Heat Treatment (8)

Question 1

Why Heat-Treatment?

Answer 1

• Achieve specific desirable properties

- Initiate Allotrophic change

Question 2

Austenization

Answer 2

Heating of iron and iron-based metals to a temperature high enough to produce changes in the crystal structure from Ferrite to Austenite.
- Capable of dissolving up to 2% Carbon

Question 3

Recovery

Answer 3

Heated to around 250°C

Material has gradual reduction in internal stress, increase in hardness

Question 4

Relaxation

Answer 4

Heated to around 650°c to allow atoms in the structure to re-arrange into a more uniform structure

Question 5

Tempering

Answer 5

Follows hardening operations, to reduce brittleness, improving ductility while also retaining hardness and strength.

• Heating steel below the transformation temperature
• Allows trapped carbon to diffuse

Question 6

Spherodizing

Answer 6

Spheroidite is the result of heating Carbon Steel into the range of 700°C temperatures, and holding it for 30 hours.
- Decrease hardness, increase ductility in High-Carbon Steels

Question 7

Recrystallization and Allotropic change

Answer 7

Above stress relief temperatures Yield Strength, Tensile Strength and Hardness rapidly decrease.

• Softer + finer Grain Structure forms
• If Cold-Worked Recrystallization will occur
• Cold-Worked crystal Grains break into smaller ones with a Non-Uniform Lattice Structure.

Question 8

Annealing

Answer 8

From Recrystallization and Allotropic change, newly formed Grains absorb old ones and replace them.

• Steel heated to 38°C above the critical temperature, held 1 hour for every inch of maximum part thickness
• Cooled slowly 20°C per hour until the temp has dropped below the knee of Time Temperature Transformation Diagram

Question 9

Normalizing

Answer 9

Max temp is 55°C above upper critical line and held long enough for Recrystallization of the Austenite to occur.

• Material is removed and cooled in still air
• After Casting, Forging, Rolling to refine Grain Size
• Creates greater Hardness and higher Tensile Strength steel than Full-Annealing due to Pearlite formation

Question 10

Quenching (Hardening)

Answer 10

Heated to remove Ferrite through transformation to Austenite and then quickly cooled to form Martensite
- Rate of cooling = Hardness of material

Question 11

Austempering

Answer 11

To form pure Bainite, quenched in molten metals or salts, cooled quickly to avoid formation of Martensite or Pearlite, held at Bainite temperature until uniform structure is formed, air cooled.

Question 12

Martempering

Answer 12

Similar to Austempering but cooled to a lower temperature that is above the Martensite formation temperature. Air cooled to create Martensite

Question 13

Age-Hardening

Answer 13

Aluminium, copper, nickel, titanium and some stainless steels cannot behardened by the same Carbon Steel heat treatments. Relies in difference in Alloy solubility temperatures.

Three stages

• Solution Treatment - Temp is raised and kept to allow Alloying elements to move into lattice structure.
• Quenching - Quenched to create unstable super saturated solution.
• Aging - Alloys out of solution form a new phase of small particles that create an obstacle to dislocation movement, hardening material.