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What is heat treatment of stainless steel?

DataTime:[2016-12-22]
Stainless steel is typically heat treated based on the improvement of processability, elimination of internal stresses and strengthening of the ductility and corrosion resistance of metals, or the generation of hard structures capable of tolerating abrasion and high mechanical stresses.

Heat treatment of stainless steel is mostly carried out under controlled conditions to avoid carburization, decarburization and scaling on the metal surface.

Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, normalizing and quenching.

The following describes the heat treatment process basically includes most of the mainstream of steel heat treatment process.

Annealing of stainless steel
Stainless steel will be heated to a certain temperature and a fixed time insulation, and then slowly cooling, known as annealing. Annealing of stainless steels are to heat the steel to the phase transition or part of the phase transition temperature, through the insulation after the slow cooling heat treatment method. The purpose of annealing is to eliminate defects in the organization, to improve the organization so that the composition of the grain homogenization and refinement to improve the mechanical properties of steel to reduce residual stress; at the same time reduce the hardness, improve the plasticity and toughness, improve the cutting performance. So the annealing is to eliminate and improve the former procedures left after the organization of defects and internal stress, but also for the follow-up process to prepare, it is semi-finished annealing heat treatment, also known as pre-heat treatment.

Annealing of stainless steels is carried out at temperatures greater than 1040°C, but certain types of steel can be annealed at very controlled temperatures of below 1010°C while considering fine grain size. The process is maintained for a short interval, in order to prevent surface scaling and control grain growth.

Normalizing of stainless steel
Normalizing is heated to above the critical temperature of stainless steel, it is all around for uniform austenite, and then in the air to cool naturally. It can improve comprehensive mechanical properties of stainless steel, but with less ductility for the same composition than full annealing.

Quenching
Quenching is a process of cooling a metal at a rapid rate. This is most often done to produce a martensite transformation.

To harden by quenching, a stainless steel must be heated above the upper critical temperature and then quickly cooled. Depending on the alloy and other considerations (such as concern for maximum hardness vs. cracking and distortion), cooling may be done with forced air or other gases, (such as nitrogen). Liquids may be used, due to their better thermal conductivity, such as oil, water, a polymer dissolved in water, or a brine. Upon being rapidly cooled, a portion of austenite (dependent on alloy composition) will transform to martensite, a hard, brittle crystalline structure. The quenched hardness of a stainless steel depends on its chemical composition and quenching method. Cooling speeds, from fastest to slowest, go from brine, polymer (i.e. mixtures of water + glycol polymers), fresh water, oil, and forced air. However, quenching a certain steel too fast can result in cracking. If the percentage of carbon is less than 0.4 percent, quenching is not possible.

Tempering
Tempering is a process of heat treating, which is used to increase the toughness of iron-based alloys. Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to some temperature below the critical point for a certain period of time, then allowing it to cool in still air. The exact temperature determines the amount of hardness removed, and depends on both the specific composition of the alloy and on the desired properties in the finished product. For instance, very hard tools are often tempered at low temperatures, while springs are tempered to much higher temperatures.

Most applications require that quenched parts be tempered. Tempering consists of heating stainless steel below the lower critical temperature, (often from 400 to 1105 ˚F or 205 to 595 ˚C, depending on the desired results), to impart some toughness. Higher tempering temperatures (may be up to 1,300 ˚F or 700 ˚C, depending on the alloy and application) are sometimes used to impart further ductility, although some yield strength is lost.

It should be noted that not all steels will respond to all heat treatment processes, summaries the response, or otherwise, to the different processes.

 

Anneal

Normalise

Harden

Temper

Low Carbon <0.3%

yes

yes

no

no

Medium Carbon 0.3-0.5%

yes

yes

yes

yes

High Carbon >0.5%

yes

yes

yes

yes

Low Alloy

yes

yes

yes

yes

Medium Alloy

yes

yes

yes

yes

High Alloy

yes

maybe

yes

yes

Tool Steels

yes

no

yes

yes

Stainless Steel (Austenitic eg 304, 306)

yes

no

no

no

Stainless Steels (Ferritic eg 405, 430 442)

yes

no

no

no

Stainless Steels (Martensitic eg 410, 440)

yes

no

yes

yes

Heat treatment of stainless steel surface
(1) surface hardening: the steel surface is quickly heated to above the critical temperature, but the heat has not yet had time to reach the heart before the rapid cooling, so that the surface layer can be quenched in the martensite, while the heart does not occur Phase change, which achieved the surface hardened and the heart of the same purpose. Applicable to medium carbon steel.

(2) Chemical heat treatment: refers to the chemical element of the atom, with the help of atomic diffusion of high temperature, it penetrated into the surface layer of the workpiece to change the chemical composition and structure of the workpiece surface layer, so as to achieve a specific surface layer of steel The required organization and performance of a heat treatment process. In accordance with the different types of infiltration of elements, chemical heat treatment can be divided into carburizing, nitriding, cyanide and percolation metal four.

Carburizing
Carburizing refers to the process of infusing carbon atoms into a steel surface layer. But also to make the surface of low-carbon steel workpiece with high-carbon steel surface, and then after quenching and low temperature tempering, the workpiece surface layer with high hardness and wear resistance, and the central part of the workpiece is still maintaining the toughness of low carbon steel and Plasticity.

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.

Cyanide
Also known as carbonitriding, refers to the steel in the same time infiltration of carbon atoms and nitrogen atoms in the process. It makes the steel surface with carburizing and nitriding characteristics.

Infiltration of metal
Refers to the process of penetration of metal atoms into the surface layer of steel. It is the surface layer of steel alloy, so that the workpiece surface with some alloy steel, special steel characteristics, such as heat, wear resistance, oxidation resistance, corrosion resistance. Production is commonly used in aluminizing, chromizing, boronizing, silicon infiltration and so on.

The above is the mainstream of steel, especially heat treatment process of stainless steel. With the continuous progress of technology, heat treatment process is also constantly updated.

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