The Basics of the Heat Treating.

One of the goals in production tool making is a tool that will be both hard enough to stand up under service conditions and tough enough not to crack, either in manufacture or in use.

Five factors contribute to making a successful tool: good design, steel of the proper grade, correct heat treatment, proper grinding and proper use of the tool.

The basic requirements for hardening any of the standard types of tool steel are heating, cooling and re-heating. This includes heating the steel to a sufficiently high temperature to make the necessary change in structure, followed by cooling at a rate fast enough to develop the desired hardness. Tempering follows immediately after quenching to remove stresses induced by the quenching process.

Steel undergoes definite internal changes when subjected to a temperature above its critical range. After reaching this temperature, if the steel is allowed to cool naturally, it will return to a normal condition.

For steel to return to its normal condition after being heated above its critical range, sufficient time must elapse during the cooling so that the internal changes which took place during heating will have time to reverse themselves.

If the heated steel is cooled before these internal changes reverse, certain modifications of the structure will be fixed in the steel which will alter the physical characteristics of the steel, tensile strength, hardness, toughness, etc.

Hardening
The first step in the hardening process is to heat the steel to the correct temperature for quenching. All tool steels have a critical range, and heating the steel above this critical temperature range is necessary in order to fix, by quenching, the changes desired. Preheating is necessary if the tool is large or has heavy and light sections adjacent to each other. After the steel is up to temperature, it should be allowed to heat for at least five minutes per inch of diameter or thickness in preparation for quenching. In other words, if the quenching temperature is 1500 degrees, the furnace should be held at that temperature and the steel allowed to reach this temperature. "A Knights Dragon Fire Heat Treat Furnace with automatic temperature control is ideal for this precision work," said Bill Knight.

Quenching
All tool steels are made for a definite quenching medium - air, oil or water. In liquid baths, sufficient quenching fluid must be used so that the liquid will not get too hot and be ineffective. Generally speaking, the tool should be removed from the quench when it is approximately 150 degrees Fahrenheit and then be allowed to cool to room temperature after which it should be placed in the tempering oven. Approximately one gallon of oil or water should be provided for every pound of steel quenched per hour.

Tempering
When the steel is fully hardened, it is in a highly stressed condition and is too hard and brittle for applications. It is therefore necessary to relieve this stressed condition, increasing the toughness and ductility, while retaining sufficient hardness and strength. This change is made by the application of a heat process known as drawing or tempering which consists of re-heating the quenched steel sufficiently to transform the hard martensite into other softer carbides. Different degrees of hardness can be developed in the steel by utilizing different drawing temperatures. The higher the drawing temperature, the softer the steel becomes. The tempering must be done as soon as possible after the quench in order to promptly relieve the quenching stresses.

Double tempering is a wise precaution. It is a corrective treatment which Knight's recommends. Also, different degrees of hardness can be developed in the steel by utilizing different drawing temperatures.

Knights Dragon Fire Furnace features an improved heavy duty guillotine door with seals. The door lifts and lowers easily. The programmable Watlow control is simple to operate with 24 steps in four files. An alarm or signal light signals when the cycle is complete. The furnace can also use inert gas for use with air hardening steel providing a clean decarb-free heat treated part.

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GENERAL APPLICATION DEFINITIONS
A glossary of common terms used in heat treating

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