The effect of stress relief annealing on the surface residual stress and organizational properties of B10 copper-nickel tees

In different annealing temperatures and holding times on the cold pushing into shape B10 copper-nickel alloy tee vacuum stress relief annealing treatment, research B10 copper-nickel alloy tee stress relief annealing before and after the mechanical properties and residual stress of the change rule. The results show that the residual stress on the surface of the main pipe and branch pipe after cold forming is compressive, and the residual stress on the surface of the transition position between the main pipe and the branch pipe is tensile; de-stressing annealing eliminates the residual stress on the surface of the B10 CuNi alloy equal tee under the premise of not damaging the mechanical properties. In the scope of this test, after annealing at 300 ℃ for 1h with the furnace cooling, pipe fittings’ axial and tangential residual stress is eliminated.

B10 copper-nickel alloy can be cold and hot processed, has good corrosion resistance and medium strength, and is widely used in important fields such as ships and ocean engineering. Its thick-walled pipe fittings are mainly used in ships’ pressurized seawater piping systems. B10 copper-nickel alloy tee fittings are often shaped using cold processing through cold pushing, in which the deformation process is accompanied by inhomogeneous deformation, which is prone to produce residual stresses inside the material, which may affect the corrosion-resistant and fatigue properties of copper-nickel alloy pipe fittings. Scholars at home and abroad focus on exploring residual stress control and abatement methods.
Residual stresses are generally reduced by pre-stretching, vibration, or heat treatment processes. Much literature has been published on the study of residual stress in metal materials; Ren Quanzhuang et al. systematically studied the residual stress distribution in the quenching process of medium carbon steel. Zhang et al. systematically investigated the effect of the non-uniformity of mechanical properties of plates in the thickness direction on eliminating residual stress in the pre-stretching process by establishing the model of the residual stress field of aluminum alloy pre-stretching plate layering. Fan Ning et al. analyzed the distribution level of residual stresses in an aluminum alloy thick plate in a pre-stretched state. They systematically investigated the abatement law of quenched residual stresses in a 7055 aluminum alloy thick plate with different pre-stretching deformation. Luo Jiahao et al. explored the process method to reduce the residual stress and dislocation density of cold rolled silicon steel by studying low temperature, low pulsed magnetic field strength short-time magnetic-thermal coupling joint action. Xie Zhipeng et al. proposed a new method to regulate the residual stress on the surface of pure titanium (TA2) anodic oxide film, focusing on the microstructure of the film layer under the regulation of residual stress. Among the many methods and measures to reduce the involved stresses, stress relief annealing treatment has been widely used in the industrial field by its high efficiency and convenience.
There need to be corresponding reports on the level and distribution of residual stresses in CuNi alloys after cold deformation and different heat treatments and the law of residual stress reduction in CuNi alloys by heat treatment. In this study, the X-ray diffraction stress method is used to characterize the surface residual stress distribution of B10 CuNi alloy thick-walled pipe fittings after different heat treatments to analyze the distribution level of residual stresses in the heat-treated state of B10 CuNi alloy thick-walled pipe fittings, and to investigate the abatement rules of residual stresses on B10 CuNi alloy thick-walled pipe fittings by different heat treatment processes.

1. Test material and method

The test material is ϕ90mm × 6mm B10 copper-nickel alloy thick-walled equal tee; the alloy composition is shown in Table 1. Cold pushing into shape thick-walled equal tee, the processing schematic diagram is shown in Figure 1.

Table.1 B10 copper-nickel alloy thick-walled tee composition (mass fraction, %)

Main components				Impurity
Ni	Fe	Mn	Cu	Pb	S	C	Zn	P
10.37	1.71	0.73	Bal.	0.005	0.0045	0.0016	0.015	0.005

The B10 copper-nickel alloy thick-walled tee generates residual stresses inside the tee fittings due to the uneven plastic deformation of the material during the room temperature forming process. Studies have shown that adequate response annealing can eliminate residual stresses while maintaining the high strength properties of the material. In this paper, the design of annealing temperatures 250, 300 ℃, annealing time of 0.5, 1h combination of tests. Heat treatment furnace selection VAF446 horizontal vacuum annealing furnace, heat treatment with the furnace cooling.

Figure.1 Equal tee cold pushed into shape schematic diagram

B10 copper-nickel alloy thick-walled equal tee stress test equipment using XStress3000 G3 X-ray stress meter. The stress test was performed by GB/T 7704-2017 “Nondestructive Testing X-ray Stress Determination Method”. Based on the sin²Ψ method, a dual-solid bit-sensitive detector was used by the radiation MnKα line, diffracted crystalline Cu(311) crystalline surface, accelerating voltage 30kV, accelerating current 6.7mA, stress constant K=-198MPa/(°), and exposure time 8s. The distribution of measurement points is shown in Fig. 2, and the same measurement point was tested for axial partial stress (σ_X) and tangential partial stress (σ_Y), respectively. The residual stress values at test points 1, 2, and 3 of the main pipe are selected as the average residual stress of the main pipe, the residual stress values at test points 4 and 5 of the branch pipe are selected as the average residual stress of the branch pipe, the residual stress values at test points 6 and 7 of the transition position between the main pipe and the branch pipe are selected as the average residual stress at the transition position between the main pipe and the branch pipe, and the mean value of the residual stress error at test points of the main pipe, the branch pipe, and the transition position is selected as the average residual stress value of the main pipe, the branch pipe, and the transition position, respectively. Stress error mean value.

Figure.2 B10 copper-nickel alloy equal tee residual stress test sampling site schematic diagram

INSTRON5587 (300kN) material testing machine was used to carry out tensile testing of tee fittings. Tensile sample from the tee main tube to take longitudinal arc specimen by GB/T 228.1-2021 “metal materials tensile test part 1: room temperature test methods” for tensile test. ZEISSObserver.Z1m metallographic microscope was used to detect the microstructure of the tee fittings. Metallographic samples from the tee main pipe and branch pipe transition area to take longitudinal samples by YS/T448-2002 “copper and copper alloy casting and processing products macro-organizational test methods” for metallographic testing. The specific sampling location is shown in Figure 3.

Figure.3 B10 copper-nickel alloy equal tee sampling parts schematic diagram

2. Test results and discussion

2.1 Residual stress on the surface of cold pushed B10 copper-nickel alloy thick-walled tee

Table 2 shows the residual stress levels on the axial and tangential surfaces of cold pushed shaped B10 CuNi alloy equal tee. According to Table 2 axial partial stress, the average axial partial stress at the test position of equal tee main pipe 1,2,3 is (-92±8.3) MPa, the average axial partial stress at the test position of equal tee branch pipe 4,5 is (-49±5) Mpa. The average axial partial stress at the test position of the main pipe and branch pipe transition 6,7 is (68±12) MPa. According to the tangential partial stress in Table 2, the residual stress level of the axial and tangential surfaces of the equal tee is (-10±2) MPa. According to Table 2, the average tangential partial stress in the test position of main pipes 1, 2, and 3 is (-36±9.3) MPa, the average tangential partial stress in the test position of equal tee branch pipes 4 and 5 is (-101±6.5) Mpa. The average tangential partial stress in the test position of the main pipe and branch pipe transition 6 and 7 is (40.5±8.5) MPa. Overall, the axial partial stress in the test position of the main pipe of the equal tee is greater than the tangential partial stress, and the axial partial stress in the test position of the branch pipe is less than the tangential partial stress. Overall, the axial partial stress is greater than the tangential partial stress in the main pipe, the axial partial stress is greater than the tangential partial stress in the branch pipe, and the axial partial stress is greater than the tangential partial stress in the transition position between the main pipe and the branch pipe.
To more intuitively analyze the trend of the residual stress on the tee surface after cold pushing into shape. According to the measured data in Table 2, the residual stress on the axial and tangential surface of the tee is plotted. Fig. 4 shows the residual stress on the axial and tangential surface of the cold pushed tee. By comparing the axial stress on the surface of cold pushed tee at different positions, it can be seen that the residual compressive stress at test points 1 and 3 of the main pipe is greater than the residual stress at 2 points, and the residual stress at the main pipe test is symmetrically distributed as a whole. The residual stresses at test points 4 and 5 of the branch pipe are close, and the residual stresses at the transition position between the main pipe and the branch pipe have a large deviation. Analyze the tee cold push forming process can be seen the tee main tube by both sides of the punch compressive stress; the main two ends of the force are greater than the middle part, so there are two ends of the test point of the residual stress is large, the middle of the test point of the phenomenon of residual stress is small; branch pipe metal flow along the axial flow uniformity, surface strain is close to the residual stress is close to the transition position due to the complexity of the flow of the metal, deformation is uneven, the residual stress of the deviation. Deviation. At the same time, the cold pushed the state of the B10 copper-nickel alloy equal tee surface main and branch pipe test points in the same stress level, between [(-101 – -120) ± 7] MPa, indicating the existence of obvious residual compressive stress. The main pipe and branch pipe transition position the stress between ((24-120) ± 12) MPa, indicating the existence of significant residual tensile stress.

Table.2 Axial and tangential residual partial stresses on the surface of B10 copper-nickel alloy equal tee in the cold pushed state.

Test points	X-direction  (axial)				Y direction  (tangential)
	Stress /MPa	Error /MPa	Mean stress /MPa	Mean error /MPa	Stress /MPa	Error /MPa	Mean stress /MPa	Mean error /MPa
1	-101	7	-92	8.3	-48	12	-36	9.3
2	-54	11			-8	6
3	-120	7			-52	10
4	-48	4	-49	5	-83	7	-101	6.5
5	-50	6			-119	6
6	112	12	68	12	23	9	40.5	8.5
7	24	12			58	8

Note: See Figure 2 for test locations

From Fig. 4, the tangential partial stress on the surface of B10 CuNi alloy equal tee in the cold pushed state; it can be seen that the residual stress at test points of main pipes 1 and 3 is larger than that at points 2 and the residual stress at main pipe test is symmetrically distributed as a whole. The residual stresses at test points 4 and 5 of the branch pipe and the transition position are close. Analyze the tee cold push forming process can be seen, tee main by both sides of the punch compressive stress, the main two ends of the force is greater than the middle part, so there are two ends of the test point residual stress is large, the middle of the phenomenon of the test point residual stress is small; branch and transition position of the metal along the tangential flow of uniformity, surface strain is close to, and thus the residual stress is close to.

Cold pushed state of B10 copper-nickel alloy equal tee surface of the main pipe and branch pipe test points are at the same stress level; there is obvious residual compressive stress. There is obvious residual tensile stress between the main and branch pipes at the transition position. Overall, the axial stress at the main pipe location is greater than the tangential stress, and the branch pipe location shows the opposite result, with the transition location being closer. During the forming process of the thick-walled equal tee, the pipe fittings are compressed under the axial pressure of the left and right punches, and the main pipe is subjected to compressive stress, and obvious plastic flow occurs. Under the hydraulic expansion, the pipe fitting along the inner wall of the mold up the full cavity, the formation of the tee branch, and the tee branch are also subject to compressive stress. Due to the deformation process occurring uneven stress-strain, pipe fittings may accumulate many crystal defects on the surface, resulting in a large residual stress at the measurement point, axial maximum of up to -120MPa. The processing and molding process determines the equal tee main and branch pipe surface by compressive stress; the main pipe and the branch pipe transition part of the plastic flow of the metal direction changes, and the surface presents a tensile stress state. Eventually, the residual stress is stored as elastic deformation energy in the B10 copper-nickel alloy equal tee. The whole tee presents compressive stress on the surface of the main pipe and branch pipe and tensile stress on the transition position.

Figure.4 Axial and tangential partial stress on the surface of B10 CuNi alloy equal tee in the cold pushed the state

2.2 Effect of stress relief annealing on surface residual stress level

Fig. 5(a) shows the distribution of axial frictional stresses on the surface of B10 copper-nickel alloy equal tee in different heat treatment states. The residual stress values of test points 1, 2, and 3 of the main pipe are selected as the average residual stress of the main pipe, and the residual stress values of test points 4 and 5 of the branch pipe are selected as the average residual stress of the branch pipe, the residual stress values of test points 6 and 7 of the transition position between the main pipe and the branch pipe are selected as the average residual stress of the transition position between the main pipe and the branch pipe, and the mean value of residual stress error of the test points of the main pipe, the branch pipe and the transition position are selected as the mean value of residual stress error of the main pipe, the branch pipe, and the transition position, respectively. B10 copper-nickel alloy equal tee cold into the form of the main surface of the axial average partial stress is (-92 ± 8) MPa, the branch surface axial average partial stress is (-49 ± 5) MPa, the transition region axial average partial stress is (68 ± 12) MPa. 250 ℃ × 0.5h of heat treatment, the main average residual stress is reduced to (-60 ± 5.6) MPa, the maximum stress reduction rate reaches 34.78%; the axial partial stress on the surface of the transition position is reduced to (59.5±7) MPa, the stress is reduced. After the heat treatment at 300℃ × 0.5h, the average residual stress of the main tube was reduced to (-26.3±6.3)MPa, with the maximum stress reduction rate of 71.41%, and the axial stress on the surface at the transition position was reduced to (2.5±4)MPa, with the stress reduction rate of 96.32%. In the same annealing holding time, the annealing temperature increases, and the residual stress is reduced. This is because, in the case of the same holding time, when the relaxation activation energy of the stresses under the temperature condition is completely relaxed and eliminated, it is necessary to increase the heat treatment temperature to enable the relaxation behavior of the stresses with higher relaxation activation energy to continue.

After heat treatment at 250℃ × 1h, compared with the cold pushed state, the axial residual stress of the main pipe is reduced to (-34±8.3)MPa, with a stress reduction rate of 63.04%; the axial stress on the surface of the branch pipe is in the range of (-28±5) MPa, with a stress reduction rate of 43.75%; and the axial stress on the surface of the transition position is reduced to (19.5±4.5) MPa, with a stress reduction rate of 71.32%. The stress reduction rate reaches 71.32%. After de-stressing annealing treatment, the residual stress on the surface of the tee in the axial direction has the same trend of change; with the increase in annealing temperature and the extension of the holding time, there is a relatively large decrease. This is because the metal material, after heating and holding time treatment, high residual stress will occur in the place of microscopic or local plastic deformation so that the stress is slowly relaxed to reduce the residual stress.

Figure.5 Stress relief annealed B10 copper-nickel alloy equal tee surface axial (a) and tangential (b) partial stresses
Fig.5(b) shows the distribution of tangential partial stress on the surface of B10 CuNi alloy equal tee in different heat treatment states. The average partial stress on the surface of the main pipe of B10 CuNi alloy equal tee in the cold-formed form is (-36±9.3) MPa, that on the surface of the branch pipe is (-101±6.5) MPa, and that on the surface of the transition area is (40.5±8.5) MPa. The main pipe and transition position tangential average partial stress is lower than the axial direction, and the branch pipe tangential average partial stress is higher than the axial direction. After 250°C × 0.5h stress relief annealing treatment, the surface tangential residual stress at the main pipe position increases, and the surface residual stress at the remaining positions decreases. When the annealing holding time is extended to 1h, the main and branch tangential average partial stress decreases to the cold forming state, and the branch transition position residual stress decreases significantly. Increase the temperature to 300 ℃ insulation 1h, tee main, and branch position residual stress is eliminated, and the transition position residual stress elimination is obvious.
After the de-stressing annealing treatment, the overall trend of the residual stress on the surface of the tee fittings in the axial and tangential direction is the same, with the increase in the annealing temperature and the extension of the holding time having a relatively large decrease. As mentioned above, after the insulation treatment of metal materials, the high residual stress location will produce a stress relaxation phenomenon through plastic deformation to eliminate the residual stress. Li Wentao et al. In the study of the effect of annealing temperature on the residual stress of cold rolled strip steel, with the help of microhardness reflecting changes in dislocation density and resistivity reflecting changes in the concentration of point defects, as well as observation of the microstructure state of the transmission electron microscope, which together revealed that the stress relaxation of the low-temperature recovery process is mainly the disappearance of excessive vacancies produced by plastic deformation, which makes the density of point defects decrease.

2.3 Effect of stress relief annealing on properties and organization of thick-walled tee

Figure 6 shows the room temperature mechanical properties of B10 copper-nickel alloy thick-walled equal tee after stress relief annealing. The annealed mechanical properties show that stress relief annealing has little effect on the strength index. Tee cold forming, tensile strength 469MPa, yield strength 341.5MPa, elongation 13.5%. When stress relieved annealed 0.5h, the strength of different temperature stress relieved annealed specimens with the cold forming state without stress relieved heat treatment compared with no significant change; the annealing time increased from 0.5h to 1h, which showed a similar pattern. In terms of material plasticity, stress relief annealing had little effect on the specimens’ elongation, and the material’s overall elongation was less than 15%. This is because of the selection of temperature range belonging to the recovery stage; copper, due to the stacking layer error energy is low, not easy to polygonization and other processes, basically does not occur softening in this recovery stage.

Fig.6 Effect of stress relief annealing on the mechanical properties of B10 Cu-Ni alloy equal tee

Figure 7 shows the microstructure of the longitudinal section of ϕ90mm × 6mm B10 copper-nickel alloy equal tee after the stress relief annealing process. As can be seen from Figure 7, the overall organization of the B10 copper-nickel alloy equal tee is distributed along the direction of the arrow in the figure after the tee is cold pushed into shape. Combined with the tee cold push forming process, it is known that the transition position of the main pipe and branch pipe along the die cavity metal plastic flow direction changes, and microstructure appears in a certain direction. After 250 ℃ × 0.5h low-temperature stress relief annealing, microstructure and cold push forming after the organization is similar, the grain along the Figure 7 (b) shows the direction of the arrow elongation distribution; this is due to the process of processing the organization does not appear to revert to the lattice distortion still retained after the cold push forming of the organization; by 250 ℃ × 1h low-temperature stress relief annealing, in addition to retaining the direction of the arrow elongation distribution of the grain, deformation of grains near the emergence of small Grain. After 300 ℃ stress relief annealing 0.5, 1h, the elongated organization gradually returns; the organization still shows a deformation state, mainly due to further increase in temperature, the original blocked dislocation slip, dislocation density is greatly reduced due to the deformation produced by the lattice distortion and the disappearance of residual stress.

Figure.7 Microstructure of B10 CuNi alloy equal tee after stress relieving annealing by different processes.
(a) initial state; (b) 250 ℃ × 0.5h; (c) 250 ℃ × 1h; (d) 300 ℃ × 0.5h; (e) 300 ℃ × 1h

3. Conclusion

• 1) ϕ90mm × 6mm B10 copper-nickel alloy equal tee cold pushed after the main pipe, branch pipe surface residual stress for compressive stress, and the main pipe and branch pipe transition position surface residual stress for tensile stress. The axial residual stress on the surface of the main pipe is greater than the tangential residual stress.
• 2) ϕ90mm × 6mm B10 copper-nickel alloy equal tee in the annealing temperature of 250, 300 ℃, annealing time of 0.5, 1h combination of annealing test, in the premise of not harming the mechanical properties of the B10 copper-nickel alloy equal tee surface residual stress elimination is obvious. With the increase of annealing temperature and the extension of annealing holding time, the residual stress on the surface of the pipe fittings showed a significant decline.
• 3) Within the scope of this test, ϕ90mm × 6mm B10 copper-nickel alloy equal tee is annealed at 300℃ in a vacuum environment for 1h with the cooling of the furnace, and the residual stresses in the axial and tangential directions are eliminated.

Author: Wang Junliang