Heat Treatment

Dr. Santosh S. Hosmani

First Stage of Tempering:

• In the first stage of tempering (100 to 200 °C), ε-carbide forms frommartensite.

• The composition of this carbide is close to Fe2 4C. In the case of alloyedThe composition of this carbide is close to Fe2.4C. In the case of alloyedsteels, the iron atoms may be replaced by other elements.

• The ε-carbide has a close-packed hexagonal structure and occurs asnarrow laths or rods on cube planes of the martensite with orientationnarrow laths or rods on cube planes of the martensite with orientationrelationship:

• After the precipitation of ε-carbide in stage I, the martensite is stillsupersaturated with carbon to certain extent and would undergo furthersupersaturated with carbon to certain extent and would undergo furtherdecomposition on heating to higher temperatures.

First Stage of Tempering:

• At low temperatures e-carbide precipitates as very fine (10–100 nm)plates or rods. With an increase in tempering temperature or time, ε-carbide particles become coarser.

FIGURE: Electron microscopic image of the ε-carbide, 50,000x

First Stage of Tempering:

Second Stage of Tempering:

• In the temperature range of 200 to 350 °C the retained austenite in the• In the temperature range of 200 to 350 C, the retained austenite in thesteel decomposes into ferrite and cementite.

• This decomposition was detected successfully by x-ray diffraction anddilatometric and specific volume measurementsdilatometric and specific volume measurements.

• The kinetics of this decomposition are related to carbon diffusion inaustenite.

Third Stage of Tempering:

I th t t f 250 t 750 °C tit i it t ithi• In the temperature range of 250 to 750 °C, cementite precipitates withinthe martensite. The composition of the cementite is Fe3C. In alloyedsteels, it is referred as M3C, where M corresponds to substitutionalalloying additions (e g Cr Mn) in addition to Fealloying additions (e.g., Cr, Mn) in addition to Fe.

• Cementite has an orthorhombic crystal structure and usually occurs asWidmansta¨tten plates. An example of tempered martensite in 300-M(medium carbon utra high strength low alloy) steel is shown in Figure(medium carbon utra-high strength low alloy) steel is shown in Figure.

FIGURE: Transmission electron micrograph of quenched and tempered 300-M steel samples after tempering at 300 °C for 2 min, showing cementiteplates in a martensite lath.

• The orientation relationship between ferrite and cementite is of the type:

The habit planes of cementite can be parallel to either {0 1 1}α or {1 1 2}α offerriteferrite.

• The nucleation of cementite may occur at ε-carbide and may grow bydissolution of the ε-carbide. In high-carbon steels, the cementiteprecipitates along the twin boundaries of martensite. Other sites forprecipitates along the twin boundaries of martensite. Other sites fornucleation of cementite are the prior austenite grain boundaries or interlathboundaries.

• With the formation of cementite, most of the carbon in martensite isremoved from solid solution. As a result, the tetragonality of bctstructure is lost. Early stages of cementite growth occur only by carbondiffusion with no significant partitioning of substitutional alloying elements.However, with extended tempering, redistribution of alloying elements alsooccurs between ferrite and cementite. In addition, the plate-like cementiteparticles may coarsen and spheroidize with extended tempering. At thisstage the reco er and recr stalli ation of martensite laths ma also bestage, the recovery and recrystallization of martensite laths may also beinitiated. In high-carbon martensite, higher order carbides such as M5C2 (x-carbide) can also form.

Tempering (contd.)

Ref.: W.D. Callister’s book

Fourth Stage of Tempering:

•Tempering at higher temperatures (>700 °C) leads to the precipitation ofp g g p ( ) p pmore equilibrium alloy carbides such as M7C3 and M23C6. In steelscontaining Cr, Mo, V, and Ti, these carbides are associated with hardeningof the steel that is called secondary hardening. The precipitation of thesecarbides also leads to the dissolution of cementite. An example of alloycarbide formation by tempering at 600°C in 300-M steel is shown in Figure.

•At this stage, the recrystallization of martensite lath is more complete, andthere is a tendency for the formation of equiaxed grains and extensivegrain growth.

FIGURE: Transmission electronmicrographs of quenched andmicrographs of quenched andtempered 300-M steel samples aftertempering at 600 °C for 1 min,showing alloy carbides.

Assignment Topic

“Temper Embrittlement”Temper embrittlement is inherent in many steels and can be characterized byreduced impact toughness. The state of temper embrittlement has practically noeffect on other mechanical properties at room temperature.

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Ref.: http://www.keytometals.com/Articles/Art102.htm

Austempering

B i iBainite

Short needles of Fe3C embedded in plates of ferrite

The austempering process consists ofaustenitization, quenching into a hotb th i t i d b t 260 450 °Cbath maintained between 260–450 °C,holding at this temperature until thetransformation of austenite to bainite iscomplete and cooling to roomcomplete, and cooling to roomtemperature.

Optical micrograph from an Fe–Cr–Csteel showing a upper bainitic sheaf

Transmission electron micrographfrom an Fe–C–Si–Mn steel showingman shea es of pper bainiticstructure. many sheaves of upper bainiticferrite.

The bainite microstructure consists of aggregates of ferrite plates separated by thinfil f t it t it tit Th t f l t ll d

The scale of individual plates of ferrite is too small to be resolved

films of austenite, martensite, or cementite. These aggregates of plates are calledsheaves.

adequately using optical microscopy, which is capable only of revealingclusters of plates.

Ref.: H.K.D.H. Bhadeshia, Bainite in Steels, 2nd ed., The Institute of Materials, London, 2001

Optical micrograph, Fe–0.8C wt%steel transformed at 300 °C,

Thin-foil electron micrographsshowing the carbide precipitation

ithi th b it f lshowing sheaves of lowerbainite.

within the sub-units of lowerbainite.

Ref.: H.K.D.H. Bhadeshia, Bainite in Steels, 2nd ed., The Institute of Materials, London, 2001

TEM micrograph TEM micrograph

Upper bainite in Fe 0 095C 1 63Si Lower bainite Fe 0 4C 2Si 3MnUpper bainite in Fe–0.095C–1.63Si–2Mn–2Cr wt% steel transformedisothermally at 400 °C.

Lower bainite, Fe–0.4C–2Si–3Mnwt%, transformed isothermally at300 °C.

Ref.: H.K.D.H. Bhadeshia, Bainite in Steels, 2nd ed., The Institute of Materials, London, 2001

FIGURE: Schematic illustration of the microstructural features relevant in thekinetic description of a bainitic microstructure.

Ref.: H.K.D.H. Bhadeshia, Bainite in Steels, 2nd ed., The Institute of Materials, London, 2001

The time required for aThe time required for asupersaturated plate of ferrite todecarburize by diffusion intoaustenite is increases with decreasein temperature At elevatedin temperature. At elevatedtemperatures the diffusion is so rapidthat there is no opportunity toprecipitate carbides in the ferrite,giving rise to an upper bainiticgiving rise to an upper bainiticmicrostructure.

FIGURE: Schematic illustration of upper and lower bainite formationmechanism. The dark regions between the plates represent carbides that form inh d l h d k l b d h f h hthe residual austenite. The dark lines represent carbides that form within theferrite plates.

Ref.: H.K.D.H. Bhadeshia, Bainite in Steels, 2nd ed., The Institute of Materials, London, 2001

Nanostructured Bainite

An alloy has been designed in this way with the approximateAn alloy has been designed in this way, with the approximatecomposition Fe–1C–1.5Si–1.9Mn–0.25Mo–1.3Cr–0.1V wt%, which ontransformation at 200◦C, leads to bainite plates which are only 20–40nm thick. The slender plates of bainite are dispersed in stable40nm thick. The slender plates of bainite are dispersed in stablecarbon-enriched austenite which, with its face-centred cubic lattice,buffers the propagation of cracks.

FIGURE: Bainite obtained by transformation at 200 °C. (a) Optical micrograph.(b)Transmission electron micrograph(b)Transmission electron micrograph

Ref.: Caballero, F. G. and Bhadeshia, H. K. D. H., Very strong bainite, Current Opinion in Solid State and Materials Science 8, 251, 2004

Nanostructured Bainite

Th b i it bt i d b t f ti t l t t i thThe bainite obtained by transformation at very low temperatures is thehardest ever (700HV, 2500 MPa),

It has considerable ductility,

It is tough and

It does not require mechanical processing or rapid cooling.

The steel after heat treatment therefore does not have long-rangeresidual stresses,

It is very cheap to produce and has uniform properties in very largeIt is very cheap to produce and has uniform properties in very large

sections.

In effect the hard bainite has achieved all of the essential objectives ofIn effect, the hard bainite has achieved all of the essential objectives ofstructural nanomaterials which are the subject of so much research, but inlarge dimensions.

Ref.: Caballero, F. G. and Bhadeshia, H. K. D. H., Very strong bainite, Current Opinion in Solid State and Materials Science 8, 251, 2004

Compared with the process of hardening and tempering, there areBack to Austempering….

the following substantial differences:

1. At austempering there is no austenite-to-martensite transformation,b t th fi l t t (b i it ) i bt i d d ll d i thbut the final structure (bainite) is obtained gradually during theisothermal transformation of austenite to bainite.

2. After austempering there is no tempering.3 While hardening and tempering is a two operation process3. While hardening and tempering is a two-operation process,

austempering is performed in one cycle only, which is an advantagefor the automation of the process.

Austempering of steel offers two primary potential advantages:

1. Reduced distortion and less possibility of cracking2. Increased ductility and toughness, especially in the range of high

strength (hardness) values between 50 and 55 HRCstrength (hardness) values between 50 and 55 HRC.

Why less distortion in austempering ?Why less distortion in austempering ….?

FIGURE: Temperature differences between surface and core of the workpiecein conventional hardening and in austempering.