Introduction to Heat Treatment. Introduction

Transcription

1 MME444 Heat Treatment Sessional Week 01 Introduction to Heat Treatment Prof. A.K.M.B. Rashid Department of MME BUET, Dhaka Introduction Can you control the microstructure that formed during cooling of an alloy of fixed composition? Yes, by controlling the transformation mechanism by varying temperature and time of transformation. Manipulating phase transformations is a major tool for controlling the properties of metals and alloys. 1

2 Introduction Phase diagrams represents microstructure that SHOULD develop, assuming that temperature is changed slowly enough to maintain equilibrium at all time. In actual practice, materials processing is rushed, and time becomes an important factor. The practical aspect of this is HEAT TREATMENT, the temperature versus time history necessary to generate a desired microstructure. The fundamental basis for heat treatment is KINETICS, which is defined as the science of time-dependent phase transformation. Time The Third Dimension Time did not appear in any quantitative way in the discussion of phase diagrams. Phase diagrams summarised equilibrium states and these equilibrium structures take time to develop, and the approach to equilibrium can be mapped on a time scale. T Time for solidification to go to completion is a strong function of temperature. completion of reaction I A B The reaction proceeds slowly near the melting point. The reaction is fastest at some intermediate temperature. The reaction becomes slow again as the temperature is decreased further. x Schematic illustration of the approach to equilibrium 2

3 Time The Third Dimension Example: Solid-State Transformation All diffusion controlled solid-state transformation processes commonly proceed by the nucleation and thermally activated growth mechanism Since these transformations involved the formation of at least one new phase that has different structure and/or composition from that of the parent one, some atomic rearrangements via diffusion are required, which is a time-dependent phenomenon. Time The Third Dimension For solid-state transformations, the fraction of material transformed y over a time period t is given by the Avrami equation: y = 1 exp (-kt n ) where k and n are time-independent constants. By convention, the rate of transformation r is taken as the reciprocal of time required for 50% completion of the transformation: r = 1 t 0.5 3

4 Temperature 10/11/2011 Time The Third Dimension Temperatures have profound influence on kinetics and, thus, on the rate of transformation. For most reactions, rate of transformation increases with temperature according to the following Arrhenius equation: y (%) r = A exp (-Q/RT) A = temperature independent constant Q = activation energy for the reaction log t (min) Recrystallisation of Copper Time The Third Dimension Now, the overall transformation rate depends on both the nucleation rate and the growth rate. growth / diffusion The nucleation rate increases with decreasing temperature. nucleation / driving force Rate (s -1 ) overall transformation Temperature dependency of overall transformation rate The growth rate is slower the lower the temperature (a diffusion-controlled process). 4

5 Temperature Temperature Temperature 10/11/2011 Time The Third Dimension In practice, a phase transformation does not always take place exactly at the transition temperature. For example, liquids may undercool before solidification commences. T 1 T 2 Transformation temperature, T t nucleation / driving force Rate (s -1 ) Temperature dependency of overall transformation rate growth / diffusion overall transformation Near T t, the overall transformation rate is slow. (faster growth rate, slower nucleation rate) few nucleus formed, grow rapidly to form coarser grains At much lower temperature than T t, the overall transformation rate is slow again. (faster nucleation rate, slower growth rate) many nuclei formed, finer grains resulted due to slower growth rate The maximum transformation rate occurs at a temperature range where the driving forces for solidification and diffusion rates are both significant. The TTT Diagram If, instead plotting the rate against temperature, we plot the start and finish times for a transformation at various temperature, we obtain a graph which is a mirror image of the overall transformation curve. Untransformed structure End of Transformation Re-plot increasing % transformation Start of Transformation Transformed structure Rate (s -1 ) log (time) This plot is known as Time Temperature Transformation (TTT) diagram. Commonly used to represent the temperature dependence of a transformation. 5

6 Temperature 10/11/2011 Types of Phase Transformations 1. Diffusion-dependent with no change in phase composition or number of phases present (e.g. melting, solidification of pure metal, allotropic transformations, recrystallization, etc.) 2. Diffusion-dependent with changes in phase compositions and/or number of phases (e.g. eutectoid transformations, where one solid transforms into a mixture of two solids) 3. Diffusion-less phase transformation produces a metastable phase by cooperative small displacements of all atoms in structure in no time!! (e.g. martensitic transformation) Types of Phase Transformations Diffusional vs. Martensitic Transformation Untransformed structure MARTENSITIC log (time) 50 % Transformed structure DIFFUSIONAL cooling curve Diffusional transformations occur relatively slowly usually result in the formation of equilibrium phases through diffusion of atoms transformation occurs only when time is increased Martensitic transformations very rapid, no diffusion involved systematic coordinated shearing of the lattice produce a metastable phase transformation occurs only when temperature is decreased 6

7 T (C) T (C) 10/11/ TTT Diagram for Eutectoid 723 Steel (0.8% C) 600 Eutectoid temperature Austenite Pearlite Coarse Fine 500 Pearlite + Bainite 400 Bainite Isothermal cooling Non-isothermal cooling Austenite M S M 90 Martensite t (s) TTT Diagram for Eutectoid Steel (0.8% C) Eutectoid temperature Pearlite 500 Continuous cooling curve (constant rate) Austenite M S Bainite 100 M 90 Martensite Curve 2 Curve t (s) 7

8 T (C) 10/11/ Eutectoid 723 Steel (0.8% C) 600 Eutectoid temperature Different Cooling Treatments M S M M 90 M + P Coarse P Fine P t (s) Formation of Pearlite Pearlite microstructure containing coarse alternate layers of ferrite (white) and cementite (black) Nucleation and growth process Heterogeneous nucleation at grain boundaries Interlamellar spacing is a function of the temperature of transformation Lower temperature finer spacing higher hardness 8

9 Formation of Bainite Bainite formed at 348 C Bainite formed at 278 C Nucleation and growth process Acicular, accompanied by surface distortions Bainite plates have irrational habit planes Ferrite in Bainite plates possess different orientation relationship relative to the parent Austenite than does the Ferrite in Pearlite FCC Austenite Formation of Martensite Possible positions of Carbon atoms Only a fraction of the sites occupied FCC Austenite Alternate choice of Cell C along the c-axis obstructs the contraction 20% contraction of c-axis 12% expansion of a-axis Tetragonal Martensite Austenite to Martensite 4.3 % volume increase In Pure Fe after the Matensitic transformation c = a 9

10 Formation of Martensite The martensitic transformation occurs without composition change. The transformation occurs by shear without need for diffusion. The atomic movements required are only a fraction of the interatomic spacing. The shear changes the shape of transforming region results in considerable amount of shear energy plate-like shape of Martensite The amount of martensite formed is a function of the temperature to which the sample is quenched and not of time. Hardness of martensite is a function of the carbon content but high hardness steel is very brittle as martensite is brittle Steel is reheated to increase its ductility this process is called TEMPERING Tempering of Martensite ' 3 ( BCT ) Temper ( BCC) Fe C ( OR) Martensite Ferrite Cementite Heat below Eutectoid temperature wait slow cooling The microstructural changes which take place during tempering are very complex Time temperature cycle chosen to optimize strength and toughness Example: For tool steel, As quenched (R c 65) Tempered (R c 45-55) 10

11 Temperature Hardness (R c ) 10/11/2011 Harness of Martensite as a function of Carbon content % Carbon Properties of 0.8% C steel Constituent Hardness (R c ) Tensile strength (MN / m 2 ) Coarse pearlite Fine pearlite Bainite Martensite 65 - Martensite tempered at 250 C It is an operation or combination of operations involving heating a metal or alloy in its solid state to a certain temperature, holding it there for some times, and cooling it to the room temperature at a predetermined rate to obtain desired properties. holding heating Time cooling All basic heat-treating processes for steels involve the transformation of austenite. the nature and appearance of these transformation products determine the physical and mechanical properties of heat treated steels. 11

12 Heating Period Heating steel to above upper critical temperature (A 3 or A cm ) to form singlephase austenite. Rate of heating is usually less important, except for [1] highly stressed materials, or [2] thick-sectioned materials. Holding / Soaking Period Holding at the austenitising temperature for complete homogenisation of structure. Usually 1 hour per 1 inch section is enough for holding. Cooling Period Cooling rate that determines the nature of transformation products of austenite. Depending on cooling rate, heat treatment of steels are classified as: [1] annealing, [2] normalising, and [2] hardening. Annealing is a generic term denoting a treatment that consists of heating to and holding at a suitable temperature followed by cooling slowly through the transformation range, primarily for the softening of metallic materials. Generally, in plain carbon steels, full annealing (commonly known as annealing) produces coarse ferrite-pearlite structures. Purposes of annealing Annealing of Steels Refining grains Inducing ductility, toughness, softness Improving electrical and magnetic properties Improving machinability Relieve residual stresses 12

13 Normalising of Steels Normalising is done by heating the steel to above the upper critical, followed by slow cooling to room temperature in still air. Cooling rate is no longer under equilibrium conditions Purposes of normalising: Phase diagram cannot predict the proportions of phases. There will be less pro-eutectic constituent and more pearlite. Finer and stronger pearlite produced. Modifying and refining cast dendritic structure Refining grains and homogenising the structure Inducing toughness Improving machinability ANNEALED coarse lamellar pearlite Cementite Ferrite NORMALISED medium lamellar pearlite Temperature for Annealing and Normalising Schematic on selection of heat treatment temperatures 13

14 Hardening of Steels Hardening is done by heating the steel approximately to 50 C above the upper critical temperatures (A 3 line) (for hypeutectoid steels). 50 C above the lower critical temperatures (A 3,1 line) (for hypereutectoid steels) followed by drastic cooling to room temperature. Purposes of hardening: to improve hardness to improve wear resistance Hardening of Steels Martensite appears microscopically as needle or acicular structure, sometimes described as a pile of straw. Martensite needles (black) in retained austenite (white background) 14

15 Hardening of Steels Quenching medium Severity of cooling medium influences the cooling rate. Air slow cooling rate low hardness Oil moderate cooling rate moderate hardness Water fast cooling rate high hardness Part geometry Thicker the sample, more variation in the cooling rate between the centre and surface of the sample. Centre slow cooling rate low hardness Surface faster cooling rate high hardness Alloy Content Addition of alloying elements slows down the diffusion process, thereby making it easier for the steel to form martensite. Tempering of Steels In the as-quenched condition, the steel is too brittle for most applications. The formation of martensite also leaves high residual stresses in the steel. Tempering is done almost immediately after hardening to relieve residual stresses and to improve ductility and toughness. The increase in ductility is attained at the sacrifice of some hardness or strength. In tempering, the hardened steel is heated and held to a temperature (which is below the lower critical), and then cooled to room temperature. The selection of heating temperature depends upon desired properties. 15

16 Tempering of Steels During tempering, the excess carbon atoms, trapped in martensite, gradually come out as extremely fine cementite particles and the metastable BCT martensite transforms into stable BCC ferrite. The resulting microstructure (fine cementite dispersed in ferrite matrix) is called tempered martensite. Tempering of Steels Effect of tempering temperature on mechanical properties of a 1050 steel 16

17 Any Question? 17