Machinability of steel

from Wikipedia, the free encyclopedia

The machinability of steel is an important manufacturing property of the various steel materials. Machinability is generally the suitability of a material to be machined ( drilling , milling , turning , ...).

Steel is the most frequently machined material. Together with cast iron (see machinability of cast iron ) , it is a ferrous material and is characterized by a carbon content of up to 2.06%, while cast iron contains over 2.06%. Steels are very diverse materials. Their machinability depends primarily on the structure, which in turn depends on the exact carbon content and the condition of the heat treatment. Numerous alloying elements also play a role. Some are deliberately added to the alloy in order to improve machinability, others in order to increase properties such as strength, whereby a deterioration in machinability is accepted for better performance properties. Other elements such as phosphorus are actually undesirable, but improve machinability.

structure

Ferritic- pearlitic structure of an unalloyed steel with 0.35% carbon (C35) with light ferrite grains and dark lamellar pearlite

The structural components occurring in steel are ferrite , cementite , pearlite , austenite , bainite and martensite . They differ greatly in terms of their strength , hardness , elongation at break and their tendency to stick to the cutting edge .

ferrite

Ferrite is a body-centered cubic phase with a maximum solubility of 0.02% for carbon. Ferrite has the lowest hardness (80–90  HV ) and tensile strength (200–300 N / mm²) of all structural components and the highest elongation at break of 70–80%. The resulting cutting forces and wear are therefore low. The problem is the high deformability. This leads to long ribbon and tangled chips that can get caught in the machine and to the formation of burrs and thus poor surface quality. In addition, at low cutting speeds it tends to stick to the cutting edge, which leads to the undesirable effect of the built- up edge .

Cementite

Cementite is an intermetallic phase that is extremely hard with over 1100 HV and is also very brittle. In practice, it cannot be machined. Cementite can occur in free form or as a component of pearlite or bainite. Because of its great hardness, cementite causes high abrasive tool wear .

Perlite

Perlite is a phase mixture of ferrite and cementite. With a carbon content of 0.8%, the entire structure consists of perlite, including perlite and ferrite, above that of perlite and cementite, the closer the carbon content is to the so-called eutectoid point of 0.83%, the greater the percentage of pearlite. The hardness is around 210 HV, the tensile strength is 700 N / mm² and the elongation at break is 48%. The values ​​are therefore in the middle range. The cementite is mostly in the form of finely divided lines, but through heat treatment it can also be in a globular (spherical) form. Because of its high hardness compared to ferrite, pearlite causes higher abrasive wear and greater cutting forces. However, it is less prone to sticking and the formation of built-up edges. The chip forms are cheaper and the surface qualities that can be achieved are better because it does not tend to form burrs.

Austenite

Austenite is a phase with a face-centered cubic structure. In unalloyed steel it only occurs above 723 ° C, but in alloyed steels it can also be present at room temperature. This is the case with many stainless steels. Austenite is also characterized by high deformability (elongation at break 50%) and medium tensile strength and hardness (180 HV, 530–750 N / mm²).

Austenite tends to form built-up edges and to stick to the edge. The tendency to adhesion is particularly pronounced with austenite. In addition, long ribbon or tangled chips are formed. Because of the high plastic deformation during processing, work hardening of the newly created surface occurs during processing. This leads to increased cutting forces during further processing. In addition, the thermal conductivity of austenite is a third lower, which hinders the dissipation of the heat generated in the chip. The cutting edge is therefore subject to higher thermal loads.

Martensite

Martensite is formed when austenite is cooled very quickly. Then, in the body-centered cubic martensite lattice, excess carbon atoms are dissolved that could not diffuse out. Martensite has a very high hardness of 900 HV and a tensile strength of 1380 to 3000 N / mm². This leads to very high cutting forces and high tool wear, which is caused by abrasion and thermal stress.

Carbon content

Iron-carbon diagram

The carbon content has a major influence on the structure. The iron-carbon diagram provides a connection .

  1. Carbon content below 0.25% : If the carbon content is very low, there is mainly ferrite, which determines the machinability, and some pearlite. Because of the high tendency to adhere, built-up edges form at low cutting speeds. In addition, the surface quality is poor because of the high deformability of the material. Tool wear and temperature increase only slowly with increasing cutting speed. The tool cutting edges should have the largest possible positive rake angle (e.g. over 6 ° when turning). Oils are mainly used as cooling lubricants , since the lubricating effect is more important than the cooling effect. Steels with a low carbon content cause problems especially in processes that require low cutting speeds such as drilling , threading , parting off and reaming . The surface qualities that can be achieved are then particularly poor.
  2. Carbon contents between 0.25% and 0.4% : With these steels, the influence of pearlite increases. They are harder and stronger, so the cutting forces, temperatures and abrasive tool wear increase. In return, however, more favorable chip shapes and surface qualities are achieved. The tendency to adhesion decreases and the formation of built-up edges shifts to lower cutting speeds. An improvement in machinability can be achieved through heat treatment. Coarse-grain annealing is advantageous for carbon contents of up to 0.35%, normal annealing above that . Steels with a carbon content between 0.25 and 0.4% are often processed by cold extrusion and then finished by machining. The work hardening that occurs during extrusion has a beneficial effect on machinability, especially with regard to chip forms.
  3. Carbon content between 0.4% and 0.8% : In this range, pearlite is mainly present and only a little ferrite. With a content of 0.83% C, only pearlite is present. The strength of the steels in the range between 0.4 and 0.8% C thus increases, which also leads to higher cutting forces, temperatures and abrasive tool wear. The temperatures are high even at low cutting speeds, and wear is also present as crater wear on the rake face . The surface qualities and chip shapes that can be achieved, however, are good.
  4. Carbon contents above 0.8% : When these steels slowly cool in air, a structure is obtained that consists of pearlite grains embedded in a matrix of cementite. These steels are therefore difficult to machine. The forces that occur are very high, wear and tear and the temperature are high even at low cutting speeds. The tools should be designed as stable as possible with a positive rake angle and a slightly negative angle of inclination of around −4 °.

Accompanying and alloying elements

Accompanying elements are usually undesirable in steel, but cannot be completely removed. Alloy elements, on the other hand, are deliberately added to change certain properties. The machinability can have a positive as well as a negative influence on accompanying and alloying elements. This is done through three different mechanisms:

  1. Change in structure. For example, high nickel proportions favor the formation of austenite.
  2. Formation of compounds that have a lubricating effect such as manganese sulphide .
  3. Formation of hard connections that result in high abrasive tool wear like most compounds of carbon and metals ( carbides ).
  • Manganese increases the strength of steel and improves its hardenability. Together with sulfur it forms manganese sulphide, which has a beneficial effect on machinability. With a low carbon content and a manganese content of up to 1.5%, the shape of the chips improves. With a high carbon content, however, tool wear increases.
  • Chromium , molybdenum , tungsten : Chromium and molybdenum are alloyed with case-hardened and heat-treated steels because they improve hardenability. Chromium, molybdenum and tungsten form hard carbides with higher carbon contents, which increase tool wear.
  • Nickel is used to increase the strength of the steel. It also favors the formation of austenite, which can also be present at room temperature with higher nickel contents. Nickel also causes increased toughness, especially at low temperatures, so that the machinability is generally negatively influenced by nickel.
  • Silicon increases the strength of ferrite, including the ferrite contained in pearlite. With oxygen it forms hard silicates, which increase tool wear.
  • Phosphorus increases the brittleness of ferrite. This is mostly undesirable. In the case of free-cutting steel , however, phosphorus up to 0.1% is deliberately added as this makes the chips easier to break. Higher proportions improve the surface quality and increase wear.
  • Titanium and vanadium form finely divided carbides and carbonitrides. These increase the strength of the steel because the structure is made much finer. They thus increase the cutting forces and worsen the chip shape.
  • Sulfur forms compounds with other alloy components. The iron sulfide is undesirable because it reduces strength and greatly increases brittleness. Manganese sulfide has a higher melting point and has a beneficial effect on machinability. It is in the form of inclusions that make it easier to break the chips, improve the surface quality and reduce the tendency to build up built-up edges.
  • Lead is insoluble in iron and is in the form of tiny inclusions that are less than a micrometer in diameter. Since lead melts at low temperatures, it forms a protective lubricating film between the tool and the chip, which reduces wear. In addition, lead leads to low cutting forces (up to 50% lower) and good chip breaking. Lead is especially added to free-cutting steels, but it is toxic and harmful to the environment, so that it is increasingly being avoided.

Machinability of steel materials

Free cutting steel

Free-cutting steel is a special type of steel that has particularly good machinability. The forces that occur are low, there is little wear, the chips are short and the surface quality is high. There are hardenable and non-hardenable free cutting steels. They are used in particular on automatic lathes for mass and large-scale production. The most important alloying elements are sulfur, lead and phosphorus. In addition there are tellurium , bismuth and antimony . The tools usually consist of coated high-speed steel or hard metal . Often there are also special profile tools.

In the area of ​​low cutting speeds and with steels with a low carbon content, adhesion plays a major role. For this reason, elements are added that form friction-reducing layers. These are lead and manganese sulfide. Phosphorus leads to embrittlement of the material and thus to easier breakage of the chips. However, lead and manganese sulphide also reduce the strength and also promote chip breaking without leading to embrittlement, so that lead and sulphide are alloyed in preference to phosphorus.

Case-hardening steel

Case- hardening steel is intended for case hardening and is characterized by a carbon content below 0.2%. They are mostly machined before hardening, sometimes also after what is known as hard machining . In the unhardened state, the ferrite predominates in the structure due to the low carbon content. This leads to long chips, low cutting forces and wear, but a built-up edge up to a cutting speed of about 200 m / min. Hard metals from the P group or high-speed steel are mostly used as cutting materials . Case-hardening steels are often subjected to heat treatment. This includes setting a certain strength or a certain structure. Alloyed case-hardening steels are often treated by coarse-grain annealing in order to reduce the tendency to adhesion. The heat treatment condition has little influence on the applicable cutting speeds for carbide tools, but a greater influence for tools made of high-speed steel. Because long chips are formed, a suitable chip breaker is important when turning. However, as with free-cutting steels, chip breaking can also be improved by adding lead or sulfur to the alloy.

For machining in the hardened state with hardnesses of more than 45 HRC, fine-grain hard metals, mixed ceramics and boron nitride are used as cutting materials.

Tempered steel

Quenched and tempered steel is intended for quenching and tempering and has a carbon content of 0.2 to 0.6%. The most important alloying elements are silicon, manganese, chromium, molybdenum, nickel and vanadium. The machinability is essentially determined by the structure, which in turn is determined by the heat treatment. The alloying elements usually have a smaller influence. With low carbon contents the wear effect is relatively low, with higher carbon contents it increases strongly. The chip length depends heavily on the structure and the state of heat treatment. It can be improved by adding lead and sulfur or by using suitable chip breakers. Steels with higher carbon contents can be annealed to produce globular cementite, which improves machinability. However, the adhesion increases. In the hardened and tempered structure, high cutting forces and temperatures act. Quenched and tempered steels are occasionally roughed prior to tempering . Most components, however, are machined in the quenched and tempered condition. This leads to high abrasive wear on the tools. The choice of cutting material depends on the hardness. Hard metals and cermets are used below 45 HRC, above cutting ceramics and boron nitride.

Nitriding steel

Nitrided steel is heat treatable steel with a carbon content between 0.2 and 0.45%. These steels are usually tempered first, then machined and finally nitrided . In this process, nitrogen combines with iron and certain elements, the so-called nitride formers vanadium and aluminum , which are specially alloyed for this purpose. This leads to an increase in hardness in the surface layer. In addition, chromium and molybdenum can be added to improve the heat treatability.

The structure that exists after tempering leads to high wear, temperatures and cutting forces, but good surface quality and short chips. If nitriding steels are machined in the soft state, poor surface quality and long chips are to be expected.

In some steels, nickel is added to the alloy in order to increase the strength, which, however, impairs the machinability. Aluminum-free grades are easier to process than aluminum-containing grades. On the other hand, it is beneficial to add sulfur.

Tool steel

Tool steel is divided into alloyed and unalloyed tool steel, as well as cold, hot and high-speed steel. The carbon content can range up to 1.5%.

Forged or rolled tool steel with a carbon content of up to 0.9% consists of lamellar perlite and ferrite, over 0.9% it is lamellar perlite and cementite. In the annealed condition, the structure consists of ferrite with embedded cementite grains, regardless of the carbon content. The cementite network cannot be completely dissolved only in the case of very high carbon contents. In the hardened state, the structure in the edge layers consists of martensite, which merges into an intermediate structure towards the center and finally into fine lamellar pearlite. Cementite grains can also occur in steels above 0.8% C if the steel has been soft-annealed beforehand.

Unalloyed tool steels between 0.5 and 1.5% carbon are usually first soft-annealed and then machined. With carbon contents below 0.8%, they can also be normalized or machined in the deformed state. However, the ferrite in the structure then creates problems such as sticking, built-up edges and poor surface quality. In the annealed condition, the ferrite also causes problems such as poor surface quality and long chips. These can be eliminated by quenching and tempering the steel, but then the cutting force and wear increase sharply. In the annealed state, all alloyed and especially high-alloy tool steels are considered to be difficult to machine due to strong adhesions and the formation of built-up edges.

Mainly hard metals with titanium and tantalum carbides of group P are used as cutting materials . Cermets are also used. Quenched and tempered steels can be machined with boron nitride .

Hardened steel

Up until the 1980s it was believed that hardened steel could only be machined by grinding. With the development of the so-called super-hard cutting materials, this has changed and machining by turning, milling, drilling and broaching is now also used in mass production. Machining by this method is then referred to as hard cutting.

Fine-grain carbides can be used for processes with an interrupted cut such as milling or processes with short cutting times such as slotting or broaching. These are characterized by a comparatively high toughness and low hot hardness.

In processes with uninterrupted cuts such as turning, the tools are subject to significantly higher temperatures. Then at least mixed ceramics must be used. However, because of their low toughness, these can only be used to a limited extent. The chip thickness should be less than 0.1 mm and the corner radii should be as large as possible; ideally, round cutting inserts are used. These measures lead to a low mechanical load on the cutting edges and thus prevent breakage. The low cutting values ​​compared to boron nitride can then be compensated for by the lower tool costs in some applications.

Boron nitride is standard for hard machining. As the workpieces press on the free surface during machining and thus generate considerable passive forces, the largest possible clearance angle is recommended. The cutting edge rounding is usually around 20 µm. Smaller chamfers or radii lead to increased tool wear, larger ones to vibrations.

The compressive stresses that occur during machining can be between 4000 and 4700 N / mm². In extreme cases, the temperatures can exceed the melting temperature of the material. The passive force is much greater than normal soft machining and can even exceed the cutting force.

Austenitic, stainless steel

According to the main requirements, stainless steel is divided into

  • corrosion-resistant steels
  • heat-resistant steels
  • creep resistant steels.

The main alloy elements are nickel and chromium, the proportions of which make up between 10% and 20%. All three groups can have different structures depending on the alloy content: ferritic-austenitic, austenitic, ferritic or martensitic. The corrosion-resistant, austenic steels , which are exclusively described in the following, are of greatest importance .

The austenitic, corrosion-resistant steels are machined either in the quenched condition or in the solution-annealed condition. Compared to ferritic-pearlitic or tempered steels, they cause much greater problems when machining. This is due to their great deformability, their high ductility, their tendency to stick to the tool as well as the strain hardening and low thermal conductivity. The latter leads to poor heat dissipation via the chip and thus to a higher tool temperature. The wear on the rake face or flank is high. It comes to sticking to the tool surfaces, to crumbling or breaking of the cutting edges and to long chips. The possible cutting speeds and also the service life are therefore relatively low. The coating on the tools can peel off, as the bond with the chip is often greater than that between the coating and the tool.

Coated or uncoated hard metals with tungsten carbide and cobalt from group M are used as cutting materials. Compared with the ferritic-pearlitic steels, the cutting speeds are about two to five times lower, i.e. in the range from 50 m / min to about 160 m / min, which leads to tool lives of about 5 to 15 minutes. The end of the service life of uncoated hard metals is due to the high crater wear, which limits the cutting speed to less than 100 m / min. Higher speeds are only possible with coated tools.

Special measures must be taken to ensure good chip breaking. These can be alloy surcharges, but these are only possible to a limited extent without limiting the usability of the workpieces. This is why chip breakers and other form elements of the cutting edges are particularly important. Sharp-edged cutting edges with a cutting edge rounding of only 30 µm for finishing lead to lower cutting forces, less plastic deformation of the workpiece edge zone and better surfaces due to reduced burr formation. With smaller roundings, these values ​​get even better, but wear increases sharply. Roundings in the range from 40 to 60 µm are also possible for roughing.

The machining is usually done with cooling lubricant. Dry machining is, however, possible, but only with deteriorated surface quality and tool life.

Individual evidence

  1. ↑ Herbert Schönherr: Machining production. Oldenbourg, Munich / Vienna 2002, ISBN 3-486-25045-0 , p. 60.
  2. ↑ Herbert Schönherr: Machining production. Oldenbourg, 2002, p. 60.
  3. ^ Fritz Klocke , Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, ISBN 978-3-540-23458-6 , p. 274.
  4. ↑ Herbert Schönherr: Machining production. Oldenbourg, 2002, p. 60.
  5. ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, p. 274.
  6. ↑ Herbert Schönherr: Machining production. Oldenbourg, 2002, p. 60.
  7. ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, p. 274 f.
  8. ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, p. 274 f.
  9. ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, pp. 274, 276.
  10. ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, p. 277.
  11. ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, p. 277.
  12. ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, p. 277 f.
  13. ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, p. 278.
  14. ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, p. 278.
  15. ↑ Herbert Schönherr: Machining production. Oldenbourg, 2002, p. 61.
  16. ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, p. 278.
  17. ↑ Herbert Schönherr: Machining production. Oldenbourg, 2002, p. 61.
  18. ↑ Herbert Schönherr: Machining production. Oldenbourg, 2002, p. 61.
  19. ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, p. 279.
  20. ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, p. 279.
  21. ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, p. 279.
  22. ↑ Herbert Schönherr: Machining production. Oldenbourg, 2002, p. 61.
  23. ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, p. 279.
  24. ↑ Herbert Schönherr: Machining production. Oldenbourg, 2002, p. 61.
  25. ↑ Herbert Schönherr: Machining production. Oldenbourg, 2002, p. 61.
  26. ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, p. 279 f.
  27. ↑ Herbert Schönherr: Machining production. Oldenbourg, 2002, p.
  28. ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, p. 280.
  29. ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, p. 288 f.
  30. ^ Association of German Ironworkers (ed.): Steel. Volume 2: Applications. Springer, 1985, ISBN 3-540-13084-5 , p. 485 f.
  31. ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, p. 291 f.
  32. ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, p.
  33. ^ Association of German Ironworkers (ed.): Steel. Volume 2: Applications. Springer, 1985, p. 137 f.
  34. ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, p.
  35. ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, p. 296 f.
  36. ^ Association of German Ironworkers (ed.): Steel. Volume 2: Applications. Springer, 1985, pp. 350-352.
  37. ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, pp. 297-299.
  38. ↑ Berend Denkena, Hans Kurt Tönshoff: Spanen - basic. 3. Edition. Springer, Berlin 2011, ISBN 978-3-642-19771-0 , p. 194.
  39. ↑ J. Paulo Davim (Ed.): Machining - Fundamentals and Recent Advances. Springer, 2008, ISBN 978-1-84800-212-8 , pp. 105-110.
  40. ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, pp. 303-306.