MP35N Springs

MP35N Springs

MP35N is a non-magnetic Cobalt-Nickel-Chromium-Molybdenum alloy having an excellent combination of very high strength, excellent corrosion resistance, and high fatigue strength.

MP35N Springs FAQs

What is MP35N and what makes it different from other spring alloys?

MP35N is a non-magnetic cobalt-nickel-chromium-molybdenum alloy. CTI's specification table gives the nominal composition as nickel 33.0 to 37.0 percent, chromium 19.0 to 21.0 percent, molybdenum 9.0 to 10.5 percent, with cobalt as the balance. Density is 0.304 lb./in3 (8.42 g/cm3) and modulus of elasticity at 70°F is 33.8x10^3 ksi. The "MP" refers to its multiphase structure, and that structure is the reason MP35N combines very high strength, high fatigue strength, and corrosion resistance in a single material rather than forcing you to trade one property against another the way most spring alloys do.

What are MP35N springs used for?

Oil field and downhole springs are a primary application, particularly where sour gas exposure would attack a lower alloyed material and where a failed spring means a costly intervention. The alloy is equally established in medical implants and surgical instruments, in marine and subsea equipment, in aerospace and defense hardware needing non-magnetic high strength parts, and in instrumentation where a spring has to survive an aggressive fluid without losing calibration. Coiling Technologies builds MP35N springs in canted coil, compression, extension, and torsion configurations along with wire forms.

How does MP35N develop its strength if it is not a conventional precipitation hardened alloy?

MP35N is strengthened by cold work followed by aging, which is a different mechanism from the gamma prime precipitation used in nickel superalloys like Inconel 718 or Rene 41. Cold drawing drives a strain induced phase transformation in the multiphase structure, producing a very fine platelet arrangement that blocks dislocation movement. A subsequent age at moderate temperature adds further strengthening on top of the cold worked structure. The practical consequence is that final strength depends on the amount of cold work in the wire as well as the aging cycle, so the wire condition you specify directly determines achievable spring load.

What is the maximum service temperature for an MP35N spring?

CTI's technical specification table lists a Max Temp of 315°C (600°F). That is far below the nickel superalloys and it is the main constraint on the alloy. Because MP35N derives much of its strength from cold work, prolonged exposure to higher temperature begins to recover that structure and the spring loses load permanently. MP35N is selected for strength and corrosion resistance, not for hot service. If your application runs above 600°F you need a precipitation hardened nickel alloy such as Inconel X-750, 718, or Nimonic 90 instead, and you accept the loss of MP35N's corrosion behavior.

What is the minimum wire size for MP35N springs?

The specification table lists a Min Size of 0.005 in (0.127 mm). Fine wire capability is one reason MP35N is common in medical device springs and small instrument mechanisms where high strength, non-magnetic behavior, and corrosion resistance all have to be met in a very small section. Because the alloy work hardens strongly, coiling fine wire in a heavily cold worked condition requires careful control of forming stress and of the aging cycle that follows. Supply the load, deflection, and envelope with your requirement rather than fixing wire size, so the design can be worked to a formable section.

Is MP35N suitable for sour gas service and NACE MR0175 requirements?

MP35N is widely specified for sour service and is used in downhole and wellhead hardware where hydrogen sulfide is present. Its high molybdenum and chromium content resists chloride pitting, crevice corrosion, and sulfide stress cracking, which is the failure mode that disqualifies high strength steels and many stainless grades in that environment. If your application falls under NACE MR0175 or ISO 15156, Coiling Technologies can build and heat treat MP35N springs to the material and condition requirements you specify and supply material certifications documenting chemistry and mechanical properties. CTI is not accredited to those standards, so the specification and acceptance criteria come from you.

How does MP35N compare to Inconel X-750 and Elgiloy?

Against Inconel X-750, MP35N offers higher strength and better ductility in downhole and sour gas conditions, which is why it displaces X-750 in those applications, but X-750 is usable far higher in temperature. Elgiloy is the closer relative, a cobalt-chromium-nickel-molybdenum alloy also strengthened by cold work and optional aging, with comparable corrosion resistance and non-magnetic behavior. Selection between them usually comes down to available wire condition, the specification your customer or regulator requires, and cost at the section you need. Both are appropriate where a stainless steel spring would pit, crack, or fatigue prematurely.

What quality documentation can Coiling Technologies provide with an MP35N spring order?

CTI can supply material certifications with chemistry and mechanical property data, dimensional inspection reports showing measured values against your drawing tolerances, first article inspection documentation, load test data, and traceability from the finished spring back to the incoming raw material. Non-destructive testing is available in house. That package matters for MP35N specifically, because the alloy's strength depends on cold work and aging history rather than composition alone, so a chemistry certificate by itself does not confirm the spring will meet load. State the documentation you require at the RFQ stage so it is planned into the job rather than reconstructed afterward.