Powder high-speed steel is a kind of metallurgical steel, which is made from molten steel by high-pressure atomization, through screening, compaction, vacuum sealing and hot-static pressing, and then forged and rolled.
Powder metallurgical steel is compacted by uniform and fine carbide particles. It overcomes a series of shortcomings caused by carbide segregation and forms a new component of high-speed steel which cannot be produced by smelting method. Compared with ordinary high-speed steel made by metallurgical process, the mechanical properties of powder metallurgical high-speed steel are greatly improved. The toughness of powder metallurgy M2 high speed steel is 20% higher than that of normal M2 high-speed steel. Powder high-speed steel has stronger hardness and more uniform structure after heat treatment. Only tools made by this process will not cause tooltip collapse because of the accumulation of carbides or the presence of large carbides.
Effect on the Technological Properties of Materials
- Improving the grindability of materials
The grindability of most powder high speed steels is doubled than that of ordinary high speed steels. It is not easy to grind ordinary high speed steel when its V content is more than 3%, and it is almost impossible to grind if its V content is more than 5%. However, the V content of powder high speed steel T1 5 is 5%, which still has good grindability. Some powder high speed steels containing 9% V can still be grinded. Powder high-speed steel also overcomes the shortcomings of common high-speed steel such as forging difficulty, easy overheating, cracking and distortion in heat treatment.
- New grade high-speed steel with higher alloying element content can be produced.
The chemical composition of powder high speed steel has made a great breakthrough because of no longer worrying about carbide segregation, grinding and forging. Firstly, the V content of steel is increased. Almost all the powder high speed steels contain more than 3% V, and some contain 9.8% V. At the same time, the carbon content of rigid steel is increased. Almost all the powder high speed steels are high carbon high speed steels with a carbon content of more than 1.2% and some of them as high as 2%.
- Higher prices
Powder high-speed steel is more expensive than ordinary high-speed steel. Only gear cutters and special material cutters with special requirements can be used.
Brand and Composition of Powder High Speed Steel
At present, the main producers of powder high-speed steel are Sweden, the United States, Japan, Austria and other countries. Their brands and chemical compositions are shown in the table below.
|Country||Materials||Chemical composition %|
|American||CPM Rex T15||1.5||12.0||/||4.0||5.0||5.0|
|CPM Rex 76||1.5||10.0||5.3||3.8||3.1||9.0|
|CPM Rex 20||1.8||12.5||6.5||4.0||5.0||/|
Heat Treatment of Powder High Speed Steel
High-speed steels in different countries have their corresponding heat treatment specifications according to their grades and chemical composition. The following are some examples of heat treatment process parameters.
The annealing specification recommended by Sweden for powder high-speed steels with four grades and chemical compositions is as follows: after heat preservation at 850 ~900, the cooling rate is less than 10 /h, and then slowly cooled to 700.
According to the basic principle of heat treatment, the heat treatment of powder high-speed steel should be the same as that of ordinary high-speed steel. The difference is that the carbide particles of powder high-speed steel are uniform and fine, and easy to dissolve into the matrix. Therefore, powder high-speed steel with the same chemical composition can choose a lower quenching temperature than that of ordinary high-speed steel, which can generally reduce the temperature by 5 ~8. Similarly, because the austenite of powder high-speed steel is easier to homogenize, a shorter holding time can be used, which can generally be shortened by 1/3 than that of ordinary high-speed steel.
Different countries have different grades and heat treatment specifications of powder high-speed steel. The quenching temperatures of four grades of powder high speed steels in Sweden are set at 1160 ~1180 C. The quenching temperature of American powder high speed steel CPM Rex42 is 1230 C and CPM Rex 20 is 1190 C.
Because the carbides of powder high speed steel are uniform and fine, austenitizing is sufficient, tempering secondary hardening is more sufficient, and tempering effect of the same specification is better. The tempering temperature and holding time of powder high speed steel are approximately the same as those of ordinary type. Twice or thrice tempering is usually used. However, tempering temperature of some brands of powder high speed steel has been adjusted. For example, the tempering temperature of Austrian S390PM powder high speed steel is 500 C.