In modern industries, stainless steel is widely used due to its exceptional corrosion resistance, heat resistance, and various superior properties. From kitchenware to aerospace components, from chemical equipment to medical devices, stainless steel is indispensable across countless fields. Forging plays a crucial role in stainless steel processing, significantly enhancing its performance and quality. This article explores the key aspects of stainless steel forging to help you better understand this important process.
Stainless steel is an alloy material with multiple outstanding properties. It not only offers corrosion resistance but also maintains excellent performance under both high and low temperatures. Some stainless steels are even non-magnetic. These features make stainless steel essential in many applications.
Chemically, stainless steel typically contains over 12 percent chromium, along with various other alloying elements. Based on alloy composition and structure, stainless steels are mainly classified into three categories: austenitic, martensitic, and ferritic. There are also intermediate and derivative types within these groups.
Austenitic stainless steels are formed by adding nickel, manganese, nitrogen, and other austenite-forming elements into chromium-based stainless steel with about 18 percent chromium. A common composition includes 18 percent chromium and 8 percent nickel. This type maintains an austenitic structure at both room temperature and high temperature and cannot be strengthened by heat treatment. It is typically used in solution-annealed condition. Austenitic stainless steel exhibits excellent ductility, toughness, formability, corrosion resistance, and oxidation resistance. Cold deformation can further improve strength.
It is widely used in parts requiring corrosion resistance, oxidation resistance, or service at elevated or cryogenic temperatures. However, it may become susceptible to intergranular corrosion after sensitization between 600 and 860°C. Improper processing or harsh environments can also trigger stress corrosion cracking and hydrogen embrittlement.
Ferritic stainless steel has a body-centered cubic structure with chromium dissolved in ferrite (α-Fe solid solution). Chromium content typically ranges from 13 to 30 percent. Although its corrosion resistance is generally inferior to that of austenitic stainless steel, its resistance to stress corrosion is superior. If interstitial elements are not well controlled, issues such as 475°C brittleness, sigma phase embrittlement, and high-temperature brittleness (above 950°C) may occur.
Modern smelting technology reduces carbon and nitrogen content below 100×10-6, preventing such brittleness. Ferritic stainless steels are expected to replace some austenitic grades. For air-melted ferritic stainless steels, processing parameters must avoid severe grain growth and brittleness during hot deformation.
Martensitic stainless steel contains 12 to 18 percent chromium. It forms austenite at high temperatures but transforms into martensite upon cooling to room temperature and can be strengthened by heat treatment. It is an ideal structural material combining corrosion resistance and heat resistance. In Cr-based steels, quenching increases strength and hardness with rising carbon content, while corrosion resistance, ductility, and toughness decrease.
Martensitic stainless steels provide high heat strength and adequate corrosion resistance, making them suitable for load-bearing parts operating below 550–600°C or in humid environments. Their corrosion resistance is lower than that of ferritic and austenitic steels, so heat treatment parameters must consider resistance to stress corrosion and the influence on heat resistance and ductility.
Precipitation-hardening stainless steels combine advantages of austenitic and martensitic types and are considered transitional grades. They can be austenitic, semi-austenitic, or martensitic. With more than 12 percent chromium and less than 0.1 percent carbon, they contain alloying elements such as Al, Ti, W, and Mo used for compound formation and strengthening.
These steels are solution-treated to form austenite (sometimes with little ferrite), cooled, and then strengthened through cold working, tempering, or cold deformation, followed by aging treatment to achieve precipitation hardening. Typical grades include 0Cr17Ni4Cu4Nb (17-4PH, martensitic), 0Cr17Ni7Al (17-7PH, semi-martensitic), and A266 (austenitic). Their forging temperature range is narrow, formability is poor, hardening tendency is high, and grains easily coarsen. Strict control of deformation parameters is required.
Forging temperature has a decisive impact on the final performance and quality of stainless steel products. Different stainless steel types require different forging temperatures.
Austenitic and ferritic stainless steels do not undergo phase transformation during heating and cooling. Grain refinement depends primarily on starting and finishing forging temperatures and total deformation. Heat treatment cannot refine grains afterward.
Excessive temperatures promote the formation of high-temperature ferrite (α or δ) in austenitic steels and δ ferrite in martensitic steels, reducing plasticity due to mechanical differences between phases, which can result in cracking along phase boundaries. Ferrite formation generally occurs between 1000 and 1300°C depending on grade.
Ferritic steels are prone to grain growth, so forging should be done at lower temperatures to ensure fine grains and reduce intergranular corrosion and notch sensitivity. Typical starting temperature is 1120°C and finishing temperature is 700–800°C, not exceeding 800°C.
Martensitic stainless steels generally start forging at about 1150°C. Finishing temperature depends on carbon content: 925°C for high-carbon grades and 850°C for low-carbon grades. Both must remain above the phase transformation temperature.
Austenitic steels usually start forging at 1150–1200°C and finish at 825–850°C. Ordinary 18-8 types start at 1200°C, while those containing molybdenum or with high silicon require below 1150°C. For 25-12 and 25-20 types, starting temperature must not exceed 1150°C, finishing not lower than 925°C.
Starting temperatures usually range from 1120 to 1180°C, and finishing from 850 to 950°C. Martensitic PH types start at 1180°C with finishing above 850°C. Complex shapes require furnace cooling. Semi-austenitic grades start at 1150°C and finish above 950°C.
Lowering starting temperature by 50–80°C during final forging helps refine grains. For corrosion resistance, stainless steel must be heated in protective, neutral, or mildly oxidizing atmospheres and never in reducing or strongly oxidizing atmospheres. Direct flame impingement is prohibited to prevent carburization or chromium depletion at grain boundaries.
High-temperature exposure time must be limited to prevent oxidation, alloy depletion, and grain coarsening. It should generally not be less than 10–20 minutes and must follow structural-steel heating requirements.
Due to microstructural characteristics, cooling conditions must be controlled.
Martensitic stainless steels should be slowly cooled to about 600°C then air cooled to avoid cracking caused by martensitic transformation or 475°C embrittlement.
Ferritic and austenitic stainless steels require rapid cooling to avoid 475°C embrittlement in ferritic types and Cr23C6 precipitation along grain boundaries in austenitic types, which promotes intergranular corrosion. Sensitization in austenitic steels occurs between 480 and 815°C and must be avoided by fast cooling.
Post-deformation processing has significant impact on corrosion resistance and forgings quality.
For martensitic stainless steels, martensite may form during post-forging cooling due to high hardenability. Pickling must be carried out after tempering to prevent cracking from stress corrosion. Untreated forgings may naturally crack, so timely tempering is mandatory.
For ferritic and austenitic grades, solution annealing at 1050–1070°C followed by water quenching prevents intergranular corrosion by keeping chromium carbides in solution. Austenitic steels used after cold deformation perform best with the sequence: solution treatment → cold deformation → sensitization treatment. Reversing this sequence severely reduces corrosion resistance.
Ferritic and austenitic steels lack phase transformation, so grain refinement depends on forging parameters. Martensitic steels can suffer coarse grains if heated excessively or deformed insufficiently, due to inherited structures. Final deformation must exceed 12–20 percent. Ferritic steels require at least 30 percent due to strong grain-growth tendency.
For stainless steel ingots, a forging ratio of 2–3 effectively breaks columnar grains and welds internal defects, improving grain refinement. For example, forging Cr23Ni8 ingots with a ratio of 2.15 improves strength by 30–50 percent compared to cast steel.
Presence of α-phase in austenitic stainless steels can cause cracking during forging along phase interfaces. When α-phase is excessive, forming operations that generate high tensile stress such as upsetting and piercing should be avoided. The α-phase volume fraction must be kept below 10 percent.
For materials with high ferrite content, preheating should include longer soaking time or solution treatment to dissolve ferrite into austenite, round particles, or convert bands into chains, improving plasticity.
Several additional considerations are required during forging:
Heating atmosphere: Stainless steel must be heated in protective, neutral, or slightly oxidizing atmospheres. Reducing or highly oxidizing environments or direct flame exposure can cause carburization or chromium depletion and reduce intergranular corrosion resistance.
High-temperature holding time: Holding time must be controlled to prevent oxidation, alloy depletion, and grain coarsening.
Cooling rate: Different cooling methods apply to different types, as described above.
Sequence of processing: Proper planning of heat treatment and cleaning prevents corrosion issues and quality defects.
Stainless steel forging is a complex and precise process that significantly affects the final performance and quality of stainless steel products. By properly controlling forging temperature, cooling behavior, and subsequent processing, manufacturers can greatly enhance product reliability and properties. Each type of stainless steel has its own forging requirements, so adjustments must be made based on the specific grade and application.
Forged stainless steel components are widely used in aerospace, chemical processing, medical equipment, and food industries, contributing greatly to modern industrial development. We hope this article helps you understand stainless steel forging better. If you have further questions or need more information, feel free to contact us.
