Department of Materials Engineering Heat Treatment Ammar R. H. Alshemary Assistant Lecturer Department of Materials Engineering University of Kufa

Introduction Most of the engineering properties of metals and alloys are related to their structure. Equilibrium structure can be predicted for an alloy with the help of an equilibrium diagram. Mechanical properties can be changed by varying the relative proportions of micro constituents (such as change from γ to P, γ to Fe3C or change in chemical composition in the case hardening treatment). In practice, change in mechanical properties is achieved by process known as heat treatment. This process consist of heating a metal or alloy to a specific predetermined temperature, holding at this temperature for required time, and finally cooling from this temperature. All operations are carried out in solid state. Sometimes, it becomes necessary to repeat these operations to impart some characteristics. Therefore, We can define the heat treatment as an operation or combination of operations ,involving heating and cooling of a metal or alloy in its solid state with the object of changing the characteristics of the material.

Heat treatment is an important operation in the final fabrication process of Many engineering components. The object of this process is to make the metal better suited, structurally and physically, for some specific applications. Heat Treatment (time and temperature) Microstructure Mechanical Properties All metals can be subjected to thermal cycling. But the effect of thermal cycling may differ from one metal to another. For example, heat treatment has significant impact on steels, and its properties may be changed considerably by definite heating and cooling cycles. In contrast, there is hardly any effect of thermal cycling on properties of hot rolled copper.

There are number of factors of paramount importance which are to be considered when heat treating a metal or alloy. Some of them are temperature up to which metal/alloy is heated, the length of time that the metal/alloy is held at the elevated temperature, the rate of cooling, and the atmosphere surrounding the metal/alloy when it is heated. Any heat treatment process can be presented graphically with temperature and time as coordinates. Figure 1.1 describes a simple heat treatment cycle, whereas figure 1.2 and 1.3 represent some complex heat treatment cycles.

Figure 1.1 is the simplest possible heat treatment cycle in which the metal/alloy is heated, held at elevated temperature for some time, and then cooled to room temperature. Figure 1.2 shows a typical heat treatment cycle suitable for a precipitation hardenable alloy. In this case, the alloy is heated and held at high temperature. This step is termed as solutionizing. The alloy is then cooled rapidly to room temperature by quenching. The quenched alloy is heated and held at a moderately high temperature above the room temperature, followed by slow cooling. The last step is termed as artificial ageing. Figure 1.3 represent a typical heat treatment cycle for carburizing process. The low carbon steel is heated in the temperature range of austenitic region, in contact with some carbonaceous material. It is held at this temperature for some time and then quenched. The quenched steel is reheated to a temperature slightly lower than the one employed in the first step. After holding for some time, it is rapidly cooled to room temperature. In the last step ,the steel is again heated to about 750 ᵒC (which is just above the lower critical temperature), held at this temperature and then quenched. By these three steps, a hard case and though core are obtained in the case of carburized steel.

Definition of heat treatment Heat treatment is an operation or combination of operations involving heating at a specific rate, soaking (holding) at a temperature for a period of time and cooling at some specified rate. The aim is to obtain a desired microstructure to achieve certain predetermined properties (physical, mechanical, magnetic or electrical). Objectives of heat treatment (heat treatment processes) The major objectives are to increase strength, hardness and wear resistance (bulk hardening, surface hardening) to increase ductility and softness (tempering, recrystallization annealing) to increase toughness (tempering, recrystallization annealing) to obtain fine grain size (recrystallization, annealing, full annealing, normalising)

to remove internal stresses induced by differential deformation by cold working, non-uniform cooling from high temperature during casting and welding (stress relief annealing) to improve machineability (full annealing and normalising) to improve cutting properties of tool steels (hardening and tempering) to improve surface properties (surface hardening, corrosion resistance-stabilising treatment and high temperature resistance) (precipitation hardening, surface treatment) to improve electrical properties (recrystallization, tempering, age hardening) to improve magnetic properties (hardening, phase transformation) Fe-cementite metastable phase diagram (Fig.1) consists of phases liquid iron(L), δ-ferrite, γ or austenite, α-ferrite and Fe3C or cementite and phase mixture of pearlite (interpenetrating bi-crystals of α ferrite and cementite)(P) and ledeburite (mixture of austenite and cementite)(LB). Solid phases/phase mixtures are described here.

Fig.1: Fe-Cementite metastable phase diagram (microstructural) L=liquid, Cm=cementite, LB=ledeburite, δ=delta ferrite, α= alpha ferrite, α’= alpha ferrite(0.00005 wt%C) γ=austenite, P=pearlite, eu=eutectic, ed=eutectoid, I=primary, II=secondary, III=tertiary δ+L L 1539˚C δ A5=1495˚C 0.09 0.53 δ+γ 0.17 1227˚C L+γI 1394˚C 4.30 L+CmI γ γI+LB LB+CmI A4=1147˚C 2.11 LB’ (γeu(γII +CmII)+Cmeu) +CmI γI’(γII+CmII)+LB’ (γ’eu(γII +CmII)+Cmeu) A B C Temperature, °C Acm 910˚C D E F A2 =668/ 770˚C A3 Cm Ledeburite=LB(γeu+Ceu) 0.77 αI+γ γII+CmII 0.0218 A1=727˚C α Pearlite P(αed+Cmed) +CmII (P(αed+Cmed)+CmII)+ LB’ (P(αed+Cmed) +CmII+Cmeu) LB’ (P(αed+Cmed) +CmII)+Cmeu)+CmI αI+ (P(αed +Cmed) αI(α’+CmIII)+(P(αed(α’ed+CmIII)+Cmed) P(αed (α’ed +CmIII)+Cmed) +CmII (P(αed(α’ed+CmIII)+Cmed) +CmII)+ LB’ ((P(αed(α’ed+CmIII)+Cmed) +CmII)+Cmeu) LB’ ((P(αed(α’ed+CmIII)+Cmed) +CmII)+ Cmeu)+CmI Ao=210˚C α’+CmIII 6.67 0.00005 Weight percent carbon

Basic Concepts in The Iron-Carbon System • Ferrous Alloys (Iron,Steel, Cast iron) Versatile and ductile. Cheap. Most widely used materials in the world. • Iron (< 0.008% C) • Steel (< 2.11% C) • Cast iron (<6.67% [mostly <4.5%]C). • The material properties are more than composition – they are dependent on how the material has been treated. • Iron melts at 1538°C. As it cools, it forms in sequence the following solid phases: Delta ferrite Austenite Alpha ferrite Cementite

Alpha ferrite or α-ferrite: Interstitial solid solution of carbon in iron of body centred cubic crystal structure (BCC) (α iron )(same as Fig. 2(a)). Maximum C solubility of 0.022% at 727°C. The stability of the phase ranges between low temperatures to 910°C, and solubility ranges 0.00005 wt % C at room Soft and ductile. Magnetic up to the Curie temperature of 768°C. Solid solution: one or more solute in the solvent, such as mixture, crystal structure remain unchanged by addition of solutes. Fig.2(a): Crystal structure of ferrite

There are two morphologies can be observed under equilibrium transformation or in low under undercooling condition in low carbon plain carbon steels. These are intergranular allotriomorphs (α)(Fig. 4-7) or intragranular idiomorphs(αI) (Fig. 4, Fig. 8) Fig. 4: Schematic diagram of grain boundary allotriomoph ferrite, and intragranular idiomorph ferrite.

Fig.5: An allotriomorph of ferrite in a sample which is partially transformed into α and then quenched so that the remaining γ undergoes martensitic transformation. The allotriomorph grows rapidly along the austenite grain boundary (which is an easy diffusion path) but thickens more slowly. Fig.6: Allotriomorphic ferrite in a Fe-0.4C steel which is slowly cooled; the remaining dark-etching microstructure is fine pearlite. Note that although some α-particles might be identified as idiomorphs, they could represent sections of allotriomorphs. Micrograph courtesy of the DOITPOMS project.

Fig.7: The allotriomorphs have in this slowly cooled low-carbon steel have consumed most of the austenite before the remainder transforms into a small amount of pearlite. Micrograph courtesy of the DoITPOMS project. The shape of the ferrite is now determined by the impingement of particles which grow from different nucleation sites. Fig. 8: An idiomorph of ferrite in a sample which is partially transformed into α and then quenched so that the remaining γ undergoes martensitic transformation. The idiomorph is crystallographically facetted.

Austenite (gamma γ phase ) Interstitial solid solution of carbon in iron of face centred cubic crystal structure (FCC) (Fig.3(a)). having solubility limit of 2.11 wt% at 1147°C with respect to cementite. The stability of the phase ranges between 727-1495°C and solubility ranges 0-0.77 wt%C with respect to alpha ferrite and 0.77-2.11 wt% C with respect to cementite, at 0 wt%C the stability ranges from 910-1394°C. Denser than ferrite, and the FCC phase is much more formable at high temperatures. Large amounts of Ni and Mn can be dissolved into this phase. The phase is non-magnetic Fig.3(a ): Crystal structure of austenite is shown at right side.

δ ferrite: Interstitial solid solution of carbon in iron of body centred cubic crystal structure(BCC) (Fig .2(a)) (δ iron ) of higher lattice parameter (2.89Å) having solubility limit of 0.09 wt% at 1495°C with respect to austenite. The stability of the phase ranges between 1394-1539°C. This is not stable at room temperature in plain carbon steel. However it can be present at room temperature in alloy steel specially duplex stainless steel. Fig.2(a): Crystal structure of ferrite

Cementite or Fe3C 100% iron carbide Fe3C (chemical compound). Very hard. Very brittle. Interstitial intermetallic compound of C & Fe with a carbon content of 6.67 wt% Orthorhombic structure consisting of 12 iron atoms and 4 carbon atoms in the unit cell. Stability of the phase ranges from low temperatures to 1227°C. Fig.9(a): Orthorhombic crystal structure of cementite. The purple atoms represent carbon. Each carbon atom is surronded by eight iron atoms. Each iron atom is connected to three carbon atoms.

Fig.9(b): The pearlite is resolved in some regions where the sectioning plane makes a glancing angle to the lamellae. The lediburite eutectic is highlighted by the arrows. At high temperatures this is a mixture of austenite and cementite formed from liquid. The austenite subsequently decomposes to pearlite. Courtesy of Ben Dennis-Smither, Frank Clarke and Mohamed Sherif

Critical temperatures: A=arret means arrest A0= a subcritical temperature (<A1) = Curie temperature of cementite=210°C A1=Lower critical temperature=eutectoid temperature=727°C A2=Curie temperature of ferrite=768/770°C A3=upper critical temperature=γ+α /γ phase field boundary =composition dependent=910-727°C A4=Eutectic temperature=1147°C A5=Peritectic temperature=1495°C

Types/morphologies of phases in Fe-Fe3C system Acm=γ/γ+cementite phase field boundary=composition dependent =727-1147°C In addition the subscripts c or r are used to indicate that the temperature is measured during heating or cooling respectively. c=chaffauge means heating, Ac r=refroidissement means cooling, Ar Types/morphologies of phases in Fe-Fe3C system Cementite=primary (CmI), eutectic (Cmeu), secondary (CmII)(grain boundary allotriomophs, idiomorphs), eutectoid (Cmed) and tertiary(CmIII). Austenite= austenite(γ)(equiaxed), primary (γI), eutectic (γeu), secondary (γII) (proeutectoid), α-ferrite=ferrite (α) (equiaxed), proeutectoid or primary (grain boundary allotriomorphs and idiomorphs)(αI), eutectoid(αeu) and ferrite (lean in carbon) (α’). Phase mixtures Pearlite (P) and ledeburite(LB)

L(0.53wt%C)+δ(0.09wt%C)↔γ(0.17wt%C) at 1495°C Important Reactions Peritectic reaction Liquid+Solid1↔Solid2 L(0.53wt%C)+δ(0.09wt%C)↔γ(0.17wt%C) at 1495°C Liquid-18.18wt% +δ-ferrite 81.82 wt%→100 wt% γ Fig.10: δ-ferrite in dendrite form in as-cast Fe-0.4C-2Mn-0.5Si-2 Al0.5Cu, Coutesy of S. Chaterjee et al. M. Muruganath, H. K. D. H. Bhadeshia

Eutectic reaction Liquid↔Solid1+Solid2 Liquid (4.3wt%C) ↔ γ(2.11wt%C) + Fe3C (6.67wt%C) at 1147˚C Liquid-100 wt% →51.97wt% γ +Fe3C (48.11wt%) The phase mixture of austenite and cementite formed at eutectic temperature is called ledeburite. Fig.11: Microstructure of white cast iron containing massive cementite (white) and pearlite etched with 4% nital, 100x. After Mrs. Janina Radzikowska, Foundry Research lnstitute in Kraków, Poland

Fig. 12: High magnification view (400x) of the white cast iron specimen shown in Fig. 11, etched with 4% nital. After Mrs. Janina Radzikowska, Foundry Research lnstitute in Kraków, Poland Fig. 13: High magnification view (400x) of the white cast iron specimen shown in Fig. 11, etched with alkaline sodium picrate. After Mrs. Janina Radzikowska, Foundry Research lnstitute in Kraków, Poland

Solid1↔Solid2+Solid3 Eutectoid reaction γ(0.77wt%C) ↔ α(0.0218wt%C) + Fe3C(6.67wt%C) at 727°C γ (100 wt%) →α(89 wt% ) +Fe3C(11wt%) Typical density α ferrite=7.87 gcm-3 Fe3C=7.7 gcm-3 volume ratio of α- ferrite:Fe3C=7.9:1 Fig. 14: The process by which a colony of pearlite evolves in a hypoeutectoid steel.

It is a mechanical mixture of ferrite and Fe3C. Pearlite Formation It is a mechanical mixture of ferrite and Fe3C. Austenite transforms to ferrite and Fe3C at Eutectoid Transformation Temperature (727°C). When slow cooled, this is Pearlite (looks like Mother of Pearl). Formed in thin parallel plates. Fig. 2.2 Schematic representations of the microstructures for an iron–carbon alloy of eutectoid composition (0.76 wt% C) above and below the eutectoid temperature.

Diffusion of Carbon in Pearlite Fig. 2.3 pealite formation and microstructure microstructuremic

A plain-carbon steel that contains 0 A plain-carbon steel that contains 0.76 percent C is called a eutectoid steel since an all-eutectoid structure of α ferrite and Fe3C is formed when austenite of this com¬position is slowly cooled below the eutectoid temperature. If a plain-carbon steel contains less than 0.76 percent C, it is termed a hypoeutectoid steel, and if the steel contains more than 0. 76 percent C, it is designated a hypereutectoid steel. Fig. 15 : The appearance of a pearlitic microstructure under optical microscope.

a) Coarse pearlite (b) Fine pearlite Fig. 2.4 Photomicrographs of pearlite types microstructuremic

Fig. 2.5 Schematic representations of the microstructures for an iron–carbon alloy of hypoeutectoid composition (containing less than 0.76 wt% C) as it is cooled from within the austenite phase region to below the eutectoid temperature.

Fig. 2. 6 Photomicrograph of a 0 Fig. 2.6 Photomicrograph of a 0.38 wt% C steel having a microstructure consisting of pearlite and proeutectoid ferrite. Fig. 2.8 Photomicrograph of a 1.4 wt% C steel having a Microstructure consisting of a white proeutectoid cementite network surrounding the pearlite colonies. 1000X.

Fig.2.7 Schematic representations of the microstructures for an iron–carbon alloy of hypereutectoid composition C1(containing between 0.76 and 2.14 wt% C), as it is cooled from within the austenite phase region to below the eutectoid temperature.

Evolution of microstructure (equilibrium cooling) Sequence of evolution of microstructure can be described by the projected cooling on compositions A, B, C, D, E, F. At composition A L δ+L δ δ+γ γ γ+αI α α’+CmIII At composition B L δ+L L+γI γ αI +γ αI+ (P(αed+Cmed) αI(α’+CmIII)+(P(αed(α’ed+CmIII)+Cmed)

At composition C L γ At composition D L L+γI γII+CmII P(αed+Cmed)+CmII P(αed (α’ed+CmIII)+Cmed)+CmII L+γI γI+LB γI’(γII+CmII)+LB’ (γ’eu(γII+CmII)+Cmeu) (P(αed+Cmed)+CmII)+ LB’ (P(αed+Cmed)+CmII+Cmeu) (P(αed(α’ed+CmIII)+Cmed) +CmII)+ LB’ ((P(αed(α’ed+CmIII)+Cmed)+CmII)+Cmeu)

At composition E L At composition F L Fe3C L+CmI LB(γeu+Cmeu+CmI LB’ (γeu(γII+CmII)+Cmeu)+CmI LB’ (P(αed+Cmed)+CmII)+Cmeu)+CmI LB’ ((P(αed(α’ed+CmIII)+Cmed) +CmII)+ Cmeu)+CmI

Bainite Bainite is a phase that exists in steel microstructures after certain heat treatments. It is one of the decomposition products that may form when austenite (the face centered cubic crystal structure of iron) is cooled past a critical temperature of 727 °C. Davenport and Bain originally described the microstructure as being similar in appearance to tempered martensite. A fine non-lamellar structure, bainite commonly consists of ferrite, carbide, and retained austenite. In these cases it is similar in constitution to pearlite, but with the ferrite forming by a displacive mechanism similar to martensite formation, usually followed by precipitation of carbides from the supersaturated ferrite or austenite. The temperature range for transformation to bainite is between those for pearlite and martensite. When formed during continuous cooling, the cooling rate to form bainite is higher than that required to form pearlite, but lower than that to form martensite, in steel of the same composition. The microstructures of martensite and bainite at first seem quite similar; this is a consequence of the two phases sharing many aspects of their transformation mechanisms. However, morphological differences do exist on the resolution level of the TEM and can be used in microstructural evaluation. Under a simple light microscope, the microstructure of bainite appears dark (i.e., it has low reflectivity).

Bainite Formation For T ~ 300-540°C, upper bainite consists of needles of ferrite separated by long cementite particles For T ~ 200-300°C, lower bainite consists of thin plates of ferrite containing very fine rods or blades of cementite In the bainite region, transformation rate is controlled by microstructure growth (diffusion) rather than nucleation. Since diffusion is slow at low temperatures, this phase has a very fine (microscopic) microstructure. Pearlite and bainite transformations are competitive; transformation between pearlite and bainite not possible without first reheating to form austenite. Fig. 2.9 Structure of Bainite microstructuremic

Martensite formation in Steel The diagram below assumes slow equilibrium cooling. Each phase is allowed to form. Time is not a variable. Since the martensitic transformation does not involve diffusion, it occurs almost instantaneously; the martensite grains nucleate and grow at a very rapid rate—the velocity of sound within the austenite matrix. Thus the martensitic transformation rate, for all practical purposes, is time independent. Martensite grains take on a plate-like or needle-like appearance, as indicated in figure below. The white phase in the micrograph is austenite (retained austenite) that did not transform during the rapid quench. As already mentioned, martensite as well as other microconstituents (e.g., pearlite) can coexist. • However; If cooling is rapid enough that the equilibrium reactions do not occur. (does not involve diffusion). • Austenite transforms into a non-equilibrium phase Called Martensite

Martensite is : Hard brittle phase Photomicrograph showing the martensitic microstructure. The needleshaped grains are the martensite phase, and the white regions are austenite that failed to transform during the rapid quench. Martensite is : Hard brittle phase Iron carbon solution whose composition is the same as austenite from which it was derived But the FCC structure has been transformed into a body center tetragonal (BCT) The extreme hardness comes from the lattice strain created by carbon atoms trapped in the BCT.

Fig 2.11 (a) FCC γ iron unit cell showing a carbon atom in a large interstitial hole along the cube edge of the cell, (b) BCC α iron unit cell indicating a smaller interstitial hole between cube-edge atoms of the unit cell. (c) BCT (body-centered tetragonal) iron unit cell produced by the distortion of the BCC unit cell by the interstitial carbon atom.