Chapter 9 Heat Treatment Flashcards
What are heat treatments associated with in Metals and alloys
Annealing
Normalising
Solution treatment
Hardening
What are heat treatments associated with in glasses
Stress reliving
Tempering
What does annealing consist of
Heating the metal to a given temperature
Holding it for a set period of time at that temperature
Slow cooking to room temperature
How does annealing remove the effects of strain hardening via recovery and recrystallisation
Decreasing yield strength
Increasing ductility
Increasing electrical conductivity
What is the annealing point of glades
The point at which annealing takes place
What does relieve tensile stresses in glasses do
It reduces the probability of fracture which would have been a result from forming or cooling from the forming temperature
What does normalising consist of
Heating the material to a given temperature
Holding it for a set period of time at the temperature
Slow cooling to room temperature in still air
What is normalising used to eliminate
Residual stresses from casting, forming or welding ( carried out after rough machining of castings and forgings but before final machining )
What does faster air cooling restrict and why can it be excessive
It restricts grain growth and it can be excessive because the metal becomes less ductile
What is solution treatment carried out on
Precipitation hardening alloys especially non ferrous alloys e.g. Al based alloys
Describe the process of solution treatment
The alloy is heated to high temperature
It then soaks for a short period of time
This dissolves the precipitates to form a solid solution
The alloy is quenched or air cooled to retain the copper in a solid solution
What are the different types of hardening processes
Strain hardening
Precipitation hardening
Quench hardening
Tempering
What does strain hardening do
Strain hardening involves cold forming operations in which their is an increase in strength of metals by strain hardening. It is the only way to harden non ferrous metals and alloys. Involves a decrease in ductility.
Describe the process of precipitation hardening (with reference to Duralumin)
Copper will slowly combine with aluminium to produce a very fine distribution of hard precipitates. Increases strength and decreases ductility. In aluminium alloys the process is slow at room temperature- natural ageing
What is artificial ageing
The process in which precipitation hardening is fastened by heating the alloy to a temperature well below that for solution treatment
What is quench hardening
The heat treatment of steels in which the crystal structure changes
Iron —> increasing temperature
Bcc–> fcc—>bcc
What does quench hardening consist of
Holding medium, high or alloy carbon steels steels at high temperatures (900c) to dissolve carbides in the FCC structure. Then cooling rapidly in brine, water or oil. This gives the structure a high hardness but with no ductility.
When is tempering carried out and what is it used to do to steel
It is carried out after quench hardening and it is used to increase the toughness of steel (accompanied by a decrease in hardness)
What heat is the quenched hardened steel heated to
The tempering temperature is relatively low at 205 - 650
In quench hardening what does hardness achieved depend on
Composition ( carbon content ) Quench rate ( brine > water > oil
Describe the process of tempering glass
Glass is heated to softening point
Glass is cooled rapidly with jets of cold air
This places the surface of the glass into compression
This increases the strength by 4x
