Heat treatment Flashcards
Why is Annealing used?
- improves ductility
- reduces hardness and strength
- relieve residual stresses
- improve machinability and dimensional stability
How does annealing work?
1) Heat the workpiece to a specific temperature in a furnace
2) Holding it at the temperature for a period of time (soaking)
3) Cooling the workpiece, in air or in a furnace
What is full annealing?
Full annealing is a term applied go ferrous alloys. Structure obtained is coarse pearlite which us soft, ductile and has small, uniform grains
- temp raised slowly to about 50C above the Austenitic Temperature line
- held so all the material to transform into Austenite or Austenite- Cementite
- cooled slowly 20C/hr in a furnace to about 50C into the ferrite- cementite rane
- cooled in room temp air with natural conventions
What is process anneal?
- used for work hardened parts made from cold forming eg extrusion drawing (low carbon steels < 0.25% Carbon
- temp raised fo just below the ferrite- austenite region and held
- ferrite phase recrystallises (stays with the same phase). Size, shape and distribution of grain structure are changed
- cooled in still air - cooling down slowly
- cheaper as not heated to a very high temp or cooled in a furnace
Stress relief annealing?
- Used to reduce or eliminate residual stresses in large castings, welded parts and cold formed parts
- Stresses due to thermal cycling or work hardening, forming, machining, volume changed during phase transformations
- parts are heated to temps up to 600- 650C
- held for an extended time
- slowly cooled in still air
What is normalising?
Used instead sometimes to avoid excessive softness
- raising the temperature to over 60C , above Austenitic temperature line and fully into the Austenite rang
- similar to full annealing but cooled in still air under natural convention
- results in pearlitic structure
- since temperature is higher than for annealing, produces more homogenised structure
- grain size is finer
- cheaper than annealing
Tempering?
- done immediately after quench hardening
- quenched hardened parts are often too brittle there tempering is done to reduce brittleness, increase stability and toughness
- this brittleness is caused bu a predominance of martensite, and the brittleness can be removed by tempering
- the mechanism of tempering depends on the steel and the tempering temperature
- martensite is metastable (somewhat unstable) and will transform to ferrite and cementite on heating
- fine cementite precipitates within the martensite
- the cementite is spheroidized by prolonged tempering to produce a fine distribution of rounded particles
- the heating for tempering is best done by immersing the parts in a medium. Heating in a bath also ensures that the entire part has the same temperature and will undergo the same tempering
Tempered martensite?
- heat to temp below eutectoid
- typically between 250-650C
- tempered martensite is much tougher than normal matensite. Tempering can result in a desired combination of hardness, ductility, toughness, strength and structural stability
- properties can be varied by altering the tempering temperature
Eg very hard tools are often tempered at low temps while springs are tempered to much higher temps
Practical uses of tempered martensite?
- very high strengths can be achieved
- parts can be formed whole soft and them hardened
- properties can optimised
- local hardening possible
Tool steels- resistance to abrasion and deformation
Machine parts and hand tools like hammers
With silicon for spring steels- high yield strength
Practical problems of martensite?
- martensite formation involves 4% volume change and creates high internal stresses
- high cooling rates also create high internal stresses and cracking or distortion of parts is common especially in small sections
- hard to cool large parts fast enough
Different microstructures in the iron-carbon system?
- pearlite
- spheroidite
- bainite
- martensite
- retained austenite
- tempered martensite
Pearlite?
Fine pearlite- if the ferrite and cementite lamellae in the pearlire structure of the eutectoud steel are thin and closely packed
Coarse pearlite - thick and widely spread
The difference between the two depends on the rate of cooling through the eutectoid temperature, which is the site of the a reaction in which Austenite is transformed into pearlite. If rate of cooling is high fine pearlite, if slow coarse pearlite
Bainite?
Visible only through electron microscopy, bainite is a very fine microstructure, consisting of ferrite and cementite, similar to pearlite
Bainite can be produced in steels with alloying elements and at cooling rates that are higher than those required by pearlite
Bainitic steel is generally stronger and more ductile than pearlite at the same hardness level
Martensite?
When austenite is cooled at a high rate, such as by quenching in water, its fcc structure is transformed into a body centred tetragonal, bct, structure called martensite
Because martensite does not have as many slip systems as a bcc structure and the carbon is in interstitial positions, it is extremely hard and brittle.
Martensite transformations takes place almost instantaneously because it involves not the diffusion process but a slip mechanism and thus involves plastic deformation. This is a time dependent phenomenon that is the mechanism in other transformations as well
Retained austenite?
If the temp to which the alloy is quenched is not sufficient enough , only a portion is transformed to martensite. The rest is retained austenite, which is visible as white areas in the structure, along with the dark, needlelike martensite. Retained austenite can cause dimensional instability and cracking and lower the hardness and strength of the alloy
