Study On The Subsurface Crack Of Supporting Roller Made Of Austempered Ductile Iron

Austempered Ductile Iron (ADI) is a new type ductile iron, which has been used extensively in the aero of aerospace, ships, vehicles and heavy industry because of its high strength, excellent toughness, good ductility, great abrasion resistance and so on. It is also beneficial to use ADI material substitute some other materials. Some foreign companies have utilized ADI material to make the supporting roller of heavy machinery, and a satisfied result was gotten. Abrasive wear and spalling are the important factors influencing the useful time of supporting roller. It can offer some methods to prolong the useful time to study the influence of the subsurface crack of ADI supporting roller on spalling.The excellent mechanical properties of ADI own to the isothermal quenching technics, especially isothermal quenching time and temperature. In this paper, LZQT500-7 DI was isothermally quenched with the treatment parameters of austenitizing temperature 900℃, austenitizing time 90min, isothermal quenching time 120min and three different isothermal quenching temperature, 260℃, 310℃, 360℃separately, so three different ADI materials were gotten. The elastic modulus, yield strength and the speed of crack propagating of these three materials were tested with the tensile test and the fatigue crack propagation experiment.The contacting model of roller and creeper tread was established in ANSYS, which could calculate the contact stress and contact size with finite element method. And the reliability of results was verified with Hertz formula. Then the contact stress values were compared with the yield strength.The model of supporting roller was simulated with the calculated results above in FRAC2D/L. The changes of stress intensity factor (SIF) were studied. Then the friction was taken into account, and the△K was calculated. And the spalling life wan predicted in the place where the crack tip had the greatest△K with Paris formula and the method of calculating effective SIF.The results show as follows:1. The yield strength of ADI decrease with the temperature arising from 260℃to 360℃. The greatest yield strength is 1329Mpa at the isothermal quenching temperature 260℃. The influence of isothermal quenching temperature on elastic modulus of is small, but it is still evident that the elastic modulus rises slightly as the isothermal quenching temperature goes up. The speed of crack propagating decreases with the rising of isothermal quenching temperature. Namely, spalling phenomenon is relatively hard to produce in supporting roller at a properly low quenching temperature.2. The calculated values of contacting stress and size were verified to be reliable by Hertz formula and the values are close to each other. The contact stress of supporting roller isothermally quenched at 360℃is prone to getting close to the yield strength3. When the crack depth rises, the value of SIF decreases. And the value goes up with the rising of crack length. The distance of the middle point of crack to the symmetric line of roller also affects SIF, the value of SIF decreases as the distance gets far. The values of KII changes greatly at the place where is about 3mm way from the symmetric line of roller, and it is caused by the contacting size of the model.4. The values of KII decrease when friction is taken into account, and the friction affects the crack propagating times. According to the crack propagating path simulated, the spalling life of the roller whose quenching temperature is 360℃is the greatest. As the quenching temperature goes down, the spalling life gets short.