Machining stainless and heat resisting steels
Stainless steel is called alloy, which is capable of long time to withstand exposure to chemically active environments, it may be unfavorable atmospheric conditions, and acidic or alkaline environment in the chemical industry. In recent years, many nodes, machines and mechanisms of carbon steels are used less and less, and they are gradually replacing elements of special steels. This is due to the fact that ordinary steel has a certain threshold — the limit above which it becomes impossible its use in an environment of increasing loads, e.g. at high temperatures, pressure or in the presence of aggressive environments. In this case, they successfully replace heat-resistant stainless steels and alloys doped with exclusive properties that will work well where ordinary steel can not cope.
Advantages of stainless steels
The heat resistance. Called high-temperature material that can withstand high temperatures without losing their mechanical strength. High-temperature steel will take and the group dispersion hardening, with the release of the alloying element is different from the basis steel, in fine shape, and its distribution throughout the volume of the metal. Heat resistance characterizes material that does not lose corrosion resistance when heated. The combination of these qualities have alloy corrosion resistant steel. High strength and toughness data materials and refer them to the class difficult, especially when cutting with removal of chips. This requires a special tool, the cutting, the selection of coolant, and the solution. important details.
The comparison of physical and mechanical properties of alloy steels and conventional, it was found that indicators such as the limit of the tensile strength, hardness they are approximately equal. But alloyed and plain steels are only the same mechanical properties, while other qualities may differ substantially, especially microstructure, corrosion resistance and ability to strengthen its positions by mechanical action. Remember the diagram of tension — compression, well-known from the course of strength of materials. The diagram starts from the area of elastic deformation, when the material, after removal of the load returns to its original state without deformation. Increasing the load results in the area of so-called «fluidity» when the material starts to deform without significant growth of the applied force. On the graph is almost a horizontal line. Followed by a sharp hardening and further strain is necessary to increase the force of impact. The same process occurs in metal cutting, only in the surface layer of metal is associated with changes to the crystal lattice under the action of mechanical loads. In the processing of ordinary steel is also typical, but the hardening of alloy steels is much more pronounced. And don’t forget the differences in properties such as thermal conductivity, melting point etc. that also have a significant impact on the treatment process.
Thus, during the cutting process, the indicators of hardening alloy steel is high enough, which requires the application of considerable forces. In addition, most alloy steels, particularly heat-resistant, highly plastic, which also makes it difficult machining. The index of plasticity is defined as the ratio of the conditional yield strength, tensile strength. The smaller the ratio, the ductility of the material, the more hardened under mechanical load. And the stainless steels belong to highly plastic. In addition, there is another side of plasticity, the so-called «viscosity» of the material. When machining alloy steel on a lathe, the chips are not broken, as for example, when machining carbon steels of the same hardness, and winds a long strip. This causes a lot of inconvenience and complicates its processing in automatic mode.
The second feature of alloy steels in machining — low thermal conductivity, which leads to higher temperatures in the working area, and requires the optimal selection of the coolant, which in addition to the effective removal of heat must facilitate cutting and to prevent hardening. Hardening occurs on the working edge of the cutting plate leads to changes in the geometry of the cutter, and eventually to its premature failure. As a rule, when machining alloyed high-temperature steels are not recommended for high-speed processing — this leads to higher details. To solve this problem by using special inserts designed exclusively for alloy steels and special coolant.
The third feature — the preservation of strength and hardness under high temperatures. This is particularly true for heat-resistant steels, which, combined with shot peening leads to accelerated wear of the cutting tool and allows you to use high revs.
Fourth — the presence in the composition of the steel solid solution second phase with an extremely solid carbide and intermetallic compounds, which, despite their microscopic size, are on the surface of the cutting tool as the abrasive material. The tool grinds and blunts much faster, which leads to the necessity of frequent regrinding and straightening of the blade geometry. As practice shows, the coefficient of friction, the processing of alloy steels to order more than ordinary carbon steels.
Fifth. Low vibration caused by the uneven processes of hardening of details as cutting — because the process of plastic deformation in the processing proceeds differently, in the beginning and in the middle of processing. If you are processing a small-sized item, in principle, this phenomenon can be neglected. When it comes to handling long items — for example — the shaft, there may already be a difficulty.
Optimization of technology
All these phenomena require a special approach to treatment of alloy steels cutting, especially if the processing is done fully automatically — for example, on sliding head machines and CNC machines with automatic feed rod. How can you reduce the impact of 'negative factors — consider the example of turning — as the most common. Turning involves the removal of a layer of seam allowance in the form of chips from rotating around its axis part. The movement of the cutter in this case occurs in two coordinates in a horizontal plane. Under the influence of the cutting forces there is a partial offset of the crystal lattice, there is a hardening — surface hardening. A substantial part of the frictional energy of the tool is converted into heat. and as we remember — the material has low thermal conductivity. The surface heats unevenly, there is the vibration, resulting in a negative effect of these factors compounded.
The tool is not dull so quickly, you can reduce the layer of cut stock and flow tool, and to increase the spindle speed. As a result, the surface will be obtained with higher roughness class. Well proven methods of treatment of alloy steels acid — this reduces the degree of occurrence of phenomena such as accelerated tool wear and work hardening, however, is extremely negative impact on lathe equipment and lathe. Optimization of treatment of alloy steels is, first and foremost, the optimal selection of the cutting tool, the improved stability, the selection of optimum cutting conditions, and proper choice of coolant and its optimal flow.
Hard alloy Т30К4, T15K6, ВК3 have high hardness and resistance to wear. Wear-resistant weld Deposit Т5К7, Т5К110 — more mating, but less wear resistant. And finally, ВК6А, VK8 different reduced wear resistance, but increased viscosity — they have proven themselves under shock loads.
A carbide insert with coating of TiC
They have high durability. A significant effect on the cutting properties of carbide inserts have different ways of handling such materials — for example, nitriding and cyanide leaching. Coating cubic boron nitride is quite expensive, but has a truly unique properties — such coating greatly increases the hardness of the tool, its durability and resistance.
Processing of high-temperature steel
Are the marks of rigid alloys as р14Ф4, Р10К5Ф5, Р9Ф5, Р9К9. The letter R in the designation indicates that the solid alloy to high speed. In such alloys added cobalt and vanadium, which significantly increases the mechanical strength of the cutting tool. The use of high-speed alloys can significantly speed up the processing of alloy steels, reducing the consumption of the tool. But these alloys have a weak spot — they are afraid of overheating. If, during the processing of steel tool with such an insert happens the interruption in coolant supply to the tool in the vast majority of cases comes into disrepair and must either be disposed of, either to burn a new plate.
The use of coolant
This is one of the treatment conditions of alloy steels. Coolant is necessary, first of all, to prevent premature tool wear, improve cutting action, higher-quality surface of the workpiece and improve the processing accuracy. For each type of processed steel, cutting plates, we select its coolant, the method of delivery in the area of cutting.
The most effective is such a method, which helps to maximize the heat removal from the cutting zone. There are proven — high-pressure coolant mostly on the rear surface of the working plate of the cutting tool, the spray coolant and is quite rare, mainly in defense plants — cooling carbon dioxide.
The choice of method of cooling
Depends on the conditions of processing and technological capabilities of the equipment. The most common high-pressure cooling — it can be applied during turning, milling multi-instrumental, by grinding, etc. This method is typical for many equipment manufacturers, both domestic and foreign. The spray liquid is fed accurately into the area of cutting. In contact with hot metal it evaporates quickly, taking the heat and effectively cooling the working surface. The drawback of this method include the high loss of coolant. The use of this method allows to increase the tool life by almost 6 times — naturally this is reflected in the cost of the part in the end.
More effective is the simultaneous supply of coolant to the cutting area and the area of the chip formation, however, technically, it is not always possible — may require modifications to the process equipment. This cooling method is suitable for medium and small batch production.
The most effective from the point of view of heat removal from the treatment zone, is cooled with carbon dioxide, wherein the temperature in the region of cut is about minus 79 °C. However, this method is the most costly, is only applicable in a single production. Usually, in the defense industry, in the manufacture of small batches of high precision and critical parts, which are manufactured from alloy steels with special properties.
The basic requirements for processing
For machining of alloy steels in the machine and system AIDS (machine — fixture — tool — item) must possess a number of qualities. It is primarily the increased stiffness of the entire system. Because alloy steels during processing can cause vibration, which is transmitted to the whole system. At low stiffness of the system AIDS it can lead to marriage and increased tool wear. Second, the system must be designed for heavy mechanical loads arising in the process — and they are much higher than in the processing of ferrous metals. Third — minimum backlash in the units and mechanisms of metal working equipment.
The motor must have a significant margin of safety, since the processing of alloy steels involves high loads. For this reason, before treatment began to check the condition of belt transmission, belts and pulleys. Devices and tools should be the most rigid and short to reduce the influence of cutting forces on the final result.
To optimize the processing of alloy steels, can be achieved by use of ultrasonic vibrations, weak currents, preheating parts, but those ways are all too expensive, require special additional equipment and rarely used. Most often used in practice special acid. Sometimes experienced turners use the normal bow, or rather its juice, which, surprisingly, significantly improves the purity of the surface of the part, facilitates the cutting process and prolongs the service life of the tool.
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