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Chapter 9 Heat treatment

(This chapter covers selective sections in Callister Chap. 9, 10 &11)

Study theme outcomes: After studying this chapter , students should or should be able to:

- know and understand the terminology - use binary phase diagrams to determine and/or calculate: - the phases present at certain temperatures and alloy compositions - the composition of phases at certain temperatures - the mass fraction of each phase at a certain temperature - draw and/or interpret microstructures for equilibrium cooling of binary alloys e.g Fe-C - draw, explain and apply cooling or heating curves for binary alloys for example for hypo-, hyper- and eutectoid steel - explain the difference in crystal structures of austenite (γ) , ferrite (α) and martensite - understand the mechanism of perlite, bainite, martensite, tempered martensite and spherodite formation and the corresponding microstructures - interpret, explain and use graphs depicting mechanical properties of the different microstructures - know and understand the terminology and heat treatment process steps i.e. austenitizing, annealing, normalizing, hardening, tempering and spherodizing

9.1 Introduction. (Callister p 109; 390 para. 11.8) Heat treatment of a material is the heating of a material to a specific temperature for a certain period of time and subsequent cooling in order to obtain some micro-structural (grain size, phase transformation etc.), property or stress state in the material. Examples:

- annealing, - hardening and tempering, - normalising, - precipitation hardening, - recrystallization , - stress relieving etc.

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9.2 Phases (Callister p 254 – 255) A phase is an atomic structural state i.e. as a gas, liquid or solid. The phase in which the material appears is dependant on composition, temperature and the pressure.

See Callister Fig. 9.2: Diagram of phases of water. 9.3 Equilibrium Phase diagrams. (Callister p 255 -260) A phase diagram at constant pressure ( 1 atm.) is either:

- a diagram of composition (x axis) and temperature (y axis) with phase areas bordered by lines in the case of alloys

- or a diagram of time (x axis) and temperature (y axis) with phase areas bordered by lines in the case of alloys and bordered by line intercepts in the case of pure elements.

An alloy is a mixture of elements on atomic scale. Consider for example the Ni – Cu phase diagram in Fig. 1. The properties of the atoms and their crystal structures are such that there is complete solubility between Cu & Ni. The element which is in the majority is the parent metal or solvent and that which is in the minority the alloying element or solute. Such a system is referred to as a binary isomorphous system. There are three phase areas on the equilibrium phase diagram:

- a liquid phase at higher temperatures - a solid phase (α) area at lower temperatures - a dual liquid + solid (α).

The phase areas are bordered by a liquidus and a solidus line . These lines are determined from cooling curves for different Cu-Ni alloys

9.4 Use of an quilibrium phase diagram. (Callister p 260– 263)

♦ The three basic “rules” Other phase diagrams, i.e. binary eutectic, binary eutectoid, binary peritectic etc. are also found. The following could be determined,

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regardless of which phase diagram is considered or used, by applying the three basic “rules” :

1. The phases present are those in whose area the alloy

composition and temperature lines intercept. 2. The compositions of the phases are obtained from the

intercept of the temperature with the borderlines. 3. The relative amounts of the phases present are

obtained by applying the lever rule. ♦ The lever rule The lever rule is used to calculate the relative amounts (as wt fraction or % ) of the phases present and will now be deduced. Consider an alloy with composition 30 wt % Ni, 70 wt % Cu at 1200 oC in Fig. 1. The phases present are L + α with the composition of each phase read from the intercept of the liquidus (Y) and solidus (X) with temperature respectively. Suspend a mass mα at X and mass mL at Y . Join the two masses by a weightless lever. Support the lever with masses at the alloy composition O.

Take moments around X: mL (XY) = (mL + mα)OX = mleg. OX ∴Mass fraction L = mL/ mleg = OX/XY = (70 – 62)/ (78 – 62) = 0,5

Take moments around Y: mα (XY) = (mL + mα)OY = mleg. OY ∴Mass fraction α = mα/mleg = OY/XY = (78 – 70)/ (78 – 62) = 0,5

Alternatve: Mass fraction α = 1 - Mass fraction L = 1 – 0,5 = 0,5 9.5 Binary eutectic systems (Callister p 269 – 282) Selfstudy: The principles discussed above are applicable. It will be explained in the following paragraphs. 9.6 The iron-iron carbide (Fe-Fe3C) or iron-carbon (Fe-C) phase diagram. (Callister p 290 – 293) The equilibrium Fe-C or Fe-Fe3C phase diagram, as shown in Callister Fig. 9.24, is an important tool in predicting the response of steel to heat treatment.

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♦ Inspection of the phase diagram show the following phases: 1. Liquid present at high temperatures above the liquidus line. 2. α or ferrite. It is Fe with a BCC crystal structure with a

maximum solubility of 0,022 wt% C at 727 oC. 3. γ or austenite. It is Fe with a FCC crystal structure with a

maximum solubility of 2,2 wt% C at 1147 oC. Pure Fe transforms from α to γ at 912 oC on heating.

4. δ- iron. It is Fe with a BCC crystal structure. This phase occurs at temperatures above that where steel is normally heat treated and will not be considered further.

5. Fe3C or cementite. It is an iron carbon intermediate compound with a fixed ratio of Fe to C namely 6,7 wt % C.

6. In between the single phase areas are dual phase areas of which the following are of relevance: α + γ, γ + L, γ + Fe3C (ledeburite) and α + Fe3C (pearlite)

Fe containing more than 6,7 % C is too brittle to use in engineering applications. Fe3C therefore forms the right hand compound on the composition scale.

♦ Phase transformations or reactions in the iron-iron carbide phase diagram

The Fe-Fe3C phase diagram contains three horizontal lines that indicate isothermal invariant reactions (phase transformations that take pace at a single temperature and composition): Peritectic reaction: This is a reaction that occur at 1493 oC where:

δ (0,1 %C)+ L (0,5 %C) → γ (0,18 %C)

Temperatures in the vicinity of the peritectic temperature is above the normal heat treating temperatures of steel and will not be further considered in this course. Eutectic reaction: At the eutectic reaction point, liquid of 4,3 %C forms γ or austenite of 2,14 %C and the intermetallic compound Fe3C or cementite with 6,7 %C. This reaction which occurs at 1147 oC, can be written as:

L (4,3 %C) → γ (2,14 %C) + Fe3C (6,7 %C)

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The eutectic mixture of γ + Fe3C in the Fe-Fe3C system is called ledeburite. Eutectoid reaction: At the eutectoid reaction point, solid austenite of 0,76 %C produces a fine eutectoid mixture of α-ferrite with 0,022 %C and Fe3C (cementite) with 6,7 %C. This reaction which occurs at 727 oC , can be written as:

γ (0,76 %C) → α ( 0,022 %C) + Fe3C (6,7 %C)

The eutectoid reaction product α + Fe3C is called pearlite. The eutectoid reaction which takes place in the solid state is important in the heat treatment of steel. 9.7 Equilibrium cooling of steel (Callister p 293 – 300)

♦ The equilibrium cooling of eutectoid plain carbon steel (0,76 %C). If a sample of a 0,76%C steel is heated to above 727oC in the austenite phase area (austenitising.) for a sufficient period of time, the microstructure will become homogeneous austenite. On slow cooling to just above 727oC, the structure will be 100 % γ with a composition of 0,76 %C. When cooled to just below the eutectoid temperature of 727 oC, it will transform to α ( 0,022 %C) + Fe3C (6,7 %C) or pearlite. The relative amounts of α or ferrite and Fe3C or cementite can be calculated by applying the lever rule.

w% α = ( 6,7 – 0,76)x100 / (6,7 – 0,022) = % w% Fe3C = 100 – = % Callister Fig. 9.27 show the characteristic lamellar microstructure of pearlite

The decomposition of γ to pearlite at the eutectoid temperature occurs by a process of nucleation and growth. The lower the transformation temperature relative to the eutectoid temperature, the higher the rate of nucleation and the rate of growth. The net effect is that fine pearlite (small inter-lamellar spacing) form at lower and course pearlite (larger inter-lamellar

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spacing) at higher transformation temperatures. See Callister Fig. 10.15 for microstructures.

♦ The equilibrium cooling of hypo-eutectoid (less than 0,76 %C) plain carbon steel. Austenitise a sample with say 0,5 %C. On slow cooling, primary or pro-eutectoid α nuclei will form on the γ grain boundaries. These nuclei will grow with further cooling, to form a layer of α on the grain boundaries. On further cooling, the γ or austenite composition, gradually moves towards the eutectoid composition or 0,76 %C just above 727 oC. The α or ferrite composition, gradually moves towards 0,022 %C just above 727 oC. The relative amounts of the phases present can be calculated by applying the lever rule. The % α increase while the %γ decrease as the temperature is decreased. When the alloy is cooled to just below 727 oC, the γ (with 0,76 %C) which remained just above 727 oC will now transform or react to form pearlite. The α which forms in this eutectoid reaction (below 727 oC) is called secondary or eutectoid α to distinguish it from the pro-eutectoid α which formed above 727 oC.

The % Fe3C and % total α can be calculated by applying the lever rule just below 727 oC. The % eutectoid α can be calculated by subtracting the % primary α from the total.

♦ The equilibrium cooling of hyper-eutectoid ( more than 0,76 %C) plain carbon steel.

Austenitise a sample with say 1,3 %C. On slow cooling, primary or pro-eutectoid Fe3C nuclei will form on the γ grain boundaries. These nuclei will grow with further cooling, to form a layer of Fe3C on the grain boundaries. On further cooling, the γ or austenite composition, gradually moves towards the eutectoid composition or 0,76 %C just above 727 oC. The Fe3C composition, remains constant at 6,7 %C.. The relative amounts of the phases present can be calculated by applying the lever rule. The % Fe3C increase while the %γ decrease as the temperature is decreased. When the alloy is cooled to just below 727 oC, the γ (with 0,76 %C) which remained just above 727 oC will now transform or react to form pearlite. The Fe3C which

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forms in this eutectoid reaction (below 727 oC) is called secondary or eutectoid Fe3C to distinguish it from the pro-eutectoid Fe3C which formed above 727 oC. The % total Fe3C and % total α can be calculated by applying the lever rule just below 727 oC. The % eutectoid Fe3C can be calculated by subtracting the % primary Fe3C from the total. 9.8 The non-equilibrium cooling of steel Alternative phase diagrams, i.e isothermal transformation and continuous cooling diagrams are applicable when the cooling rate is too rapid for equilibrium phases to form. The non-equilibrium phases that do occur when steel is rapidly cooled are: ♦ Bainite transformation (Callister p 328 – 329) Bainite forms between approximately 250 oC and 550 oC . It has a feathery or needle-like structure (as shown in Fig. 2) Bainite is nucleated by α or ferrite, which is followed by the precipitation of Fe3C in the ferrite. This process leads to a fine dispersion of Fe3C in a matrix of α. A distinction is made between upper and lower bainite. Upper or feathery bainite is bainite which formed at temperatures between 350 oC to 550 oC, and has fine Fe3C plates generally parallel with the long axis of the α needles that make up the matrix. Lower bainite forms at temperatures between 250 oC and 350 oC and consist of very fine Fe3C plates usually oriented at an angle of 60 o to the long axis of the α needles. Fig .3 shows the difference between upper and lower bainite. ♦ Martensite transformation (Callister p 331 – 332) With slow cooling of steel from the γ temperature range, carbon atoms are able to diffuse out of the FCC austenite structure which subsequently form a BCC α structure. With an increase in the cooling rate, insufficient time is allowed for the carbon to diffuse out of the solution, the structure cannot become BCC while the carbon is trapped in the solution. The resultant structure is BCT and is called martensite. Callister Fig. 10.21 shows the straw like microstructure of martensite. The highly distorted lattice structure is

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the primary reason for the high hardness and low ductility of martensite. Since no atomic diffusion takes place during martensite formation, the reaction is not time dependent and occurs very rapidly. The amount of γ to martensite transformation is temperature dependent. The transformation start at Ms and is completed at Mf . 9.9 Some heat treatments of plain carbon steel. (Callister p 388 – 390; 343 – 345)

♦ Annealing is a heat treatment process by which the steel is heated up to the austenite phase area and then allowed to cool very slowly. It is very often allowed to cool with or in the furnace. Equilibrium phases as discussed in para. 9.7 are obtained. The microstructures render soft ductile material with large grain sizes and course pearlite.

♦ Normalising is a heat treatment whereby the plain carbon steel is heated up to the austenite phase area for hypo-eutectoid and eutectoid steels and then allowed to cool at an intermediate cooling rate like being cooled in air. The equilibrium phase diagram can therefore not be used to determine the amount of phases present. Normalising renders a finer pearlite and lower % pro-eutectoid component than annealing. It is thus harder and less ductile than the annealed steel Hyper-eutectoid steels are normalised at temperatures just above 727 oC This limits the amount of pro-eutectoid Fe3C at the grain boundaries. Fe3C is hard and brittle and will cause embrittlement of the steel if excessive amounts form on the grain boundaries. The decrease of the relative amount of Fe3C will also improve machine-ability of the steel.

♦ Hardening is the heat treatment process whereby the steel is austenitised and then very rapidly cooled (quenched) to form martensite..

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♦ Tempering Martensite is hard and brittle and has high residual stresses. Tempering is a heat treatment whereby the physical properties of the martensite is altered to comply to certain requirements. The steel is heated to a temperature range below the eutectoid temperature , kept at the temperature for a certain period of time and then cooled to room temperature. Residual stresses are relieved and hardness and ductility improved during tempering. Ductility increase but hardness and strength decrease. The mechanism of tempering: Martensite is a metastable phase, and as thermal energy is supplied during tempering, carbon will tend to precipitate as rod-shaped carbide (Fe3C) and the iron, now depleted of carbon, will assume a BCC ferrite structure. The process is diffusion controlled. The amount of diffusion is dependent on temp. and time at temp. Extended tempering at relatively high tempering temp. will result in the rod-like Fe3C particles coalescing to from spherical Fe3C particles within the ferrite. This is known as spheroidising. The corresponding microstructure is shown in Callister Fig. 10.19.

♦ Stress relieving Internal residual stresses are introduced into a metal component by:

- cold working - quenching - welding - machining and grinding

These internal stresses could result in premature failure of the component. A stress relieve heat treatment consist of heating the material to a temperature below the eutectoid temperature for a controlled period of time.

♦ Spheroidising Prolong heating (i.e. ± 24 hours at 600 oC) of either tempered martensite, bainite or pearlite will result in the Fe3C or cementite particles, which are present in all the mentioned phases, taking on a spherical geometry or shape in a α or ferrite matrix. This structure is called sheroidite and is more ductile and softer and less strong than

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the phases from which it originated. It also has good machine-ability. The driving force for spheroidising is the minimisation of the inter-phase energy. The interface between two phases has a surface energy (J/ m2). The microstructure with the lowest internal energy is therefore a structure where the surface area between phases is a minimum. The geometry or shape which has the lowest surface to volume ratio is a sphere. 9.10 Some mechanical properties of heat treated steel See Callister Fig. 10.30 & Fig 10.32 EXAMPLES Class examples PROBLEMS See clickUP

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