Materials Laboratory Archives

By: Tony Studer

The need for confirmation: A Case Study

As plants age, the need for inspection for service related damage to ensure unit reliability increases. There are several approaches that plants can take to reduce the risk of premature failures and proactively manage their DMWs. First is metallurgical sampling. Based on temperature profiles across the boiler, operating conditions, and operating history, DMWs can be selected for laboratory analysis. This will provide some insight into possible damage accumulation; however, the better approach, if damage is suspected, is to perform an ultrasonic inspection of the DMWs. This allows inspection of all the DMWs, and only requires access and surface preparation. If indications are detected, then tube sampling should be performed. It is critical to perform a metallurgical analysis of several of the DMWs suspected of containing service damage to confirm that the indications are service related and to help establish the extent of the damage compared to ultrasonic testing results. Typical DMW damage is described in the Featured Damage Mechanism article. The importance of the metallurgical analysis is demonstrated in the three following case studies.

By: Wendy Weiss

Large utility-type steam generators inevitably contain a large number of pressure part welds that join components fabricated from different alloys.

Background
The welds made between austenitic stainless steel tubing and the lower-alloyed ferritic grades of tubing (T11, T22) deserve special mention because of the early failures that developed in some of these dissimilar metal welds (DMWs) soon after their introduction in superheater and reheater assemblies. Prior to the mid-1970s, many DMWs were fabricated either as standard fusion welds using an austenitic stainless filler metal, such as TP308, or as induction pressure welds, in which the tubes were fused directly to each other without the addition of filler metal. Some of these welds failed after less than 40,000 hours of operation, with the earliest failures being associated with DMWs that operated “hot” in units that cycled heavily and were subjected to bending stresses during operation.

After the mid-1970s, and in response to extensive research carried out by EPRI and other organizations, an increasing number of DMWs in superheater and reheater tubes were fabricated as fusion welds using nickel-based filler metals, such as the INCO A, INCO 82, INCO 182, etc.

News & Views, Volume 43 | Metallurgical Lab: Dissimilar Metal Welds (DMW) in Boiler Tubing

News & Views, Volume 43 | Metallurgical Lab Featured Damage Mechanism: Failure of Dissimilar Metal Welds (DMW) in Steam-Cooled Boiler Tubes