Relevance

Stainless is an alloy that can withstand the effects of chemically active environment for a long time, it may be adverse atmospheric conditions, and acid or alkaline environment in chemical production. Recently, in many units, machines and mechanisms, carbon steel grades are used less and less often, and they are gradually replaced by elements of special steels. This is due to the fact that normal steel has a certain threshold - a limit above which it becomes impossible to use in conditions of increasing loads, such as high temperatures, pressure or in the presence of aggressive media. In this case, they are successfully replaced by heat-resistant and resistant stainless steels and alloyed alloys with exclusive properties, which will work well where ordinary steel will fail.

Advantages of stainless steels

Heat Resistance. Heat resistant is a material that can withstand exposure to high temperatures without losing its mechanical strength. Refractory steels also belong to the group of dispersion hardening, with the allocation of the alloying element other than the steel base, in a finely dispersed form, and its distribution throughout the metal. Heat resistance characterizes a material that does not lose corrosion resistance when heated. Corrosion-resistant alloy steels have a combination of these qualities. The high strength and toughness of these materials classifies them as hard-to-machine, which is especially evident during cutting and chip removal. This requires a special tool, cutting mode, the selection of coolant, and other important details.

Machining

When comparing the physical and mechanical properties of alloyed steel and conventional steel, it was found that such indicators as the ultimate tensile strength and hardness are approximately equal. But alloyed and mild steels have the same mechanical properties only while other properties may differ significantly, particularly in microstructure, corrosion resistance, and mechanical strengthening capabilities. Recall the tensile - compression diagram, well known from the course of the strength of materials. The diagram begins with a section of elastic deformation, when the material, after the load is removed, returns to its original state without deforming. Increasing the load leads to a zone of so-called "fluidity" when the material begins to deform without a significant increase in the applied force. In the graph, this is practically a horizontal line. This is followed by a sharp hardening - and for further deformation, it is necessary to significantly increase the applied force. The same process occurs in metal cutting, only in the surface layer of the metal - this is due to changes in the crystal lattice under mechanical load. In the machining of ordinary steel, this is also typical, but the hardening of alloyed steels is much more pronounced. And we should not forget the differences in properties such as thermal conductivity, melting point, etc., which also have a significant effect on the machining process.

Machining

So, in machining, the hardening values of alloyed steel are quite high, which requires considerable force to be applied. In addition, most alloyed steels, especially heat-resistant steels, are very ductile, which also makes machining difficult. 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, the more it hardens under mechanical stress. And stainless steels are highly ductile. In addition, there is another side to ductility, the so-called "toughness" of the material. When machining alloyed steel on a lathe, the chips do not break as they do when machining carbon steels of the same hardness, but curl in a long ribbon. This causes a lot of inconvenience and complicates its machining in automatic mode.

The second feature of alloyed steel in machining is a low thermal conductivity, which 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 scalloping. This causes the cutting edge of the cutting insert to become chipped and leads to changes in the cutter geometry and, ultimately, to early failure of the cutter. As a rule, high machining speeds are not recommended for machining alloyed heat-resistant steels, because they make the part more expensive. This problem can be solved by using special cutting inserts designed exclusively for alloy steels and special coolant.

The third peculiarity is preservation of strength and hardness under the influence of high temperatures. This is especially characteristic of heat-resistant steels, which, in combination with the overlap leads to accelerated wear of the cutting tool and does not allow the use of high revolutions.

Fourth - the presence in the steel composition of the solid solution of the second phase with extremely hard intermetallic and carbide compounds, which, despite their microscopic size, act on the surface of the cutting tool as an abrasive material. The tool wears down and becomes blunt much faster, which leads to the need for frequent resharpening and dressing of the cutting edge geometry. As practice shows, the coefficient of friction when machining alloyed steel is an order of magnitude greater than when machining conventional carbon steel.

Fifth. Low vibration resistance is caused by non-uniformity of hardening processes in the process of cutting - as plastic deformation process during machining proceeds differently in the beginning and in the middle of processing. If you are machining a small workpiece, then in principle this phenomenon can be neglected. When it comes to machining a long workpiece such as a shaft, however, there can be difficulties.

Optimizing the process

All these phenomena require a special approach to alloyed steel machining, especially if the machining is carried out in fully automatic mode - for example, on Swiss-type lathes and CNC machines with automatic bar feeder. How to reduce the influence of 'negative factors - let's consider the example of turning - as the most common. Turning involves removing a layer of allowance in the form of chips from a workpiece rotating around its axis. Cutter motion in this case occurs in two coordinates in the horizontal plane. Under the influence of cutting forces a partial displacement of the crystal lattice occurs - naklep - a surface hardening. In this case, a significant part of the friction energy of the tool is converted into thermal energy, and as we remember - the material has a low thermal conductivity. The surface of the part is heated unevenly, vibration occurs and, as a result, the negative effects of the factors mentioned above are aggravated.

To make the tool not blunt so quickly, you can reduce the allowance layer and the tool feed, and increase the spindle speed. This will result in a surface with a higher roughness class. Acid-assisted machining of alloy steels has proven to be a good method of reducing phenomena such as accelerated tool wear and build-up, but it has an extremely negative effect on the turning equipment and the turner himself. Optimization of alloyed steel processing means, first of all, optimal selection of the cutting tool, increased durability, choice of optimum cutting conditions, and the correct choice of coolant and its optimum supply.

Grades of cutters

Hard alloys T30K4, T15K6, BK3 have high hardness and wear resistance. Wear-resistant tips 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 tool hardness, durability and wear resistance 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 hard 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 and reduce tool consumption. But such alloys have a weak point - they are afraid of overheating. If a cutting tool with such a cutting insert fails when machining steel, the tool in the vast majority of cases becomes unusable, and it has either to be scrapped or soldered with a new insert.

The use of coolant

This is one of the conditions of machining of alloyed steels. Coolant is necessary, first of all, to prevent premature tool wear, improve cutting characteristics, obtain a better surface of the machined part and improve the machining accuracy. For each type of processed steel, type of cutting insert, the coolant is selected and the way of its supply into the cutting 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 have recommended 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 common - it can be used for turning, milling multitool machining, grinding, etc. This method is typical for many equipment manufacturers, both domestic and foreign. The liquid is sprayed precisely into the cutting area. When in contact with the heated metal, it quickly evaporates, taking away the heat and effectively cools the working surface. The disadvantage of the described method is high losses of coolant. The use of this method allows to increase the tool life period 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 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 method of cooling is suitable for medium and small-scale production.

The most effective, from the point of view of heat removal from the machining zone, is of course cooling with carbon dioxide, where the temperature in the cutting area is about minus 79 °C. However, this method is the most expensive and can only be used in individual production. It is usually used in the defence industry, for small batches of high-precision and critical parts that are made of alloyed steels with special properties.

Basic machining requirements

For machining alloyed steel, the machine itself and the aids system (machine - fixture - tool - part) must have a number of qualities. This is, first of all, increased rigidity of the entire system. After all, alloyed steels can cause vibration during machining, which is transmitted to the entire system. If the rigidity of the AIDS system is low, it can lead to scrap and increased tool wear. Second, the system must be designed to withstand the considerable mechanical stresses that occur during machining, which are much higher than in ferrous metals. Third - minimum backlash in the nodes and mechanisms of metalworking equipment.

The electric motor must have a significant safety margin, because the processing of alloyed steel involves high loads. For the same reason it is necessary to check the condition of the V-belt transmission, belts and pulleys themselves before starting steel processing. Fixtures and tools should be as stiff and as short as possible to reduce the influence of cutting forces on the final result.

Alternative Directions

Optimizing the machining of alloyed steels is possible through the use of ultrasonic vibrations, weak currents, and preheating of parts - but these methods are all too expensive, require special additional equipment, and are rarely used. Most often, special acids are used in practice. Sometimes experienced turners use the most common onion, or rather its juice, which, surprisingly, markedly improves the cleanliness of the surface of the part, facilitates the cutting process and increases the life of the tool.

Buy, price

Evek GmbH has a wide range of stainless steel products in stock. We value our customers time, so we are always ready to help with the optimum choice. At your service experienced managers-consultants. The quality of the products is guaranteed by strict compliance with the norms of production. The terms of fulfilling orders are minimal. Wholesale customers receive preferential discounts.

Relevance

Stainless is an alloy that can withstand the effects of chemically active environment for a long time, it may be adverse atmospheric conditions, and acid or alkaline environment in chemical production. Recently, in many assemblies, machines and mechanisms, carbon steel grades are used less and less often, and they are gradually replaced by elements of special steels. This is due to the fact that normal steel has a certain threshold - a limit above which it becomes impossible to use in conditions of increasing loads, such as high temperatures, pressure or in the presence of aggressive media. In this case, they are successfully replaced by heat-resistant and resistant stainless steels and alloyed alloys with exclusive properties, which will work well where ordinary steel will fail.

Advantages of stainless steels

Heat Resistance. Heat resistant is a material that can withstand exposure to high temperatures without losing its mechanical strength. Refractory steels also belong to the group of dispersion hardening, with the allocation of the alloying element other than the steel base, in a finely dispersed form, and its distribution throughout the metal. Heat resistance characterizes a material that does not lose corrosion resistance when heated. Corrosion-resistant alloy steels have a combination of these qualities. The high strength and toughness of these materials classifies them as hard-to-machine, which is especially evident during cutting and chip removal. This requires a special tool, cutting mode, the selection of coolant, and other important details.

Machining

When comparing the physical and mechanical properties of alloyed steel and conventional steel, it was found that such indicators as the ultimate tensile strength and hardness are approximately equal. But alloyed and mild steels have the same mechanical properties only while other properties may differ significantly, particularly in microstructure, corrosion resistance, and mechanical strengthening capabilities. Recall the tensile-compression diagram, well known from the course of the strength of materials. The diagram begins with a section of elastic deformation, when the material, after the load is removed, returns to its original state without deforming. Increasing the load leads to a zone of so-called "fluidity" when the material begins to deform without a significant increase in the applied force. In the graph, this is practically a horizontal line. This is followed by a sharp hardening - and for further deformation, it is necessary to significantly increase the applied force. The same process occurs in metal cutting, only in the surface layer of the metal - this is due to changes in the crystal lattice under mechanical load. In the machining of ordinary steel, this is also typical, but the hardening of alloyed steels is much more pronounced. And we should not forget the differences in properties such as thermal conductivity, melting point, etc., which also have a significant effect on the machining process.

Machining

So, in machining, the hardening values of alloyed steel are quite high, which requires considerable force to be applied. In addition, most alloyed steels, especially heat-resistant steels, are very ductile, which also makes machining difficult. 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, the more it hardens under mechanical stress. And stainless steels are highly ductile. In addition, there is another side to ductility, the so-called "toughness" of the material. When machining alloyed steel on a lathe, the chips do not break as they do when machining carbon steels of the same hardness, but curl in a long ribbon. This causes a lot of inconvenience and complicates its machining in automatic mode.

The second feature of alloyed steel in machining is a low thermal conductivity, which 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 scalloping. This causes the cutting edge of the cutting insert to become chipped and leads to changes in the cutter geometry and, ultimately, to early failure of the cutter. As a rule, high machining speeds are not recommended for machining alloyed heat-resistant steels, because they make the part more expensive. This problem can be solved by using special cutting inserts designed exclusively for alloy steels and special coolant.

The third peculiarity is preservation of strength and hardness under the influence of high temperatures. This is especially characteristic of heat-resistant steels, which, in combination with the overlap leads to accelerated wear of the cutting tool and does not allow the use of high revolutions.

Fourth - the presence in the steel composition of the solid solution of the second phase with extremely hard intermetallic and carbide compounds, which, despite their microscopic size, act on the surface of the cutting tool as an abrasive material. The tool wears and dulls much faster, which leads to the need for frequent resharpening and dressing of the cutting edge geometry. As practice shows, the coefficient of friction when machining alloyed steel is an order of magnitude greater than when machining conventional carbon steel.

Fifth. Low vibration resistance is caused by non-uniformity of hardening processes in the process of cutting - as plastic deformation process during machining proceeds differently in the beginning and in the middle of processing. If you are machining a small workpiece, then in principle this phenomenon can be neglected. When it comes to machining a long workpiece such as a shaft, however, there can be difficulties.

Optimizing the process

All these phenomena require a special approach to alloyed steel machining, especially if the processing is carried out in fully automatic mode - for example, on Swiss-type lathes and CNC machines with automatic feed of bars. How to reduce the influence of 'negative factors - let's consider the example of turning - as the most common. Turning involves removing a layer of allowance in the form of chips from a workpiece rotating around its axis. Cutter motion in this case occurs in two coordinates in the horizontal plane. Under the influence of cutting forces a partial displacement of the crystal lattice occurs - naklep - a surface hardening. In this case, a significant part of the friction energy of the tool is converted into thermal energy, and as we remember - the material has a low thermal conductivity. The surface of the part is heated unevenly, vibration occurs and, as a result, the negative effects of the factors mentioned above are aggravated.

To make the tool not blunt so quickly, you can reduce the allowance layer and the tool feed, and increase the spindle speed. This will result in a surface with a higher roughness class. Acid-assisted machining of alloy steels has proven to be a good method of reducing phenomena such as accelerated tool wear and build-up, but it has an extremely negative effect on the turning equipment and the turner himself. Optimization of alloyed steel processing means, first of all, optimal selection of the cutting tool, increased durability, choice of optimum cutting conditions, and the correct choice of coolant and its optimum supply.

Grades of cutters

Hard alloys T30K4, T15K6, BK3 have high hardness and wear resistance. Wear-resistant tips 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 tool hardness, durability and wear resistance 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 hard 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 and reduce tool consumption. But such alloys have a weak point - they are afraid of overheating. If a cutting tool with such a cutting insert fails when machining steel, the tool in the vast majority of cases becomes unusable, and it has either to be scrapped or soldered with a new insert.

The use of coolant

This is one of the conditions of machining of alloyed steels. Coolant is necessary, first of all, to prevent premature tool wear, improve cutting characteristics, obtain a better surface of the machined part and improve the machining accuracy. For each type of processed steel, type of cutting insert, the coolant is selected and the way of its supply into the cutting 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 have recommended 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 common - it can be used for turning, milling multitool machining, grinding, etc. This method is typical for many equipment manufacturers, both domestic and foreign. The liquid is sprayed precisely into the cutting area. When in contact with the heated metal, it quickly evaporates, taking away the heat and effectively cools the working surface. The disadvantage of the described method is high losses of coolant. The use of this method allows to increase the tool life period 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 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 method of cooling is suitable for medium and small-scale production.

The most effective, from the point of view of heat removal from the machining zone, is of course cooling with carbon dioxide, where the temperature in the cutting area is about minus 79 °C. However, this method is the most expensive and can only be used in one-off production. It is usually used in the defence industry, for small batches of high-precision and critical parts that are made of alloyed steels with special properties.

Basic machining requirements

For machining alloyed steel, the machine itself and the aids system (machine - fixture - tool - part) must have a number of qualities. This is, first of all, an increased rigidity of the entire system. After all, alloyed steels can cause vibration during machining, which is transmitted to the entire system. If the rigidity of the AIDS system is low, it can lead to scrap and increased tool wear. Second, the system must be designed to withstand the considerable mechanical stresses that occur during machining, which are much higher than in ferrous metals. Third - minimum backlash in the nodes and mechanisms of metalworking equipment.

The electric motor must have a significant safety margin, because the processing of alloyed steel involves high loads. For the same reason it is necessary to check the condition of the V-belt transmission, belts and pulleys themselves before starting steel processing. Fixtures and tools should be as stiff and as short as possible to reduce the influence of cutting forces on the final result.

Alternative Directions

Optimizing the machining of alloyed steels is possible through the use of ultrasonic vibrations, weak currents, and preheating of parts - but these methods are all too expensive, require special additional equipment, and are rarely used. Most often, special acids are used in practice. Sometimes experienced turners use the most common onion, or rather its juice, which, surprisingly, markedly improves the cleanliness of the surface of the part, facilitates the cutting process and increases the life of the tool.

Buy, price

Evek GmbH has a wide range of stainless steel products in stock. We value our customers time, so we are always ready to help with the optimum choice. At your service experienced managers-consultants. The quality of the products is guaranteed by strict compliance with the norms of production. The terms of fulfilling orders are minimal. Wholesale customers receive preferential discounts.