Heat resistant steel refers to steel with thermal stability and thermal strength under high temperature conditions. Thermal stability refers to the chemical stability (corrosion resistance, non-oxidation) of steel under high temperature conditions. Thermal strength refers to sufficient strength of steel under high temperature conditions. The heat resistance is mainly guaranteed by alloying elements such as chromium, molybdenum, vanadium, titanium and niobium, so the selection of welding materials should be determined according to the alloying element content of the base metal. Heat resistant steel is widely used in the installation construction of petroleum and petrochemical industry. We can often come into contact with pearlite heat resistant steel with low alloy content, such as 15CrMo, 1Cr5Mo and so on.
Generally, the alloy element content of less than 50% is called heat resistant steel. Japanese heat resistant steel standard JIS G4311, G4312, in addition to several SUH series standards. According to the different microstructure of the parent phase, the heat resistant steel can be divided into ferritic heat resistant steel, martensitic heat resistant steel, austenitic heat resistant steel and precipitate hardening heat resistant steel. JIS G5122 provides for SUH series heat resistant cast steel, but does not classify the grade of steel according to the parent phase structure, and mixes the ferritic heat resistant steel, martensitic heat resistant steel and austenitic heat resistant steel together.
Ferritic heat resistant steel
The representative ferritic heat resistant steel widely used is SUS430 with low C-17%Cr. Cr is the element which can improve the high temperature corrosion resistance of steel and is an indispensable element in heat resistant steel. SUS430 has good oxidation resistance. SUS430 is cheaper because there are no other elements in the steel. However, SUS430 does not harden after high temperature quenching, and its high temperature strength is low, so it can only be used for parts that do not require much strength. On the other hand, because the thermal expansion coefficient of SUS430 is small, and the thermal expansion coefficient of austenitic heat resistant steel is large, so, for the temperature changes repeatedly, easy to produce thermal fatigue parts, SUS430 is better. In addition, when SUS430 is used for a long time at about 500℃, embrittlement will occur due to the precipitation of brittle phase, which should be paid attention to. In addition, Al is also an element that enhances antioxidant capacity. At high temperature, Al forms Al2O3 on the surface of the oxide sheet, which becomes a strong protective film and plays a role in improving the oxidation resistance. The heat-resistant steel using this effect of Al is FCH1. FCH1 is a heat resisting steel with 5%Al added in 25%Cr steel. It has good oxidation resistance below 1200℃.
Martensitic heat resistant steel
Typical martensitic heat resistant steels are 12%Cr steels SUS403 and SUS410J1 with about 0.1% carbon content. These heat-resistant steels are hardened by high temperature quenching and then tempered to precipitate M23C6 on the martensite of the parent phase, maintaining high strength at 500-600℃. If Mo is added to improve the temper softening resistance, the high strength can be further maintained. Martensitic heat resistant steel will soften at high temperature above 500-600℃, resulting in a sharp decline in strength. Therefore, martensitic heat resistant steel is suitable for working temperature below 500-600℃, requiring high temperature strength components. In addition, because the chromium content of martensitic heat resistant steel is less (12%), and part of the chromium is still in the carbide, can not guarantee the chromium content in the parent phase, so the oxidation resistance of martensitic heat resistant steel is often less than that of ferritic heat resistant steel and austenitic heat resistant steel. Si and Al can also form protective film on the oxide sheet of martensitic heat resistant steel. SUH3 and SUH11 martensitic heat resistant steels with Si added to improve their oxidation resistance. These heat-resistant steels are used for engine intake valves and heat-resistant bolts.
Austenitic heat resistant steel
The addition of Cr to steel is accompanied by the addition of Austenite stabilizing element Ni, which makes the steel stable austenite structure at all temperatures. Ordinary austenitic structure steels are SUS304 and SUS310. As we all know, SUS304 is corrosion resistant stainless steel, but SUS304 can also be used as heat resistant steel. Below 600℃, the strength of austenitic heat resistant steel is between martensitic heat resistant steel and ferritic heat resistant steel, and above 600℃, the strength is greater than martensitic heat resistant steel. In addition, SUS304 under 800℃, SUS310 under 1000℃, repeated heating - cooling, has good oxidation resistance. However, when used for a long time at 700-900℃, there will be brittle phase precipitation, so that the material becomes brittle. In addition, because the thermal expansion coefficient of SUS304 and SUS310 is greater than that of martensitic and ferritic heat resistant steels, thermal fatigue damage is easy to occur, and attention should be paid to these two points.
When high temperature strength is required, the strength of austenitic heat resistant steel can be further enhanced by precipitation strengthening and solution strengthening. The austenitic heat resistant steel used for the engine exhaust valve is SUH35. The high temperature strength of SUH35 was improved by carbide precipitation strengthening and solid solution strengthening by adding C to steel. The austenite structure can be obtained even if the content of Ni is 4% by increasing the content of Mn. The SUH660 used for heat-resistant bolts and springs has been strengthened by the precipitation of γ 'phase (Ni3 (Al, Ti)) due to the addition of Al and Ti.
Precipitate reinforced heat resistant steel
The representative brand of martensitic heat resistant steel is SUS630. After aging at 500℃, ε phase (Cu phase) precipitate in low C martensite parent phase of SUS630 to improve the strength of steel. However, when the temperature exceeds 500℃, the ε phase becomes coarser and the martensite structure changes, which results in the decrease of steel strength. Therefore, the SUS630 is mainly used for turbine components below 500℃. The main component of SUS630 steel is 17Cr-4Ni-4Cu, the Ni content is not too high, and considering the austenite stability, the Ni content can not be reduced, so it is not a resource-saving development steel.
