Difficulties in processing ASTM A182 F51 duplex stainless steel
Yaang has undertaken the trial production of a fluid rotary head installed in tens of meters deep sea water under the sea. When the fluid rotating head works, it should meet the requirements of no leakage inside and outside, strong corrosion resistance, and be able to adapt to a variety of fluid components and high flow rate. The socket weld flange of one of its components is shown in Fig. 1.
Non standard socket weld flange material is ASTM A182 F51. For this kind of difficult to machine material, the surface roughness value should reach Ra=0.2 μm, and the dimensional accuracy should reach IT5 level, which was first encountered by our company. Yaang’s technical personnel and master workers worked together to deeply and carefully analyze the processing difficulties of the material and the part design drawing, carried out a number of technical breakthroughs in the key processes, and finally successfully processed the products meeting the requirements of the drawings and delivered them on time. Now the processing difficulties are summarized as follows, for your reference.
Analysis of material cutting difficulties
Table of Contents
- 1 Analysis of material cutting difficulties
- 2 The material has the following characteristics in the cutting process
- 3 Analysis of machining difficulties in part design drawings
- 4 Solutions to processing difficulties
- 5 Conclusion
ASTM A182 F51 is austenite + ferrite duplex stainless steel according to American ASTM standard. Its mechanical properties are: minimum tensile strength 620MPa, minimum yield strength (0.2% residual deformation) 450MPa, minimum elongation 25%, minimum reduction of area 45%, minimum average impact energy 45j. Its machinability is the worst among stainless steels. Taking the machinability of 45# steel as 100%, the relative machinability of austenite + ferrite duplex stainless steel is less than 40%.
The material has the following characteristics in the cutting process
When the plastic deformation is serious, the lattice of the material is distorted, and the strengthening coefficient is large; and the austenite is not stable enough, part of austenite will transform into martensite under the action of cutting stress; in addition, compound impurities are easy to decompose into dispersion distribution under the effect of cutting heat, which makes the cutting process produce hardening layer; the depth of hardened layer can reach 1/3 or more of the cutting depth, and the hardened layer is hard 4 ~ 2.2 times higher than the original. The work hardening phenomenon caused by the previous feeding or the previous working procedure seriously affects the smooth progress of the subsequent process.
Large cutting force
The material has large plastic deformation in the cutting process, and its elongation is more than 1.5 times of 45# steel, which increases the cutting force. At the same time, the work hardening is serious, the thermal strength is high, the cutting resistance is increased step by step, and the chip curling and breaking are difficult. Therefore, the cutting force of the material is large.
High cutting temperature
In addition, the thermal conductivity of the material is about 1/4 ~ 1/2 of that of 45# steel, and a large amount of cutting heat is concentrated on the cutting zone and the interface between the tool and chip, resulting in poor heat dissipation. Under the same conditions, the cutting temperature of the material is about 200 ℃ higher than that of 45# steel.
The chip is not easy to break and bond
The plasticity and toughness of the material are very large. The continuous cutting during turning not only affects the smooth operation, but also damages the added t surface. Under high temperature and high pressure, the material has strong affinity with other metals, which is easy to cause adhesion and form chip accretion, which not only aggravates the tool wear, but also causes tearing phenomenon, which worsens the machined surface.
The tool is easy to wear
In the process of cutting the material, the affinity between the tool and the chip will produce adhesion and diffusion, which will cause the tool to produce crescent shaped craters on the front face of the tool, and the cutting edge will also form tiny spalling and notches. In addition, the hardness of carbide particles (such as tic) in the material is very high, so it can directly contact and rub with the tool, scratch the tool, and add the following Hardening phenomenon will aggravate tool wear.
Large coefficient of linear expansion
The linear expansion coefficient of stainless steel is about 1.5 times of that of carbon steel. Under the action of cutting temperature, the workpiece is easy to produce thermal deformation, and the dimensional accuracy is difficult to control.
Analysis of machining difficulties in part design drawings
As shown in Figure 1, the bottom surface roughness value of the non-standard socket weld flange with fluid rotating head relative to the inner ring is required to be Ra=0.2 μm, the diameter is 200 mm, and the dimensional tolerance grade is IT5. Other dimensional accuracy and geometric tolerance accuracy are also very strict. The roughness requirement of Ra=0.2 μm is also rare for general carbon steel (Ra=0.8 μm is more commonly used). Moreover, it can be seen from the above that the material is the most difficult to process in stainless steel. During the trial cutting, it is found that the thickness of hardened layer and the large cutting force can easily lead to tool breaking; the chip is not easy to break and the cutting temperature is high, which leads to the sticking of the tool and aggravates the tool wear and scratch the machined surface. The difficult machining characteristic of the material determines that the traditional cutting method and cutting tool can not be used to process general materials, especially for the roughness of Ra=0.2 μm and the requirements of high-precision dimension, shape and position tolerance. Therefore, the process arrangement of machining process and the selection of tool material, tool geometric parameters and cutting tools become the key.
Solutions to processing difficulties
Working procedure of machining process
Arrangement → blank pretreatment → rough turning → ultrasonic flaw detection → boring → heat treatment (solution treatment and quenching) → rough turning → failure → semi finishing turning → boring → semi finishing turning → finishing turning → penetrant testing → three-dimensional and roughness testing.
The traditional machining method should use grinding or ultrasonic vibration cutting machine to achieve the roughness requirement of Ra=0.2 μm, but there is no grinding in the process arrangement, only turning. Because through our repeated experiments, the surface roughness value of CNC grinding machine can reach Ra=0.8 μm, and the surface roughness value processed by ultrasonic vibration cutting machine can reach Ra=0.3 μm. We use the MAG CNC turning and milling center machine tool (the diameter of the rotary table is 2m, the load of the worktable is 15t, the height of the z-axis is 2m, and the one-way movement distance of the x-axis is 1600mm). With the turning method, the surface roughness value reaches Ra=0.2 μm.
In the process arrangement, the principle of separation of rough machining and finish machining is followed. After rough machining and before finishing process, aging treatment is carried out to fully release the clamping force and cutting force acting on the workpiece. The final finish machining is carried out after the deformation of the workpiece is stable, so as to prevent the size error and shape error caused by the transition of rough machining deformation to finishing I sequence.
Selection of tool material
The selection of suitable tool material is the basis of machining high precision parts. The dual phase stainless steel ASTM A182 F51 is difficult to machine, which requires that the tool material should have good heat resistance, high wear resistance and little affinity with the processed material. At present, high speed steel and cemented carbide are commonly used as cutting tool materials. It is concluded that cemented carbide is more suitable for machining this material. We choose coated cemented carbide material, the substrate is ultra-fine grain non alloy material with good strength and toughness, and the coating is wear-resistant and high-temperature resistant AlTiN material.
Selection of tool geometry parameters
Rake angle: according to the difficult machining characteristics of the material mentioned above, on the premise of ensuring sufficient strength of the cutter, the larger rake angle of 15 ° to 25 ° is selected, which can not only reduce the plastic deformation of the metal to be cut, but also reduce the cutting force and cutting temperature, and reduce the depth of hardened layer.
Back angle: increasing the rake angle can reduce the friction between the flank and the machined surface, but it will reduce the strength and heat dissipation of the cutting edge. The reasonable value of the back angle depends on the cutting thickness. When the cutting thickness is small, the larger back angle should be selected. We take 10 ° to 20 ° for finish machining or 6 ° to 10 ° for rough machining.
Main deflection angle and secondary deflection angle: reducing the main deflection angle can increase the working length of the cutting edge, which is conducive to heat dissipation. However, the radial force increases during the cutting process, which is easy to generate vibration. The main deflection angle is 45 ° to 75 ° and can be appropriately increased if the rigidity of the machine tool is insufficient. The secondary deflection angle is 8 ° to 15 °. In order to enhance the strength of tool tip, 0.5 ~ 1.0 mm tool tip arc should be grinded.
Edge angle: in order to enhance the strength of the tool tip, the inclination angle is – 8 ° – 3 ° and – 15 ° to – 5 ° for intermittent cutting.
Another important point is the use of peripheral grinding blade. By grinding the edge and shape of both sides of the blade, the tool can cut under the action of large shear force, which can reduce the friction force which increases the cutting force and cutting temperature. The cutting edge after peripheral grinding can retain a 0.0005in wide cutting edge, which is 1/5 ~ 1/3 of the cutting edge width of the ordinary only pressed blade. The ideal surface roughness of parts can be processed with this kind of blade.
Selection of cutting parameters
After the cutter material and geometric parameters are determined, the selection of cutting parameters plays a decisive role in the product processing quality, production efficiency and tool life. For this material, the medium and low speed 50 ~ 80m/min cutting should be adopted. Our final choice of cutting parameters is as follows.
- Semi finish turning: cutting speed 50 m/min, feed rate 0.3 mm/r, back feed 0.3 mm;
- Finish turning: cutting speed 80 m/min, feed rate 0.1 mm/r, back feed 0.1 mm, multiple processing to remove the surface hardening layer;
- Superfinishing turning: cutting speed 80m/min, feed rate 0.05mm/r, back feed 0.1mm.
Material of nonstandard socket weld flange: ASTM A182 F51 has the worst cutting performance in stainless steel. It has the characteristics of severe work hardening, large cutting force, high cutting heat, low thermal conductivity, easy to stick and wear tools, and has high precision requirements. However, through our in-depth research, innovative test and hard research, we have finally processed the parts which are fully qualified by the CMM and roughness meter and delivered on schedule, which is very satisfactory to users.
Source: China Socket Weld Flange Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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