Effects of processing parameters on the properties of nano-sized cemented carbide

cemented carbide encountered in the process of research and development of the contradiction between the strength and hardness, the high hardness, low strength, high strength, low hardness. The traditional method of preparation of WC cemented carbide particle size is always greater than its original particle size, usually in several microns.This large particles alloy apparent hardness and strength of contradiction and softening problem such as larger brittle and processing. To achieve a cemented carbide and high hardness and high strength, need on the basis of the particle size of micron grade further refine grain.Reduce the grain size of WC, can increase the contact number and area of carbide, strength, hardness and toughness can be obtained theoretically with better alloy material. The preparation of nanometer structure of the alloy material is considered to be the best way to solve this problem. The United States, Japan, Sweden and other countries have been produced with high hardness (HRA is greater than 90) and high strength (greater than 3200 mpa) of nano-sized cemented carbide.

 

However, many countries in the nano-sized cemented carbide research have encountered difficulties, there were two main problems, one is the preparation of nanometer WC - Co composite powder activity is very big, is very sensitive to temperature,in a high temperature will quickly grow up; 2 it is difficult to solve the carbide in the process of sintering density increased at the same time, the problem of grain growth. The current development trend is to improve the preparation of ultra-fine WC - Co composite powder sintering process and improvement.

 

In the preparation of nanometer WC - Co composite powder, developed spray heat conversion method, comprehensive mechanical - thermal activation method,the direct reduction carbonization method and plasma method, etc.The spray heat conversion technology and related technology most mature, comprehensive mechanical - thermal activation method development potential is the largest, direct reduction carbonization method and plasma method is difficult to effectively control the carbon content and the limitations of high production cost.

Spray heat conversion method is the most widely industrial application of nanometer WC - Co composite powder preparation methods,the basic process is directly to make good the WC powder, Co powder, solid solution and forming agent for charge calculation, add the alcohol into the precursor solution in wet on wet grinding, make the raw material blending, then spray drying. The WC grain size can be obtained for more than 40 nm. Spray heat conversion method of the main advantages:

1. because the precursor in solution exists in the form of liquid,molecular diffusion fully and evenly distributed, makes easy to control grain size in subsequent sintering process of grow up.

2. the technique, less waste of materials, save resources and has little pollution to the environment.

3. the spray drying process is in the closed loop system, under the condition of nitrogen protection, so after spray drying of mixture stable performance and good liquidity.

 

In sintering process, the development of many new techniques, including low pressure hot isostatic pressing sintering, the discharge plasma sintering, microwave sintering and selective laser sintering, etc. Now the most widely used is low pressure hot isostatic pressing sintering. The process is lower than that of conventional hot isostatic pressing under pressure (about 6 mpa), dewaxing, presintering, vacuum sintering, hot isostatic pressing and so on several process to a process within a device. Low-pressure hot isostatic pressing sintering process of the preparation of cemented carbide high density, good uniformity, good performance,and the process has a short production cycle, less process,low energy consumption, small waste of materials, etc. Experiments show that compared with other process, the process of WC - Co alloy,uniform microstructure, did not produce bulky WC grain size,its performance has reached the requirement of hardness and intensity combination.