Free-cutting stainless steel

Easy cutting stainless steel: resistant to air, steam and other weak corrosive media and acid, alkali and other chemical etched media corrosion of steel

Easy cutting stainless steel refers to the steel resistant to air, steam, water and other weak corrosive media and acid, alkali, salt and other chemical etched media. Also known as stainless acid resistant steel. In practical application, the steel with weak corrosion resistance is often called stainless steel, and the steel with chemical corrosion resistance is called acid-resistant steel. Because of the difference in chemical composition, the former is not necessarily resistant to corrosion of chemical media, while the latter is generally rust proof. The corrosion resistance of stainless steel depends on the alloying elements contained in the steel. Chromium is the basic element to make stainless steel corrosion resistance, when the chromium content in the steel reaches about 1.2%, chromium and the oxygen in the corrosive medium, the formation of a very thin oxide film on the surface of the steel (self-passivation film), can prevent further corrosion of the steel matrix.

Element classification

First, sulfur free cutting steel

Sulfur forms manganese sulfide inclusions with manganese and iron in steel. Such inclusions can interrupt the continuity of matrix metal, and promote the chip breaking to form a small and short crimping radius during cutting, which is easy to eliminate, reduce tool wear, reduce machining surface roughness, and improve tool life. Generally, the machinability of steel increases with the increase of sulfur content in steel. But the longitudinal and transverse mechanical properties of steel are different, transverse plastic, toughness, fatigue and corrosion resistance are also reduced. When the sulfur content in steel is too high, it will lead to thermal brittleness, which causes difficulties in hot working and deteriorates the mechanical properties of steel. Usually the sulfur content is 0.08% ~ 0.30%, some can be increased to 0.4%, the sulfur content in easy cutting tool steel and stainless steel should be between 0.06% ~ 0.10%, phosphorus and sulfur compound added to steel, usually phosphorus content in 0.04% ~ 0.12%, phosphorus soluble in ferrite will improve hardness and strength, reduce toughness, The chip is easy to break and remove, so as to obtain a good machining surface roughness, but the high content of phosphorus will significantly reduce the plasticity, improve the hardness, but harmful to the machinability of steel.

Two, lead free cutting steel

Lead in steel is in the form of fine metal particles, uniformly distributed or attached to sulfide around. Due to the low melting point of lead, melting seepage plays a lubricating role in cutting, reduce friction, improve machinability, but does not affect the mechanical properties at room temperature. Lead content in steel is generally 0.10% ~ 0.35%. Because of the heavy ratio of lead, such as high content, it is easy to cause serious segregation and the formation of large particle inclusion, but reduce the beneficial effect of lead on machining. Lead and sulfur compound added to low carbon structural steel, improve the steel cutting effect is more significant.

Three, calcium free cutting steel

Calcium in steel and aluminum, silicon combined to form a low melting point of composite oxides (mainly CaO·Al2O3·SiO2), high-speed cutting, calcium oxide attached to the surface of the cutting tool lubrication and friction reduction, so as to improve the service life of the tool. If it contains sulfur, lead and other elements at the same time, their compound effect will make the cutting effect better. Since the 80 s with the improvement of cutting tools, on the calcium free-cutting steel coated with T | N coated tools, for the tool of the high cost of rolling shaping cutter, gear shaper cutter, such as gear machining tools, have a significant effect.

Selenium, tellurium, bismuth free cutting steel

The content of tellurium and bismuth is about 0.03%-0.10%, and the content of selenium is up to 0.15%. Selenium exists in steel in the form of selenides such as FeSe and MnSe, and its effect is similar to sulfur. For steel requiring both high machinability and good plasticity, adding selenium to steel is better than sulfur. Tellurium can be added to steel alone or at the same time with lead or sulfur to form complex inclusions, so as to reduce cutting resistance and cutting heat, so that the chip is easy to be removed, significantly improve the machinability of steel, and obtain good machining surface roughness. However, tellurium can reduce the ductility and toughness of steel slightly. Selenium and tellurium are commonly used in alloy steels. Bismuth acts like lead in steel as fine metallic particle inclusions, uniformly distributed or attached to sulfide.

5. Other free-cutting steels

Effects of Other Elements on the machinability of steel In addition to the above free-cutting elements, other elements also have certain effects on the machinability of steel:

(1) Carbon. The carbon content in steel is related to the machinability of steel. Carbon content is too low, there will be a large number of ferrite in the organization, the hardness and strength of the steel is very low, the chip is easy to stick on the blade to form a knife tumor, and the chip is torn off, so that the machinability is reduced, the processing surface roughness is very high. Carbon content is too high, the pearlite volume in the organization increases, the hardness and strength increase, so that the cutting resistance increases, the machinability deteriorates. The carbon content of free cutting structural steel should be 0.15% ~ 0.25%.

(2) Manganese. Manganese and sulfur in steel form manganese sulfide inclusion, so that chips easy to fracture, improve the machinability of steel, but also eliminate or weaken the thermal brittleness caused by sulfur. Therefore, the manganese content in the free-cutting steel should be kept between 0.60% and 1.60%, and the appropriate Mn/S ratio should be maintained.

(3) Silicon and aluminum. Both silicon and aluminium are harmful to the machinability of steel. Silicon is partially soluble in ferrite, which increases the hardness of steel and makes cutting difficult. Moreover, silicon combines with oxygen in steel to form silicon oxide inclusions with higher hardness, which increases tool wear and reduces tool life. Therefore, the silicon content in the easy cutting steel should be low. Aluminum is generally added to steel as a deoxidizer, and most of it combines with oxygen to form fine brittle alumina inclusion, which increases tool wear. The addition of silicon and aluminum will also reduce the oxygen content of steel, make sulfide inclusions distributed in elongated strips, and worsen the machinability of steel.

(4) Oxygen and nitrogen. Oxygen is generally harmful in steel because it reduces the mechanical properties of steel, but the increase of oxygen content in free-cutting steel will cause the sulfide to be spindle-shaped distribution, improving the machinability of steel. Although nitrogen can improve the strength of steel, but increase the brittleness, the cutting process will form short broken chips. Trace nitrogen content in steel (≤0.002%) plays a favorable role in machinability and workpiece surface quality, but when the content is too high, the strengthening effect of steel increases, which is unfavorable to tool life.