2Heat 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
3Heat 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
4Heat 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
5Heat 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.
6Heat 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
7FULL 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.
8Heat Treatment Normalizing Normalizing involves: Heating Cooling in still air
9NORMALIZINGCooling 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.
10PROCESS 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.
11STRESS-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.
12SPHEROIDIZATION- 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
13no 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
14Stress-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
15Heat Treatment Furnaces batch furnaceinsulated chamberheating systemaccess door