Experience of Difficulties in Processing Stainless Steel Pipes

The main difficulties in processing stainless steel pipes are as follows:

1. Large cutting force and high cutting temperature: This type of material has high strength, large tangential stress, and plastic deformation during cutting, so the cutting force is large. In addition, the thermal conductivity of the material is extremely poor, which causes the cutting temperature to rise, and the high temperature is often concentrated in the narrow area near the cutting edge of the tool, thereby accelerating the wear of the tool.

2. Severe work hardening: Austenitic stainless steel pipes and some high-temperature alloy stainless steel pipes are austenitic structures, and have a large tendency to work hardening during cutting, which is usually several times that of ordinary carbon steel. The tool cuts in the work hardening area, which shortens the tool's life.

3. Easy to stick to the tool: Both austenitic stainless steel pipes and martensitic stainless steel pipes have the characteristics of strong and tough chips and high cutting temperatures during processing. When the tough chips flow through the front cutting edge, sticking phenomena such as bonding and welding will occur, affecting the surface roughness of the processed parts.

4. Accelerated tool wear: The above materials generally contain high melting point elements, high plasticity, and high cutting temperature, which accelerates tool wear and requires frequent sharpening and tool replacement, thus affecting production efficiency and increasing tool use costs. Mainly reduce cutting line speed and feed. Use tools specially designed for processing stainless steel pipes or high-temperature alloys. It is best to use internal cooling for drilling and tapping.

Stainless steel pipe processing technology

Through the above processing difficulties, the processing technology and related tool parameter design of stainless steel pipes should be quite different from those of ordinary structural steel materials. The specific processing technology is as follows:

1. Drilling: During drilling, due to the poor thermal conductivity and small elastic modulus of stainless steel pipes, hole processing is also difficult. To solve the hole processing problem of such materials, it is mainly to select suitable tool materials.

(1) Tool material selection: Due to the high cutting force and high cutting temperature when processing stainless steel pipes, the tool material should be selected as high-strength and high-thermal-conductive cemented carbide as much as possible. For the processing of hardened parts of this type of material, CBN (cubic boron nitride) blades can be used. The hardness of CBN is second only to diamond and can reach 7000-8000HV. Therefore, it has high wear resistance. Compared with diamond, the outstanding advantage of CBN is that its heat resistance is much higher than that of diamond, which can reach 1200℃, and it can withstand very high cutting temperatures. In addition, it is very chemically inert and does not react chemically with iron metals at 1200-1300℃, so it is very suitable for processing stainless steel pipes. Its tool life is dozens of times that of cemented carbide or ceramic tools.

(2) Tool geometry parameters Tool geometry parameters play an important role in its cutting performance. In order to make cutting light and smooth, cemented carbide tools should use a larger rake angle to increase tool life. Generally, the rake angle is 10°-20° for rough machining, 15°-20° for semi-finishing, and 20°-30° for finishing. The selection of the main deflection angle is based on the fact that when the rigidity of the process system is good, 30° to 45° can be taken; if the rigidity of the process system is poor, 60° to 75° can be taken. When the ratio of the length to the diameter of the workpiece exceeds 10 times, 90° can be taken.

When boring stainless steel pipes with ceramic tools, in most cases, ceramic tools use negative rake angles for cutting. The rake angle size should generally be -5° to -12°. This is conducive to strengthening the blade and giving full play to the superiority of the high compressive strength of ceramic tools. The size of the back angle directly affects the wear of the tool and also affects the blade strength. Generally, 5° to 12° is selected. The change in the main deflection angle will affect the changes in the radial cutting force and the axial cutting force, as well as the size of the cutting width and cutting thickness. Because the vibration of the processing system is extremely unfavorable to ceramic tools, the selection of the main deflection angle should be conducive to reducing this vibration, and generally, 30° to 75° is selected. When CBN is selected as the tool material, the tool geometry parameters are rake angle 0° to 10°, back angle 12° to 20°, and main deflection angle 45° to 90°.

(3) The roughness value of the front cutting edge should be small when grinding. In order to avoid the phenomenon of chips sticking to the tool, the front and back cutting edges of the tool should be carefully ground to ensure that they have a small roughness value, thereby reducing the resistance to chip outflow and avoiding chips sticking to the tool.

(4) The cutting edge of the tool should be kept sharp. The cutting edge of the tool should be kept sharp to reduce work hardening. The feed rate and back-cutting amount should not be too small to prevent the tool from cutting into the hardened layer and affecting the service life of the tool.

(5) Pay attention to the grinding of the chip breaker: Since the chips of stainless steel pipes are strong and tough, the chip breaker groove on the front cutting edge of the tool should be properly ground to facilitate chip breaking, chip storage, and chip removal during the cutting process.

(6) Selection of cutting amount: According to the characteristics of stainless steel pipes, low speed and large feed rate should be used for cutting during processing.

(7) The cutting fluid should be selected appropriately: Since stainless steel pipes are very easy to produce adhesion and have poor heat dissipation, it is very important to select cutting fluids with good anti-adhesion and heat dissipation properties during boring, such as cutting fluids with higher chlorine content, and aqueous solutions that do not contain mineral oil or nitrite and have good cooling, cleaning, rust prevention and lubrication effects, such as H1L-2 synthetic cutting fluid.