Heat Treatment

Heat Treatment of Steel Alloys

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Heat Treatment of Steel Alloys

Diffusive vs. Displacive Transformations of P Pure Iron I (Fe) (F ) Role of Dissolved Carbon in Fe Transformations Definition of “Steel” Steel Heat Treatment of Steel Alloys

Diffusion Process

Other diffusion mechanisms h i Interstitial diffusion Grain boundary 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 F is Fe i body b d centered t d cubic bi (BCC). (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, off 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

Nucleation Rate The rate of nucleation is critically dependent on temperature. rr*= 2 γ/|ΔG| and ΔG = ΔH (Te - T)/Te Grain boundary nucleation will not occur in pure Fe unless it is cooled below perhaps 910°C. If the temperature is further cooled to 900°C the rate of nucleation is further increased as shown semischematically at the right.

Diffusive Transformation of f.c.cÆ 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”. Th TTT diagram The di for f the h diffusive f.c.c.->b.c.c. transformation of pure Fe is shown at the right. 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 h f.c.c. f F from Fe f 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. bcc 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. t l The lens shaped crystals stop when they hit the next grain b boundary. d This kind of transformation is called a Martensitic T Transformation. f ti

Martensite transformation

Crystal Structures of f.c.c. Fe and b.c.c. Martensite The details of how pure f.c.c. iron transforms by translation is shown below.

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 h below. b l 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 Th C b atoms t cannott fit into i t interstitial i t titi l spaces in i the th BCC ferrite structure like they can in the FCC Austinite and produce a BCT ( schematicallyy shown below). p ) 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. 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 water quench

Pearlite Pearlite

(d) Hot-quench at 580°C & hold 2 sec; water-quench Bainite

(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 f begins to form. f This is called pro proeutectoid (or pre pre--eutectoid) ferrite since it forms before the eutectoid temperature. p

Hypoeutectoid Isothermal Transformation Curve

Quenched & Tempered Steel Alloys Heatt Treatment H T t t off Steel St l All Alloys (Tempering) (T i ) Microstructure of Fe-C Martensites Mechanical Properties of Fe-C Martensites Mi Microstructural t t l Changes Ch in i Martensite M t it with ith Tempering

Tempering p g Tempering is the process of heating a martensitic steel at a temperature below the eutectoid transformation temperature. This makes it “softer” and more “ductile”. ductile .

Microstructure of Fe-C Martensites

Mechanical Properties of Fe-C M t Martensites it

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 th is there i a high hi h di dislocation l ti ddensity, it and d th these dislocations provide lower energy sites for carbon atoms than there regular interstitial positions positions. This process can take place between 20° and 200°C.

Microstructural Changes in Martensite with Tempering p g For martensitic plain-carbon steels with more than 0.2% carbon tempering produces Cementite, Cementite Fe3C. C The shapes are diffenent at different temperatures. The important point is that the Fe matrix returns to its BCC form found in Ferrite. p 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 p g

Austempering p g

Typical Mechanical Properties & Applications pp of Plain-Carbon Steels