Processing and Inspection of Alloy 625 Nickel Base Alloy Tube Sheet
Alloy 625 belongs to the Ni-Cr-Mo-Nb alloy and is a solid solution strengthened nickel based alloy, co mmonly known as Inconel Alloy 625. Due to its high content of Ni, Cr, Mo, and other elements and its unidirectional austenite structure, this alloy exhibits excellent corrosion resistance in both oxidizing and reducing media, making it co mmonly used in special environments with high temperatures and strong corrosion. In early 2006, a certain company selected SB564 UNS NO6625, which had a very high material cost, as the tube sheet material for the gas/gas heat exchanger. In processing this high hardness and high toughness nickel-based high alloy material, low speed, small feed rate, and the use of sharp wear-resistant tools are required. However, the specific cutting parameters and tools still need to mature experience. A series of cutting tools and processing experiments were carried out to complete the turning and drilling processing of the tube plate according to the design requirements.
This article combines manufacturing practices to discuss the machining experience and manufacturing inspection points of nickel-based high alloy tube sheets obtained through simulation experiments.
2. Characteristics of Alloy 625 material
The unified numerical code of the material and the comparison of national grades are shown in Table 1, the main chemical composition of the material is shown in Table 2, and the mechanical properties are shown in Table 3.
3. Dimension and processing requirements of Alloy 625 tube plate
3.1 625 tube plate structure
The structure of the tube plate is shown in Figure 1, with a thickness of 140 mm and a diameter of 1600 mm. The number of openings for the heat exchange tube is 1496, and the size of the tube holes for the heat exchange tube is shown in Figure 2. The arrangement type of the heat exchange tube is shown in Figure 3.
3.2 Technical requirements
(1) The sealing surface of the tube plate should be perpendicular to the axis, with a verticality tolerance of 0.300.30 mm;
Table.1 Unified numerical codes and comparison of national grades
|Unified numerical code||H03360|
|GB/T||(Recommended national standard in China)||0Cr20Ni65Mo10Nb4 (NS336)|
|ASTM||(ASTM Standards)||NO6625 (Inconel 625)|
|DIN||(Former Federal German National Standard)||NiCr22Mo9Nb (W.-Nr.2.4856)|
|UK (BS)||(British Standard)||NA21Ni-Cr-Mo-Nb (NA21)|
|ISO||(International Organization for Standardization Standards)||NiCr22Mo9Nb|
Table.2 Main Chemical Components of Inconel 625 (%)
|Project||Minimum value||Maximum value||Warranty letter||Retest value|
Table.3 Mechanical Properties of Inconel 625
|Project||Specified value||Warranty Letter||Retest value|
|Heat treatment State||Solution annealing||Solution annealing||—|
|Tensile strength (MPa)||≥758||803||890|
|Yield strength (MPa)||≥345||481||530|
|Hardness (HB)||≥ 240 (reference value)||—||310 – 330 (measured)|
Figure.1 Structure of Inconel 625 Tube Sheet
Figure.2 Tube hole size of heat exchange tube
(2) The spacing between the heat exchange tube holes is 32 mm, and the corners are arranged in a regular triangular shape;
(3) The tube plate hole of 1496 – Ø 25.3+0.1 should be strictly perpendicular to the sealing surface of the tube plate, with a verticality tolerance of 0.08 mm;
(4) After drilling the pipe plate, the width of the pipe hole bridge must not be less than 5.59 mm for no less than 96% of the pipe holes, and the minimum width of the hole bridge allowed for no more than 4% is 3.55 mm and no more than 5 places.
Figure.3 Arrangement of heat exchange tubes
4. Processing difficulties of Inconel 625 tube plate
(1) What materials and cutting parameters can be used to achieve the turning and drilling processing of this high hardness and high toughness solid solution strengthened nickel-based high alloy;
(2) How to ensure that the tube plate hole is perpendicular to the sealing surface of the tube plate and has a vertical tolerance of 0.08 mm, and how to detect it;
(3) How to ensure the width requirement of the pipe bridge when drilling and processing pipe plate holes with a length/diameter ratio greater than 5, and the center line of the hole is prone to deviation.
In response to the abovementioned processing difficulties, it is necessary first to determine the process testing plan, turning and drilling conditions, and detection methods.
5. Process testing of tube sheets
5.1 Turning Test of Inconel 625 Tube Plate
(1) Test on C620 lathe.
The cutting tools are machine-clamped 75 ° hard alloy turning tools with hard alloy grades of YG8, YT15, and YW2, respectively. The test plate is ＞ 200 mm × 46 mm UNS NO6 625. When processing the end face and outer circle of the test plate, the rotational speed n=400 rpm, the feed rate F=0.15 mm/r, the cutting depth t=1.5 mm, and the tool γ Front=8 °, α Back=10 °, the front blade faces inverted R3 crescent depression.
- 1) When using YG8 blades, the tool wear is extremely severe, and the durability is extremely low;
- 2) When using the YT15 blade, the blade severely collapses;
- 3) When using YW2 blades with excellent comprehensive mechanical properties, the tool wear is normal and meets the requirements for machining accuracy.
(2) On a 2.5m vertical lathe, the test was conducted directly with a workpiece tube plate. The cutting parameters were changed to rotational speed n=15 revolutions per minute, feed rate F=0.25m/r, and cutting depth t=1.5 mm using a tool that passed the test on a C620 lathe (with the tool brand and angle unchanged). The test results showed that the tool met the machining requirements.
5.2 Drilling Test of Inconel 625 Tube Sheet
The test plate is Ø 400 mm × 46 mm, with a surface finish of level 3 on the upper and lower surfaces, and the material and hardness are the same as those of the tube sheet.
6. Key points for processing and inspection of Inconel 625 tube sheet
(1) Forging: Tube plate blank Ø 1605 mm × 145 mm, chemical composition, and mechanical properties retested according to ASME SB564.
(2) Vertical lathe: Use a 2.5m vertical lathe to complete the processing of the tube plate shape, as shown in Figure 1.
1) Using reasonable cutting parameters and cutting tools to process the sealing surface and outer circle of the upper end face of the tube plate, ensuring that the sealing surface of the upper end face is parallel and coaxial, perpendicular to the outer circle, meeting the requirements of section 3.2 (1). At the same time, the upper-end face of the tube plate can serve as the basis for drilling and inspection;
2) Flip the workpiece 180 °, use the machined upper end face as the positioning reference, align according to the outer circle, and process the lower end face of the tube plate to ensure that the parallelism tolerance of the upper and lower end faces is not greater than 0.05 mm;
3) Install a magnetic micrometer on the vertical lathe tool holder, rotate the workpiece at an extremely low speed, move the tool holder, and check the parallelism value. The measured value is 0.04 mm.
4) Scribing: Place the tube plate vertically, position the outer circle, and draw perpendicular centerline lines on the upper surface of the tube plate to determine the center of the tube plate circle;
(3) Drilling: The 1496 – Ø 25.3 + 0.1 hole drilling process was completed on the HTB55 CNC deep hole drilling machine, and the emulsion confirmed in Section 5.2 (1) was used to cool the Alloy 625 during the process.
- 1) By positioning the tube plate’s lower-end face and outer circle, locate the horizontal and vertical centerline, and then level the upper-end face to determine the drilling center. Use auxiliary tools to fasten the tube plate to the special bending plate of the deep hole drilling machine so that the upper-end face of the tube plate is perpendicular to the feed direction of the drill pipe.
- 2) Drill holes using tested coated deep hole drills and verify suitable cutting parameters according to the compiled data program.
- 3) Use a go/no go inspection gauge to check the aperture, ensuring the through end passes. If the upper-end stops, the aperture size accuracy is qualified. Otherwise, it is necessary to stop the machine and replace the drill bit with a new one. Based on the drilling hole as the benchmark, use the designed perpendicularity inspection tool to randomly check the perpendicularity tolerance of the hole on the upper end face of the tube plate, and the measured value is 0.05 mm. After drilling the hole, use a vernier caliper to measure the pipe bridge’s width on the plate’s lower-end face. The measured width of the pipe bridge is between 6.5 and 6.7 mm.
- (5) Chamfering: Complete the chamfering process of pipe holes on a radial drilling machine.
- (6) Check the dimensions and keep records.
Author: Zhang Liang