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Heat Treating

How you heat treat stainless steel depends on the type and grade of stainless steel as well as the reason for the treatment.

Cleaning: Before any heat treating, remove all oils, grease, and other residues from the stainless steel. Residues that remain on the steel result in carburization that reduces corrosion resistance properties.

Annealing: Austenitic stainless steels can not be hardened by heat treatment. They do, however, harden by cold working. Annealing, or solution treatment, is employed for recrystallizing the work-hardened austenitic stainless steels and drawing chromium carbides, precipitated around the work-hardened stainless steels, into the solution.

Anneal stainless steels at temperatures greater than 1904°F, but certain types of steel can be annealed at very controlled temperatures of below 1850°F for a fine grain size. Keep the time at temperature short in order to prevent surface scaling and control grain growth.

Quench Annealing: Quench annealing of austenitic stainless steel is a process of rapidly cooling the metal from 1900°F to below 1100°F or preferably below 900°F by water quenching. Stabilized and extra-low carbon grades of stainless steel may be very rapidly cooled by fan accelerated gas or water quenching.

Stabilizing Anneal: A stabilizing anneal is often carried out following conventional annealing of grades 321 and 347 which contain titanium or niobium. The carbon content of these grades is combined with titanium in grade 321, and niobium in grade 347, during annealing. A second anneal between 1598°F to 1652°F for 2 to 4 hours, followed by rapid cooling prevents precipitation of chromium carbide.

This treatment can be performed under rigorously corrosive operating conditions or conditions that involve temperatures ranging from 750°F to 1600°F.

Process Annealing: All Ferritic and martensitic stainless steels can be processed or subcritical annealed by heating in the ferrite temperature range, or fully annealed by heating above the critical temperature in the austenite range. Follow with slow cooling. Sub-critical annealing can be carried out in temperatures from 1400° to 1525°F. The soft structure of spheroidised and ferrite carbides can be produced by cooling the material slowly at a rate of 75°F per hour or holding the material for one hour or longer at the subcritical annealing temperature. Stainless steel that has been cold-worked following full annealing can be subcritically annealed in less than 30 min.

The Ferritic steel grades retaining single-phase structures throughout the operating temperature range require nothing more than short recrystallization annealing at temperatures of 1400° to 1750°F.

Bright Annealing: Bright anneal all grades of stainless steels in highly reducing controlled atmosphere furnaces to reduce scaling. This treatment can be carried out in a salt bath, but bright annealing performed under hydrogen, dissociated ammonia, or nitrogen/hydrogen atmospheres at dew points less than -60°F is mostly preferred to prevent or minimize surface oxidation. Martensitic grades and some ferritic grades are susceptible to hydrogen embrittlement.

Hardening: Like low alloy steels, martensitic stainless steels are hardened using tempering, quenching, and austenitizing. Austenitizing temperatures range from 1800° to 1850°F. At an austenitizing temperature of 1800°F, as-quenched hardness tends to increase first and then drops, following retention. The optimum austenitizing temperature for certain steel grades may be based on the temperature of the following process tempering.

Heat slowly or preheat at 1450°F before austenitizing to prevent cracking in hard carbon grades and in intricate sections of low carbon types.

Cooling and Quenching: Air cool martensitic stainless steels to produce full hardness. For larger sections, oil quenching is sometimes used. Temper as soon as parts reach room temperature. When tempering at temperatures above 950°F, cool rapidly to below 750°F to avoid embrittlement.

Tempered immediately after cooling at room temperature, particularly if oil quenching has been used to prevent cracking. In some cases, components are frozen at -103°F prior to tempering. Temper martensitic steels at temperatures greater than 950°F followed by rapid cooling to below 750°F to avoid embrittlement.

Some precipitation-hardening stainless steels require rigorous heat treatments when compared to standard martensitic types. For example, aging, sub-zero cooling, trigger annealing, and annealing may require a semi-austenitic precipitation-hardening type. Martensitic precipitation-hardening types, on the other hand, often require only aging treatment.

Stress Relieving: Achieve moderate stress relief below 750°F or significant stress relief at temperatures of up to 800° to 1700°F. One hour of stress relief at 1600°F relieves about 85% of residual stresses. However, this temperature range can impair corrosion resistance. Stabilized stainless steels or low-carbon type steels are preferred to avoid these effects.

Full solution treatment of stainless steels, by heating to about 1975°F followed by rapid cooling, eliminates all residual stresses. However, it is not practical for most large or complex fabrications.

Low-Temperature Stress Relieving: Cold-working austenitic stainless steels improves strength, however, compressive yield strength and proportional limit will increase with low-temperature stress relieving at temperatures of up to 325° to 775°F. Higher temperatures will degrade the material strength and, hence, they are not preferred for stress-relieving cold-worked products.

Nitriding: Austenitic stainless steels can be surface hardened by nitriding. This process has very limited application, as the stainless steel core is soft and has very low strength for heavy applications. Another major limitation is that the nitrided steel is less resistant to corrosion when compared to the original stainless steel.

Physical Vapour Deposition (PVD): Physical vapor deposition enables the deposition of thin, hard layers on many materials including stainless steels. Titanium nitride (TiN) is the most commonly applied coating, available in aesthetically pleasing gold color. Owing to its appearance, this coating is commonly applied on No. 8 mirror polished surface for producing architectural panels embedded with gold panels.