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MP35N Cobalt UNS R31233 Round Bar and Rod
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MP35N (UNS R31233) is a cobalt-chromium-nickel-molybdenum-tungsten alloy built around a different design philosophy from nickel alloys: it derives its strength from a fine dispersion of carbides in a cobalt matrix, and it retains that strength at temperature where nickel-based spring materials begin to relax. That combination of hot strength, wear resistance and general corrosion resistance makes it a standard material for high-strength springs, valve components and wear parts in aerospace, oilfield and process equipment. Hefei Tailan Metal supplies MP35N round bar and rod to ASME SB-815, with plate and sheet to ASME SB-818.
Material Specifications and Grade Comparison
The table below sets out the governing material specification and the grade comparison for MP35N Cobalt UNS R31233 Round Bar and Rod. All composition and mechanical values are taken directly from the ASME BPVC Section II specifications named in each row.
| Property | MP35N (UNS R31233) | Elgiloy (Co-Cr-Ni-Mo) | Hastelloy C-276 (N10276) |
|---|---|---|---|
| Base metal | Cobalt | Cobalt | Nickel |
| Chromium, % | 23.5–27.5 | not specified in SB-815 | not specified in SB-815 |
| Molybdenum, % | 4.0–6.0 | not specified in SB-815 | not specified in SB-815 |
| Tungsten, % | 1.0–3.0 | not specified in SB-815 | not specified in SB-815 |
| Nickel, % | 7.0–11.0 | not specified in SB-815 | not specified in SB-815 |
| Tensile strength min | 130 ksi (896 MPa) | not specified in SB-815 | not specified in SB-815 |
| Specification | ASME SB-815 / SB-818 | not an ASME Section II spec | ASME SB-366 / SB-462 |
Product Forms, Sizes and Material Route
Product forms and size range
MP35N is supplied as hot-finished, annealed and descaled rod, as hot-finished annealed and ground or turned rod, and as wire for spring manufacture. SB-815 Table 4 gives permissible diameter variations separately for hot-finished annealed and descaled rod and for hot-finished annealed ground or turned rod, and rod is furnished in random mill lengths or in multiples of a specified unit length within the stated length limits. Wire for spring applications is supplied on enquiry. State the diameter, condition and length basis when you enquire.
How MP35N is used
The dominant application is high-strength spring wire and spring components, including valve springs, aircraft and gas-turbine spring elements, and instrumentation springs in corrosive service. Beyond springs, MP35N serves in wear and erosion parts, valve trim, and fasteners where both strength and corrosion resistance are required. The strength is delivered by a carbide dispersion within the cobalt matrix, which is why the material is valued where a nickel-based spring alloy would soften at the working temperature.
Material route and certification
MP35N is supplied under ASME SB-815 for rod and SB-818 for plate, sheet and strip. Chemical composition is controlled to SB-815 Table 2 with product check analysis tolerances applied. Rod is supplied in the solution-annealed condition specified in the scope of SB-815, either pickled or mechanically descaled below 3/4 in. diameter and ground or turned at 3/4 in. and above. Full mill test report traceability is supplied with the material.
Specification and Inspection Requirements
| Item | Requirement |
|---|---|
| Material specification | ASME SB-815 (rod), SB-818 (plate, sheet and strip) |
| Material designation | Cobalt-chromium-nickel-molybdenum-tungsten alloy, UNS R31233 |
| Tensile strength, min | 130 ksi (896 MPa) |
| Yield strength, min | 55 ksi (379 MPa) |
| Elongation in 2 in. (50.8 mm) or 4D, min | 15 % |
| Size range, rod | 3/16 to 3/4 in. (9.76 to 19.05 mm) exclusive: hot or cold finished, solution annealed, pickled or mechanically descaled. 3/4 to 3 1/2 in. (19.05 to 88.9 mm) inclusive: hot or cold finished, solution annealed, ground or turned |
| Ends | Rods ordered to random or nominal lengths shall be furnished with either cropped or sawed ends; rods ordered to cut lengths shall be furnished with square saw cut or machined ends |
| Dimensional tolerances | SB-815 Table 4 permissible variations in diameter and out-of-roundness; Tables 1 and 3 govern rod length |
Chemical Composition
Chemical composition shall conform to the applicable ASME specification. The table below reproduces the composition limits for the grades covered by MP35N Cobalt UNS R31233 Round Bar and Rod.
| Element | Composition limits, % (SB-815 Table 2) |
|---|---|
| Cobalt | Remainder |
| Chromium | 23.5–27.5 |
| Molybdenum | 4.0–6.0 |
| Nickel | 7.0–11.0 |
| Tungsten | 1.0–3.0 |
| Iron | 1.0–5.0 |
| Boron | 0.015 max |
| Carbon | 0.02–0.10 |
| Manganese | 0.1–1.5 |
| Nitrogen | 0.03–0.12 |
| Silicon | 0.05–1.00 |
| Phosphorous | 0.030 max |
| Sulfur | 0.020 max |
Mechanical Property Requirements
| Property | Requirement (SB-815 Table 3) |
|---|---|
| Tensile strength, min | 130 ksi (896 MPa) |
| Yield strength, min | 55 ksi (379 MPa) |
| Elongation in 2 in. (50.8 mm) or 4D, min | 15 % |
| Rounding of results | Per Practice E29 — chemical composition and hardness to the nearest unit in the last stated place of the specified limit; tensile and yield strength to the nearest 1000 psi (7 MPa); elongation to the nearest 1 % |
Applications and Service Environments
High-strength springs
Valve springs, aircraft landing-gear and engine spring elements, and precision instrument springs where high tensile strength, fatigue performance and corrosion resistance are needed together. This is the market that defines MP35N.
Valve components and trim
Springs, seats and wear parts in steam and process valves, where the combination of hot strength and wear resistance matters more than bulk corrosion resistance.
Oilfield and subsea equipment
Spring-loaded and wear components in high-pressure and sour-service equipment, where the cobalt matrix provides both strength and a useful degree of general corrosion resistance.
Aerospace and defence hardware
Fasteners, pins and retainers requiring high strength with good corrosion and wear behaviour, including downhole and aerospace instrumentation.
Certification, Testing and Inspection
Chemical composition
Full analysis to SB-815 Table 2, with product (check) analysis tolerances applied, and the certificate required before shipment.
Tensile properties
Tensile strength 130 ksi (896 MPa) minimum, yield strength 55 ksi (379 MPa) minimum, elongation 15 % minimum per SB-815 Table 3.
Dimensional inspection
Permissible variations in diameter and out-of-roundness per SB-815 Table 4, and length per SB-815 Tables 1 and 3.
Surface condition
Rod free of scale and surface defects. Rods 3/16 to 3/4 in. diameter are supplied hot or cold finished, solution annealed, pickled or mechanically descaled; rods 3/4 to 3 1/2 in. diameter are supplied hot or cold finished, solution annealed, ground or turned.
Ends
Cropped or sawed ends for random and nominal lengths; square saw cut or machined ends for cut lengths.
Documentation
Mill test report with heat number traceability, and supplementary inspection on request.
Ordering Information
Please specify the product form (rod, bar or wire), nominal diameter with the tolerance basis, condition (hot-finished annealed and descaled, or annealed ground and turned), length basis (random mill length or multiples of a specified unit length), quantity, and the end preparation required. If a specific heat treatment condition applies to your application, state it and we will confirm what can be supplied under ASME SB-815. We will then issue a quotation on request.
