Metal Casting Technologies : MCT DEC 2017 (4TH QRT)
22 www.metals.rala.com.au METAL Casting Technologies 4th Quarter 2017 23 TECHNICAL FEATURE In producing irons based on pure charge materials the presence of Ce and other Rare Earth (RE) elements, which may arise from nodularization and/or inoculation treatments, can be counteracted by carefully controlled additions of subversive elements such as Sb and Bi [6-8, 19, 23]. These two elements can also be used to increase nodule number. In the use of antimony to offset any RE effects and hence prevent chunky graphite at slow cooling rates. an optimum RE/Sb ratio of 2 has been suggested with the % Sb addition of 0.03% . This work also found that with a 0.04% Sb addition some spiky graphite was formed. Other work suggests that 0.012% Sb additions can be successfully used to increase nodule number and prevent chunky graphite in 90mm thick sections . When attempting to increase nodule number in sections over 50mm thick in essentially ferritic grades of ductile iron it is suggested that any Sb addition should be limited to 0.005% and to situations where some pearlite presence is acceptable . Opinion on safe and optimum Sb additions is divided. Sb itself, if not correctly counteracted by Ce, will have a deleterious effect on graphite morphology Safe levels of Sb addition will vary with charge make-up, melt preparation & treatments, and the type of castings being produced, hence they must be determined by each foundry. Foundries must also be aware that any build-up of Sb content in foundry returns might also lead to future problems in microstructure control since Sb is not lost during re-melting. In addition, Sb, like Bi and Pb, can be absorbed in the hot refractory linings of melting furnaces and ladles and can be transferred back into subsequent melts. Compositional control of trace elements is a problem for most foundries since conventional spectrographic analysis may not have sufficient accuracy for trace but nevertheless significant and possibly damaging levels of Sb, Bi, Pb, Bi etc. Specialist analytical techniques are needed such as inductively coupled plasma mass spectrometry (ISP-MS), the use of which is covered in an ISO standard for the determination of the elements Sn, Sb, Ce, Pb and Bi in steel and iron . Antimony and its compounds can cause a number of health problems such as skin and mucous membrane irritations . Foundries using Sb additions must be aware of dangers posed by contact with Sb or from fume and dust during melting and other operations. Research on trace elements There is still considerable scope for research into the behaviour of trace elements such as Sb in cast irons, for example, towards improved understanding of their effects on graphite nucleation and growth, on eutectic carbide formation, and on matrix structure. Such research needs to further examine the cumulative and synergistic effects of the various trace elements including gaseous elements hydrogen, nitrogen and oxygen, and their influences on solidification and solid-state phase transformations at different cooling rates (section size effects). Like most casting alloys, cast irons are “again and again” materials being produced by continually recycling worn out or scrapped parts, steel scrap, foundry returns, etc. Much of the bought-in steel scrap used by iron-foundries is sourced as clippings or off-cuts from the auto industry which is now making increasing use of micro-alloyed low C steels (e.g. HSLA and dual-phase), re-phosphorized steels and Zn coated steels. Hence, both in steel scrap sources and produced iron castings, the levels of residual elements, especially carbide formers such as V, Mo, Ti, Cr, B, Nb and P etc. are expected to gradually increase. This increase is likely to give rise to greater incidence of intercellular eutectic and pearlitic carbides, to increase the difficulty of producing ferritic ductile grades and may also influence shrinkage behaviour. Figure 3. Ductile iron showing the presence of large amount of “chunky” graphite. X100 . Due to the hardness of their carbides contamination with Ti, Nb and V will adversely affect machinability. In solidification of ductile irons most elements, other than Ni and Si, tend to segregate to the intercellular regions. In these regions higher concentrations of residual elements i.e. from micro-alloyed steel scrap may also interact, especially in thicker casting sections, with original segregated trace subversive elements such as Sb, Pb, Bi and Ti and may increase the latter’s deleterious influences on graphite morphology. There is a need for statistical analysis studies to quantify any interaction effects of alloying and residual elements on the microstructures and properties of ductile and compacted graphite irons. One such study has examined how volume fraction of chunky graphite in a ductile iron can be related to composition in terms of the effects of Ce, Cu, La, P, Sb and Sn contents . Studies of the effects of Sb and other elements on the crystallography and formation of growth faults during graphite formation [e.g. 27-29] can lead to clearer understanding of the behaviour of both subversive and nodularizing elements. For example, from work on air and vacuum melts of Fe-C-Ce and Fe-C-Sb ternary alloys, it has been suggested that Ce has an indirect influence on graphite growth via its action as an oxygen scavenger, whereas Sb has a direct effect on graphite structure leading to formation of irregular bending of flake forms [28-29]. Transmission electron microscopy (TEM) of such flakes indicate that each flake comprises of several areas with different orientations resulting from changes in growth direction due to growth faulting or twinning which could have been promoted by the presence of Sb. This latter observation may be of relevance in studying the mechanism of modification by Sb of eutectic Silicon in Al-Si base casting alloys [30-32]. Sb can be used as an alternative to Strontium (Sr) to provide more persistent longer lasting modifying effects than Sodium (Na). Since both graphite and silicon are faceted phases with limited growth directions it would be interesting to compare the possibility and nature of any impurity induced twinning in each due to the presence of Antimony. n REFERENCES 1) J.F. Meredith: “The Influence of Deleterious Elements in Ductile Iron Castings”. METAL Casting Technologies (2004) Vol.50 September pp.32-33 . 2) J.F. Meredith: “The Influence of Common Elements in Ductile Iron”. METAL Casting Technologies (2011) Vol.57 March pp.42-45. 3) P.M . Cabanne et Al: “Production of Heavy and Thick Ductile Iron Castings: Process Review and Potential Defects”. Indian Foundry J. (2010) Vol.56 February pp.33-42. 4) J.T.H . Pearce: “Review of the effects of antimony on the wear resistance and mechanical properties of grey cast iron”. BCIRA Journal (1969) Vol.17 September pp.444-446. 5) R.H . Abhorn: “What Antimony May Do for You in Gray and Ductile Iron”. AFS Transactions (1976) Vol.84 pp.503-506. 6) J.D. Mullins: “Chunk Graphite Defects in Ductile Iron”. Sorelmetal News No. 93 (2006) March 2pp. 7) P. Larranaga et Al: “Effect of antimony on the eutectic reaction of heavy section spheroidal graphite castings”. Int. J. of Cast Metals Res. (2009) Vol.22 pp. 192-195. 8) X.G . Diao et Al: “Effect of antimony addition and section size on formation of chunky graphite in ductile iron”. Mat. Sc. & Technology (2011) Vol.27 pp.834-838. 9) I.M . Kovalov: “Antimony Cast Iron for Wear Resistance”. Russian Castings Production (1967) May pp.239-240. 10) J.F. Wallace et Al: “Factors Influencing the Ferrite Layer on the surface of Gray Iron Castings”. AFS Transactions (1975) Vol.83 pp. 531-549. 11) P.J. Rickards: “Ferrite in shell-moulded grey iron castings -a summary paper”. BCIRA Journal (1976) Vol.24 January pp.54-62. 12) V.V. Chistayakov; “Influence of Antimony on the growth and scale resistance of gray irons”. Russian Castings Production (1970) July pp.338 -339. 13) S. Dawson et Al: “The Effect of Metallurgical Variables on the Machinability of Compacted Graphite Iron”. Proceedings of SAE World Congress (2001) Detroit, US. Paper 2001-01-0409 pp.4 -16. 14) “Abnormal graphite forms in cast irons”. BCIRA Broadsheet 138-2 (1979) 4pp. 15) J.T.H . Pearce: “Microstructure in heavier section ductile irons”. METAL Casting Technologies (2014) Vol.60 December pp.26-29. 16) R. Barton: “Experience in the prevention of defects in nodular iron”. BCIRA Journal (1968) Vol.16 November pp.554-565. 17) “The effects of cerium in nodular (SG) iron”. BCIRA Broadsheet 211-6 (1986) 3pp. 18) R. Kalbom et Al: “On the solidification sequence of ductile iron castings containing chunky graphite”. Mat. Sc. & Eng. A (2005) Vol.413-414 pp.346-351. 19) M.J. Fallon: “Experiences in the Manufacture of Ductile Irons”. The Foundryman (1995) Vol.88 pp.308-318. 20) J. Lacaze et Al: “Effect of carbon equivalent on graphite formation in heavy section ductile iron parts”. Materials Science Forum (2010) Vol. 636-7 pp.523-530. 21) I. Karsay et Al: “Nickel alloyed ductile iron graphite structures”. AFS Transactions (1961) Vol.69 pp.672-679. 22) R. Logan et Al: “An Investigation of Chunky Graphite Defects in SiMo Iron used for High Temperature Applications”. Indian Foundry Journal (2011) Vol.57 pp.41-48. 23) L. Zhe et Al: “Influence of cooling rate and antimony addition content on graphite morphology and mechanical properties of a ductile”, China Foundry (2012) Vol.9 pp.114-118. 24) “Steel and iron – Determination of nine elements by the inductively coupled plasma mass spectrometric method: Part 1: Determination of tin, antimony, cerium, lead and bismuth”. ISO 16918-1:2009 (en), International Organization for Standardization. 25) R.G Cooper & A.P. Harrison: “The exposure to and health effects of antimony”. Indian J. of Occupational & Environmental Medicine (2009) Vol.13 pp.3-10. 26) J. Sertucha et Al: “Statistical Analysis of the Influence of Some Trace Elements on Chunky Graphite Formation in Heavy Section Nodular Iron Castings”. Met. & Mat. Trans. A (2013) Vol. 44 pp. 1159-1162. 27) J. Lacaze. “Trace elements and graphite shape degeneracy in nodular graphite cast irons”. International Journal of Metalcasting (2017) Vol. 11 pp. 44 -51. 28) K. Theuwissen et Al: “Effect of Ce and Sb on Primary Graphite Growth in Cast Irons”. Trans. Indian Institute of Metals (2012) Vol. 65 pp. 707-712. 29) K. Theuwissen et Al: “Structure of graphite precipitates in cast iron”. Carbon (2016) Vol. 96 pp.1120-1128. 30) A.K . Dahle et Al: “Eutectic modification and microstructure development in Al-Si Alloys”. Mat. Sc. & Eng. A (2005) Vol.413-414 December pp. 243-248. 31) H. Li and Q. Chai: “Study on Modification Mechanism of Antimony in Al-Si cast alloys”. Advanced Materials Research (2011) Vol.197-198 pp.1079-1085. 32) M. Tokar et Al: “The effect of Strontium and Antimony on the mechanical properties of Al-Si alloys”. Mat. Sc. & Eng. (2014) Vol.39 pp.69-79. THERE IS STILL CONSIDERABLE SCOPE FOR RESEARCH INTO THE BEHAVIOUR OF TRACE ELEMENTS SUCH AS SB IN CAST IRONS, FOR EXAMPLE, TOWARDS IMPROVED UNDERSTANDING OF THEIR EFFECTS ON GRAPHITE NUCLEATION AND GROWTH, ON EUTECTIC CARBIDE FORMATION, AND ON MATRIX STRUCTURE.
MCT MAR 2018 (1ST QRT)