Presentation on theme: "H EAT T REATMENTS OF F ERROUS A LLOYS. H EAT T REATMENT Heat treatment – controlled heating and cooling of metals for the purpose of altering their properties."— Presentation transcript:
H EAT T REATMENTS OF F ERROUS A LLOYS
H EAT T REATMENT 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
H EAT T REATMENT 1) Hardening Two-step process: 1. Heating above a critical temperature 2. 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
H EAT T REATMENT 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
H EAT T REATMENT 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.
H EAT T REATMENT 2) 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: 1. Heating for a period of time 2. Slow cooling
FULL ANNEALING - Imposes uniform cooling conditions at all locations which produces identical properties Steps: 1.Metal is heated. Hypoeutectoid (<0.77% Carbon): 30-60 o C above the A 3 line Hypereutectoid steels (>0.77% Carbon): 30-60 o C above the A 3 line 2. Temperature is maintained until the material transforms to austenite. 3. Cooled at a rate of 10-30 o C per hour until it reaches about 30 o C below A 1 4. Metal is air cooled to room temperature.
H EAT T REATMENT Normalizing Normalizing involves: 1. Heating 2. Cooling in still air
NORMALIZING -Cooling is non-uniform, resulting to non-uniform properties Steps: 1.Metal is heated 60 o C above line A 1. 2.Held at this temperature until material transforms to austenite. 3.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: 1.Temperature is raised slightly below A 1. 2.Held in this temperature to allow recrystallization of the ferrite phase. 3.Cooled in still air at any rate.
STRESS-RELIEF ANNEAL - Reduces residual stress in large castings, welded assemblis and cold-formed parts Steps: 1.Metals are heated to temperatures below A 1. 2.Temperature is held for an extended time 3.Material is slowly cooled.
SPHEROIDIZATION - Produces a structure where the cementite is in form of small spheroids dispersed throughout the ferrite matrix Three ways: 1.prolonged heating at a temperature below the A 1 then slowly cooling the material 2.cycling between temperatures slightly above and below the A 1 3.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
H EAT T REATMENT F URNACES batch furnace insulated chamber heating system access door
H EAT T REATMENT F URNACES continuous furnace parts are heat treated continuously through the furnace on conveyors or various designs that use trays, belts, chains and other mechanisms
D ESIGN C ONSIDERATIONS Heat treating Part design Sharp internal or external corners Quenching method nonuniform cooling Thickness, holes, grooves, keyways, splines, asymmetrical shapes, Cracking and warping