Analysis of the overheating fracture of 40CrNiMoA steel crankshaft:
1. Quality issues:
The forging process is: free forging, die forging, 860 ° C normalizing and 660 ° C tempering. Forgings heated by the same batch of batches, due to different actual heating temperatures, different degrees of overheating fractures appear after forging, and the performance is degraded. Eight forgings with different degrees of superheat were tested for fracture, conventional mechanical properties and fatigue strength.
The test data shows that overheating is not sensitive to the influence of the strength index of this steel, and has a significant effect on the plasticity index. When the temperature is too hot, it is significantly lower and lower than the technical standards. As the degree of superheat increases, the rotational bending fatigue strength also decreases significantly.
When the forging is heated normally and the fracture is fibrous, the performance does not change much. When the forgings are generally overheated and the fractures are partially stoned, the plasticity index is reduced. When the forging is severely overheated and the stone is broken, the plasticity index is significantly reduced.
2. Defect analysis:
The root cause of the overheating fracture is mainly because the heating temperature is too high, the specific reasons are:
1) When the heating is too high, the temperature is too long, or too much is charged in one furnace, and the forging time of the furnace is too long, so that the heating temperature is too high.
2) The temperature inside the furnace is not uniform, the blank is improperly placed in the furnace, and the part directly exposed to the flame is easily overheated.
3) The part with a small cross section after the preform is heated up quickly when reheating, and it is easy to overheat. If the deformation amount is too small when the die forging, and the stop forging temperature is high, the superheated structure will be retained.
3. Prevent countermeasures:
1) Strictly control the heating temperature and heating time. Do not charge too much in one furnace. When stopping forging in the middle, the furnace temperature should be lowered in time.
2) Do not approach the nozzle (or the nozzle) when installing the billet to prevent the flame from being directly sprayed onto the billet.
3) Do not pull the rod too thin during the manufacturing process to ensure a large amount of deformation during final forging.
4) When the superheated structure of 40CrNiMoA steel forgings is difficult to eliminate by general heat treatment, multiple recrystallization heat treatment or high temperature diffusion annealing treatment may be used. Forgings that are severely overheated should be scrapped.