The Research On Fatigue Property Of The Materials Used In Lorry Coupler Knuckle With Increased Speed And Heavyload

The article researches the influence of heat treatment process on fatigue property of low alloy cast steel ZG25MnCrNiMo used on coupler knuckle for the purpose of meeting the lorry requirement of increased speed and heavy load. In this research, three different kinds of heat treatments are carried out: quenching plus high temperature tempering (normalization grade C steel) and normalization plus high temperature tempering plus surface quenching (quenching and tempering grade C steel) except for normalization plus high temperature tempering (normalization grade C steel); and their laws of effect on the rotary bending fatigue property, low cycle fatigue property and fatigue crack growth rate of lorry coupler knuckle are researched. The results show that the life of normalization grade C steel and quenching plus high temperature tempering grade C steel differs little; while the fatigue life N50 and safety fatigue life N95 of surface quenching grade C steel are obviously higher than that of normalization grade C steel and quenching plus high temperature tempering grade C steel; the fatigue life limit of normalization grade C steel and quenching plus high temperature tempering grade C steel differ little, but compared to the normalization grade C steel and quenching plus high temperature tempering grade C steel, the fatigue limit (σ-1)50 of surface quenching grade C steel is increased by 29%~34% and safety fatigue limit (σ-1)95 is increased by 16 %~21.5 %. The low cycle fatigue property of surface quenching grade C steel is higher than that of normalization grade C steel and quenching and high temperature tempering grade C steel. When the value of strain is large, the low cycle fatigue life of quenching and high temperature tempering grade C steel and normalization grade C steel is long, while when the value of strain is small, the low cycle fatigue life of surface quenching grade C steel is long and the higher the strength of material is, the longer the low cycle fatigue life is. The fatigue crack length, the fatigue crack growth doorstep value and fatigue crack growth rate of normalization grade C steel and quenching and high temperature tempering grade C steel is almost the same, while the fatigue crack length is longer, fatigue crack growth rate is higher and the fatigue crack growth doorstep value is lower in surface quenching grade C steel. This shows that after quenching the crack of the low alloy cast steel ZG25MnCrNiMo easily grows larger in low stress strength coefficient. On the basis of experiment, according to the shape of coupler knuckle, the inductor designed and made is used to surface quenching of coupler knuckle. After quenching, the hardness layer with thickness over 4mm and hardness over 40HRC can be attained and form remnants pressure stress on the surface of coupler knuckle. This makes coupler tongue have longer fatigue life. The author thinks that coupler knuckle with low alloy cast steel ZG25MnCrNiMo firstly quenched and tempered, quenching plus high temperature tempering grade C steel can be gained that has higher low temperature impact ductility and when it is surface quenched, it not only has higher low temperature brittleness fracture resistance, but also has higher wear resistance and fatigue strength. This procedure is an economical heat treatment method to improve coupler knuckle service property without changing the current materials.