Research of the application of nickel-chrome-silicon bronze BrNHK for the manufacture of helical compression springs

The results of a study of the mechanical characteristics of the BrNHK bronze alloy using the acoustic emission method are presented. The mechanical characteristics of BrNHK-2.5-0.6-0.7 wire were studied after various heat treatment modes. The widespread use of bronze alloy BrKMts3-1 is due to its low price, while the mechanical and operational properties satisfy both consumers and spring manufacturers, since the technology for manufacturing springs from this alloy is not fundamentally different from the technology for manufacturing springs from cold-worked wire. In turn, the BrB2 alloy is considered a scarce alloy and is used only for springs intended for special equipment, since the properties of this alloy after hardening heat treatment are not inferior to cold-worked carbon steels. Given the increased cost of beryllium, work is underway both to reduce the beryllium content and to switch to beryllium-free alloys.


Introduction
Today, one of the main problems faced by spring manufacturers is the deterioration of the quality of the source material.One of the factors influencing the quality of the finished product (spring) is the quality of the source material, especially for springs operating under long-term cyclic loads.Defects on the surface of the material of the finished spring are stress concentrators and during operation contribute to the development of cracks, which leads to the destruction of the spring (figure 1) and failure of the entire mechanism.
The main operational properties of helical coil springs are their resistance to stress relaxation and creep during operation.As a rule, relaxation resistance and creep depend on the selected material and the maximum permissible tangential stresses.Creep is one of the main properties of materials; its rate largely depends on the operating temperature.Creep of metals is a process caused by the movement of crystal dislocations [20].Under prolonged load in the cross section of the spring coil, creep appears under the influence of a constant torque and over time, the distribution of tangential stresses in the cross section of the spring coil becomes nonlinear.The rate of decrease in the shear stress in the spring cross-section is faster in the edge area than in the center.Because of this, the stress at the center increases to ensure a constant compressive force.When the spring is unloaded due to the nonuniformity of the creep rate, the tangential stresses are redistributed and residual stresses appear in the cross section of the spring coil.It can be seen that increased residual stresses occur near the center.
Residual stresses decrease and are redistributed with increasing relaxation time.When the spring is kept under load for a long time, the residual stresses can stabilize and turn into plastic deformation.Thus, we observe the manifestation of residual stresses when the spring is unloaded, when its height in the free state is different from the initial height of the spring [1].Springs are important parts of many technical devices operated at high temperatures.The greater the value of the total deformation of the spring S3, the higher the rate of creep deformation in the cross section of the spring coil.When the spring is unloaded, residual stresses arise in the cross section of the coil, which affect the further operation of the spring.Today, the study of residual stresses in a spring at room temperature refers to the technology of captivity [2,3].The use of confinement allows you to avoid destruction or damage to the springs due to the high shear stress that occurs in the transverse coil during loading.But the study of residual stresses as a result of creep at elevated temperatures is not sufficiently covered in the literature.Of practical importance is the analysis of residual stresses in the cross section of a coil of a cylindrical helical spring operating at high temperatures.The magnitude of residual stresses significantly affects the stability of the springs when operating under variable loads.Relaxation of force in a compressed spring is a negative factor during the operation of the fuel assembly of a power reactor, which is caused by the development of irreversible creep deformations in the spring material under the influence of high temperature.The relaxation resistance of a spring is assessed mainly by the rate of force drop in a compressed spring, which is usually represented by relaxation curves.The main defects encountered during the incoming inspection of materials are deep scratches, mechanical damage, small and large ulcers, foreign inclusions, rolled scale, cracks and many others (figure 2).

Materials and research methods
At the incoming inspection, a test for technological winding is provided, which allows you to evaluate the quality of the material without resorting to additional testing.If the sample is destroyed during testing, the entire coil is rejected.Such cases are rare and most often occur when testing bronze alloys used in spring production, such as BrKMts3-1 and BrOTs4-3 (figure 3), steel wire most often has surface defects.As an exception, surface defects can be smoothed out or removed using additional surface treatment (sandblasting or shot blasting, tumbling), which will increase the quality and reliability of the finished springs (figure 4).The use of bronze alloys [1][2][3][4][5][6][7][8][9][10] in spring production is not large and amounts to no more than 500 kg per year, steel wire (rods) is many times more, but, as a rule, steel wire can be replaced with imported one, but with bronze alloys there is no such possibility.At the incoming inspection, out of 10 deliveries of bronze wire, about 5-7 deliveries are rejected, most often these are small sections from 0.5 to 3 mm and often due to cracks opening after the coiling test, on wire with a cross-section of 5 mm or more after the coiling test surface cracking and scuffing appears.One of the disadvantages of the incoming inspection of the material is that the wire is accepted according to a sample cut from one end of the coil; there are cases when the received wire coil has internal defects that cannot be detected in any way during the incoming inspection, but they appear during the manufacture of springs (figure 5).Presumably the appearance of defects is caused by the wire manufacturing technology.As a rule, bronze wire must be supplied in a solid state (work-hardened) with a tensile strength of at least 850-800 MPa and a ductility of at least 2%.When conducting a tensile test, the tensile strength (σв) is more than 1000 MPa and the relative elongation (δ) is not more than 1%.Therefore, it can be assumed that the material lacks ductility during the coiling test.An urgent task is to find a more technologically advanced material for the manufacture of springs from bronze alloys used in domestic production, for example, BrNHK bronze alloy [1][2][3].

Results and discussion
One of the problems solved in the practice of producing springs is the choice of heat treatment mode in order to obtain optimal mechanical properties, since the material arrives in an unstrengthened state and has low mechanical properties.To test the optimal heat treatment mode, manufactured samples of 250 mm wire, cut from a coil, were subjected to experimental heat treatment modes followed by tensile testing; the research results are listed in table 1.Heat treatment modes are based on literature data applied to the heat treatment of BrNHK sheet material.After the heat treatment regime, scale formed on the surface of the samples, which affects the aesthetic appearance, but it turned out that after heat treatment two additional operations can be applied -blowing and subsequent electropolishing (figure 6).6. Springs made of bronze alloy BrNHK: aafter aging, bafter aging + blowing + electropolishing.To visually represent the effect of holding time during aging of the BrNHK bronze alloy [6][7][8][9] on the mechanical properties, diagrams were constructed in the coordinates "Stressholding time" presented in figure 7.
Analysis of the experimental results showed that at a temperature of 380 °C and 480 °C and a holding time of 1.5 hours, the minimum strength, yield strength and elongation are obtained.Increasing the holding time to 2.5 hours increases the level of the yield strength to the maximum recorded result of 848 MPa at a temperature of 380 °C, and for a temperature of 480 °C a decrease in mechanical properties is observed, as well as for a temperature of 465 °C.Thus, the optimal temperature for aging can be considered 440 °C, and the holding time is 2.5 hours, since during this holding the maximum yield strength value is 835 MPa and the yield strength value is σ0.05 = 823 MPa, while the ultimate strength is 856 MPa, although with exposure for 3.0 hours and a temperature of 440 °C, the maximum value of the tensile strength was recorded equal to 866 MPa.It should be noted that in all aging modes the relative elongation is more than 2%.
The temperature and duration of aging are selected experimentally in each individual case, taking into account the required properties of a particular semi-finished product or product.Depending on the regime, structural changes and the resulting set of properties, aging is divided into: complete, incomplete and stabilizing.When deciding which aging mode -incomplete or complete aging -to give preference to, one should be guided by those properties that are decisive in the operation of the product.This may be increased impact strength, satisfactory ability to shape after aging, stability in operation, increased electrical and thermal conductivity, deformation during aging, high corrosion resistance, etc. Incomplete aging -leads to greater ductility at a given level of strength.This is due to the faster increase in tensile strength during aging compared to the yield strength.The holding time for incomplete aging is relatively short, and in some cases requires careful monitoring.Complete aging makes it easier to control during long exposures, since the properties of the semi-finished product do not depend greatly on time.The greater completeness of the release of the strengthening phase determines a higher level of electrical conductivity γ, a minimum level of residual stresses, a relatively high elastic modulus and increased ductility.In most cases, heat treatment for more than 2 hours is not economically profitable, and less than 10 minutes does not allow for effective control.Stepped or double aging can be carried out according to two schemes: 1. Incomplete aging followed by re-aging.This process makes it easy to control ductility, strength and hardness, and consists of long-term heating below 320˚C or short-term heating above 320˚C, followed by short aging above the initial temperature; 2. Aging for maximum strength at high temperature followed by long-term aging (8-24 hours).With this treatment, a slightly larger amount of the strengthening γ phase is released, resulting in an increase in strength, hardness, and electrical conductivity.
The cooling rate after aging is not limited.Residual stresses after aging can be reduced by lowtemperature tempering without changing strength and hardness.The product is heated to 150-200˚C for 15-30 minutes.This treatment is often used to relieve stresses arising during the formation of the spring geometry and thus stabilize the shape and size of the product.

Conclusions
Based on the results of the study, it was concluded that the optimal heat treatment mode is aging at a temperature of 440 °C and holding for 2.5 hours.Since the plasticity of the material is preserved in this mode, this mode can be recommended to the manufacturer as a final treatment, but at the same time, the wire must be cleaned of scale using blowing followed by electropolishing.Further research will be aimed at studying the microstructure of the BrNHK bronze alloy and conducting operational tests on manufactured springs.

Figure 1 .
Figure 1.Spring destruction during cyclic tests due to the presence of a defect on the surface of the material.

Figure 2 .
Figure 2. Surface defects in the material.

Figure 3 .Figure 4 .
Figure 3. Defects on the surface of the material after the technological winding test.

Figure 5 .
Figure 5. Defects in the bronze alloy BrkMts3-1 that appeared during the manufacturing process of springs.

Table 1 .
Mechanical properties of bronze alloy BrNHK after experimental heat treatment conditions.