HEAT TREATMENTS OF FERROUS ALLOYS

HEAT TREATMENT

Heat 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 strength Improving machining characteristics Reducing forming forces and energy consumption Restoring ductility

HEAT TREATMENT

1) HardeningTwo-step process:

1. Heating above a critical temperature2. 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

2) Annealing - used to reduce hardness, alter toughness, ductility, or other mechanical or electrical properties

Full annealing: results in a soft and ductile materialFull Annealing Process involves:1. Heating for a period of time2. Slow cooling

FULL ANNEALING

- Imposes uniform cooling conditions at all locations which produces identical properties

Steps:

1. Metal is heated.

Hypoeutectoid (<0.77% Carbon): 30-60oC 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

NormalizingNormalizing involves:1.Heating 2.Cooling in still air

NORMALIZING

- Cooling is non-uniform, resulting to non-uniform properties

Steps:

1. Metal is heated 60oC above line A1.

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 A1.

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 A1.

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 matrixThree ways:

1. prolonged heating at a temperature below the A1

then slowly cooling the material

2. cycling between temperatures slightly above and below the A1

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

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