Heat Treating Background:

A Powerpoint Presentation Outlining Heat Treating Basics

The following is background information about the steels we heat treat and the

processes we use to change their physical properties.

Steel grades are defined by a four-digit designation.  The last two digits 

represent the weight percent carbon in the steel.  For example:

        10      30             means there is .30% carbon in the steel.

        10      80             means there is .80% carbon in the steel.

The first two numbers represent an alloy grade or family.  For example:

        41      40             means there is chromium and molybdenum in the steel. 

                               The last two digits mean there is .40% carbon.

        86      20             means there is chromium, nickel and molybdenum in the steel.

                               The last two digits mean there is .20% carbon. 

(There are many different alloy grades--too numerous to list here.  If you would like a

listing of the various grades of alloy, please call Rich Polin at 517.787.6312 x 120.)

The definitions, as described below, apply only to steel.  They may vary

somewhat for other metals.

ANNEALING     

        Annealing consists of heating to, and holding at, an elevated temperature

        (1300-1550F)followed by slow cooling to approximately 1100F.  Cooling rate below

        1100F is not important.

        Why anneal?

               -       To improve machinability

               -       To improve formability for cold working, such as cold heading

                       and stamping

               -       To improve electrical properties

               -       To improve dimensional stability

               -       To homogenize the microstructure 

               -       To reduce hardness

CARBURIZING

        Carburizing is a process whereby carbon atoms are diffused into the surface of parts

        to create an enriched case.  The depth of the case is dependent upon temperature, time

        and carbon potential.

        Why carburize?

               -To increase wear resistance    

               -To increase strength

               -To increase hardness

               -To impart favorable residual stress patterns

CARBONITRIDING

        Carbonitriding is a process whereby carbon and nitrogen atoms are diffused into

        the surface of parts to create an enriched case.  The depth of the case is dependent

        upon temperature, time and carbon potential.  The nitrogen from an ammonia source is

        added to increase the hardenability of the case.  This is especially helpful for

        low hardenability grades like 10__, 11__, and 12__ grade steels.

        Why carbonitride?         

               -To increase wear resistance

               -To increase strength

               -To increase hardness

               -To impart favorable residual stress patterns

CARBON RESTORATION   

        For our applications, Carbon Restoration consists of processing parts in a neutral

        to slightly carbon enriching atmosphere to compensate for carbon lost from the     

        surface during air atmosphere annealing operations at the steel mill.   

DIFFUSION 

        Diffusion is the movement of atoms within a material from high potential to low

        potential.  For our processes, we deal with the diffusion of carbon atoms (carburizing)

        and carbon and nitrogen atoms (carbonitriding).

HARDENABILITY

        Hardenability is a relative description of the ability of a steel grade to harden

        based on its alloy content and grain size.  As an example, we compare 1040 steel to

        4140 steel.

        Given the same heat treat process variables, 4140 steel can be hardened in a 

        much larger section size than 1040 steel because it contains alloying content   

        which promotes ease of hardening.  In other words, a 3.5" diameter piece of 

        4140 steel can be hardened to the same hardness as a 0.5" diameter piece of

        1040 steel.  Therefore, 4140 has higher hardenability than 1040.

MICROSTRUCTURE 

        The structure of polished and chemically etched steel samples as seen 

        through a microscope.

NEUTRAL HARDENING

        Neutral Hardening consists of processing parts in an atmosphere which will neither

        enrich nor deplete the surface of carbon.  For our furnaces, this means maintaining a  

        carbon potential in the furnace that is the same as the % carbon in the parts. 

        For example:  

                               4140    steel   Furnace atmosphere should be 0.40%C   

                               52100   steel   Furnace atmosphere should be 1.00%C   

        Note:          Heat treating 52100 steel is a neutral hardening process. 

NORMALIZING 

        Normalizing consists of heating to, and holding at, elevated temperatures (usually 

        1600-1700F) followed by still air or forced air cooling.

        Why normalize? 

               -       To improve machinability

               -       To refine the grain structure

               -       To increase (or decrease) the strength of the part depending

                       on steel grade and prior thermal and mechanical history

               -       To homogenize the microstructure

STRESS RELIEVING   

        Stress relieving ia a process whereby parts are heated between 300-1300F,

        followed by air cooling. 

        Why stress relieve? 

               -To reduce stress due to welding or cold working 

               -To develop dimensional stability 

TEMPERING    

        Tempering is a process whereby parts are heated between 300-1300F following 

        neutral hardening or case hardening.

        Why temper?  

               -To relieve stress from the hardening operation

               -To soften the parts to a desired hardness

               -To develop dimensional stability