The solution treatment of stainless steel is closely related to its resistance to intergranular corrosion. This process directly determines the long-term stability of the material in a corrosive environment.

I. The core role of solution treatment

1. Process principle

High temperature and rapid cooling: Heat the stainless steel to 1020~1100℃ (austenitic stainless steel) or 920~980℃ (duplex stainless steel), keep the temperature to make the alloy elements (such as Cr, Ni, Mo) and carbides completely dissolved in austenite, and then rapidly cool (water quenching) to form a single supersaturated solid solution.

Key goal: Eliminate the precipitation of chromium carbide (Cr₂₃C₆) and restore the homogeneity of the material.

2. Inhibition mechanism of intergranular corrosion

Without solution treatment: carbides precipitate at the grain boundary; form a chromium-poor area (Cr<12%); the chromium-poor area corrodes preferentially (anode).

After solution treatment: carbon atoms are dissolved in the austenite matrix; the chromium content of grain boundaries is uniform (≥16%); the overall passivation film is complete (cathodic protection).

Scientific essence: Intergranular corrosion is caused by the precipitation of chromium carbide at 450~850℃ (sensitization temperature), which leads to the depletion of chromium near the grain boundaries. Solution treatment dissolves carbides at high temperature and quickly cools to "freeze" uniform structure, avoiding the formation of chromium-poor areas.

II. Key process parameters of solution treatment

1.Temperature: 1050±30℃ for austenitic steel

2.Hot-holding time: ≥1 hour for every 25mm thickness

3.Cooling rate: water quenching (≥30℃/s)

III. Verification of intergranular corrosion resistance after solution treatment

Testing standard

1. Sulfuric acid-copper sulfate corrosion test (Strauss method): GB/T 4334.5

The specimen is immersed in boiling 16% H₂SO₄ + 6% CuSO₄ solution for 24 hours. It is qualified if there is no crack after bending.

2. Nitric acid corrosion test (Huey method): ASTM A262 Practice C

65% HNO₃ boiling solution for 48 hours per cycle, a total of 5 cycles, and the corrosion rate is ≤0.6 mm/month.

IV. Limitations and supplementary processes of solution treatment

1. Limitations

Weld sensitization cannot be completely eliminated: the heat affected zone of the weld may re-enter the sensitization temperature range.

Not suitable for stabilized steel (such as 321/347): such steels rely on titanium/niobium fixed carbon (TiC/NbC).

2. Collaborative protection process

Stabilized annealing: 850~900℃ insulation to promote the formation of TiC/NbC, suitable for 321/316Ti.

Ultra-low carburization: C≤0.03%, reduce carbides from the source, suitable for 304L/316L.

Laser surface remelting: rapid melting-solidification, grain refinement, suitable for welding repair areas.

Solution treatment is the cornerstone process for improving the intergranular corrosion resistance of stainless steel. It dissolves carbides at high temperature and rapidly cools to maintain the supersaturated state of austenite, fundamentally eliminating the chromium-poor area. However, its effect needs to be combined with material composition design (ultra-low carbon/stabilization), welding control and surface treatment. In harsh environments such as chemical industry and nuclear power, a complete solution treatment process is a key prerequisite for the long-term safe operation of equipment.