Heat Treatments - Part 2 Flashcards
What effects on a steel will softening have?
- Reduce strength
- Remove residual stresses
- Improve toughness
- Restore ductility
- Refine grain size
List four softening processes.
- Annealing
- Spheroidising
- Normalising
- Tempering (Austempering and Martempering)
What is annealing?
Heating to a desired temperature, soaking at that temperature and then cooling to room temperature.
What changes does annealing make to a steel?
Annealing changes the shape, size and distribution of the grain structure.
What are the 4 types of annealing?
- Stress-relief annealing
- Normalising
- Isothermal annealing
- Spheroiding annealing
What is stress-relief annealing?
Annealing below the transformation temperature to reduce internal residual stresses without intentionally modifying its structure and mechanical properties. Recrystallisation may take place.
For plain and low carbon steels the temp is 450-650C. For how working tool steels and high spped steels, the temp is 600-750C.
Note: during the process, machining allowance should be sufficient to compensate for any warping resulting from stress relieving.
State and describe the three stages of annealing.
- Recovery - stresses and relieved and dislocations start to move
- Recrystallisation - deformed grains replaced by new strain-free grains, which nucleate and grow until the original grains are entirely consumed
- Grain growth - microstructure coarsens
What is the recrystallisation temperature?
The temperature at which recrystallisaion just reaches completion in 1 hour. It is typically between one third and one half of the melting temp of the alloy.
List three causes of residual stresses.
- Thermal Factors - caused by temp gradients within the material during heating or cooling e.g. quenching.
- Mechanical Factors - cold-working
- Metallurgical Factors - transformation of the microstructure
How would you reduce residual stress?
Residual stress can only be reduced by a plastic deformation in the microstructure - which means the yield strength must be lowered below the value of residual stress.
The lower the yield strength the greater the plastic deformation and the greater the possibility of reducing residual stress.
Yield strength and the ultimate tensile strength of steel both decrease with increasing temperature.
Explain Normalising.
Steel is normalised to refine grain size (make structure more uniform) or to improve machinability. Usually grains have become coarse as a result of being heated to a high temperature e.g. for forging or welding.
The steel is heated to above the austenitising temperature, but cooling is accomplished by natural air.
What are the similarities and differences between normalising and annealing heat treatments?
- The normalising holding temperature is higher.
- Normalising cooled by air, annealing is cooled in a furnace. Therefore normalising cooling rate is faster.
- Normalising kept at holding temperature for longer
- Same heating rate
- Both heated to fully austenitic
- Both form pearlite and ferrite
What is Spheroidising?
Spheroidising converts medium and high carbon steels into ductile and machinable alloys. The carbon is heated at temps below the eutectoid, followed by a slow cooling process.
List 3 applications for the spheroidising heat treatment.
- Railroad tracks
- Bridge cables
- Tyre cords
What is tempering?
Tempering is reheating a quenched steel to a suitable temperature below transformation temperature for an appropriate time, and then cooling back to room temperature.
It is done to make martensite more ductile, with a small loss in strength.
