Details should be paid attention to when performing solution treatment duplex stainless steel pipe

The solid solution treatment of duplex stainless steel pipes requires attention to details such as temperature, insulation time, and cooling rate to ensure tissue uniformity and corrosion resistance, as follows:

1、 Temperature control

Accurately set temperature range:

Conventional duplex steel (such as S32205) needs to be controlled at 1050-1100 ℃, while super duplex steel (such as S32750) needs to be controlled at 1100-1150 ℃. If the temperature is too low, it can easily lead to residual precipitates (such as σ phase), while if it is too high, the proportion of austenite phase will decrease, affecting the performance.

Requirements for furnace temperature uniformity:

The temperature difference inside the furnace is ≤± 10 ℃. It is recommended to use a vacuum furnace or a protective atmosphere furnace (such as nitrogen) to avoid surface chromium removal caused by high temperature oxidation, which affects corrosion resistance.

2、 Control of insulation time

Adjust the duration according to the wall thickness:

Calculated based on the wall thickness of the pipe, δ (mm), the insulation time is usually 1-2 minutes/mm. For example, when δ=10mm, the insulation time is 10-20 minutes to ensure that the carbides are fully dissolved and the two-phase structure is homogenized.

Avoid excessive insulation:

Exceeding the optimal time can lead to coarse grains, reduced ductility, and increased energy consumption and oxidation risk.

3、 Cooling rate control

Quickly cool to below the phase transition temperature:

Quickly cool from the solid solution temperature to below 400 ℃ (such as water cooling or air cooling+forced air cooling) to prevent brittle zone (ferrite precipitation induced hardening) and precipitation of σ phase (Cr Fe intermetallic compound) at 475 ℃, with a cooling rate of ≥ 50 ℃/s.

Cooling medium selection:

Thin walled tubes (δ ≤ 5mm) can be air-cooled, while thick walled tubes (δ>5mm) must be water-cooled to avoid uneven cooling leading to uneven organization or stress concentration.

4、 Surface protection and cleaning

Prevent oxidation and decarbonization:

Remove surface oil stains and oxide scales before entering the furnace, and spray high-temperature antioxidants; If an air furnace is used, acid washing (such as 10% hydrofluoric acid+30% nitric acid solution) should be carried out after cooling to remove the oxide layer and restore the passivation film.

Avoid residual pollution:

The tooling that comes into contact with pipes should be made of stainless steel or ceramic materials to prevent iron ion contamination (such as residual iron filings causing local corrosion).

5、 Stress and deformation control

Optimization of furnace loading method:

The pipes are vertically suspended or horizontally placed on heat-resistant supports to avoid stacking and squeezing, and to prevent elliptical deformation due to their own weight at high temperatures. The spacing between supports is ≤ 1.5 meters.

Stress relief treatment after cooling:

For precision fittings, low-temperature annealing at 200-300 ℃ can be performed after solid solution treatment to eliminate cooling stress and avoid deformation caused by stress release during subsequent processing.

6、 Performance testing verification

Metallographic examination:

Sampling samples were observed to determine the austenite ferrite two-phase ratio (target 45% -55%), with no residual precipitates and a grain size of 5-8 levels (ASTM standard).

Hardness test:

After solid solution, the hardness should be ≤ HB290. If it exceeds this limit, the temperature or holding time needs to be readjusted to avoid exacerbating the work hardening tendency.

Strictly controlling the above details can ensure that a uniform two-phase structure is obtained after solid solution treatment of duplex stainless steel pipes, improving corrosion resistance and processing performance.