Heat Treatment • Heat treatment – controlled heating and cooling of metals for the purpose of altering their properties • at least 90% of all heat-treating operations are carried out on steel. • Heat treatment uses: • Increase strength • Improving machining characteristics • Reducing forming forces and energy consumption • Restoring ductility
Heat Treatment • Hardening • Two-step process: • Heating above a critical temperature • Rapid cooling (quenching) • Effect of carbon content of steel on hardness • 0 to 0.3 percent: not practical to harden • 0.3 to 0.7 percent: hardness obtainable increases very rapidly • above 0.7 percent: hardness obtainable increases only slightly with increased carbon content
Heat Treatment • for most purposes, 0.35 to 0.65 percent carbon produces high hardness and gives fair toughness, which is lost if high carbon content is used • Quenching media • oil: mild quenching • water: cheap, fairly good; vaporizes easily, forms air bubbles causing soft spots • brine: more severe than water; may lead to rusting hydroxides: very severe quenching
Heat Treatment • Hardenability - 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.
Heat Treatment • Annealing - used to reduce hardness, alter toughness, ductility, or other mechanical or electrical properties • Full annealing: results in a soft and ductile material Full Annealing Process involves: • Heating for a period of time • Slow cooling
FULL ANNEALING - Imposes uniform cooling conditions at all locations which produces identical properties • Steps: • Metal is heated. • Hypoeutectoid (<0.77% Carbon): 30-60oC above the A3 line • Hypereutectoid steels (>0.77% Carbon): 30-60oC above the A3 line • 2. 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 A1 • 4. Metal is air cooled to room temperature.
Heat Treatment • Normalizing Normalizing involves: • Heating • Cooling in still air
NORMALIZING • Cooling is non-uniform, resulting to non-uniform properties • Steps: • 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.
PROCESS ANNEAL • Used to treat low-Carbon Steels (<0.25% Carbon) • Metal produced is soft enough to enable further cold working without fracturing • Steps: • Temperature is raised slightly below A1. • Held in this temperature to allow recrystallization of the ferrite phase. • Cooled in still air at any rate.
STRESS-RELIEF ANNEAL - Reduces residual stress in large castings, welded assemblis and cold-formed parts • Steps: • Metals are heated to temperatures below A1. • Temperature is held for an extended time • Material is slowly cooled.
SPHEROIDIZATION - Produces a structure where the cementite is in form of small spheroids dispersed throughout the ferrite matrix • Three ways: • prolonged heating at a temperature below the A1 then slowly cooling the material • cycling between temperatures slightly above and below the A1 • for high-alloy steels, heating to 750-800oC or higher and holding it for several hours
no significant phase transformations like that of steel • Three purposes: • 1. produce a uniform, homogenous structure • 2. provide stress relief • 3. 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
Stress-relief annealing – reduces tendency for stress-corrosion cracking • Tempering – reduce brittleness, increase ductility and toughness, reduce residual stress • Austempering – provides high ductility and moderately high strength • Martempering – lessens tendency to crack, distort and develop residual stresses during heat treatment • Ausforming – ausformed parts have superior mechanical properties
Heat Treatment Furnaces • batch furnace • insulated chamber • heating system • access door
Heat Treatment Furnaces • continuous furnace • parts are heat treated continuously through the furnace on conveyors or various designs that use trays, belts, chains and other mechanisms
Design Considerations • Heat treating • Part design • Sharp internal or external corners • Quenching method • nonuniform cooling • Thickness, holes, grooves, keyways, splines, asymmetrical shapes, • Cracking and warping