What details should be paid attention to when conducting the solution treatment for duplex stainless

What details should be paid attention to when conducting the solution treatment for duplex stainless steel pipes? 

For the solution treatment of duplex stainless steel tubes, attention should be paid to details such as temperature, holding time, and cooling rate to ensure uniformity of the structure and corrosion resistance. The specific requirements are as follows: 

I. Temperature Control

Precisely set the temperature range:

For conventional duplex steel (such as S32205), it should be controlled within 1050-1100℃; for super duplex steel (such as S32750), it should be 1100-1150℃. If the temperature is too low, it may lead to the residue of precipitated phases (such as σ phase), while if it is too high, the proportion of austenite will decrease, affecting the performance.

Uniformity of furnace temperature requirements:

The temperature difference within the furnace should be ≤ ±10℃. It is recommended to use a vacuum furnace or a protective atmosphere furnace (such as nitrogen), to avoid surface decarburization caused by high-temperature oxidation and affect the corrosion resistance. 

II. Control of Insulation Time

Adjust the duration based on the wall thickness:

Calculate the insulation time using the pipe wall thickness δ (mm). Generally, the insulation time is 1-2 minutes per mm. For example, when δ = 10 mm, the insulation time should be 10-20 minutes to ensure that the carbides are fully dissolved and the two-phase structure is homogenized.

Avoid excessive insulation time:

Insulating for longer than the optimal duration can lead to coarse grains, reduce ductility, and increase energy consumption and oxidation risks. 

III. Cooling Rate Control

Quickly cool to below the phase transformation temperature:

From the solution temperature, quickly cool to below 400℃ (such as water cooling or air cooling + forced air cooling), to prevent the formation of the brittle zone at 475℃ (formation of ferrite leading to hardening) and the precipitation of σ phase (Cr-Fe intermetallic compound), the cooling rate should be ≥ 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 which may result in uneven microstructure or stress concentration. 

IV. Surface Protection and Cleaning

Prevention of Oxidation and Decarburization:

Before entering the furnace, remove surface oil stains and oxide scale. Apply high-temperature anti-oxidant coating; if using an air furnace, perform acid washing (such as 10% hydrofluoric acid + 30% nitric acid solution) after cooling to remove the oxide layer and restore the passivation film.

Avoiding Residual Contamination:

The tooling in contact with the pipe material should be made of stainless steel or ceramic to prevent iron ion contamination (such as residual iron filings causing local corrosion). 

V. Stress and Deformation Control

Furnace Loading Optimization:

The pipe materials are vertically suspended or placed horizontally on the heat-resistant supports to avoid stacking and squeezing. This prevents elliptical deformation caused by self-weight at high temperatures. The spacing between the supports should be no more than 1.5 meters.

Post-Cooling Stress Relief Treatment:

For precision pipe components, low-temperature annealing at 200-300℃ can be performed after solution treatment to eliminate cooling stress and prevent deformation during subsequent processing due to stress release. 

VI. Performance Testing and Verification

Metallographic structure inspection:

Inspect the ratio of austenite to ferrite in the sample (target 45%-55%), with no residual inclusions and grain size ranging from 5 to 8 levels (in accordance with ASTM standards).

Hardness test:

The hardness after solution treatment should be ≤ HB290. If it exceeds this value, the temperature or holding time needs to be adjusted to prevent the tendency of processing hardening from intensifying. 

Strictly controlling these details can ensure that the duplex stainless steel tubes obtain a uniform two-phase structure after solution treatment, thereby enhancing corrosion resistance and processing performance.

Tags: Duplex stainless steel, Knowledge of Stainless Steel, Explanation of Stainless Steel