Masters Theses

Wei Liu

Date of Award

8-1996

Degree Type

Thesis

Degree Name

Master of Science

Major

Metallurgical Engineering

Major Professor

Carl D. Lundin

Committee Members

Charlie R. Brooks, Raymond A. Buchanan

Abstract

The nature and the effect of the unmixed zone in dissimilar fusion welds, especially its significance with regard to corrosion resistance, has been studied in this Welding Research Council sponsored program. Three types of super-austenitic stainless steel (high Mo), (AL-6XN, 254SMO and 654SMO) and five types of fully austenitic nickel based weld filler metals (Inconel 625, C-22, C- 276, 686CPT and P16) were evaluated. Shielded Metal Arc (SMA), Gas Metal Arc (GMA), Gas Tungsten Arc (GTA) (filler added) and Gas Tungsten Arc (GTA) autogenous welding processes were utilized for fabricating weldments of each super-austenitic stainless steel. The employment of these welding techniques provided a full range of unmixed zone formation. The unmixed zone formation was metallographically evaluated using both Optical Light Microscopy (OLM) and Scanning Electron Microscopy (SEM) with varied etchants and etching techniques. It was found that an unmixed zone, with the identical chemical composition of the base metal, is distributed along and adjacent to the fusion boundary in the dissimilar welds. The distribution (width) of the unmixed zone along the weld interface is widest near the root of the SMA Bead-on-plate (BOP) welds and near the mid-plane of the sheet for GTA and GMA open root welds. The formation of the unmixed zone is dependent on a number of factors which are highly reflective of the welding process, welding heat input and the physical properties of filler metal, such as melting range. The unmixed zone in GTA welds is wider than that in GMA welds for the same welding heat input. A wider unmixed zone has also been observed for welds fabricated using a filler metal with a higher melting range than that with a lower melting range. Depletion of Mo in the core of the cellular dendrites in the unmixed zone has been clearly defined by energy dispersive spectroscopy (EDS) analysis. It is believed that depletion of Mo in the core of the cellular dendrites in the unmixed zone is the major reason for reduced localized corrosion resistance in the unmixed zone. However, upon proper solution heat treatment, the extent of Mo segregation is reduced and the localized corrosion resistance of the unmixed zone is improved. The pitting corrosion behavior of the unmixed zone was evaluated using immersion corrosion tests in 10% FeCl3. For GTA BOP welds, the critical pitting temperature (CRT) of the unmixed zone is lower than that of respective base metal. However, GTA autogenous BOP welds show that the CRT of the fusion zone is lower than that of the unmixed zone (the entire fusion zone of the autogenous weld is an unmixed zone). These experiments indicate that the unmixed zone has an inferior pitting corrosion resistance compared to the base metal. The pitting corrosion behavior of the unmixed zone was confirmed using cyclic polarization scanning tests. The pitting corrosion resistance of the unmixed zone was evaluated based on measurement of the hysteresis loop area and passive current density in comparison with the respective base metal. The results obtained from the cyclic polarization scanning tests are consistent with that obtained from the immersion corrosion tests. Pit initiation and propagation behavior was investigated for AL-6XN SMA BOP welds (C-22 filler metal) in 10% FeCI3 at the unmixed zone OPT of 80°C as a function of time. Pits initiated at the core of the cellular dendrites in the unmixed zone and a "cave propagation" behavior beneath the surface was observed. Upon an increased extent of "cave propagation" beneath the surface, corrosion breaks through to the surface at the other locations due to continuous corrosion along the core of the cellular dendrites. In addition, pit propagation is always oriented vertically downward, independent of the weld or base metal orientation. The crevice corrosion behavior of the unmixed zone was evaluated for GTA filler added open root welds. Test results showed that a deeper and wider corroded area is characteristic of the unmixed zone compared to the HAZ and base metal. These results define that the unmixed zone has a lower crevice corrosion resistance than that of the base metal and HAZ. This is the case for all the three super-austenitic stainless steels. It was found that surface condition has a significant influence on the pitting and crevice corrosion behavior of the unmixed zone. Greater surface roughness exacerbates pitting and crevice corrosion in the unmixed zone because a greater surface roughness creates micro-crevices and a higher surface energy condition, which can cause instability of the passive film on the specimen surface. Surface oxidation (i.e. HAZ tinting) also exacerbates the localized corrosion. It is believed that there are some physical defects, such as pores and/or cracks in the oxide film of the heat tinted zone, which can trigger and facilitate the localized corrosion in the heat tinted zone. Reducing surface roughness before welding by grinding (600 grit) and removal of heat tint after welding by pickling causes a significant reduction in the corrosion sensitivity of the unmixed zone. Solution heat treatments at 2100°F, 2150°F and 2200°F for 30 minutes were performed on all weldment types to define the effect of solution treatment on the localized corrosion behavior in the unmixed zone (related to reduction of the extent of Mo segregation). A thirty minute solution heat treatment at 2100°F is recommended for both dissimilar and autogenous welds to eliminate the pitting corrosion sensitivity in the unmixed zone. After solution treatment at 2100°F for 30 min, the pitting resistance of the unmixed zone is equivalent to that of the base metal, while the pitting resistance of autogenous fusion zone is close to that of the respective base metal. Under the crevice condition, a thirty minute solution heat treatment at 2100°F is recommended for dissimilar welds to eliminate the crevice corrosion in the unmixed zone. A 2200°F solution heat treatment for 30 min is suggested for autogenous weld to avoid the crevice corrosion in autogenous fusion zone. After the recommended solution treatments, the crevice corrosion resistance of the unmixed zone and the autogenous fusion zone is basically equivalent to that of the base metal. Bend tests and hardness measurements on AL-6XN welds show that there is virtually no influence of the unmixed zone on mechanical properties (although there is a significant effect of the unmixed zone on localized corrosion behavior). The bend ductility and hardness of the unmixed zone were equivalent to that of the base metal for all three super-austenitic stainless steels.

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