Hardening and Tempering

Hardening And Tempering

The use of this treatment will result in an improvement of the mechanical properties, as well as an increase in the level of hardness, producing a tougher and stronger component. Alloys are heated above the critical transformation temperature for the material, then cooled rapidly to form Martensite. Alloys may be air-cooled, oil-quenched, water-quenched, or marquenched, depending upon the carbon content and the amount of alloying elements in the material. Hardened materials are usually tempered or stress-relieved to improve their dimensional stability and toughness.

Steel parts often require a heat treatment to obtain improved mechanical properties, such as increasing hardness, strength, and toughness. The hardening process consists of heating the components above the critical (normalising) temperature, holding them at this temperature until fully equalised, and then cooling at a rate fast enough to allow the material to transform to Martensite and then tempered to achieve the required properties.

process

Tempering is done to develop the required combination of hardness, strength, and toughness or to relieve the brittleness of fully hardened steels. Steels are never used in the quenched condition. The combination of quenching and tempering is important to improve toughness.

Tempering is generally considered effective in relieving stresses induced by quenching in addition to lowering hardness to within a specified range or meeting the customer’s specification.

Tempering is the process of reheating the steel at a relatively low temperature leading to precipitation and spheroidization of the carbides present in the microstructure. The tempering temperature and times are generally controlled to produce the final properties required of the steel. The result is a component with the appropriate combination of hardness, strength, and toughness for the intended application.

MTE Carries out hardening in molten salt at temperatures ranging from 760 – 900oC.  We also utilize various quenching media such as water, oil, and marquench facilities which ensures that we can process a wide range of engineering materials.

All our furnaces are also used for carburizing and therefore operate at carbon potentials of 0.6 – 0.8% C, but due to the rapid rate of heating in salt, most components for hardening do not spend a significant time in the furnace and the resultant carburization of the surface layers is usually negligible.

MTE has 5 Hardening furnaces capable of a throughput of +/- 200kg/hour.  However, we usually only run 4 furnaces at any one time while the 5th is kept as a backup in the event of a breakdown.   

Benefits

There are several benefits of hardening, depending on the steel type:

  • Heavy loaded parts can be given an optimal combination of high strength, toughness and, if applicable, temperature resistance
  • Such parts can be made lighter and more stiff, due to higher strength.
  • Tools and dies get the required high wear and/or heat resistance while maintaining toughness.
  • Parts that need grinding to low roughness, acquire the required machinability.

Tool steels: the desired properties of high hardness, wear resistance, heat resistance, and machinability can only be achieved by hardening.

Martensitic stainless steels: these steels only get their maximum corrosion resistance by hardening.

For all steel types, most parts are machined prior to hardening when the material is relatively soft and therefore easy to machine. A grinding operation may follow after heat treatment if tight tolerances are required.

Application

Almost all steels that require superior strength, hardness and toughness are hardened.

Examples of engineering steels and components are as follows:

  • Highly loaded parts, such as drive shafts, gears, pistons, carrier bars, frames, forklift forks, nuts and bolts, lifting eyes etc.
  • Similar parts, intended for elevated temperatures
  • Springs of any kind, and any dimension
  • Tools: cutting, hammering, rolling i.e. any kind of tooling for cold, as well as for hot working
  • Dies: cutting, rolling, stamping, hammering, but also plastic and aluminium casting, and extrusion dies
  • Stainless steel parts needing high corrosion resistance (food and medical industry)