tempering


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tempering,

process involving slow and moderate heating to increase the hardness and toughness of metals that have undergone previous heat treatment. Metals are usually hardened (see hardeninghardening,
in metallurgy, treatment of metals to increase their resistance to penetration. A metal is harder when it has small grains, which result when the metal is cooled rapidly.
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) by being heated to high temperatures and quenched rapidly. This treatment causes brittleness, which is reduced by tempering. Steel is notably responsive to tempering, and makers of tools, weapons, armor, and other articles of steel have long had great skill in the process. Tempering is not necessary for such products as razors and files, in which hardness is sought but brittleness is not a serious disadvantage. Other products, e.g., swords and saws, require tempering for toughness. In the handicraft process of tempering, the condition of the steel during heating is judged by its color, caused by an oxide film. A desired hardness can be achieved by plunging the steel into a bath when it has cooled to the right shade of yellow or brown or blue. To secure a bath of the right temperature, various liquids are used, e.g., pure water, saltwater, oil, and molten metal. The process of softening steel that is harder than desired is called annealing. In modern mass production the processes of tempering are guided by scientific tests in place of the artisan's skill. Comparable to tempering is the process of hastening the cooling of a surface of a casting to increase the hardness of the part so "chilled."

Tempering

 

(in metallurgy), a type of heat treatment, in which a quenched alloy is reheated to a temperature below the first critical point, is held at that temperature, and is subsequently cooled. The term “tempering” is mainly applied to steels. For other alloys, the term “quench aging” is usually used to refer to the processes by which the state of a metal is altered after quenching.

The main purpose of tempering is to produce steels with certain desired properties, particularly the optimum combination of strength, plasticity, and impact strength. As temperature is increased, the overall properties of steel change gradually; however, it is possible to define narrow temperature ranges in which these changes occur rapidly. Three transformations are distinguished: the first occurs between 100° and 150°C, the second between 250° and 300°C, and the third between 325° and 400°C. The first transformation is accompanied by a reduction in the volume of the metal, the second by an increase, and the third by a large reduction.

Using X-ray crystallography, G. V. Kurdiumov made a large contribution to the elucidation of tempering processes. His studies showed that the first and third transformations involve the decay of martensite, while the second transformation is related to the destruction of the residual austenite (seeMARTENSITE, AUSTENITE). Two phases arise in the decay of martensite between 100° and 150°C during the course of tempering: one phase consists of a solid solution of the metal at the initial concentration, while a second phase contains 0.25–0.3 percent carbon. As tempering proceeds in the temperature range of 200°–300°C, a low-temperature form of iron carbide appears; tempering at higher temperature leads to the formation of cementite.

The three transformations do not necessarily occur when steel is tempered. The first transformation is absent in low-carbon steels, which contain up to 0.2 percent carbon. Alloying with Cr, Mo, W, V, Co, and Si shifts the second transformation toward higher temperatures. When tempered between 450° and 550°C, steels alloyed with Mo, W, and V undergo a separation of particles that consist of dispersed carbides of Mo, W, and V. This separation results in secondary hardening. Ultimately, a high degree of tempering transforms a steel’s structure into a ferritecarbide mixture.

Based on recent experimental data, it appears that quenched steel undergoes the following processes during storage and heating: (1) redistribution of carbon atoms in martensite, which involves a flow of carbon atoms toward the dislocations and toward the boundaries of martensite crystals, as well as a displacement of carbon atoms into the pores of the crystal lattice; (2) decay of martensite with the formation of precipitates of one of the carbide fractions, depending on the conditions of alloying and the temperature of tempering as well as on the actual structure of the martensite crystals; (3) relaxation of internal microscopic stresses as a result of microscopic plastic deformation; (4) transformation of the residual austenite into perlite or bainite, depending on alloying conditions and the temperature range; and, finally (5) transformation of residual austenite during cooling after tempering (secondary hardening).

Although increasing the temperature of tempering lowers hardness and strength, it also increases plasticity and impact strength. Raising tempering temperatures also reduces the critical temperature of cold embrittlement. Tempering up to 300°C increases the resistance toward small plastic deformations. Tempering in the temperature ranges from 300° to 400°C and from 500° to 600°C, particularly in alloyed steels, leads to a decrease in impact viscosity and an increase in the critical cold embrittlement temperature; these are phenomena of irreversible and reversible temper embrittlement. Rapid cooling after tempering at 600° to 650°C in steel that is alloyed with Mo and W reduces reversible embrittlement. Low-temperature tempering (120°-250°C) mainly serves to decrease the tendency toward brittle fracture and is employed in the thermal treatment of steels that are used for tools, of case-hardened steels, and of high-strength structural steels. Tempering at 300°-400°C is used for the thermal treatment of coil springs and leaf springs, while tempering at 450°-650°C is employed in the heat treatment of machine parts that are subject to dynamic and vibrational stresses.

REFERENCES

Kurdiumov, G. V. Iavleniia zakalki i otpuska stali. Moscow, 1960.
Kurdiumov, G. V. O kristallicheskoi strukture zakalennoi stali. In the collection Problemy metallovedeniia i fiziki metallov, vol. 9. Moscow 1968.
Guliaev, A. P. Termicheskaia obrabotka stali, 2nd ed. Moscow, 1960.

R. I. ENTIN

tempering

[′tem·pə·riŋ]
(materials)
Impregnating wood fibers or composition board with an oxidizing resin or drying oil followed by heat treatment, to improve strength, durability, and water resistance.
(metallurgy)
Heat treatment of hardened steels to temperatures below the transformation temperature range, usually to improve toughness.
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