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Metal Casting Technologies : March 2011
METAL Casting Technologies March 2011 33 toughness and in general cause wide property variations. For improved cleanliness, integrity and reliability the steel for high strength investment castings must be melted and refined under vacuum or by using chemical fluxes in electroslag refining. Fatigue resistance is increased by any alloy addition or treatment which raises the yield strength and is also improved by surface hardening treatments such as carburizing, nitriding, flame and induction hardening of ferrous alloys and by surface deformation such as shot peening. These surface treatments introduce residual compressive stresses into the surface layers which tend to oppose the formation of fatigue cracks. Fatigue resistance will also be affected by any pre-loading effects from residual stresses or assembly stresses. Damaging residual tensile stresses can arise in Al alloys as a result of water quenching during solution treatment: to avoid this problem forced air in place of water as a quenchant has been recommended for some time . In the work  on Al cylinder heads fatigue cracking was reduced when solution treatment was carried out using polymer-water quenching media rather than conventional water quenching. Fatigue resistance of Al alloys can be improved by squeeze casting and by semi-solid processing [18, 19] both techniques providing finer dendrite arm spacings and improved soundness. Hot isostatic pressing (HIP) can also be used to remove internal porosity other than insoluble gas pores and thus improve densification in most types of casting alloys . Castings are heated to a suitable temperature and then subjected to inert gas pressure to heal pores by plastic deformation. This gives significant improvements in ductility and in fatigue life. Although Hipping cannot remove oxides there is some evidence that it can deactivate entrained oxide bifilm defects as initiating sites for fatigue cracking in Al alloy . However hipping will only provide maximum improvements in ductility and fatigue resistance in castings made from clean relatively oxide free metal. It must be remembered that the combined effects of service stresses and corrosive environments can be very damaging. Tensile stresses can interact with specific ions to cause stress corrosion cracking e.g. the chloride ion in stainless steels. Fatigue life can be considerably reduced under corrosive conditions: corrosion produces pitting and certain ions can increase crack growth rates such that corrosion can remove the fatigue endurance limit in ferrous alloys. ■ References. "Entrainment Defects". J. Campbell: Material Science & Technology 22 (2006) 127-145. "Casting defects and fatigue strength of a die cast aluminium alloy: a comparison between standard specimens and production components". M. Avalle, G. Belingardi, M.P. Cavatorta, R. Doglione: International Journal of Fatigue 24 (2002) 1-9. "Influence of casting defects on the fatigue limit of nodular cast iron". Y. Nadot, J. Mendez, N. Ranganathan: International Journal of Fatigue 26 (2004) 311-319. "Prediction of the Fatigue Life of Cast Steel Containing Shrinkage Porosity". R.A. Hardin, C. Beckermann: Met. & Materials Transactions A 40A (2009) 581-597. "An Atlas of Metal Damage". L. Engle & H. Klingele: (1981) Wolfe Publishing-Carl Hanser Verlag, Munich, Germany. "Critical material issues in cast aluminium cylinder heads". Y. Birol & A.A. Ebrinc: Foundry Trade Journal 181 (2008) 196-199. "Mechanical Testing and Properties", Ch.6: The Science & Engineering of Materials. D.R. Askeland (1989) Van Nostrand Reinhold International. "The use of fatigue data in the design of iron and steel castings" BCIRA Broadsheet 257-1 (1986). "An Introduction to Fracture Toughness". J. Pearce: Metal Casting Technologies 52 (2006) 16-20. "Cast irons and steels in fatigue stress applications - effects of notches". BCIRA Broadsheet 257-2 (1987) "Cast irons and steels in fatigue stress applications - effects of surface condition and treatments". BCIRA Broadsheet 257-3 (1987). "Porosity and the fatigue behaviour of hypoeutectic and hypereutectic aluminium- silicon casting alloys". H.R. Ammar, A.M. Samuel & F.H. Samuel: International Journal of Fatigue 30 (2008) 1024-1035. "Inclusions in Castings". J. Pearce: Metal Casting Technologies 50 (2004) 17-22. "The Revolution in Casting Production". J. Campbell: Foundryman 92 (1999) 313-317. "The Concept of the Fatigue Potential of Cast Alloy". C. Nyahumwa & N. Green: Foundryman 93 (2000) 257- 260. "Commentaries on 'Entrainment defects' by J. Campbell". P.R. Beeley, J.R. Griffiths, N.R. Green & C.J. Newton: Material Science and Technology 22 (2006) 999-1008. "An Overview of the Development of Al-Si Alloy Based Material for Engine Applications". H. Ye: Journal of Materials Engineering & Performance 12 (2003) 288-297. "Casting -- current practice and future potential". G.A. Chadwick: Metals & Materials (1986) 693-698. "Fatigue Properties of a semi-solid cast Al-7Si- 0.3Mg-T6 alloy". C.J. Davidson, J.R. Griffiths, M. Badiali & A. Zanada: Metallurgical Science & Technology 18 (2000) 27-31. "Fundamental Aspects of Hot Isostatic Pressing: An Overview". H.V. Atkinson & S. Davies: Met. & Materials Transactions A 31A (2000) 2981-3000. "Influence of casting technique and hot isostatic pressing on the fatigue of an Al-7Si-Mg Alloy". C. Nyahumwa, N.R. Green & J. Campbell: Met. & Materials Transactions A 32A (2001) 349-358. CASTINGS ARE HEATED TO A SUITABLE TEMPERATURE AND THEN SUBJECTED TO INERT GAS PRESSURE TO HEAL PORES BY PLASTIC DEFORMATION.