Interstitial-Free Steels:Structure of Spot Welds

Interstitial-free steel

Fe-0.002C wt%

Interstitial-free and unalloyed

Extremely fast transformations

Extremely fast cooling rates

Phase diagram similar to pure iron

Difficult to interpret structure

Interstitial-free steel

Fe-0.002C wt%

0 50 100 150 200

Pressure / kbar

800

400

0

γ

α ε

1200

1600δ

0 50 100 150 200

Pressure / kbar

800

400

0

γ

α ε

1200

1600δ Bundy (1965)

J. Khan, L. Xu and Y. J. Chao, Science and Technology of Welding and Joining 4 (1999) 201

Completed in a time period

much less than a second

austenite

ferrite

cementite

Bhadeshia, Vitek and David, Materials Science and Technology 7 (1991) 50

austenitic stainless ferritic stainless

Physical metallurgy of spot welds in interstitial-free

steels

Bhadeshia, Vitek and David, Materials Science and Technology 7 (1991) 50

Diffusion of all atoms during nucleation and growth.Sluggish below about 850 K.

Invariant-plane strain shape deformation with large shear component.No iron or substitutional solute diffusion.Thin plate shape.

Cooperative growth of ferrite & cementite.

No change in bulk composition.

Diffusionless nucleation & growth.

Carbon diffusion during paraequilibrium nucleation. No diffusion during growth.

Carbon diffusion during paraequilibrium nucleation & growth.

ALLOTRIOMORPHIC FERRITE

IDIOMORPHIC FERRITE

MASSIVE FERRITE

PEARLITE

WIDMANSTATTEN FERRITE

..

BAINITE & ACICULAR FERRITE

MARTENSITE

RECONSTRUCTIVE DISPLACIVE

UPPER BAINITE(High Temperature)

LOWER BAINITE(Low Temperature)

Carbon supersaturated plate

Carbon diffusion into austenite

Carbon diffusion into austenite and carbide precipitation in ferrite

Carbide precipitation from austenite

Ohmori and Honeycombe

log{time}

Ms

ferrite

pearlite

bainite

martensite

Ae3

upper bainite

lowerbainite

Widmanstatten ferriteBs

Ws

Widmanstättenferrite

Hayat, Demir, Acarer, Aslanar, Kovove Materialy 47 (2009) 11

Reproduced with permission

Bayraktar, Kaplan, Devillers and Chevalier, J. Mat. Proc. Techn. 189 (2007) 114

Reproduced with permission

Bhadeshia, Metal Science 16 (1982) 159

12

3 45

6

1. Fe-0.4C2. Fe-0.4C-2Si3. Fe-0.4C-1Ni4. Fe-0.4C-1Mn5. Fe-0.4C-1Mn-1Cr6. Fe-0.4C-2Mn

0 1 2 3 4 5 10 10 10 10 10 10

Time / s

800

700

600

500

400

300

200

Tem

pera

ture

/

C

calculations

Mukhopadhyay, Bhattacharya and Ray,

J. Mat. Proc. Techn. 209 (2009) 1995

Reproduced with permission

Mukhopadhyay, Bhattacharya and Ray,

J. Mat. Proc. Techn. 209 (2009) 1995

Reproduced with permission

Mukhopadhyay, Bhattacharya and Ray,

J. Mat. Proc. Techn. 209 (2009) 1995

Reproduced with permission

Bhadeshia, Svensson & Gretoft,

Acta Metallurgica

33 (1985) 1271