Grade Identification and Standard Basis
18MnMoNbR is a low alloy steel plate for boilers and pressure vessels. The grade designation is a compact record of its composition: the leading 18 records a nominal carbon content of about 0.18 percent, Mn and Mo stand for the manganese and molybdenum additions, Nb for the niobium microalloying, and R marks the steel as pressure vessel material. Plates are produced and accepted under GB/T 713-2014, the Chinese standard for steel plates for pressure vessels, which governs the composition limits, the heat treatment condition, the tensile requirements and the acceptance rules of the grade.
The steel was developed for heavy wall equipment. Manganese provides solid solution strengthening and hardenability, molybdenum raises the resistance to temper embrittlement and improves strength at elevated temperature, and niobium refines the grain through niobium carbonitride precipitation. The combination allows designers to hold a yield strength in the 390 MPa class while keeping the toughness and weldability that pressure equipment codes require.
Delivery Condition and Heat Treatment
The standard delivery condition is normalized plus tempered. Normalizing produces a uniform microstructure, and tempering at a temperature not lower than 650 C restores toughness and relieves the internal stresses left by normalizing and by any forming operation. The minimum tempering temperature matters for two reasons: it gives the plate the required combination of strength and toughness, and it keeps the base metal properties close to those of the post weld heat treated condition used on finished vessels, so that the design values stay valid after fabrication.
| Item | Requirement |
|---|---|
| Standard | GB/T 713-2014 |
| Nominal carbon | About 0.18 percent |
| Delivery condition | Normalized plus tempered |
| Tempering temperature | Not lower than 650 C |
| Ultrasonic examination | Plate by plate |
| Packing and marking | According to GB/T 247 |
Service Applications
The grade is used where pressure, temperature and wall thickness make plain carbon plate uneconomic or unsafe. Typical equipment includes reactors, heat exchangers, separators, spherical tanks, oil and gas storage tanks, liquefied gas tanks, nuclear reactor pressure shells, liquefied petroleum gas cylinders and turbine housings. In the petrochemical, coal conversion and power generation sectors the steel is selected for the thicker sections of vessels and drums, including the shell courses and heads of boiler drums where the combination of internal pressure and thermal load governs the design.
Because the grade is often used in the as fabricated, post weld heat treated condition, the mill and the fabricator have to agree on the heat treatment schedule early. Simulation of the post weld heat treatment on test coupons is a common requirement, and the results are reported with the plate certificate so that the vessel designer can confirm that the properties still meet the code after fabrication.
Inspection, Fabrication and Quality Control
Ultrasonic testing is performed plate by plate rather than on a sample basis, following the method and acceptance level agreed at order stage, to detect laminations, inclusions and internal discontinuities before rolling and forming operations.
Tensile testing establishes yield strength, tensile strength and elongation for the delivered thickness, and impact testing checks toughness at the temperature required by the design code.
Chemical analysis confirms the carbon, manganese, molybdenum and niobium content together with the phosphorus and sulphur limits, which are kept low to protect toughness and to reduce cracking risk during welding.
Forming of heads and shells is carried out in the normalized condition with adequate preheat where necessary, followed by the final heat treatment that restores the specified properties.
Packing, marking and the quality certificate follow GB/T 247, so that each bundle is traceable and the documentation can be presented for third party or code inspection.
Cutting of thick plate is normally done by thermal cutting with edge preparation controlled so that no hard untempered layer remains on edges that will be welded. Weld metal selection follows the strength and toughness class of the base metal, and procedure qualification records are supplied when the contract calls for them.
Frequently Asked Questions
Q: Which standard governs 18MnMoNbR supply?
A: GB/T 713-2014, the Chinese standard for steel plates for pressure vessels. It fixes the chemical composition, the heat treatment condition and the mechanical property requirements of the grade.
Q: What is the delivery condition of this plate?
A: Normalized plus tempered, with a tempering temperature of at least 650 C. The plate is normally delivered ready for fabrication, and post weld heat treatment follows the vessel code.
Q: Why is ultrasonic testing done on every plate?
A: Because the plate is used in pressure equipment where internal discontinuities cannot be tolerated. Plate by plate examination detects laminations and inclusions before forming and welding, when repair or replacement is still economical.
Q: How is the quality certificate organized?
A: Packing, marking and the certificate follow GB/T 247. The documents carry the heat number, chemical composition, tensile and impact results, heat treatment records and the ultrasonic examination report.
Q: Can the grade be used for boiler drums as well as reactors?
A: Yes. The good toughness, weldability and strength of the steel make it suitable for both pressure vessels and boiler components, including drums and similar heavy wall equipment.
