Grade Identification and Standard Basis
12Cr1MoV is a low alloy heat resistant steel used for components that operate at elevated temperature under pressure. The designation records its composition: about 0.12 percent carbon, around 1 percent chromium, a molybdenum addition and a vanadium addition. In Chinese practice the grade is specified in GB/T 3077, the standard for alloy structural steels, which fixes the chemical composition limits and the heat treatment and mechanical property requirements. When the steel is supplied as plate for boiler or pressure equipment, it is ordered to the same chemistry and property basis and produced by a normalizing plus tempering route.
The steel belongs to the family that carries the middle range of elevated temperature duty in power plants and process industries. It is more heat resistant than plain carbon or manganese steel, and less expensive than the higher chromium grades, which is why it remains one of the most widely used materials for superheater and reheater components, steam pipework and headers.
Composition and Heat Treatment
The alloy design depends on carbide forming elements. Chromium and molybdenum form stable carbides that resist coarsening, and vanadium forms fine vanadium carbide particles that are particularly effective at blocking dislocation movement at temperature. Together they give the steel its creep strength, that is, its ability to carry load for long periods without slowly stretching to failure. The chromium content also builds up an adherent oxide layer that protects the surface from oxidation and from attack by flue gas and steam.
| Element | Typical GB/T 3077 range |
|---|---|
| Carbon | 0.08 to 0.15 percent |
| Silicon | 0.17 to 0.37 percent |
| Manganese | 0.40 to 0.70 percent |
| Chromium | 0.90 to 1.20 percent |
| Molybdenum | 0.25 to 0.35 percent |
| Vanadium | 0.15 to 0.30 percent |
| Typical mechanical properties | Yield strength 245 MPa minimum, tensile strength 490 MPa minimum, elongation 22 percent minimum |
The delivered condition is normally normalizing followed by high temperature tempering. The normalizing treatment around 970 to 1010 C dissolves the carbides and refines the structure, and tempering in the region of 720 to 760 C produces the tempered bainitic or ferritic structure with dispersed alloy carbides that gives the required combination of strength, toughness and creep resistance. Fabrication heat treatments such as post weld heat treatment are kept below the tempering temperature of the base metal so that the carbide structure is not altered.
Applications
The grade is used where metal temperatures reach into the creep range. Typical applications are superheater and reheater tubes and headers, main steam pipework, steam drums and boiler components, and hot sections of petrochemical plant such as hydrogen equipment, high temperature piping and reactors. Power generation and the chemical and petrochemical industries account for most of the consumption, and the grade is also found in nuclear plant components that operate at high temperature.
Fabrication, Welding and Quality Control
Hot forming and bending are carried out at temperatures above the tempering range, followed by a fresh normalizing and tempering treatment so that the specified properties are restored.
Welding uses matching chromium molybdenum consumables, with preheat, interpass temperature and post weld heat treatment specified in a qualified procedure.
Post weld heat treatment relieves stresses in the joint and the heat affected zone, and the temperature is controlled so that the base metal carbide structure is preserved.
Chemical analysis confirms the chromium, molybdenum and vanadium contents together with the low phosphorus and sulphur levels that protect creep ductility.
Tensile testing confirms yield strength, tensile strength and elongation, and impact testing verifies toughness in the delivered condition.
Ultrasonic examination and dimensional checks are applied according to the order, and the plate or component is marked with heat number and heat treatment identification.
Because the steel is used at temperature, the service record depends as much on correct heat treatment and on joint quality as on the composition of the plate. Documentation that ties the delivered material to its heat treatment record is therefore part of the acceptance of the product.
Frequently Asked Questions
Q: Which standard covers 12Cr1MoV?
A: GB/T 3077, the standard for alloy structural steels, sets the composition and property requirements. Plate and component supply is agreed on the same basis, with heat treatment and test requirements stated in the order.
Q: Why does the steel contain vanadium?
A: Vanadium forms very fine carbides that resist coarsening at temperature, which raises the creep strength of the steel. This is what allows the grade to work in the creep range of steam plant.
Q: What is the delivery condition?
A: Normalizing followed by high temperature tempering, typically normalizing around 970 to 1010 C and tempering around 720 to 760 C, producing a stable tempered structure with dispersed carbides.
Q: What are the typical mechanical properties?
A: A minimum yield strength of 245 MPa, a minimum tensile strength of 490 MPa and a minimum elongation of 22 percent, verified by tensile testing on the delivered material.
Q: How is the heat affected zone protected during welding?
A: By using a qualified procedure with controlled preheat and interpass temperature, matching consumables, and post weld heat treatment at a temperature that relieves stress without altering the base metal carbide structure.
