Graphitisation Cast Iron

Solid→Solid Phase Transformationsin Inorganic Materials 2005

THE GRAPHITISATION PROCESS IN MEDIUM-

CARBON STEEL

David V Edmonds and Kejian HeInstitute for Materials Research,University of Leeds, Leeds, UK


Research Programme Objectives

• To study graphitisation in steels, with the overall aim of identifying a new route to the development of a plain carbon cold-forging steel with good machinability.

• To accelerate the kinetics of graphitisation.

• To examine the mechanism of graphite formation in steels by high-resolution microanalytical electron microscopy.


Outline of Presentation

• Background to overall research programme objectives.

• Choice of steel alloying - thermodynamic modelling.

• Examination of graphite nodule formation using transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS) and energy-filtered transmission electron microscopy (EFTEM).


Machining Operation (after Metal Cutting, EM Trent, Butterworths, 1977)


Traditional Free-Machining(Free-Cutting) Steels

• The world market is very large –from threaded screws and bolts to accurately machined components e.g. in the automotive industry.

• Plain carbon steels are alloyed with elements such as Pb, S, Te, Bi and P – to act as a lubricant at the tool/workpiece interface, and additionally, to assist with chip break-up.

• Disadvantages can be toxicity, impairment of cold forgeability, and, at least for Te and Bi, steel that is more expensive and difficult to recycle.


Free-Machining Plain Carbon Steels– An Alternative

• Why not anneal plain carbon steels to exchange cementite for graphite?

• The presence of graphite (rather than cementite) in the microstructure of a plain carbon cold-forging steel would act as an internal lubricant during machining, and also assist chip break-up, as in the case of grey cast irons which have customarily exhibited excellent machinability. Forgeability should also be improved.

• However, the annealing time required to convert cementite to graphite in steels has traditionally been too long (of the order of 100+ hours) to integrate successfully into a production heat treatment schedule.

• Thus, consider whether graphitisation can be influenced by alloying, to reduce these heat treatment times.


Thermodynamic Assessment of Alloying

The effect of various alloying elements on graphitisation were evaluated in terms of the driving force for the precipitation of graphite in an Fe-0.54wt%C system at 680ºC using Thermo-Calc.

•Si and Al have strong and roughly equivalent effects.

•Ni and Co are similar but less strong.

•Cu has a positive effect but is much weaker.

•Mo is mildly suppressive, Mn more so.

•Cr is strongly suppressive.


Experimental Steels

Alloying Philosophy•Increase Si and Al alloying•Minimise Mn alloying•(Avoid alloying with Cr)

C Si Al Mn Ni Time for graphite to appear in

microstructure at 680°C

(hours)

Approx. time for

completion of

graphitisation at 680°C

(hours)

Steel 1 0.47 0.19 - 0.32 1.5 18 115

Steel 2 0.38 1.82 1.44 0.07 - 0.5 3.5Note faster graphitisation in Si-Al steel.


Light Microscopy

Steel 1 Steel 2 : Si-Al steel

Annealed 115 hours Annealed 3.5 hours

Light micrographs of steels 1 & 2, austenitised at 1150oC, quenched to martensite, and annealed at 680oC.


Experimental – further examination of graphitising Si-Al steel (Steel 2)

50 m50 m

As-quenched (martensite) Quenched and annealed 0.5 hours

Graphitisation has started


Si-Al steel (Steel 2)

50 m 50 m

50 m

0.5 hours 3.5 hours

55 hours

Progress of graphitisation at 680°C.


Transmission Electron Microscopy

0.5µm0.5m

Bright field TEM image

showing coarsened cementite

particles located mainly at the

interfaces of ferrite laths after

0.5 hours at 680°.

5 µm

Bright field TEM image showing only graphite nodules present in a more equiaxed ferrite structure after 1.5 hours at 680°.


Further Electron Microscopy Observations – Particle Nucleation

Nucleation of graphite (G) on an aluminium oxide inclusion (O) after 0.5 hours. Note also the carbide particle dispersion and remnants of the martensite structure.

Nucleation on an AlN particle. Note the irregular graphite morphology and structure.

The inner ring of the diffraction pattern is (002) graphite, and the single crystal reflections are from the [111] zone of AlN.

G


Graphite Nodule Morphology

____0.5μm

Small, ~4µm diameter, regular spheroidal graphite nodules, apparently without a coring oxide or nitride particle, after 1.5 hours at 680°C.

5 µm


Graphite (002) Lattice Fringes Within Conical Segments


Structure of Spheroidal Nodules

BF image DF image: diametric section

DF image: non-diametric

section

Schematic diagram illustrating cone-helix growth model for graphite nodules in cast irons [after DD Double and A Hellawell, Acta Metall., 22(1974)481].


(002) Lattice Fringes in a Spheroidal Graphite Nodule

Away from centre Near centre

Central region of nodule (BF image)


High-Resolution Images - Pitch Graphitization Series

200200ooCC 600600ooCC 12001200ooCC

27302730ooCC 20002000ooCCAll images show (002) fringes.

HowardDaniels,IMR,Leeds University


lowloss2

-0 10 20 30 40 500

10

20

30

40

50

60

70

Energy Loss (eV)

Counts x 1000

Plasmon Position / HTT

2323.5

2424.5

2525.5

2626.5

0 1000 2000 3000HTT (oC)

Plasmon Energy (eV)

EELS- Shifting of the π+σ Plasmon as a Function of Graphitisation

• Plasmon peak energy closely follows the change in density.

• Not a perfect match due to the effect of crystallite size.


EFTEM – Plasmon Mapping

By taking the ratio of the intensities in the windows shown for a large number of different plasmon positions, it is possible to calibrate the intensity in the image.

27/22ev using 3ev windows

0

0.5

1

1.5

2

21 23 25 27 29

Plasmon energy (eV)

Ratio 27/22ev

lowloss2

-0 10 20 30 40 500

10

20

30

40

50

60

70

Energy Loss (eV)

Counts x 1000


EFTEM Plasmon Imaging of a Graphite Nodule

0.5m

23 eV

24 eV

25 eV

26 eV

27 eV

Bright Field ( montage of 25 images) Plasmon (27eV/22eV) Ratio Map (montage of 25 images)

Plasmon ratio map suggests a more amorphous core.


Coarsened Carbide Particles

0.5m

100nm


BF TEM Images of Surviving Carbides

100nm

Annealed 50 min. Annealed 58 min. Annealed 58 min.

Are these carbide particles?


Carbon K-edge ELNES – Pitch Graphitization Series

750750ooCC

15001500ooCC

27302730ooCC

Howard Daniels,IMR, Leeds University

As order within the carbon increases, the electronic structure follows suit, resulting in higher definition of the unoccupied states.


Carbon K edge EELS Spectra

-0.3

0.2

0.7

1.2

1.7

280 285 290 295 300 305 310 315 320 325 330

Energy Loss (eV)

Counts

Fe3C

centre

centre1

edge 1

edge

680 graphite

• EELS spectra collected from coarse particles, and cementite and graphite for comparison.• Carbon content - 30 atom% in crystalline cementite part, 70 atom% in amorphous part.


EFTEM Jump Ratio Images of a Particle

MnFe

C O

Remaining cementite

The particle is not simple cementite – it consists of crystalline cementite and a more amorphous part.

TEM BF image; C K- jump ratio image; O K- jump ratio image; Fe L2,3- jump ratio image and Mn L2,3 - jump ratio image.


0

20

40

60

80

100

120

140

160

0 5 10 15 20

Distance from quenched end (mm)

Particle area (µm

2 )

Si-Al

Si-Al-B

0

200

400

600

800

1000

1200

0 5 10 15 20


No. of particles (mm

-2)

Si-Al

Si-Al-B

100

150

200250

300

350

400

450500

550

600

0 5 10 15 20 25 30


Hardness (HV30)

Si-Al

Si-Al-B

Si-Al 6hr

Martensite

Lower bainite

Upper bainite / acicular ferrite

Ferrite + pearlite /Widmanstatten ferrite

100

150

200250

300

350

400

450500

550

600

0 5 10 15 20 25 30


Hardness (HV30)

Si-Al

Si-Al-B

Si-Al 6hr

Martensite

Lower bainite

Upper bainite / acicular ferrite

Ferrite + pearlite /

Widmanstatten ferrite

Jominy Bar Analysis after Annealing for 6 hours at 680°C

Different graphite nucleation kinetics and microstructural dispersions result from different starting microstructures, possibly related to the different routes for carbide formation between martensite, bainite and pearlite in the Si-Al experimental steels.


Conclusions• Graphitisation of carbon steels in the tempered martensitic condition can be achieved after short annealing times (2-3 hours) by alloying.

• Regular spheroidal nodules appear to consist of cone-shaped segments radiating from a central core.

• Within the cones the circumferential stacking of the graphite layers during growth is very regular, equivalent to that which can be achieved in the graphitisation of carbonaceous materials only at temperatures around two thousand degrees higher.

• Microanalysis by high-resolution TEM suggests that, in the experimental steels, either cementite dissolution is accompanied by loss of crystallinity and the formation of amorphous regions, or these regions form on the decomposing cementite.

• Observations of a more amorphous centre to the small spheroidal nodules (lacking an obvious nucleating particle), suggests that the amorphous carbon regions associated with the decomposing cementite may be the nuclei for these graphite nodules – an intermediate stage in graphite nodule formation.

Graphitisation Cast Iron

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