Heat Treatment
Content
Heat Treatment of Steel Alloys
YELLOW ALERT
Heat Treatment of Steel Alloys
Diffusive vs. Displacive Transformations of Pure Iron (Fe) Role of Dissolved Carbon in Fe Transformations Heat Treatment of Steel Alloys
Diffusion Process
Other diffusion mechanisms
Grain boundary Interstitial diffusion
Surface
Diffusive Transformation of FCC to BCC in Pure Fe
Above 914° C pure Fe is face centered cubic (FCC). Below 914° C the thermodynamically stable phase of pure Fe is body centered cubic (BCC). Note that the speed of the “interface” in this transformation is zero at 914° C.
Less thermal energy Increasing driving force
Speed of the interface
Why this shape?
Temperature
Nucleation in the Diffusive Transformation of f.c.c.-> b.c.c. in Pure Fe Nucleation is very important The more nuclei : The more Volume Transformed In a diffusive transformation: – Volume transforming per second increases linearly with the number of nuclei.
Grain Boundary Nucleation
The grain boundaries in the f.c.c. pure Fe are the most common site for nucleation of the b.c.c. phase.
Homogeneous vs. Heterogeneous Nucleation
The critical radius, r*het, of a heterogeneous nucleus is much larger than the critical radius, r*hom, of a homogeneous Crystal radius nucleus of the same phase. For the same critical radius the heterogeneous nucleus contains far fewer atoms.
heterogeneous
homogeneous
Absolute temperature
Diffusive Transformation of f.c.c b.c.c. in Pure Fe
The overall rate of transformation depends both on nucleation and growth The semi-schematic diagram below shows that the rate of transformation starts below the equilibrium temperature, 914°C, and increases until approximately 700°C. The slowing rate of diffusion dominates below 700°C.
Time-Temperature-Transformation (TTT) Diagram
The standard practice to display diffusive transformations is with the “Time-TemperatureTransformation” (TTT) diagram. It is also known as the “IsothermalTransformation” diagram or “C-curve”. The TTT diagram for the diffusive f.c.c.->b.c.c. transformation of pure Fe is shown at the right.
The two curves are related
Consider the 1% transformation line (1% of the fcc to transform to bcc)
1)
The transformation rate is zero both at 914 and –273 C so the time required for the transformation is infinite at these temperatures
2) The transformation rate is a maximum at 700 C so the time for the 1% transformation must be a minimum at 700 C
Displacive Transformation of f.c.c. -> b.c.c. in Pure Fe
If we quench f.c.c. Fe from 914°C at a rate of about 105°Cs-1, we expect to prevent the diffusive The TTT diagram for the diffusive f.c.c.->b.c.c. transformation from taking place. In reality, below 550°C the Fe will transform to b.c.c. by a displacive transformation.
Martensite Plates form in f.c.c. Lattice
The displacive transformation of f.c.c. -> b.c.c. in pure Fe is shown schematically. Lens shaped crystals of b.c.c. Fe nucleate at the grain boundaries of the f.c.c. Fe and grow out into the f.c.c. crystal. The lens shaped crystals stop when they hit the next grain boundary. This kind of transformation is called a Martensitic Transformation.
Martensite transformation
Complete TTT Diagram for Pure Fe
The is shown below. The “Ms” stands for “Martensite Start Temperature” and the “Mf” stands for “Martensite Finished Temperature”. If a sample is cooled fast enough to prevent the diffusive transformation from taking place, then martensite will be formed as schematically shown at the left.
Martensite Transformation in Steels
The Martensite in Steel is Not Cubic
The crystal structure of 0.8% Carbon martensite is shown below. To make room for the carbon atoms the lattice stretches along on crystal direction. This produces a face centered tetragonal unit cell. Note that only a small proportion of the labelled sites actually contain a carbon atom.
BCT formation
Fe-C Interstitial Solid Solution in Austinite
The Carbon atoms fit into interstitial spaces in the FCC Austinite structure schematically shown below. Note the distortion of the Fe atoms [0.258-nm diameter] around the Carbon atoms [0.154-nm diameter] since the voids are 0.104-nm diameter.
Fe-C Interstitial Solid Solution in Ferrite & Martensite The Carbon atoms cannot fit into interstitial spaces in the BCC ferrite structure like they can in the FCC Austinite and produce a BCT ( schematically shown below). Note in the BCT the Carbon atoms force the unit cell to be alongated in the c-direction. The largest interstitial void in BCC iron has a diameter of 0.072-nm.
Isothermal Transformation Experiments
An Example (Assume a Eutectoid Low Carbon Steel)
(a) Water-quench to room Temperature. (b) Hot-quench at 690°C & hold 2 hr; water-quench (c) Hot-quench at 610°C & hold 3 min; water-quench (d) Hot-quench at 580°C & hold 2 sec; water-quench
Bainite Pearlite Pearlite
(e) Hot-quench at 450°C & hold 1 hr; water-quench
50% pearlite + 50 martensite
All martensite
Another one...
Formation of Bainite
Perlite + Martensite
Bainite + Martensite
Martensite
Hypoeutectoid Phase Diagram
If a steel with a composition x% carbon is cooled from the Austenite region at about 770 °C ferrite begins to form. This is called proeutectoid (or pre-eutectoid) ferrite since it forms before the eutectoid temperature.
Hypoeutectoid Isothermal Transformation Curve
Quenched & Tempered Steel Alloys
Heat Treatment of Steel Alloys (Tempering) Microstructure of Fe-C Martensites Mechanical Properties of Fe-C Martensites Microstructural Changes in Martensite with Tempering
Tempering
Tempering is the process of heating a martensitic steel at a temperature below the eutectoid transformation temperature. This makes it “softer” and more “ductile”.
Microstructure of Fe-C Martensites
Mechanical Properties of Fe-C Martensites
Microstructural Changes in Martensite with Tempering
Martensite is a metastable structure, and it decomposes when reheated. In lath martensites of low-carbon plain-carbon steels there is a high dislocation density, and these dislocations provide lower energy sites for carbon atoms than there regular interstitial positions. This process can take place between 20° and 200°C.
Microstructural Changes in Martensite with Tempering
For martensitic plain-carbon steels with more than 0.2% carbon tempering produces Cementite, Fe3C. The shapes are diffenent at different temperatures. The important point is that the Fe matrix returns to its BCC form found in Ferrite. The electron micrographs below show the microstructure for two treatments.
Variation of Hardness with Tempering Treatment
The curves below show the reduction of hardness for various treatments of a quenched low-carbon plain-carbon steel with 0.35% carbon.
Martempering
Austempering
Typical Mechanical Properties & Applications of Plain-Carbon Steels
YELLOW ALERT
Heat Treatment of Steel Alloys
Diffusive vs. Displacive Transformations of Pure Iron (Fe) Role of Dissolved Carbon in Fe Transformations Heat Treatment of Steel Alloys
Diffusion Process
Other diffusion mechanisms
Grain boundary Interstitial diffusion
Surface
Diffusive Transformation of FCC to BCC in Pure Fe
Above 914° C pure Fe is face centered cubic (FCC). Below 914° C the thermodynamically stable phase of pure Fe is body centered cubic (BCC). Note that the speed of the “interface” in this transformation is zero at 914° C.
Less thermal energy Increasing driving force
Speed of the interface
Why this shape?
Temperature
Nucleation in the Diffusive Transformation of f.c.c.-> b.c.c. in Pure Fe Nucleation is very important The more nuclei : The more Volume Transformed In a diffusive transformation: – Volume transforming per second increases linearly with the number of nuclei.
Grain Boundary Nucleation
The grain boundaries in the f.c.c. pure Fe are the most common site for nucleation of the b.c.c. phase.
Homogeneous vs. Heterogeneous Nucleation
The critical radius, r*het, of a heterogeneous nucleus is much larger than the critical radius, r*hom, of a homogeneous Crystal radius nucleus of the same phase. For the same critical radius the heterogeneous nucleus contains far fewer atoms.
heterogeneous
homogeneous
Absolute temperature
Diffusive Transformation of f.c.c b.c.c. in Pure Fe
The overall rate of transformation depends both on nucleation and growth The semi-schematic diagram below shows that the rate of transformation starts below the equilibrium temperature, 914°C, and increases until approximately 700°C. The slowing rate of diffusion dominates below 700°C.
Time-Temperature-Transformation (TTT) Diagram
The standard practice to display diffusive transformations is with the “Time-TemperatureTransformation” (TTT) diagram. It is also known as the “IsothermalTransformation” diagram or “C-curve”. The TTT diagram for the diffusive f.c.c.->b.c.c. transformation of pure Fe is shown at the right.
The two curves are related
Consider the 1% transformation line (1% of the fcc to transform to bcc)
1)
The transformation rate is zero both at 914 and –273 C so the time required for the transformation is infinite at these temperatures
2) The transformation rate is a maximum at 700 C so the time for the 1% transformation must be a minimum at 700 C
Displacive Transformation of f.c.c. -> b.c.c. in Pure Fe
If we quench f.c.c. Fe from 914°C at a rate of about 105°Cs-1, we expect to prevent the diffusive The TTT diagram for the diffusive f.c.c.->b.c.c. transformation from taking place. In reality, below 550°C the Fe will transform to b.c.c. by a displacive transformation.
Martensite Plates form in f.c.c. Lattice
The displacive transformation of f.c.c. -> b.c.c. in pure Fe is shown schematically. Lens shaped crystals of b.c.c. Fe nucleate at the grain boundaries of the f.c.c. Fe and grow out into the f.c.c. crystal. The lens shaped crystals stop when they hit the next grain boundary. This kind of transformation is called a Martensitic Transformation.
Martensite transformation
Complete TTT Diagram for Pure Fe
The is shown below. The “Ms” stands for “Martensite Start Temperature” and the “Mf” stands for “Martensite Finished Temperature”. If a sample is cooled fast enough to prevent the diffusive transformation from taking place, then martensite will be formed as schematically shown at the left.
Martensite Transformation in Steels
The Martensite in Steel is Not Cubic
The crystal structure of 0.8% Carbon martensite is shown below. To make room for the carbon atoms the lattice stretches along on crystal direction. This produces a face centered tetragonal unit cell. Note that only a small proportion of the labelled sites actually contain a carbon atom.
BCT formation
Fe-C Interstitial Solid Solution in Austinite
The Carbon atoms fit into interstitial spaces in the FCC Austinite structure schematically shown below. Note the distortion of the Fe atoms [0.258-nm diameter] around the Carbon atoms [0.154-nm diameter] since the voids are 0.104-nm diameter.
Fe-C Interstitial Solid Solution in Ferrite & Martensite The Carbon atoms cannot fit into interstitial spaces in the BCC ferrite structure like they can in the FCC Austinite and produce a BCT ( schematically shown below). Note in the BCT the Carbon atoms force the unit cell to be alongated in the c-direction. The largest interstitial void in BCC iron has a diameter of 0.072-nm.
Isothermal Transformation Experiments
An Example (Assume a Eutectoid Low Carbon Steel)
(a) Water-quench to room Temperature. (b) Hot-quench at 690°C & hold 2 hr; water-quench (c) Hot-quench at 610°C & hold 3 min; water-quench (d) Hot-quench at 580°C & hold 2 sec; water-quench
Bainite Pearlite Pearlite
(e) Hot-quench at 450°C & hold 1 hr; water-quench
50% pearlite + 50 martensite
All martensite
Another one...
Formation of Bainite
Perlite + Martensite
Bainite + Martensite
Martensite
Hypoeutectoid Phase Diagram
If a steel with a composition x% carbon is cooled from the Austenite region at about 770 °C ferrite begins to form. This is called proeutectoid (or pre-eutectoid) ferrite since it forms before the eutectoid temperature.
Hypoeutectoid Isothermal Transformation Curve
Quenched & Tempered Steel Alloys
Heat Treatment of Steel Alloys (Tempering) Microstructure of Fe-C Martensites Mechanical Properties of Fe-C Martensites Microstructural Changes in Martensite with Tempering
Tempering
Tempering is the process of heating a martensitic steel at a temperature below the eutectoid transformation temperature. This makes it “softer” and more “ductile”.
Microstructure of Fe-C Martensites
Mechanical Properties of Fe-C Martensites
Microstructural Changes in Martensite with Tempering
Martensite is a metastable structure, and it decomposes when reheated. In lath martensites of low-carbon plain-carbon steels there is a high dislocation density, and these dislocations provide lower energy sites for carbon atoms than there regular interstitial positions. This process can take place between 20° and 200°C.
Microstructural Changes in Martensite with Tempering
For martensitic plain-carbon steels with more than 0.2% carbon tempering produces Cementite, Fe3C. The shapes are diffenent at different temperatures. The important point is that the Fe matrix returns to its BCC form found in Ferrite. The electron micrographs below show the microstructure for two treatments.
Variation of Hardness with Tempering Treatment
The curves below show the reduction of hardness for various treatments of a quenched low-carbon plain-carbon steel with 0.35% carbon.
Martempering
Austempering
Typical Mechanical Properties & Applications of Plain-Carbon Steels
