Where the confusion starts
SUS303 and SUS304 look identical and sit at a similar price, but they machine very differently. The most common outcome of choosing the wrong one: the drawing says 304, the quote is based on 304, and the parts come off the machine with poor finish, heavy tool wear and an out-of-control cost.
The core difference
SUS303 (free-machining stainless) Sulphur (or selenium in some grades) is added to 304, forming inclusions that break chips and improve evacuation.
- Pros: good machinability, consistent surface finish, long tool life — well suited to automatic lathes and Swiss-type machines in volume
- Cons: slightly lower corrosion resistance than 304; poor weldability, so it is generally not used for welded assemblies
- Typical parts: shafts, pins, nuts, fittings, precision hardware
SUS304 (general-purpose austenitic stainless)
- Pros: good corrosion resistance, toughness, weldability and formability; the most versatile grade
- Cons: strong work-hardening tendency, stringy chips that tangle, built-up edge, torn surfaces; higher cutting forces and faster tool wear than 303
- Typical parts: food equipment, chemical fittings, welded or deep-drawn components
SUS316 adds molybdenum to 304, giving better resistance to chloride pitting. It is common in medical, marine and chemical environments, and is a step harder to machine than 304.
SUS410 / SUS420 are martensitic grades that can be hardened by heat treatment, suited to wear parts. They machine better than 304 but resist corrosion less well than the 300 series.
How to choose
- Will it be welded? If yes, do not use 303
- Is there a corrosive medium? For normal atmospheric service 303 is fine; for acids, alkalis or coastal environments prefer 304 / 316
- Are strength or hardness required? If heat treatment is needed, consider 410 / 420 or an alloy steel
- Cost sensitivity — for high-volume, complex or tight-tolerance parts, the machining advantage of 303 usually outweighs the material price difference
Process guidance when 304 is mandatory
- Reduce cutting speed to keep the cutting zone from overheating and hardening further
- Do not feed too lightly — a very light feed rubs the edge against the hardened layer and wears it faster
- Keep tools sharp and change them on time; a dull edge is the main source of poor finish and dimensional drift
- Use generous coolant, and high-pressure through-tool coolant where available
- Select a coated insert grade intended for stainless steel
If a material substitution or cost reduction is worth exploring, raise it at quotation stage — we will suggest alternative grades and assess the machining impact.