Study on the Corrosion Resistance of 39SiCrVTiA High strength and high toughness spring steel

39SiCrVTiA spring steel is heat-treated and compared with the existing high-strength spring steels 60Si2CrVA and SAE9254 for electrochemical impedance spectroscopy (EIS), polarization curve and slow strain rate testing (SSRT). The test results of electrochemical impedance spectroscopy (EIS), polarization curve show that the corrosion resistance of 60Si2CrVA was the best, followed by that of SAE9254 and 39SiCrVTIA.However, the test results of the SSRT test show that the three spring steels in 5% NaCl solution possess high SCC susceptibility. The SCC susceptibility of 39SiCrVTiA steel is slightly lower and the stress corrosion ability is better than the other two steels which may be related to its containing Ti, V elements and lower carbon content.


Introduction
High strength spring steel is widely used in railway, automobile, engineering machinery and other machinery manufacturing [1].This kind of spring steel, has been implemented in the high-stress design, such as the spiral spring for automobile suspension, but the further improving the strength index of steel has been greatly restricted. With the increase of spring stress, the sensitivity of spring steel to corrosion has increased rapidly, and the problem of corrosion fatigue failure during the process has attracted attention. Therefore, higher strength and better corrosion fatigue resistance will become the main research direction of new spring materials. According to the current high-performance, highreliability and lightweight development trends of various types of mechanical devices, this paper develops alloy spring steel with high strength, high toughness and good corrosion resistance. In the research and development of the current high-end spring products such as automotive and rail transit damping spring and stabilizer rod, the latest concept requirements of performance are put forward, enhancing the strength and toughness of the material at the same time pay attention to improve the spring material corrosion resistance to meet the needs of high reliability and lightweight mechanical products. At present, 55SiCrA and 60Si2CrVAT are used as spring steels. The 55SiCrA steel has unstable strength and poor corrosion resistance, while the 60Si2CrVAT steel has the characteristics of high strength but insufficient plastic toughness, hydrogen embrittlement resistance and corrosion fatigue resistance. Therefore, the research and development of spring steel with high strength, high toughness and corrosion resistance has important practical engineering significance and good academic value. In view of the above situation, in order to improve the lowtemperature toughness of the material and ensure the durability of the coil spring in the corrosive environment, the 39SiCrVTiA spring steel with high strength and toughness has been developed.

Test Method
This paper mainly uses various testing methods to comprehensively evaluate the various properties of the 39SiCrVTiA spring steel developed, with special attention to corrosion resistance. And compared with the high-strength spring steel 60Si2CrVA and SAE9254 commonly used today, it provides relevant reference basis and data support for the promotion and application of this material. The experimental materials are 39SiCrVTiA, 60Si2CrVA and SAE9254 spring steels with ϕ 15.8 mm. The material is quenched and tempered, and the structure is tempered troostite. The specific chemical components are shown in table 1. 39SiCrVTiA also contains Ni 0.22, Cu 0.23 and Ti 0.049.
In the formula, δE and ψE are the elongation and sectional shrinkage of spring steel in solution, and δ0 and ψ0 are the elongation and sectional shrinkage of spring steel in air, respectively.

Electrochemical test.
Electrochemical impedance spectra test. Figure1 shows the electrochemical impedance spectra of three materials in a 5% NaCl solution. The equivalent circuit Rs (QdlRt) in Figure2 is used to fit the impedance spectrum, where Rs is the solution resistance, Qdl is the electric double layer capacitance at the non-ideal electrode/solution interface, and Rt is the charge transfer resistance. In the actual electrochemical system, the frequency response characteristics of the double-layer capacitor at the electrode/solution interface are different from that of the pure capacitor, so the constant phase angle Q is often used to represent the double-layer capacitor. Figure3 shows the fitted charge transfer resistance Rt. It can be seen from Figure3 that the value of the charge transfer resistance Rt of different materials is different. 39SiCrVTiA has the smallest Rt value, and the other two materials are closer. The smaller Rt is, the smaller the resistance during charge transfer is [2], and the lager the electrochemical reaction rate is, the lager the corrosion rate is [3]. Therefore, the order of the corrosion resistance of different materials is: 60Si2CrVA> SAE9254> 39SiCrVTiA. Polarization curves test. Figure 4 shows the polarization curves of different spring steels in 5% NaCl solution. Table 2 shows the corrosion potential Ecorr and corrosion current density Icorr of different spring steels in 5% NaCl solution fitting from the polarization curves in Figure5. From the table 2, it can be seen that the corrosion potential sequence from positive to negative is 60Si2CrVA, SAE9254 and 39SiCrVTiA, and the corrosion current density sequence from small to large is 60Si2CrVA, SAE9254 and 39SiCrVTiA.The more negative the corrosion potential is, the greater the thermodynamic tendency of corrosion is. The larger the corrosion current density is, the higher the corrosion rate is [4,5]. Therefore, the corrosion resistance order of different materials is 60Si2CrVA > SAE9254 > 39SiCrVTiA. Based on the above test results, spring steel 60Si2CrVA has the best corrosion resistance, while the steel SAE9254 and 39SiCrVTiA has poor corrosion resistance. According to the chemical composition of the three spring steels, the composition of steel 39SiCrVTiA contains Cu, Ti, Ni and other elements. Relevant studies have shown that the addition of these three elements in material can improve the electrode potential [6][7][8][9], refine the grain and improve the compactness of the rust layer thus material corrosion resistance can be improved. Additionally, the reduction of carbon content in steel 39SiCrVTiA is also beneficial to improving its corrosion resistance. However, the actual test results show that the corrosion resistance of steel 39SiCrVTiA is worse than that of steel 60Si2CrVA. This may be related to the content of the other two elements. According to the chemical composition comparison, steel 60Si2CrVA has higher Cr and V content (1.02% and 0.16%), while the Cr and V content of steel 39SiCrVTiA is only 0.73% and 0.06% respectively. As is known to all, element Cr is the key element to improving the corrosion resistance of materials [10,11], and its content greatly affects the corrosion resistance performance. Meanwhile the element V is useful for refining grains and reducing segregation, thus improving corrosion resistance to a certain extent. Therefore, the higher content of element Cr and V may account for the better corrosion resistance of spring steel 60Si2CrVA.   Stress corrosion test. Figure 5 shows the stress-strain curves of three spring steels in air and 5%NaCl solution.
According to the figure, the tensile strength and yield strength 0.2 of spring steel 39SiCrVTiA, 60Si2CrVA and SAE9254 is 1855Mpa and 1767Mpa, 2127Mpa and 1900 Mpa, 2073 Mpa and 1845 Mpa respectively. It can be observed that the stress-strain behavior of samples in air and solution (applying -1v vs.SCE) is significantly different, and the elongation of samples in air is much higher than that in solution. When we carried on the slow strain rate tensile test (SSRT) of the three materials in 5%NaCl solution, the samples fracture during the elastic deformation phase. This indicates that spring steels have a high stress corrosive cracking sensitivity under corrosion test condition.   Figure 6. Elongation-loss rate Iδ and area reduction loss rate Iψ of the three spring steels in 5% NaCl solution Figure 6 shows the elongation loss and section shrinkage loss of three spring steels in 5% NaCl solution.  [12,13]. After observing of Figure7(e) in detail, it can be seen that the cracks in the local region are wide and deep, and the short cracks are connected with each other to grow into long cracks, and the extension path is similar to the linear form, which are the obvious hydrogen embrittlement cracking characteristics (HE) [14]. Therefore, the SCC process presents the characteristics of intergranular brittle cracking and hydrogen embrittlement cracking at the same time, and its SCC sensitivity is relatively high. Fracture morphologies of steel 39SiCrVTiA tested in air (a,b)and 5% NaCl solution(c-f) Figure 8 shows the fracture morphology of spring steel 60Si2CrVA in air and 5% NaCl solution. It can be seen from Figure 8(a) and(b) that the tensile fracture of steel 60Si2CrVA in air also has obvious necking and plastic deformation phenomenon, and large number of dimples and micropores of different sizes and depth which shows ductile fracture characteristics. Figure8(c) and (d) show that the fracture morphology of steel 60Si2CrVA in 5%NaCl solution is significantly different from that in air. The fracture in air shows obvious ductile fracture characteristics like necking phenomenon; however, in 5%NaCl solution, the fracture shows no necking phenomenon. Figure8(d) shows the typical ice-sugar-like intergranular cracking morphology, with many cracks extending along grain boundaries and obvious grain separation, which are obvious characteristics of Intergranular Stress Corrosion Crack (IGSCC). After observing the Figure8(e) and 8(f) in detail, it also can be seen that apart from obvious characteristics of IGSCC , there are also existing some characteristics of Transgranular Stress Corrosion Cracking (TGSCC) [15,16,17] in the local region. Transgranular cracks are long, deep and wide when they almost extend in a continuous linear form, which show the obvious hydrogen embrittlement cracking characteristics (HE).Then, if combined with Figure7 and 8, it can be seen that when -1v (vs.SCE) is applied in SSRT process, the spring steel 60Si2CrVA has a high SCC sensitivity in 5%NaCl solution, and presents obvious characteristics of intergranular brittle cracking and hydrogen embrittlement cracking.

Discuss
According to the polarization curve (Figure 4), when the potential reaches -1V (vs, the steels will react as the cathode as shown below. 2H2O+2e-→2H+2OH-(3) H+H→H2 (4) The partly H generated in the process of cathode hydrogen evolution will be adsorbed, then it penetrates into the steel and spreads along the grain boundary. In the SSRT process, on the influence of stress, the diffusion of H atom is intensified, and it tends to gather at the grain defect (grain boundary), which will lead to stress concentration and generate a large number of tiny grain boundary cracks in the steel (Figure7(d), 8(d) and 9(d)).In addition, due to the stress-induced diffusion, H atom is more likely to diffuse and accumulate at the crack tip, and it will promote the rapid propagation of the crack tip, then the tendency of hydrogen embrittlement increases and it will lead to the occurrence of transgranular brittle fracture (Figure7 (e), 8(e), 8(f) and 9(e)). If we combine the brittle fracture characteristics in Figure7, 8 and 9 with the relatively large values of section shrinkage loss and elongation loss in Figure6, it can be inferred that when spring steel is applied to -1V (vs.SCE) in the 5% NaCl solution, the SCC mechanism is mainly reflected as hydrogen embrittlement mechanism (HE). Additionally, it can be seen from table 1 that the main difference in chemical composition of spring steel 60Si2CrVA and SAE9254 is that steel 60Si2CrVA contains V element, while steel SAE9254 does not. In addition to refining grains, reducing segregation and improving strength and toughness, the element V is also the element forming the strong carbide, stable tiny precipitates which avail to the formation of hydrogen capture traps and then affect the diffusion and distribution of hydrogen in steel leading to the effectively reducing the sensitivity of delayed fracture of steel [18,19]. This may be the reason for the higher SCC sensitivity of steel SAE9254 under the experimental conditions in this paper. While spring steel 39SiCrVTiA contains element Ti, which has a similar function to V. The addition of Ti can increase the number of hydrogen traps and improve toughness, thus improving its resistance to hydrogen brittleness. Additionally, the steel 39SiCrVTiA has the lowest carbon content, which is also conducive to improving the hydrogen brittleness resistance. Therefore, the SCC sensitivity of 39SiCrVTiA steel is lower than that of the other two materials.

Conclusion
The results of the long period immersion test show that the corrosion rates of the three spring steels are similar, and the steel SAE9254 has the highest corrosion rate, followed by 39SiCrVTiA and 60Si2CrVA steels. Electrochemical ac impedance spectrum and polarization curve tests all show that the corrosion resistance sequence of spring steel materials from strong to weak was 60Si2CrVA > SAE9254 > 39SiCrVTiA. When -1V (vs.SCE) was applied in SSRT process, all three spring steels had high SCC sensitivity in 5%NaCl solution. The SCC sensitivity of steel 39SiCrVTiA was slightly lower, while the SCC sensitivity of steel SAE9254 was largest. The SCC mechanism is mainly hydrogen embrittlement (HE). The lower SCC sensitivity of 39SiCrVTiA steel may be related to its elements Ti, V and low carbon content. To sum up, 39SiCrVTiA spring steel is a kind of high strength and high toughness spring steel, which has excellent plastic toughness especially under low temperature condition and it also has the better hydrogen embrittlement resistance. However, its corrosion resistance remains to be improved.