Heat Treatment of Steels MME 293 Heat Treatment of Steels #2: Hardening and tempering Department of MME BUET, Dhaka

Today’s Topics  Hardening of steels  Hardenability of steels  Tempering of steels Reference: SH Avner. Introduction to Physical Metallurgy, 2nd Ed., Ch. 8.

Hardening of Steels  Hardening is done by heating the steel approximately  50 C above the upper critical temperatures (A3 line) (for hypoutectoid steels).  50 C above the lower critical temperatures (A3,1 line) (for hypereutectoid steels) followed by drastic cooling to room temperature.  Purposes of hardening:  to improve hardness  to improve wear resistance

Hardening of Steels What happens during hardening?  If cooling rate is very fast (as in during water quenching):  austenite cannot transforms into ferrite due to insufficient time is given for C atoms to diffuse out of austenite.  most of the C atoms remain trapped in austenite to distort the structure; the c-axis becomes elongated.  a supersaturated solid solution of C atoms in a body- centred tetragonal (BCT) structure is formed.  this structure is called MARTENSITE.

Hardening of Steels What happens during hardening? Structure of martensite (BCT) where the vertical axis is slightly expanded because of the trapped carbon atoms

Hardening of Steels What happens during hardening?  The highly distorted lattice structure is the prime reason for the high hardness and brittleness of martensite .  The basic aim of most hardening operations is to obtain 100% full martensitic structure. The minimum cooling rate that is required to produce a full martensitic structure is call the critical cooling rate (CCR).  Since atoms of martensite (BCT) are less densely packed than in austenite (FCC), an expansion occurs during this transformation  This causes high localised stresses, which results in plastic deformation (shear) of the matrix.

Martensite needles (B) in retained austenite (W) Hardening of Steels What does martensite look like?  Martensite appears microscopically as white needle or acicular structure, sometimes described as a pile of straw. Pearlite Martensite needles (B) in retained austenite (W)

Hardening of Steels Characteristics of martensitic transformation  Diffusionless (time independent) transformation.  Small volume of austenite suddenly changes its crystal structure into martensite by a combination of two shearing actions; No change in chemical composition.  Transformation proceeds only during cooling  Transformation depends upon the decrease in temperature; start at MS temperature, ends at MF temperature. MS MF Per cent Martensite Temperature  If cooling is interrupted, transformation ceases.

Hardening of Steels Characteristics of martensitic transformation  The start of martensite transformation cannot be suppressed.  MS/MF temperature cannot be changed by changing cooling rate.  MS temperature is a function of composition only.  Martensite transformation never completes; there is always some retained austenite in the structure.

Hardening of Steels Characteristics of martensitic transformation  Martensite is a metastable structure.  It will change into a stable ferrite (BCC), if given the opportunity.  Need sufficient carbon to obtain extreme hardness.  The max. hardness obtainable from a steel depends on the C content only.

Tempering Temperature (C) Tempering of Steels  In the as-quenched condition, the steel is too brittle for most applications. The formation of martensite also leaves high residual stresses in the steel.  Tempering is done almost immediately after hardening to relieve residual stresses and to improve ductility and toughness. The increase in ductility is attained at the sacrifice of some hardness or strength. 100 80 60 40 20 0 200 400 600 800 Tempering Temperature (C) Toughness (ft-lb) Hardness (RC) Toughness Hardness  In tempering, the hardened steel is heated and held to a temperature (which is below the lower critical), and then cooled to room temperature. The selection of heating temperature depends upon desired properties.

Tempering of Steels Effect of tempering temperature on mechanical properties of a 1050 steel

Tempering of Steels What happens during tempering?  During tempering, the excess carbon atoms, trapped in martensite, gradually come out as extremely fine cementite particles and the metastable BCT martensite transforms into stable BCC ferrite.  The resulting microstructure (fine cementite dispersed in ferrite matrix) is called tempered martensite.