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Heat-resistant Steel Castings and Their Heat Treatment

Heat-resistant steel castings are parts cast from heat-resistant alloy steel as raw materials. Under high temperature conditions, steel with oxidation resistance, sufficient high temperature strength and good heat resistance is called heat resistant steel. Heat-resistant steel can be divided into two types: oxidation-resistant steel and heat-strength steel according to its properties. Anti-oxidation steel is also referred to as non-skinned steel. Hot-strength steel refers to steel that has good oxidation resistance at high temperatures and high high-temperature strength. In the case of the same chemical composition, the as-cast state has higher thermal strength than the rolled state. Cast heat-resistant steel parts occupy a large proportion in the field of heat-resistant steel.

Some high-alloy heat-resistant steels are difficult to process and deform. The production of castings is not only more cost-effective than rolled products, but also castings have higher endurance strength. Therefore, heat-resistant cast steel occupies a considerable proportion of heat-resistant steel. In addition to sand casting, precision casting techniques can also be used to obtain products with smooth surfaces and precise dimensions. Centrifugal casting is often used for high temperature furnace tubes for the cracking of synthetic ammonia and ethylene

According to the metallographic structure in the normalized state, heat-resistant cast steel can be divided into four categories: pearlite heat-resistant steel, martensitic heat-resistant steel, ferritic heat-resistant steel, and austenitic heat-resistant steel.

 

1. Pearlitic Heat-resistant Cast Steel

The alloying elements of pearlite heat-resistant cast steel are mainly chromium and molybdenum, and the total amount generally does not exceed 5%. In addition to pearlite and ferrite, its structure also has bainite. This kind of steel has good high-temperature strength and process performance at 500-600℃. They are widely used to make heat-resistant parts below 600℃, such as boiler steel pipes, steam turbine impellers, rotors, fasteners, high-pressure vessels, pipelines, etc. Typical grades are: 16Mo, 15CrMo, 12Cr1MoV, 12Cr2MoWVTiB, 10Cr2Mo1, 25Cr2Mo1V, 20Cr3MoWV, etc.

The heat treatment of pearlite heat-resistant steel castings is mainly normalized or quenched and tempered in order to obtain a stable structure, good comprehensive mechanical properties and required strength under high temperature conditions.

 

2. Martensitic Heat-resistant Cast Steel

The chromium content of martensitic heat-resistant cast steel is generally 7-13%. It has higher high-temperature strength, oxidation resistance and water vapor corrosion resistance below 650℃, but its weldability is poor. 1Cr13, 2Cr13 containing about 12% chromium, and steel grades developed on this basis such as 1Cr11MoV, 1Cr12WMoV, 2Cr12WMoNbVB and other alloys are usually used to make steam turbine blades, discs, shafts, fasteners, etc. In addition, 4Cr9Si2, 4Cr10Si2Mo, etc. used in the manufacture of exhaust valves for internal combustion engines are also martensitic heat-resistant steels.

The common heat treatment process for martensitic heat-resistant steel castings is normalizing + tempering.

 

3. Ferritic Heat-resistant Cast Steel

Ferritic heat-resistant cast steel contains more chromium, aluminum, silicon and other elements, forming a single-phase ferrite structure, which has good oxidation resistance and high temperature gas corrosion resistance, but their high temperature strength is low, at room temperature The brittleness is greater and the weldability is poor. Such as 1Cr13SiAl, 1Cr25Si2, etc. Ferritic heat-resistant cast steel is generally used to make parts that bear low load and require high-temperature oxidation resistance.

The heat treatment of ferritic heat-resistant steel castings generally adopts annealing in order to relieve stress, and then rapid cooling (in order to quickly pass through the brittle zone of 400-500 ℃).

 

4. Austenitic Heat-resistant Cast Steel

Austenitic heat-resistant cast steel contains more austenite-forming elements such as nickel, manganese, nitrogen, etc., at temperatures above 600°C, it has good high-temperature strength and structural stability, and good welding performance. Usually used as a heat-strength material that works above 600°C. Typical grades are 1Cr18Ni9Ti (321), 1Cr23Ni13 (309), 0Cr25Ni20 (310S), 1Cr25Ni20Si2 (314), 2Cr20Mn9Ni2Si2N, 4Cr14Ni14W2Mo, etc.

Austenitic anti-oxidation steel can be treated with high temperature solution heat treatment to obtain good cold deformation. Austenitic hot-strength steel is first treated with high-temperature solution treatment, and then aging treatment is carried out at 60-100℃ higher than the use temperature to stabilize the structure and precipitate the second phase to strengthen the matrix.

 

Uses of Heat-resistant Steel Castings

Heat-resistant steel is often used to manufacture parts and components that work at high temperatures in industrial sectors such as boilers, steam turbines, power machinery, industrial furnaces, and aviation and petrochemical industries. In addition to high-temperature strength and high-temperature oxidation and corrosion resistance, these parts also require sufficient toughness, good workability and weldability, and a certain degree of structural stability according to different uses.

 

Some Grades of Heat Resistant Alloy Steel and Their Applications

Steel Grade Workable Temperature and Applications
00Cr12 Anti-oxidation temperature 600~700℃, used as high temperature, high pressure valve body, burner
0Cr13Al Applicable temperature range 700~800℃, gas turbine compressor blade
1Cr17 Anti-oxidation at temperatures below 900℃, used as high temperature parts and nozzles for furnaces
1Cr12 Oxidation resistance and high high temperature strength in the temperature range of 600~700℃ and used for high temperature parts of steam turbine blades, nozzles, and boiler burner valves.
1Cr13 The oxidation resistance temperature is 700~800℃, and its use is the same as that of 1Cr12 steel
0Cr18Ni9,1Cr18Ni9Ti Anti-oxidation temperature below 870℃, can be used as boiler heating surface pipe, heating furnace parts, heat exchanger, muffle furnace, converter, nozzle
0Cr18Ni10Ti, 0Cr18Ni11Nb It is resistant to high temperature corrosion and oxidation in the temperature range of 400~900℃, and can be used for pipe fittings whose working temperature is below 850℃
0Cr23Ni13 Anti-oxidation temperature up to 980℃, used for burner fire tube, steam turbine blade, heating furnace body, methane conversion device, high temperature separation device
0Cr25Ni20 Anti-oxidation temperature up to 1035℃, used for heating furnace parts; gas transmission system parts with working temperature below 950℃
0Cr17Ni12Mo2, 0Cr19Ni13Mo2 The anti-oxidation temperature is not lower than 870℃, the working temperature is 600~750℃, the heat exchanger tubes and furnace fittings for chemical industry and oil refining.
0Cr17Ni7Al High temperature load-bearing parts with working temperature below 550℃

 

 


Post time: Sep-10-2021