Deep hole machining test of nickel based superalloy tubesheet

Pressurized water reactor nuclear power plant steam generator tube sheet generally take the form of composite, that is, high-strength low-alloy steel base material + nickel-based overlay layer. Its purpose is to rely on low-alloy steel to withstand the pressure of the working medium, so that the surface weld layer to play a role in the primary side of the radioactive fluid corrosion resistance, as well as the tube sheet and heat exchanger tube welding isolation transition role. At present, such composite structure of the tube sheet deep hole processing technology is relatively mature.
A type of steam generator, because of its high operating temperature, design pressure, harsh working environment, the design of the tube sheet through the body with high strength at high temperatures, nickel-based alloy forgings, and strict requirements for the accuracy of the tube sheet hole. The material grade of the tube sheet is SB-564 UNS N08810, and the mechanical properties are listed in Table 1. The holes are arranged in a triangular shape, with a hole center distance of 27 mm and a hole depth of 320 mm. 800H tube sheet hole size accuracy is stricter than that of the pressurized water reactor steam generator, and this material is the first application of steam generator tube sheet in engineering, which brings challenges to the deep hole processing process of the tube sheet.
Table.1 SB-564 UNS N08810 mechanical properties of forgings

Project	Tensile strength Rm at 550 °C, MPa	550 °C yield strength Rp0.2, MPa	Elongation%	Surface hardness HB
Numerical value	477	147	46-50	99-113

1. Nickel-based high-temperature alloy cutting characteristics

Conventional cutting of nickel-based high-temperature alloy has the following characteristics: good material plasticity, large cutting deformation, severe machining hardening; high material strength, high cutting force; small thermal conductivity of the material, high cutting temperature, fast tool wear. For the drilling of nickel-based high-temperature alloy holes, because in a closed or semi-closed processing state, making the hole surface quality, hole size and position accuracy are poor.
Moreover, the deep holes in nickel-based high-temperature alloy forgings are mostly single holes in the rotary body, which are commonly processed by horizontal lathes through the external chip discharge method of drilling and boring. However, the processing efficiency of these traditional methods is low, and the accumulated errors caused by multiple positioning and repeated positioning make the hole position accuracy poor, which is not suitable for this type of steam generator tube sheet hole processing.

2. Test conditions

2.1 Test piece material

In order to be more representative, the test metal material, hole thickness and the same product tube sheet, and the product tube sheet with the same melting furnace number, similar forging ratio and the same heat treatment process.

2.2 Machining process and tools

There are three kinds of deep hole processing processes: gundrilling (external chip removal), BTA drilling (internal chip removal) and spray suction drilling (internal chip removal). Gundrilling and BTA drilling processes have been applied in the deep hole processing of nuclear power pipe plate and have certain experience. Considering the processing accuracy and efficiency of the tube sheet hole, the BTA process was adopted.

2.3 Machining equipment

Test using imported horizontal CNC deep hole drilling, the main parameters of the machine tool: X axis (horizontal) stroke 6500mm, Y axis (vertical) stroke 4500mm, Z axis (spindle level) stroke 1100mm; positioning accuracy: X axis 0.03mm, Y axis 0.02mm, Z axis 0.01mm; repeat positioning accuracy: 0.01mm.

3. Test items and results

3.1 Effect of drill diameter on reaming volume

The hole diameter of BTA drill bit will gradually become smaller as the tool wears during the machining process. Since the designed hole diameter tolerance range is small (0.05mm), the range of BTA drill diameter selection is also very small, not only to prevent the first hole diameter from exceeding the upper limit of the difference, but also to take into account the tool wear after the hole diameter becomes smaller than the lower limit.
Figure 1 shows the enlargement of the first hole diameter for four drill sizes of Φ19.16/19.27/19.34/19.42mm. From the data, the average reaming volume is about 0.035mm for Φ19.42mm and 0.04mm for individual holes, while the maximum reaming volume is 0.06mm and the average reaming volume is 0.04mm for Φ19.16mm. It can be seen that the reaming volume of nickel-based alloys decreases with the increase of drill diameter.

Fig.1 Variation of reaming volume of nickel-based alloy material with drill diameter

3.2 Influence of rotational speed on machining accuracy

When the BTA drill bit of Φ19.16mm is used to drill holes with the same feed speed and different rotational speeds (800-1400r/min), the hole diameter fluctuates within the range of Φ19.18-19.20mm, except for some holes reaching Φ19.22mm, which can meet the requirements of the design drawings. For this target product, it can be considered that the rotational speed has no effect on the bore diameter within the selected parameters.
However, as the speed increases, the surface roughness of the hole wall becomes significantly better. This is because when the spindle speed is small, part of the chips are not directly cut out by the cutting edge, but are formed by the tool extrusion, which leads to the poor quality of the hole wall.
However, as the speed increases, the tool wear increases significantly. The main reason is that, with the increase of spindle speed, the cutting process of frictional heat generation time will be very short, the heat is too late to the chip and tool internal conduction, resulting in higher cutting temperature, and because of the poor thermal conductivity of nickel-based high-temperature alloy itself, so that heat accumulation in the cutting position, tool wear faster.

3.3 The impact of feed rate on machining quality

Feed rate is the main factor affecting the efficiency of deep hole machining, in the above selected cutting speed, using different feed rates for testing. Figure 2 shows the shape of the chip when using a large feed of more than 70mm/min, obviously thick and wide, but also not easy to break the chip. This is because in the case of a certain speed, when the feed rate increases, the depth of cut will increase, the chip is removed from the workpiece after curling, due to the better toughness of nickel-based alloy, although the chip has undergone cold hardening, but still not easy to break the chip, it is very easy to block the chip channel.
Figure 3 shows the shape of the chip when the small feed of less than 70mm/min is used, although it is long but not easy to break the chip, it is narrow and thin folded long strip shape, which can be discharged smoothly with the cutting fluid. Moreover, the cutting is smooth and the impact of chip breaking is avoided, and the durability of the drill bit can be improved to a certain extent.

3.4 Drill life test

The first hole diameter is the upper limit of the design value or exceeds the upper limit; with the increase of the number of holes, the drill bit wears more and the hole diameter decreases gradually. For the tail hole, most of the holes have reached the lower limit of the design value, and some of them have exceeded the lower limit.

Fig. 2 Chip pattern at large feed Fig. 3 Chip pattern at small feed
Among the three teeth of the BTA drill, the outer teeth are the most worn at the farthest distance from the center of the drill because the outer teeth have the highest linear velocity. As for the wear of the guide bar, the color of the surface coating of the first guide bar and the second guide bar changed significantly, and the color of the first guide bar changed less than that of the second guide bar, and the top of the first guide bar was worn to different degrees, while the wear of the second guide bar was not obvious.

4. Conclusion

The following conclusions were obtained from the process test of machining nickel-based high-temperature alloy 800H tube sheet hole by BTA drill bit.

• (1) The reaming volume of BTA drill bit decreases with the increase of drill diameter, for the drill bit of Φ19.16-19.42mm, the maximum reaming volume reaches 0.06mm, and the average reaming volume is about 0.04mm.
• (2) At the same feed rate, the speed of 800-1400r/min has little effect on the hole diameter, but the surface roughness of the hole wall becomes better as the speed increases.
• (3) The same speed, with the increase in feed rate, the chip thickened and not easy to break the chip, easy to plug the chip phenomenon; using a small feed, the chip is narrow and thin pleated long, can be smoothly discharged.
• (4) Choose the cutting parameters with the best machining accuracy, continuous processing of 40 holes, the tail hole is close to or exceeds the lower limit of the hole diameter tolerance, the outer edge of the BTA drill and the first guide strip are seriously worn.

Authors: Wang Fuchun, Wang Qiang, Li Wei, Zhao Yue