Machining of stainless and heat-resistant steels
Characteristics of machining
Plasticity. Most heat-resistant steels are very ductile, which makes it difficult to process by cutting. The ductility index is determined by the ratio of the yield strength to the tensile strength. The lower the ratio, the more ductile the material is.
Low thermal conductivity of alloyed steel leads to higher temperatures in the working area and requires optimal selection of coolant, which in addition to effective heat removal should facilitate cutting and prevent chips on the working edge of the cutting blade. This problem can be solved by using a special coolant.
Hardness retention under high temperatures combined with riveting leads to accelerated wear of the cutting edge and prevents the use of high RPMs.
Second phase solid solution. The presence of an extremely hard intermetallic and carbide second phase solid solution in the steel acts like an abrasive material on the cutting edge surface. The tool blunts much faster.
Low vibration resistance results from uneven hardening of the workpiece as it is cut — because the plastic deformation process occurs differently during machining, in the beginning and in the middle of machining.
Optimising the process
To reduce tool blunting, you can reduce stock removals and tool feed, and increase spindle speeds. Acid surface treatment has proven to be a good method of reducing wear on the cutting edge. Optimization of the technology means, first of all, optimal selection of cutting tools, selection of optimum cutting conditions and correct choice of coolant.
Grades of cutters
Hard alloys T30K4, T15K6, BK3 have high hardness and wear resistance. Wear-resistant T5K7, T5K110 are more ductile, but less wear-resistant. And, finally, BK6A, BK8 have lower wear resistance, but higher toughness — they have proven themselves under shock loads.
Tungsten carbide plates with TiC coating.
They are distinguished by their high wear resistance. The cutting properties of carbide inserts are significantly affected by various methods of treatment of such materials, such as nitriding and cyanidation. Coating with cubic boron nitride is quite expensive, but has truly unique properties — such coating increases the hardness of the cutting edge many times.
Machining of heat-resistant steels
Such grades of hard alloys as Р14Ф4, Р10К5Ф5, Р9Ф5, Р9К9 are applied. The letter P in the designation indicates that the alloy belongs to the high-speed. In such alloys cobalt and vanadium are added, which greatly increases the mechanical resistance of the cutting tool. The use of high-speed alloys allows significantly accelerate the processing of alloyed steels.
Coolant application
Special coolant prevents premature tool wear, improves cutting characteristics and increases accuracy. Each type of processed steel requires its own coolant, a method of its supply to the working area. The most effective method is the one that promotes maximum heat removal from the cutting zone. Here, high-pressure coolant supply mainly to the back surface of the cutting tool working plate, coolant spraying and, rare enough, mainly at defensive enterprises, cooling by carbon dioxide proved themselves well.
Choice of cooling method
It depends on the processing conditions and the technological capabilities of the equipment. High-pressure cooling is the most widespread and can be used for turning, milling, grinding, etc. This method is typical for many equipment manufacturers, both domestic and foreign. The liquid is sprayed precisely into the working area. Coming in contact with the heated metal, it evaporates quickly, taking away the heat and effectively cooling the working surface. This method makes it possible to increase tool life by almost 6 times, which has an impact on the cost of the part in the end.
Coolant supply
More effective is the simultaneous supply of coolant in the cutting area and in the area of chip formation, however, technically it is not always possible — it may require modifications of technological equipment. This cooling method is suitable for medium and small-scale production.
Cooling with carbon dioxide
The most effective way to dissipate heat from the machining zone is of course cooling with carbon dioxide, where the temperature in the cutting zone is around minus 79 °C. However, this method is the most expensive and is only suitable for individual production. It is usually used in the defense industry.
Furnizor
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