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Tests establish dilution effect on weld overlay performance

TWI conducted a series of tests to thoroughly assess the effect of dilution on the performance of a corrosion-resistant weld overlay.

Corrosion-resistant weld overlays are used to improve the service life of components made with an otherwise corrosion-prone material, but one of the major concerns affecting their use is the impact of excessive dilution – the filler metal mixing with the parent metal.

Restrictive standards

Codes and standards for qualification of weld overlay procedures generally attempt to control dilution by restricting the heat input, but as this can be achieved by proportionally varying both the welding current and the welding speed, the resulting dilution can be different even if the heat input is controlled.

Due to these uncertainties, industry takes a conservative view when specifying the permissible dilution. For Inconel Alloy 625 weld overlay, for example, the permissible amount of iron in the overlay is restricted to five per cent: the same level as that permitted in Alloy 625 wrought product, resulting in substantial productivity losses, an increase in cost, and associated issues such as distortion of components.

 

Exhaustive testing

TWI launched a joint industry project to establish exactly how the weld overlay integrity is affected when the iron content exceeds five per cent, exploring the effects of a range of dilution levels – up to 50 per cent – on carbon steel substrate. Welding processes including TIG, MIG and tandem MIG were used. The weld overlays were subjected to a comprehensive range of corrosion and corrosion fatigue tests to establish their performance.

The testing confirmed that Alloy 625 could sustain a significantly higher iron content without any significant loss of corrosion or corrosion fatigue performance. These results should provide more confidence in relaxing the current dilution restrictions, potentially resulting in significant cost savings.

For more information on this project and to discuss the implications of its findings, please email contactus@twi.co.uk.

Figure 1 (a) Photographs of the specimen assemblies used for pitting/crevice and stress corrosion cracking testing; (b) micrograph of a seawater crevice corrosion tested sample
Figure 1 (a) Photographs of the specimen assemblies used for pitting/crevice and stress corrosion cracking testing; (b) micrograph of a seawater crevice corrosion tested sample
Avatar Vinod Kumar Consultant – Arc Processes, Fabrication and Welding Engineering

For more information please email:


contactus@twi.co.uk