The steel alloy known as Grade 91 has achieved broad acceptance within the modern power industry for use in fabricating a variety of critical pressure part components, including tubing, piping and headers. As with all of the creep strength-enhanced ferritic (CSEF) steels, its attractiveness to designers is based on the fact that within a specific temperature range, and when properly processed, it provides superior elevated temperature strength at substantially lower cost than the austenitic stainless steels, all while maintaining the advantageous thermal-physical properties of a ferritic alloy.
Grade 91 was co-developed by the Oak Ridge National Laboratory (ORNL) and Combustion Engineering (CE, currently named Alstom Power Inc.) at the Metallurgical and Materials Laboratory in Chattanooga, Tenn. Extensive study under Department of Energy sponsorship in the 1975-1980 timeframe had demonstrated the alloy’s excellent mechanical properties. That attracted the attention of boiler designers and pressure vessel fabricators, and in 1983 Grade 91 gained initial acceptance in
The ensuing years have seen broad application of the material in both the power and petrochemical industries. However, in-service experience demonstrated that cracking has been observed in Grade 91 steel components very early in the operating life. There are many reasons why this cracking occurs, including:
- When best practice fabrication and erection procedures are not followed, a wide range of problems have been documented due to improper heat treatment of weldments and components. Composition of the base metal and weld metal also can be a factor in some cases.
- Design and construction practices can result in local stress concentrations in creep-weak regions, such as the heat affected zones (HAZ) in weldments. These issues can be a consequence of the approaches used for reinforcement of penetrations or because of the influences of other local stress concentrations.
- Local operating conditions can be unexpectedly severe due to excessive thermal transients and/or the presence of sustained system loading.
- Difficulties associated with accelerated creep damage developing at dissimilar metal welds or in the HAZ of similar metal welds.
A recent EPRI project has successfully established the background technology necessary to underpin significant improvements in approaches for ensuring that high quality components are supplied and installed. The information is contained in an EPRI report, “Guidelines and Specifications for High Reliability Fossil Power Plants – Best Practice Guidelines for Manufacturing and Construction of Grade 91 Steel Components” (EPRI document #1023199), which details more than 30 years of experience with Grade 91 material in the laboratory, shop and field.
The EPRI document was created to address issues associated with manufacture of Grade 91 steel components. A chapter provides specific details associated with purchase of Grade 91 steel components. Every attempt has been made to ensure that the details of the specification are consistent with industry best practices, which have been modified as required by research from the EPRI Life Assessment of Creep Strength-Enhanced Ferritic Steels project. Appendices in the guide provide summaries of the technical background linked to specific points within the best practice section. These summaries allow a rapid referral to key information. It should be recognized, however, that the metallurgy and performance of Grade 91 steel are complex, so a full review of references is recommended to provide a complete understanding. In addition, work is still in progress to provide additional details in key areas, and the appendices will be updated and supplemented as necessary.
The issues addressed in the EPRI guidelines include:
Influence of composition
From the metallurgical standpoint, it is apparent that there have been issues associated with poorer-than-expected creep performance of Grade 91 steel from both low hardness and, at least inferentially, an influence of the level of aluminum (Al) which can modify the level of free nitrogen (N) available. In the extreme, there are concerns that available nitrogen levels can be so low, the size and density of nitrides and carbo-nitrides required to contribute to creep strength can be compromised. It is apparent that Grade 91 steels with a relatively high concentration of Al are more susceptible to forming cracks early in life. For this reason, the EPRI guideline recommends careful control of elements beyond those contained in the ASME/ASTM material specifications.