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Steel Material Properties and Heat treatment Overview

One of the main factors contributing to the utility of steels is the broad range of mechanical properties which can be obtained by heat treatment.  For example, easy formability and good ductility may be necessary during fabrication of a part.  Once formed very high strength part may be needed in service.  Both of these material properties are achievable from the same material.

All steels can be softened to some degree by annealing.  The degree of softening depends on the chemical composition of the particular steel.  Annealing is achieved by heating to and holding at a suitable temperature followed by cooling at a suitable rate.

Similarly, steels can be hardened or strengthened.  This can be accomplished by cold working, heat treating, or an appropriate combination of these.

Cold working is the technique used to strengthen both low carbon low alloyed steels and highly alloyed austenitic stainless steels.  Only reasonably high strength levels can be attained in the carbon low alloyed steels, but the highly alloyed austenitic stainless steels can be cold worked to rather high strength levels.  Most steels are commonly supplied to specified minimum strength levels.

Heat treating is the primary technique for strengthening the remainder of the steels.  Some common heat treatment of steels are listed below:

  •   Martensitic hardening
  •   Pearlitic transformation
  •   Austempering 
  •   Age hardening

TTT Diagram

Figure 1 Schematic of Time Temperature Transformation Diagram

Carbon and alloy steels are Martensitic hardened by heating to the Austenitizing temperature followed by cooling at the appropriate rate.  One requirement for full transformation to Martensite is that cooling must occur prior to the nose of the transformation start curve in Figure 1.  Cooling frequently occurs by quenching in oil or water.  Some steels are capable of Martensitic transformation when air cooled.   

Ms is when the Martensite transformation starts.  Mf is when the Martensite transformation finishes.  Martempering is a martensitic transformation where the part is cooled rapidly to above the Ms and held until the temperature becomes uniform across the cross section.

After Martensitic transformation the steel is then tempered.  Tempering consists of reheating the steel to an intermediate temperature.  Tempering causes microstructural changes in the steel in addition to relieving internal stresses and improving toughness.

The maximum hardness of carbon and alloy steels, after rapid quenching to avoid the nose of the isothermal transformation curve, is a dependent on the alloy content, predominantly the carbon content.  The maximum thickness for complete hardening or the depth to which an alloy will harden is measure of a steels hardenability.

Pearlitic transformation is another transformation for austenite during cooling.  If cooling of austenite is not quick enough some or all of the steel may transform to Pearlite instead of Martensite.  While Pearlite is not as hard as Martensite, the steels properties are still quite good and Pearlitic structures are used in many applications.

Austempering is another heat treatment for steels.  In this heat treatment steels are Austenitized followed by rapidly quenching avoid transformation of the austenite to Pearlite.  The steel is held at a temperature below temperatures that promote Pearlite formation and above the Martensite start transformation range.  While held at this temperature range the austenite transforms isothermally to a completely Bainitic microstructure.  Finally the steel is cooled to room temperature.  The intention of Austempering is to acquire increased ductility or notch toughness at high hardness levels, or to decrease the possibility of cracking and distortion that might occur by traditional quenching and tempering.

Some steels have been developed that are strengthened by age hardening.  These steels are heat treated to dissolve certain constituents in the steel into solution followed by cooling.  Subsequently these steels are age hardened to precipitate the constituents in some favored particle size and distribution.  

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Last modified: January 25, 2012