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DEVELOPMENT OF BAINITIC NODULAR IRON FOR THE CONSTRUCTION OF SPEED GEARS FOR THE CAR INDUSTRY

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CONSIDERABLE ENERGY SAVINGS COULD BE ACHIEVED IF BAINITIC NODULAR (CAST) IRON COULD BE USED FOR GEAR APPLICATIONS (E.G. GEAR BOX DIFFERENTIAL) INSTEAD OF HEAT TREATED 18 CR MO 4 STEEL. THE FIAT STUDY INTENDS TO ANALYSE THE INDUSTRIAL FEASIBILITY AND TO INVESTIGATE THE POSSIBLE PROBLEMS CONNECTED WITH THE USE OF CAST IRON.

IN ORDER FOR BAINITIC NODULAR IRON GEARS TO HAVE THE SAME MECHANICAL PROPERTIES (SUCH AS SURFACE HARDNESS AND CORE STRENGTH) AS STEEL COMPONENTS AFTER A CARBONITRIDING TREATMENT, IT WAS NECESSARY TO TREAT THE MACHINED PARTS BY FERRITIZATION AND AUSTEMPERING CYCLES. THE STRENGTH PROPERTIES OF THIS MATERIAL HAD ALSO TO BE IMPROVED BY A SHOT-PEENING SURFACE TREATMENT.

THE USE OF BAINITIC NODULAR IRON, HOWEVER, RESULTED IN SAVINGS ON TOOL COSTS IN THE MACHINING STAGE. A SET OF LABORATORY TESTS WAS DESIGNED TO SIMULATE DIFFERENT TYPES OF MACHINING OPERATION WHICH WOULD PERMIT ONE TO ESTABLISH A COMPARISON AMONG THE VARIOUS TEST MATERIALS IN A SHORT TIME AND AT LOW COST. THE TWO OPERATIONS MOST SUITABLE FOR A MACHINABILITY EVALUATION OF THE BEVEL GEARS FOR A REARDRIVE VEHICLE DIFFERENTIAL, ARE MILLING AND DRILLING. WHEN THE COMPARISON IS MADE FOR MILLING BETWEEN TWO CAST IRON VARIETIES AND STEEL, IT CAN BE SEEN THAT PERLITIC CAST IRON HAS AN ADVANTAGE OVER STEEL OF ABOUT 100 %, WHILE FERRITIC CAST IRON REACHES THE 200 % LEVEL. THE DRILLING TESTS SHOWED THE ADVANTAGES OF FERRITIC CAST IRON OVER BOTH PERLITIC CAST IRON AND STEEL (FROM 130 % TO 1500 %). AT THE SAME TIME THE STUDY INDICATED THE OCCURENCE OF INITIAL DEFORMATION ON THE FINISHED PARTS. FOR THIS REASON IF BAINITIC IRON IS TO BE USED FOR GEAR APPLICATIONS, FURTHER EFFORTS ARE NEEDED IN THE AREAS OF HEAT AND MECHANICAL TREATMENT IN ORDER TO OBTAIN SATISFACTORY CHARACTERISTICS OF THE NEW MATERIAL.

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Fiat Auto SpA
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