Three Different Families Under One Name
Chromium-nickel-molybdenum steel is a broad description that covers three quite different materials. The first is the quenched and tempered alloy structural steel represented by 40CrNiMo, used for shafts, gears and highly stressed machine parts. The second is the austenitic stainless steel family represented by 316 and 316L, used for chemical and process equipment. The third is the duplex stainless steel family, whose pressure vessel and piping grades include the 2205 and 2507 compositions, in which the ferrite and austenite phases each occupy roughly half of the structure.
All three contain chromium, nickel and molybdenum, but they do not resist corrosion in the same way. Only the two stainless families rely on a chromium rich passive film. The alloy structural steel relies on its mechanical properties and must be protected by coating, plating or a corrosion allowance whenever it is exposed to a wet environment.
The Role of Each Alloying Element
Chromium: above roughly 10.5 percent by mass it forms a stable, self-repairing oxide film on the surface. This film is the basis of stainless behaviour.
Nickel: stabilises the austenitic phase, improves toughness and ductility at low temperature and contributes to resistance in reducing acids.
Molybdenum: substantially improves resistance to pitting and crevice corrosion in chloride bearing environments, and improves resistance to reducing acid attack.
Nitrogen: strengthens the austenite in duplex and high alloy austenitic grades and contributes strongly to pitting resistance.
The combined effect of the three pitting resistant elements is expressed as the pitting resistance equivalent number, PREN, calculated as chromium plus 3.3 times molybdenum plus 16 times nitrogen, using the mass percentages of each element.
Corrosion Performance Compared
| Family and grade | Typical composition | PREN | Service notes |
|---|---|---|---|
| 40CrNiMo alloy steel | 0.40 C, 0.6 to 0.9 Cr, 1.25 to 1.65 Ni, 0.15 to 0.25 Mo | Not applicable | No passive film; requires coating or a corrosion allowance |
| 316L austenitic | About 17 Cr, 12 Ni, 2.5 Mo, low carbon | About 24 | Good general corrosion resistance; pitting and stress corrosion cracking risk in hot chlorides |
| 2205 duplex, UNS S32205 | About 22 Cr, 5 Ni, 3 Mo, 0.17 N | 34 or higher | High strength with excellent chloride pitting and stress corrosion resistance |
| 2507 super duplex, UNS S32750 | About 25 Cr, 7 Ni, 4 Mo, 0.27 N | 40 or higher | Seawater and aggressive chloride service |
The equivalent Chinese duplex grades are S22053 for the 022Cr23Ni5Mo3N composition and S25073 for the 022Cr25Ni7Mo4N composition. Compared with standard austenitic stainless steel, the duplex family has significantly higher strength and much better resistance to chloride stress corrosion cracking and to intergranular corrosion, while retaining good toughness and weldability.
Mechanical Properties of 40CrNiMo
40CrNiMo to GB/T 3077 is a chromium-nickel-molybdenum alloy structural steel with carbon in the range 0.37 to 0.44 percent, silicon 0.17 to 0.37 percent, manganese 0.50 to 0.80 percent, chromium 0.60 to 0.90 percent, nickel 1.25 to 1.65 percent and molybdenum 0.15 to 0.25 percent. Oil quenched from about 850 deg C and tempered at about 600 deg C it develops a minimum tensile strength of 980 MPa and a minimum yield strength of 835 MPa, with the fatigue strength and toughness that make it suitable for large stressed components such as shafts, gears and connecting parts.
Its corrosion behaviour is that of an alloy steel, not a stainless steel. The chromium content is far below the level needed for a stable passive film, so unprotected exposure to moisture, chloride or acidic condensate will produce rapid general corrosion. In practice the material is protected by plating, painting, oiling or by designing in a corrosion allowance.
Selection Rules and Common Pitfalls
Austenitic 316L is not immune to chloride stress corrosion cracking. Risk increases with temperature, chloride concentration and applied stress, and the grade is usually limited to moderate chloride levels well below its atmospheric boiling point.
Duplex grades offer high strength with chloride resistance, but their service temperature range is limited because the ferrite phase is subject to embrittlement at elevated temperature.
Never mix families in a wet system without considering galvanic coupling. A stainless component coupled to carbon or alloy steel will accelerate attack on the less noble material.
Sensitisation in the heat affected zone of unstabilised grades can be avoided by using low carbon or stabilised material and by controlling welding heat input.
The passive film can be destroyed in crevices and under deposits, so drainage, cleaning and joint design matter as much as the alloy selection.
Specify the alloy by number rather than by description. Chromium-nickel-molybdenum steel can mean any of the three families above, and the corrosion consequences of choosing the wrong one are severe.
FAQ
Q: Is 40CrNiMo a stainless steel?
No. It is a quenched and tempered alloy structural steel with less than one percent chromium. It has high strength and good fatigue resistance but no stainless passive film, so it must be protected against corrosion.
Q: What does PREN mean?
The pitting resistance equivalent number, calculated as chromium plus 3.3 times molybdenum plus 16 times nitrogen. Higher values indicate better resistance to pitting and crevice corrosion in chloride environments.
Q: Why is duplex stainless steel better than 316L in chlorides?
Duplex grades contain more chromium, molybdenum and nitrogen, which raises the PREN to 34 or above for 2205 and 40 or above for 2507. They also have a two phase structure that resists chloride stress corrosion cracking far better than austenitic grades.
Q: What is the service temperature limit for duplex stainless steel?
The upper limit is set by embrittlement of the ferrite phase and is generally kept below about 250 to 300 deg C depending on the grade and the applicable specification. The lower limit is set by toughness requirements.
Q: Does molybdenum improve resistance to all corrosion types?
No. It is most effective against pitting and crevice corrosion in chlorides and against reducing acids. It does not prevent attack by strongly oxidising media or by high temperature oxidation, which depend on other elements.
Q: How should a chromium-nickel-molybdenum grade be specified on an order?
By the exact grade designation and product form standard, for example 316L to ASTM A240, or 2205 duplex to ASTM A240 with UNS S32205. A descriptive name alone is not sufficient for material selection.
