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ME124 Experiment #7: ME124 Experiment #7: The ASTM Tensile TestThe ASTM Tensile Test
Lecture 2:Lecture 2:Effects of Heat Treatment on Material Effects of Heat Treatment on Material
Properties of 4130 SteelProperties of 4130 Steel
Spring 2003Spring 2003
Lecture 2Lecture 2
•• Brief review of heat treatment process and TTT diagrams Brief review of heat treatment process and TTT diagrams for 4130 steelfor 4130 steel
•• Sample results from previous tensile tests with untreated Sample results from previous tensile tests with untreated and quenched specimensand quenched specimens
•• Interpretation and discuss of resultsInterpretation and discuss of results
•• Effects of tempering Effects of tempering
Heat Treatment of SteelHeat Treatment of Steel•• Why?Why? Alter steel microstructure to obtain desired mechanical
properties
• Original structure of HR/annealed 4130 is some Fe (α) and Fe3C combination
• For increased strength/hardness, martensitic microstructure is desired– Martensite formation requires rapid cooling (quenching)
– Critical cooling rate increases with carbon content
•• Compromise!Compromise! Increased strength accompanies reduced ductility and toughness
Impact of Carbon ContentImpact of Carbon Content• Heat “treat-ability” varies upon carbon content
– Low carbon steels unresponsive to heat treatment; cold-working is most effective strategy
– High carbon steels are difficult to heat treat owing to high quenching rates required; steels are already strong and hard due to high %C
– Medium carbon steels can be heat-treated to improve strength; alloying elements (Ni, Cr, Mo) alter ITT diagrams, reduce quenching rates
MicrostructuralMicrostructural Effects of QuenchingEffects of Quenching
quenching
FCC structure BCC structure
ITT Diagram ITT Diagram –– 4130 Steel4130 Steel
4130 Composition (%wt)0.33% C0.90% Cr0.18% Mo0.53% Mn
4130 Steel 4130 Steel HardenabilityHardenability ((JominyJominy Test)Test)
Thermal Modeling (Lumped Capacitance)Thermal Modeling (Lumped Capacitance)
**length scale derived from Volume/Area for specimen
Thermal Modeling (cont’d)Thermal Modeling (cont’d)Approximate thermal trajectory (h=500 W/mK)
Martensite formation “zone”
Heat Treatment Process for 4130 SteelHeat Treatment Process for 4130 Steel
Maintain at 900°C (1652°F) for 4+ hours
“Austenitization”
Rapidly quench the specimen in a water bath
The quenched specimen; note the presence of “scaling” on the surface resulting from oxidation
Raw Tensile Test Data (4130 steel)**Raw Tensile Test Data (4130 steel)**
Young’s modulus ~unchanged (atomic-level property!)
**Fall 2000, ENGR1 data**Fall 2000, ENGR1 data
Raw Tensile Test Data (4130 steel)**Raw Tensile Test Data (4130 steel)**
Young’s modulus unchanged (atomic-level property!)
**Spring 2003 **Spring 2003 ME124 preME124 pre--lab test datalab test data
Specimen Appearance after FailureSpecimen Appearance after Failure
Quenched Untreated
Impact of Tempering Impact of Tempering MartensiteMartensite• Martensite is strong and hard, but brittle • “Tempering” is a post-processing heat-treatment used to recover some degree
of ductility • Diffusion-based process (carbon atoms) performed at elevated, sub-eutectoid
temperature of ~250-600°Cdual-phase: α + Fe3CBCT, 1-phase
Heat + Time(diffusion) Fe3C
α matrix
Tempering (cont’d)Tempering (cont’d)• Increasing Fe3C particle size reduces boundary area, and
determines amount of ductility recovered• Fe3C particle size determined by diffusion (elapsed time)• Diffusion process can be accelerated by increasing temperature
Tempering of quenched 1080 steel
Tensile Test Data (Tempered 4130 steel)Tensile Test Data (Tempered 4130 steel)
Specimen Length ComparisonSpecimen Length Comparison
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