A.E., Cer.E., Chem.E, Met.E., And Phys. 377 Fall 2001 Principles of Engineering Materials

Steels:Part 3Designation Scheme:AISISAE number systemGenerally a 4-digit numberRead as 2 two-digit pairs1080 ten eighty4140 forty-one forty8620 eighty-six twenty

The first two digits represent chemical family10 = plain carbon (no intentional alloy additions)Other = alloyed steel Last two digits designate the points of carbon (i.e. carbon content)One point = 0.01%

1080 = plain carbon steel with 0.80% carbon4140 = alloy steel with 0.40% carbon

Heat Treatment:Any elevated temperature process whose intended purpose is to alter the structure and properties of a material

Typically associated with establishment of final properties

A family of treatments exists to facilitate ease of manufactureWeaken (less force required for shaping)Enhance ductilityEnhance machinabilityEquilibrium conditions produce the weakest, most ductile structuresSlow coolsLong times at elevated temperaturesGuiding Tool: equilibrium phase diagramProcessing Heat Treatments:Full annealNormalizeProcess annealSpheroidizeIsothermal annealAustemper

Processing Heat Treatments:Full annealapplicable to all steelsErase the prior structure by reforming austenite and then transform it under near-equilibrium conditions by slowly cooling in a furnaceRequires lengthy furnace treatment and changes of furnace temperatureGives weakest, most ductile of all possible products (coarse pearlite in eutectoid steel)Produces a uniform structure and properties at all locations since the cooling rate was set by the furnaceProcessing Heat Treatments:Normalizeapplies to all steelsErase the starting structure by reforming austenite and then transform it by removal from the furnace and cooling in still airThe resulting structure is not as weak or as ductile as full anneal (fine pearlite for eutectoid steel)

Processing Heat TreatmentsProcess annealUsed only with low carbon steels (< 0.25 C -- structure is largely ferrite)Recrystallizes cold worked ferrite to restore ductilityStay well below A1 temperatureChanges the form of the microstructure not the microstructure itself. We do not want to transform back to austenite!Processing Heat TreatmentsSpheroidizeUsed to enhance the machinability of high carbon steels, >0.6 C (structures with > 75% pearlite)Heat and hold for a long time below the A1Carbide sheets break up into spheriodal particles of Fe3CSimple Heat Treatments

Isothermal Heat TreatmentsOnly valid use of the T-T-T diagramIsothermal anneal:Isothermal heat treatment below A1 but above the nose to produce pearliteAustemper:Isothermal heat treatment below the nose but above Ms to produce bainiteIsothermal Heat Treatments

Final Heat Treatments:Desire is almost always to strengthenGenerally involves non-equilibrium conditionsRapid cools or quenchesThe equilibrium phase diagrams will not be a useful tool

Quench and Temper Heat TreatmentConsider the plot of Ms and Mf versus carbon content for plain carbon steelsNOTE: For high carbon steels, a sub-zero cooling may be necessary to complete the transformation

Retained austeniteResults from incomplete transformationProduct is soft and weak when we want strongIt can subsequently transformDimensions change with transformation

Residual stressesIn martensite, the surface is in tension Can cause surface cracking (quench cracks)Martempering can be used to reduce stresses and cracking

CCT diagrams -- (Continuous Cooling Transformation)Provide for more realistic cooling of reasonable sized itemsCan now account for different cooling of surfaces and centerTransformations shift to longer timesEnables identification of the critical cooling rate necessary to produce all martensite

CCT Diagram