Alloying Design for High Wear-Resistant Cast Hot-Forging Die Steels

The alloying design of cast hot-forging die steels was analyzed. The relationship of the life of cast hot-forging dies with the failure patterns was studied. The thermal wear resistance was believed to be the key property for the alloying design of cast hot-forging die steels. The alloying design parameters were selected and optimized for the cast hot-forging die steel with high wear resistance. The wear resistance of the optimized cast die steel was evaluated in comparison with commercial H13 steels and 3Cr2WSV steel. In the new cast hot-forging die steel, VC is predominant carbide with Cr and Mo as the main solution elements in α-Fe. It is found that the cast die steel has significantly lower wear rate than normal H13 steel and 3Cr2W8V steel, almost the same as that of high purity H13 steel. The high wear resistance of the new cast hot-forging die steel can be attributed to its reasonable alloying design and nonsensibility to the detrimental function of S and P.

Selection of Heat Treatment Process and Wear Mechanism of High Wear Resistant Cast Hot-Forging Die Steel

Dry sliding wear tests of a Cr-Mo-V cast hot-forging die steel was carried out within a load range of 50--300 N at 400℃ by a pin-on-disc high temperature wear machine. The effect of heat treatment process on wear resistance was systematically studied in order to select heat treatment processes of the steel with high wear resistance. The morphology, structure and composition were analyzed using scanning electron microscopy （SEM）, X-ray diffraction （XRD） and energy dispersive spectroscopy （EDS） ; wear mechanism was also discussed. Tribo-oxide layer was found to form on worn surfaces to reduce wear under low loads, but appear inside the matrix to increase wear under high loads. The tribo-oxides were mainly consisted of Fe3O4 and Fe2O3, FeO only appeared under a high load. Oxidative mild wear, transition of mild-severe wear in oxidative wear and extrusive wear took turns to operate with increasing the load. The wear resistance strongly depended on the selection of heat treatment processes or microstructures. It was found that bainite presented a better wear resistance than martensite plus bainite duplex structure, martensite structure was of the poorest wear resistance. The wear resistance increased with increasing austenizing temperature in the range of 920 to 1 120 ℃, then decreased at up to 1 220 ℃. As for tempering temperature and microstructure, the wear resistance increased in following order： 700℃ （tempered sorbite）, 200 ℃ （tempered martensite）, 440 to 650 ℃ （tempered troostite）. An appropriate combination of hardness, toughness, microstructural thermal stability was re- quired for a good wear resistance in high-temperature wear. The optimized heat treatment process was suggested for the cast hot-forging steel to be austenized at 1020 to 1 120 ℃, quenched in oil, then tempered at 440 to 650℃ for 2 h.

Wear Behavior and Mechanism of Spheroidal Graphite Cast Iron

Wear behavior and mechanism of spheroidal graphite cast iron were studied on a pin on-disk elevated tem- perature wear tester. The phase and morphology of worn surfaces were examined by X ray diffraction and scanning electron microscopy. Results show that with an increase of load, wear rate of spheroidal graphite cast iron gradually increases under low loads, rapidly increases or potentially increases under high loads; wear rate increases with in- creasing ambient temperature. At 25 200 ℃, adhesive wear prevails; oxidative wear and adhesive wear coexist at 400 ℃. As load surpasses 150 N at 400 ℃, extrusive wear appears. The elevated-temperature wear of spheroidal graphite cast iron is a physical and chemical process including the following reactions： xFe＋y/2O2-FexOy , 2C＋ O2- 2CO and Fex Oy ＋yCO-xFe＋yCO2. Hence, at 400 ℃, the amount of graphite and tribo oxides are substan- tially reduced because of reductive function of graphite. It can be suggested that wear reduced effect of graphite and tribo-oxides is impaired.