BABARIA INSTITUTE OF TECHNOLOGY DESIGNED BY: HUSAIN Y MALEK. E. NO:140050119038 ROLL NO:14ME38

Heat Treatment on steel

HEAT TREATMENT Heat treatment is a method used to alter the physical, and sometimes chemical properties of a material. The most common application is metallurgical It involves the use of heating or chilling, normally to extreme temperatures, to achieve a desired result such as hardening or softening of a material It applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally

HEAT TREATMENT Fundamentals: Fe-C equilibrium diagram. Isothermal and continuous cooling transformation diagrams for plain carbon and alloy steels. Microstructure and mechanical properties of pearlite, bainite and martensite. Austenitic grain size. Hardenability, its measurement and control.

TTT diagrams TTT diagram stands for “time-temperature-transformation” diagram. It is also called isothermal transformation diagram. Definition: TTT diagrams give the kinetics of isothermal transformations. TTT diagram is a plot of temperature versus the logarithm of time for a steel alloy of definite composition. It is used to determine when transformations begin and end for an isothermal heat treatment of a previously austenitized alloy. TTT diagram indicates when a specific transformation starts and ends and it also shows what percentage of transformation of austenite at a particular temperature is achieved.

Determination of TTT diagram for eutectoid steel Davenport and Bain were the first to develop the TTT diagram of eutectoid steel. They determined pearlite and bainite portions whereas Cohen later modified and included MS and MF temperatures for martensite. There are number of methods used to determine TTT diagrams. These are salt bath (Figs. 1-2) techniques combined with metallography and hardness measurement, dilatometry (Fig. 3),electrical resistivity method, magnetic permeability, in situ diffraction techniques (X-ray, neutron), acoustic emission, thermal measurement techniques, density measurement techniques and thermodynamic predictions. Salt bath technique combined with metallography and hardness measurements is the most popular and accurate method to determine TTT diagram.

Fig. 1 : Salt bath I -austenitisation heat treatment. Fig. 2 : Bath II low-temperature salt-bath for isothermal treatment.

Fig. 3(b) : Dilatometer equipment Fig . 3(a): Sample and fixtures for dilatometric measurements

Determination of TTT diagram for eutectoid steel In bath I number of samples are austenitised at AC1+20-40°C for eutectoid and hypereutectoid steel, AC3+20-40°C for hypoeutectoid steels for about an hour. Then samples are removed from bath I and put in bath II and each one is kept for different specified period of time say t1, t2, t3, t4, tn etc. After specified times, the samples are removed and quenched in water. The time taken to 1% transformation to, say pearlite or bainite is considered as transformation start time and for 99% transformation represents transformation finish. On quenching in water austenite transforms to martensite. But below 230°C it appears that transformation is time independent, only function of temperature

Therefore after keeping in bath II for a few seconds it is heated to above 230°C a few degrees so that initially transformed martensite gets tempered and gives some dark appearance in an optical microscope when etched with nital to distinguish from freshly formed martensite (white appearance in optical microscope). Followed by heating above 230°C samples are water quenched. So initially transformed martensite becomes dark in microstructure and remaining austenite transform to fresh martensite (white). Quantity of both dark and bright etching martensites are determined. Here again the temperature of bath II at which 1% dark martensite is formed upon heating a few degrees above that temperature (230°C for plain carbon eutectoid steel) is considered as the martensite start temperature (designated MS). The temperature of bath II at which 99 % martensite is formed is called martensite finish temperature ( MF). Transformation of austenite is plotted against temperature vs time on a logarithm scale to obtain the TTT diagram. The shape of diagram looks like either S or like C.

Hardness Temperature Log time Fig.4: Time temperature transformation (schematic) diagram for plain carbon eutectoid steel 100 T2 At T1, incubation period for pearlite=t2, Pearlite finish time =t4 Minimum incubation period t0 at the nose of the TTT diagram, T1 % of Phase 50% Ae1 Austenite +pearlite T2 Pearlite start Coarse Pearlite 50% Transformation Pearlite finish Pearlite T1 t1 t2 t3 t4 Fine pearlite t5 t0 Hardness 50% very fine pearlite + 50% upper bainite MS=Martensite start temperature M50=temperature for 50% martensite formation MF= martensite finish temperature Austenite+upper bainite Upper bainite Temperature Metastable austenite Bainite finish Bainite start Austenite +lower bainite Lower bainite MS, Martensite start temperature M50,50% Martensite Metastable austenite +martensite MF, Martensite finish temperature Martensite Log time

Fig. 7(a) :Schematic TTT diagram for plain carbon hypoeutectoid steel Ae3 γ=austenite α=ferrite CP=coarse pearlite P=pearlite FP=fine pearlite UB=upper Bainite LB=lower Bainite M=martensite MS=Martensite start temperature M50=temperature for 50% martensite formation MF= martensite finish temperature γ+α Ae1 α+CP α+P γ+P FP t0 Hardness FP + UB Temperature UB Metastable γ LB MS M50 Metastable γ + M MF M Log time

Fig. 7(b): Schematic TTT diagram for plain carbon hypereutectoid steel Aecm γ=austenite CP=coarse pearlite P=pearlite FP=fine pearlite UB=upper Bainite LB=lower Bainite M=martensite MS=Martensite start temperature M50=temperature for 50% martensite formation γ+Fe3C Ae1 Fe3C+CP γ +P Fe3C+FP t0 very FP +UB Hardness UB Temperature γ+UB Metastable γ γ+LB LB MS M50 Metastable γ + M Log time

Fig. 5(b): Optical micrograph showing colonies of pearlite. Fig. 5(a) : The appearance of a (coarse) pearlitic microstructure under optical microscope. Fig. 5(b): Optical micrograph showing colonies of pearlite. 14

Fig. 5(c): Transmission electron micrograph of extremely fine pearlite. Fig. 5(d): Optical micrograph of extremely fine pearlite from the same sample as used to create Fig. 5(c). The individual lamellae cannot now be resolved. 15

Upper bainite (550-350°C) Lower bainite (350-250°C) Martensite

What’s a CCT Diagram? Phase Transformations and Production of Micro constituents takes TIME. Higher Temperature = Less Time. If you don’t hold at one temperature and allow time to change, you are “Continuously Cooling”. Therefore, a CCT diagram’s transition lines will be different than a TTT diagram.

Slow Cooling Time in region indicates amount of microconstituent!

Medium Cooling Cooling Rate, R, is Change in Temp / Time °C/s

Fast Cooling This steel is very hardenable… 100% Martensite in ~ 1 minute of cooling!

TTT diagram gives Nature of transformation-isothermal or athermal (time independent) or mixed Type of transformation-reconstructive, or displaciv Rate of transformation Stability of phases under isothermal transformation conditions. Temperature or time required to start or finish transformation Qualitative information about size scale of product Hardness of transformed products

Types of heat treatment on steel We have noted that how TTT and CCT diagrams can help us design heat treatments to design the microstructure of steels and hence engineer the properties. Thermal (heat treatment) Or a combination (Thermo-mechanical, thermo-chemical) Treatments Mechanical Chemical Bulk Surface

An overview of important heat treatments BULK SURFACE ANNEALING HARDENING & TEMPERING THERMAL THERMO- CHEMICAL NORMALIZING Full Annealing Carburizing MARTEMPERING Flame Recrystallization Annealing Induction Nitriding AUSTEMPERING Stress Relief Annealing LASER Carbo-nitriding Spheroidization Annealing Electron Beam

Ranges of temperature where Annealing,Normalizing and Spheroidization treatment are carried out for hypo- and hyper-eutectoid steels. Full Annealing 910C Acm Normalization Normalization A3 Full Annealing 723C A1 Spheroidization Stress Relief Annealing  T Recrystallization Annealing Wt% C 0.8 %

Recrystallization Annealing Full Annealing The steel is heated above A3 (for hypo-eutectoid steels) | A1 (for hyper-eutectoid steels) → (hold) → then the steel is furnace cooled to obtain Coarse Pearlite Coarse Pearlite has ↓ Hardness, ↑ Ductility Not above Acm → to avoid a continuous network of proeutectoid cementite along grain boundaries (→ path for crack propagation) Recrystallization Annealing Heat below A1 → Sufficient time → Recrystallization Cold worked grains → New stress free grains Used in between processing steps (e.g. sheet rolling)

Stress Relief Annealing Annihilation of dislocations, polygonization Residual stresses → Heat below A1 → Recovery Welding Differential cooling Machining and cold working Martensite formation Spheroidization Annealing / Process Annealing Heat below/above A1 (long time) Cementite plates → Cementite spheroids → ↑ Ductility Used in high carbon steel requiring extensive machining prior to final hardening & tempering Driving force is the reduction in interfacial energy

NORMALIZING Heat above A3 | Acm → Austenization → Air cooling → Fine Pearlite (Higher hardness) Purposes Refine grain structure prior to hardening To harden the steel slightly To reduce segregation in casting or forgings In hypo-eutectoid steels normalizing is done 50oC above the annealing temperature In hyper-eutectoid steels normalizing done above Acm → due to faster cooling cementite does not form a continuous film along GB

HARDENING Quench produces residual strains Heat above A3 | Acm → Austenization → Quench (higher than critical cooling rate) Quench produces residual strains Transformation to Martensite is usually not complete (will have retained Austenite) Martensite is hard and brittle Tempering operation usually follows hardening; to give a good combination of strength and toughness

Schematic of Jominy End Quench Test Typical hardness test survey made along a diameter of a quenched cylinder Variation of hardness along a Jominy bar (schematic for eutectoid steel) Jominy hardenability test

Quenching media Heat Treatments Brine (water and salt solution) Water Oil Air Turn off furnace Heat Treatments A – Normalising B – Annealing or Hardening C – Spheroidising or Process Annealing D - Tempering

Tempering 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 Tool steel: As quenched (Rc 65) → Tempered (Rc 45-55)

MARTEMPERING AUSTEMPERING To avoid residual stresses generated during quenching Austenized steel is quenched above Ms for homogenization of temperature across the sample The steel is then quenched and the entire sample transforms simultaneously Tempering follows Austenite Pearlite Pearlite + Bainite Bainite Martensite 100 200 300 400 600 500 800 723 0.1 1 10 102 103 104 105 Eutectoid temperature Ms Mf t (s) → T →  + Fe3C Martempering Austempering AUSTEMPERING To avoid residual stresses generated during quenching Austenized steel is quenched above Ms Held long enough for transformation to Bainite

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