Stainless steel
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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. The 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 stress or abrasion in rvice. 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 recrystallis the work hardened grains but also takes chromium carbides (precipitated at g
rain boundaries in nsitid steels) back into solution in the austenite. The treatment also homogenis dendritic weld metal structures, and relieves all remnant stress from cold working. Annealing temperatures usually are above 1040°C, although some types may be annealed at cloly 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 becau the metal must be cooled rapidly, usually by water quenching, to prevent nsitisation (except for stabilid 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 niobiu
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m in grade 347 when the 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 subquent precipitation of chromium carbide. This special protective treatment is sometimes uful when rvice conditions are rigorously corrosive, especially when rvice 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, grea and other carbonaceous residues. Such residues lead to carburisation during heat treating, which degrades corrosion resistance.
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Process Annealing生产工艺英文>adagio
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All martensitic and most ferritic stainless steels can be subcritical annealed (process ann
ealed) 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 prent 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 spheroidid 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-pha 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.star fighter
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 subquent scale removal; the product is as bright after as before annealing. The products are often referred to as "BA".
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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 increas with austenitising temperature to about 980°C and then decreas due to retention of austenite. For some grades the optimum austenitising temperature may depend on the subquent tempering temperature.
Preheating before austenitising is recommended to prevent cracking in high-carbon types
and in intricate ctions 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 becau of their high alloy content. Air cooling from the austenitising temperature is usually adequate to produce full hardness, but oil quenching is sometimes ud, particularly for larger ctions. Parts should be tempered as soon as they have cooled to room temperature, particularly if oil quenching has been ud, 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.ceac
