Grade 303 represents the optimum in machinability among the austenitic stainless steels. It is primarily used when production involves extensive machining in automatic screw machines. Machinability Rating (compared to B1212) is approximately 78%.
303 is also available as a "Ugima" Improved Machinability grade, with machinability even higher than that of the standard 303.
The sulphur addition which is responsible for the improved machining and galling characteristics of Grade 303 lowers its corrosion resistance to below that of Grade 304. As for other austenitic grades the structure gives 303 excellent toughness, although the sulphur in 303 reduces its toughness slightly.
Grade 303Se (UNS S30323) has a selenium rather than sulphur addition, improving the hot and cold forming characteristics over those of 303 and providing a smoother machined surface finish. The machinability rate is also slightly reduced. Grade 303Se is not readily available in Australia.
These properties are specified for long product (bar) in ASTM A582. Similar but not necessarily identical properties are specified for other products such as wire and forgings in their respective specifications. Grade 303 is not produced in flat rolled products.
Typical compositional ranges for grade 303 stainless steels are given in table 1.
Table 1. Composition ranges for 303 grade stainless steel
Typical mechanical properties for grade 303 stainless steels are given in table 2.
Table 2. Mechanical properties of 303 grade stainless steel
Typical physical properties for annealed grade 303 stainless steels are given in table 3.
Table 3. Physical properties of 303 grade stainless steel in the annealed condition
Approximate grade comparisons for 303 stainless steels are given in table 4.
Table 4. Grade specifications for 303 grade stainless steel
Possible alternative grades to grade 303 stainless steels are given in table 5.
Table 5. Possible alternative grades to 303 grade stainless steel
Good resistance to mildly corrosive atmospheres, but significantly less than Grade 304 due to the sulphur addition; the sulphide inclusions act as pit initiation sites. Grade 303 should not be exposed to marine or other similar environments, as these will result in rapid pitting corrosion. Because the sulphide inclusions in 303 are primarily aligned along the rolling direction the corrosion resistance is particularly reduced in cross-sections.
Grade 303, like other common austenitic stainless steels, is subject to stress corrosion cracking in chloride containing environments above about 60°C.
Good oxidation resistance in intermittent service to 760°C and in continuous service to 870°C. Continuous use in 425-860°C range not usually recommended due to carbide precipitation - 303 usually does not have a low carbon content so is susceptible to sensitisation.
As well as reducing the corrosion resistance, the sulphur additions in 303 also result in poor weldability and reduced formability compared to Grade 304. Sharp bends should not be attempted in 303. A practical compromise alternative may be a 304 Ugima Improved Machinability grade - this does not machine as readily as 303, but does offer better formability (as well as better weldability and corrosion resistance).
Solution Treatment (Annealing) - Heat to 1010-1120°C and cool rapidly. This grade cannot be hardened by thermal treatment.
Not generally recommended but, if unavoidable and a lower strength can be tolerated, use Grade 308L or 309 electrodes. AS 1554.6 does not pre-qualify welding of 303. Welds must be annealed for maximum corrosion resistance.
A "Ugima" improved machinability version of grade 303 is available in round bar products. This machines significantly better even than standard 303, giving very high machining rates and lower tool wear in many operations.
Typical applications include:
• Nuts and Bolts
• Aircraft Fittings
• Electrical Switchgear Components
• In general any component that is heavily machined and where the corrosion resistance and fabrication properties of 303 are viable.