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August 31, 2021

ATI 316 stainless steel is susceptible to precipitation of chromium carbides

ATI 316 stainless steel is susceptible to precipitation of chromium carbides in grain boundaries when exposed to temperatures in the 800°F to 1500°F (425°C to 815°C) range. Such “sensitized” steels are subject to inter-granular corrosion when exposed to aggressive environments. ATI 316L alloy is available to avoid the hazard of intergranular corrosion. ATI 316L alloy provides resistance to intergranular attack even after short periods of exposure in the 800-1500°F (425-815°C) temperature range. Stress relieving treatments falling within these limits can be employed without affecting the corrosion resistance of the metal. Accelerated cooling from higher temperatures for the “L” grades is not needed when very heavy or bulky sections have been annealed. ATI 316LN alloy possesses the same mechanical properties as the corresponding higher-carbon ATI 316, and offers the resistance to intergranular corrosion of ATI 316L alloy. Although the short duration heating encountered during welding or stress relieving does not produce susceptibility to intergranular corrosion, continuous or prolonged exposure at 800-1200°F (422- 650°C) can produce sensitization of ATI 316LN (and of ATI 316L) stainless steels.

 

The influence of molybdenum reduces the resistance of ATI 316LN stainless steel to highly oxidizing environments including the nitric acid environment of the ASTM A 262 practice C "Huey" test.

 

Stress Corrosion Cracking

 

Austenitic stainless steels are susceptible to stress corrosion cracking (SCC) in halide environments. Although the ATI 316, ATI 316L and ATI 316Ti alloys are more resistant to SCC than the 18 Cr-8 Ni alloys, they still are quite susceptible. Conditions that produce SCC are:

 

(1) Presence of halide ion (generally chloride),

(2) Residual tensile stresses, and

(3) Temperature in excess of about 140°F (60°C)

 

Stresses result from cold deformation or thermal cycles during welding. Annealing or stress relieving heat treatments may be effective in reducing stresses, thereby reducing sensitivity to halide SCC. Although the low carbon ATI 316L and ATI 316LN alloys offer no advantage as regards SCC resistance, they are better choices for service in the stress relieved condition in environments which might cause intergranular corrosion. If SCC resistance is desired, use of duplex stainless steels such as ATI 2205™ or ATI 2003® duplex stainless alloys should be considered.

 

FABRICATING AND WELDING

 

Fabrication

 

The austenitic stainless steels, including the ATI 316LN alloy, are routinely fabricated into a variety of shapes ranging from the very simple to very complex. These alloys are blanked, pierced, and formed on equipment essentially the same as used for carbon steel. The excellent ductility of the austenitic alloys allows them to be readily formed by bending, stretching, deep drawing and spinning. However, because of their greater strength and work hardenability, the power requirements for the austenitic grades during forming operations are considerably greater than for carbon steels. Attention to lubrication during forming of the austenitic alloys is essential to accommodate the high strength and galling tendency of these alloys.

 

Annealing

 

The austenitic stainless steels are provided in the mill-annealed condition ready for use. Heat treatment may be necessary during or after fabrication to remove the effects of cold forming or to dissolve precipitated chromium carbides resulting from thermal exposures. For the ATI 316LN alloy the solution anneal is accomplished by heating in the 1900- 2150°F (1040-1175°C) temperature range followed by air cooling or a water quench, depending on section thickness. ATI 316LN stainless cannot be hardened by heat treatment.

 

Welding

 

The austenitic stainless steels are considered the most weldable of the stainless steels. They are routinely joined by all fusion and resistance welding processes. Two important considerations for weld joints in these alloys are (1) avoidance of solidification cracking, and (2) preservation of corrosion resistance of the weld and heat-affected zones. ATI 316LN stainless steel often is welded autogenously. If filler metal must be used for welding ATI 316LN stainless, it is advisable to utilize the low carbon ATI 316L or E318 filler metals. Contamination of the weld region with copper or zinc should be avoided, since these elements can form low melting point compounds, which in turn can create weld cracking.

 

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