Flashcards in Objective 03: Heat Treatment of Steels Deck (38)
What is the purpose of Heat Treating a metal?
To force a physical and/or chemical transformation in the alloy and then cool it at a rate, and in such a manner, that it retains the desired properties.
What is the main application of Heat Treatment for power engineers and inspectors?
Heat treatments that produce pearlitic microstructures (ferrite, pearlite, and cementite)
What are the effects of Annealing Processes?
1. Homogenize the microstructure
2. Increase ductility
3. Remove residual stresses
4. Improve machinability
What is the process of Annealing?
1. Heat the steel to just above the transition temperature required to produce austenite
2. Hold it at that temperature to allow uniform crystal restructuring
3. Cool it very slowly to room temperature at a maximum rate of 38ºC/hour by leaving it in the furnace with the heat shut off or packing it in sand or another material that is a good heat insulator
What is the effect of annealing high carbon steel?
It can induce brittleness and reduce toughness and ductility
What aspect of the Annealing Proces encourages grain enlargement?
What causes high carbon steels to become brittle in the Annealing Process?
Large grain formation
How does Sub-Critical/Process Annealing differ from Full Annealing?
The steel is heated to just below its austenite transformation temperature and then cooled slowly to reduce internal stresses in the metal
What is the purpose of Normalizing carbon steel with less than 0.8% carbon?
1. Relieve internal stresses embedded in the metal from welding, machining, or forging
2. Refine grain size and promote uniform composition to increase strength and toughness
3. Improve machinability
What is the process of Normalizing?
1. Raise the temperature of the steel to approximately 55ºC above the upper transition temperature into the austenite region
2. The steel is held at that temperature just long enough to ensure even heating throughout
3. It is then allowed to cool in still air at a rate not exceeding 100ºC/hour
If a furnace is used to normalize steel, what precaution should be taken?
The furnace should have a reducing atmosphere (no free oxygen present) to prevent oxide scale forming on the surfaces
What is the benefit of normalizing as compared to any other heat treatment process?
It produces tougher steel
How hard does normalizing make low carbon steels?
Just hard enough to machine freely, leaving the surface free of tears
Any process of prolonged heating and cooling of steel, similar to anealing, that converts the carbide content of the matrix into a rounded or spheroid structure
What characterizes metal that has been spheroidized?
It is the softest and most workable
What is the process of Hardening?
1. Mild steel is heated to a temperature above its transformation range (austenizing)
2. Then it's cooled quickly to increase hardness by the formation of Martensite
Structure of fine carbide needle-like grains that are extremely hard
How is Martensite formed?
They are formed during the transformation from austenite. If the temperature is dropped quickly, the carbon in the austenite does not have time to precipitate as pearlite but instead forms distorted needle-like grains of carbide in the ferrite matrix
Critical Cooling Rate
Cooling during the hardening process
Is the Critical Cooling Rate the same for all materials?
What are different mediums that can be used for cooling?
How can cooling be promoted?
Agitation of the liquid or sample
Heat Treatment process that produces Martensite in the outer layer only, leaving the interior to retain a tough ferrite-pearlite composition
Which metal parts typically undergo Case Hardening?
2. Machine Tools
8. Hand Tools
What are 2 important thermochemical case hardening processes for low alloy steels?
What is the process of Carburizing?
1. The part is heated to its transformation temperature in an atmosphere of carbon monoxide
2. Carbon diffuses into the skin of the metal increasing martensite formation in this area when the part is later quench hardened
What is the process of Nitriding?
1. The part is heated in a furnace at a temperature below the transformation range of iron (approximately 500ºC to 600ºC) in an atmosphere of ammonia
2. At this temperature, ammonia dissociates into nitrogen and hydrogen
3. Atomic nitrogen diffuses into the surface layer of the metal forming iron nitrides which are extremely hard
What distinguishes Nitriding from Carburizing?
It does not require subsequent quench hardening