English Views: 0 Author: Eternal Tech Team Publish Time: 2026-09-27 Origin: Site
When a stainless steel forging comes off the forging press, its size and shape are basically in place. But from a metallurgical point of view, forging changes the shape and also has a big effect on the material's microstructure: grains are stretched and broken, carbides precipitate along grain boundaries during cooling, and working stress is introduced into the matrix. These problems will not go away just because the forging "looks fine." Solution treatment is the microstructure repair step that a forging plant must complete after forging. Its core operation is: heat the forging again to a high-temperature single-phase zone, hold it so the precipitates fully dissolve, then cool it quickly to get a supersaturated, uniform solid solution.
It is important to note that the timing of solution treatment after forging directly affects the result. In the traditional process, the forging is usually air-cooled to room temperature, rough-machined, and then heated again for solution treatment. But during this process, the alloy carbide Cr₂₃C₆ precipitates at grain boundaries. Based on many years of stainless steel forging experience, Jiangyin Eternal Heavy Industry Co., Ltd. believes a better way is to put the forging into the furnace right after forging, use its residual heat to quickly raise it to the solution temperature, and shorten the time window for carbide precipitation.
After forging, stainless steel forgings face three metallurgical problems that need to be solved.
First, carbide precipitation and sensitization. During cooling after forging, austenitic stainless steel passes through the sensitization temperature range of 450–850°C. When it stays in this range, carbon and chromium combine at grain boundaries to form Cr₂₃C₆ carbides. The formation of these carbides uses up chromium near the grain boundaries and creates a “chromium-depleted zone.” The chromium content in this zone may drop below 12%, and the steel loses the passivation ability that makes stainless steel corrosion-resistant. Cases of forged high-pressure valves show that if the solution temperature is not high enough or the cooling rate is not fast enough, the material will be in a sensitized state, and the risk of intergranular corrosion will rise clearly. Research on the effect of cooling rate on large 316H forgings also confirms that as the cooling rate decreases, precipitates gradually increase, intergranular corrosion sensitivity increases, and resistance to intergranular corrosion decreases.
Second, sigma phase precipitation. During forging and later cooling, duplex stainless steel and super austenitic stainless steel may precipitate sigma phase in the 600–900°C range. Sigma phase is an intermetallic compound rich in chromium and molybdenum. It is very hard and brittle, and its presence can seriously reduce the toughness and corrosion resistance of the material. Solution treatment after forging suppresses the formation and growth of sigma phase by quickly passing through this temperature range.
Third, uneven grains and working stress. Forging deformation is not evenly distributed in different positions, which leads to differences in grain size. Solution treatment has a homogenizing effect on grain size control after forging. But the solution temperature must be controlled with a precise balance: if the temperature is too high, grains grow too much; if it is too low, Cr₂N phase precipitates, which affects ductility and corrosion resistance.
Austenitic stainless steel forgings have the most rigid demand for solution treatment. Take AISI 304/304L and 316/316L as examples. Both ASTM A276 and A479 specify that their standard heat treatment state is solution treatment. The typical solution temperature range is 1040–1120°C. The holding time is estimated at 1–1.5 min/mm based on effective thickness. After coming out of the furnace, they must be quickly water-quenched.
For large-section forgings, the conflict between holding time and grain growth is especially serious. Tests show that when austenitic stainless steel forgings are held at 1050°C or above for more than 4 hours, the grains begin to grow clearly. Therefore, for large-tonnage forgings, the recommended process route is to put them into the furnace right after forging and heat them quickly, instead of the traditional way of air cooling and then reheating.
321 and 347 are stabilized austenitic stainless steels containing Ti and Nb. After solution treatment (about 1000–1100°C) dissolves the carbides, a stabilization treatment at 850–900°C is usually still needed. This makes carbon combine first with Ti/Nb to form stable MC-type carbides, thereby “locking” the carbon and preventing it from combining with chromium again during later high-temperature service.
High-nitrogen austenitic stainless steel is an important development direction in recent years. A 2025 study shows that the nitrogen content of new high-nitrogen non-magnetic austenitic stainless steel has exceeded 0.85%, filling the domestic technical gap in high-nitrogen stainless steel melted at atmospheric pressure. The solution treatment temperature for this type of material usually needs to be raised to 1100–1180°C, with a holding time of 2–8 hours, to ensure sufficient solution of nitrogen.
The purpose of solution treatment for duplex stainless steel is fundamentally different from that for austenitic steel. For 2205 (UNS S31803/S32205), the forging solution treatment temperature range is 1020–1100°C, followed by water quenching. The core goals are: dissolve sigma phase, restore the austenite/ferrite two-phase ratio to close to 50:50, and obtain a uniform microstructure.
ASTM A182 has clear rules for the solution treatment of F51 and F60 forgings. It also requires verification of ferrite content (40–60%) and a pitting corrosion test according to ASTM A923 Method C. This means the effect of solution treatment on duplex steel forgings can be checked not only by metallographic examination, but also must be confirmed by corrosion testing.
One process rule that needs special attention is: duplex steel forgings must not be given a simple stress-relief annealing after welding. This is because the stress-relief temperature of 600–900°C falls exactly in the sigma phase precipitation range, and this will reduce local corrosion resistance. If post-weld heat treatment is really needed, the forging must be solution-treated again above 1020°C and water-quenched.
For super duplex stainless steel (such as 2507/UNS S32750), the solution temperature usually needs to be raised to 1050–1125°C, and the cooling rate requirements are even stricter.
17-4PH (UNS S17400, AISI 630) is the most widely used precipitation-hardening martensitic stainless steel. Its heat treatment logic is a “two-step” process: first solution treatment, then aging. ASTM A564-13 specifies that solution treatment is held at 1040°C ± 15°C for 30 minutes, then air-cooled to below 32°C, followed by aging at 480–620°C. The core purpose of solution treatment is to obtain a supersaturated low-carbon martensitic matrix and at the same time dissolve carbides and copper-rich phases precipitated during forging. A 2024 study further verified this process route, using solution treatment at 1050°C × 0.5 h, then air cooling, followed by later aging treatment.
It is worth noting that the forging temperature of 17-4PH should not exceed 1038°C, and hot working should not be done below a certain lower limit temperature. After forging, it must be cooled to room temperature before solution treatment, and then aged. This is the consistent requirement of ASTM A564 and AMS 5643.
For martensitic stainless steels (such as 410, 420, and 440C), the post-forging heat treatment is similar in operation to solution treatment—heating to the austenitizing temperature and then oil quenching or air cooling—but the purpose is completely different. Its goal is not to obtain supersaturated austenite, but to obtain a martensitic structure, which must then be tempered to adjust hardness and toughness. ASTM A473-24a, as a general standard for stainless steel forgings, covers the solution heat treatment and annealing requirements for austenitic, austenitic-ferritic, ferritic, and martensitic stainless steel forgings.
Ferritic stainless steels (such as 430 and 446) have no martensitic transformation. They generally do not undergo solution treatment in the strict sense. Instead, they are annealed at 750–850°C. Air cooling after annealing is enough, because ferritic steel is much less prone to sensitization than austenitic steel.
As a stainless steel forging plant, when performing solution treatment, we need to anchor to the following standards:
ASTM A484/A484M-24b is the general requirements specification for stainless steel bars, billets, shapes, and forgings. It explicitly specifies that austenitic stainless steels and austenitic-ferritic stainless steels shall be delivered in the solution-annealed condition and shall meet the requirements for temperature, permissible annealing procedures, quenching, and rapid cooling.
ASTM A473-24a is the product specification for stainless steel forgings and covers austenitic, austenitic-ferritic, ferritic, and martensitic stainless steel forgings. For different grades, this specification details the specific temperatures, holding times, and cooling methods for solution treatment, stabilization treatment, annealing, and quenching and tempering.
ASTM A182/A182M applies to forged or rolled stainless steel pipe flanges, forged fittings, and valve parts for high-temperature service. The solution treatment requirements for F51 and F60 (1020–1100°C, water quenching) are specified in this standard.
ASTM A564/A564M applies to precipitation-hardening stainless steel forgings and specifies the process parameters for solution annealing and age hardening for grades such as 17-4PH.
ASTM A705/A705M-23 likewise applies to age-hardening stainless steel forgings. The 2023 edition further clarifies the tensile property requirements to be met after solution annealing and aging.
In actual production, the effect of solution treatment depends on precise control of several key variables.
Temperature window: The solution temperature differs a lot between grades. For 304/304L, it is 1040–1120°C; for 316L, it is a little higher to compensate for the added Mo; for 2205, it is 1020–1100°C; for 17-4PH, it is 1040 ± 15°C; and for high-nitrogen steel, it needs to be 1100–1180°C. If the temperature is off by more than ±15°C, grains may become coarse or precipitates may remain.
Holding time: It is calculated by effective thickness, usually 1–1.5 min/mm. But for large-section forgings, it is necessary to check whether the core has reached the solution temperature. Too long holding time (for example, more than 4 hours above 1050°C) will cause grains to grow and lower mechanical properties.
Cooling rate: This is the variable most easily underestimated. Austenitic stainless steel and duplex steel must be water-quenched or force-air-cooled. Research on large 316H forgings shows that when the cooling rate is below 0.042°C/s, precipitates begin to appear; when the cooling rate drops to 0.032°C/s, severe cracks and surface peeling appear on the bending specimen surface. For thick-wall forgings, it is necessary to make sure the core cooling rate meets the requirement. If needed, use a circulating water tank and stirring to strengthen cooling.
Time between forging and solution treatment: For grades with higher carbon content or stabilized steels containing Nb or Ti, during the process of air cooling after forging to room temperature and then reheating, Cr₂₃C₆ will precipitate at grain boundaries. The recommended practice is to put the forging directly into the furnace after forging and heat it quickly, to shorten the time spent in the sensitization range.
Jiangyin Eternal Heavy Industry Co., Ltd., as a professional stainless steel forging plant, from a forging plant's cost point of view, solution treatment means extra heating energy, water quenching equipment, and process time. But the cost of not doing solution treatment or not doing it well is higher: forgings fail the corrosion resistance test at the customer's end and need rework or even scrap; intergranular corrosion cracks appear in the heat-affected zone after welding, causing the whole batch of parts to be returned; the ferrite content of duplex steel forgings deviates from the standard range, and they cannot pass the ASTM A923 corrosion test.
For forging suppliers serving end markets such as oil and gas, chemical, nuclear power, and marine engineering, the ability to do solution treatment is itself a qualification. ASTM A182 requires F51 forgings to undergo ferrite content verification and corrosion testing, and the NORSOK M-650 standard clearly requires that super austenitic 6Mo alloy forgings must undergo post-forging solution treatment. These standard requirements mean that solution treatment is not an auxiliary process that a forging plant can outsource, but should be built and maintained as a core process capability.
