The effects of peak temperature and cooling rate on the susceptibility to intergranular corrosion of alloy 690 by laser beam and gas tungsten arc welding

doi:10.1016/j.corsci.2009.01.002

Corrosion Science

Volume 51, Issue 3, March 2009, Pages 439-445

https://doi.org/10.1016/j.corsci.2009.01.002 Get rights and content

Abstract

This study investigates the effects of welding method, peak temperature, and cooling rate on the susceptibility to intergranular corrosion of alloy 690 weldments. The experimental results reveal that the laser beam welding process with cooling rate of around 212.6 °C/s can be produced with much less mass loss and a lower value of maximum reactivation current density/maximum anodic current density than with the gas tungsten arc welding process, where cooling rate is at around 17–20.6 °C/s. This is because the very rapid cooling rate during welding leads to an insufficient exposure time of around 2.1 s within the chromium (Cr)-carbide precipitation temperature range, suppressing Cr-carbide precipitation and Cr-depletion along grain boundaries in the weld decay region of the heat affected zone.

Introduction

Austenitic nickel-based alloys are extensively employed in components of nuclear power plants. They are typically fabricated using a conventional arc welding process, such as gas tungsten arc welding (GTAW) or shielded metal arc welding (SMAW). However, such welding techniques generate a tremendous amount of heat within the fusion zone (FZ) and the heat affected zone (HAZ) of the weldment, and therefore prompt the precipitation of chromium (Cr)-rich carbides at the grain boundaries during the cooling process. Therefore, a significant increase in susceptibility to intergranular corrosion (IGC) of the weldment is observed when the weldment is exposed to a corrosive environment [1], [2], [3], [4], [5], [6]. It has been reported that the precipitation of Cr-rich carbides and their distribution along grain boundaries markedly affect the susceptibility of welded nickel-based alloy to IGC behavior [1], [2], [5], [6]. The changes in Cr-carbide types and distribution which take place within the nickel-based alloy weldments are fundamentally dependent on both the total amount of heat supplied to the weldment and the cooling rate of various regions (i.e., the FZ, the HAZ, and the base metal (BM)) following the movement of the heat source. Recent studies have suggested that laser beam welding (LBW), characterized by a concentrated heat input, a high welding speed and a rapid cooling rate, results in a significant decrease in the susceptibility to IGC [7], [8].

The influence of factors such as peak temperature and cooling rate of thermal cycle is significant on IGC phenomenon of weldment. The objective of the current study is to examine the effects of the welding method, the peak temperature, and the cooling rate on the susceptibility of nickel-based alloy 690 weldments to IGC effects. Accordingly, a series of welding trials were performed to investigate the thermal cycles of alloy 690 butt weldments fabricated using the GTAW and the LBW techniques. In the welding tests, the temperature was recorded continuously at various points within the HAZ and BM of the weldment, and the resulting thermal profiles were then correlated with microstructural observations of the tested specimens in order to establish the influence of the peak temperature and cooling rate on the susceptibility of the GTAW and LBW weldments to IGC.

Section snippets

Experimental

The welding trials were fabricated using alloy 690 plates with a thickness of 3 mm. The plates had been solution annealed at 1050 °C for 5 min and then quenched in water prior to their delivery. The chemical composition of the alloy 690 plates is summarized in Table 1. The alloy 690 butt weldments were fabricated using a GTAW process with filler metal or a LBW process without filler metal, as shown in Fig. 1(a) and (b). The GTAW process was performed in a pure argon shielding gas using each

Results and discussion

Fig. 2(a) and (b) illustrate the thermal cycles at various positions within the GTAW and LBW weldments, respectively. In general, the LBW process has both a higher energy density (10⁴–10⁵ W/mm²) than the GTAW process (10² W/mm²) and a higher welding speed [10]. Overall, therefore, the difference in the energy densities of the two heating sources causes a significant difference in the characteristics of the heating rate, peak temperature, and cooling rate of the thermal cycles of the two weldments.

Conclusions

The CGZs of the GTAW and LBW weldments have a relatively lower susceptibility to IGC than either the FZs or the WDZs. This is due to its peak temperatures being above 1000 °C, which dissolved and suppressed Cr-carbide precipitation at the grain boundaries during subsequent cooling. The FZ and WDZ of the LBW weldment are significantly less susceptible to the effects of IDC and IGC, respectively, than the corresponding regions of the GTAW weldment. It has been demonstrated that the WDZ of the GTAW

Acknowledgement

The authors gratefully acknowledge the financial support of National Science Council, Taiwan (ROC), under Contract No. NSC 96-NU-7-006-001.

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LetterThe effects of peak temperature and cooling rate on the susceptibility to intergranular corrosion of alloy 690 by laser beam and gas tungsten arc welding

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgement

Mater. Sci. Eng. A

J. Mater. Process. Technol.

Mater. Sci. Eng. A

The influence of thermal treatment on the chemistry and structure of grain boundaries in inconel 600

Metall. Mater. Trans. A

The effects of heat treatment on the chromium depletion, precipitate evolution, and corrosion resistance of INCONEL alloy 690

Metall. Mater. Trans. A

Alloy optimization for PWR steam generator heat-transfer tubing

JOM

Letter
The effects of peak temperature and cooling rate on the susceptibility to intergranular corrosion of alloy 690 by laser beam and gas tungsten arc welding