Heat resistant case steel
Advantages: retains strength and other desired properties at high temperatures
Typical applications: power conversion
Effects of molybdenum: helps to resist both creep and high temperature corrosion
The need for higher efficiencies in combustion engines and power plants means increased operating temperatures and pressures. Consequently, cast steel materials must be adapted to these demanding boundary conditions. Principally, ferritic steels and austenitic steels are capable candidates for this challenge.
Important aspects of a successful material are its creep and oxidation resistance. Due to alloying cost aspects, ferritic grades are preferred where technically possible. The relatively low coefficient of thermal expansion of the ferritic steels as compared to austenitic grades also favors thermal fatigue properties. Molybdenum is an established alloying element in such materials as it improves resistance both to creep and high temperature corrosion.
For large power plant components, 9-10% chromium steels with either a ferritic or martensitic matrix play an important role. Conventional martensitic steels such as X20CrMoV12-1 do not fulfill the increased endurance demand of 100,000 hours at a 600°C operating temperature and 100 MPa stress.
An improved cast steel (G-X12CrMoWVNbN10-1-1) was developed to tolerate an operational condition of 620°C and 100 MPa. The steel contains 1% molybdenum, up to 0.07% niobium and 0.2% vanadium. Niobium and vanadium form mixed MX type carbides whereas molybdenum and tungsten form M23C6 type carbides and Laves phase together with chromium and iron. These precipitates represent strong barriers against creep at high operating temperatures.
Effect of Mo on hot strength and creep resistance limit strength
Austenitic cast steel for operation at even higher temperatures contains 18-38% nickel, 15-26% chromium, up to 2.5% silicon and carbon from 0.15 to 0.6%. This chemical composition guarantees a stable austenitic structure at both high and ambient temperatures.
Other alloy additions aim at a simultaneous improvement of the following three parameters: creep resistance, operating temperature, and resistance to aggressive gas environments. In that sense, molybdenum alloying to austenitic irons has similar effects as in ferritic SiMo alloys. The chemical compositions of commonly used stainless steel castings are classified into “C” and “H” series alloys. In the H series, both phase stability and mechanical properties at high operating temperatures are the factors of primary importance. In the C series, the aim is to attain the most corrosion resistant microstructure. Molybdenum is typically added to a level of 0.5 in C series alloys, while selected H series alloys contain molybdenum to much higher levels (e.g., Type 316 stainless steel: 2-3% molybdenum, Type 303: 1.5% molybdenum, Type 317: 3-4% molybdenum).
Table: Nominal compositions of stainless steel castings