Stainless Alloy Steel MS-198 (MRC) Bars/ MS-198 (MRC) Rods / MS-198 (MRC) Rings

Product Description

MS-198 Stainless Alloy Steel (MRC/SKF)

MS-198 is a specialized, high-performance martensitic stainless steel specification primarily associated with MRC Bearings (a brand of the SKF Group). This material is engineered specifically for the manufacture of high-precision aerospace bearings, where a combination of extreme hardness, fatigue life, and corrosion resistance is required.

It is closely related to AISI 440C, but like many proprietary aerospace "MS" (Material Specifications), it features tighter controls on melt chemistry, inclusion limits, and heat-treatment response to ensure the reliability required for flight-critical applications.

Technical Specifications & Equivalents

MS-198 is typically utilized in environments where standard bearing steels (like 52100) would succumb to corrosion, but where the mechanical loading remains high.

Chemical Composition (Typical)

The high carbon content allows for maximum hardness, while the high chromium provides the "stainless" characteristic.

• Chromium (Cr): 16.0% – 18.0%

• Carbon (C): 0.95% – 1.20%

• Molybdenum (Mo): 0.75% max

• Manganese (Mn): 1.0% max

• Silicon (Si): 1.0% max

Mechanical Properties & Performance

MS-198 is designed to be heat-treated to a very high hardness level, which is essential for rolling-element bearing surfaces.

• Hardness (As Heat Treated): 58–62 HRC

• Operating Temperature: Maintains stability up to approximately 150°C (300°F); above this, hardness may begin to degrade unless specialized tempering cycles are used.

• Corrosion Resistance: Excellent resistance to fresh water, steam, crude oil, and gasoline. It is one of the most corrosion-resistant of the high-hardness martensitic steels.

Product Forms & Applications

1. Round Bars & Rods

MS-198 rods are generally supplied in the spheroidized annealed condition to allow for initial machining or grinding.

• Applications: Bearing rollers, precision shafts, and valve needles for aerospace fuel systems.

2. Rings & Forgings

Rings are used as the primary structural component for ball and roller bearing assemblies.

• Applications: Bearing inner and outer races (rings) for aircraft engines, gearboxes, and auxiliary power units (APUs).

Key Attributes for Aerospace Use

• Material Cleanliness: Under the MS-198 specification, the steel is often produced via Vacuum Induction Melting (VIM) and Vacuum Arc Remelting (VAR). This removes non-metallic inclusions that could act as starting points for fatigue cracking.

• Wear Resistance: The high volume of primary carbides in the microstructure provides exceptional resistance to abrasive wear.

• Dimensional Stability: The material undergoes specialized sub-zero treatments (cryogenic cooling) during processing to transform retained austenite, ensuring the rings do not "grow" or change shape during service.

Fabrication Considerations

• Machinability: Due to its high carbon and chromium content, MS-198 is difficult to machine. It is typically machined in the annealed state using rigid setups and then finish-ground after hardening.

• Welding: Generally not recommended for welding. The high carbon content makes it extremely prone to cracking.

• Finishing: To achieve maximum corrosion resistance, surfaces must be ground and polished to a mirror finish to remove any surface scale or impurities.