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.

Effects of Boron on the Microstructure, Ductility-dip-cracking, and Tensile Properties for NiCrFe-7 Weld Metal

“…Filler metal composition, such as S, P, H, C, Ti, Ta, Mo, La, Ce, and Nb, has a strong effect on DDC formation in austenitic weldments. Segregation of S, P, and H to GBs is a dominant factor for DDC aggravation in Ni-base alloys, whereas Ti, Ta, Mo, La, Ce, and Nb addition to a filler metal reduces the sensitivity of DDC [13,17,20,[27][28][29][30][31] .…”

Method for predicting risk of cracking during weld repair of heat resistant cast steels

“…[7][8][9][10][11] However, in the case of heat resistant cast steels, the low ductility is intrinsically due to their characteristic microstructure, resulting from both the manufacturing process and their composition. These materials are prepared by the sand casting technique and are stabilised using an aging treatment of several hours at y950uC.…”

Microcracking susceptibility in dissimilar multipass welds of alloy 690 to type 316L stainless steel using La added filler metals

“…Cracks occurring in the spot Varestraint testing can be classified into three types; cracks occurring apart from the fusion line (regarded as ductility dip cracking), those occurring adjacent to the fusion line (regarded as liquation cracking) and those occurring in the arc spot (regarded as solidification cracking). 9 Figure 6 shows an example of the effect of the La content in the weld metal on the cracking susceptibilities of ductility dip, liquation and solidification cracks evaluated by the spot and transverse Varestraint tests (base metal: type 316LA stainless steel, dilution ratio: 40% (single pass weld Figure 7 shows the effect of the (PzS) content in the weld metal on the microcracking susceptibility of the reheated weld metal with varying the amounts of impurity elements in the base metal (type 316LA, 316LC, 316LD stainless steels). Cracking susceptibilities of ductility dip, liquation and solidification cracks increase with increasing the (PzS) content in the weld metal when the amount of La in the weld metal is relatively small being less than y0?012 mass-%.…”

“…Analysis of EPMA indicate that La, S and/or P seem to be enriched in the fine products. 9 Cross-sectional view of multipasss weld of type 316L stainless steel using alloy 690 filler metal and distribution of dilution ratio measured in multipass weld metal Products in the La added weld metals were analysed by the electron diffraction and X-ray diffraction analyses. The identified products of La in the weld metals are summarised in Table 2.…”