Heat Treatment

• Quench and Temper– Heating steel to a high enough temperature to dissolve carbon, cooling rapidly enough to make the steel very hard and strong and then to temper the steel to give it toughness as well as substantial strength.
• Softening called annealing which involves heating the steel so the carbon dissolves, soaking it at temperature, turning off the furnace off and letting the steel cool in the furnace.
• Normalizing as with annealing, the load is taken above the critical temperatures and homogenised and then removed from the furnace to naturally cool;
• Sub-critical annealing is when components are held below the critical temperature and the cementite forms as spheres which makes the steel very formable;
• Case hardening which produces a hard surface and a softer core.
• Short hardened is where part of the component is heated to the desired temperature, held to soak and quenched. This process is performed in a molten pot furnace or a radio frequency device;
• Solution Heat Treatment– holding the alloy at an elevated temperature to essentially produce a homogenous alloy.
• Protective atmosphere heat treatment which may be in a salt bath furnace, fluidized bed furnace or vacuum furnace. When heat treatment conditions are controlled so that the surface is not oxidized and carbon is not lost to the atmosphere, components have higher fatigue strength.
• Stress Relieving – Heating metals below the critical temperature which reduces what metallurgists call the dislocation density without substantial changes to the mechanical properties of the metal. Some welds require this treatment.  Some of the Stainless steels and Copper based alloys are susceptible to Stress Corrosion Cracking and by performing a stress relieving treatment the failure mode of stress corrosion cracking is eliminated.
• Removing Hydrogen –Some processes, such as electroplating can result in hydrogen damage known as hydrogen embrittlement for steels.  The heat treater can remove the hydrogen from alloys that are susceptible to hydrogen damage.
• Cryogenic – Quenched and tempered tool steel may have a significant amount of retained austenite which is softer than martensite.  The cryogenic treatment transforms retained austenite to martensite which increases the strength of the tool steel.  Because the transformed martensite is untempered the component should have a tempering operation to temper the martensite generated by the cryogenic treatment.

• Heat Treatment is used to unlock the potentials of alloys required for various engineering alloy applications. By this we mean we can produce metallurgical structures from selected alloys to achieve superior strength, wear resistance, toughness, machinability, weldability, fatigue strength, impact resistance, formability, homogeneity, and corrosion resistance.  We cannot achieve all these properties at the same time in the one alloy, but in consultation with our customers we achieve the desired outcome.
• To make the properties of materials comply with the design requirements principally this is strength which we measure using hardness testers. Customers set an acceptable hardness range say in Rockwell C.  The customer may want components in the range 40 – 45 Rockwell C;
• To remove undesirable constituents. Some castings may have undesirable constituents, such as massive carbides,  that can be removed by homogenization; and
• To take advantage of the carbon solubility difference between the low temperature form of steel [Ferrite] and the high temperature form of steel [Austenite.] Remember that carbon is the element that can give the most potent changes to the mechanical properties of steel.

Generally, case hardening of steel is low carbon steel.  When the steel is placed in a furnace the temperatures are in the critical range that is the austenitic range.  The phase that the steel comes in contact with needs to have a higher carbon potential than the steel.  If the steel has 0.2 wt. % carbon and the carbon potential of the phase is 1.0% then the carbon will produce a surface hardness of 1.0 %.  Holding the load at this elevated temperature for a long period will enable carbon to diffuse into the steel.

We can case harden using a process that is either solid, liquid or gaseous.  The components to be heat treated must be heated into the austenite region.

Quenching involves transforming the high temperature phase of steel, austenite, into martensite.  The component is placed in the quenchant and allowed to cool to room temperature.  The transformation of austenite only occurs through sufficient cooling.

From the most rapid to the slowest quenching medium we have:

Brine (salt water) > water >polymer > oil > forced gaseous >still air

If you like the rules of quenching are:

1. We have to quench fast enough to make the steel fully hard;
2. Quench slow enough to avoid quench cracks; and

Select the quenchant that achieves both rule 1 and rule 2.