425 Mod is a hardenable, straight-chromium martensitic stainless steel that sits between 420 and 440A on the carbon scale, at roughly 0.50–0.55% carbon with 13.0–14.0% chromium and a molybdenum addition (0.80–1.20%) that the standard 420 grade doesn't have. That extra carbon and molybdenum give 425 Mod a meaningfully higher hardening ceiling than 420 — typically 57–60 HRC after a proper quench and temper — along with better resistance to localized chloride attack than 420, thanks to the combined chromium-molybdenum content. Like other martensitic 4xx grades it carries little to no nickel and is not usable in the annealed condition for corrosion-critical service; its strength and wear resistance come specifically from heat treatment.

This combination of high hardness, good edge retention, and moderate corrosion resistance makes 425 Mod a popular cutlery and knife-blade steel, occupying a niche between the more common 420HC and the higher-carbon 440A/440C grades. It also sees use in valve trim, pump components, and wear parts where a hardened stainless surface is needed. Because it's supplied and machined almost exclusively in the softer annealed condition before final heat treatment, safe machining speeds are considerably lower than for lower-carbon 4xx grades — manufacturer guidance puts annealed turning speeds around 18–24 m/min (60–80 SFM), closer to a tool steel than to 410 or 416.

425 Mod machines nothing like the lower-carbon 4xx grades on this site. Its carbon content is roughly three to four times that of 410, and its added molybdenum promotes fine, hard carbide particles throughout the microstructure — both factors that push cutting forces and abrasive tool wear well above what a straight 12% chromium grade like 403 or 410 would produce, even before heat treatment. In practice, this alloy is essentially always machined in the annealed condition, with final hardening reserved for after the part is shaped.

Because of the higher carbide volume, 425 Mod tends to segment into short, well-broken chips more readily than lower-carbon martensitic grades, which is a small silver lining. The bigger practical concern is edge wear: a sharp, rigid setup with adequate coolant and conservative feed rates will get more consistent results than pushing speeds the way you might with 410 or 403. Insert selection should favor a tougher substrate that can handle the higher cutting forces without chipping, rather than the sharper, more fragile edge preparations sometimes used on easier-machining stainless grades.

At stable cutting conditions, a semi-hard substrate with a thin CVD coating is a reasonable general-purpose starting point for turning, with the caveat that speeds need to come down substantially from what works on 410 or 416. Below roughly 390 SFM (120 m/min) — which for this grade means most practical cutting speeds — a semi-hard substrate with a PVD coating is generally the safer choice.