Analysis of Forging Technology for Stainless Steel

Stainless steel is widely used in national defense industry, chemical industry, petroleum industry and power sector, and many products require not only corrosion resistance, but also high strength, so most stainless steel must be used after forging. Compared with carbon steel, stainless steel has different characteristics: low thermal conductivity, narrow forging temperature range, strong superheat sensitivity, high resistance at high temperature and low plasticity, which bring many difficulties to forging production, and different types of stainless steel forging process are also different.

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Forging of Austenitic Stainless Steel
Forging of Ferritic Stainless Steel
Forging of martensitic stainless steel
Precipitation hardening stainless steel

20190805155851 55132 - Analysis of Forging Technology for Stainless Steel

1. Forging of Austenitic Stainless Steel

The forging of austenitic stainless steel is more difficult than that of ordinary steel, but surface defects rarely occur. Most austenitic stainless steels can be forged in a wide temperature range above 927 ℃. Because austenitic stainless steel has no phase transformation in high temperature range, the forging temperature should be higher than martensitic stainless steel, but high chromium or low carbon stainless steel does not have the above properties, because high chromium or low carbon stainless steel will produce different content of ferrite according to the composition, and ferrite is harmful to forgability.
For austenitic stainless steel, the control of final forging is equally important. Except for stable and ultra-low carbon stainless steels, the final forging temperature of almost all austenitic stainless steels should be controlled above the sensitive temperature range and cooled rapidly below 871 ℃. This is because austenitic stainless steel is easy to form hot cracks and phase at low temperature.
For stable or ultra-low carbon steels, which are not sensitized by sensitizing treatment, forging is carried out at temperatures lower than those at higher forging temperatures, and strain hardening sometimes occurs with small reduction. Strain hardening temperature usually occurs at 538~649 ℃. When the hardness requirement is very low, solid solution annealing is required after forging.
Sulfur or selenium can improve mechanical properties of austenitic stainless steel. The harmful lath structure of selenium is less likely to exist. Titanium-added 06Crl8NillTi (321) section steel also has segregated slab structure. It is easy to cause surface cracking during forging. Cobalt-stabilized 347 steel is not easy to produce slab segregation, so it is a kind of stable steel suitable for forging.
When austenitic steel is heated, the atmosphere in the furnace should be slightly oxidized; both decarbonizing and peroxide atmosphere will produce harmful inclusions or chromium-poor phenomena, thus reducing the corrosion resistance of the steel. Chromium depletion is particularly serious in 16Cr23Nil3 (309) and 20Cr25Ni20 (310) profiles.
1) Forging process characteristics
The types and forging process characteristics of stainless steel commonly used are introduced below.
18-8 austenitic stainless steel is often used to make boiler and steam turbine parts which work for a long time below 610 C, as well as various parts in chemical production. Its forging process features are as follows:

  • (1) The surface of 18-8 austenitic stainless steel is easy to carburize when it is heated in coal furnace. Therefore, it is necessary to avoid contact with carbon-containing substances and adopt oxidizing medium to heat it so as to reduce the carburization phenomenon of steel and prevent intergranular corrosion.
  • (2) The thermal conductivity of austenitic steel should be slowly heated at low temperature, and the initial forging temperature should not be too high. Excessive high temperature has the tendency to form S-phase. At the same time, the grain size also grows rapidly. Generally, 1150 ~ 1180 ℃ is chosen.
  • (3) Surface defects of billet must be removed before forging heating, in order to prevent further expansion and scrap of forgings.
  • (4) The final forging temperature should not be too low. At the same time, slow cooling at 700 900 will precipitate. Continuous forging will produce cracks.
  • (5) Air cooling is adopted after forging, and solid solution treatment is also necessary.

2) Forging temperature range
The initial forging temperature of austenitic stainless steel is generally not more than 1200 and the final forging temperature is 825 ~ 850 ℃. The final forging temperature is mainly limited by the carbide precipitation sensitive temperature (480 ~ 820 ℃). Once the final forging temperature is within this temperature range, the carbide precipitation will increase the deformation resistance and reduce the plasticity, leading to forging cracking.

2. Forging of Ferritic Stainless Steel

After quenching, the hardness of pure chromium ferrite stainless steel does not increase significantly. In the forging process, work hardening will occur. The degree of work hardening varies with temperature and deformation. The requirement of cooling temperature is not strict after forging.
The forgeable temperature range of ferritic stainless steel is very wide, but at higher temperatures, the range is limited to a certain extent due to grain growth and fragile structure. The final forging temperature must be strictly controlled for 06O13A1 (405) section steel. The existence of a small amount of austenite in 06Crl3AK405 steel leads to the fragile grain boundary, which should be paid special attention to. Generally, ferritic steels are forged at temperatures below 704 degrees Celsius. For 16Cr25N (446) section steel, when the total reduction is 10%, the temperature must be lower than 871 ℃, so that the grain can be refined and the steel has toughness at room temperature. Ferritic stainless steel is best annealed after forging.
The forging performance of ferritic stainless steel is better. Compared with austenitic stainless steel, ferrite steel has higher recrystallization rate and lower recrystallization temperature, so the tendency of grain growth during plastic deformation is greater. The upper limit temperature of forging should be strictly limited, and the initial forging temperature of general ferrite stainless steel is 1040 ~ 1120 ℃. In order to obtain fine grain structure and prevent brittleness, the deformation amount and final forging temperature should be properly controlled. The minimum deformation amount required for grain refinement depends on the temperature. The minimum deformation amount is about 5% at 700 ℃ and 10% at 800 ~ 900 ℃ and 10% at 800 ~ 900 ℃ 15%. The final forging compression amount should not be less than 12% and 20% at 800 ℃. In order to avoid cold work hardening due to low temperature, the final forging temperature should not be lower than 705 ℃. The final forging temperature of the forging process formulated by a factory is 705 ~ 790 ℃.
Because of the poor thermal conductivity of ferritic stainless steel, when surface defects are cleaned by grinding wheel, local overheating can cause cracks, and surface defects need to be removed by air shovel cleaning or peeling.
Ferritic stainless steel can not be strengthened by heat treatment because phase transformation does not occur when it is heated and cooled in a certain temperature range. The purpose of heat treatment is to eliminate the internal stress produced during cold deformation processing and welding and improve the processing performance; secondly, to eliminate the segregation produced during solidification of castings by heat treatment, to obtain a single and homogeneous ferrite structure, and to eliminate the transformation products formed during welding and the brittleness at 475 ℃. Sex.
1) Forging process characteristics

  • (1) The recrystallization temperature of ferritic stainless steel is lower and the recrystallization speed is faster, so the tendency of grain growth during plastic deformation is greater. The grain growth of ferritic stainless steel is faster than 950X.
  • (2) Forging properties of ferritic stainless steel are limited by grain growth and structural weakening. For example, for American 405 steel (similar to 06Crl3Al), the appearance of a small amount of austenite will weaken the grain boundary, so the final forging temperature should be strictly controlled.
  • (3) In order to obtain fine grain structure, the final forging compression should not be less than 12% ~ 20%, and the final forging temperature should not be higher than 800 ℃. In order to avoid cold work hardening due to low temperature, the final forging temperature should not be lower than 705 ℃.

2) Forging temperature range

  • (1) The initial forging temperature of ferritic stainless steel should not be too high, all of which should be below 1200 ℃, especially the last heat of the blank should not exceed 1120 ℃.
  • (2) The final forging temperature of ferritic stainless steel is usually 720 ~ 800 ℃ in production, which is not allowed to be higher than 800 ℃.
  • (3) The grain growth tendency of ferrite is larger than that of austenite.

20190805160517 87681 - Analysis of Forging Technology for Stainless Steel

3. Forging of martensitic stainless steel

Martensitic stainless steel has high hardenability, namely air cooling hardening. Therefore, when cold forging martensitic steels, especially those high carbon steels, protective measures must be taken to prevent cracking. Generally, martensitic steel should be covered with insulating material or cooled uniformly in the furnace, so that it can be slowly cooled to 593 ℃. If the forgings are directly cooled by water spraying like cooling forging dies, the phenomenon of forging cracks will be caused.
Generally, martensitic stainless steel forgings are tempered after forging to reduce the hardness of the steel so that it can be machined. After machining, it is hardened by ignition and tempered.
The maximum forging temperature of martensitic stainless steel should be lower than the temperature produced, otherwise cracks will easily occur. Usually formed at 1093 ~ 1260 ℃. In the forging process, special attention should be paid not to exceed this temperature, and the phenomenon of local overheating caused by rapid metal movement should be avoided. Moreover, surface decarbonization will promote the formation of ferrite, so it should be limited.
With the increase of chromium content, the formation temperature of ferrite decreases, while a small amount of ferrite will significantly reduce its malleability. When the ferrite increases to more than 15%, the malleability will gradually increase until the structure is completely transformed into ferrite. The final forging temperature is limited by the isomorphic transformation (which starts at about 816 ℃), and this steel usually stops forging at 927 ℃ because it is difficult to form at too low temperature.
Adding sulphur or smashing to Y12Crl3 (416) section steel can improve its machinability, but these elements can cause forging problems, especially when they form surface plate structure, they are easy to crack. However, it can be eliminated by adjusting forging temperature and process. If sulfur is added, it is impossible to eliminate such cracks. From this point of view, selenium is better.
The formation of S-ferrite should be avoided in martensitic stainless steel during forging heating, because the appearance of ferrite will cause cracks in forgings. Stainless steel forgings should be avoided from overheating due to excessive heating. The decarbonization of forgings during heating mainly promotes the formation of ferrite, so the surface decarbonization of forgings should be minimized. There is no special requirement for the deformation of martensitic stainless steel at the last fire. This kind of steel is prone to cracking after forging. The reason is that martensite and carbide structure occur during air cooling after forging, and the internal stress is large. Therefore, cooling after forging must be carried out slowly. Generally, it must be slowly cooled in the sand pit or slag at 200 ℃. Isothermal annealing must be carried out in time after removing the sand pit to prevent cracking.
Martensitic stainless steels, especially Crl3 martensitic stainless steels, are widely used in less corrosive media (such as water vapor) and require high mechanical properties because of their low price. 12Crl3 steel is semi-martensitic stainless steel. In addition to martensite structure, there are ferrite structure in steel.
1) Forging process characteristics

  • (1) The isomeric transformation of martensitic stainless steel (20Crl3, 40Crl3, 14Crl7N12, etc.) occurs during heating and cooling. For this kind of steel, there is no special requirement for the final-fire deformation.
  • (2) The formation of 8-ferrite should be avoided when martensitic stainless steel forging is heated, because the appearance of ferrite will cause cracks in forgings, and the metal heating speed should be avoided to cause overheating. The surface decarbonization of forgings will cause excessive ferrite formation, so surface decarbonization should be minimized.
  • (3) Martensitic stainless steel is prone to crack after forging, because martensite and carbide structure will appear after air cooling, resulting in larger internal stress, so after forging, it should be slowly cooled, generally in the sand pit or slag around 200 ℃, annealing in time after taking out the sand pit to prevent fracture.

The forging temperatures of various stainless steels are shown in the table.

Forging temperature of stainless steel
Types of Stainless Steel Chemical composition/% Forging temperature/%
Austenitic stainless steel C Cr Ni Preheat Initial forging temperature Final forging temperature
<0.2 17~20 8~9 815 1175~1205 870~925
<0.2 22~26 12~15 815 1150~1175 925
<0.25 25~26 19~22 815 1150~1175 925
<0.25 19~21 24~26 815 1150~1175 925
Ferritic stainless steel >0.12 14~16 760 1040~1120 705~790
>0.35 18~27 760 1040~1120 705~790
<0.15 11.5~14 815 1120~1180 870~925
Martensitic stainless steel >0.15 12~14 815 1095~1150 870~925

2) Forging temperature range
The initial forging temperature of martensitic stainless steel is influenced by high temperature ferrite formation temperature and ferrite state. If ferrite is banded, cracks will easily occur. For example, when ferrite is fine spherical, plasticity will increase significantly (see table).
The initial forging temperature of martensitic stainless steel varies with the carbon content. The initial forging temperature of martensitic stainless steel varies with the carbon content, and the final forging temperature of martensitic stainless steel varies with the carbon content. The initial forging temperature of martensitic stainless steel varies with the carbon content. The final forging temperature of martensitic stainless steel varies with the carbon content.
20190805170228 58427 - Analysis of Forging Technology for Stainless Steel

4. Precipitation hardening stainless steel

1) Forging process characteristics
Both semi-austenite precipitation hardening stainless steel and Martensite Precipitation Hardening Stainless steel can obtain high hardness by heat treatment combined with martensite transformation and precipitation. These steels are the most difficult to forge and will crack if temperature regulations are not strictly observed. Their forging temperature range is very narrow. If the forging temperature is lower than 982 ℃, it must be reheated again. During grain growth and ferrite formation, these steels exhibit poor plasticity (better rigidity) at any set of forging temperatures. Therefore, heavy hammers and more impact times are required for forging to achieve the same plastic deformation as other types of stainless steel.
During dressing, the forging must be kept at a high enough temperature to prevent the occurrence of trimming cracks. In order to avoid the occurrence of these cracks, the forgings are usually subjected to mild reheating between final forging and dressing operations. In addition, the cooling of forgings, especially stainless steel in martensite system, must be controlled to prevent cracking.
The following example shows a typical forging process for slender forgings with materials of 17-4PH.
The long and slender forgings 1168.4 mm in length were forged with 396.6 mm short bar blank. It needs six steps of operation and two steam hammers forging. The first step is to stretch it to 65.5mm billet; the second step is to forge it to 50.8mm billet; the third step is to forge it to the width of the final forging die; the fourth step is to rotate it 90 degrees and cut the head; the fifth step is to pre-forge; and the sixth step is to complete the forging.
Because this type of steel is forged in a very narrow temperature range, secondary heating is required before final forging. The heating temperature is 117 ℃ and the heating time is LH for the first time and 1.5h for the second time. The heating atmosphere is slightly oxidized and the lubricant is graphite oil.
2) Forging temperature range
The initial forging temperature of martensitic precipitation hardening stainless steel is 1180 ℃, and that of Semi-austenitic precipitation hardening stainless steel is 1150 ℃.
Because the plasticity of precipitation hardening stainless steel is poor, the final forging temperature should not be too low, otherwise forging cracks will occur. The final forging temperature should not be less than 950 ℃. When it is lower than 950 ℃, it should be reheated back to the furnace.
The effect of temperature on the strength of several stainless steels is shown in the table.
20190805171120 57764 - Analysis of Forging Technology for Stainless Steel

Source: China Flange Manufacturer – Yaang Pipe Industry (www.steeljrv.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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analysis of forging technology for stainless steel - Analysis of Forging Technology for Stainless Steel
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Analysis of Forging Technology for Stainless Steel
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Compared with carbon steel, stainless steel has different characteristics: low thermal conductivity, narrow forging temperature range, strong superheat sensitivity, high resistance at high temperature and low plasticity, which bring many difficulties to forging production, and different types of stainless steel forging process are also different.
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