Heat treatments Flashcards
What is spherodizing?
A heat treatment meant to provide more ductility through forming spherical cementite particles instead of lamellae. This increases machinability.
This process relies on diffusion.
What is the recommended temperature for carrying out spherodizing?
Just below the eutectoid point so under 723 degrees.
What are the mechanical properties achieved through spherodizing?
Toughness (ability to absorb energy and deform plastically) and machinability improve.
Strength and hardness decrease.
What is normalization?
A heat treatment meant to reduce grain size to achieve a homogeneous grain size and stronger material.
What is the recommended temperature for carrying out normalization?
Right above the eutectoid point, over 723 degrees.
What is the issue of raising the temperature too much during normalization?
Grain growth can occure - this is not beneficial as the purpose is to reduce grain size.
What is the purpose of normalization?
To strengthen the material by decreasing grain size.
What is martensitic hardening?
After austenitizing quenching fast into the martensite region.
Very hard and brittle material as a result.
What are some issues with martensitic hardening?
It creates a very brittle material with residual stresses.
How can the brittleness of martensite be solved?
Through tempering.
What is tempering?
A heat treatment for brittle materials.
Following quenching, the material is heated up to an intermediate temperature to create a controlled diffusion of carbon. This results in improved wear resistance and a more ductile material, while maintaining hardness and strength.
Why is tempering better than precipitate hardening?
The purpose of precipitation hardening is to increase hardness and strength leading to a hard and brittle material.
However, tempering maintains strength and hardness while increasing ductility and machinability.
Both are meant to harden the material
What is hardenability?
A material’s ability to form martensite.
What is the Jominy end-quench test?
It is meant to test the hardenability of a specimen and how it is formed.
A specimen is austenitized and quenched in water. After that indentations are made along the specimen to test the hardness (Rockwell hardness).
In conclusion cooling rate diminishes away from the tip leading to softer material (atoms had more time to diffuse when not quenched).
Another consequence is a softer core compared to the outer part.
How do alloying elements influence the hardness of a material?
Alloying elements lead to slower transformation behavior (DUE TO THE NEED OF SUBSTITUTIONAL DIFFUSION OF THE ALLOYING ELEMENTS) - they shift the perlite and bainite regions (see figures).
By adding alloying elements a larger hardening depth can be achieved (see figures).
