Heat Treatment Flashcards
Explain annealing
Where a material is exposed to a elevated temp for an extended period of time then slowly cooled to room temp
Why is the annealing cooling process slow
To avoid warping/cracking
What is the purpose of annealing
To relive internal stress
Increase ductility, toughness and softness
Produce specific microstructure
What is process annealing
Used to soften and increase ductility of a previously strain hardened metal.
Used during fabrication that require plastic deformation to prevent fracture
What is stress relief annealing
Stresses may develop, distortion and warping
The sample is heated and held to attain uniform temp before cooling
What is normalising
Sample heated to at least 50 degrees above the upper critical temp.
After sufficient time has been allowed for alloy to completely transform to austenite, the treatment is terminated by cooling in air
Purpose of normalising
Used to decrease the average grain size and produce a more uniform and desirable size distribution
Fine grains are tougher
What is full annealing
Sample heated to form austenite, 50 degrees above upper critical temp
Alloy then furnace cool which takes several hours
Forms coarse Pearlite that is relatively soft and ductile
Full annealing time consuming yet forms microstructure having small grains and a uniform grain structure
What is spherodising
Prolonged heating to 15-25 hours just below the eutectoid temp
Results in soft spherodite structure
What is quenching
Heating to austenite temp and then cooling fast enough to avoid the formation of ferrite, Pearlite or bainite to Obtain pure martensite
What does martensite hardness depend on
Carbon content of the steel
What is tempering
Reheating martensite steels to temps between 150-509 to force some carbide precipitation
Why do we temper
Increases the ductility and toughness of martensite
Hardness and strength is lost
What is martempering
Rapid cooling and held until the inner and outer core of the steel are the same temp. Then cooled
Low carbon steels
0.1 - 0.25 % C Pro eutectoid ferrite and Pearlite High ductility and formability Low strength, can’t be strengthened Pipes, sheets panels, wires
Medium carbon steel
0.25 - 0.55 % C
Good combination of strength and ductility
Strengthened by treatment
Bolts, gears and cranes
High strength low allow steels
Carbon content reduced
Loss of strength compensated by increasing Mn content and by microalloying
Large welded structures, oil rigs, ocean liners
High carbon steel
0.6 - 0.8 % C
Eutectoid Pearlite, strengthened and hardened by treatment
High strength moderate toughness
Tool steels
0.8 - 1.2 % C Pro eutectoid cementite and Pearlite High hardness and low toughness Hammers, knives and tools Poor weldability and machinability
Name the three basic classes of stainless steel
Ferritics Chromium and iron alloy 12 - 25 % C Cheap Martensitics Iron chromium alloy Hard cutting materials Austensitics Iron chromium alloy Corrosion resistant
Cast irons
Content between 2- 5.3% C
Tertiary alloy system with the third element being silicon
White cast iron
Relatively less carbon and silicon
Cool rapidly and do not have enough silicon the cast iron solidifies as white
Cementite makes the alloy hard and brittle
Intermediate product for producing malleable irons
Slurry pipe elbows
Grey cast irons
2-3% silicon
Cool the iron reasonably slow and silicon will cause carbon to form as graphite flakes
Adding more silicon gets carbon out of austenite and into flakes
Cheap, castable
Unsuitable for taking tensile loads
Malleable cast iron
Heat white cast iron above critical line, convert carbide to graphite which will produce a rough climb of graphite
Stronger, more ductile than gray cast iron
Expensive
Gear box casing
