310 Stainless steel coil tube chemical component ,Effect of Surface Defects in Oil-Hardened Steel Wire on the Fatigue Life of Valve Springs in Automotive Engines

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Stainless steel 310 coiled tubes /coiled tubing Chemical Composition and composition

The following table shows the chemical composition of grade 310S stainless steel.

10*1mm 9.25*1.24 mm 310 Stainless steel capillary coiled tube suppliers 

Physical Properties

The physical properties of grade 310S stainless steel are displayed in the following table.

Mechanical Properties

The following table outlines the mechanical properties of grade 310S stainless steel.

Thermal Properties

The thermal properties of grade 310S stainless steel are given in the following table.

Other Designations

Other designations equivalent to grade 310S stainless steel are listed in the following table.

The purpose of this study is to evaluate the fatigue life of a valve spring of an automobile engine when applying microdefects to an oil-hardened wire of 2300 MPa grade (OT wire) with a critical defect depth of 2.5 mm in diameter. First, the deformation of the surface defects of the OT wire during the manufacture of the valve spring was obtained by finite element analysis using subsimulation methods, and the residual stress of the finished spring was measured and applied to the spring stress analysis model. Second, analyze the strength of the valve spring, check for residual stress, and compare the level of applied stress with surface imperfections. Third, the effect of microdefects on the fatigue life of the spring was evaluated by applying the stress on surface defects obtained from the spring strength analysis to the SN curves obtained from the flexural fatigue test during rotation of the wire OT. A defect depth of 40 µm is the current standard for managing surface defects without compromising fatigue life.
The automotive industry has a strong demand for lightweight automotive components to improve the fuel efficiency of vehicles. Thus, the use of advanced high strength steel (AHSS) has been increasing in recent years. Automotive engine valve springs mainly consist of heat-resistant, wear-resistant and non-sagging oil-hardened steel wires (OT wires).
Due to their high tensile strength (1900–2100 MPa), the currently used OT wires make it possible to reduce the size and mass of engine valve springs, improve fuel efficiency by reducing friction with surrounding parts1. Due to these advantages, the use of high-voltage wire rod is rapidly increasing, and ultra-high-strength wire rod of 2300MPa class appears one after another. Valve springs in automotive engines require a long service life because they operate under high cyclic loads. To meet this requirement, manufacturers typically consider fatigue life greater than 5.5×107 cycles when designing valve springs and apply residual stress to the valve spring surface through shot peening and heat shrink processes to improve fatigue life2.
There have been quite a few studies on the fatigue life of helical springs in vehicles under normal operating conditions. Gzal et al. Analytical, experimental and finite element (FE) analyzes of elliptical helical springs with small helix angles under static load are presented. This study provides an explicit and simple expression for the location of maximum shear stress versus aspect ratio and stiffness index, and also provides analytical insight into maximum shear stress, a critical parameter in practical designs3. Pastorcic et al. The results of the analysis of the destruction and fatigue of a helical spring removed from a private car after failure in operation are described. Using experimental methods, a broken spring was examined and the results suggest that this is an example of corrosion fatigue failure4. hole, etc. Several linear regression spring life models have been developed to evaluate the fatigue life of automotive helical springs. Putra and others. Because of the unevenness of the road surface, the service life of the helical spring of the car is determined. However, little research has been done on how surface defects that occur during the manufacturing process affect the life of automotive coil springs.
Surface defects that occur during the manufacturing process can lead to local stress concentration in valve springs, which significantly reduces their fatigue life. Surface defects of valve springs are caused by various factors, such as surface defects of the raw materials used, defects in tools, rough handling during cold rolling7. The surface defects of the raw material are steeply V-shaped due to hot rolling and multi-pass drawing, while the defects caused by the forming tool and careless handling are U-shaped with gentle slopes8,9,10,11. V-shaped defects cause higher stress concentrations than U-shaped defects, so stringent defect management criteria are usually applied to the starting material.
Current surface defect management standards for OT wires include ASTM A877/A877M-10, DIN EN 10270-2, JIS G 3561, and KS D 3580. DIN EN 10270-2 specifies that the depth of a surface defect on wire diameters of 0.5–10 mm is less than 0.5–1% of the wire diameter. In addition, JIS G 3561 and KS D 3580 require that the depth of surface defects in wire rod with a diameter of 0.5–8 mm be less than 0.5% of the wire diameter. In ASTM A877/A877M-10, the manufacturer and purchaser must agree on the allowable depth of surface defects. To measure the depth of a defect on the surface of a wire, the wire is usually etched with hydrochloric acid, and then the depth of the defect is measured using a micrometer. However, this method can only measure defects in certain areas and not on the entire surface of the final product. Therefore, manufacturers use eddy current testing during the wire drawing process to measure surface defects in continuously produced wire; these tests can measure the depth of surface defects down to 40 µm. The 2300MPa grade steel wire under development has higher tensile strength and lower elongation than the existing 1900-2200MPa grade steel wire, so the valve spring fatigue life is considered to be very sensitive to surface defects. Therefore, it is necessary to check the safety of applying existing standards for controlling the depth of surface defects for steel wire grade 1900-2200 MPa to steel wire grade 2300 MPa.
The purpose of this study is to evaluate the fatigue life of an automotive engine valve spring when the minimum flaw depth measurable by eddy current testing (i.e. 40 µm) is applied to a 2300 MPa grade OT wire (diameter: 2.5 mm): critical flaw depth . The contribution and methodology of this study are as follows.
As the initial defect in the OT wire, a V-shaped defect was used, which seriously affects the fatigue life, in the transverse direction relative to the wire axis. Consider the ratio of the dimensions (α) and length (β) of a surface defect to see the effect of its depth (h), width (w), and length (l). Surface defects occur inside the spring, where failure occurs first.
To predict the deformation of initial defects in OT wire during cold winding, a sub-simulation approach was used, which took into account the analysis time and the size of surface defects, since the defects are very small compared to OT wire. global model.
The residual compressive stresses in the spring after two-stage shot peening were calculated by the finite element method, the results were compared with the measurements after shot peening to confirm the analytical model. In addition, residual stresses in valve springs from all manufacturing processes were measured and applied to spring strength analysis.
Stresses in surface defects are predicted by analyzing the strength of the spring, taking into account the deformation of the defect during cold rolling and the residual compressive stress in the finished spring.
The rotational bending fatigue test was carried out using an OT wire made from the same material as the valve spring. In order to correlate the residual stress and surface roughness characteristics of the fabricated valve springs to the OT lines, SN curves were obtained by rotating bending fatigue tests after applying two-stage shot peening and torsion as pretreatment processes.
The results of the spring strength analysis are applied to the Goodman equation and the SN curve to predict valve spring fatigue life, and the effect of surface defect depth on fatigue life is also evaluated.
In this study, a 2300 MPa OT grade wire with a diameter of 2.5 mm was used to evaluate the fatigue life of an automotive engine valve spring. First, a tensile test of the wire was carried out to obtain its ductile fracture model.
The mechanical properties of OT wire were obtained from tensile tests prior to finite element analysis of the cold winding process and spring strength. The stress-strain curve of the material was determined using the results of tensile tests at a strain rate of 0.001 s-1, as shown in fig. 1. SWONB-V wire is used, and its yield strength, tensile strength, elastic modulus and Poisson’s ratio are 2001.2MPa, 2316MPa, 206GPa and 0.3 respectively. The dependence of stress on flow strain is obtained as follows:
Rice. 2 illustrates the ductile fracture process. The material undergoes elastoplastic deformation during deformation, and the material narrows when the stress in the material reaches its tensile strength. Subsequently, the creation, growth and association of voids within the material lead to the destruction of the material.
The ductile fracture model uses a stress-modified critical deformation model that takes into account the effect of stress, and post-necking fracture uses the damage accumulation method. Here, damage initiation is expressed as a function of strain, stress triaxiality, and strain rate. The stress triaxiality is defined as the average value obtained by dividing the hydrostatic stress caused by the deformation of the material up to the formation of the neck by the effective stress. In the damage accumulation method, destruction occurs when the damage value reaches 1, and the energy required to reach the damage value of 1 is defined as the destruction energy (Gf). The fracture energy corresponds to the region of the true stress-displacement curve of the material from necking to fracture time.
In the case of conventional steels, depending on the stress mode, ductile fracture, shear fracture, or mixed mode fracture occurs due to ductility and shear fracture, as shown in Figure 3. The fracture strain and stress triaxiality showed different values ​​for the fracture pattern.
Plastic failure occurs in a region corresponding to a stress triaxiality of more than 1/3 (zone I), and the fracture strain and stress triaxiality can be deduced from tensile tests on specimens with surface defects and notches. In the area corresponding to the stress triaxiality of 0 ~ 1/3 (zone II), a combination of ductile fracture and shear failure occurs (i.e. through a torsion test. In the area corresponding to the stress triaxiality from -1/3 to 0 (III), shear failure caused by compression, and fracture strain and stress triaxiality can be obtained by upsetting test.
For OT wires used in the manufacture of engine valve springs, it is necessary to take into account the fractures caused by various loading conditions during the manufacturing process and application conditions. Therefore, tensile and torsion tests were carried out to apply the failure strain criterion, the effect of stress triaxiality on each stress mode was considered, and elastoplastic finite element analysis at large strains was performed to quantify the change in stress triaxiality. The compression mode was not considered due to the limitation of sample processing, namely, the diameter of the OT wire is only 2.5 mm. Table 1 lists the test conditions for tensile and torsion, as well as stress triaxiality and fracture strain, obtained using finite element analysis.
The fracture strain of conventional triaxial steels under stress can be predicted using the following equation.
where C1: \({\overline{{\varepsilon}_{0}}}^{pl}\) clean cut (η = 0) and C2: \({\overline{{\varepsilon}_{0} }}^{pl}\) Uniaxial tension (η = η0 = 1/3).
The trend lines for each stress mode are obtained by applying the fracture strain values ​​C1 and C2 in the equation. (2); C1 and C2 are obtained from tensile and torsion tests on samples without surface defects. Figure 4 shows the stress triaxiality and fracture strain obtained from the tests and the trend lines predicted by the equation. (2) The trend line obtained from the test and the relationship between stress triaxiality and fracture strain show a similar trend. The fracture strain and stress triaxiality for each stress mode, obtained from the application of trend lines, were used as criteria for ductile fracture.
Break energy is used as a material property to determine the time to break after necking and can be obtained from tensile tests. The fracture energy depends on the presence or absence of cracks on the surface of the material, since the time to fracture depends on the concentration of local stresses. Figures 5a-c show the fracture energies of samples without surface defects and samples with R0.4 or R0.8 notches from tensile tests and finite element analysis. The fracture energy corresponds to the area of ​​the true stress-displacement curve from necking to fracture time.
The fracture energy of an OT wire with fine surface defects was predicted by performing tensile tests on an OT wire with a defect depth greater than 40 µm, as shown in Fig. 5d. Ten specimens with defects were used in the tensile tests and the average fracture energy was estimated at 29.12 mJ/mm2.
The standardized surface defect is defined as the ratio of the depth of the defect to the diameter of the valve spring wire, regardless of the surface defect geometry of the OT wire used in the manufacture of automotive valve springs. OT wire defects can be classified based on orientation, geometry, and length. Even with the same defect depth, the level of stress acting on a surface defect in a spring varies depending on the geometry and orientation of the defect, so the geometry and orientation of the defect can affect fatigue strength. Therefore, it is necessary to take into account the geometry and orientation of defects that have the greatest impact on the fatigue life of a spring in order to apply stringent criteria for managing surface defects. Due to the fine grain structure of OT wire, its fatigue life is very sensitive to notching. Therefore, the defect that exhibits the highest stress concentration according to the geometry and orientation of the defect should be established as the initial defect using finite element analysis. On fig. 6 shows the ultra-high strength 2300 MPa class automotive valve springs used in this study.
Surface defects of OT wire are divided into internal defects and external defects according to the spring axis. Due to the bending during cold rolling, compressive stress and tensile stress act on the inside and outside of the spring, respectively. Fracture can be caused by surface defects that appear from the outside due to tensile stresses during cold rolling.
In practice, the spring is subjected to periodic compression and relaxation. During the compression of the spring, the steel wire twists, and due to the concentration of stresses, the shear stress inside the spring is higher than the surrounding shear stress7. Therefore, if there are surface defects inside the spring, the probability of the spring breaking is the greatest. Thus, the outer side of the spring (the location where failure is expected during the manufacture of the spring) and the inner side (where the stress is greatest in the actual application) are set as the locations of the surface defects.
The surface defect geometry of OT lines is divided into U-shape, V-shape, Y-shape, and T-shape. Y-type and T-type mainly exist in the surface defects of raw materials, and U-type and V-type defects occur due to careless handling of tools in the cold rolling process. With regard to the geometry of surface defects in raw materials, U-shaped defects arising from non-uniform plastic deformation during hot rolling are deformed into V-shaped, Y-shaped and T-shaped seam defects under multi-pass stretching8, 10.
In addition, V-shaped, Y-shaped and T-shaped defects with steep inclinations of the notch on the surface will be subjected to high stress concentration during the operation of the spring. Valve springs bend during cold rolling and twist during operation. Stress concentrations of V-shaped and Y-shaped defects with higher stress concentrations were compared using finite element analysis, ABAQUS – commercial finite element analysis software. The stress-strain relationship is shown in Figure 1 and Equation 1. (1) This simulation uses a two-dimensional (2D) rectangular four-node element, and the minimum element side length is 0.01 mm. For the analytical model, V-shaped and Y-shaped defects with a depth of 0.5 mm and a slope of the defect of 2° were applied to a 2D model of a wire with a diameter of 2.5 mm and a length of 7.5 mm.
On fig. 7a shows the bending stress concentration at the tip of each defect when a bending moment of 1500 Nmm is applied to both ends of each wire. The results of the analysis show that the maximum stresses of 1038.7 and 1025.8 MPa occur at the tops of V-shaped and Y-shaped defects, respectively. On fig. 7b shows the stress concentration at the top of each defect caused by torsion. When the left side is constrained and a torque of 1500 N∙mm is applied to the right side, the same maximum stress of 1099 MPa occurs at the tips of the V-shaped and Y-shaped defects. These results show that V-type defects exhibit higher bending stress than Y-type defects when they have the same depth and slope of the defect, but they experience the same torsional stress. Therefore, V-shaped and Y-shaped surface defects with the same depth and slope of the defect can be normalized to V-shaped ones with a higher maximum stress caused by stress concentration. The V-type defect size ratio is defined as α = w/h using the depth (h) and width (w) of the V-type and T-type defects; thus, a T-type defect (α ≈ 0) instead, the geometry can be defined by the geometric structure of a V-type defect. Therefore, Y-type and T-type defects can be normalized by V-type defects. Using depth (h) and length (l), the length ratio is otherwise defined as β = l/h.
As shown in Figure 811, the directions of surface defects of OT wires are divided into longitudinal, transverse and oblique directions, as shown in Figure 811. Analysis of the influence of the orientation of surface defects on the strength of the spring by the finite element method.
On fig. 9a shows the engine valve spring stress analysis model. As an analysis condition, the spring was compressed from a free height of 50.5 mm to a hard height of 21.8 mm, a maximum stress of 1086 MPa was generated inside the spring, as shown in Fig. 9b. Since the failure of actual engine valve springs mainly occurs within the spring, the presence of internal surface defects is expected to seriously affect the fatigue life of the spring. Therefore, surface defects in the longitudinal, transverse and oblique directions are applied to the inside of engine valve springs using sub-modeling techniques. Table 2 shows the dimensions of surface defects and the maximum stress in each direction of the defect at maximum spring compression. The highest stresses were observed in the transverse direction, and the ratio of stresses in the longitudinal and oblique directions to the transverse direction was estimated as 0.934–0.996. The stress ratio can be determined by simply dividing this value by the maximum transverse stress. The maximum stress in the spring occurs at the top of each surface defect, as shown in Fig. 9s. The stress values ​​observed in the longitudinal, transverse, and oblique directions are 2045, 2085, and 2049 MPa, respectively. The results of these analyzes show that transverse surface defects have the most direct effect on the fatigue life of engine valve springs.
A V-shaped defect, which is assumed to most directly affect the fatigue life of the engine valve spring, was chosen as the initial defect of the OT wire, and the transverse direction was chosen as the direction of the defect. This defect occurs not only outside, where the engine valve spring broke during manufacture, but also inside, where the greatest stress occurs due to stress concentration during operation. The maximum flaw depth is set to 40 µm, which can be detected by eddy current flaw detection, and the minimum depth is set to a depth corresponding to 0.1% of the 2.5 mm wire diameter. Therefore, the depth of the defect is from 2.5 to 40 µm. Depth, length, and width of flaws with a length ratio of 0.1~1 and a length ratio of 5~15 were used as variables, and their effect on the fatigue strength of the spring was evaluated. Table 3 lists the analytical conditions determined using the response surface methodology.
Automotive engine valve springs are manufactured by cold winding, tempering, shot blasting and heat setting of OT wire. Changes in surface defects during spring fabrication must be taken into account to evaluate the effect of initial surface defects in OT wires on the fatigue life of engine valve springs. Therefore, in this section, finite element analysis is used to predict the deformation of OT wire surface defects during the manufacture of each spring.
On fig. 10 shows the cold winding process. During this process, the OT wire is fed into the wire guide by the feed roller. The wire guide feeds and supports the wire to prevent bending during the forming process. The wire passing through the wire guide is bent by the first and second rods to form a coil spring with the desired inside diameter. The spring pitch is produced by moving the stepping tool after one revolution.
On fig. 11a shows a finite element model used to evaluate the change in the geometry of surface defects during cold rolling. The forming of the wire is mainly completed by the winding pin. Since the oxide layer on the surface of the wire acts as a lubricant, the friction effect of the feed roller is negligible. Therefore, in the calculation model, the feed roller and the wire guide are simplified as a bushing. The coefficient of friction between the OT wire and the forming tool was set to 0.05. The 2D rigid body plane and fixation conditions are applied to the left end of the line so that it can be fed in the X direction at the same speed as the feed roller (0.6 m/s). On fig. 11b shows the sub-simulation method used to apply small defects to wires. To take into account the size of surface defects, the submodel is applied twice for surface defects with a depth of 20 µm or more and three times for surface defects with a depth of less than 20 µm. Surface defects are applied to areas formed with equal steps. In the overall model of the spring, the length of the straight piece of wire is 100 mm. For the first submodel, apply submodel 1 with a length of 3mm to a longitudinal position of 75mm from the global model. This simulation used a three-dimensional (3D) hexagonal eight-node element. In the global model and submodel 1, the minimum side length of each element is 0.5 and 0.2 mm, respectively. After analysis of sub-model 1, surface defects are applied to sub-model 2, and the length and width of sub-model 2 is 3 times the length of the surface defect to eliminate the influence of the sub-model boundary conditions, in addition, 50% of the length and width is used as the depth of the sub-model. In sub-model 2, the minimum side length of each element is 0.005 mm. Certain surface defects were applied to the finite element analysis as shown in Table 3.
On fig. 12 shows the distribution of stress in surface cracks after cold working of a coil. The general model and submodel 1 show almost the same stresses of 1076 and 1079 MPa in the same place, which confirms the correctness of the submodeling method. Local stress concentrations occur at the boundary edges of the submodel. Apparently, this is due to the boundary conditions of the submodel. Due to stress concentration, sub-model 2 with applied surface defects shows a stress of 2449 MPa at the tip of the defect during cold rolling. As shown in Table 3, the surface defects identified by the response surface method were applied to the inside of the spring. The results of the finite element analysis showed that none of the 13 cases of surface defects failed.
During the winding process in all technological processes, the depth of surface defects inside the spring increased by 0.1–2.62 µm (Fig. 13a), and the width decreased by 1.8–35.79 µm (Fig. 13b), while the length increased by 0.72–34.47 µm (Fig. 13c). Since the transverse V-shaped defect is closed in width by bending during the cold rolling process, it is deformed into a V-shaped defect with a steeper slope than the original defect.
Deformation in Depth, Width and Length of OT Wire Surface Defects in the Manufacturing Process.
Apply surface defects to the outside of the spring and predict the likelihood of breakage during cold rolling using Finite Element Analysis. Under the conditions listed in Table. 3, there is no probability of destruction of defects in the outer surface. In other words, no destruction occurred at the depth of surface defects from 2.5 to 40 µm.
To predict critical surface defects, external fractures during cold rolling were investigated by increasing the defect depth from 40 µm to 5 µm. On fig. 14 shows fractures along surface defects. Fracture occurs under conditions of depth (55 µm), width (2 µm), and length (733 µm). The critical depth of a surface defect outside the spring turned out to be 55 μm.
The shot peening process suppresses crack growth and increases fatigue life by creating a residual compressive stress at a certain depth from the spring surface; however, it induces stress concentration by increasing the surface roughness of the spring, thus reducing the fatigue resistance of the spring. Therefore, secondary shot peening technology is used to produce high strength springs to compensate for the reduction in fatigue life caused by the increase in surface roughness caused by shot peening. Two-stage shot peening can improve surface roughness, maximum compressive residual stress, and surface compressive residual stress because the second shot peening is performed after the first shot peening12,13,14.
On fig. 15 shows an analytical model of the shot blasting process. An elastic-plastic model was created in which 25 shotballs were dropped into the target local area of ​​the OT line for shot blasting. In the shot blasting analysis model, surface defects of the OT wire deformed during cold winding were used as initial defects. Removal of residual stresses arising from the cold rolling process by tempering before the shot blasting process. The following properties of the shot sphere were used: density (ρ): 7800 kg/m3, elastic modulus (E) – 210 GPa, Poisson’s ratio (υ): 0.3. The coefficient of friction between the ball and the material is set to 0.1. Shots with a diameter of 0.6 and 0.3 mm were ejected at the same speed of 30 m/s during the first and second forging passes. After the shot blasting process (among other manufacturing processes shown in Figure 13), the depth, width, and length of surface defects within the spring ranged from -6.79 to 0.28 µm, -4.24 to 1.22 µm, and -2 .59 to 1.69 µm, respectively µm. Due to the plastic deformation of the projectile ejected perpendicular to the surface of the material, the depth of the defect decreases, in particular, the width of the defect is significantly reduced. Apparently, the defect was closed due to plastic deformation caused by shot peening.
During the heat shrinking process, the effects of cold shrinkage and low temperature annealing can act on the engine valve spring at the same time. A cold setting maximizes the tension level of the spring by compressing it to its highest possible level at room temperature. In this case, if the engine valve spring is loaded above the yield strength of the material, the engine valve spring plastically deforms, increasing the yield strength. After plastic deformation, the valve spring flexes, but the increased yield strength provides the elasticity of the valve spring in actual operation. Low temperature annealing improves heat and deformation resistance of valve springs operating at high temperatures2.
Surface defects deformed during shot blasting in FE analysis and the residual stress field measured with X-ray diffraction (XRD) equipment were applied to sub-model 2 (Fig. 8) to infer the change in defects during heat shrinkage. The spring was designed to operate in the elastic range and was compressed from its free height of 50.5 mm to its firm height of 21.8 mm and then allowed to return to its original height of 50.5 mm as an analysis condition. During heat shrinkage, the geometry of the defect changes insignificantly. Apparently, the residual compressive stress of 800 MPa and above, created by shot blasting, suppresses the deformation of surface defects. After heat shrinkage (Fig. 13), the depth, width, and length of surface defects varied from -0.13 to 0.08 µm, from -0.75 to 0 µm, and from 0.01 to 2.4 µm, respectively.
On fig. 16 compares deformations of U-shaped and V-shaped defects of the same depth (40 µm), width (22 µm) and length (600 µm). The change in width of U-shaped and V-shaped defects is larger than the change in length, which is caused by closing in the width direction during the cold rolling and shot blasting process. Compared to U-shaped defects, V-shaped defects formed at a relatively greater depth and with steeper slopes, suggesting that a conservative approach can be taken when applying V-shaped defects.
This section discusses the deformation of the initial defect in the OT line for each valve spring manufacturing process. The initial OT wire defect is applied to the inside of the valve spring where failure is expected due to the high stresses during operation of the spring. The transverse V-shaped surface defects of the OT wires slightly increased in depth and length and sharply decreased in width due to bending during cold winding. Closing in the width direction occurs during shot peening with little or no noticeable defect deformation during the final heat setting. In the process of cold rolling and shot peening, there is a large deformation in the width direction due to plastic deformation. The V-shaped defect inside the valve spring is transformed into a T-shaped defect due to width closure during the cold rolling process.