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Metal Casting Technologies : Dec 2009
METAL Casting Technologies December 2009 31 www.synchro32.com All Inclusive All Modules All System Upgrades All Technical Support No Upfront Fee Training One-on-one via Internet On site group sessions Foundry and Cast Metal Industry ERP Software Production Control, Incorporating Management Information Accounts Link & Shop Floor Data Collection Making specific production control accessible and affordable! WEB: www.synchro32.com EMAIL: firstname.lastname@example.org USA UK EU BRAZIL +1 800 323 2808 +44 845 370 3232 +44 845 3703232 +55 31 3581 1516 AUD $100.00 UK £50.00 New Module Touch Screen Shop Floor Data Collection Specific austenite, is favoured. As seen in Figure 1(d), when the Cr content is increased to 36%, for a Cr/C ratio of 17, the iron solidifies in the primary ferrite phase field to give a microstructure of eutectic M7C3 carbides in a relatively soft ferritic matrix. High Cr irons with 1-2%C and 30-40%Cr are used for heat and corrosion resistance  Heat treated microstructures The Ni-Cr martensitic irons must always be heat treated before service to remove retained austenite: to achieve this Ni-Hards 1 & 2 are given low temperature tempering type treatments, at 275oC and 450oC, but due to its higher alloy content Ni-Hard 4 needs a destabilisation treatment at 850oC followed by slow cooling to precipitate secondary carbides for maximum hardening . In some applications as-cast austenitic High Cr irons can be used without any hardening heat treatment since the austenite can work harden to provide self-replacing wear resistant surfaces . Austenite can transform to martensite at wear surfaces under impact loads locally raising hardness to around 900Hv. The surrounding matrix of austenite normally has sufficient plasticity to accommodate the volume changes caused by transformation. In most cases the irons are given suitable heat treatments [2, 3]: ● To soften the casting for machining e.g. for roller tyres and pump parts. This produces pearlitic and secondary carbides in a ferrite matrix, lowering hardness levels down to 350-400Hv. ● To harden via destabilisation treatment (at 950-1050oC), air quenching and tempering (at 450-550oC). This gives a distribution of secondary carbides in a tempered martensite matrix with small amounts (ideally <5%) of residual austenite: hardness levels are 700-850Hv. 5 Fig 5. As cast microstructure of a 30%Cr iron showing austenite to martensite transformation in the eutectic matrix regions adjacent to the M7C3 eutectic carbides. (SEM x2000)
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