Ferritic stainless steel forging & austenitic stainless steel forging

Basic knowledge of ferritic stainless steel forging

basic knowledge of ferritic stainless steel forging - Ferritic stainless steel forging & austenitic stainless steel forging

Characteristics of ferritic stainless steel forging Types of Ferritic Stainless Steel Chemcial Composition of Ferritic Stainless Steel
Mechanical Properties of Ferritic Stainless Steel Physical Properties of Ferritic Stainless Steel Applications of Ferritic Stainless Steel
Austenitic stainless steel Characteristics of austenitic stainless steel forging Applications of austenitic stainless steel

Pure chromium ferritic stainless steel has no significant increase in hardness after quenching. During the forging process, work hardening will occur and the hardening degree varies with temperature and shape variables. This kind of steel has no rigorous requirement for the cooling temperature after forging.
Ferritic stainless steel has a wide range of forging temperatures, but at higher temperatures, its range is limited to a certain extent due to the grain growth and the weak structure. For 06O13A1(405) steel, the final forging temperature must be strictly controlled. Generally, ferritic steels are forged at temperatures below 704 °C. For 16Cr25N (446) steel, the temperature must be lower than 871 °C when the total reduction is 10%, so as to refine the grain and let the steel have good toughness at room temperature. Ferritic stainless steel is preferably annealed after forging.
Compared with austenitic stainless steel, the forging properties of ferritic stainless steel are better since ferritic steel has higher recrystallization rate and lower recrystallization temperature, the tendency of grain growth during plastic deformation is greater. The upper limit temperature of forging should be strictly limited, generally, the initial forging temperature of the ferritic stainless steel is 1040 °C to 1120 °C. In order to obtain fine grain structure and prevent brittleness, the amount of deformation and the final forging temperature should be properly controlled. The minimum amount of deformation necessary to refine the grains depends on the temperature, which is about 5% at 700 °C, and about 10% to 15% at 800 to 900 °C. The final forging compression should not be lower than 12% – 20
%, and the final forging temperature should not higher than 800 °C. In order to avoid cold work hardening due to the low temperature, the final forging temperature should not be lower than 705 °C.
Due to the poor thermal conductivity of ferritic stainless steel, when the surface defects are cleaned by a grinding wheel, local overheating can cause cracks, and it is necessary to remove surface defects by using methods like peeling. Ferritic stainless steel does not undergo a phase change when heated or cooled in a certain temperature range, and therefore cannot be strengthened by heat treatment. The purpose of the heat treatment is firstly, to eliminate the internal stress generated during cold deformation processing and welding, and to improve the processing performance. Secondly, the segregation generated during solidification of the casting is eliminated by heat treatment to obtain a single, uniform ferrite structure.

Characteristics summary of the ferritic stainless steel forging process

  1. The ferritic stainless steel has a lower recrystallization temperature and a faster speed, so the tendency of grain growth during plastic deformation is greater.
  2. Forging properties of ferritic stainless steel are limited by grain growth and weakening of the structure. For example, for 405 stainless steel, the presence of a small amount of austenite will lead to the weakening of the grain boundary, so the final forging temperature should be strictly controlled.
  3. In order to obtain fine grain structure, the compression amount of the last forging should not be lower than 12%~20%, and the final forging temperature is not higher than 800 °C. In order to avoid work hardening due tolow temperature, the final forging temperature should not be lower than 705 °C.

Forging temperature range

  • The initial forging temperature of ferritic stainless steel should be lower than 1200 °C.
  • The final forging temperature of ferritic stainless steel is usually set at 720~800 °C in production, and is not allowed to be higher than 800 °C;
  • The grain growth of ferrite tends to be larger than that of austenite.

Types of Ferritic Stainless Steel

  • Group 1 (type 409/410L): These have the lowest chromium content of all stainless steels and are ideal for slightly corrosive environments where localized rust is acceptable. The least expensive of all stainless steels, type 409 was initially created for automotive exhaust systems silencers, but can now be found in automotive exhaust tubing and catalytic converter casings. Type 410L is often used for containers, buses and LCD monitor frames.
  • Group 2 (type 430): The most commonly used ferritic steel, type 430 has a higher chromium content and is, consequently, more resistant to corrosion by nitric acids, sulfur gasses, and many organic and food acids. In some applications, this grade can be used as a replacement for austenitic grade 304. Type 430 is often found in interior appliances, including washing machine drums, kitchen sinks, cutlery, indoor panels, dishwashers, and other cooking utensils.
  • Group 3 (type 430Ti, 439 and 441): Having better weldability and formability characteristics than Group 2 ferritic sheets of steel, Group 3 steel can be used to replace austenitic type 304 austenitic in a wider range of applications, including in sinks, exchange tubes, exhaust systems and welded parts of washing machines.
  • Group 4 (type 434, 436, 444): With a higher molybdenum content, these ferritic stainless steel grades have enhanced corrosion resistance and are used in hot water tanks, solar water heaters, exhaust system parts, electric kettles, microwave oven elements, as well as the automotive trip. Grade 444, in particular, has a pitting resistance equivalent (PRE) to Grade 316, allowing it to be used in more corrosive outdoor environments
  • Group 5 (446, 445, 447): This group of specialty stainless steels is characterized by a relatively high chromium content. The result is steel with excellent corrosion and scaling (or oxidation) resistance. In fact, the corrosion resistance of Grade 447 is equivalent to that of titanium metal. Molybdenum is also commonly added to improve corrosion resistance. Applications for Group 5 steel are found in highly corrosive coastal and offshore environments.

Chemcial Composition of Ferritic Stainless Steel

The chemical composition of different grades of ferritic stainless steel are summarised in the following table.

Table 1. Chemical composition of different grades of ferritic stainless steel

Chemical Composition

AWM 404GP™

304

Chromium

21 %

18.2 %

Nickel

0.2 %

8.2 %

Carbon

0.009 %

0.05 %

Titanium

0.3 %

Copper

0.4 %

Mechanical Properties of Ferritic Stainless Steel

The mechanical properties of different grades of ferritic stainless steel are summarised in the following table.

Table 2. Mechanical properties of ferritic stainless steel

Mechanical Properties

AWM 404GP™

304

Yield Stress (MPa)

310

290

Tensile Strength (MPa)

475

600

Elongation %

31

55

Hardness (HV)

157

157

Physical Properties of Ferritic Stainless Steel

The physical properties of different grades of ferritic stainless steel are summarised in the following table.

Table 3. Physical properties of ferritic stainless steel

Physical Properties

AWM 404GP™

304

Density ( Kg / m3 )

7,750

8,027

Young’s Modulus (MPa)

199,000

193,000

Thermal Conductivity (W/m.°C)

22.5

16.3

Thermal Conductivity (X 10-6 /°C )

10.0

15.9

Magnetism

Magnetic

Non-Magnetic

Applications of Ferritic Stainless Steel

Basic knowledge of austenitic stainless steel forging

an introduction of austenitic stainless steel forging - Ferritic stainless steel forging & austenitic stainless steel forging

Stainless steel is widely used in chemical industry, petroleum industry, power plant, etc., and many products require not only corrosion resistance, but also high strength, so most stainless steels are used after forging. Compared with carbon steel, stainless steel has characteristics such as low thermal conductivity, narrow forging temperature range, strong heat sensitivity, high resistance and low plasticity at high temperature, so on and so forth, which bring many difficulties to forging production. And there are discrepancies in different types of stainless steel forging processes.
Forging of austenitic stainless steel is more difficult than 
those of ordinary steels, but it rarely produces surface defects. Most austenitic stainless steels can be forged at temperatures above 927 °C. Because austenitic stainless steel has no phase transition in the high temperature range, its forging temperature is higher than that of martensitic stainless steel. But high chromium or low carbon stainless steels do not have the above properties, when forging high chromium or low carbon stainless steels, if the temperature is higher than 1093 °C, different levels of ferrite will be produced depending on the composition, which is harmful to the forgeability.
For austenitic stainless steels, controlling final forging is pretty important. Except for stable and ultra-low carbon stainless steel, the final forging temperature of almost all austenitic stainless steels should be controlled above the temperature sensitive zone and rapidly cooled below 871 °C. This is because thermal cracks and transformation can be easily produced on austenitic stainless steel at low temperatures. Forging of stable and ultra-low carbon steels is performed at a temperature lower than the regular forging temperature. The strain hardening usually occurs at 538 °C to 649 °C. When the hardness requirement is relatively low, solution annealing should be performed after forging.
Sulfur or selenium can play a role in improving the mechanical properties of austenitic stainless steels. The harmful slat-like structure of selenium is less likely to exist. The 321 steel with titanium also has segregated band, so when forging, it is easy to cause surface cracking. 347 stainless steel with cobalt is suitable for forging because it is not easy to produce segregation. When heating austenitic steel, the furnace should be slightly oxidative; the decarburizing and peroxidizing circumstances will produce harmful inclusions or chromium depletion, thereby reducing the corrosion resistance of the steel. The chromium depletion is particularly serious in 309 and 310 
steels.
Characteristics of the austenitic stainless steel forging process
18-8 austenitic stainless steel is often used to make long-term-used boiler and steam turbine parts that function below 610 °C, as well as various parts in chemical production. Firstly, 18-8 type austenitic stainless steel is easy to carburize when being heated in the coal stove, so avoid contact with carbon-containing substances during heating, and use oxidizing media to reduce carburization of steel and prevent intergranular corrosion. Secondly, the initial forging temperature of austenitic steel should not be too high, it should be heated slowly at 1150 – 1180 °C. Thirdly, the surface defects of the blank must be removed before heating, so as to prevent further expansion during forging and the forging from being scrapped. Fourthly, the final forging temperature should not be too low, otherwise, precipitation will occur at 700 – 900 °C and lead to cracking. Fifthly, the part must be air-cooled and solution treated after the forging process.
Temperature range
The initial forging temperature of austenitic stainless steel generally does not exceed 1200 °C; the final forging temperature is usually at  825 – 850 °C, which is mainly limited by the sensitive temperature of carbide precipitation. Once the final forging temperature is within this temperature range (480 – 820 °C), the deformation resistance is increased due to the precipitation of carbides, and the plasticity is reduced which leads to cracking.

Applications of austenitic stainless steel

Austenitic stainless steels are used in a wide range of applications, including:

  • Automotive trim
  • Cookware
  • Food and beverage equipment
  • Industrial equipment

Applications by Steel Grade

304 and 304L (standard grade):

  • Tanks
  • Storage vessels and pipes for corrosive liquids
  • Mining, chemical, cryogenic, food and beverage, and pharmaceutical equipment
  • Cutlery
  • Architecture
  • Sinks

309 and 310 (high chrome and nickel grades):

  • Furnace, kiln, and catalytic converter components
  • 318 and 316L (high moly content grades):
  • Chemical storage tanks, pressure vessels, and Customized Piping System

321 and 316Ti (“stabilized” grades):

  • Afterburners
  • Super heaters
  • Compensators
  • Expansion bellows

200 Series (low nickel grades):

  • Dishwashers and washing machines
  • Cutlery and cookware
  • In-house water tanks
  • Indoor and nonstructural architecture
  • Food and beverage equipment
  • Automobile parts

Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry (www.metallicsteel.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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References

  • https://www.yaang.com

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