10 Questions
What is the purpose of martempering in hardened steel?
To reduce thermal stresses
In the process of martempering, when does the cooling stop?
Just before the Ms temperature
What type of microstructure is expected from the austempering process?
Bainite
What is the expected hardness range after tempering tool steel from Rc65 to Rc45-55?
Rc40-45
Which temperature is used for heating in both martempering and austempering processes?
50°C above A3 (Hypoeutectoid)
Which process aims to achieve a microstructure with lower stresses?
Martempering
What type of transformation occurs during austempering?
Isothermal transformation in bainite region
At what temperature does cooling stop before quenching in martempering?
$Ms + 50°C$
Which microstructure is associated with good strength and ductility in this context?
$Bainite$
In austempering, what is the purpose of more than critical cooling before isothermal transformation?
To prepare for bainite transformation
Study Notes
Heat Treatment Processes
- Bulk Annealing: Heating temperature above A3 (Hypoeutectoid) or A1 (Hypereutectoid), cooling in furnace; resulting in coarse microstructure, lower strength, and higher ductility.
- Normalizing: Heating temperature above A3 (Hypoeutectoid) or Acm (Hypereutectoid), cooling in air; resulting in fine microstructure, more strength, and lower ductility.
- Spheroidization Annealing: Heating temperature below A1, cooling not specific; resulting in globular cementite in ferrite matrix, increasing both strength and ductility.
- Stress Relief Annealing: Heating temperature below recrystallization temperature (~550°C), cooling not specific; relieving stresses developed during cold working, machining, or welding.
Hardening and Tempering
- Hardening: Heating temperature above A3 (Hypoeutectoid) or A1 (Hypereutectoid), cooling rapidly; resulting in martensite, high hardness, and poor ductility.
- Tempering: Heating temperature 400-500°C, cooling not specific; resulting in martensite with carbon precipitates, improving ductility at the cost of strength.
Other Heat Treatment Processes
- Martempering: Heating temperature above A3 (Hypoeutectoid) or A1 (Hypereutectoid), cooling rapidly, then quenching; reducing thermal stresses in hardened steel, resulting in martensite with lower stresses.
- Austempering: Heating temperature above A3 (Hypoeutectoid) or A1 (Hypereutectoid), cooling rapidly, then isothermal transformation in bainite region; resulting in bainite, good strength and ductility.
Critical Temperatures
- A1: Isotherm at eutectoid alloying.
- A3: Phase boundary between austenite and two-phase (austenite + ferrite).
- Acm: Phase boundary between austenite and two-phase (austenite + cementite).
Test your knowledge on heat treatment processes such as bulk annealing, normalizing, surface hardening, tempering, mar tempering, and more. Explore different types of annealing techniques and thermomechanical processes. Understand holding temperatures and critical temperatures for various heat treatment methods.
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