Heat treating is the process of heating or cooling metals to alter their physical or chemical properties, without affecting size and shape.

The goal of heat treating is to create a heat treated part strong enough to perform a special task. The steps for achieving these goals are consistent for all heat treatment processes:


Heat treating processes vary depending on the type of metal being used and the intended strength of the treated part. Furnace temperature has the most significant impact on the results of heat treatment. Some parts require furnace temperatures up to 2375 °F, while others involve special "heat treatment" processes of 120 °F below zero.

Heat treating results are sometimes dependent on heating rate, with some heating processes requiring rapid or slow heating. Improper heat treatment techniques can affect the integrity of the heated part, potentially leading to severe industrial accidents.


Cooling rates from high furnace temperature are also an important part of the heat treating process. Leaving the heat treated part in the furnace to cool at a controlled rate is the slowest ordinary cooling rate. Fast cooling generally involves some type of liquid and is known as "quenching." There are many different liquid types used in quenching, including:

Each liquid provides a different cooling rate, so it is essential that the correct quenching liquid is used to achieve the proper heat treating cooling levels to avoid cracking. Other cooling methods include forced air or gases such as nitrogen.


Heat treating generally involves steels, but alloys of copper and titanium are also commonly treated. Steels differ in composition based on the amount of carbon or other alloys mixed in when the liquid steel was formed. "Plain steels" are mainly iron plus carbon, while "alloy steels" consist of iron and carbon plus one or more metals. The amount of carbon in any steel determines the resulting strength achieved from heat treating.

The Society of Automotive Engineers (SAE) and the American Iron and Steel Institute (AISI) use a four number system to classify steel types. The first two numbers identify the kind steel, and the last two numbers tell the amount of carbon. For example, in SAE (or AISE) 1040, the 10 signifies plain carbon steel and the 40 describes 40 "points" of carbon. Carbon points are a way of indicating quantity.

Each steel type will have its own number designation. Some common steel types are:

10 - Plain Carbon

11 - Free Cutting

31- Nickel-Chromium

41- Chromium-Moly

46 - Nickel-Moly

52 - Chromium

61 - Chrome-Vanadium

86 - Nickel-Chromium-Moly


"Tool steels" are special purpose steels generally created to perform processes on other materials. They have their own classification system divided by letter types:

W - Water Hardening

H41-H43 - Moly Hot Work

S - Shock Resisting

T - Tungsten High Speed

O - Oil Hardening

M - Moly High Speed

A - Air Hardening

L - Low Alloy Special

D - High Carbon, High Chrome

F - Carbon-Tungsten

H11-H16 - Chromium Hot Work

P - Low Carbon Mold

H20-H26 - Tungsten Hot Work

Several - Miscellaneous

These tool steels are categorized by their intended work, what they consist of, or the type of heat treatment they require.


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