Development Of Class 14.9 Bolt For Automotive Industry And Study On Its Properties

An increasingly urgent task for automotive industry is to reduce the weight and energy consumption of automobile. Further strength improvement of high-strength bolt could benefit not only the weight reduction of bolt itself, but also the weight reduction of bolted parts and even the entire automobile. With this background, some concentrative researches on automotive high-strength bolt steel and bolt have been taken in industrial developed countries in recent years, and a series of ultra-high-strength bolt steels and bolts have been developed. Therefore, it is significant to research ultra-high-strength bolt steel and to develop bolt with higher strength.Based on the previous researches on ADF1 steel, this paper has conducted studies as below:(1)Studying the effects of heat treatment on ADF1 steel's mechanical properties, to obtain the optimal heat treatment process for class 14.9 high-strength bolt.(2)Taking a comparative study between ADF1 steel and traditional 42CrMo steel, SCM435 steel for class 12.9 bolt, to review the feasibility of ADF1 steel for class 14.9 bolt.(3)Studying the property requirements of class 14.9 bolt and key manufacturing technologies, and trial-manufacturing class 14.9 high-strength bolt.(4)Studying properties of class 14.9 bolt, especially its fatigue property and delayed-fracture property, to evaluate if class 14.9 bolt could meet the demands for practical application.Results of studies on heat treatment and comparative studies between ADF1 steel and traditional class 12.9 bolt steels show that ADF1 steel's key properties, such as delayed fracture property, fatigue property, notch sensitivity property, etc., can all meet the demands for class 14.9 bolt. Therefore, trial-manufacture of class 14.9 bolt with ADF1 steel has been conducted. And through studies on key manufacturing technologies, the reasonable manufacturing process has been found.To manufacture class 14.9 bolt with ADF1 steel, 940℃might be chosen as the austenizing temperature, and 610℃as the tempering temperature. With such heat treatment parameters, the tensile strength can be controlled in the range of 1440MPa-1500MPa, and other mechanical properties are satisfied as well.Results of studies on bolt, such as tensile test, hardness test, creep behavior study at room temperature, fatigue test, tightening experiment, environmental corrosion and delayed fracture experiment, etc., show that the newly developed bolt can provide satisfied performance. The fatigue limit of class 14.9 bolt has been improved by 47.7%, compared with class 12.9 bolt of same size made of 42CrMo steel. And the environmental corrosion resistance and delayed fracture resistance are also better than class 12.9 bolt.The developed class 14.9 bolt has passed the rig test and small scale application validation,which means that the application of class 14.9 high-strength bolt as key bolt in automotive engine is feasible.