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