The designers of modern thermal power plants continue to work hard to improve plant efficiency by increasing the heat rate as a function of main-steam pressure and temperature.
As competition with wind and solar platforms continues to intensify, so does the need to eke out the maximum Mw output possible in a coal or combined cycle generating unit. For nearly three decades, the application of creep strength enhanced ferritic (CSEF) materials (e.g. P 91, P92, P911, P122, Gr 23, etc.) has been employed to lessen plant construction costs, while maintaining/improving piping system performance.
The 9-12% Cr (typ.), ferritic-martensitic steels depend heavily on the interplay between the metallurgically complex alloying elements and processing parameters to achieve the desired mechanical properties. (High creep-rupture strength and fracture toughness)
This is the primary reason there has been a significant learning curve in applying these materials. Documented failures have occurred as a function of microstructure degradation.
Degradation has been linked with fabrication of these materials and in construction using them, particularly in the areas of chemical composition (base metal and weld metal), heat treatment (including pre- and post-weld heat treatment (PWHT)) and hardness.
Note that, for ASME code construction, chemical composition and heat treatment parameters are included in the applicable Sections of the ASME Code. Customer specifications also may, and do, further limit these parameters, based on the purchaser’s requirements.
1. Chemical Composition
Control of chemical composition, particularly of alloying elements such as chromium, nickel, manganese, silicon and trace elements such as copper, tin and antimony, factor into long-term performance. Control of nickel and manganese is critical for the welding consumables utilized in fabrication and/or repair welding.
Incorporating tight specifications in procurement documents, combined with careful review of certified material test reports and related documentation, has become the norm in purchasing CSEF materials. Most owner-users, OEMs and EPCs have well-developed specifications they impose on the material suppliers, fabricators, constructors and manufacturers that provide CSEF materials or employ them in the products they manufacture.
Recent developments have led to recommendations reducing the maximum nickel content, raising the minimum chromium content, and imposing tighter controls on the Ni+Mn (weld metal) and tramp element values.
2. Heat Treatment/PWHT
When properly applied, CSEF materials deliver excellent performance in the demanding high-pressure/temperature services typically found in power plant applications. That includes power boilers, HRSGs, main steam piping, reheat piping, tubing, headers and valves, etc.
The material properties are achieved by proper heat treatment during the material fabrication process, and subsequent controls on PWHT throughout the manufacturing and installation of the piping, tubing or component. Obviously, chemical composition plays into the effects of PWHT on the weld- and heat-affected zone (HAZ), particularly the Ni+Mn value. Extreme care in pre-heat, maintenance of interpass temperature, PWHT within the required temperature parameters and heating/cooling rates must be taken to ensure development of the optimal mechanical properties. Some specifications also include post-weld baking requirements.
Hardness is a property that is also affected by chemical composition and application of heat treatment(s). As in the differing ways that chemical composition and PWHT requirements are restricted, the specifying organizations may also have a varying hardness range requirement. Although there are minor differences in the required “delivered” hardness for Type 91 materials, most consider a range of 200-250 HBW acceptable. (Note again that this is a “delivered” range.) Manufacturers must take into consideration the effects that welding and PWHT will have on the material’s hardness throughout the manufacturing process.
VALVE MANUFACTURERS MET THE CHALLENGE
Valve manufacturers are subject to the concerns and controls applicable to CSEF materials. The materials used for valve bodies, bonnets, discs, wedges, equalizing piping, etc. in main-steam, by-pass and vent/drain applications (to name a few) have been specified as CSEF materials since the early 1990s (in some cases earlier).
When this was first introduced, some manufacturers chose to supply valves of the traditional 2 ¼ chrome, 1 moly (WC9 castings and F22 forgings) materials, adding Type 91 stub ends to the valve bodies so that a similar metal installation weld could be made in the field. However, as demand increased, the valve industry reacted accordingly and (for the most part) currently supplies complete valve bodies of (S)A217 Gr C12A (castings) and/or (S)A182 F91 (forgings).
As the desire to increase operating temperatures continued, Type 92 and other CSEF materials have been employed in high-temperature/pressure applications. Valve manufacturers (as well as other piping/component manufacturers/suppliers) have worked hard to identify sources for the new materials and developed welding, PWHT and other process procedures to properly manufacture their products accordingly.
The necessity for proper, effective training of personnel that perform welding, heat treatment(s) and nondestructive examinations in the application of the CSEF materials is becoming more and more recognized. There is a wealth of material available as source documents for such training, as well as several subject matter experts who are available for consultation.
CODES AND STANDARDS
The utilization of CSEF materials is addressed in ASME Section I (power boilers), Section II (materials), Section III (nuclear components), Section IX (welding, brazing and fusing qualifications) and in the B31.1 (power piping) and B31.3 (process piping) codes. Revisions to material specifications have been made dividing Type 91 into two types, Type 1 and Type 2, with the Type 2 material having tighter controls on chemical composition.
A concern as to the allowables specified in ASME Section II – Part D for the CSEF materials has given rise to the formation of an ASME Section II working group. This working group on CSEF materials is making recommendations as to the development of a code case and code revisions that would restrict the current allowables as deemed necessary. These code activities have the attention of the owner-user/supplier/manufacturer community as to what next direction these materials take. Stay tuned for further developments.
The CSEF materials have provided a means to achieve plant performance efficiencies that would have been difficult (or costly) to attain without them. But this has come at a cost to the industries in which these materials have been applied.
The corrective action processes and resultant research and development that has ensued have provided us with some assurance that, given the proper controls, CSEF materials can continue to be utilized safely and effectively in the tough applications for which they are best suited. In the future, organizations involved with CSEF materials need to keep close tabs, both on developments in the code community and the requirements in customer specifications that further restrict (or add to) code requirements, in order to ensure continued proper and safe applications.
Advanced Pressure Boundary Materials – National Energy Technology – Michael Santella, Oak Ridge National Laboratory; John Shingledecker, EPRI
ASME Boiler & Pressure Vessel Code – American Society of Mechanical Engineers, New York, N.Y.
Welding and Postweld Heat Treatment of P91 Steels, William F. Newell, Jr. in the Welding Journal