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Metal Casting Technologies : Dec 2009
30 www.metals.rala.com.au In the Ni-Cr martensitic irons containing 4%Ni and 2%C (Ni-Hards 1 & 2) the eutectic carbide is the continuous M3C type but if the %Cr level is increased to 8-9% then a mixed carbide structure of (M3C plus M7C3) with reduced continuity is obtained thus increasing fracture resistance. This is the basis of the Ni-Hard type 4 Grade which contains around 8.5%Cr and 4.5-6.5%Ni. In Ni-Hards the matrix has sufficient hardenability to prevent pearlite formation in sections up to 100mm. and on cooling in the mould after casting around 50% of the austenite transforms to martensite. The typical structure of as cast Ni-Hard type 2 is shown in Figure 4, the martensite appearing as dark needles in a retained austenite matrix. All grades of Ni-Hard must be heat treated to convert retained austenite to martensite and bainite before being put into wear service to avoid spalling at wear surfaces. Retained austenite can transform to martensite at wear surfaces and the resultant volume expansion can result in cracking since the surrounding structure has insufficient plasticity. In high Cr irons with 12-30%Cr, depending on composition and cooling rate, the austenite formed on solidification may be retained on cooling in the mould or it may partially or fully transform to ferrite and carbides (secondary and/or pearlitic), or partially transform to martensite. From Figures 1(a & b) and 2(a) it is seen that the 0.11%, 5.54% and 14.68% Cr containing irons have pearlitic matrices but the 30%Cr iron, with a higher Cr/C ratio of 12.5 has an austenitic matrix Fig. 1(c). However, due to local C and Cr depletion adjacent to the eutectic carbide, there is always some transformation of the eutectic austenite to martensite in as-cast austenitic irons, as illustrated in Figure 5. As-cast austenitic structures, if required (e.g. if hardening of a casting raises concerns about cracking), are promoted by faster cooling rates, by high Cr/C ratios, and by Ni, Cu and Mo additions. In Fig 1(c) and 2(b) it is seen that the eutectic carbides in the 30%Cr iron are relatively fine. In general the eutectic carbides are coarser at 12-25%Cr and 35-40% levels. At high Cr-low C levels primary ferrite, not primary 2b 4 2a 3 Fig 2. Eutectic carbide morphology as revealed by deep etching. (a) Continuous form of M3C eutectic carbide in a 3.7%-5.5%Cr iron with pearlitic matrix partially removed during etching (SEM x500) Fig 3. Coarse primary M7C3 carbides and finer eutectic M7C3 carbides in an as-cast hyper-eutectic 25%Cr iron. The matrix is austenite. SEM back scattered electron image to show carbides in dark con- trast (x300). (b) Very fine rod-like nature of M7C3 eutectic carbide in a 2.4%C- 30%Cr iron with primary austenite dendrite arms and eutectic austenite removed by etching (SEM x500). Fig 4. As-cast microstructure of Ni-Hard (nominal 2.8%C-4%Ni- 2%Cr) showing continuous M3C eutectic carbide in a matrix of austenite of which around 50% has transformed to martensite. (x500) TECHNICAL FEATURE
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