THE GRAPHITISATION PROCESS IN MEDIUM-CARBON STEEL

1. THE GRAPHITISATION PROCESS IN MEDIUM-CARBON STEEL David V Edmonds and Kejian He Institute for Materials Research, University of Leeds, Leeds, UK Solid→Solid Phase Transformations in Inorganic Materials 2005

2. 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. Solid→Solid Phase Transformations in Inorganic Materials 2005

3. 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). Solid→Solid Phase Transformations in Inorganic Materials 2005

4. Machining Operation (after Metal Cutting, EM Trent, Butterworths, 1977) Solid→Solid Phase Transformations in Inorganic Materials 2005

5. 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. Solid→Solid Phase Transformations in Inorganic Materials 2005

6. 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. Solid→Solid Phase Transformations in Inorganic Materials 2005

7. 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. Solid→Solid Phase Transformations in Inorganic Materials 2005

8. Experimental Steels Alloying Philosophy • Increase Si and Al alloying • Minimise Mn alloying • (Avoid alloying with Cr) Note faster graphitisation in Si-Al steel. Solid→Solid Phase Transformations in Inorganic Materials 2005

9. Light Microscopy Steel 2 : Si-Al steel Steel 1 Annealed 115 hours Annealed 3.5 hours Light micrographs of steels 1 & 2,austenitised at 1150oC, quenched to martensite, and annealed at 680oC. Solid→Solid Phase Transformations in Inorganic Materials 2005

10. Experimental– further examination of graphitising Si-Al steel (Steel 2) 50 m 50 m As-quenched (martensite) Quenched and annealed 0.5 hours Graphitisation has started Solid→Solid Phase Transformations in Inorganic Materials 2005

11. 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. Solid→Solid Phase Transformations in Inorganic Materials 2005

12. Transmission Electron Microscopy 0.5µm0.5m 5 µm Bright field TEM image showing coarsened cementite particles located mainly at the interfaces of ferrite laths after 0.5 hours at 680°. Bright field TEM image showing only graphite nodules present in a more equiaxed ferrite structure after 1.5 hoursat 680°. Solid→Solid Phase Transformations in Inorganic Materials 2005

13. Further Electron Microscopy Observations – Particle Nucleation G 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. Solid→Solid Phase Transformations in Inorganic Materials 2005

14. Graphite Nodule Morphology ____ 0.5μm 5 µm Small, ~4µm diameter,regularspheroidal graphite nodules, apparently without a coring oxide or nitride particle, after 1.5 hours at 680°C. Solid→Solid Phase Transformations in Inorganic Materials 2005

15. Graphite (002) Lattice Fringes Within Conical Segments Solid→Solid Phase Transformations in Inorganic Materials 2005

16. Structure of Spheroidal Nodules DF image: diametric section BF image Schematic diagram illustrating cone-helix growth model for graphite nodules in cast irons [after DD Double and A Hellawell, Acta Metall., 22(1974)481]. DF image: non-diametric section Solid→Solid Phase Transformations in Inorganic Materials 2005

17. (002) Lattice Fringes in a Spheroidal Graphite Nodule Central region of nodule (BF image) Away from centre Near centre Solid→Solid Phase Transformations in Inorganic Materials 2005

18. High-Resolution Images - Pitch Graphitization Series 200oC 600oC 1200oC All images show (002) fringes. Howard Daniels, IMR, Leeds University 2730oC 2000oC Solid→Solid Phase Transformations in Inorganic Materials 2005

19. EELS- Shifting of the π+σ Plasmon as a Function of Graphitisation Plasmon peak energy closely followsthe change in density. Not a perfect match due to the effect of crystallite size. Solid→Solid Phase Transformations in Inorganic Materials 2005

20. 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. Solid→Solid Phase Transformations in Inorganic Materials 2005

21. EFTEM Plasmon Imaging of a Graphite Nodule 0.5m 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. Solid→Solid Phase Transformations in Inorganic Materials 2005

22. Coarsened Carbide Particles 0.5m 100nm Solid→Solid Phase Transformations in Inorganic Materials 2005

23. BF TEM Images ofSurviving Carbides 100nm Annealed 50 min. Annealed 58 min. Annealed 58 min. Are these carbide particles? Solid→Solid Phase Transformations in Inorganic Materials 2005

24. Carbon K-edge ELNES – Pitch Graphitization Series 750oC 1500oC 2730oC As order within the carbon increases, the electronic structure follows suit, resulting in higher definition of the unoccupied states. Howard Daniels, IMR, Leeds University Solid→Solid Phase Transformations in Inorganic Materials 2005

25. Carbon K edge EELS Spectra 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. Solid→Solid Phase Transformations in Inorganic Materials 2005

26. EFTEM Jump Ratio Images of a Particle C Remaining cementite Fe Mn O 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. Solid→Solid Phase Transformations in Inorganic Materials 2005

27. 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. Solid→Solid Phase Transformations in Inorganic Materials 2005

28. 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. Solid→Solid Phase Transformations in Inorganic Materials 2005