Steel Heat Treatments Flashcards
What are the mechanical properties of Fe3C?
Hard and brittle
Why don’t you want a material to fully turn to Fe3C?
It is too brittle to be useful for engineering but small amounts in alloys are good for strength, just reduces ductility.
What is stronger, coarse or fine pearlite?
Fine, which happens at a lower temperature.
What annealing treatments of pearlite are carried out below A1 temperature?
Subcritical (process) annealing & spheroidising
What annealing treatments of pearlite are carried out above A1 temperature?
Full annealing & normalising
What is process or subcritical annealing and what are the advantages of it?
80-170C below eutectoid (A1) temp.
Used to eliminate effect of cold working in steels less than 0.25wt%C - mild steels
Causes recovery and recrystallisation of the ferrite without changing distribution of pearlite - just gets rid of the dislocations
Advantages: lower temp so cheaper, less risk of distortion, cooling rate doesn’t matter as not producing any austenite
What is spheroidising and what are the advantages?
Used on medium to high carbon steels that contain lots of Fe3C which are difficult to machine
The lamellae pearlite represents a large interfacial area and surface energy
Pearlitic structure is held for several hours below eutectoid (A1) temp.
Fe3C plates adopt a spherical shape to reduce surface energy
Advantages: structure is toughened and more ductile than with full annealing
Disadvantages: loss of strength
What is full annealing and what are the advantages?
Steel is heated into y phase (austenitising), held at temp then cooled very slowly to form a completely new structure
Hypoeutectoid steels - carried out at 30C above A3 in y phase field (austenite)
Hypereutectoid steels - carried out at 30C above A1 in y + Fe3C phase field
Prevents formation of a brittle, continuous film of Fe3C at the grain boundaries
Advantages: good ductility
Disadvantages: lower strength
What is normalising and what are the advantages?
Similar to full annealing - steel is heated above A3, but held for a short time and cooled faster in air
Faster cooling = finer pearlite and higher strength
Easy in industrial environment as furnace can remain at temp however steel may contain greater residual stresses
Advantages: steel can be converted from coarse to fine pearlite solely by thermal treatment without mechanical deformation
What is the main trade off when heat treating martensite?
Martensite is very hard due to the fact its a supersaturated solid solution of carbon in ferrite
We want to increase the ductility without sacrificing too much of the hardness gained
What are the 4 stages of tempering plain carbon steels?
1) up to 250C - carbon precipitates a e-carbide as narrow rods, initial strength increase at 100C
2) 200-300C - decomposition of any retained austenite to untempered martensite
3) 200-350C - e-carbide transforms to Fe3C as plates up to 200nm x 15nm, full loss of tetragonality - matrix is BCC ferrite
4) above 450C - Fe3C precipitates, coarsens and spheroidises, ferrite is recrystallised
