Chapter 6 Flashcards
What is Heat Treatment?
It is a combination of heating and cooling operations on a metal or alloy in its solid state to produce desired properties
What are the three General Heat Treatment cycle?
- Austenitising (Heating)
- Soaking (Holding)
3.Quenching
What are the effect on alloying elements on Austenite Transformation?
Effect on Cooling Rate - Steel becomes easier to harden
Effect on Heat Treatment Temperature - Austenite Temperature will be raised by carbide formers
Austenite Temperature will be lowered by Ni and Mn
All elements will lower the MS and MF temp.
Effect on Shape of TTT diagram - 2 Extra Noses will appear on the graph
Effect on complex carbides - Complex carbides are very difficult to dissolve in austenite hence requiring a higher heating temp
What causes Retained austenite (RA)?
Large amount of carbons and alloying elements in the austenite that will depress the MS and MF temperature.
If the MF falls under room temperature, quenching will only transform some of the austenite to martensite
What are the effect on properties of RA?
Reduces overall hardness of the steel and cracks may occur.
Steel becomes dimensionally unstable
How to Eliminate retained austenite?
By double tempering and subzero tempering
What is the process of double tempering?
First Temper - Temper the martensite after which the cooling process after tempering will transform some of the retained austenite to martensite
Second Temper- Tempers the newly formed martensite and the process is repeated if necessary
What happens to steels when they are cooled?
Steel goes through isothermal contraction when cooled and the surface cools down faster than the core of the steel.
How are residual stresses produced?
Residual stresses are produced by non-uniform contraction which then relieved by plastic deformation hence distortion occurs
What will result in the cracking of steels during heat treatment
When the steel cools down from MS to MF, Austenite will transform the Austenite to martensite(BRITTLE) which will result in cracking.
What are some ways to prevent distortion and cracking during heat treatment?
1) Ensure that the component is well secured in the furnace
2) Avoid using stress raisers
3) To counter size effect use martempering
What are the steps of Martempering?
1) Quench in metal bath kept at temp above MS and MF.
2) Hold for a short time to equalize the surface and core temp.
3) Air cool from Ms to MF and temper steel
What are the limitations of Martempering and Austempering?
Restricted to small components
What are the steps for Austempering(Bainite hardening)
1) Quench in metal bath kept at temp above MS and MF
2) Hold for sufficient time to allow isothermal transformation of austenite to bainite
3) Air cool the steel
How is surface hardness obtained
By diffusing an element into steel, surface volume increases.
