Please enable JavaScript.
Coggle requires JavaScript to display documents.
Highlight of EBB225, Improve machinibility and hardness by refine grain…
-
- Improve machinibility and hardness by refine grain and get uniform structure.
- Heat above critical temp(more than annealing)
- Done on medium or high C steel to improve machinability by produce coarse sheroidal cementite particle in ferritic matrix
- Heat to just below eutectoid & hold for a long period of time.
- Relieve stress,increase softness,ductility & toughness and produce uniform microstructure
- Heat to austenization T then phase transormation occcur
- Often in low or medium steel
- negate effect of cold working
- Heating to T lower critical line allows recovery & recrystallization
- Does not austenize and no phase change but change of grain morphology
- No change of microstructure
- Relieve some of entangled dislocation & regain ductility
- Heating to temp lower critical line and uniform cooling in air.
- Austenize for min 1/2 hour
- Quench to temp below Ms
- Reheat to temper to desired properties
- Heat treatment with minimal temp gradient & transformation stress to avoid crack.
- Incubation period above Ms to equalize T & was held in salt bath to reduce thermal stress.
- Holding at T 370 C for a longer period whle it pass through bainite
- Once bainite form,steel quench to RT
- Cooling rate necessary to obtain full martensite(hard)
- accomplish by immersion in water,oil,polymer solution or salt
-
-
- Orthohombic
- metastable phase
- hard & brittle compound
- Form when solubility limit of C in exceed the temperature below 727 C
- FCC
- non-magnetic
- When cooled slowly ,separate into ferrite and cementite
- consist of carbide platelets making up a fine grain structure.
- eutectoid mix of ferrite & cementite.
- Nucleation start with ferrite crystal
- Rapid cool form bainite and martensite
- superior ductility &toughness but lower hardness &strength
- hard & brittle phase
- Form when steel rapidly cool from austenite state
- Nucleation start with carbide crystal.
- Alloy element move the nose of TTT diagram to the right,slower cooling(obtain martensite) and increase hardenability
- Reduce rate of tempering
- Effect phase stability(phase diagram is altered)
Various element(Cr,Mn,Ni) is added to plain carbon steel
- Chemical comosition
- Austenite grain size
-
-
-
-
- BCC structure
- Low carbon content
- soft and ductile
-
-
- At low temp,slow rate of diffusion increase time need or the process to start and complete.
- Pearlite-larger T (Ttrans below TE)make the difffusion faster produce coarser pearlite - Bainite -Diffusion controlled
- Spheroidite -Diffusion dependant
- Below TE = Nucleation low,Growth high
- Way below TE = Nucleation high ,Growth low
- Factor effect:
i)Growth rate increase with degree of supercooling until diffusion slow down the transformation
ii)Microstructure finer when transformation rates increase. iii)acceleration of transformation possible if transfer and matter are fast
