AMS 5842 MP159 Cobalt-Nickel Alloy Bar: Condition Selection and Procurement Boundaries

For aerospace fastener, oilfield tool, and medical implant design engineers and buyers — supplied by ZYTC Alloy, ISO 9001 certified manufacturer and exporter

AMS 5842 Covers One Specific Condition — MP159 Has Three

AMS 5842 is an SAE specification covering MP159 bar in the solution heat treated and work strengthened condition, in nominal diameters up to 1-3/4 in (44 mm). This condition is neither soft nor the final service condition.

The full MP159 condition system has three levels:

The procurement trap: If the PO states "MP159 bar, AMS 5842," you receive material at approximately 38 HRC that has not been aged. If the design strength was calculated against AMS 5843 (≥44 HRC), AMS 5842 bar will not meet it directly. Conversely, if you need subsequent cold forming or machining, AMS 5843's hardness makes processing very difficult. The condition must be selected based on your process route — not simply "pick the strongest one."

ZYTC Alloy supplies MP159 round bar to AMS 5841 / 5842 / 5843, with the condition stated on the certificate.

Where the Strength Comes From: Cold Reduction Is the Core Variable

MP159's ultra-high strength is not achieved by a single heat treatment. The strengthening path is: solution treatment → cold working (strain-induced ε phase) → aging (γ′/η precipitation).

Cold reduction is the key variable determining strength. Research explicitly states that MP159's cold reduction must be tightly controlled within 48 ± 1%: too little deformation gives insufficient strength; too much raises strength but reduces ductility. This control window is what distinguishes MP159 from ordinary cold-worked alloys, and it is the core of a supplier's process capability.

AMS 5842's "work strengthened" condition corresponds to this cold deformation step. But the specification itself does not tell you whether the supplier's actual cold reduction falls within the 48 ± 1% window. If the certificate states only "conforms to AMS 5842" without reporting cold reduction or measured hardness, you cannot confirm the material is in the optimal strengthening range.

Procurement action: For AMS 5842 bar, require the certificate to report measured hardness (HRC) and, where available, cold reduction ratio. Lots below 38 HRC may not achieve the strength expected from AMS 5842.

Mechanical Properties: Data Varies Significantly by Condition

MP159 mechanical properties change markedly with condition. The following are cross-verifiable typical values from published data:

Sources: Aircraft Materials technical data and Lork product data. AMS 5842 tensile values are incomplete in published sources; hardness ≥38 HRC is the more reliable acceptance criterion.

Academic research further shows that MP159 after simple thermomechanical processing can reach over 2.2 GPa tensile strength at room temperature, and aged MP159 retains about 1.8 GPa tensile strength with 12.5% elongation at intermediate temperatures. This indicates that MP159's performance ceiling depends on process control, not chemistry alone.

Service Temperature: 600°C Class, Not 1000°C Class

The original page stated "up to 1100°F (593°C)," which is broadly accurate but needs boundaries: MP159's long-term service temperature is approximately 600°C, with short-term excursions to 650–700°C. It is not a substitute for nickel-base superalloys (such as 718 or Waspaloy); above 700°C in long-term service, nickel-base alloys show a clear creep-strength advantage.

Procurement implication: If the service involves continuous operation above 600°C, or requires creep rupture data as a design basis, MP159 may not be the optimal choice. Its advantage lies in the combination of ultra-high strength from room temperature to 600°C + corrosion resistance + non-magnetic behavior, not in long-term creep performance at extreme temperatures.

Corrosion Resistance and Non-Magnetic Behavior: MP159's Real Differentiation

MP159's high chromium and molybdenum content gives it excellent corrosion resistance in seawater, chlorides, and acidic environments. It shows good crevice corrosion resistance in ferric chloride testing, and outstanding hydrogen embrittlement resistance. For H₂S service in the oilfield industry, the high Cr-Mo combination provides sulfide stress cracking (SSC) resistance, and it can be supplied per NACE MR0175/ISO 15156.

Non-magnetic behavior is another frequently overlooked differentiator. MP159 has a magnetic permeability of approximately 1.00265 at 25°C — essentially non-magnetic. For medical components requiring MRI compatibility, or aerospace instruments sensitive to magnetic fields, this is a key material selection reason. Although 316L is also austenitic, cold working can generate slight magnetism; MP159's cobalt-base matrix maintains low permeability after cold working and aging.

Supply Information

ZYTC Alloy supplies MP159 round bar in selectable conditions:

Please provide in your inquiry: condition (5841 / 5842 / 5843), diameter, length, quantity, downstream operation (cold forming / machining / direct service), and whether NACE compliance documentation or measured hardness data is required.

If downstream cold forming or machining is involved, note this in the inquiry — we will recommend AMS 5842 rather than AMS 5843 to avoid processing difficulties from excessive hardness.

📧 dominic@zytc-steel.com
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