Heat Resistant Steels & Superalloys (marketing department of Shanghai HY Industry Co., Ltd)

Heat-resistant steel and superalloy （ Heat Resistant Steels & Superalloys ）, these materials are used in high temperature conditions, how high temperature is called high temperature? What are the characteristics of high temperature working conditions? Why call it a different name? What is the difference between heat-resistant steel, heat-resistant alloy, and high-temperature alloy?

Let’s talk about heat-resistant steel and heat-resistant alloys first. The difference between the two is not clearly defined. Usually, those with alloy element content less than 50% are called heat-resistant steel, and those with alloy element content greater than 50% are called (iron-based) heat-resistant alloys. The difference between the two mainly lies in the content of alloying elements

Let’s talk about heat-resistant steel and high-temperature alloys. Heat-resistant steel refers to steel with good high-temperature oxidation resistance and high-temperature strength. The maximum service temperature of heat-resistant steel and heat-resistant alloys under relatively high loads can generally only reach 750°C to 850°C. For components used at higher temperatures, nickel-based, cobalt-based and refractory metal-based superalloys are used. The difference between the two is mainly in the high temperature range.

What challenges do high temperature conditions pose to the heat resistance of metals? What is the specific meaning of heat resistance?

The heat resistance of metal materials includes two aspects: high temperature oxidation resistance and high temperature strength.

① High temperature oxidation resistance

The high temperature oxidation resistance of metal refers to the resistance of steel to oxidation under high temperature conditions, and is an important guarantee condition for whether steel can work at high temperature for a long time.

Oxidation is a typical chemical corrosion. In an oxidizing atmosphere such as high-temperature air and combustion exhaust gas, a chemical reaction occurs between the metal and oxygen, that is, oxidation corrosion, and the corrosion product (oxidation film) adheres to the surface of the metal. As the oxidation progresses, the thickness of the oxide film continues to increase. Whether the metal continues to oxidize after oxidation to a certain extent depends directly on the performance of the oxide film on the metal surface. If the formed oxide film is dense and stable, has a high binding force with the base metal, and has a high strength of the oxide film, it can prevent the diffusion of oxygen atoms into the metal and reduce the oxidation rate, otherwise it will accelerate the oxidation, causing the metal surface to peel and Falling off, etc., leading to early failure of parts.

The composition of the oxide film on the steel surface is related to the temperature. Below 570°C, the oxide film is composed of Fe2O3+Fe3O4, which is relatively dense, can effectively hinder the diffusion of oxygen, and has good oxidation resistance. When heated above 570°C, the oxide film is composed of FeO+Fe2O3+Fe3O4. FeO is close to the steel surface, followed by Fe3O4 and Fe2O3 outward. FeO is loose and porous, accounting for about 90% of the entire oxide film thickness. Metal atoms and oxygen atoms are very small. It is easy to diffuse through the FeO layer and accelerate oxidation. In the presence of FeO at high temperature, the oxidation resistance of steel is greatly reduced, and the higher the temperature, the faster the atomic diffusion and the faster the oxidation rate.

The main way to improve the oxidation resistance of steel is alloying, adding Cr, Si, Al and other alloying elements to the steel, so that when the steel is in contact with oxygen at high temperature, it will preferentially form a dense high melting point oxide film Cr2O3, SiO2, Al2O3, etc., tight Cover the surface of the steel to prevent oxidation from proceeding.

② High temperature strength

The high-temperature strength of metal refers to the resistance of metal materials to mechanical loads at high temperatures, that is, the ability of metal materials to resist plastic deformation and destruction at high temperatures.

The mechanical properties of metals at high temperatures are quite different from those at room temperature. When the working temperature is higher than the recrystallization temperature, in addition to plastic deformation and work hardening caused by external forces, the metal will also undergo recrystallization and softening processes. Therefore, parts that can normally serve at room temperature are difficult to meet the requirements at high temperatures.

The mechanical properties of metals at high temperatures are related to factors such as temperature, time, and structural changes.

The phenomenon of “creep” often occurs when metals work at high temperatures, that is, when the working temperature is higher than the recrystallization temperature and the working stress exceeds the elastic limit at this temperature, the metal slowly deforms over time. The more resistant a metal is to creep, the higher its high temperature strength.

The high temperature strength of metals is generally expressed by creep limit and endurance strength. The creep limit refers to the stress value when the residual deformation of a metal reaches a certain value after a period of time at a certain temperature. Endurance strength refers to the stress value when the metal material breaks and breaks after a certain period of time at a constant temperature.

The grain boundary strength of metal materials is lower than that of the grain at high temperature, so the addition of alloy elements to increase the recrystallization temperature to form stable special carbides, and the use of coarse grain materials to reduce grain boundaries can effectively improve the high temperature strength of steel

Shanghai HY Industry Co., Ltd has international first-class material production equipment such as high temperature alloy isothermal forging production line, CONSARC vacuum melting furnace, VAR vacuum arc remelting furnace etc., with strong professional ability and highly responsible management team. We are a member of China Application of nuclear energy materials, Member of the International Council of the Aeronautical Sciences. Years of experience in Heat Resistant Steels & Superalloys. When you have superalloy material inquiry, please contact us without hesitation. Shanghai HY Industry Co., Ltd is Corrosion & fatigue nickel alloy professional manufacturer,We will provide you with sincere and thoughtful service.

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