Inspecting Aberrant P91 Components for Integrity
12/01/2014 | Dr. Ahmed Shibli
Many utilities around the world have been discovering abnormal or aberrant P91 base and weld metal microstructures in their plants, leading to, or with a potential to lead to, early cracking and failure in critical plant components. Although it is widely recognized that proper heat treatment is critical in achieving full-strength, high-chromium martensitic steels, it is clear that for various reasons many material suppliers, plant manufacturers, and welding companies have failed to perform this important step, resulting in serious consequences.
Research Is Lacking
Few studies have been carried out on aberrant P91 materials/welds, and the little information available in the public domain tends to suggest that, for certain material conditions, the creep rupture strength level could be as low as that of ASME (The American Society of Mechanical Engineers) P9 steel, or even the Type IV strength of P9 welded joints. Some users have applied the rupture strength of P22 steel for predicting the rupture strength of aberrant P91 steels. However, there is a genuine concern that life prediction based on P9 or P22 steels rupture strength may be overly conservative for many of the aberrant P91 base metal and weld variants.
Although the Electric Power Research Institute is known to have carried out limited and short-term rupture strength studies on aberrant P91, the study entitled “Long Term Rupture Strength of Aberrant P91” is believed to be the first comprehensive and systematic study on more than 30 variants of aberrant P91 base and weld metals of the type frequently found in power plants. This recently started project—aimed at testing base metal and cross weld specimens to test durations of up to 30,000 hours—is currently sponsored by ETD Consulting, GDF Suez (and its subsidiaries Laborelec and Electrabel), and a group of Japanese utilities under the umbrella of the Tokyo-based Central Research Institute of Electric Power Industry. Discussions to gain the involvement of plant manufacturers, such as Alstom, and some U.S. power industry companies are still in progress.
A Variety of Concerns
Typical examples of aberrant P91 base metal variants that plant operators have come across include the following:
■ Material over-tempered (approximately 15C above Ac1—Ac1 is the temperature at which austenite begins to form on heating).
■ Material slightly over-tempered (approximately 10C below Ac1) for longer periods of about 6 hours. Note: Over-tempering can take place at temperatures just below the Ac1 temperature.
■ Material tempered at the bottom of the new ASME allowable range (730C) or the old ASME allowable range (704C).
■ Fully ferritic P91 (too slow cooling from austenitizing).
■ Mixed 30% to 70% ferrite to 70% to 30% martensite (too slow cooling from austenitizing).
■ Lean alloy composition (some or all of the chromium [Cr], molybdenum [Mo], vanadium [V], and niobium [Nb] near lower limits).
■ Soft spots and soft bands found in some piping material.
Examples of aberrant weld metal P91 variants that plant operators have come across include the following:
■ Under-tempered weld metal (<730C).
■ Over-tempered at roughly 10C below Ac1 or 15C above Ac1 (that is, close to Ac1), or slightly higher than the ASME range (775C).
Similarly, various weld repairs and weld repair configurations/geometries, or repeat weld repairs, have also exhibited problems in power plants in Europe, the Middle East, Asia, and North America. Again, in the absence of rupture strength data for these types of joints, plant operators have had difficulty determining how to deal with potentially vulnerable components in their plants.
Other welding problems faced in some of the plants have been low-alloy root runs, which some welding companies have carelessly introduced to avoid the purging needed with high-alloy root runs. These root runs are notoriously difficult to detect.
Plant operators have also come across problems with dissimilar metal welds (both ferritic-to-ferritic and ferritic-to-austenitic). With a range of post-weld heat treatments and no long-term rupture data, little information exists to predict their long-term integrity. Another problem exists when bends are found with no, or inappropriate, post-bend heat treatment. Understanding the integrity and expected behavior of such components is difficult.
An example of how the creep rupture strength of an abnormal P91 material may compare with that of ASME P9 and P91 steels at 600C is shown in Figure 1, while Figure 2 offers an example of how P91 welded component rupture strength may decrease with aberrant material or heat treatment.