Metal Casting Technologies : MCT MAR 2018 (1ST QRT)
28 www.metals.rala.com.au METAL Casting Technologies 1st Quarter 2018 29 TECHNICAL FEATURE 4.3 -4 .5 for sizes over 50mm. C levels of 3.5-3 .8% with Si at 2.3 - 2.7% have been suggested as optimal . Unless a carbidic iron is to be produced the cast structure should be free from intercellular eutectic carbide. Ni, Cu and Mo additions should only be made when necessary, i.e. to achieve sufficient hardenability to avoid pearlite formation during quenching to the austempering temperature. Maximum levels of 2% and 0.8% are suggested for Ni and Cu respectively. Any Mo addition must be below 0.3% since, like Mn, it segregates to eutectic cell boundaries forming carbides which are difficult to decompose during austenitisation. Segregation of Mo and Mn tends to promote martensite formation in the inter-cellular regions in austempered microstructures. A general recommendation is that Mn levels should be limited to 0.6% max. in sections <13mm in thickness, and since nodule numbers tend to decrease and segregation is enhanced as section size increases, Mn content should be <0.35% in larger sections. Other work suggests that for all sections %Mn should be less than 0.4 . The lower nodule numbers and more severe segregation obtained in microstructures as section size increases, lengthens the austenitising time required for satisfactory homogenisation of the austenite prior to austempering of thicker-sectioned castings. Heat treatment cycles must take account of casting design, chemical composition, and as-cast microstructure. Precautions during heat treatment include control of: z Heating rate to austenitising temperature to avoid distortion and danger of cracking z Furnace atmosphere to prevent oxidation and decarburisation during austenitising and transfer z Austenitising time and temperature z Quench (heat transfer) characteristics and circulation in the salt bath for austempering z Austempering time and temperature z Amount of retained austenite z Dimensional changes For efficient heat treatment sealed quench furnace designs, of the type used for carburising of steel components, have been adapted to provide specialised sealed quench austempering furnaces in which a molten salt bath replaces the oil quench tank [12-14]. The newer furnaces can process maximum casting loads of 3.5 to 7.5 tonnes . Castings must be austempered at a suitable temperature for the correct time to produce an ausferrite matrix with a controlled amount of retained austenite. Any transformation to carbides and/or martensite must be avoided. In considering the “process window” for austempering in the higher temperature range to produce ausferrite matrices, the first part of austenite transformation. i .e. to ferrite with remaining austenite becoming enriched in C has been termed the Stage 1 austempering reaction . This is: Austenite Ferrite + Austenite (High C). This reaction is said to be complete when the ferrite fraction and the austenite C content reach their maximum values and hence when no martensite is formed on cooling to ambient temperature. As austempering time is increased a second reaction, Stage 2, eventually occurs in which the untransformed austenite begins to decompose to ferrite and carbides. This is: Austenite (High C) Ferrite + Carbides. The time period between the completion of the Stage 1 reaction and the onset of Stage 2 has been described as the “processing window” for austempering . During this window the amount of ferrite and the C content in the austenite remain constant. The length of the window depends chemical composition and on the extent of micro-segregation . The proportion of retained austenite in the resultant ausferrite microstructures depends essentially on austempering temperature with 35-40% retained after treatment at 375- 400°C, about 20% at 325-350°C and 10-15% at 300°C . In general, maximum levels in impact resistance and fracture toughness correspond with maximum retained austenite levels in final microstructures. As outlined above at low austenitising temperatures, due to lower diffusion rates of C, some carbides also form during the Stage 1 reaction. Compared to higher temperatures, more austenite decomposes to ferrite, less austenite is retained (before the Stage 2 reaction begins to form more ferrite and carbide) and much of this austenite transforms to martensite on cooling. Austempered microstructures cannot easily be characterised using conventional optical (light) microscopy. Scanning electron microscopy (SEM) of etched structures can reveal the morphology of ferrite and austenite phases but thin foil transmission electron microscopy must be used to study crystallographic relationships between phases and to identify the nature of carbides that may be present. Micro-analysis associated with SEM and/or TEM can determine local alloy elemental levels in determining the extent and effects of inter- cellular segregation on transformation behaviour. Retained austenite levels can be determined by suitable X-ray diffraction (XRD) techniques. There is interest in developing micro-scale models to predict austenite transformation to ausferrite. Recent modelling has shown that transformation rate depends on graphite nodule count and austempering temperature while the amount of ferrite formed increases with decrease in austenitising and/or austempering temperatures [25,26] Transformation during austempering produces dimensional growth of up to 0.5% . Dimensional changes can be predicted if consistent as-cast matrix structures are obtained and if consistent proportions of ferrite and austenite are present in ausferritic structures. Castings are usually at least rough machined before austempering with higher hardness grades being fully machined. Where parts require greater precision, final machining of critical dimensions is carried out after austempering for some lower hardness grades provided their compositions are closely controlled [23,27] A ferritising anneal has been suggested prior to the first machining since subsequent dimensional changes, although higher, are said to be more predictable if the starting matrix is ferrite rather than ferrite + pearlite mixtures . Together with dimensional control in ADI components, the special problems faced when machining austempered structures, e.g. high work hardening with strain induced transformation of austenite to martensite, vibration and high workpiece-tool interface temperature, have been the subjects of considerable research and practical studies [14,27-30]. Other developments To improve resistance to abrasive wear in agricultural, mining, and civil engineering applications etc. ADI microstructures can include controlled volume fractions of as-cast eutectic M3C carbides. This material is called “Carbidic Austempered Ductile Iron (CADI)” [13-15], an example of microstructure is shown in Figure 7 . This material can compete with alloy white irons for some agricultural wear parts in that it can provide abrasion resistance with improved toughness to avoid premature impact failures [15, 31]. The standard grades of ADI listed earlier in Table 1 have minimum tensile strengths ranging from 900 to 1600 MPa. Lower strength grades that offer improved machineability can be produced by partial austenitising heat treatment at temperatures in the (ferrite + austenite) phase field rather than complete austenisation before austempering [14,16,32,33] This is known as intercritical treatment and provides “IADI” grades with minimum tensile strengths of 750 and 800 MPa. This type of ADI is sometimes referred to as “Dual phase ADI” [e.g. 33] and has matrix microstructures of pro-eutectoid ferrite + ausferrite. Such microstructures not only give similar levels of ductility to ferritic ductile grades but also similar strength and hardness levels to fully pearlitic grades. Towards ductility improvement in ADI there is interest in two -step austempering treatments [34-37] in which the iron is initially held for a short time at a lower austempering temperature to give nucleation of fine ferrite. Then, to encourage C partition into the austenite, the austempering temperature is progressively raised by 25-35°C during treatment. Such step-up treatment is also said to improve abrasion resistance [35,37]. There have long been attempts at producing high strength bainitic matrix structures in ductile irons directly in the as-cast condition thus avoiding the need for heat treatment [e.g. 38-40]. Further developments and applications were limited by the cost of alloying (up to 4%Ni and up to 1%Mo) needed to prevent pearlite formation, and by the tendency for formation of intercellular carbides and/or martensite due to Mo segregation. Also, these irons quite often needed tempering to achieve suitable properties. More recently there has been renewed interest in obtaining as-cast ausferritic structures by controlled (engineering) cooling whereby shakeout and isothermal transformation temperatures are designed to achieve pearlite free ausferritic matrices in all sections of a casting [41-43]. To achieve the required continuous cooling transformation (CCT) characteristics irons containing 3-5%Ni, up to 0.2%Mo and up to 1%Cu have been studied to model the processing window needed to achieve ausferritic microstructures [41,43]. For all austempering treatments it is critical that predictable microstructures and properties are obtained in all sections of a casting. Simulation and microstructural modelling will play an increasingly important role in achieving this objective. One example is the combination of process simulation with virtual Design of Experiments to optimise the austempering process . Also, for the future, to provide more design information for potential users of ADI, ideas for new material standards have been proposed . Rather than basing standards just on minimum tensile values of strength and ductility it is suggested that a quality index value which covers strength and ductility should also be included together with the strain hardening profile of the tensile stress-strain relationship and more information on fracture and fatigue properties. FIGURE 7. The microstructure of Carbidic ADI contains both graphite nodules and as-cast eutectic M3C carbides in an ausferritic matrix, x300 approx. After Hayrynen  SCANNING ELECTRON MICROSCOPY (SEM) OF ETCHED STRUCTURES CAN REVEAL THE MORPHOLOGY OF FERRITE AND AUSTENITE PHASES BUT THIN FOIL TRANSMISSION ELECTRON MICROSCOPY MUST BE USED TO STUDY CRYSTALLOGRAPHIC RELATIONSHIPS BETWEEN PHASES AND TO IDENTIFY THE NATURE OF CARBIDES THAT MAY BE PRESENT.
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