Grades 321 and 347 are the basic austenitic 18/8 steel (Grade 304) stabilized by Titanium (321) or Niobium (347) additions. These grades are used because they are not sensitive to intergranular corrosion after heating within the carbide precipitation range of 425-850 °C. Grade 321 is the grade of choice for applications in the temperature range of up to about 900 °C, combining high strength, resistance to scaling and phase stability with resistance to subsequent aqueous corrosion.

Grade 321H is a modification of 321 with higher carbon content, to provide improved high-temperature strength.

A limitation with 321 is that titanium does not transfer well across a high-temperature arc, so it is not recommended as a welding consumable. In this case grade 347 is preferred - the niobium performs the same carbide stabilization task but can be transferred across a welding arc. Grade 347 is, therefore, the standard consumable for welding 321. Grade 347 is only occasionally used as parent plate material.

Like other austenitic grades, 321 and 347 have excellent forming and welding characteristics, are readily brake or roll-formed and have outstanding welding characteristics. Post-weld annealing is not required. They also have excellent toughness, even down to cryogenic temperatures. Grade 321 does not polish well, so is not recommended for decorative applications.

Composition of 321 Stainless Steel

Typical compositional ranges for grade 321 stainless sheets of steel are given in table 1.

Table 1.  Composition ranges for 321-grade stainless steel

Mechanical Properties of 321 Stainless Steel

Typical mechanical properties for grade 321 stainless sheets of steel are given in table 2.

Table 2.  Mechanical properties of 321-grade stainless steel

Grade Specification Comparison

Approximate grade comparisons for 321 stainless sheets of steel are given in table 4.

Table 4.  Grade specifications for 321-grade stainless steel