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Metal Casting Technologies : March 2008
Fig 5. Back scattered electron image (BEI) and X ray mapping for some elements in a sample taken from cast duplex stainless steel after solution treatment. In the BEI image (top left) the austenite appears light grey, the ferrite grey and non-metallic inclusions black. The maps show that the austenite areas are richer in Ni, Cu, Mn and C while the ferrite is richer in Cr, Mo and Si. The inclusions contain Mn and Si. There are small white areas that are rich in Mo and Si. The small isolated higher C levels on the C map are probably due to specimen surface contamination numbers over 10 unless a special detector is fitted. Modern EPMA instruments are fitted with both WDS and EDS facilities so that WDS can be used for analysis of light elements and for trace elements, leaving the other elements to be analysed by EDS. EXAMPLE OF USE OF EPMA IN DUPLEX STAINLESS STEEL Microanalysis is useful in improving the performance of duplex stainless steel castings. We need to know how alloying elements such as Ni, Cr and Mo, etc. are distributed between the austenite and ferrite phases since this partition will affect the corrosion pitting resistance of each phase . It is also important to characterise the various carbides and other intermetallic phases that can form as precipitates during cooling in the mould, during cooling at different rates from solution METAL Casting Technologies March 2008 43