Study on Drilling and Tapping Technology of 1Cr18Ni9Ti Stainless Steel Slot

0 Introduction In view of the excellent corrosion resistance and toughness of 1Cr18Ni9Ti material, it has become increasingly widely used in liquid gas transmission pipe fittings. Especially in the case of high pressure delivery of ACID and alkaline liquids, this type of material is often used to make valve fittings. Such parts have complex shapes, high precision requirements, and large batch sizes. Since 1Cr18Ni9Ti is an austenitic stainless steel material, it has characteristics such as large toughness, good high-temperature mechanical properties, strong chip adhesion, poor thermal conductivity, strong work hardening, and poor cutting performance, especially for drilling and threading of small holes. More difficult. The use of traditional technology, tool consumption, scrap rate is extremely high. Therefore, through repeated practice, the process scheme has been continuously improved and good results have been achieved. In this type of valve body parts, there are a large number of hole diameters below Ø8mm, hole length above 240mm, length/diameter ratio l/d>30, surface roughness Ra1.2, cylindricity error 0.05mm, and Many are smaller than M6 internal threads. In the processing of such elongated holes and small-diameter internal threads, if the traditional processing method is used, the stability of the extended bit is firstly poor, and it is difficult to meet the technical requirements. In particular, it is difficult to remove chips, which can easily cause breakage of drills and scrapping of workpieces. Secondly, the wear of drills and taps is aggravated. Generally, the drills and taps need to be updated when several parts are processed, which increases the processing cost and reduces the processing efficiency. 1 Drilling process design and parameters determine the lengthening of the drill pipe and improve the stability of the drill. Because it is a deep-hole machining, it needs some extended drills. The lengthened drill is not only costly, but also has a long cycle. In the absence of special fixtures, When welding a long handle with a universal standard bit, it is not easy to weld positive and cannot meet the technical requirements. To this end, design a set of special fixtures, as shown in Figure 1, to make small deformation after welding. The principle is: using a V-shaped clamp seat to ensure the concentricity of the drill bit and the drill shank. The drill bit and the drill shank are positioned and clamped for welding using a bolt presser plate and brazed with a copper material. The extended drill shank should be accurately sized and guided to improve drill bit stability during drilling.

1. V-shaped seat 2. Drill, drill shank 3. Pressure plate 4. Bolt Figure 1 Welding drill pipe clamp

The geometric parameters of the drill bit are determined because 1Cr18Ni9Ti belongs to austenitic stainless steel difficult-to-machine materials, according to conventional geometric parameters: front angle 116°~120°; back angle 10°~14°; secondary back angle a=0°, drill bits are drilled. Extremely low durability, poor processing quality and other issues are very prominent. After analyzing the cutting process, the geometric parameters of the cutting part of the drill bit are changed from the following five aspects. Change the back angle and front angle: Through a large number of tests, it is proved that the change of the back angle is 12°~15°, and the angle of 135°~140° is ideal. Grinding edges: The original secondary relief angle a = 0 ° ground a into 3 ° ~ 5 °, leaving a width of the edge of the edge. This not only retains the guiding action of the original edge, but also reduces the friction between the edge and the hole wall, thereby reducing the cutting heat and the "biting" phenomenon of the drill and the workpiece. Open the swarf groove: use the drill grinding machine to grind the swarf, its purpose is to reduce the width of the chip, improve the cutting conditions, and facilitate chip removal. Through a large number of tests, one to two flutes can be drilled for drills with a diameter of Ø6 to 12 mm. The width of the comminutor c is 0.05 to 0.06 d. The depth t of the kerf is 0.04 to 0.05 d, where d is the diameter of the drill. . Trimming the chisel edge: The chisel edge should be shortened as much as possible, the sharpness should be reduced, and the rake face at the chisel edge should be repaired so that the negative rake angle of the sharpening part can be changed to the rake angle. The cutting speed v is 8 to 10 m/min, and the feeding amount f is 0.12 to 0.20 mm/r.

Figure 2 Tap Profile

2 Improvement Measures for Taps The characteristics of austenitic stainless steels, such as 1Cr18Ni9Ti, use W18Cr4V as the tool material in the tapping process, and use a smaller lead angle Ø. The calibration part is shorter and the number of chip flutes is larger. Taps with angle g=15° to 20° and relief angle a=8° to 12°. According to the use situation, the tap can be processed with more than M6 threaded holes, but when processing the following M6 threads, it is easy to attack, tap, die, and break. Analysis of the reasons is mainly due to the characteristics of stainless steel, due to the small tap, open the chip flutes after the cross-section is very small, hardened after quenching, high hardness and poor toughness, low torsional strength, easy to produce broken phenomenon. Through experimental analysis, the existing general-purpose tap is improved and developed into a triangular-shaped tap, as shown in FIG. 2 , which has a larger guide angle Ø, an anterior angle of g=20° to 25°, and a posterior angle a=8°. ~10°. The cross-sectional area of ​​the tap is larger than that of a tap with the same size, its rigidity is good, the torsional strength is large, the service life is improved, and the effect is good after use. 3 After selecting the proper cutting fluid to analyze the causes of broken drills and taps, we found that the following factors play a major role: First, the problem of lubrication, the accumulation of built-up edge when machining, the difficulty of sticking knife and chips, high temperature lubrication is not good. Second, the cooling is not sufficient, and heat cannot be emitted in time. To solve the above problems, it is most effective to choose a cutting fluid with good anti-adhesion and cooling performance. Through a large number of experiments, the requirements for cutting fluids are: good fluidity; good high temperature lubrication. That is, the fluidity of the cutting fluid is good, and the cutting fluid can be flowed to the processing surface in a timely and continuous manner to cool and lubricate, so as to reduce the occurrence of built-up edge and non-adhering knife. In this regard, through comparative experiments with multiple groups of formulations, it was found that the cutting fluid with a distribution ratio of oil, alcohol, and molybdenum disulfide is used in a high-speed full-loss system, and the use effect is good. Its formula is: 70# extreme pressure industrial gear oil 30%, low viscosity, good fluidity, low temperature lubrication, cooling effect; molybdenum disulfide 25%, good lubricity, extreme pressure agent, its lubricating effect under high temperature and high pressure Good; 45% alcohol, low viscosity, good fluidity, better cooling effect. 4 Conclusion The process is characterized by continuous exploration and repeated practice, design of a special process scheme and technical parameters, and based on the tool structure, material improvement and cutting practice proved that the process to other austenitic stainless steel materials For example, when parts of 1Cr18Ni9Ti, 1Gr18Ni9 and 0Gr18Ni12Mo2 are processed, the process has a good adaptability, which can significantly reduce the overall cost of processing.

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