API Technical Report 938-B

This report is only applicable to the 9Cr-1Mo-V alloy steel, Grade 91. There are newer, higher strength grades, namely 9Cr-2W (Grade P92) and 9Cr-1Mo-1W-Cb (Grade 911), however, because little fabrication and welding experience is available and application history for these grades is still evolving, they are not included in the scope of this report. Some data on these grades is included for comparison with Grade 91.

It covers the basic material and metallurgical properties of 9Cr-1Mo-V steel, including a summary of the physical and mechanical properties, corrosion and oxidation resistance, indicating possible corrosion and/or mechanical failure mechanisms and how to avoid them. The appropriate base metal heat treatment is also given.

This report provides guidelines on the proper specifications for base metal and welding consumables and successful fabrication, including welding and heat treatment requirements for use of 9Cr-1Mo-V alloy steel in the oil refinery services. This includes guidelines for preheat, postweld heat treatment, procedure qualification, and mechanical and nondestructive testing.

This document also defines hardness limits for the base material and welds in order to avoid cracking failures due to wet sulfide stress corrosion cracking or due to other possible failure mechanisms.

A discussion of both proper and improper refinery service applications for these steels is also provided.