Background

Stainless steels are often heat treated; the nature of this treatment depends on the type of stainless steel and the reason for the treatment. These treatments, which include annealing, hardening and stress relieving, restore desirable properties such as corrosion resistance and ductility to metal altered by prior fabrication operations or produce hard structures able to withstand high stresses or abrasion in service. Heat treatment is often performed in controlled atmospheres to prevent surface scaling, or less commonly carburisation or decarburisation.

Annealing

The austenitic stainless steels cannot be hardened by thermal treatments (but they do harden rapidly by cold work). Annealing (often referred to as solution treatment) not only recrystallises the work hardened grains but also takes chromium carbides (precipitated at grain boundaries in sensitised steels) back into solution in the austenite. The treatment also homogenises dendritic weld metal structures, and relieves all remnant stresses from cold working. Annealing temperatures usually are above 1040°C, although some types may be annealed at closely controlled temperatures as low as 1010°C when fine grain size is important. Time at temperature is often kept short to hold surface scaling to a minimum or to control grain growth, which can lead to "orange peel" in forming.

Quench Annealing

Annealing of austenitic stainless steel is occasionally called quench annealing because the metal must be cooled rapidly, usually by water quenching, to prevent sensitisation (except for stabilised and extra-low carbon grades).

Stabilising Anneal

A stabilising anneal is sometimes performed after conventional annealing for grades 321 and 347. Most of the carbon content is combined with titanium in grade 321 or with niobium in grade 347 when these are annealed in the usual manner. A further anneal at 870 to 900°C for 2 to 4 hours followed by rapid cooling precipitates all possible carbon as a titanium or niobium carbide and prevents subsequent precipitation of chromium carbide. This special protective treatment is sometimes useful when service conditions are rigorously corrosive, especially when service also involves temperatures from about 400 to 870°C, and some specifications enable this treatment to be specified for the product.

Cleaning

Before annealing or other heat treating operations are performed on austenitic stainless steels, the surface must be cleaned to remove oil, grease and other carbonaceous residues. Such residues lead to carburisation during heat treating, which degrades corrosion resistance.

Process Annealing

All martensitic and most ferritic stainless steels can be subcritical annealed (process annealed) by heating into the upper part of the ferrite temperature range, or full annealed by heating above the critical temperature into the austenite range, followed by slow cooling. Usual temperatures are 760 to 830°C for sub-critical annealing. When material has been previously heated above the critical temperature, such as in hot working, at least some martensite is present even in ferritic stainless steels such as grade 430. Relatively slow cooling at about 25°C/hour from full annealing temperature, or holding for one hour or more at subcritical annealing temperature, is required to produce the desired soft structure of ferrite and spheroidised carbides. However, parts that have undergone only cold working after full annealing can be sub-critically annealed satisfactorily in less than 30 minutes.

The ferritic types that retain predominantly single-phase structures throughout the working temperature range (grades 409, 442, 446 and 26Cr-1Mo) require only short recrystallisation annealing in the range 760 to 955°C.

Controlled Atmospheres

Stainless steels are usually annealed in controlled atmospheres to prevent or at least reduce scaling. Treatment can be in salt bath, but the best option is "bright annealing" in a highly reducing atmosphere. Products such as flat rolled coil, tube and wire are regularly bright annealed by their producers, usually in an atmosphere of nitrogen and hydrogen. The result is a surface requiring no subsequent scale removal; the product is as bright after as before annealing. These products are often referred to as "BA".

Hardening

Martensitic stainless steels are hardened by austenitising, quenching and tempering much like low alloy steels. Austenitising temperatures normally are 980 to 1010°C, well above the critical temperature. As-quenched hardness increases with austenitising temperature to about 980°C and then decreases due to retention of austenite. For some grades the optimum austenitising temperature may depend on the subsequent tempering temperature.

Preheating before austenitising is recommended to prevent cracking in high-carbon types and in intricate sections of low-carbon types. Preheating at 790°C, and then heating to the austenitising temperature is the most common practice.

Cooling and Quenching

Martensitic stainless steels have high hardenability because of their high alloy content. Air cooling from the austenitising temperature is usually adequate to produce full hardness, but oil quenching is sometimes used, particularly for larger sections. Parts should be tempered as soon as they have cooled to room temperature, particularly if oil quenching has been used, to avoid delayed cracking. Parts sometimes are frozen to approximately -75°C before tempering to transform retained austenite, particularly where dimensional stability is important, such as in gauge blocks made of grade 440C. Tempering at temperatures above 510°C should be followed by relatively rapid cooling to below 400°C to avoid "475°C" embrittlement.

Some precipitation-hardening stainless steels require more complicated heat treatments than standard martensitic types. For instance, a semi-austenitic precipitation-hardening type may require annealing, trigger annealing (to condition austenite for transformation on cooling to room temperature), sub-zero cooling (to complete the transformation of austenite) and aging (to fully harden the alloy). On the other hand, martensitic precipitation-hardening types (such as Grade 630) often require nothing more than a simple aging treatment.

Stress Relieving

Stress relieving at temperatures below 400°C is an acceptable practice but results in only modest stress relief. Stress relieving at 425 to 925°C significantly reduces residual stresses that otherwise might lead to stress corrosion cracking or dimensional instability in service. One hour at 870°C typically relieves about 85% of the residual stresses. However, stress relieving in this temperature range can also precipitate grain boundary carbides, resulting in sensitisation that severely impairs corrosion resistance in many media. To avoid these effects, it is strongly recommended that a stabilised stainless steel (grade 321 or 347) or an extra-low-carbon type (304L or 316L) be used, particularly when lengthy stress relieving is required.

Full solution treatment (annealing), generally by heating to about 1080°C followed by rapid cooling, removes all residual stresses, but is not a practical treatment for most large or complex fabrications.

Low Temperature Stress Relieving

When austenitic stainless steels have been cold worked to develop high strength, low temperature stress relieving will increase the proportional limit and yield strength (particularly compressive yield strength). This is a common practice for austenitic stainless steel spring wire. A two hour treatment at 345 to 400°C is normally used; temperatures up to 425°C may be used if resistance to intergranular corrosion is not required for the application. Higher temperatures will reduce strength and sensitise the metal, and generally are not used for stress relieving cold worked products.

Annealing After Welding

Stainless steel weldments can be heated to temperatures below the usual annealing temperature to decrease high residual stresses when full annealing after welding is impossible. Most often, stress relieving is performed on weldments that are too large or intricate for full annealing or on dissimilar metal weldments consisting of austenitic stainless steel welded to low alloy steel.

Stress relieving of martensitic or ferritic stainless steel weldments will simultaneously temper weld and heat affected zones, and for most types will restore corrosion resistance to some degree. However, annealing temperatures are relatively low for these grades, and normal subcritical annealing is the heat treatment usually selected if the weldment is to be heat treated at all.

Surface Hardening

Only limited surface hardening treatments are applicable to the stainless steels. In most instances hardening of carbon and low alloy steels is due to the martensitic transformation, in which the achievable hardness is related to the carbon content - as most martensitic stainless steels have carbon contents ranging from fairly low to extremely low, this hardening mechanism is of little use.

Nitriding

It is possible to surface harden austenitic stainless steels by nitriding. As in nitriding of other steels the hard layer is very hard and very thin; this makes the process of limited use as the underlying stainless steel core is relatively soft and unsupportive in heavily loaded applications. A further drawback is that the nitrided case has a significantly lower corrosion resistance than the original stainless steel.

A number of alternative, proprietary surface hardening processes for austenitic stainless steels have been developed but these have not as yet become commercially available in Australia.

Physical Vapour Deposition (PVD)

An interesting recent development is the PVD (Physical Vapour Deposition) process. This enables very thin but hard layers to be deposited on many materials, including stainless steels. The most commonly applied coating is Titanium Nitride "TiN", which in addition to being very hard is also an aesthetically pleasing gold colour. Because of its appearance this coating has been applied, generally on No8 mirror polished surface, to produce gold mirror finished architectural panels.