With the need to reduce costs and environmental protection, thermal power plants face pressure from two sides. The conventional power generation system developed increasing the steam parameters of the unit, namely, supercritical (SC) and ultra-supercritical (USC) of the unit has been put into the power grid in recent years. One of the most effective ways to improve the efficiency of thermal power plants is to improve the boiler steam temperature, pressure and other parameters, and the main problem to improve the steam parameters is the problem of metal materials, especially the thermal strength, resistance to high-temperature corrosion and oxidation, cold processing and hot processing properties. In simple terms, boiler steel refers to the special steel used to manufacture pressure elements of steam boilers, mainly boiler steel plates and boiler seamless steel tubes, which not only bear pressure loads but also are heated by direct fire, requiring the chemical composition and mechanical properties of steel to meet certain conditions. Here are some of the most common and suitable steel types used for boiler systems in power plants:
Carbon Steel and Low Alloy Steel
Low carbon steel like ASTM A179, and A201C has good plasticity, toughness and weldability, is an economical choice of heating surface pipes, economizers, superheaters, reheaters used thermal power plant. Boiler drums and tubing often use low alloy steels like 2.25Cr-1Mo or 9Cr-1Mo steel. They provide high strength at elevated temperatures. This is a commonly used material for boiler components like heat exchangers, economizers, water walls, ducting, etc. Specific grades like P22, P91, and T22 are designed for high-temperature, and pressure applications.
- SA-210C: Mild steel. Ductility, toughness, good weldability, sufficient strength below 450 ° C, satisfactory oxidation resistance below 530 ° C, but long-term use above 450 ° C will occur pearlite spheroidization and graphitization, creep limit and durable strength reduction, resulting in leakage. It is generally used for heating surface pipes of low and medium pressure boilers (working pressure is generally not greater than 5.88Mpa, working temperature below 450 ° C) and heating surface pipes of high pressure boilers (working pressure is generally above 9.8Mpa, working temperature is between 450 ° C and 650 ° C), economizer, superheater, reheater, pipe for petrochemical industry, etc.
- T11, T12, P11, P12: the addition of chromium elements improves the stability of carbides and effectively prevents the tendency of graphitization, but the phenomenon of pearlite spheroidization and alloying element redistribution will lead to a decrease in the thermal strength of the material. When the temperature exceeds 550℃, the thermal strength decreases significantly and the oxidation resistance deteriorates.
- T22, P22: high thermal strength and durable plasticity, the surface forms a dense oxide protective film at 580℃, has sufficient oxidation resistance and good weldability, but long-term operation will appear pearlitic spheroidization and alloying element redistribution phenomena; and the thermal strength is reduced.
- T23: The material is improved based on T22 combined with the advantages of steel 102, by reducing the C content and adding W, V, Nb, B to obtain low-carbon, multi-component, high-strength, high-toughness Bainitic heat-resistant steel. At 600℃, the strength is 93% higher than T22, and the weldability and processability are better.
Stainless Steel Tube
Martensitic and Austenitic stainless steel is a common material in thermal power plants and is used in many parts of boilers and steam turbines, including boiler tubes, header, pipes, rotors, cylinders, etc. Austenitic stainless steels like 304 and 316 are widely used for superheater and reheater tubes, piping, casing, etc. due to good heat resistance and corrosion resistance.
- SUPER304H: It is an improved version of TP304H by adding 3% Cu and 0.4% Nb. The fine grain structure and the precipitation strengthening of the fine copper phase have a very high creep strength, and the allowable stress is 30% higher than TP304H at 600 ~ 650℃. It has excellent mechanical properties, steam oxidation resistance and heat corrosion resistance at high temperatures, and can run for a long time below 650℃. It is the preferred material for super (super) critical boiler superheater and reheaters.
- TP347HFG: Type TP347H stainless steel is obtained by a specific hot working and heat treatment process. After refining the grain, the allowable stress is increased by more than 20%, and the steam oxidation resistance of the material is greatly improved.
- HR3C steel (25Cr-20Ni-Nb-N steel): is a new type of stainless steel developed in Japan. By limiting the content of C, adding 0.20% ~ 0.60% Nb, 0.15% ~ 0.35% N, and using the dispersed precipitation of the strengthened phase, the material has excellent high-temperature strength and high-temperature steam oxidation resistance and is one of the main heat-resistant steel pipes for the final superheater and reheater in the 650℃ super (super) critical power station boiler.
- T91: Improved 9Cr-1Mo type high-strength Martensitic heat-resistant steel, is a 9%Cr steel with excellent comprehensive performance. The steel by reducing the carbon content, adding alloying elements V and Nb, control the content of N and Al, so that it has high impact toughness, thermal strength and corrosion resistance. The T91 steel tubing has a small linear expansion coefficient and good thermal conductivity and is mainly used for headers and steam pipe with sub-critical parameter and supercritical parameter boiler wall temperature not more than 600℃.
- T92: is a new 9%Cr Martensitic heat-resistant steel obtained on the basis of T91 by reducing Mo, increasing the content of W, and controlling the content of B, the mechanical properties are equivalent to T91, and the welding performance has been improved. The creep strength at 600 ~ 650℃ is greatly increased, the allowable stress is 34% higher than that of T91, and the strength is 1.12 times that of TP347H.
- T122:12%Cr Martensitic heat-resistant steel, that is, adding 2% W, 0.07%Nb and 1% Cu, the steel has higher thermal strength and corrosion resistance, the reduction of carbon content also makes the welding performance improved, mainly used for the manufacture of main steam pipelines below 620℃.
Nickel-based alloys Tube
If the steam parameters are above 700 °C, many components of the unit will only be superalloys, and nickel-based alloys, including directional solidification and single crystal alloys, are being evaluated for use in steam turbines.Nickel-based alloys like Inconel 617 or Haynes 230 allow very high-temperature operation for superheater and reheater sections. But they are more expensive.
It is generally believed that the supercritical boiler design with a steam temperature of about 700 ° C will require the header and the main steam pipe to work at a maximum of 750 ° C, which is far beyond the capacity of Ferritic steel, and the thermal fatigue problem of austenitic steel makes their use for this thick-walled part unlikely. Although the requirement of creep strength is not excessive for nickel-based superalloys, other requirements such as weldability, formability and corrosion resistance are not easy to achieve.