6.2 - Heat treatment of steel (quenching & tempering)

Why do we want to harden metals?

In general, materials are only technically usable as long as no plastic deformation occurs (permanent, irreversible deformation)

→ usable in elastic region

• small for softer materials

• Large for harder materials

What’s the elastic region in the stress-strain diagram?

Draw the stress-strain curve for both soft and hard materials

The linear region of the stress-strain curve is the elastic region

What are the stages of the hardening process?

Quenching (hardening) & tempering (reheating at temperatures up to 650°C)

Describe the full process of heat treatment to harden steel.

Draw a simple diagram to illustrate it.

1. Austenitisation: heating up to ~850°C to get austenite (homogeneous) microstructure → necessary for the subsequent (next, upcoming) hardening

2. Quenching: rapid cooling → austenite transforms into martensite hard & brittle)

3. Tempering: slight reheating up to ~500°C → reduces brittleness & residual stresses while retaining hardness

What are the purposes/effects of tempering?

• improve deformation capacity

• Reduce residual stresses while retaining hardness

• Reduce brittleness & risk of cracking

What is the transformation process of austenite into martensite called?

When does it happen in the heat treatment process flow?

Martensitic transformation

→ occurs during quenching

Explain how the martensitic transformation arises during the heat treatment process.

Draw the martensite unit cell

Austenite microstructure

Austenite has a FCC structure

Interstitial sites:

- between the atoms at the corners of each face

- in the center of the austenite unit cell (fcc)

Carbon atoms randomly occupy interstitial sites (they’re not all necessarily occupied!)

Martensite microstructure

BCT: body centered tetragonal (not cubic!)

Shorter side = ½ of the diagonal of the face of the FCC

Longer side = side of the face of the FCC

Centered atom = atom at the center of the face of the FCC

Carbon atoms between the atoms in the longer sides → they cause the distortion of the unit cell (not cubic)

Austenite → martensite

1. Quenching

   as temperature decreases, FCC austenite lattice becomes metastable → “wants” to transform into a more stable structure

2. Bain mechanism / distortion

   1 axis of the cube of the FCC elongates

   The 2 other axes shrink

   → form the BCT structure

   iron atoms slightly shift but don’t move far → no diffusion, no precipitation of a new phase

3. Adaptation mechanism (local atomic adjustments)

   As the axis of the FCC elongated/shrunk, iron and carbon atoms slightly moved

   → carbon atoms stabilise the BCT structure & create internal stresses

How does the martensitic transformation increase the strength of steel?

• Higher distortion of the lattice due to the supersaturation of carbon (the steel holds more carbon than it would at equilibrium) → forces between iron and carbon atoms make the axis elongate or shrink even more

• As carbon increase inner tensions within the lattice, it is harder for dislocation to move through

• the boundaries between martensite grains form and are also obstacles for dislocation movements