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Failure Analysis of a 4-Inch 25Cr-35Ni Welded Fitting

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Up until the 1940s, typical furnace tube materials consisted of wrought chromium steels and austenitic stainless steels. But the low carbon content led to increased creep.

Product Number: 51323-19079-SG
Author: Sudhakar Mahajanam, Chris Miller, Scott Harding
Publication Date: 2023
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A 4-inch HP-modified fitting that cracked in the weld between the pipe and the reducer was examined. The fitting was operating in a cracking/pyrolysis operation (carburizing/reducing environment) with hydrocarbon (gas phase) on the inside surface and furnace atmosphere on the outside surface. Several examinations of the fitting were performed including visual inspection, fractography, metallography, chemical analysis and hardness testing. It was found that the crack occurred in the reducer side heat affected zone (HAZ) region of the girth weld from stress relaxation cracking (SRC). SRC is known to occur at elevated temperatures (500-750°C or 930-1380°F) in welded joints of heavy wall sections of low alloy steels, austenitic stainless steels and nickel-based alloys. The crack morphology was intergranular/interdendritic and chromium/niobium carbides were identified at the austenite dendrite boundaries. Additionally, high hardness values were recorded in the cracked region.

A 4-inch HP-modified fitting that cracked in the weld between the pipe and the reducer was examined. The fitting was operating in a cracking/pyrolysis operation (carburizing/reducing environment) with hydrocarbon (gas phase) on the inside surface and furnace atmosphere on the outside surface. Several examinations of the fitting were performed including visual inspection, fractography, metallography, chemical analysis and hardness testing. It was found that the crack occurred in the reducer side heat affected zone (HAZ) region of the girth weld from stress relaxation cracking (SRC). SRC is known to occur at elevated temperatures (500-750°C or 930-1380°F) in welded joints of heavy wall sections of low alloy steels, austenitic stainless steels and nickel-based alloys. The crack morphology was intergranular/interdendritic and chromium/niobium carbides were identified at the austenite dendrite boundaries. Additionally, high hardness values were recorded in the cracked region.

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