1. Heat Treatments of Ferrous Alloys

2. 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

3. 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

4. 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

5. 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.

6. 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

7. 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.

8. Heat Treatment Normalizing Normalizing involves: Heating
Cooling in still air

9. 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.

10. 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.

11. 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.

12. 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 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 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

14. 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

15. Heat Treatment Furnaces
batch furnace
insulated chamber
heating system
access door

17. 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

19. 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