Cooling down a metal Flashcards
What is the meaning of cooling down a metal
When metals cools down the atoms group together and lattice forms in several different locations at the same time. Each of these lattices has its own orientation and as the sooling continues we have several lattices in different directions forming the GRAINS.
What is grains and its purpose
The grains are formed from the lattices. The smaller grains the stronger material and that is obtained from rapid cooling.
What is the consequence of rapid cooling?
The rate of the cooling influences the formation of the different phases. Rapid cooling results in the formation of martensite, while slow cooling results in formation of perlite and bainite.
Cooling affects the grainsize while fast cooling makes them smaller.
What is Quenching?
A process to cool down a metal by placing it in colder water or oil. The process goes quite fast and avoids the occurence of diffusion driven transformation (eutectoid transformation) the phase martensite appears.
What is TTT diagram?
A TTT diagram Represents isothermal (constant temperature) transformations.
The steel is rapidly cooled to a specific temperature and held at that temperature.
If the cooling rate is fast enough, martensite transformation only depends on the temperature, while perlite depends on time and temperature and this is why we want a fast cooling for martensite.
What influences the TTT diagram. What happens with an increased carbon content when the metal is cooled down?
Different carbon content influences the TTT diagrams.
When carbon content increases:
* The field of metastable austenite widens
* Ms and Mf lower
* The perlite transforms later
* Adding alloying elements to the plain carbon composition always shifts the TTT diagram to the right - makes it easier to obtain martensite.
What the CCT diagram?
The CCT diagram shows the effects of cooling rate in steel as a function of temperature.
What is hardenability an hardenability band?
Hardenability: The capability of a steel to achieve the deepest hardening effect.
Hardenability band: is a graphical representation that illustrates the ability of a steel alloy to be hardened through quenching over a range of section sizes or thicknesses. It depicts the relationship between the critical cooling rate required to achieve a specific level of hardness and the section size of the steel component.
Steeper Hardenability Band: A steeper band indicates a more sensitive steel to cooling rates. A small change in cooling rate can result in a significant difference in hardness.
Shallower Hardenability Band: A shallower band indicates a less sensitive steel to cooling rates. The hardness will be more uniform throughout the part, even with variations in cooling rates.
Hardness at Different Distances: The hardness generally decreases with increasing distance from the quenched end due to slower cooling rates at greater distances.
What is the tempering curve? What is the tempering process?
Tempering curve shows the relationship between tempering temperature and the release of residual stresses or hardness of the steel.
Reheating the quenched martensite and then quenching it again for a lower temperature.
Less carbon content = softer metal
High Tempering Temperature (Right Side): At higher tempering temperatures, the quenched microstructure partially transforms from martensite to softer phases like ferrite and cementite. This reduces hardness (or increases tempering if representing stress) as the microstructure becomes less brittle.
Low Tempering Temperature (Left Side): At lower tempering temperatures, there’s minimal change in the microstructure, and the hardness (or residual stress) remains relatively close to that of the quenched state.
Explain the phenomenon embrittlement during tempering
When steel looses its toughness after hardening.
350 degree steel is a forbidden temperature to temper your steel → will become glass.
Very ductile metal - 600 degree
Very brittle metal - 200 degree
What is happening before the heat treatments?
All treatments start at room temperatures and goes up to A3
Depending on the metal you are working with, above A3 it starts soaking → austenization → expansion of the grain size. → soaking time no longer than 1 hour, otherwise very brittle and weak material.
After soaking you can decide what heat treatment to do
Describe the heat treatments annealing, normalizing and quenching
- Annealing
Heating up to austenitizing (above A3 or Acm) and then slow cooling into a furnace. TARGET: workability - Normalizing
The process involves heating the steel to a temperature above its critical transformation temperature (typically around 800-950°C, or 50-80°C above A3, depending on the steel composition), holding it at this temperature for a specific period, and then allowing it to cool in still air.
Target: refine the grain structure and improve the mechanical properties of the material.
Remember: as the carbon content increase, the working temp range gets higher. What type of phase we obtain depends on the carbon content
- Quenching
Rapidly cool a heated metal component, typically from a high temperature, by immersing it in a quenching medium, such as water, oil, or polymer solution.
What is complete annealing and Spheroidizing annealing?
Complete annealing: Heating up to austenitizing (above A3 or Acm) slow cooling into a furnace TARGET: workability
Spheroidizing annealing: heating slightly above or slightly below Ac1 or by a pendulum across A1 ; subsequent slow cooling in furnace TARGET: workability & better microstructure for following HT
How are the grainsizes affected by the austenization temperarure during heat treatment?
- The higher is the soaking temperature the bigger is the grain size achieved
- The higher is the cooling rate the finer becomes inter-lamellar space in pearlite.
- If you use to high temperature for to long time you risk to get to large grains.
