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Metal Casting Technologies : Dec 2009
32 www.metals.rala.com.au TECH N ICAL FEA TURE References: 1. J.T.H. Pearce: "Abrasive wear behaviour of alloy cast irons". British Foundryman (1985) Vol. 78, 13-23. 2. J.T.H. Pearce: "High Chromium Cast Irons to Resist Abrasive Wear". Foundryman (2002) Vol.95, 156-166. 3. J.T. H. Pearce: "Heat Treatment of High Chromium Cast Irons". METAL Casting Technologies (2001) Vol.47, June, 41-43. 4. J.T.H. Pearce: "Materials Characterisation -- Which technique should we use?" METAL Casting Technologies (2004) Vol.50, March, 22-26. 5. J.T.H. Pearce: "Ferrous Casting Alloys for Heat Resistance". METAL Casting Technologies (2009) Vol.55, March, 36-41. 6. "Ni-Hard Material Data and Applications" (1996) Nickel Development Institute Publication 11 017. 7. J.T.H. Pearce: "The use of transmission electron microscopy to study the effect of abrasive wear on the matrix structure of a high chromium cast iron". Wear (1983) Vol.89, 333-344. 8. A. Wiengmoon et Al: "Microstructural and crystallographical study of carbides in 30wt.% Cr cast irons". Acta Mat. (2005) Vol.53, 4143-4154 9. J. Wang et Al: "The precipitation and transformation of secondary carbides in a high chromium cast iron". Materials Characterization (2006) Vol.56, 73-78. 10. S.D. Carpenter et Al: "XRD and electron microscope study of a heat treated 26% chromium white iron microstructure". Mat. Chem. & Physics (2007) Vol.101, 49-55. 11. R.W. Durman: "Progress in abrasion resistant materials for use in comminution processes". Int. J. Minerals Processing (1988) Vol.22, 381-399. 12. S.K. Hann & J.D. Gates. "Transmission electron microscopy of a transformation toughened white cast iron". J. Materials Science (1997) Vol. 32, 3443-3450. The effect of annealing for 4 hours at 750oC on the original austenite matrix in a 2.4%C-30%Cr iron is shown in Figure 6. This allows machining with conventional carbide tools. One disadvantage of Ni-Hard is the inability to be softened in this way so Ni-Hards have to be ground or machined with special tool tips such as cubic boron nitride. Annealed structures in Cr irons are re-austenitised and are destabilised in the same way as as-cast structures by holding for around 4 hours circa 1000oC. During destabilisation, depending on composition and holding time, M7C3 and/or M23C6 secondary carbides precipitate within the matrix reducing the C and alloy content of the austenite matrix. This raises the Ms Temperature, and at the same time allows the austenite to retain sufficient hardenability (sufficient C and Cr etc.) to form martensite during forced air cooling, if the Cr/C ration is high enough or if the iron contains Mo. To save costs grinding balls with 12-15%Cr and minimal Mo are hardened by oil quenching. The effects of destabilisation temperature and subsequent tempering conditions have been explained in an earlier MCT paper . Figure 7 illustrates the microstructures in a destabilised 2%C-20%Cr-1.7%Mo iron. The microstructure in view (b) contains insufficient secondary carbide precipitation (due to too short a holding time) and considerable retained austenite has been retained on air hardening. There is ongoing interest [e.g.8-10] in understanding how the nucleation and growth of secondary carbides during destabilisation & cooling, and their crystallography, can influence both their distribution and the transformation of the surrounding austenite and the final properties after tempering. Special high temperature treatment at 1130-1180oC can be applied where extra toughness is needed, e.g. in hammers. This produces a controlled amount of secondary carbide precipitation in a fully austenitic matrix and provides fracture toughness levels of 40-45 MPa Ãm as against 20-30 MPa Ãm for as-cast or normally hardened irons . There is also a reduction in angularity of the eutectic carbides. The secondary carbides encourage cracks to pass through matrix as well as through the eutectic carbides thus improving toughness . In all materials microstructural control is the key to achieving optimum properties and performance. Although alloy white irons have been around for a long time there is still considerable scope for microscopists to further explore the structural features of these alloys. ■ 6 7a 7b Fig 6. The effect of annealing at 750oC on the microstructure of the 2.4%C-30%Cr iron seen in the as-cast condition in Figure 5. The austenite matrix has transformed to a mixture of pearlitic carbides in a ferrite matrix. Hv is 350-400. (x1000) (a) Destabilised at 900oC for 4 hours and forced air cooled. Hv is 710. (x1000). Fig 7A & 7B. Effect of destabilisation and forced air cooling on the microstructure of 2%C-20Cr-1.7%Mo Iron. (No tempering treatment). (b) Destabilised for insufficient time resulting in insufficient precipitation of secondary carbides (x2500). TECHNICAL FEATURE
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