2 Heat TreatmentHeat treatment – controlled heating and cooling of metals for the purpose of altering their propertiesat least 90% of all heat-treating operations are carried out on steel.Heat treatment uses:Increase strengthImproving machining characteristicsReducing forming forces and energy consumptionRestoring ductility
3 Heat Treatment Hardening Two-step process: Heating above a critical temperatureRapid cooling (quenching)Effect of carbon content of steel on hardness0 to 0.3 percent: not practical to harden0.3 to 0.7 percent: hardness obtainable increases very rapidlyabove 0.7 percent: hardness obtainable increases only slightly with increased carbon content
4 Heat Treatmentfor most purposes, 0.35 to 0.65 percent carbon produces high hardness and gives fair toughness, which is lost if high carbon content is usedQuenching mediaoil: mild quenchingwater: cheap, fairly good; vaporizes easily, forms air bubbles causing soft spotsbrine: more severe than water; may lead to rusting hydroxides: very severe quenching
5 Heat TreatmentHardenability - the ability of a steel to develop its maximum hardness when subjected to the normal hardening heating and quenching cycle. A steel is said to have good hardenability when it can be fully hardened with relatively slow cooling.
6 Heat TreatmentAnnealing - used to reduce hardness, alter toughness, ductility, or other mechanical or electrical propertiesFull annealing: results in a soft and ductile materialFull Annealing Process involves:Heating for a period of timeSlow cooling
7 FULL ANNEALING- Imposes uniform cooling conditions at all locations which produces identical propertiesSteps:Metal is heated.Hypoeutectoid (<0.77% Carbon): 30-60oC above the A3 lineHypereutectoid steels (>0.77% Carbon): 30-60oC above the A3 line2. Temperature is maintained until the material transforms to austenite.3. Cooled at a rate of 10-30oC per hour until it reaches about 30oC below A14. Metal is air cooled to room temperature.
8 Heat Treatment Normalizing Normalizing involves: Heating Cooling in still air
9 NORMALIZINGCooling is non-uniform, resulting to non-uniform propertiesSteps:Metal is heated 60oC above line A1.Held at this temperature until material transforms to austenite.Metal is cooled to room temperature using natural convection.
10 PROCESS ANNEALUsed to treat low-Carbon Steels (<0.25% Carbon)Metal produced is soft enough to enable further cold working without fracturingSteps:Temperature is raised slightly below A1.Held in this temperature to allow recrystallization of the ferrite phase.Cooled in still air at any rate.
11 STRESS-RELIEF ANNEAL- Reduces residual stress in large castings, welded assemblis and cold-formed partsSteps:Metals are heated to temperatures below A1.Temperature is held for an extended timeMaterial is slowly cooled.
12 SPHEROIDIZATION- Produces a structure where the cementite is in form of small spheroids dispersed throughout the ferrite matrixThree ways:prolonged heating at a temperature below the A1 then slowly cooling the materialcycling between temperatures slightly above and below the A1for high-alloy steels, heating to oC or higher and holding it for several hours
13 no significant phase transformations like that of steel Three purposes:1. produce a uniform, homogenous structure2. provide stress relief3. bring about recrystallization- process is usually slowly heating the material to moderate temperatures, holding it for a certain time to allow change in desired properties to take place then is slowly cooled
14 Stress-relief annealing – reduces tendency for stress-corrosion cracking Tempering – reduce brittleness, increase ductility and toughness, reduce residual stressAustempering – provides high ductility and moderately high strengthMartempering – lessens tendency to crack, distort and develop residual stresses during heat treatmentAusforming – ausformed parts have superior mechanical properties
15 Heat Treatment Furnaces batch furnaceinsulated chamberheating systemaccess door