Corrosion of antibacterial Cu-bearing 316L stainless steels in the presence of sulfate reducing bacteria

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 mult...

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

In this second of two papers, the microstructural and microchemical evolution of Alloy 600 (A600), Alloy 690 (A690), Filler Metal 82H (FM82H), and Filler Metal 52 (FM52) during the weld thermal cycle was investigated and compared to the hot ductility data presented in the first paper (Ref. 1). The Gleeble® hot ductility test was used to subject these four alloys to a simulated weld thermal cycle. Water quenching was conducted at select temperatures so that the elevated temperature microstructure could be subsequently characterized. Microstructural and microchemical characterization was carried out using scanning electron microscopy, transmission electron microscopy, and analytical electron microscopy techniques. Complete dissolution of intergranular carbides was observed in A690 and FM52 at ∼2400°F (1316°C), both of which exhibit an on-cooling ductility minimum at 1600°F (871°C). Of all four alloys, the greatest resistance to ductility dip cracking (DDC) was observed in A600 and A690 during on-heating, which had coarse, homogenously distributed intergranular carbides. FM82H, which formed NbC intergranular carbides, had the most stable intergranular microstructure and serrated grain boundaries, which corresponded to the best overall DDC resistance. Modifications to the thermal cycle that resulted in increased intergranular carbide coverage in FM82H and FM52 also reduced DDC susceptibility. AEM analysis did not reveal any sulfur or phosphorous segregation in FM52 at 1600°F (871°C) on-heating, on-cooling, or after a 60s hold. Samples with microstructures that consisted of coarsened carbides and/or serrated grain boundaries, which are expected to decrease grain boundary sliding, were found to be resistant to DDC. Based on the results of this work and the results previously presented in Paper I (Ref. 1), grain boundary sliding contributes to DDC. Conversely, sulfur and phosphorous embrittlement do not play a role in DDC of FM52 at the concentrations investigated. The dynamic precipitation of partially coherent intergranular M 23 C 6 carbides at intermediate temperatures may exacerbate DDC in A690 and FM52, but requires further investigation.

Metallurgical investigation into ductility dip cracking in Ni based alloys

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 ...

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

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

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

Microcracking behaviour in the gas tungsten arc multipass weld metal of alloy 690 was investigated. The majority of microcracks occurred within about 300 μm from the fusion line of the subsequent weld bead and propagated along the solidification boundaries in the multipass weld metal. The morphology of the crack surface indicated the characteristic texture of ductility dip cracking. The microcracking susceptibility of the reheated weld metal was evaluated via the spot Varestraint test using three different filler metals having varying contents of impurity elements such as P and S. Microcracking occurring in the spot Varestraint tests consisted predominantly of ductility dip cracking, with a small amount of liquation cracking. The ductility dip cracking temperature range was about 1350–1600 K in the weld metal FF1, and narrowed in the order of weld metals FF1>FF3>FF5. The ductility dip cracking susceptibility was reduced with decreasing contents of impurity elements in the filler metal. It was conc...

Microcracking in multipass weld metal of alloy 690 Part 1 - Microcracking susceptibility in reheated weld metal

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...

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

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) 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.

H. Okauchi

Papers

Microcracking in multipass weld metal of alloy 690 Part 1 - Microcracking susceptibility in reheated weld metal

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

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

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