TWI Technology Briefing 622 - September 1997
P Woollin
FULL REPORT
Trials using the TIG process indicate that high nitrogen superaustenitic stainless steels can be welded with overalloyed filler addition to give pitting resistance approaching parent steel levels. For the highest molybdenum grades, control of arc energy is essential to minimise intermetallic phase formation.
Background
Austenitic stainless steels have excellent toughness, ductility and corrosion resistance. The highest alloy grades currently available, sometimes referred to as superaustenitics, contain up to 7-8% molybdenum and 0.5% nitrogen. Production of mechanically sound joints in the superaustenitic alloys is straightforward but obtaining a weld area corrosion resistance matching that of the parent steel may be problematic. Causes of poor corrosion resistance include
- molybdenum segregation in weld metal and fusion boundary unmixed zones,
- intermetallic phase formation, and
- loss of nitrogen from the weld pool.
Previous work has shown that the established 6% molybdenum superaustenitic grades typically require use of overalloyed filler to obtain acceptable corrosion resistance. However, work at TWI has shown that more recently developed grades with 4.5 and 7.5% molybdenum and 0.4-0.5% nitrogen may be welded autogenously at low arc energy to give acceptable corrosion performance.
The present work was undertaken to examine the corrosion resistance of TIG weldments made with overalloyed filler in two of the recently developed superaustenitic stainless steels with 0.4-0.5% nitrogen, ie UNS S32654 (7.5%Mo, 0.5%N) and S34565 (4.5%Mo, 0.4%N).
Objective
- To quantify and optimise the corrosion resistance of weldments in new high nitrogen superaustenitic stainless steels made using the TIG process with addition of overalloyed nickel-based filler metal.
Experimental approach
Two superaustenitic steels were used meeting UNS S32654 and S34565, both being in sheet form with thickness of 3mm. A series of mechanised TIG bead-on-plate runs was performed and square butt joints were produced. Nickel base filler to AWS type ER NiCrMo-4, which is overalloyed in Mo with respect to the two parent steels, was used throughout. Arc energies in the range 0.3-1 kJ/mm were used to give varying cooling rates, together with two shielding gases, namely (i) Ar and (ii) Ar+20%He+2.25%N.
Metallographic sections were taken through each of the melt runs and joints for examination under an optical microscope. The extent of weld metal dilution by the parent steel was estimated from the weld profile. Coupons were removed from all melt runs and joints for pitting corrosion testing in ferric chloride solution, following a method based on ASTM G48. The samples were exposed to the test solution at progressively higher temperatures until pitting attack was identified visually or by weight loss measurements, to define a critical pitting temperature (CPT). In addition, chemical analyses were performed on all-weld metal samples.
Results and discussion
For both steels examined, the critical pitting temperatures obtained were in a similar range to those previously obtained for autogenous welds produced at comparable arc energies. In addition, the highest CPTs obtained approached those of the parent steels examined for both weld types. For the S32654 (7.5%Mo, 0.5%N) steel, there was a trend for decreasing CPT with increasing arc energy which was apparently accompanied by increasing levels of intermetallic precipitation both in weld metal and HAZ. The S34565 (4.5%Mo, 0.4%N) steel did not show this trend of decreasing CPT in the range examined and it was less susceptible to intermetallic formation than S32654.
No systematic difference was noted between the use of the two shielding gases in terms of critical pitting temperature of the final weldment. A range of dilution from 28-88% was obtained in the weldments. However, little trend with dilution was noted, although highest CPT levels were associated with lowest dilution.
Conclusions
- Use of overalloyed 15%Mo nickel-based filler gave TIG weldments in S32654 (7.5%Mo, 0.5%N) and S34565 (4.5%Mo, 0.4%N) steels with pitting corrosion resistances approaching those of the parent steels at 0.3 kJ/mm and within 20°Cof the parent material at 0.7-1 kJ/mm.
- Use of overalloyed 15%Mo nickel-based filler addition for S32654 and S34565 steels gave critical pitting temperatures which were generally higher than for autogenous TIG welds. However, the difference between the two weld types wasless pronounced than for the established 6%Mo stainless steels.
Recommendations
- TIG welding with overalloyed nickel-based filler is recommended when joining the recently developed high nitrogen superaustenitic stainless steels.
- Autogenous TIG or laser welding may be considered also when joining the recently developed high nitrogen superaustenitic stainless steels, as they give pitting resistance approaching that of welds made with filler.
- TIG welding of superaustenitic stainless steels with or without filler addition should employ low arc energy, especially for the higher molybdenum steels: low arc energy is preferred to limit both weld metal segregation and HAZintermetallic precipitation.
Member Report No. 622-1997
TIG welding high nitrogen superaustenitic stainless steels with filler addition
