O’Donnell Consulting was asked to provide an assessment of the welding metallurgy and details on how metallurgy drives the welding procedures for P91. This is a specially modified and heat treated steel that performs quite well at elevated temperature – usually 1,00 F and higher. Since it is a highly hardenable alloy, it is subject to hydrogen cracking. The performance of Grade 91 welds depends entirely on having the correct chemical analysis in the weld metal.
Appendages such as lifting lugs and alignment fixtures are welded to heavy wall pipe to facilitate installation of piping in a power plant. After these appendages have served their purpose, they are removed. These appendages are generally located adjacent to circumferential welds that are subjected to post weld heat treatment (PWHT) as required for the P91 steel.
P91 is a creep resistant martensitic steel used extensively in the generation of electric power. It is nominally a 9%Cr-1%Mo-I/4%V plus other small additions. The martensite structure is developed by heating the steel to a high temperature (austenitizing temperature) and then rapidly cooling to a temperature where the austenite transforms to martensite; the second step tempers the martensite to soften the microstructure.
The optimum properties for P91 steels are achieved in the quenched and tempered condition. To achieve the fully martensitic structure in weldments, the steel must be cooled to a temperature where the entire parent structure, austenite, has transformed.
This martensite finish temperature is about 200 F. In addition, the interpass temperature must be kept below about 600 F to prevent the austenite from being stabilized; a condition that prevents full transformation to martensite. Because martensite formed during welding is very hard and prone to cracking, the weldment must be subjected to a PWHT following any welding. The PWHT provides tempering of the newly formed martensite. Another characteristic is that the P91 is prone to hydrogen induced cracking. Prevention of this cracking requires high preheat temperatures and the use of low hydrogen electrodes. The combinations of high preheat temperatures and a low interpass temperatures makes the weldability “window” small.
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