303 is the most common “free-machining” grade using sulphur to produce sulphides allowing easier chip-breaking during machining. Basically it is 304 with sulphur.
Approximate Composition – 17% Cr, 8% Ni, 0.15% minimum S (sulphur) (Exact composition ranges vary between EN and ASTM standards).
This grade combines the following characteristics:
- Fair pitting corrosion resistance but significantly lower than 304.
- Improved machinability compared to 304. Machinability index 60% improvement compared to 304
- Poor weldability
- Higher leaching rates of Cr and Ni than normal austenitic stainless steels
303 is a basic stainless steel where high productivity is required. Normally, austenitic stainless steels like 304 are difficult to machine. The high work hardening rate of austenitic stainless steels produces greater tool wear and a tendency to produce “stringy” chips during machining. The situation is made worse by the lower thermal conductivity and higher thermal expansion of these steels.
303 uses sulphur (0.15/0.35%) to produce a large number of manganese sulphide inclusions. These inclusions allow increased speeds and reduced tool wear.
The disadvantages of sulphide inclusions are reduced corrosion resistance, poor weldability and greater propensity to leach chromium and nickel ions in contact with aqueous fluids. The last item means that 303 is not authorised to be used in contact with drinking water nor for jewellery. Nevertheless, 303 is used in many applications where basic corrosion resistance is required.
Applications which illustrate these features include:
Threaded fasteners, shafts, valve bodies, precision measurement devices, pneumatic manifolds.