Background

Grade 316 is the standard molybdenum-bearing grade, second in importance to 304 amongst the austenitic stainless steels. The molybdenum gives 316 better overall corrosion resistant properties than Grade 304, particularly higher resistance to pitting and crevice corrosion in chloride environments.

Grade 316L, the low carbon version of 316 and is immune from sensitization (grain boundary carbide precipitation). Thus it is extensively used in heavy gauge welded components (over about 6mm). There is commonly no appreciable price difference between 316 and 316L stainless steel.

The austenitic structure also gives these grades excellent toughness, even down to cryogenic temperatures.

Compared to chromium-nickel austenitic stainless steels, 316L stainless steel offers higher creep, stress to rupture and tensile strength at elevated temperatures.

Key Properties

These properties are specified for flat rolled product (plate, sheet and coil) in ASTM A240/A240M. Similar but not necessarily identical properties are specified for other products such as pipe and bar in their respective specifications.

Composition

Table 1. Composition ranges for 316L stainless steels.

Mechanical Properties

Table 2. Mechanical properties of 316L stainless steels.

Physical Properties

Table 3. Typical physical properties for 316 grade stainless steels.

Grade Specification Comparison

Table 4. Grade specifications for 316L stainless steels.

Note: These comparisons are approximate only. The list is intended as a comparison of functionally similar materials not as a schedule of contractual equivalents. If exact equivalents are needed original specifications must be consulted.

Possible Alternative Grades

Table 5. Possible alternative grades to 316 stainless steel.

Corrosion Resistance

Excellent in a range of atmospheric environments and many corrosive media - generally more resistant than 304. Subject to pitting and crevice corrosion in warm chloride environments, and to stress corrosion cracking above about 60°C. Considered resistant to potable water with up to about 1000mg/L chlorides at ambient temperatures, reducing to about 500mg/L at 60°C.

316 is usually regarded as the standard “marine grade stainless steel”, but it is not resistant to warm sea water. In many marine environments 316 does exhibit surface corrosion, usually visible as brown staining. This is particularly associated with crevices and rough surface finish.

Heat Resistance

Good oxidation resistance in intermittent service to 870°C and in continuous service to 925°C. Continuous use of 316 in the 425-860°C range is not recommended if subsequent aqueous corrosion resistance is important. Grade 316L is more resistant to carbide precipitation and can be used in the above temperature range. Grade 316H has higher strength at elevated temperatures and is sometimes used for structural and pressure-containing applications at temperatures above about 500°C.

Heat Treatment

Solution Treatment (Annealing) - Heat to 1010-1120°C and cool rapidly. These grades cannot be hardened by thermal treatment.

Welding

Excellent weld ability by all standard fusion and resistance methods, both with and without filler metals. Heavy welded sections in Grade 316 require post-weld annealing for maximum corrosion resistance. This is not required for 316L.

316L stainless steel is not generally weldable using oxyacetylene welding methods.

Machining

316L stainless steel tends to work harden if machined too quickly. For this reason low speeds and constant feed rates are recommended.

316L stainless steel is also easier to machine compared to 316 stainless steel due its lower carbon content.

Hot and Cold Working

316L stainless steel can be hot worked using most common hot working techniques. Optimal hot working temperatures should be in the range 1150-1260°C, and certainly should not be less than 930°C. Post work annealing should be carried out to induce maximum corrosion resistance.

Most common cold working operations such as shearing, drawing and stamping can be performed on 316L stainless steel. Post work annealing should be carried out to remove internal stresses.

Hardening and Work Hardening

316L stainless steel does not harden in response to heat treatments. It can be hardened by cold working, which can also result in increased strength.

Applications

Typical applications include:

•         Food preparation equipment particularly in chloride environments.

•         Pharmaceuticals

•         Marine applications

•         Architectural applications

•         Medical implants, including pins, screws and orthopaedic implants like total hip and knee replacements

•         Fasteners

Background

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 a 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 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.

Grade 304L is more readily available in most product forms, and so is generally used in preference to 321 if the requirement is simply for resistance to intergranular corrosion after welding. However 304L has lower hot strength than 321 and so is not the best choice if the requirement is resistance to an operating environment over about 500°C.

Key Properties

These properties are specified for flat rolled product (plate, sheet and coil) in ASTM A240/A240M. Similar but not necessarily identical properties are specified for other products such as pipe and bar in their respective specifications.

Composition

Typical compositional ranges for grade 321 stainless steels are given in table 1.

Table 1. Composition ranges for 321 grade stainless steel

Mechanical Properties

Typical mechanical properties for grade 321 stainless steels are given in table 2.

Table 2. Mechanical properties of 321 grade stainless steel

Physical Properties

Typical physical properties for annealed grade 321 stainless steels are given in table 3.

Table 3. Physical properties of 321 grade stainless steel in the annealed condition

Grade Specification Comparison

Approximate grade comparisons for 321 stainless steels are given in table 4.

Table 4. Grade specifications for 321 grade stainless steel

Possible Alternative Grades

Possible alternative grades to grade 321 stainless steels are given in table 5.

Table 5. Possible alternative grades to 321 grade stainless steel

Corrosion Resistance

Equivalent to Grade 304 in the annealed condition, and superior if a weldment in these grades has not been post-weld annealed or if the application involves service in the 425-900°C range. Subject to pitting and crevice corrosion in warm chloride environments, and to stress corrosion cracking above about 60°C. Considered resistant to potable water with up to about 200mg/L chlorides at ambient temperatures, reducing to about 150mg/L at 60°C.

Heat Resistance

Good oxidation resistance in intermittent service to 900°C and in continuous service to 925°C. These grades perform well in the 425-900°C range, and particularly where subsequent aqueous corrosive conditions are present. 321H has higher hot strength, and is particularly suitable for high temperature structural applications.

Heat Treatment

Solution Treatment (Annealing) - heat to 950-1120°C and cool rapidly for maximum corrosion resistance.

Stabilizing - heat to 870-900°C for 1 hour per 25mm of thickness and air cool. Stabilization is recommended for most severe service conditions (above 425°C) and particularly for material annealed at the upper side of the annealing temperature range.

Stress Relief - Heat to 700°C for 1 to 2 hours and air cool.

These grades cannot be hardened by thermal treatment.

Welding

Excellent weld ability by all standard fusion methods, both with and without filler metals. AS 1554.6 pre-qualifies welding of 321 and 347 with Grade 347 rods or electrodes; high silicon version of 347 is also pre-qualified for welding of 321.

Applications

Typical applications include:

•         Aircraft exhaust manifolds

•         Expansion joints

•         Bellows

•         Furnace parts

•         Heating element tubing

•         Heat Exchangers

•         Woven or welded screens for high temperature mineral processing

•         Spiral Welded tube for burner pipes and flues