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Heat Treat Process Definitions
These are general, generic definitions only, designed with various types of steel in mind. Temperatures and cooling rates may differ with various types of materials.


STRESS RELIEVING
 
Does not affect mechanical properties. Generally done at 1250F & below.

Heating and holding at a relatively low temperature, then cooling at a rate slow enough (usually in still air or a furnace) not to develop new residual stresses. Process is used to reduce the residual stresses (both mechanical & thermal) in a piece of material and have no appreciable change in its mechanical properties (i.e., tensile strength, yield strength, hardness, etc.).

Applications: To be used for torch-cut material with greater than 0.30% carbon (ex. 4140) prior to subsequent hardening; any and all welded fabrications; heavily machined parts or intricate designs prior to subsequent hardening; or any parts that may be moving during finish grinding.



ANNEALING

Affects mechanical properties. Generally done at 1300F to 1500 F

Heating and holding at a relatively moderate temperature, then cooling at a very slow rate (usually in the furnace). This process is used primarily for softening of material, as well as to improve machinability and alter the material's mechanical properties.

Applications: Done prior to machining a previously hardened part or to soften material that is to be formed without cracking (stamping or bending). Also, all "pre-hardened" material (i.e. 4140 PHT) must be annealed before it is heat treated (hardened) again.



NORMALIZING

Affects mechanical properties. Generally done at 1600F to 1650 F

Heating and holding at a relatively high temperature, then cooling at a moderate rate (usually in still or circulated air). This process is used to refine the material's grain structure, homogenize (create uniform grain structure throughout) the material, and improve machinability. Process will result in a very tough, moderate-hardness material. Process may be followed by further thermal processing (such as tempering) to achieve a desired hardness.

Applications: May be used on weldments to achieve uniform grain structure between the parent material and the weld material, as well as on castings to improve grain structure. Also used to toughen parts used in relatively moderate to low hardness applications.



HARDENING

Affects mechanical properties. Generally done at 1400F to 1850 F

Heating and holding at a relatively high temperature, then cooling (quenching) at a relatively quick rate (usually in water, oil or circulated air). This process results in the maximum attainable hardness for a particular steel, as well as maximum brittleness and maximum residual stress in the part. At this point the material is generally useless and must be followed by one or more tempering processes.

Applications: First operation of Harden & Temper heat treat process.



TEMPERING (or DRAWING)

Affects mechanical properties. Generally done at 1150F & below.

Re-heating a hardened piece of material and holding it at a relatively low temperature, then cooling in still air. This process always immediately follows the hardening process. It is used to decrease the material's hardness to customer specifications, while at the same time greatly increasing its toughness (reducing brittleness) and reducing the stresses. The higher the tempering temperature, the lower the resulting hardness. Some materials require a minimum of two or three tempering processes after hardening.

Applications: The second operation of the Harden & Temper heat treat process. Also used to decrease the hardness of a previously hardened & tempered part. (Note: Tempering cannot be used to increase hardness, only decrease.)


CARBURIZING (or CASE HARDENING)

Affects mechanical properties. Generally done at 1550F to 1700 F.

Heating and holding at a relatively high temperature for an extended period of time in a carbonaceous (high carbon) atmosphere. This can be followed either by a rapid cooling (quench) rate for Carburizing & Hardening, or a slow cooling rate for Carburizing Only. This process is reserved for low carbon (less than .30%) steels only. The process allows carbon to absorb into the surface of the steel; thus allowing the surface area (called the Case) to harden upon quenching, and the center of the part (called the Core) to remain soft and tough. Case depths commonly range from .020"-.125". For case depths in the .003"-.020" range, nitrogen is often added to the atmosphere, resulting in a very similar process called CARBONITRIDING.

Applications:
Used to achieve a hard, wear resistant surface while maintaining a tough core on otherwise non-hardening steels.


NITRIDING

Affects mechanical properties. Generally done at 700F to 1000 F.

Heating and holding at a relatively low temperature for an extended period of time in a nitrogen-rich atmosphere. This process allows nitrogen to absorb into the surface of the steel, resulting in a very hard, very thin case (usually in the .002"-.010" range). The process is used strictly to provide excellent wear resistance; however, the thin case will not withstand any type of impacting load. This process is normally applied to previously hardened parts. It is important that the material to be nitrided receive no less than a 1000oF temper prior to the nitriding process. Thus, it is important that you notify your heat treater when hardening a part that is to be nitrided later on.

Applications: Used strictly to provide a wear resistant surface on parts with little distortion due to the relatively low processing temperature.


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