HEAT TREATING PROCESSES


 

AGE HARDENING

A process of aging/hardening that changes the properties of certain steels, and structural alloys. This process increases the hardness and strength, and typically decreases ductility. Age hardening usually follows rapid cooling or cold working.

 

ANNEALING

Annealing is used to induce ductility, soften material, and relieve internal stresses of the metal. Annealing reduces the hardness of the metal, improves machinability, and facilitates cold working by refining the chemical structure, making it homogeneous.

 

AUSTENITIZING

Austenitizing is a heat-treating process by which a ferrous alloy is heated to a temperature at or above the transformation range of the material. At this temperature the material forms austenite.

 

BRIGHT HARDENING

Bright Hardening is a process of hardening exotic alloys, stainless steel or steel alloys in a vacuum furnace. This process is performed on parts capable of hardening without water, or oil quenching. Tool steels made of air hardening alloys are typically bright hardened in batch vacuum furnaces.

 

CARBONITRIDING

Carbonitriding is a process that introduces carbon and nitrogen into a ferrous alloy at the Austenitizing temperature. This case hardening technique is performed in a gaseous atmosphere of such composition as to cause absorption of carbon and nitrogen into the surface of the steel.

 

CARBURIZING

Carburizing is a process in which an austenitizable steel is brought to the desired temperature in a carbon rich atmosphere causing absorption of carbon into the surface of the steel. Carburizing can be performed to various depths of the steel’s surface.

 

INDUCTION HARDENING

Induction Hardening is a process in which the specified material is heated to the critical temperature range by electromagnetic induction. The material is then immediately quenched to produce the required hardness.

 

NORMALIZING

Normalizing is a process of heating steel to a temperature 1000oF above the critical temperature and cooling in air. Normalizing steel eliminates the effects of prior process, rendering a more homogenous chemical composition of the steel.

 

STRESS RELIEVING

Stress Relieving is a process of heating the material to a suitable temperature and holding that temperature to reduce residual stresses in the steel caused by casting, quenching, welding, cold work, or machining.

 

SUB-CRITICAL ANNEALING

Sub-Critical Annealing is a process of heating the material to a temperature below that which austenite is formed, but above the temperature used for stress relieving.

 

TEMPERING

Tempering is a process of reheating material, that has been quench hardened or normalized, to a temperature below the Austenitizing range to achieve the specific mechanical properties and hardness. 

 

VACUUM HEAT TREATING

Vacuum Heat Treating is a process of heating steel in a vacuum atmosphere to avoid oxidation. A vacuum furnace removes the oxygen and prevents oxidation and scaling of the steel.

 

 

HEAT TREATING TERMS


ALLOY 

A substance having metallic properties and composed of two or more chemical elements of which at least one is metal. 

 

ALLOY STEEL

Steel that has had alloying elements added to achieve specific properties. 

 

ATMOSPHERE

Atmosphere refers to the gases surrounding the work in an Endothermic or Vacuum furnace. The furnace atmosphere varies by type of material and the work being carried out.

 

AUSTENITE

A solid solution of iron and carbon attained by heating the metal above the critical temperature. This high temperature must be attained to allow the proper microstructure to form and achieve the full hardness. 

 

BRIGHT ANNEALING 

A process of annealing carried out in a controlled vacuum atmosphere so that surface oxidation is reduced to a minimum and the surface remains relatively bright. 

 

CARBIDE 

A compound composed of carbon and one or more less electronegative metallic elements. Carbides are generally classified by their chemical bonding type.

 

CARBON STEEL

Steel whose properties are attributed mainly to the presence of carbon, without substantial amounts of other alloying elements. 

 

CASE HARDENING

A heat treatment in which the surface layer of a steel is made substantially harder than the interior by altering its composition and properties. 

 

CAST IRON

Cast Iron is an alloy of iron and carbon. Cast Iron typically has a carbon content that is above 1.7%.

 

COLD ROLLED

Cold Rolled refers to material that has been finished by rolling at or near room temperature. 

 

DECARBURIZATION

Loss of carbon from the surface of a ferrous alloy due to heating. The resulting loss of carbon reduces the strength of the metal and reduces the surface hardness.

 

DEFORMATION

A change in dimensions of a component as the result of an applied stress. 

 

DIFFUSION

The action of atoms migrating due to their random thermal motion. Diffusion typically happens from regions of high concentration towards regions of low concentration. 

 

DUCTILITY

The ability of a material to deform without fracturing or breaking. Ductility can be determined by the elongation or reduction of an area in by a tensile test.

 

ELASTIC DEFORMATION

Temporary changes in dimensions caused by stress. When the stress is removed, the material returns to the original dimensions.

 

ELONGATION

Measured during tensile testing. Measured after fracture of the component. Usually measured as a percentage of the original component’s length. 

 

FATIGUE

The maximum stress that can be sustained for a specified cycle without failure. The stress being completely reversed within each cycle unless otherwise stated. 

 

FERRITE

A solid solution in which alpha iron is the solvent, and which is characterized by a body-centered cubic crystal structure. 

 

GRAIN

A grain is a particle of metal or of a metal alloy in which the space lattice pattern is continuous except for small irregularities. 

 

HARDENABILITY

Hardenability refers to the ability of steel to harden deeply upon quenching. Hardenability takes into consideration the overall size of the part and the method of quenching.

 

HARDNESS

The ability of metal to resist abrasion or indentation. For heat treating - Hardness is usually defined in terms Rockwell, Brinnell, Knoop, or Vickers. 

 

MARTENSITE

The body centered tetragonal structure of steel attained upon quenching the steel from the Austenitic phase of heat treating. This transformation phase occurs only upon cooling. 

 

MECHANICAL PROPERTIES

Mechanical Properties refers to the properties of a material that reveal the elastic and inelastic reaction when force is applied. 

 

MICROHARDNESS TEST

A hardness test used to test small parts using a microscope. Usually measured in Knoop or Vickers.

 

MICROSTRUCTURE 

The structure of polished and etched metal or alloy specimens as revealed by the microscope at a magnification of over ten diameters. 

 

PRECIPITATION HARDENING

The hardening of a metal caused by precipitation of a constituent from a supersaturated solid solution. 

 

PROCESS ANNEALING

The heating of a ferrous alloy to a temperature close to, but below, the lower limit of the transformation range. Followed by cooling to soften the alloy. 

 

QUENCHING

The process of rapidly cooling the metal from the Austenitic temperature. Typically performed in Oil or Water. 

 

SCALE

The oxide formed on a metal by heat treating meta in and atmosphere of air or other oxidizing atmospheres. 

 

SOAK

The holding time required, at temperature, to assure that all parts are at the required temperature for sufficient time. Ensuring the correct metallurgical reaction will occur. 

 

STEEL

An alloy of iron and carbon. Steel may contain other elements in which the carbon content does not exceed about 2.0%. 

 

STRESS RISERS

Factors such as sharp edges, exaggerated changes in contour, or surface defects which can concentrate stresses locally. 

 

RESIDUAL STRESSES

Caused by stresses that are applied to the component as a result of a non-uniform plastic deformation. 

 

TENSILE STRENGTH 

The value obtained by dividing the maximum load by the original area in tensile testing. 

THERMAL STRESSES

Stresses in metal resulting from non-uniform heating and cooling. 

TOUGHNESS

Measured as the amount of energy absorbed before fracturing.