Martensitic Stainless
Steel Machining

Martensitic stainless steels are a family of chromium-alloyed steels that can be hardened by heat treatment — quenching and tempering — to achieve high hardness values up to HRC 60. Unlike austenitic grades (304, 316) which are non-hardenable and prized for corrosion resistance, martensitic grades trade some corrosion resistance for the ability to achieve high hardness, wear resistance, and edge retention. They are the only stainless steel family that can be through-hardened like tool steel, making them suitable for knives, bearings, valve components, and wear-critical precision parts.

The three grades differ primarily in carbon content, which determines maximum achievable hardness and wear resistance. 410 has the lowest carbon (0.08–0.15%), making it the most weldable and toughest — used for valve bodies, pump shafts, and structural parts where moderate hardness suffices. 420 has higher carbon (0.15–0.40%) enabling hardness to HRC 50–52, with better wear resistance — common for cutlery, surgical instruments, and dental tools. 440C has the highest carbon (0.95–1.20%) of any standard stainless steel, achieving HRC 58–60 after heat treatment — the hardest stainless available, used for ball bearings, high-quality knives, valve seats, and precision wear surfaces.

Martensitic stainless steels can be welded but require pre-heat and post-weld heat treatment to avoid cold cracking and hardened heat-affected zones. 410 has the best weldability of the three grades. 420 requires more care. 440C is generally considered difficult to weld and is rarely specified for welded assemblies — its high carbon content makes HAZ cracking a significant risk. If your part requires welding and stainless properties, consider austenitic 304L or 316L instead.

After quench and temper heat treatment: 410 achieves HRC 35–42 depending on tempering temperature. 420 achieves HRC 50–52. 440C achieves HRC 58–60 — the highest hardness of any standard stainless steel grade. These hardness values are achievable after proper austenitizing, quenching, and tempering. We coordinate heat treatment as part of the machining order, with hardness testing verification provided.

Parts are typically rough-machined in the annealed or pre-hardened condition, heat-treated to final hardness, then finish-ground or precision-turned on critical surfaces to recover final tolerance after heat treatment distortion. Machining in the hardened condition (particularly 440C at HRC 58–60) requires CBN tooling or grinding and is significantly more expensive. Our standard sequence is: rough machine → heat treat → finish grind critical dimensions → CMM verification. This sequence is coordinated and managed by us as a single-source process.

Unlike austenitic grades, martensitic stainless steel cannot be passivated as effectively due to lower chromium content and the presence of carbides. Common surface treatments include: passivation (improves corrosion resistance modestly — ASTM A967), black oxide (light protection, minimal dimensional change), PVD coating (for 440C cutting tools and wear surfaces — adds TiN or CrN wear layer), and polishing or superfinishing (for bearing surfaces and sealing faces). Hard chrome plating is also used on 410 valve stems and pump shafts.

440C is primarily used in applications requiring the highest achievable hardness in a stainless steel combined with moderate corrosion resistance. CNC machining applications include: precision ball bearings and bearing races, high-quality knife blades and cutting tools, valve seats and valve balls requiring corrosion resistance plus wear resistance, pump impeller wear rings, precision dies and forming tools, dental and surgical instruments requiring edge retention, and nozzle seats in fluid control systems.

Yes. Unlike austenitic stainless steels (304, 316) which are non-magnetic in the annealed condition, martensitic stainless steels (410, 420, 440C) are ferromagnetic — strongly magnetic in both the annealed and hardened condition. This is due to their body-centered cubic (BCC) or body-centered tetragonal (BCT in hardened state) crystal structure. If non-magnetic properties are required for your application, specify an austenitic grade instead.