by clicking the arrows at the side of the page, or by using the toolbar.
by clicking anywhere on the page.
by dragging the page around when zoomed in.
by clicking anywhere on the page when zoomed in.
web sites or send emails by clicking on hyperlinks.
Email this page to a friend
Search this issue
Index - jump to page or section
Archive - view past issues
button in toolbar for more information.
Metal Casting Technologies : March 2009
TECHNICAL FEATURE Ferritic high chromium irons High Cr irons containing 28-35% Chromium and 1-2%Carbon solidify as ferrite dendrites which remain stable on cooling producing as cast structures of ferrite and interdendritic eutectic carbides. A typical microstructure is shown in Figure 4. They have UTS values of 400-550N/mm2 and hardness of 250- 350 Hb. The ferritic High Cr Irons have much lower wear resistance than the austenitic and martensitic grades  but they have good oxidation resistance at temperatures up to 1100o C and also Figure 4. General microstructure of a ferritic high chromium iron showing ferrite dendrites with interdendritic Cr rich eutectic carbides (x150)  strength is reduced by about 25% and ductility by about 50%. It is generally found  that to avoid chunky graphite in thicker sections of austenitic iron the composition should obey the formula: [TC% + 0.2Si% + 0.06Ni%] < 4.4. As for all ductile irons, it is important to use correct Mg treatment and inoculation procedures and to avoid the effects of fading. To minimize the chance of distortion during use at temperatures above 450-500o C the austenitic irons can be heat treated prior to service in order to improve their growth resistance, and hence their dimensional stability, by removal of excess carbon from the austenite matrix. This involves slow heating and holding for at least 2 hours at 900o to 550o C followed by furnace cooling C. good corrosion resistance in oxidizing acids. Due to their high resistance to scaling and growth they are used for heat resistance in sinter plants, for furnace parts, for recuperator tubes, and for burner nozzles, etc. Control of the Cr/C ratio avoids the formation of austenite so that the ferritic matrix is stable. However with time in service, depending on composition, sigma phase and carbide precipitates may form leading to a reduction in toughness . To minimize the formation of such damaging constituents the %Manganese and %Silicon contents are each limited to 0.5%, and the Phosphorus level is kept below 0.05%. Sigma phase formation is outlined later in relation to austenitic steels. (a) As cast structure: austenite matrix with interdendritic network of eutectic carbides. (x500) Figure 5. Microstructures of HN grade 20%Cr- 25%Ni steel  40 www.metals.rala.com.au (b) After soaking at 980oC for around 75 days. Precipitation of secondary carbides within the matrix and coarsening of the interdendritic/grain boundary carbides.(x500)