HRSG User's Group: Improving steam-plant reliability, durability and profitability    

Welding Grade-91 steel Without Post-Weld Heat Treatment

William F. Newell, Jr., PE, PEng, IWE, Euroweld, Ltd

For more than a decade, the National Board of Boiler and Pressure Vessel Inspectors (NBIC) Code has required that weld-repairs to Grade-91 steel must be followed by post-weld heat treatment (PWHT), for the purpose of tempering the weld-metal and the heat-affected zone. In practice, HRSG users and welding specialists have found that it’s often difficult to perform a proper PWHT in the confined space of a heat-recovery steam generator (HRSG) (Fig 1), or that performing it might cause material degradation.

Figure 1: The limited space and tight pipe-bends in HRSGs make PWHT nearly impossible to perform.

Additionally, tens of thousands of hardness measurements taken after PWHT have demonstrated that PWHT may not have been achieving its purpose of tempering the material in all cases.

In response, the NBIC, along with the Electric Power Research Institute (EPRI), and other research groups, investigated alternative methods of tempering this creep-strength enhanced steel. The researchers considered such alternative methods as weld-evaluations, controlled deposition, tempering sequence vs. the temper-bead welding method, the groove-geometry effect, and the various options for weld-filler metals (Fig 2).

Figure 2: The codes keep changing as we learn more and more about Grade 91 steel.

One result of all these research efforts was revision of the NBIC Code. The new Code, titled “NB-23 Part 3, Repairs and Alterations,” now permits welders to waive the PWHT requirement, if PWHT for that specific repair would be impracticable, and if they use the specialized welding method of ‘bead-sequencing,’ which actually tempers the material while it is being welded.

This new option for welding Grade 91 steel—referred to as the PWHT waiver—is not—repeat—not—intended for use if PWHT is practicable to perform on that specific repair. Nor is it intended to be used merely to improve schedule, or to reduce cost. No, sir! 

The PWHT waiver may be used only if:

  • You are unable to perform a code-compliant PWHT due to access limitations
  • PWHT would cause distortion
  • PWHT would create undesirable stresses in components because of system-restraints
  • It is impracticable to set up the load-bearing, temporary support that PWHT requires
  • Hardness values in the base-metal—measured prior to that specific repair—are equal, or nearly equal, to the acceptable values for Grade-91 steel (see sidebar).

Hardness Testing of Grade 91 Steel
Grade-91 steel is an advanced, creep-strength enhanced ferritic steel composed of nine percent chromium, and one percent molybdenum—hence the number ’91’. Grade-91 steel often is used in modern HRSGs, because its enhanced creep-strength enables it to withstand the severe conditions that HRSGs experience when they’re operated in the cycling mode.

Testing the hardness of Grade 91 components after field-repairs is an inexpensive and quick way to confirm that the repairs didn’t damage its micro-structure.

Acceptable values of hardness range from a minimum of 190 HBW (the Hardness-Brinell, Wolfram scale), to a maximum of 300-350 HBW.

  • Excessively high hardness values—greater than 350 HBW—indicate that the material may be too brittle, meaning it hasn’t been tempered enough for reliable service.
  • Excessively low hardness readings—below 190 HBW—indicate over-tempering, meaning that its creep-strength is too low for reliable service.

Unfortunately, there can be substantial variability in hardness tests performed in the field, so the technicians performing these tests must be highly trained, and diligent.