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Cutting Processing of Stainless Steel

Stainless steel, as a corrosion-resistant material, is widely used in many industrial sectors and daily life, and its usage is increasing with the development of industry. Therefore, understanding its properties and mastering its cutting methods are becoming increasingly important.

There are various types of stainless steel with different properties, but according to their metallographic structure characteristics, they can be divided into the following categories:

Martensitic stainless steel and ferritic stainless steel:

Their alloy composition is mainly Cr, with a content of 12-8%. Common steels include 1Cr13, 2Cr13, 3Cr13, 4Cr13, 9Cr18, 30Cr13Mo, etc. After quenching and tempering, these steels have appropriate hardness, strength, and good oxidation resistance. During cutting, chips are prone to abrasion and tool wear. But the carbon content increases to 0.4-0. At 5%, the machinability of martensitic stainless steel improves. The main alloy component of ferritic stainless steel is also Cr, with a content similar to that of martensitic stainless steel. In cutting processing, its performance is similar to before, but its hardness is lower and its toughness is increased. In short, as long as the cutting tool material is selected appropriately and the appropriate geometric angle is matched during the cutting process, the difficulty of cutting these two types of stainless steel is still not high.

Austenitic stainless steel and austenitic ferritic stainless steel:

These two types of stainless steel not only contain chromium, but also a considerable amount of nickel (usually 7-20%). Due to the high content of nickel or manganese in these steels, their microstructure is stable, and heat treatment is difficult to strengthen them. This type of steel has continuous chips during cutting, making it difficult to break and prone to work hardening. Austenitic ferritic stainless steel only contains a certain amount of ferrite in its structure, and there are also a certain amount of high hardness intermetallic compounds. Its other properties are similar to austenitic steel, so the machining difficulty of these two materials is relatively high in cutting. The grades of austenitic stainless steel include 1Cr18N9Ti, 00Cr18Ni10, O0Cr18Ni14M02Cu2, 0Cr18Ni12M02Ti, 2Cr13Mn9Ni4, etc. Common austenitic ferritic stainless steels include 0Cr21N95Ti, 1Cr18Mn10Ni5, 1Cr18Ni1Si4A1Ti, etc. There is no precipitation hardening type of stainless steel available. This type of steel not only contains high levels of chromium and nickel, but also alloying elements such as thallium, aluminum, titanium, and molybdenum that can cause precipitation hardening, giving the steel high strength and hardness. Stainless steel of this type includes 0Cr17Ni4Cn4Nb and oCr15Ni7M02A1. The cutting difficulty of this type of stainless steel is also relatively high. The poor machinability of stainless steel lies in its difficulty. Below, we will introduce its characteristics and solutions.

1.Stainless steel cutting characteristics

The processing difficulty of stainless steel is in the order of ferrite → martensite → austenite → austenite with ferrite → precipitation hardening. The cutting characteristics of stainless steel are described as follows:

Serious trend of work hardening

There is a tendency for work hardening in the processing of stainless steel, especially in austenitic and austenitic ferritic stainless steels. The hardness of the hardened layer can reach HV560, which is more than twice the hardness of the raw material. The depth of the hardened layer can reach one-third or more of the cutting depth. The reason for hardening is that stainless steel has good plasticity (§>35%), such as 0Cr18Ni9, 1Cr18Ni9Ti, 2Cr18Ni19, Cr18Mn10Ni5M03, whose elongation is greater than 40%, which is 210-240% of 40Cr and more than 150% of 45 # steel. Therefore, during plastic deformation, lattice distortion is severe and the strengthening coefficient is large.

Low thermal conductivity

Stainless steel has a low thermal conductivity, which means its ability to conduct heat is poor. Austenitic stainless steel, for example, is only about 28% of ordinary steel. Therefore, during the cutting process, the cutting cannot pass through the workpiece in a timely manner, and the chips cannot be conducted out, resulting in a large amount of cutting heat concentrated near the cutting edge, greatly increasing the cutting temperature. For example, the cutting temperature of 18-8 stainless steel can reach 1000-1100°C. The cutting temperature of 45 # steel is only 700-750°C.

High cutting force

Stainless steel has high high-temperature strength and hardness. Taking austenitic stainless steel as an example, when its temperature reaches 700°C, its comprehensive mechanical properties are still higher than general structural steel. In addition, its plasticity and toughness are good, so it consumes more energy in cutting processing, which increases the cutting force. For example, the unit cutting force of turning 1Cr18Ni19Ti is 25% higher than that of 45 # steel.

Chips are not easy to break and are prone to forming chip lumps

Due to the high toughness and plasticity of stainless steel, continuous chips are generated during turning, which not only affects the smooth operation and causes safety accidents, but also squeezes and damages the machined surface. Stainless steel contains elements such as Cr, Ni, Ti, Mo, etc. These elements have strong affinity with other metals, are prone to adhesion, and form chip nodules. The blue brittle zone of austenitic stainless steel occurs at around 200°C or lower, while the blue brittle zone of carbon steel occurs at around 300°C, which means that the temperature at which chip deposits occur during cutting austenitic stainless steel is lower than that of carbon steel.

In the cutting process of stainless steel, the cutting temperature is high, the cutting force is large, and the alloy elements such as Cr, Ni, Ti have good compatibility with other metals, which makes the tool prone to bonding and diffusion wear, thus easily forming crescent shaped grooves on the front cutting surface. Causing a decrease in blade strength and resulting in minor peeling and gaps; Furthermore, due to the hard points of carbides in stainless steel, the cutting tools experience severe abrasive wear, resulting in particularly severe tool wear during the cutting process of stainless steel.

2. Selection of tool materials and tool geometry angles

Selection of cutting tool materials

Stainless steel is an alloy strengthened by high melting point and high activation energy elements, especially its complex composition and high alloy element content. This leads to high plasticity, good toughness, and low thermal conductivity of the material. During cutting, the deformation resistance of the cut layer is high, and the hardening depth and degree of the machined surface increase. At the same time, the deformation temperature increases, and the tendency of chip adhesion increases. Based on these characteristics, when selecting hard alloy cutting tool materials, the main considerations are their high-temperature strength, high-temperature hardness, and ensuring sufficient toughness. Therefore, in stainless steel cutting, K-type alloys are generally selected, or as much as possible, hard alloys that do not contain titanium carbide or contain less titanium carbide and add tantalum carbide (niobium) and other refractory alloy elements are used. The main reason is that K-type alloys have high bending strength, which can ensure that the cutting tool adopts a larger rake angle and a sharp edge. Secondly, K-type alloys have good thermal conductivity, which can avoid the concentration of cutting heat at the cutting edge and reduce the cutting temperature.

According to this viewpoint, we recommend the following alloys for general stainless steel cutting:

YG6A、YG8N、YW1、YW2

In recent years, the performance of materials and the accuracy of workpieces have improved rapidly, so the demand for tool materials has also increased accordingly. In order to achieve better results, we recommend using:

YW4, YS2T, YD15 and other new alloy grades

The opinions on the selection of hard alloys for stainless steel cutting are not completely consistent. Some people have suggested that P-type alloys should be used for cutting stainless steel, and many experiments have been conducted to prove that P-type alloys are better. Based on this theory, our YS25 milling test as stainless steel has proven to have excellent performance. After careful analysis, both viewpoints have some merit, but neither is comprehensive. We believe that K-type alloys can be used for low-speed intermittent cutting, while P alloys must be used for high-speed cutting.

Selection of  for cutting tools

Front angle:

Although the hardness and strength of stainless steel are not very high, it has good plasticity, high toughness, and high thermal strength. The chips are not easily cut off during cutting. The main principle is to use a larger front angle as much as possible while ensuring that the cutting edge does not collapse. The main reason for doing this is: increasing the rake angle within the range of 250 or below can reduce the unit cutting force and save energy consumption; Reduce the adhesion between chips and cutting tools, and improve the friction on the front cutting surface; Reduce cutting temperature and minimize the diffusion wear of the blade. Therefore, when turning stainless steel, the approximate range of the rake angle is 150-300. Take the smaller value during rough machining and the larger value during precision machining; Stainless steel that has not undergone quenching and tempering treatment, or has undergone quenching and tempering treatment but has lower hardness, can take a larger value; If the diameter of the workpiece is small or thin-walled, it is also advisable to take a larger value. When precision machining austenitic stainless steel, the rake angle can be selected from 200 to 250. For rough machining, a larger rake angle with a chamfer of -300 and a chamfer width of (0.5 to 1) feed rate can be used. This not only enhances the strength of the tool tip, but also does not increase a lot of cutting force.

Rear angle:

In metal cutting, the rear angle is also an important angle, and its selection is reasonable or not, which has a significant impact on the cutting process. Generally speaking, the choice of the back angle mainly depends on two aspects: one is the thickness of the cutting layer, the smaller its value, the larger the back angle should be; On the other hand, it depends on the strength of the tool material, with high strength resulting in a larger back angle, and vice versa, a smaller back angle. In the cutting process of stainless steel, the back angle value of hard alloy cutting tools is mostly adopted as follows:

Rough machining is 40-60, slightly larger than 60 during precision machining

In order to increase the strength of the cutting edge, when cutting stainless steel, the inclination angle λ S is generally taken as -20 to -60. When cutting intermittently, the inclination angle λ S is smaller, usually taken as -50 to -150. This is because tools with a large rake angle should have a smaller negative inclination angle to ensure tool strength. In production practice, in order to better improve the strength and heat dissipation ability of the cutting edge, a double-edged inclined turning tool is usually used, which can achieve ideal results.

Selection of cutting parameters

When cutting stainless steel, the cutting amount is generally: the feed rate should not be less than 0.1mm/revolution, and trace feed should be avoided to avoid cutting in the work hardening zone; The selection of cutting depth for the original cap is to avoid the cold hard layer, but sometimes it also depends on the machining allowance of the workpiece. The choice of cutting speed generally depends on the tool material, and tool materials with good thermal stability can have higher cutting speeds. However, it should also be noted that when choosing the cutting speed, the vibration zone should be avoided. This is because the vibration caused by the friction of the back cutting surface and the formation of chips is particularly severe at certain cutting speeds. Therefore, we need to avoid this vibration zone speed to prevent the cutting edge from micro collapsing and improve tool durability.

In recent years, a large amount of research has been conducted on cutting stainless steel, and there is a deeper understanding of its cutting theory. If someone suggests that cutting stainless steel should be carried out at a certain temperature, experiments on cutting austenitic stainless steel have shown that it is most suitable for cutting at around 800°C. The main reason is that in the austenitic cutting process, bonding and diffusion wear are important factors affecting tool durability, and in the temperature range of around 800°C, it can significantly reduce the bonding between the tool and the workpiece, as well as between the tool chips, while diffusion wear does not increase significantly. And at this temperature, it is conducive to the plastic deformation of the workpiece, significantly reducing the cutting force and making the cutting process light and agile. According to this viewpoint, it is advisable to use a higher cutting speed when cutting stainless steel. In order to achieve a cutting temperature of around 800°C, the corresponding cutting speed is 80-15m/min, accompanied by appropriate cutting depth and feed rate, and it is recommended to use metal ceramic blades.