316L / 316 Steel

As the second most commonly used stainless steel and with the addition of 2-2.5% of molybdenum, 316L/316 exhibit better corrosion resistance, even in the presents of chlorides, and are stronger at elevated temperatures. Both, low temperature applications are possible, as well as applications up to 1022°F (550°C). Both alloys provide higher creep, stress rupture and tensile strength at elevated temperatures. Typical for austenitic stainless steels they are both non-hardenable by heat treatment and provide excellent processing capability and formability.

AISI 316L, differs from AISI 316 in that it has a lower carbon content, is highly resistant to sensitization (grain boundary carbide precipitation) and therefore is used extensively in heavy gauge welded components and allows the use of this grade in the as-welded condition in a wide variety of corrosive applications.

Corrosion resistance

General corrosion

316L/316 has considerably more resistance than other chromium-nickel grades, like for example 304, to most complex sulfur compounds used for example in the paper industry. In temperatures as high as 120°F (48°C) alloy 316L/316 is resistant to concentrations of sulfuric acid up to 5%. At temperatures under 100°F (30°C) they have excellent resistance to higher concentrations.

Gas corrosion

Corrosion resistance is reduced in sulphurous flue gases. In such environments, 316L/316

can be used at temperatures up to 1110-1380°F (600-750°C), depending on service conditions. Consideration must be given to weather the atmosphere is oxidizing or reducing, e.g. the oxygen content, and weather impurities such as sodium and vanadium are present.

Crevice/Pitting corrosion

In the presents of chloride or halide ions, austenitic stainless steels are resistant to pitting and/or crevice corrosion when higher chromium, molybdenum and nitrogen content is present. The molybdenum in these steels will handle water with up to about 0.017lb/gal. (2,000 ppm) of chloride. Although considered adequate for some marine environment applications such as boat rails or facades of buildings near the ocean, it is not recommended in seawater containing 0.159lb/gal. (19,000ppm) of chloride.

Intergranular corrosion

Intergranular corrosion is the result due to the precipitation of chromium carbides at the grain boundaries for 316 when exposed to a temperature range of 800-1500°F (427-816°C).

To avoid intergranular corrosion, 136L with its lower carbon content can be used for applications where heavy cross-sections cannot be annealed after welding or if lower temperatures for stress relieving are wanted or when exposed to short periods of temperatures of 800-1500°F (427-816°C).

Without affecting the normal excellent corrosion resistance of 136L, short stress relieving treatment can be applied. When heavy and bulky parts have been annealed, it is not necessary to cool them rapidly from high temperatures.

Atmospheric corrosion

With the addition of molybdenum stainless steel develops a maximum resistance to staining and pitting, therefore austenitic stainless steels surpasses virtually all other engineering materials. In atmospheres where chlorides, sulfur compounds and solids are found either individually or in combination, these grades are therefore regularly used.

Oxidation

316L/316 stainless steel used for short periods at temperatures up to 1600°F (870°C) or a constant temperature of 1700°F (925°C) have good oxidation resistance. Temperature ranges of 850-1550°F (454-845°C) are not recommended for 316 stainless steel due to the possibility of carbide precipitation but performs well in varying temperatures above or below that range. 316L should be used in these applications.

Stress corrosion cracking

Austenitic steels are prone to stress corrosion cracking, as can be encountered at temperatures above 140°F (60°C) when the steel is exposed to tensile stresses or comes into contact with solutions containing chlorides for example at the same time.

Heat treatment

316L/316 are not hardenable by heat treatment, cold working will increase tensile strength as well as hardness though. To eliminate the effects of cold forming or to dissolve precipitated chromium carbides due to thermal exposure, heat treatment may be needed during or after fabrication.

Annealing

To anneal heat 316L/316 to 1870-2050°F (1020-1120°C) followed by quenching in water or air, depending on the dimension of the part.

Stress relieving

316L as the lower carbon steel grade can be stress relieved at 850-1100°F (454-593°C) for 60 minutes with minimum danger of sensitization.

316 should be done at a lower temperature not exceeding 750°F (399°C) but with a longer soaking time. With the risk of grain boundary sensitization occurring, resulting in the loss of its corrosion resistance, stress relieving above 1100°F (593°C) is not recommended.

Machinability

During deformation, 316L/316 is subject to work hardening and chip breaking. For the best machining results, it is better to use slower speeds, excellent lubrication, sharp tooling, heavier feeds and high performance equipment.

Welding

Stainless steel 316 has a medium weldability and can be welded by all standard methods. Fillers should be equal or slightly better alloyed than the base. 316 should be annealed after welding to remove any chromium carbide precipitation. Weld discoloration should be removed by pickling and passivation to restore maximum corrosion resistance.