Does duplex steel pipe need to be solution treated after hot forming

Duplex steel pipe is a stainless steel material containing a two-phase structure of ferrite and austenite, with a typical phase ratio of about 50:50. This two-phase structure combines high strength with good corrosion resistance and is widely used in high-corrosion and high-stress environments such as oil and gas, chemical industry, and marine engineering. Common grades such as UNS S31803 and S32205 are extremely sensitive to hot working processes due to their special metallurgical structure.

Hot forming process characteristics of duplex steel pipe
Hot forming refers to the process of heating the steel pipe to a high temperature and then forming it through a mold. For duplex stainless steel, the hot forming temperature is generally controlled between 1100°C and 1250°C. Within this temperature range, the material has good plasticity and is easy to form. However, there is also the risk of phase change, coarsening of the structure, and precipitation of harmful phases, especially when the high temperature stays for too long.
If the cooling control is not appropriate during the hot forming process, it may lead to an increase in the ferrite phase, a decrease in the austenite phase, and even the formation of brittle phases such as σ phase (sigma phase) and χ phase (chi phase), which will seriously weaken the toughness and corrosion resistance of the material.

Definition and purpose of solution treatment
Solution treatment refers to heating the material to an appropriate temperature (usually 1020°C to 1100°C), keeping it warm for a certain period of time, and then cooling it rapidly, so that the previously precipitated harmful phases are dissolved into the austenite matrix and a uniform duplex structure is obtained.
The main purposes of this treatment include:
Restore the standard austenite/ferrite ratio;
Dissolve harmful intermetallic compounds such as σ phase and χ phase precipitated during hot working;
Refine the grains and improve the overall mechanical properties;
Improve the resistance to pitting, crevice corrosion and stress corrosion cracking;
Restore the toughness and ductility of the material.

Is solution treatment necessary after hot forming of Duplex steel pipes?
Whether solution treatment is required should be determined based on the specific process parameters of hot forming:
Case 1: Reasonable hot forming temperature control and fast cooling speed
If the hot forming temperature is strictly controlled between 1100–1200°C, the heating time is short, and rapid cooling (such as air cooling or water cooling) is performed immediately after forming, the precipitation of harmful phases can be avoided. In this case, the austenite phase can be spontaneously regenerated to a reasonable proportion, and additional solution treatment may not be required.
Applicable to products with thin wall thickness, small geometric changes, and small forming deformation, such as elbows, short sections, reducers, etc.

Case 2: The hot forming temperature is too high or the holding time is too long
If the hot forming temperature is higher than 1250°C, or the retention time at high temperature exceeds 10 minutes, it is very easy to cause grain coarsening, σ phase formation and uneven structure. Especially in the process of hot forming with large wall thickness or large deformation, this kind of organizational change is difficult to repair by natural cooling, and subsequent solution treatment must be used to restore the ideal organizational state.
Applicable to key application areas such as thick-walled pipe fittings, pressure vessels, flanges and forged special-shaped parts.

Case 3: Natural cooling or slow cooling after hot forming
Natural cooling or slow cooling (especially cooling in the furnace) can easily cause the organization to stay in the 850°C–950°C temperature zone for too long. This temperature zone is a highly sensitive area for the formation of σ phase, and solution treatment is indispensable. It must be reheated to above 1050°C for complete dissolution.

Process specification requirements for solution treatment
The solution treatment process should strictly comply with the following process requirements:

Heating rate control: prevent temperature difference cracks;
Housing temperature: 1050°C to 1100°C;
Housing time: depends on the wall thickness, generally 15–30 minutes for every 25mm wall thickness;
Cooling method: water quenching or rapid air cooling to ensure that it does not stay in the σ phase formation zone;
Subsequent testing: metallographic analysis or magnetic induction method is used to confirm that the phase ratio meets the standard.
To ensure product quality, it is recommended to implement the heat treatment clauses in international standards such as ASTM A240, ASME SA789, and EN 10088.

Solution treatment improves the performance of Duplex steel pipes
Solution treatment after hot forming can significantly improve the following properties:
Improved pitting resistance index (PREN);
Enhanced resistance to chloride stress corrosion cracking;
Significantly improved elongation and impact toughness;
Extended service life and reduced failure risk;
Improved overall welding performance and structural stability.
For demanding engineering projects such as submarine transmission pipelines, seawater heat exchangers, refinery pressure vessels, etc., solution treatment is a necessary quality assurance link.