, but at high temperatures just below the melting temperature of alloy 690. The computer simulation of microsegregation suggested that the deterioration of hot ductility is caused by the grain boundary segregation of impurity elements during the multiple thermal cycling. The ductility dip cracking in the reheated weld metal resulted predominantly from the embrittlement of grain boundaries due to the imbalance between intergranular strength and intragranular strength at high temperature.

Diode laser brazing of aluminium alloy to steels with aluminium filler metal

Abstract Diode laser brazing of aluminium alloy (A5052) to interstitial free steel (IF steel) or type 304 stainless steel (SUS304) was conducted using aluminium filler metal (BA4047) with Nocolock flux. The processing parameters of laser power, wire feed rate and travel speed were varied. The strength of lap joints of A5052 on steels was evaluated by tensile shear test. The joint strength of A5052/steels was increased with increasing laser power and reached the maximum strength, more than approximately 80% of the A5052 base metal strength, at a laser power of 1300 W. Voids and incomplete penetration of filler metal were observed at the A5052/braze layer interface when the laser power was below 1100 W. The Fe–Al intermetallic compounds were formed at the steel/braze layer interfaces and grew drastically when the laser power exceeded 1300 W. Superior brazability of A5052/steels was found at brazing conditions corresponding to a temperature of filler metal droplet of 1050–1250 K.

Microcracking in multipass weld metal of alloy 690 Part 3 – Prevention of microcracking in reheated weld metal by addition of La to filler metal

Abstract The effect of addition of La to a filler metal on microcracking (ductility dip cracking) in the multipass weld metal of alloy 690 was investigated with the aim of improving its microcracking susceptibility. The susceptibility to ductility dip cracking in the reheated weld metal could be greatly improved by adding 0·01–0·02 wt-%La to the weld metal. Conversely, excessive La addition to the weld metal led to liquation and solidification cracking in the weld metal. Hot ductility of the weld metal at the cracking temperature was greatly improved by adding 0·01–0·02 wt-%La to the weld metal, implying that the ductility dip cracking susceptibility was decreased as a result of the desegregation of impurity elements of P and S to grain boundaries due to the scavenging effect of La. The liquation and solidification cracking resulting from excessive addition of La to the weld metal is attributed to the formation of liquefiable Ni–La intermetallic compound. A multipass welding test confirmed that microcracks in the multipass weldment were completely prevented by using a filler metal containing an addition of 0·01 wt-%La.

Influences of phosphorus and sulphur on ductility dip cracking susceptibility in multipass weld metal of alloy 690

Abstract The influence of P and S on ductility dip cracking susceptibility in the reheated weld metal of alloy 690 was evaluated by the spot Varestraint test using different alloy 690 filler metals, while varying the contents of P and S. The ductility dip cracking susceptibility was reduced with a decrease in the content of P and S in the filler metal; the amount of (P+1·2S) in the weld metal should be limited to 30 ppm in order to prevent microcracking in the multipass weld metal. A numerical simulation of cosegregation behaviour of P and S revealed that both elements were segregated at the grain boundary in the ductility dip temperature range during multipass welding. A molecular orbital analysis has suggested that ductility dip cracking can be attributed to grain boundary embrittlement due to grain boundary segregation of P and S.

Fluxless laser brazing of aluminium alloy to galvanized steel using a tandem beam – dissimilar laser brazing of aluminium alloy and steels

Tandem beam brazing with aluminium filler metal (BA4047) was conducted in order to develop the fluxless laser brazing technique of aluminium alloy (AA6022) to galvanized steels (GA and GI steels). Laser powers of tandem beam and offset distance of preheating beam from the root to the steel base metal were varied. Sound braze beads could be obtained by optimizing the preheating and main beam powers under the offset distances of 0–1 mm. A small amount of zinc remained at the braze interface between galvanized steels and the braze metal. The reaction layer consisting of Fe–Al intermetallic compounds was also formed at the steel interface, and the thickness of reaction layer could be predicted during the laser brazing (thermal cycle) process based on the growth kinetics with the additivity rule. The metal flow analysis of the melted filler metal on joints revealed that wettability and spreadability of the filler metal on the GI steel joint were superior to those on the GA steel joint. The fracture strength of the lap joint attained approx. 55–75% of the base metal strength of aluminium alloy. It was concluded that fluxless laser brazing could be successfully performed by using a tandem beam because the zinc coat layer acted as the brazing flux.

Hot cracking behaviour and susceptibility of extra high purity type 310 stainless steels

Abstract Recent progress in the refining technology has enabled the production of highly pure commercial stainless steels. The hot cracking behaviour of these stainless steels was investigated with respect to type 310 stainless steel. For comparison, four types of 310 stainless steels with various amounts of minor and impurity elements such as C, P and S were used. The purity of type 310 stainless steels used was enhanced in the order of type 310<type 310S<type 310ULC<type 310EHP steels. The hot cracking susceptibility was evaluated by the transverse Varestraint test. Two types of hot cracks occurred in these steels by Varestraint test; solidification and ductility-dip cracks. The solidification cracking susceptibility was significantly reduced as the amount of C, P and S decreased, and that in type 310EHP steel reached a level so low that solidification cracking did not occur in practical welding. On the other hand, the ductility-dip cracking susceptibility adversely increased as the purity of the steels was enhanced. However, the ductility-dip cracking susceptibility of type 310EHP steel was sufficiently low as not to yield ductility-dip cracking in practical multipass welding. The experimental and numerical analyses on the solidification brittle temperature range have revealed that the reduced solidification cracking susceptibility upon decreasing the amount of C, P and S in stainless steel can be attributed to the reduced BTR due to the suppression of minor and impurity elements, such as C, P and S, in the finally solidified liquid film between the dendrite.

Prevention of microcracking in dissimilar multipass welds of alloy 690 to type 316L stainless steel by Ce addition to filler metal

Abstract The microcracking susceptibility in dissimilar multipass weld metals was investigated by a multipass weld test using different type 316L stainless steels with varying P and S contents and using different alloy 690 filler metals with varying Ce contents. The relation between microcracking susceptibility and (P+S) and Ce contents in every weld pass of the multipass weld was investigated. Ductility dip cracks occurred in the compositional range of Ce/(P+S)<0·22, and solidification/liquation cracks occurred in that of Ce/(P+S)>1·1, while no cracks occurred at Ce/(P+S) between 0·22 and 1·1. The ductility dip cracking susceptibility could be improved by adding Ce due to scavenging of impurity elements. Microcracking could be completely prevented in dissimilar multipass weld metals using two kinds of filler metals containing 0·077 wt-%Ce for the weld passes beside the stainless steel base metal (320 ppm P and 183 ppm S) and containing 0·032 wt-%Ce for the other weld passes.

Quantitative influence of minor and impurity elements on solidification cracking susceptibility of extra high purity type 310 stainless steel

Abstract To evaluate the influence of minor and impurity elements on the solidification cracking susceptibility in austenitic stainless steel welds, the transverse Varestraint test was conducted using several extra high purity type 310 stainless steels with different amounts of C, Mn, P and S. The characteristic influence on solidification cracking could be expressed by the ratio of P/S/C≈1∶1·3∶0·56, while Mn affected solidification cracking to a negligible extent. A theoretical approach revealed that the reduced solidification cracking susceptibility with decreasing C, P and S contents could be attributed to the suppression of solidification segregation.

Prediction of Sigma Phase Precipitation in Super Duplex Stainless Steel Weldments

In order to predict the sigma (σ) phase embrittlement in multipass weldment of duplex stainless steels, the precipitation behaviours of the σ phase in the base metal and the weld metal were investigated using super duplex stainless steels (SAF2507 and DP3W) and a conventional duplex stainless steel (NAS64). In all the steels, the amount of the σ phase precipitated during an isothermal ageing process sigmoidally increased with the length of the ageing time. The kinetics of σ phase precipitation in the base metal and the weld metal could be expressed approximately by the Johnson-Mehl type equation, however, the initiating time of σ phase precipitation in the weld metal was longer than that in the base metal. The precipitation of the σ phase in the DP3W base metal was much delayed and depressed compared with the SAF2507 and NAS64 base metals. Increasing the amount of the σ phase drastically decreased the absorbed impact energies of aged steels. They fell to as low as 1/4 of the unaged values when the amount of the σ phase exceeded 5%. The precipitation of the σ phase in duplex stainless steels during multiple thermal cycle process were predicted using the additivity rule. We predicted that the largest amount of the σ phase and the most significant degradation of impact toughness would occur in the base metal region (HAZ) for peak temperatures of 1 100–1 300 K in multiple thermal cycle.

Laser brazing of alloy 600 with precious filler metals

Abstract The diode laser brazing of Ni base heat resistant alloy with precious filler metals has been conducted using the tandem beam for preheating and brazing. A couple of 1 mm thick plates of alloy 600 (Inconel 600) were butt brazed using Au–18Ni, Ag–10Pd and Ag–21Cu–25Pd filler metals of 0·5 mm diameter with a brazing flux. Sound butt joints which were free from brazing defects such as porosity and lack of penetration could be obtained at brazing clearances of 0·1–1·5 mm. The tensile strength of the braze joint produced using Ag–Pd filler metal increased with decreasing brazing clearance and reached ∼70% of the base metal strength at a brazing clearance of 0·1 mm while those obtained by using Au–Ni and Ag–Cu–Pd filler metals were comparable with the base metal strength at any clearances between 0·1 and 1·5 mm. The laser brazing technique could be successfully applied to the brazing of Ni base superalloy to attain a joint with high performance and reliability.