On intergranular corrosion of stainless steel

07Cr17Ni7Al/UNS S17700/AISI 631 is an austenite-martensite precipitation-hardening stainless steel developed on the basis of 18-8CrNi; it is also known as a phase-controlled transformation stainless steel. After solution treatment, an unstable austenitic structure is formed, which possesses good plasticity, toughness, and workability. By adjusting the composition so that carbides precipitate from the austenite, and subsequent martensitic transformation, most of the microstructure transforms into low-carbon tempered martensite with improved toughness. This state is suitable for using the steel, as it offers good mechanical properties at medium temperatures. Its corrosion resistance is superior to that of ordinary martensitic stainless steels. Chemical composition: Carbon (C) ≤ 0.09, Manganese (Mn) ≤ 1.00, Nickel (Ni) 6.50–7.75, Silicon (Si) ≤ 1.00, Phosphorus (P) ≤ 0.035, Sulfur (S) ≤ 0.030, Chromium (Cr) 16.00–18.00, Aluminum (Al) 0.75–1.50

If the austenite structure is unstable, does that mean this material has a tendency to suffer from intergranular corrosion, and will it fail intergranular corrosion tests?

Intergranular corrosion is mainly caused by a high carbon content, which leads to chromium depletion at the grain boundaries. 07Cr17Ni7Al belongs to the precipitation-hardening stainless steels; after solution treatment, it has a martensitic or semi-austenitic structure, and adjustment treatments are generally required to obtain a martensitic structure. (The specific methods for adjustment can be found in the information on precipitation-hardening stainless steels.) The corrosion resistance is primarily determined by the matrix structure; the matrix structure of 07Cr17Ni7Al is martensitic rather than austenitic, and martensitic materials have better resistance to intergranular corrosion than austenitic ones.

Taken from the internet for reference: There are several heat treatment processes for precipitation-hardening stainless steels. 1. Solution treatment: The microstructure obtained after solution treatment at 1000–1050°C for 1 hour followed by air cooling is austenite with a small amount of ferrite. During the subsequent treatment at 500–800°C, since atoms diffuse more rapidly in ferrite than in austenite, and because ferrite contains a higher amount of chromium, carbides (Cr23C6) tend to precipitate along the phase boundaries between α(δ) and r. This also reduces the carbon and alloy element content in austenite, thereby raising the Ms point of this type of steel and allowing more martensite to form. The amount of α(δ) ferrite should not be excessive, as this is unfavorable for hot working; moreover, it does not participate in the martensitic transformation, which reduces the strength of the steel. 2. Adjustment treatment: An intermediate treatment carried out after solution treatment, also commonly referred to as adjustment treatment, aims to obtain a certain amount of martensite in order to strengthen the steel. The following three methods are commonly used: (1) Intermediate aging method (abbreviated as T-treatment): After solution treatment, the material is reheated to (760±15)°C and held at that temperature for 90 minutes. As Cr23C6 carbides precipitate from the austenite, the carbon and alloy element content in the austenite decreases, raising the Ms point to 70°C. Subsequent cooling to room temperature results in a microstructure consisting of martensite + α-ferrite + residual austenite; the residual austenite is completely decomposed only after aging at 510°C. (2) After solution treatment by high-temperature adjustment and cryogenic treatment (R treatment), it is first heated to 950°C and held there for 90 minutes. By raising the Ms point and cooling to room temperature, a small amount of martensite can be obtained ； After that, cold treatment at -70°C for 8 hours can yield a certain amount of martensite. (3) The cold deformation method (C treatment method) involves cold deformation at room temperature after solution treatment; the amount of martensite formed during cold deformation is related to the degree of deformation and the composition of the stainless steel. Generally, a deformation amount of 15% to 20% is sufficient to obtain the required amount of martensite; excessive deformation will cause work hardening of the martensite, resulting in a significant decrease in plasticity. 3. Aging treatment (H treatment): After the adjustment treatment, aging treatment must be carried out. Aging treatment is another method for strengthening this type of steel. When the aging temperature is above 400°C, intermetallic compounds (such as Ni3Ti, etc.) precipitate from martensite and are distributed in a highly dispersed manner, thereby inducing precipitation hardening. Aging is generally carried out at around 500°C to achieve high strength and hardness.

Martensite has better resistance to intergranular corrosion than austenite; can it be inferred that when conducting intergranular corrosion tests on 07Cr17Ni7Al, the intergranular corrosion tendency should not be taken into account?

Yes, corrosion indicators should be evaluated based on a combination of multiple indicators. Pitting resistance/Uniform corrosion resistance/Intergranular corrosion resistance, etc