Alloy 20 and Alloy 31 pipe plate manufacturer supplier. Contact us to get free quote and sample!
Alloy 20 (also known as Carpenter 20 or UNS N08020) and Alloy 31 (UNS N08031) are two important corrosion-resistant alloys. With their excellent comprehensive performance, they have become key materials for the industry to cope with extreme corrosion challenges. They have significant differences in composition design, performance characteristics and application scenarios. HT PIPE is a alloy 20 seamless Pipe supplier with 15+ export experience. Contact us for more information and quotes for free!
alloy 20 and alloy 31 Metallurgical structure and phase stability
When Alloy 20 is exposed to the temperature range of 650-900℃ for a long time, the precipitation of σ phase or carbide may lead to decreased toughness and increased intergranular corrosion sensitivity. Especially in the medium temperature range (such as the heat affected zone of welding), the precipitation of chromium carbides (Cr23C6) along the grain boundaries will cause "sensitization".
Alloy 31 effectively inhibits the precipitation kinetics of the intermetallic phase by adding nitrogen. Nitrogen atoms occupy interstitial positions in the lattice, reducing the diffusion rate of elements such as chromium and molybdenum, thereby delaying the precipitation of harmful phases such as σ phase and χ phase. This feature makes it have better organizational stability within the common operating temperature range of chemical equipment (300-600℃), and is suitable for long-term high-temperature service environment.
alloy 20 alloy 31 Chemical composition comparison
| elements | Alloy 20 | Alloy 31 |
|---|---|---|
| Ni | 32.0-38.0 | 30.0-32.0 |
| Cr | 19.0-21.0 | 26.0-28.0 |
| Mo | 2.0-3.0 | 6.0-7.0 |
| Cu | 3.0-4.0 | 1.0-1.4 |
| N | - | 0.15-0.25 |
| Nb | 8×C-1.0 | - |
| Fe | remainder | remainder |
| C | ≤0.07 | ≤0.015 |
uns n08031 n08020 Mechanical & Physical Properties
| Property | Alloy 20 | Alloy 31 |
|---|---|---|
| Tensile Strength | 550–620 MPa | 650–750 MPa |
| Yield Strength | 240–300 MPa | 300–350 MPa |
| Elongation | 40–45% | 35–40% |
| Hardness (HB) | 150–180 | 180–220 |
| Impact Toughness | ≥80 J (-196°C) | ≥100 J (-196°C) |
uns n08020 and uns n08031 Overall corrosion performance
Alloy 20 performs well in medium-concentration sulfuric acid environment (40-60%, <50°C), thanks to the synergistic effect of its 3-4% copper content. The copper element promotes the formation of a protective copper sulfide film on the surface, which significantly reduces the corrosion rate. However, under higher concentration or temperature conditions, its protective film is easily destroyed and the corrosion rate rises sharply.
Alloy 31 shows a wider acid resistance spectrum, especially in phosphoric acid, mixed acid and high-concentration sulfuric acid environments. For example, in a 95°C, 60% sulfuric acid environment, the corrosion rate of Alloy 31 is less than 0.1 mm/year, while that of Alloy 20 exceeds 0.5 mm/year. This is due to the stable passive film formed by its high molybdenum content (6-7%), which can resist the corrosion of strongly reducing media.
carpenter 20 and carpenter 31 Localized corrosion resistance
The performance of the two alloys is most different in chloride-containing environments. Alloy 31, with its high chromium-molybdenum-nitrogen design, has a pitting resistance equivalent value (PREN = %Cr + 3.3×%Mo + 16×%N) of an astonishing 45 or more, far exceeding the PREN 30-35 of Alloy 20.
nickel alloy 20 and nickel alloy 31 Stress corrosion cracking (SCC)
Alloy 20 is at risk of stress corrosion cracking in chloride-containing hot water (>60°C) and sulfide environments, especially in cold-worked areas or welding residual stress areas.
Alloy 31 significantly improves its SCC resistance through high nickel content and nitrogen alloying. Experimental data show that in a boiling 42% magnesium chloride solution, the fracture time of Alloy 31 is two orders of magnitude longer than that of Alloy 20.
20 alloy steel pipe and alloy 31 steel pipe Intergranular corrosion sensitivity
The upper limit of Alloy 20's carbon content (0.07%) makes it prone to sensitization after welding or hot working, that is, carbides precipitate along the grain boundaries, resulting in a decrease in corrosion resistance. Therefore, thick-walled equipment often requires post-weld solution treatment (about 1120°C water quenching) to restore performance.
The ultra-low carbon design (≤0.015%) of Alloy 31 combined with the stabilizing effect of nitrogen enables it to maintain excellent intergranular corrosion resistance even in the welded state, and successfully pass the ASTM G28 A method test. This feature greatly simplifies the on-site manufacturing and maintenance process.
