Creep resistant steels

Dr. Dmitri Kopeliovich

Creep resistant steels are steels designed to withstand a constant load at high temperatures.
The most important application of creep resistant steels is components of steam power plants operating at elevated temperatures (boilers, turbines, steam lines).

• Supercritical and Ultra Supercritical power plants
• 9-12 Cr martensitic creep resistant steels
• Austenitic creep resistant steels

About 70% of electricity in the world is produced by fossil fuel (coal, natural gas, oil) power plants where the burning fuel heats water in the boiler converting it into a steam. The steam drives the turbine, which rotates the electrical generator.
Maximum theoretical efficiency of the thermal power plant is determined by the ratio between the the “hot” and “cold” temperatures (T_C and T_H) of the steam (Carnot cycle):

η = 1 - T_C/T_H

One of the factors reducing the actual efficiency of the steam turbine power plant is the latent heat of vaporization.
Latent heat of vaporization of water at normal atmospheric pressure and 212ºF (100ºC) is 972 BTU/lb (2260 kJ/kg).
The value of the latent heat of vaporization decreases when the pressure increases. At the pressure 3205 psi (22.1 MPa) there is no clear distinction between the liquid and gaseous phases of water. Since there is no phase transformation in this state the latent heat of vaporization becomes equal to zero. This state of substance is called critical.

Conventional (subcritical) power plants operate at steam pressures below 3205 psi (22.1 MPa). Their efficiency is about 35%.
The efficiency may be considerably increased at the steam pressure above the critical (supercritical). The efficiency of the modern supercritical power plants operating at 4350 psi (300 bar) and 1112ºF (600ºC) reaches 45%. Green gas (CO₂) emission in the supercritical power plants is 30% less than in conventional subcritical plants.
The next generation of steam power plants (ultra supercritical ) operating at the temperature 1200ºF (650ºC) will have the efficiency 50%.

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Creep resistance of 9-12 Cr steels is provided by the presence of the chromium carbides Cr₂₃C₆.
One of the most popular ferritic/martensitic 9-12 Cr Creep resistant steels widely used in subcritical steam power plats is P91 developed in USA in 1970s.
P91 is differs from the previous 9-12 Cr steel by additions of niobium (Nb) and a controlled amount of Nitrogen (N).
Niobium and vanadium react with carbon and nitrogen during heat treatment forming precipitating particles of carbides and nitrides (V,Nb)(C,N).

Creep resistance of the 9-12 Cr steels was further enhanced in the steel P92 by introduction of tungsten (W) and boron (B).
Boron inhibits coarsening M₂₃C₆ (eg. Cr₂₃C₆) carbides. Tungsten in P92 causes hardening the steel due to both solid solution hardening and formation of Laves phase particles (Fe₂W), which are more stable than molybdenum Laves phase characteristic for the microstructure of P91 steel.

Further improvement of creep resistance of ferritic/martensitic 9-12 Cr steels was achieved (steels T/P122, NF12, SAVE12) by an increase of Cr content from 9 to 11%. In order to prevent incomplete austenitization during normalizing treatment caused by larger amount of ferrite stabilizing Cr, austenite stabilizing elements such as cobalt or copper are added.
Despite of its excellent austenite stabilizing effect nickel leads to coarsening M₂₃C₆ particles and therefore is not used in the martensitic 9-12 Cr steel.

Besides higher creep resistance the steels containing 11% of Cr has also better oxidation resistance particularly in the steam containing atmosphere at a temperature above 1112ºF (600ºC). Increase of the chromium content from 9 to 11% results in better oxidation resistance of the steel due to the formation of a tightly adherent protective Cr reach oxide scale.
Oxidation resistance may be also improved by additions of other (than chromium) oxidizing elements (silicone, manganese) combined with special surface treatments (pre-oxidation, Shot peening).

Typical heat treatment of 9-12 Cr ferritic/martensitic steels includes Normalizing for 2-3 hours at 1922ºF (1050ºC) with air cooling followed by Tempering at 1256-1436ºF (680-780ºC).
During normalizing the steel structure becomes austenitic. Most of carbides and nitrides dissolve in austenite at this stage. Air cooling after normalizing results in transformation of austenite into hard and brittle martensite, which is then soften during tempering.
Carbide and nitride particles precipitate along the grains of prior austenite.

9-12 martensitic steels are used in the applications with operating temperature up to 1148ºF (620ºC).

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Austenitic Creep resistant steels contain chromium (Cr) and nickel (Ni).
Chromium increases the resistance to oxidation and corrosion. However chromium is ferrite stabilizing element therefore it inhibits austenitization. Nickel is added into the austenitic creep resistant steel in order to stabilize the austenitic structure in the presence of large amount of chromium.
Titanium (Ti) and niobium (Nb) form carbides and nitrides enhancing creep resistance of the steels.
Molybdenum (Mo) and tungsten (W) harden the steel due to both effects solid solution hardening and formation of strengthening  σ and Laves phases.
Aluminium (Al) and silicon (Si) improve the oxidation resistance promoting together with chromium the formation of a protective oxide film on the steel surface.

Austenitic creep resistant steels may be used at the operating temperatures up to 1202-1292ºF (650-700ºC).
Nickel base superalloys (eg. Inconel 617, Inconel 671, Inconel 740, Haynes 230) combine high creep resistance with good Corrosion and oxidation resistance and may be used even at higher temperatures.

The disadvantages of the austenitic creep resistant steels as compared to the martensitic steels:

• Lower thermal conductivity;
• Greater coefficient of thermal expansion;
• Higher cost.

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• Iron-carbon phase diagram
• Effect of alloying elements on steel properties
• Classification of steels and cast irons
• Carbon steels
• Alloy steels
• Stainless steels
• Tool and die steels
• Cast irons
• Welding stainless steels
• Creep
• Heat-treatable nickel-chromium-iron alloys
• Steel strip processing
• Steel making (introduction)
• Basic Oxygen Furnace (BOF)
• Electric Arc Furnace (EAF)
• Ladle refining
• Desulfurization of steel
• Non-metallic inclusions in steel
• Structure of killed steel ingot
• Macrosegregation in steel ingots
• Fabrication of large steel ingots
• Electroslag Remelting

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