Analysis of the difficulties in processing stainless steel materials

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The main difficulties in processing stainless steel materials include the following aspects: 1. High cutting force and high cutting temperature. This type of material has high strength; as a result, high tangential stresses and significant plastic deformation occur during cutting, leading to high cutting forces. Furthermore, the material has extremely poor thermal conductivity, which leads to an increase in cutting temperature; moreover, the high temperatures tend to concentrate in a narrow area near the tool edge, thereby accelerating tool wear. 2. Severe work hardening: Austenitic stainless steels as well as some superalloy stainless steels have an austenitic structure, which results in a high tendency to work hardening during cutting – typically several times that of ordinary carbon steels. Cutting within these areas of work hardening reduces the tool life. 3. Prone to sticking to the cutting tool: Both austenitic and martensitic stainless steels exhibit the characteristics of tough chips and high cutting temperatures during processing. When tough chips flow over the rake face, tool adhesion phenomena such as bonding and welding occur, affecting the surface roughness of the machined parts. 4. Accelerated tool wear: The aforementioned materials generally contain elements with high melting points and exhibit high plasticity; as a result, the cutting temperature is high, which accelerates tool wear. This leads to frequent sharpening and tool replacement, thereby affecting production efficiency and increasing the cost of using tools. Mainly, it’s to reduce the cutting speed and feed rate. Use tools specifically designed for machining stainless steel or high-temperature alloys; internal cooling is preferred when drilling and tapping. Processing technology for stainless steel parts: Based on the above analysis of processing difficulties, the processing technology for stainless steel and the design of related tool parameters should differ significantly from those used for ordinary structural steel materials. The specific processing steps are as follows: 1. Drilling processing: During drilling, due to the poor thermal conductivity and low elastic modulus of stainless steel, it is rather difficult to drill holes in this material. To overcome the difficulties in hole machining of such materials, it is mainly necessary to select appropriate tool materials, determine reasonable tool geometry parameters, and set suitable cutting conditions. When drilling the aforementioned materials, drill bits made of materials such as W6Mo5Cr4V2Al and W2Mo9Cr4Co8 are generally recommended. The downside of these types of drill bits is that they are relatively expensive and difficult to obtain. When using conventional W18Cr4V standard high-speed steel drill bits for drilling, drawbacks such as a small rake angle, chips that are too wide to be removed from the hole in a timely manner, and insufficient coolant to cool the drill bit arise. Coupled with the poor thermal conductivity of stainless steel, this leads to an increase in the cutting temperature at the blade edge, which can easily cause damage to the two flank surfaces and the main cutting edge as well as chip breaking, thereby reducing the lifespan of the drill bit. 1) Design of tool geometric parameters: When using a drill bit made of ordinary high-speed steel W18Cr4V for drilling, the cutting forces and temperatures are concentrated at the tip of the drill. To improve the durability of the cutting portion of the drill bit, it is possible to increase the rake angle appropriately; this angle is usually set between 135° and 140°. An increased rake angle also results in a reduced front angle at the outer edge, which makes the chips narrower and facilitates their removal. However, increasing the top angle widens the cross edge of the drill bit, resulting in increased cutting resistance; therefore, it is necessary to grind the cross edge. After grinding, the bevel angle of the cross edge should be between 47° and 55°, while the rake angle should be between 3° and 5°. When grinding the cross edge, the corner where the cutting edge meets the cylindrical surface should be rounded to enhance the strength of the cross edge. Due to the low elastic modulus of stainless steel, the metal beneath the chip layer undergoes significant elastic recovery. Coupled with severe work hardening during processing, an excessively small clearance angle accelerates the wear of the drill bit’s rear surface, while also increasing the cutting temperature and reducing the drill bit’s lifespan. Therefore, the clearance angle must be increased appropriately; however, if it is too large, the cutting edge of the drill bit will become thin, reducing its rigidity. Hence, a clearance angle of 12° to 15° is appropriate. To narrow the drill chips and facilitate their removal, it is also necessary to provide interlaced chip flutes on the two rear cutting edges of the drill bit. 2) Selection of cutting parameters: When drilling, the selection of cutting parameters should be based on the need to reduce the cutting temperature. High-speed cutting leads to an increase in cutting temperature, and high cutting temperatures accelerate tool wear; therefore, the most important parameter among the cutting parameters is the cutting speed. Under normal circumstances, a cutting speed of 12–15 m/min is appropriate. The feed rate has little effect on tool life; however, choosing a feed rate that is too low will cause the tool to cut within the hardened layer, thereby accelerating wear ； If the feed rate is too high, it will also degrade the surface roughness. Taking these two factors into account, a feed rate of 0.32–0.50 mm/r is appropriate. 3) Selection of cutting fluid: During drilling, to reduce the cutting temperature, an emulsion can be used as a cooling medium. 2. Reaming process 1) Tool geometry parameter design: For reaming stainless steel, tungsten carbide reamers are mostly used. The structure and geometric parameters of this reamer differ from those of ordinary reamers. To enhance the strength of the cutting teeth and prevent chip clogging during reaming, reamers generally have a relatively small number of teeth. The rake angle of a reamer is generally between 8° and 12°, but in certain specific situations, a rake angle of 0° to 5° can also be used to achieve high-speed reaming ； The back angle is generally 8° to 12° ； The selection of the main deflection angle varies depending on the type of hole; generally, it is 15°–30° for through-holes and 45° for blind holes ； When reaming, to ensure that chips are discharged forward, the helix angle can also be appropriately increased; generally, this angle ranges from 10° to 20° ； The land width is 0.1–0.15 mm ； The reverse taper on a reamer should be larger than that of a regular reamer; for cemented carbide reamers, it is generally 0.25–0.5 mm/100 mm, while for high-speed steel reamers, it is 0.1–0.25 mm/100 mm ； The corrected length of the reamer is generally 65%–80% of that of a regular reamer; within this, the length of the cylindrical portion is 40%–50% of that of a regular reamer. 2) Selection of cutting parameters: For reaming, the feed rate is 0.08–0.4 mm/r, and the cutting speed is 10–20 m/min. The rough reaming allowance is generally 0.2–0.3 mm, while the fine reaming allowance is 0.1–0.2 mm. For rough reaming, cemented carbide tools should be used, while for fine reaming, high-speed steel tools can be employed. 3) Selection of cutting fluid: When reaming stainless steel, oil for a full-loss system or molybdenum disulfide can be used as the cooling medium. 3. Boring process 1) Selection of tool material: Since high cutting forces and high cutting temperatures occur when machining stainless steel parts, it is advisable to use cemented carbides of the YW or YG type, which possess high strength and good thermal conductivity. YT14 and YT15 cemented carbide inserts can also be used for finishing. When machining parts made of the aforementioned materials in bulk, ceramic tools can be used. Given that these materials are characterized by high toughness and severe work hardening, the chips generated during cutting take the form of individual fragments, which causes vibration in the tool and can lead to micro-chipping of the cutting edge. Therefore, when choosing ceramic tools for machining such materials, micro-toughness should be considered first and foremost. Currently, Sialon is a good choice, especially α/β Sialon materials, which have attracted attention due to their excellent resistance to high-temperature deformation and diffusion wear. They have been successfully used in cutting nickel-based alloys, where their service life is much longer than that of Al2O3-based ceramics. In addition, SiC whisker-reinforced ceramics is also a very effective tool material for cutting stainless steel or nickel-based alloys. For the machining of quenched parts made from such materials, CBN (cubic boron nitride) inserts can be used. CBN has a hardness second only to that of diamond, ranging from 7000 to 8000 HV; as a result, it possesses excellent wear resistance. Compared to diamond, CBN’s key advantage is its much higher heat resistance, which can reach 1200°C, enabling it to withstand very high cutting temperatures. Furthermore, it has high chemical inertness and does not react chemically with ferrous metals at 1200–1300°C; therefore, it is highly suitable for processing stainless steel materials. Its tool life is dozens of times that of cemented carbide or ceramic tools. 2) Design of tool geometric parameters: The geometric parameters of a tool play an important role in its cutting performance. To ensure smooth and efficient cutting, cemented carbide tools should have a larger rake angle in order to extend their service life. Generally, for rough machining, the rake angle is set at 10°–20°, and for semi-finishing machining, it is set at 15°–20° ； During finishing, use 20° to 30°. The choice of the principal deviation angle is such that, when the rigidity of the machining system is good, a value of 30° to 45° can be adopted ； If the rigidity of the process system is low, a value of 60–75° should be used; when the ratio of the workpiece’s length to its diameter exceeds 10, 90° can be adopted. When boring stainless steel materials with ceramic tools, in the vast majority of cases, the ceramic tools are used with a negative cutting edge angle. The rake angle is generally selected to be between –5° and –12°. This helps to strengthen the blade, allowing full utilization of the advantage of ceramic cutting tools’ high compressive strength. The size of the rear angle directly affects tool wear and also influences the strength of the cutting edge; generally, a value of 5° to 12° is chosen. Changes in the principal cutting angle affect the variations in the radial and axial cutting forces, as well as the size of the cutting width and thickness. Since vibration in the machining system is extremely detrimental to ceramic cutting tools, the choice of principal cutting edge angle should help reduce such vibration; generally, an angle of 30° to 75° is selected. When CBN is used as the tool material, the tool geometry parameters are a rake angle of 0° to 10°, a clearance angle of 12° to 20°, and a cutting edge angle of 45° to 90°. 3) The roughness value of the cutting edge surface should be low during grinding. To prevent chips from sticking to the tool, both the front and back cutting edges of the tool need to be carefully ground to ensure a low roughness value, thereby reducing the resistance to chip expulsion and avoiding chip adhesion to the tool. 4) The cutting edge of the tool should remain sharp. It is important to keep the cutting edge sharp in order to reduce work hardening; the feed rate and depth of cut should not be too low, as this will prevent the tool from cutting through the hardened layer and thus extend the tool’s service life. 5) Pay attention to the grinding of the chip breakers. Due to the tough nature of stainless steel chips, the chip breakers on the cutting edge of the tool must be properly shaped to facilitate chip breaking, chip containment, and chip removal during the cutting process. 6) Selection of cutting parameters: Given the characteristics of stainless steel, it is advisable to use low speeds and larger feed rates during machining. When using ceramic tools for boring, the proper selection of cutting parameters is one of the key factors in fully leveraging the performance of these ceramic tools. When using ceramic cutting tools for continuous cutting, the cutting parameters can be selected based on the relationship between wear resistance and cutting parameters ； For intermittent cutting, reasonable cutting parameters should be determined based on the tool wear pattern. Due to the superior heat and wear resistance of ceramic tools, the impact of cutting parameters on the tool’s wear life is less significant compared to cemented carbide tools. Under normal circumstances, when using ceramic cutting tools, the feed rate has the most significant impact on tool wear. Therefore, depending on the properties of the workpiece material, and provided that the machine tool’s power, the stiffness of the machining system, and the strength of the cutting tools allow it, when boring stainless steel parts, it is advisable to choose a high cutting speed, a large depth of cut, and a relatively low feed rate. 7) The cutting fluid should be chosen appropriately. Given that stainless steel tends to stick easily and has poor heat dissipation properties, it is very important to use a cutting fluid that offers good anti-sticking effects and excellent heat dissipation capabilities during boring operations. Cutting fluids with a high chlorine content, as well as water-based solutions that provide good cooling, cleaning, rust prevention, and lubrication properties without containing mineral oils or sulfites, such as H1L-2 synthetic cutting fluid, are suitable choices. By adopting the aforementioned processing methods, it is possible to overcome the difficulties associated with working with stainless steel. This leads to a significant increase in the tool life when drilling, reaming, and boring stainless steel, as well as a reduction in the number of times tools need to be sharpened or replaced. As a result, satisfactory improvements can be achieved in terms of production efficiency and the quality of hole machining, while also reducing the workload on workers and production costs.