What 06Ni9DR Is and Which Standard Covers It
06Ni9DR is a nickel-alloyed steel plate for low-temperature pressure vessels defined in GB 3531, the standard for low-temperature pressure vessel plates. The designation can be read directly: 06 for a maximum carbon content of 0.06 %, Ni9 for a nominal nickel content of 9 %, D for low-temperature duty and R for pressure vessel. The material corresponds to the 9 % nickel steels specified in ASTM A553/A553M and ASME SA-553, and it is one of the few commercial plate steels that retains usable toughness at the boiling point of liquefied natural gas. Impact qualification is normally carried out at -196 °C, which gives a margin below the -162 °C service temperature of LNG.
Chemical Composition and Mechanical Properties
The alloy design is deliberately lean in carbon and very low in phosphorus and sulphur, since both elements degrade low-temperature toughness.
| Element | Requirement |
|---|---|
| C | 0.06 % max |
| Si | 0.15-0.35 % |
| Mn | 0.30-0.80 % |
| P | 0.010 % max |
| S | 0.005 % max |
| Ni | 8.50-10.00 % |
| Yield strength (min) | 585 MPa |
| Tensile strength | 680-820 MPa |
| Elongation | 18 % min |
| Charpy V-notch at -196 °C | 34 J min on the plate axis |
Chemistry is verified heat by heat, and tensile plus low-temperature impact testing is carried out on each production lot because the toughness of 9 % nickel steel is sensitive to both heat treatment and residual element level.
Heat Treatment and Delivery Condition
Delivery is normally quenched and tempered, a route that produces a fine ferrite plus tempered martensite structure with finely dispersed retained austenite; that austenite is what stabilises ductility at cryogenic temperature. For thicker plate, a double-normalizing route followed by tempering below 540 °C can also be applied, and plate ordered this way must be tested in the as-delivered condition because the treatment path changes the toughness response. Plate outside the quenched and tempered condition is not interchangeable for cryogenic duty. Thickness is commonly available from 5 mm to 60 mm, with wider plate preferred by tank fabricators to reduce seam length.
Applications in LNG and Cryogenic Service
The dominant use is LNG containment: membrane and Type C cargo tanks on LNG carriers, onshore storage tanks ranging from small peak-shaving units to full-containment tanks of 160,000 m3 and above, LNG fuel tanks on dual-fuel vessels, ethane and ethylene carriers, and cryogenic process pipework, headers and supports inside cold boxes. Because the steel is ferritic and therefore magnetic, it can be handled with magnetic lifting equipment and inspected with magnetic particle methods, which is an advantage on tank construction sites where austenitic stainless would require different tooling.
Welding, Cutting and Heat Treatment Control
Welding uses nickel-based filler metals of the AWS ERNiCrMo-3 or ERNiMo-3 classification for both root and fill passes. Heat input is kept low, interpass temperature is held below about 150 °C, and no preheat is applied. Consumables must be dry and low in hydrogen, and tools used on carbon steel must not be reused on the nickel surfaces to avoid contamination. Post-weld heat treatment is generally avoided: soaking in the 400-600 °C band can promote temper embrittlement of the martensitic constituent and reduce the amount of stabilising retained austenite. Thermal cutting is followed by grinding to remove the heat-affected layer, and plasma or waterjet cutting is preferred where the edge will be subject to high strain.
Quality Control and Ordering Points
Plates are examined ultrasonically to GB/T 2970 or to EN 10160 classes agreed at enquiry, and are marked in line with GB/T 247 with heat number, grade, plate identification and thickness. Certification is issued to EN 10204 type 3.1, with type 3.2 when third-party witnessing is specified. Orders should state the impact test temperature and whether impact testing is required on both the base metal and the simulated heat-affected zone, the acceptance level for ultrasonic examination, the required flatness, and the maximum permitted non-metallic inclusion content. Weld repair of the plate surface is normally subject to written approval because repair cycles consume part of the tempering tolerance of the steel.
Frequently Asked Questions
Q: Why is 06Ni9DR tested at -196 °C when LNG is stored at -162 °C?
A: The lower test temperature provides design margin for temperature stratification, boil-off and upset conditions, and it is the qualification temperature used by the standard and by tank designers.
Q: Is 06Ni9DR magnetic?
A: Yes. In the quenched and tempered condition the microstructure is ferritic-martensitic and the steel is ferromagnetic, so magnetic lifting and magnetic particle inspection can be used.
Q: Can post-weld heat treatment be applied to 06Ni9DR?
A: It is normally avoided. Holding the steel in the 400-600 °C range can embrittle the matrix and destabilise the retained austenite that provides cryogenic toughness, so fabricators design weld sequences to avoid the need for it.
Q: Which filler metal is used for welding?
A: Nickel-based consumables of the AWS ERNiCrMo-3 or ERNiMo-3 type, welded with low heat input, no preheat, and strict cleanliness control.
Q: What thickness range is normally available?
A: Production is commonly offered from 5 mm to 60 mm, with quenched and tempered delivery for the full range and the double-normalized plus tempered route used for heavier plate.
Q: What certificate and testing data are supplied?
A: An EN 10204 type 3.1 mill certificate with heat analysis, tensile results and Charpy results at -196 °C, ultrasonic examination reports to the ordered class, and dimensional records.
