Specific Heat And P91

Driving you to know more about Mechanical field: Challenges of P91 ...

With combined-cycle plants operating at tougher steam conditions and in more rigorous cycling duty, plant designers increasingly are specifying modified 9Cr-1Mo steel for critical sections of the heat-recovery steam generator (HRSG). This high-temperature alloy can drastically reduce thermal stresses and improve service life. Unfortunately it also is difficult to fabricate and repair, and is vulnerable to Type IV cracking.

The frequent startups, shutdowns, and load changes imposed by cycling duty typically cause only minor trouble for a combined-cycle plant's gas turbine, but they pose very serious challenges to the HRSG. The most serious is thermal fatigue. Cycling causes rapid rates of change in steam temperatures, creating severe thermal gradients in thick-walled HRSG components such as main steam piping and superheater headers. If the thermal gradients are large enough and are repeated enough times, the result is thermal-fatigue cracking.

The best way to fight thermal fatigue is to use modified 9Cr-1Mo steel, which allows pressure-containing components to be made with thinner sections. Thinner components have smaller temperature gradients across the wall thickness and require less time to reach thermal equilibrium, thus they are less prone to thermal-fatigue damage. Some existing plants are now using or planning to use modified 9Cr-1Mo as a replacement material for failed components, and virtually all manufacturers now use the alloy for the superheater sections in new HRSGs. Note: In addition to the thermal fatigue induced by rapid changes in steam conditions, the superheater section suffers creep damage because it operates at the highest metal temperatures. The interaction of creep and thermal fatigue significantly accelerates the overall damage process in today's superheaters.

The family of 9Cr-1Mo steels traces its roots to the late 1970s, when new alloys were being studied for the Clinch River Breeder Reactor. Researchers found that 9Cr-1Mo steels, compared to 2.25Cr-1Mo ferritic steels and 300-series austenitic stainless steels, possessed lower thermal expansion and higher thermal conductivity, thus they could minimize thermally-induced stresses in power plant components. Furthermore, with the addition of niobium, vanadium, and nitrogen, the "standard" 9Cr-1Mo (ASTM P9/T9) was found to exhibit a substantial increase in creep-rupture strength, thus giving birth to the "modified" version of 9Cr-1Mo.

Advances in materials technology for fossil power plants, proceedings from the fourth international conference, October 25-28, 2004, Hilton Head Island, South Carolina

Experience with creep-strength enhanced ferritic steels and new emerging computational methods, presented at the 2004 ASME/JSME Pressure Vessels and Piping Conference : San Diego, California, USA, July 25-29, 2004

Key Science, Physics : Teachers Guide

Proceedings of the Institution of Mechanical Engineers, Journal of materials, design and applications

Environmental engineering dictionary