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Heat Treatment & Microstructure Evolution in Metals (MM-504) Lecture # 3a Compiled for M.E. (Materials Engg.) by: Engr. Fawad Tariq Email: t_fawad@hotmail.com Powerpoint Templates Materials Engineering Department, NED University of Engineering and Technology

Crystal Structure BCC structure of ferrite FCC structure of austenite Cementite – Orthorhombic structure

Solubility of C in BCC & FCC • Octahedral & • tetrahedral • interstitial voids • in fcc structure • Octahedral & • tetrahedral • interstitial voids • in bcc structure

Solubility of C in FCC • There are two types of interstitial voids that may become sites for carbon atoms in bcc and fcc structures. • In austenite, assuming spherical Fe atoms in contact, an octahedral site could accommodate an atom 0.052 nm (0.52 A°) in radius, but a tetrahedral site could accommodate an atom only 0.028 nm (0.28A°) in radius (Ref 3.9). • Carbon atoms have radii of 0.07 nm (0.7 A°), and are therefore more readily accommodated in the octahedral voids even though some lattice expansion is required.

Solubility of C in BCC • In ferrite the interstitial sites are much smaller, thus explaining the very limited solubility of C. • A tetrahedral site in ferrite could accommodate an interstitial atom 0.035 nm (0.35 A°) in radius and an octahedral site, an atom only 0.019 nm (0.19 A°) in radius. • The octahedral sites in ferrite, however, are not symmetrical, and a C atom would severely displace only the two atoms at a distance of a/2, not those at a distance a/_2. • C atoms appear to prefer the octahedral sites in ferrite and do produce a severe distortion of the lattice in <100> directions.

Solubility of C in BCC • In ferrite, because of the limited number of carbon atoms that can be accommodated, the lattice remains essentially cubic. • If large numbers of carbon atoms present in austenite are trapped in bcc octahedral sites by rapid cooling, the cubic structure may actually become tetragonal. • The latter structure typifies the phase “martensite”.

Normalizing • Normalizing is the process of raising the temperature to over 60ºC, above line A3 (for hypo) or line ACM (for hyper) fully into the γ range. • Held at this temperature to fully convert the structure into γ, and then removed form the furnace and cooled at room temperature under natural convection. • This results in a grain structure of fine Pearlite with excess of α or Fe3C. • Soaking is usually 1 hour/inch of cross-sectional area but not less than 2 hours at temp.

Normalizing • Holding time can also be roughly estimated by: T (mins) = 60 + D • Low-carbon steels typically do not require normalizing. • Castings with relatively uniform wall thickness and section sizes are usually annealed rather than normalized. • The structure is more homogenous since the temp. involve in normalizing is higher than annealing.

Normalizing

Purposes of Normalizing • For more hardness and strength • Improve machinability • Reduces internal stresses induced by operations (forging, machining, welding, etc.) • Improve dimensional stability • Modify and/or refine the grain structure • Produce a homogeneous microstructure • Reduce banding (alternate regions of ferrite and pearlite due to segregation of Mn in ingots) • Improve ductility • Provide a more consistent response when hardening or case hardening

Grain Refinement Benefit Recall Hall-Petch Equation

Grain Refinement Benefit Neighboring grains interfere dislocation movement by slip !

Differnce b/w Normalizing and Annealing • Fully annealed parts are uniform in softness (and machinablilty) throughout the entire part; since the entire part is exposed to the controlled furnace cooling. • But in normalized part, depending on the part geometry, the cooling is non-uniform resulting in non-uniform material properties across the part. • Thin pieces cool faster and are harder after normalizing than thicker ones. By contrast, after furnace cooling in an annealing process, the hardness of the thin and thicker sections are about the same.

Differnce b/w Normalizing and Annealing

Differnce b/w Normalizing and Annealing Fig. – Microstructure after (a) hot rolling 20MnCr5 steel and (b) normalizing at 880°C  More homogenous γ during heating of steel in normalizing means more finely dispersed will be the α and Fe3C.

Microstructure after Normalizing

Structure formation during Normalizing • At normalizing, the steel is first subjected to pearlite  γ transformation and then on cooling transformed to γ  α (+ pearlite) • We need to understand the decomposition of γ and nucleation, growth of pearlite Fig. - Schematic representation of grain size formation after normalization

Austenite to Pearlite Reactions • On cooling from Ac3, proeutectoid α forms on γ GB. • Remember, GBs are potent sites for nucleation because of high defect energy + Diffusion rate is high at GB, so heterogeneous nucleation occurred! • Slow cooling rate increases the precipitation of pro- α • Relatively high cooling rate may results in the formation of needle like α (Widmannstatten) • The ejected C from pro- α goes into the γ • The C % increases in unstable γ

Austenite to Pearlite Reactions • α nucleates in C-lean regions • As the temperature is lowered below Ac1, the remaining γ (of 0.8% C) transform to pearlite • Pearlite is a lamellar, regular array of mechanical mixture of α and Fe3C. • Grow in colonies till impinge by other colony • Ferrite etched dark • % of α and Fe3C can be determine by Lever rule i.e. approx. 89% α remaining Fe3C. • Colony of pearlite nucleate from either α or Fe3C.

Austenite to Pearlite Reactions  Diffusion of C is low at lower temps.

Driving force for Pearlite • At the Ac1 the free energy of γ is exactly equal to the free energy of α and Fe3C and there is no incentive for transformation to occur. • Interfaces accommodate structural and chemical discontinuities between phases, and therefore make positive contributions to raise the energy of a system. • Below Ac1, the free energy of a unit volume of a mixture of α and Fe3C becomes much less than that of γ. • Lowering of Gibbs free energy is the driving force

Driving force for Pearlite • This free energy difference is frequently referred to as the driving force for transformation and increases with decreasing temperature or undercooling below the Ac1. • A larger driving force makes possible not only the development of more colonies of pearlite but also a finer lamellar spacing within a pearlite colony, structural changes that result in increased interfacial area of two types. • A higher density of pearlite colonies results in increased γ/pearlite interfacial area, and a reduced interlamellar spacing results in increased α-Fe3C interfacial energy within the colonies.

Driving force for Pearlite • Many relationships for the change in interlamellar spacing with undercooling have been proposed, but the one most closely related to the above considerations was developed by Zener and Hillert.

JML Equation for Pearlite Fraction • Johnson and Mehl, assuming that the pearlite colonies are spherical and randomly nucleated as a function of time, developed the following equation for isothermal pearlite formation: • where f (t) is the volume fraction pearlite formed at any time t at a given temperature, N is the nucleation rate of the colonies, and G is the rate at which the colonies grow into the γ. •  The JML equation describes mathematically the rate at which austenite is converted to a pearlitic microstructure by the nucleation and growth of pearlite colonies.

JML Equation for Pearlite Fraction

Nucleation of Pearlite

Formation of Pearlite • In ternary systems and steels, the effects of alloying elements must also be considered. • Mn and Ni do not partition themselves between the α and Fe3C and that, therefore, pearlite formation in these alloys is dependent primarily on volume diffusion of C in γ. • Any reduction in the rate of pearlite growth in these systems is due to the effect of Mn and Ni on the diffusion of carbon in austenite. • However, Cr & Mo which are strong carbide-forming elements, are considered to partition to the carbide lamellae by interface diffusion.

Formation of Pearlite • In the Fe-C-Cr and Fe-C-Mo systems, then, pearlite growth is retarded because Cr and Mo atoms must diffuse, a process that is much more sluggish than the diffusion of C because of the much larger size of the alloying element atoms co • Steels with < 0.4%C would contain more proeutectoid α; those with more C would contain more pearlite. • Depending on the %C of the steel, then, it is possible to have microstructures consisting of 100% ferrite (if the carbon content is less than or equal to 0.02%) or 100% pearlite (if the carbon content is equal to 0.77%).

Pearlite in Hypoeutectoid steel

Pearlite in Hypereutectoid steel

Orientation Relationships •  On nucleation, α forms a O.R. with one of the γ GB. • This relationship is called “Kurdjumov Sachs” relationship • Sometime the ferrite may deviate slightly from having close-packed parallel directions, as shown by the Nishiyama–Wasserman (NW) orientation relationship:

Orientation Relationships •  This habit plane is low energy habit plane, therefore it does not move during the growth of the nucleus • Therefore, α grain grows only in one of the two γ grains associated with the boundary. • The boundary b/w the α nucleus and γ grain in which it grows, is the high energy boundary and it is non-coherent.

Orientation Relationships • Ferrite grows in grain with which it does not have any orientation relationship. • In a similar way, the α component of a pearlite colony formed at the γ grain boundary should also be related to one of the austenite grains, irrespective of whether it is nucleated before or after the cementite. • Consequently, the pearlite colony will be able to grow by the movement of an incoherent α - γ interface i.e. into the γ grain to which the α is unrelated. •  Orientation relationship between the α and cementite in the pearlitic colonies is closer to Pitsch-Petch.

Orientation Relationships • Pitsch-Petch Relationship is given as: • Another O.R. reported by Bagaryatski is called Baaryatski Relationship is:

Pro-eutectoid Ferrite Growth • There are two types of boundaries which separate α lath from the γ grain into which the lath grow: • the parallel side of the lath • the edge or end of the lath • The sides of lath are normally low energy or coherent boundaries and thus immobile. • The boundary at the end of the lath is high-energy boundary and thus mobile. • Lath grow lengthwise more than thickness • However, high cooling rate in normalizing reduces the pro-eutectoid phases as compared to annealed structures.

Pro-eutectoid Ferrite Growth Fig. – Growth of proeutectoid α by the movement of ledges (in the form of small steps) along low-energy γ-α boundary

Growth of Pearlite • Growth primarily depends upon diffusion of C and Fe atoms. • C atoms moves from α towards Fe3C. • Other factors are degree of supersaturation and free energy advantage during the α formation. • Generally carbide plates grow faster than ferrite laths. • Normally, compact α grows on γ GBs whereas Widmannstatten α grows inside the grains. • Widmannstatten α forms in steels with <0.4%C. Coarse γ grains also favors formation of Widmannstatten α

Diffusion of C during transformation a g g a a Carbon diffusion Growth of Pearlite Fig.- Schematics of interface growth fronts associated with the transformation of γ to (a) pearlite and (b) dispersed Fe3C particles in α.

Growth of Pearlite Fig. – The volume and interface diffusion process during growth of pearlite colony

Growth of Pearlite

Growth of Pearlite • When a pearlite colony nucleates at a grain boundary, it grows into only one of the two grains. • With no Proeuctectoid cementite the orientation of pearlite structure is controlled by orientation of γ1. • Motion is into grain 2 since opposite end of Pearlite is higher energy than at GB. • If pearlite nucleates out of cementite on GB the direction can be both along the GB and also into grain 2.

Growth of Pearlite • Nucleation is followed by the growth of pearlite colonies. • In the Zener-Hillert model which assumes that volume diffusion of carbon is the rate-controlling mechanism for the growth of pearlite. • The growth rate GV is given by: Where, Kv = Geometric constant, λα and λθ are lamellar thickness of ferrite and pearlite, Dcv is the volume diffusion coefficient of C, Ceq is the equilibrium C concentration and λc is the critical spacing

Branching of Pearlite Fig. – Branching of Pearlite  Yield strength of pearlite depends upon the relative vol% of ferrite and cementite in the mixture.

Pearlitic Structures

Pearlite (med) Cementite (hard) C0 > 0.76 wt% C Hypereutectoid Pearlitic Structures Pearlite (med) ferrite (soft) C0< 0.76 wt% C Hypoeutectoid

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