Technical Report: Grade 91: Is it passing the test?


The use of modified 9-Cr (Grade 91) steel in modern power plants is derived from the unique properties of the material in comparison to carbon steels or lower chrome materials. Assets such as superior creep and tensile strength characteristics allow for the use of thinner materials in the design and manufacturing of piping systems and pressure vessels. These are desirable characteristics in terms of thermal cycling, hence the wide spread use of the material in newer combined cycle plants.

Unfortunately, since the use of Grade 91 began in the UK in the late 1980’s many complications associated with this particular material have been revealed over its relatively short lifetime. Grade 91 materials provide significant field welding challenges in terms of backing, preheat, and post weld heat treat programs. There is little margin for error when welding and/or heat treating this material. With increased knowledge of these issues, Thielsch Engineering, Inc. has worked with various clients throughout the country to educate on the use of this material and how to rectify problems caused by its improper usage.

It is vital for plant and maintenance managers to understand the importance of proper handling of this modern material. Our team of expert engineers at Thielsch are working daily with our clients who are experiencing costly and unforeseen complications with its use. To learn more about the positive and negative findings per our experience with P91 and T91 Click Here to read the Technical Report in it’s entirety.

To discuss how Thielsch can assist you with your metallurgical and condition assessment needs, please contact Peter Kennefick at or by phone at (401)467-6454.