HEAT TREATMENT -I

Heat treatment Defined as the controlled heating and cooling of metals for the primary purpose of altering their properties (strength, ductility, hardness, toughness, machinability.

Purpose of Heat Treatment To relieve internal stress To improve machinability To refine grain size To soften the metal To improve the hardness To improve mechanical properties To increase resistance to wear, heat and corrosion. To improve ductility and toughness To change the chemical composition.

Who uses Heat Treating ? Aircraft Industry Automobile Manufacturing Defense Sector Forging Foundry Heavy Machinery Manufacturing Powder Metal Industries

Steps in Heat Treating Operation Loading Heating Preheating Soak & diffusion Pre-cooling Cleaning Pre-wash with coalescence De-phosphate system Spray rinse Quenching (Cooling) Post-wash Tempering Surface coating Unloading

Steps in Heat Treatment Requires three basic steps: Heating to a specific temperature Holding (soaking) at that temperature for the appropriate time Cooling according to a prescribed method heating temperature range to 24000F (1316 °C) soaking times vary from a few seconds to 3 to 4 days cooling may be slowly in the furnace or quickly (quenched) into water, brine, oils, polymer solutions, molten salts, molten metals or gases 90% of metal parts are quenched in water, oil, polymers, or gases

Heat Treating Processes

Annealing It refers to a heat treatment in which the material is exposed to an elevated temperature for an extended time period and then slowly cooled. When an annealed part is allowed to cool in the furnace, it is called a "full anneal" heat treatment.

Types of Annealing Full Annealing Process Annealing Stress Relief Annealing Recrystallization Annealing Spheroidise Annealing

Full annealing Soften the metal Relieve the stress Main Objective: Soften the metal Relieve the stress Refine the structure.

Full Annealing Temp is 30 - 50° C above the upper critical temp for hypo eutectoid steel. 30 - 50° C above the lower critical temp for eutectoid steel. Cooling is done at the furnace at the rate of 10-30°C per hour. For hypo eutectoid steel the resulting microstructure is coarse pearlite and ferrite.

For hypereutectoid steel annealing temp is 30-50°C above the lower critical temp. For hyper eutectoid steel the resulting microstructure is coarse pearlite and cementite. This process provides high ductility and toughness.

Stress Relief Annealing Stress relief or recovery annealing. Annealing temp is at the range of 550-700°C. Uniform cooling is mandatory. It eliminates the stress formed during welding, cold working, casting, quenching, machining.

Need for SR Annealing Causes for stress: Plastic deformation during machining Non-uniform cooling Phase transformations between phases with different densities. Effect of Stress War page Crack Distortion

Recrystallization Annealing It is a process in which distorted grains of cool worked material are replaced by strain free new grains. Recrystallization annealing is an annealing process at temperatures above the recrystallization temperature of the cold-worked material, without phase transformation. The recrystallization temperature is not a constant for a material but depends on the amount of cold work, the annealing time, and other factors. T(recrystallization) = 0.4 T (melting)

It reduces the Dislocation density and converts elongated grains to equ axied . (a) after cold worked (b) after recrystallization annealing

Spheroidizing Converts Lamellar Pearlite  Globular Pearlite Plates of Cementite  Spheriods of Cementite

Main objectives of Spheriodising: To soften the steel Increase ductility and toughness Improves machinability and formability Reduces hardness, strength and wear resistance. Materials mainly concentrated Medium carbon steel High carbon (tool steel) Not used for Low carbon Steel

Three ways of Spheriodising Prolong heating below Lower critical temperature and slow cooling.

Cycling between temperature and then relatively slow cooling.

For tool and high speed steel heating at the temperature range between 750° - 800°C then hold at this temperature and then slow cooling.

Normalizing When an annealed part is removed from the furnace and allowed to cool in air, it is called a "normalizing" heat treatment. Main Objectives: To refine grain structure. To remove strains To remove internal stress To remove dislocations To improve mechanical properties( strength, hardness and toughness) To improve machinability of low carbon steels.

Hardening Increases the hardness Heat treatment which is used to produce Martensite predominately. Objective: To improve hardness To improve wear resistance

Hardening Steps: Heating Soaking  Complete γ Cooling.

Factors affecting Hardness Carbon Content Increasing Carbon  Increasing Hardness Quenching Medium Faster the cooling  Greater the hardness. Specimen Size Increase the specimen size Decreases the hardness. Other Factors Geometry Quenching medium Degree of agitation Surface conditions Alloy elements

Tempering

Tempering Martensite Tempered Martensite Heavily Tempered Martensite

Tempering Martensite formed in the hardening process is converted to TEMPERED MARTENSITE To improve ductility and toughness. Objective To improve ductility To improve toughness To reduce hardness To remove internal stress (rapid cooling) To impart wear resistance.

Tempering Heated to 250˚ - 650˚C and cooled slowly to room temp This is also done to relieve internal stress. Martensite(BCT) Tempered Martensite BCT supersaturated carbon TM stable ferrite and cementite Hardness decreases with increasing in tempering time.

Types of tempering Low temp T 150-250˚C Retain hard martensite Relieve internal stress Med TEMP T 350-450˚C Increases endurance limit and elastic limit For spring steel High temp T 500-650˚C On structural steel

TTT of a Eutectoid Steel

TTT

TTT Time temp transformation (or) isothermal transformation (or) S curve (or) C curve (or) Bain’s Curve. It is created three process: Austenitizing Isothermal Heat treatment Quenching Isothermal transformation

TTT 723˚-550˚C Pearlite is formed. Transformation temp decreases pearlite change from coarse to fine. On rapid quenching Martensite is formed. 550˚C- 250˚C  Bainite is formed. Bainite  Non-lamellar eutectoid structure.

TTT Bainite Upper Bainite 550˚- 350˚C  has large rod like cementite regions Lower Bainite 350˚ - 250˚C  has much finer cementite particles.

CCT

CCT The time required for a reaction to start and to end is delayed. The isothermal curves are shifted to longer times and lower temperatures. No transformation below 450˚C.

CCT A Coarse pearlite B Fine Pearlite C Martensite and Pearlite  Split Transformation D Martensite E Tangent to Nose of CCT critical cooling rate 100% Martensite.

Hardenability In a ferrous alloy, the property that determines the depth and distribution of hardness induced by quenching from elevated temperatures Hardenability Vs Hardness:- Hardness : Function of only carbon content Hardenability : C & Alloy content,   grain size, Component size, Quenchant etc

2. Jominy Hardenability (quench) test # Steel Sample: 25mm dia and 100mm long # Heating the test piece to an austenitising temperature and quenching from one end with a controlled and standardised jet of water # Take a sample from the furnace and place it on the Jominy test fixtures and observe the cooling pattern # The cooling rate along the Jominy test specimen varies from about 225 °C s-1 (at bottom) to 2 °C s-1 # After quenching the hardness profile is measured at (0.16 mm) intervals from the quenched end after the surface has been ground back to remove any effects of decarburisation (0.38mm is removed from the surface)

# The hardness variation along the test surface is a result of microstructural variation which arises since the cooling rate decreases with distance from the quenched1e”nd # Measure the distance from the quench end at where the hardness is equal to 54 HRc in the units of 0.16mm

Relationship between the Jominy distance and critical diameters for an ideal quench medium

Martempering

Induction hardening Induced eddy currents heat the surface of the steel very quickly and is quickly followed by jets of water to quench the component. A hard outer layer is created with a soft core. The slideways on a lathe are induction hardened.

Flame hardening Gas flames raise the temperature of the outer surface above the upper critical temp. The core will heat by conduction. Water jets quench the component.