Hardening and Heat Treatment

Heat treatment is used for many different types of metal and for many different effects. Here is an overview of the heat treatment methods along with links to more in-depth details.

Annealing

Heating to and holding at a suitable temperature and then cooling at a suitable rate, for such purposes as reducing hardness, improving machinability, facilitating cold working, producing a desired microstructure, or obtaining desired mechanical, physical or other properties.

Types of annealing include blue, black, box, bright, full, intermediate, isothermal, quench and recrystallization.

When applied to ferrous alloys, the term “annealing,” without qualification, implies full annealing.

When applied to nonferrous alloys, the term “annealing” implies a heat treatment designed to soften a cold worked structure by recrystallization or subsequent grain growth or to soften an age-hardened alloy by causing a nearly complete precipitation of the second phase in relatively coarse form.

The more important purposes for which steel is annealed are:

  • remove stresses
  • induce softness
  • alter ductility
  • alter toughness
  • alter electric, magnetic, or other physical and mechanical properties
  • change the crystalline structure
  • to remove gases
  • to produce a definite microstructure.

Copper: Heat until it glows dull red, then cool (either slowly or by plunging it in water), the copper becomes very soft and easy to bend and work.

Flame Annealing

Annealing in which the heat is applied directly by a flame.

Sub-Critical Annealing

A heat treating operation used to relieve or dissipate stresses in weldments, heavily machined parts, castings and forgings. The parts are heated to 620°C, uniformly heated through, and are either air cooled from temperature or slow cooled from temperature depending on the type of part and subsequent finishing or heat treating operations.

Hardenability

In a ferrous alloy, the property that determines the depth and distribution of hardness induced by quenching from a temperature above the upper critical temperature.

Hardening

Increasing the hardness by suitable treatment, usually involving heating and cooling. When applicable, the following more specific terms should be used: age hardening, case hardening, flame hardening, induction hardening, precipitation hardening,, quench hardening and through hardening.

Mild Steel: Can be case hardened to increase the surface hardness. Case hardening can be difficult to get hold of, but EKP Supplies Beta 1 compound.

Silver Steel: Heat uniformly to 770-780°C until heated through. Quench in water. Sizes up to 8mm or 5/16” dia may be oil hardened from 800-810°C.

Oil Hardening

A process of hardening a ferrous alloy of suitable composition by heating within or above the transformation range and quenching in oil.

Steel: Heat the steel to 770 to 800°C and soak at this temperature for 3/4 hour per inch (25.4mm) thickness of material. Quench in a bath of oil inserting the heaviest end of the piece first (this way relieves stresses and reduces distortion). Further heat treatment at 100 to 120°C for a short while and then allowing the piece to cool naturally in air will reduce the stresses further and the possibility of cracking. Note: the higher the carbon content the lower the hardening temperature of steel.

Precipitation Hardening

Increase the hardness of a supersaturated solid solution by heat treating it to cause a second phase to precipitate out. Coherency of the precipitate/matrix interface and how well the two lattices match up greatly influence the effect of precipitate

Quenching

Rapid cooling. When applicable, the following more specific terms should be used: direct quenching, jog quenching, hot quenching, interrupted quenching, selective quenching, spray quenching and time quenching.

The quenching media may be water, brine, oil, special solutions, salts or metals; and the intensity of the quench is determined by the temperature, volume and velocity of the media. In the case of air hardening tool steels the quenching medium is air at room temperatures.

The procedure consists of heating the material to the proper austenitizing temperature, holding at that temperature for a sufficient time to effect the desired change in crystalline structure and then quenching in a the appropriate medium. After quenching, the material is reheated to a predetermined temperature below the critical range and then cooled at a suitable rate (tempering).

Direct Quenching

Quenching carburized parts directly from the carburizing operation.

Interrupted Quenching

Quenching in which the metal object being quenched is removed from the quenching medium while the object is at a temperature substantially higher than that of the quenching medium

Quenching Oil

A high-quality, oxidation-resistant petroleum oil used to cool metal parts during their manufacture, and is often preferred to water because the oil′s slower heat transfer lessens the possibility of cracking or warping of the metal.

Tempering

Reheating a quench-hardened or normalized ferrous alloy to a temperature below the transformation range and then cooling at any rate desired.

Silver Steel: After quenching, heat the part back to around 100°C to 150°C for around 30 minutes.

References

  1. What is Quenching? – Sheffield Gauge Plate blog post on quenching and a comparison of the different quenchants: air, water, oil, vegetable oil
  2. Oil hardening steel – Sheffield Gauge Plate blog post on the subject of oil hardening steel. Overview, benefits and selecting the correct oil.

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