Deep hole machining of stainless steel tube sheet

In 2012, a reflux condenser will be manufactured for a 400000 T urea plant of a Biochemical Co., Ltd. in Shandong. The outer diameter of the tube plate of the equipment is 1960mm, the thickness is 150mm, and the material is 00Crl7Nil4Mo2. The drawing requires that the tubesheet shall be manufactured and accepted in accordance with class II forgings specified in JB4728-2000 stainless steel forgings for pressure vessels. There are 2 pieces tube sheets and 9 pieces baffle plates in the equipment. The baffle plate size is 1790mm × 20mm. The number of tube holes on each tube plate is 2576, and the tube holes are arranged in a triangle.

The chamfer is marked with chamfer major diameter and chamfer depth. In this way, when finishing the T-face, the large diameter of the chamfer will act as the rake angle cutting. Selecting the rake angle of 15 ° – 20 ° can reduce the cutting force, reduce the friction between the rake face and the chips, make the cutting light and fast, cut the generated chips smoothly at the large diameter of the chamfer, and basically control the burr turning into the spline.

The pipe hole shall be strictly perpendicular to the sealing surface of the tube plate, and the perpendicularity tolerance shall be 0.08mm. Because the length diameter ratio of the pipe hole is 6, it belongs to the category of deep hole processing. At that time, our company did not have numerical control drilling machine and deep hole drilling machine, so we could only use the existing Z3080 radial drilling machine for drilling. The accuracy of tubesheet processing, especially the tube hole spacing and diameter tolerance, perpendicularity and surface roughness, greatly affect the assembly and service performance of heat exchangers. Therefore, there is a great risk when we use the ordinary radial drilling machine to drill holes, and a slight carelessness in control may lead to dimensional out of tolerance. In order to complete the drilling task of tubesheet with high quality under the existing equipment, we analyzed the processing difficulties and formulated a detailed processing plan.

Analysis of difficulties in deep hole machining of stainless steel tube sheet

• (1) The number of pipe holes is large, and the requirements for the width of the hole bridge are strict. Therefore, the requirements for the marking accuracy are very strict.
• (2) The perpendicularity of the pipe hole is very strict when drilling. If the pipe plate is thick and slightly deviated when drilling, the error on the back of the pipe plate will be greatly enlarged, and eventually the position size and perpendicularity will be out of tolerance. There are many layers of baffles, and the concentricity of the tube sheet and the baffle tube hole is strictly required. Once the concentricity cannot be guaranteed, the difficulty of the heat exchange tube passing through the tube will be greatly increased in the equipment manufacturing.
• (3) The tubesheet material is 00Crl7Nil4Mo2, which is a class II forging. The metal material has high plasticity and high cutting resistance. In addition, the material structure is more compact during the forging process, which further enhances the toughness of the material. The material itself has work hardening phenomenon. Therefore, the adhesive wear of the drill bit is large during drilling; In addition, the thermal conductivity of stainless steel is poor, the drilling depth is very deep, and the cutting heat is not easy to dissipate, which aggravates the wear of the drill bit, and the service life of the drill bit is very low.
• (4) The stainless steel chips are discharged in a belt shape and are not easy to break. They may even wind on the drill bit, scratch the inner wall of the pipe hole and affect the surface roughness of the pipe hole.

Deep hole processing scheme and precautions for stainless steel tube sheet

In view of the above analysis of processing difficulties, we decided to use the processing technology of drilling first and expanding later, as follows:
(1) Scribing: in order to ensure the concentricity of the tubesheet and baffle plate after drilling, we adopt the processing technology of scribing on one baffle plate, drilling positioning holes with 6mm drill bit after passing the inspection, and using the baffle plate as a template to drill two pieces tubesheets and eight pieces other baffle plates respectively. That is to say, the baffle plate to be scribed shall be processed into an outer circle on the vertical lathe, then leveled, and then scribed. When marking, the baffle plate shall be placed horizontally, and the surface to be marked shall be colored to make the marking part clear and eye-catching. Manual scribing: because the lines drawn by the scribing needle have a certain width, there are inevitably certain cumulative errors. Since the pipe holes are arranged in a regular triangle, if they are drawn from the center outward, the original error will be gradually diffused and amplified as the crossing line of the regular triangle extends outward, resulting in the deviation of the center of the external pipe holes and the error of the center distance exceeding the tolerance.
In order to avoid this situation, we use the method of drawing lines from the outside to the inside. The specific method is to use a scribe to find the center of the baffle plate and draw a cross line. Be sure to ensure that the cross lines are accurate and perpendicular to each other. Then, take the center of the baffle plate as the center, draw a circle with the theoretical value of the distance between the center of the pipe hole on the outermost vertex and the center of the baffle plate as the radius, make a circle inscribed regular hexagon, first draw the center point of the pipe hole on each side of the regular hexagon, and then connect the center point of the pipe hole on each side with the center point of the pipe hole on the corresponding side according to the drawing. The intersection point of the connecting line and the connecting line is the center of other pipe holes. After the line is drawn, the center distance error of each pipe hole shall be checked first. After the inspection is qualified, sampling and punching can be carried out and positioning holes can be drilled. With this method, the position error of the outer pipe hole can be controlled within 0.2mm, and the position error of the inner pipe hole can be controlled within 0.2mm. Attention must be paid to ensure the sharpness of the needle and sample punch to minimize the error.
(2) Drilling: before drilling, fix the baffle plate as the drilling jig on a tube plate to be drilled, check the concentricity and fit of the baffle plate and the tube plate, and use a 06mm drill bit to guide the positioning hole on the baffle plate to the tube plate after it is qualified. The drilling depth is 8-10mm. After completion, remove the baffle plate and drill holes with a 23mm taper shank fried dough twist drill. Note that when drilling, the tube sheet must be fixed on the workbench to ensure the levelness of the tube sheet and the verticality of the tube hole after drilling. In addition, when drilling the 3mm bottom hole, each hole should carry out 2-3 times of drilling and chip return, which is very beneficial to the smooth discharge of chips and the cooling of the cutting edge of the drill bit.
(3) Reaming: after the first drilling with 623mm fried dough twist drill, we will use 25.4mm reamer to ream. Since the first drilling has made the pipe hole size reach 23mm, the cutting amount is relatively small during the reaming process, so that the surface quality of the pipe hole is greatly guaranteed. When reaming, pay attention to check the wear of the drill and keep the cutting edge of the drill sharp.
Selection of cutting fluid: in the processing of stainless steel deep hole tube holes, lubrication is not the main contradiction. The main contradiction is the heat dissipation of the tube plate and the drill bit. We use 10% emulsion as the cutting fluid, and inject it into the cutting edge with increased flow to take away the cutting heat as much as possible; So as to prolong the service life of the drill and improve the surface quality of the pipe hole.
Selection of cutting parameters: reasonable cutting parameters are conducive to the improvement of pipe hole accuracy. When drilling, the rotation speed is set to 105r/min and the feed rate is 0.32-0.4mm/r; The speed of reaming is 200r/min and the feed rate is 0.45-0.6mm/r. Practice has proved that selecting such cutting parameters can not only improve work efficiency, but also ensure drilling quality.
After inspection, the pipe holes processed by the above process scheme meet the technical requirements of the tube sheet. Pay attention to the following points during processing:

• On the premise of meeting the requirements of drilling depth, the length of the drill bit shall be shortened as far as possible to obtain high rigidity of the drill bit. The drill must be installed correctly, keep the drill sharp, and grind it in time after being blunt. During the drilling process, observe the chip discharge condition in time. If the chips are found to be disorderly and tangled, immediately return the tool for inspection to prevent the chips from clogging.

Source:  China Tube sheet Manufacturer: 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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