Quick answer: these grades are not a simple strength ranking. 409 and 430 are ferritic stainless steels, 410/410S and 420 are martensitic families with different heat-treatment responses, 440 grades target higher hardness, and 446 is a high-chromium ferritic grade for elevated-temperature oxidation service. Choose by environment, heat treatment, fabrication and the governing plate specification.
| Grade | Family / main direction | Selection caution |
| 409 | Ferritic; economical heat and exhaust service | Appearance and general corrosion resistance are below 430 in many environments |
| 410 | Martensitic; heat treatable for moderate strength/hardness | Properties depend strongly on hardening and tempering |
| 410S | Low-carbon 410 modification; limited hardenability | Do not substitute where hardened 410 properties are required |
| 420 | Martensitic; higher carbon for greater hardness | Grade variant and heat treatment must be defined |
| 430 | Ferritic; decorative and mild-corrosion applications | Lower weld toughness/formability than common austenitic grades in some conditions |
| 440 family | High-carbon martensitic grades for wear and hardness | 440A, 440B and 440C are different grades; heat treatment and section control are critical |
| 446 | High-chromium ferritic; elevated-temperature oxidation resistance | Not a general-purpose corrosion or high-toughness grade |
Define the actual medium, chloride level, condensation, cleaning chemistry and temperature cycle. Oxidation resistance in hot gas does not guarantee resistance in wet acids or chloride-bearing water.
Martensitic grades require a complete heat-treatment specification. Plate thickness affects achievable properties and distortion. Ferritic grades are not strengthened by conventional quench-and-temper treatment and need appropriate welding and forming controls.
Review stainless steel plate options; final substitution should be approved against the design specification.