1, Morphology of impact fracture

We analyzed the impact fracture morphology of two kinds of tungsten carbide dies for cold heading screws at home and abroad. Results show that the alloy porosity of the tungsten carbide die made by the two manufacturers in China is higher, and the alloy porosity of the same kind in Germany is the lowest and the size is the smallest. The porosity will lead to a serious reduction in the life of the cold heading die.

 

 

The surface porosity (inclusions, non-compounded carbon and equivalent) in alloy structure will accelerate the wear failure, while the internal pore will accelerate the process of crack propagation. It is found that the porosity of German tungsten carbide die is the lowest, so it is considered that the low porosity is a reason for the longer life of German die. One of the efforts to improve the life of this die is to reduce its porosity, which means to lower its A04 level and even produce non-porous alloy.

 

2, Metallurgical Structure

By microscopic observation shows that the Chinese tungsten carbide die is moderately coarse grain structure, where the distribution of WC and CO is uniform. The grain size of WC is slightly higher than that of CO, that the average grain size of WC grain is 2um (the largest size is 6-7um).

 

YG15 tungsten carbide die is composed of 2 kinds of coarse fine grain. Its average grain size of WC grain is 1.4um (the largest size is 6-7um), and there is also the phenomenon of adjacency between WC and WC and the aggregation of local grains. The microstructure of German die can be characterized by heterogeneous grain structure.

 

The tungsten carbide with non-uniform WC grain structure has good comprehensive performance such as high CO strength, high toughness, high wear resistance. Reasons are that due to the complete crystallization of WC coarse crystals and the denser accumulation of coarse WC grains, the coarse grains impede the crack propagation and make the crack branching, making the distribution of the CO phase be more uniform. So, this kind of alloy has been used in mining tools and has achieved good results in long service life.

 

Through the analysis, it is found that the die alloy used in the cold heading of the German cold heading is also an inhomogeneous WC grain structure, which may be the foundation of the die's service life of 10 million times. The high degree of WC/WC adjacency is easy to produce and expand cracks, which is unfavorable to the life of cold heading dies. It is suggested that Chinese manufacturers should strive for more precise product size processing and more precise control of heat treatment process.