Effect of Austempering Temperature on Stress Corrosion Resistance of 52CrMoV4 Spring Steel

Abstract

Stable service performance of spring steel under complex working conditions is essential, and under the synergistic effect of corrosive environment and stress, stress corrosion cracking is very likely to occur. Austempering can change the microstructure of spring steel and thereby improve its stress corrosion sensitivity. In this study, methods such as scanning electron microscopy and slow strain tensile tests were adopted to investigate the effects of different austempering temperatures on the microstructure and stress corrosion resistance of 52CrMoV4 spring steel. The results show that crack initiation during stress corrosion cracking in the test steel is mainly related to anodic dissolution. The coarsening of bainite laths and the increase in the area of martensite–austenite islands accelerate stress corrosion cracking once stress concentration occurs. The steel austempered at 310 °C has the best resistance to stress corrosion, with a stress corrosion sensitivity value of only 2.4%, which is much lower than those of specimens under traditional processes (15.5–23.7%).

1. Introduction

The 52CrMoV4 (according to EN 10089: 2002 [1]) is a high-strength alloy spring steel, which is widely used in spring components with high stress, fatigue resistance, and high-temperature resistance, such as automotive suspension systems, spring plates, and torsion bars [2,3]. The synergistic effect of alloying elements Cr and Mo can improve the hardenability and high-temperature tempering stability of the steel. The micro alloying element V in 52CrMoV4 can form nano-scale VC precipitates, which can inhibit grain growth and improve fatigue strength. However, the corrosive environment containing chloride ions formed by deicing salts or seawater atmospheres on coastal roads leads to the deterioration of the mechanical properties of 52CrMoV4 spring steel and reduce the alloy’s resistance to crack initiation and propagation, which in turn causes stress corrosion cracking (SCC) [4,5,6]. SCC primarily proceeds via two distinct mechanisms: anodic dissolution at propagating crack tips, and cathodically absorbed hydrogen leading to embrittlement [7]. The susceptibility of the steel to SCC largely depends on three material factors: (i) the chemical composition of the alloy, (ii) crystallographic grain characteristics, and (iii) distribution of age-hardening precipitates [8,9,10,11]. Pan et al. investigated the correlation between the heat treatment and the polarization potentials with the SCC in duplex stainless steel and found that ferrite is the weakest region for SCC initiation and extension, and lower Cr and Mo contents in ferrite induced the formation of pitting SCC [12]. With the negative shift in the potential, hydrogen increased the SCC susceptibility by deteriorating the passivation film, inhibiting re-passivation, and causing severe damages [13]. Huang et al. combined the low-cycle fatigue (LCF) with SCC, investigated the effect of LCF on the SCC susceptibility of CrNiMoV steels, and found that the reduction in residual stress, phase transformation, and increase in low-energy structures improved the resistance to SCC. Based on the linear relationship between the kernel-averaged misorientation (KAM) and the coefficient of stress corrosion susceptibility, they found that the effect of fatigue damage on the stress corrosion susceptibility was predictable [6].

Thermomechanical processing (e.g., heat treatment [14,15]) and compositional adjustment via microalloying synergistically modify the microstructure [16,17], thereby dictating the resultant mechanical properties of engineered materials. Obtaining the bainitic microstructure via austempering with salt bath can improve the toughness of steel, but at the expense of its strength and hardness [18]. Su et al. found that the pearlitic content of GCr15 steel increased with prolonged austempering duration, and the rate of increase gradually became slower [19].

Currently, most of the studies on SCC are focused on the field of alloys and engineering structural steels, and studies on austempering are mainly focused on its impact on microstructural evolution and mechanical performance. However, little research has been conducted on the effect of austempering temperatures (AT) on SCC, especially in the field of spring steels. In fact, spring steels have a high probability of undergoing stress corrosion cracking in their service conditions, and there is still great potential for their performance enhancement, which should be explored by investigating the effect of heat treatment on SCC of spring steel to improve its overall performance.

The 52CrMoV4 spring steel used in automotive suspension systems often experiences SCC. In order to study its SCC mechanism, this study performed austempering treatments at varying temperatures on 52CrMoV4 steel. Through slow strain rate testing (SSRT), we systematically investigated stress corrosion susceptibility under different heat treatment processes. Fractography analysis via scanning electron microscopy (SEM) elucidated how microstructural evolution governs stress cracking behavior in corrosive environments. The findings provide fundamental guidance for engineering applications of 52CrMoV4 spring steel in corrosive environments.

2. Materials and Methods

2.1. Materials

The chemical composition of the experimental steel is shown in

Table 1. Chemical compositions of the 52CrMoV4 spring steel (wt.%).

Element	C	Mn	Si	Cr	Mo	P	S	V	Fe
Content	0.49	0.80	0.35	1.12	0.22	0.02	0.16	0.15	balance

, which was tested by an optical emission spectrometer model OES8000S from Skyray Instrument (Jiangsu Skyray Instrument Co., Ltd., Kunshan, China).

The supply status of this steel is hot rolling (final rolling temperature 850–900 °C) and annealing, with an intercepting area of 30 × 90 mm. The conventional heat treatment routes are illustrated in Figure 1a. The specimens were heated to 860 °C and held for 0.5 h in a resistance furnace, followed by oil quenching to room temperature. Subsequently, the specimens were tempered either at 450 °C for 2 h or 470 °C for 3 h. These specimens are designated as Oil Quenching-450 °C (OQ-450 °C) and Oil Quenching-470 °C (OQ-470 °C), respectively. The bainitic heat treatment processes are illustrated in Figure 1b. The specimens were heated to 860 °C and held for 0.5 h, followed by austempering in salt baths at varying temperatures ranging from 270 °C to 400 °C for 2 h. Finally, they were subjected to tempering at 440 °C for 1 h.

2.2. Observation of Microstructure

The microstructure was observed using a Hitachi SU-8200 cold field emission SEM (Hitachi High-Technologies Corporation, Tokyo, Japan) and an ICX41M Optical Microscope (OM) (Ningbo Sunny Instruments Co., Ltd., Ningbo, China). An industrial camera was used to observe the macroscopic appearance of the fracture, which was observed immediately after the specimen was fractured. The microstructure of the surface and fracture characterization was observed by SEM. The specific parameters for SEM observations are as follows: the acceleration voltage was 10 KV, the working distance was 8.0–9.2 mm, and the magnification was 400–3000 times. The fracture morphology was observed as soon as possible after fracturing, and those that could not be observed in time were immersed in an air-insulating medium such as anhydrous ethanol, cleaned with pure water, and washed and dried with anhydrous ethanol before observation.

2.3. SCC Test

The electrochemical test was conducted using a CHI660E electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China). In this paper, a three-electrode system was adopted for measurement. The electrolyte selected was a 3.5 wt.% NaCl solution and the specimen size was 5 × 10 × 10 mm. The polarization curve test experiment was repeated three times to ensure the stability and reproducibility of the electrochemical state. For the SCC test, the specimens were tested at room temperature using the WDW-100MF slow strain rate stress corrosion testing machine (Jinan Shichuang Testing Equipment Co., Ltd., Jinan, China). The experimental parameters were designed to simulate marine service environments for 52CrMoV4 spring steel applications. The strain rate of 1.3 × 10⁻⁶ s⁻¹ was selected in strict accordance with GB/T 15970.7-2017 [20,21]. The 3.5 wt.% NaCl solution directly corresponds to the average chloride concentration in oceanic environments as defined by ASTM D1141-98 [22]. For each process, slow strain rate tensile tests were required to be conducted, respectively, in the air and in the corrosive medium. Each group of experiments was conducted at least three times, and the average results were selected for presentation. We reduced the impact of random errors through multiple repeated experiments and key data were reported in the form of mean ± standard deviation (mean ± SD) in the paper. The specimen dimensions are shown in Figure 2. The elongation loss (I_δ) parameter was used to evaluate the SCC sensitivity of the test steel. The larger the I_δ value, the weaker the specimen’s ability to resist stress corrosion. I_δ was calculated according to the following equation [23,24]:

$$I_{\delta }=\left\{1-{\displaystyle \frac{{\delta }_E}{{\delta }_0}}\right\}\times 100\%$$

(1)

where δ_E and δ₀ are the elongations of test steel in solution and in air, respectively.

3. Results

3.1. Microstructure

Different heat treatment processes cause the specimens to present different microstructures. The SEM images of the heat-treated specimens at varying AT are presented in Figure 3. The black plate-like microstructure is bainitic ferrite (BF) and the blocky area is the Martensite–Austenite (M/A) island. From Figure 3a–f, it can be seen that the area of the M/A island in the microstructure increases gradually with the increase in AT, and it increases further after an AT of 360 °C. This is mainly because when the AT is relatively high, the migration tendency of carbon atoms strengthens. The out-diffusion of carbon from bainite and ferrite enriches the adjacent austenite, stabilizing it and leading to M/A retention during cooling. The bainite slats generated at 270 °C are fine and disordered. When the AT is between 300 °C and 360 °C, the bainite slats are almost parallel and arranged in slat bundles, and the white undissolved carbides (UC) are uniformly distributed inside the slats. With the increase in AT, the length and width of bainite slats are increased. The bainite slats are further roughened after austempering at 400 °C.

Microstructural features characteristic of traditional processed spring steel are captured in the SEM images of Figure 4, revealing tempered martensite with uniformly dispersed carbides. The primary microstructure is tempered martensite, exhibiting distinct differences from the austempering microstructure (Figure 3). After oil quenching, the microstructure of the steel predominantly consists of martensite with minor M/A island area. The redistribution of carbon atoms will occur in subsequent tempering process, and the microstructure of the martensite and M/A gradually decompose into ferrite and carbides [25]. Globular carbides and ferrite collectively form the tempered martensite microstructure.

3.2. SCC Test Results

The stress–strain (SS) curves of the test steel obtained in both air and 3.5 wt.% NaCl solution after different heat treatments are displayed in Figure 5. The difference between the SS behavior of the specimens in air and in solution is obvious, and the difference between the tensile strength of the specimens in air and in the corrosive medium is basically within 100 MPa. For the 270 °C austempered test steel in the corrosive medium for slow strain tensile testing, the specimen fractured in the plastic deformation stage. The material’s loss of plasticity is obvious, and its resistance to fracture is significantly reduced, which indicates that the processed spring steel materials in this test conditions have a high susceptibility to stress corrosion cracking [26]. With the gradual increase in AT, the slope of the SS curve in the elastic deformation stage gradually decreases.

The elongations (%) from the SSRT experiments of the specimens under different medium after different heat treatments are shown in

Table 2. Elongation (%) of the SSRT experiments of the specimens under different medium after austempering or quenching treatment.

	Temperature/°C	270	300	310	320	360	400	OQ-450	OQ-470
Medium
Air		9.42 ± 0.18	10.67 ± 0.20	9.91 ± 0.16	11.0 ± 0.22	12.30 ± 0.12	11.46 ± 0.18	10.48 ± 0.24	10.35 ± 0.18
NaCl		6.87 ± 0.20	9.44 ± 0.15	9.67 ± 0.08	9.54 ± 0.23	9.55 ± 0.17	10.73 ± 0.14	8.89 ± 0.32	7.90 ± 0.24

. The elongations of the steels in the air are significantly greater than those in the NaCl medium. The influence of the corrosive medium varies with heat treatment: in the corrosive environment, elongation first increases and then decreases with AT, and it rises again after austempering at 400 °C. The elongation of conventional processed test steel in the corrosive medium is lower than that of most bainitic process test steels.

The stress corrosion sensitivity coefficients of the specimen after different heat treatments are shown in

Table 3. Stress corrosion sensitivity coefficients of the specimens after austempering or quenching treatment (I_δ: elongation loss).

Temperature/°C	270	300	310	320	360	400	OQ-450	OQ-470
Iδ/%	27.1 ± 2.1	11.5 ± 1.4	2.4 ± 0.8	13.3 ± 2.1	22.4 ± 1.4	6.4 ± 1.2	15.5 ± 3.1	23.7 ± 2.3

and Figure 6. With the increase in AT, the stress corrosion sensitivity (SCS) coefficient first decreases and then increases, and decreases again after austempering at 400 °C. The data in the table shows that the SCS of austempered specimens is significantly lower than that of oil-quenched specimens overall, indicating that austempering can significantly improve the SCC resistance of the specimens. The test steel austempered at 270 °C has the highest stress corrosion sensitivity coefficient, meaning its resistance to SCC is lowest; in contrast, the steel austempered at 310 °C has the lowest SCS coefficient, indicating the strongest resistance to stress corrosion cracking.

It can be seen from Figure 7 that the sample shows obvious dissolution during the anodic polarization stage, as indicated by the blue arrow in the figure.

3.3. Fracture Morphology

The macroscopic fracture morphologies of tensile specimens under different heat treatments are presented in Figure 8. These air-tested specimens exhibit predominantly toughness fracture with discernible necking deformation and distinctively brighter fracture surfaces characterized by high reflectivity. The specimens exposed to the NaCl medium exhibited complete surface corrosion with black pitting. Distinct striations caused by corrosion and tensile deformation were observable beneath the rust layer. The fracture surface of the specimen austempered at 270 °C displayed brittle fracture characteristics. The specimens austempered at 310 °C and 400 °C and tested in the corrosive medium showed limited necking at the fracture zone; however, the degree of necking was still significantly small compared with specimens tested in air.

The SEM images of microscopic fracture morphologies for these specimens austempered at different temperatures and tested in air are presented in Figure 9. The fracture surfaces exhibit extensive dimple morphologies, indicating ductile fracture mechanisms. The specimen austempered at 270 °C displays reduced surface undulation with shallower dimples, signifying less pronounced plastic deformation during fracture. In contrast, the specimen austempered at 300 °C exhibits larger and deeper dimples compared to the specimen austempered at 270 °C, demonstrating an enhanced plastic deformation during fracture [27]. With increasing austempering temperature, the fracture surfaces become increasingly irregular, suggesting greater plasticity.

The tensile fracture morphologies of specimens austempered at different temperatures in NaCl corrosive medium are presented in Figure 10. The images indicate that the fracture surfaces retain trace amounts of corrosion products while exhibiting abundant dimples. Compared to fractures produced in air, the specimens tested in NaCl solution display smaller dimples with an increased number of white tearing ridges, accompanied by generally flatter fracture surfaces. The specimens austempered at 360 °C and 400 °C show relatively smooth fracture surfaces with inconspicuous dimples.

The fracture morphologies of SSRT specimens subjected to conventional heat treatments under different mediums are displayed in Figure 11. These fracture surfaces exhibit fewer dimples and river-patterned cleavage facets, showing a marked contrast to those of the austempered specimens. Notably, the specimen treated with OQ-470 °C processing demonstrates distinct transgranular fracture characteristics on its corrosion fracture surface [28].

4. Discussion

Deformation will accelerate the dissolution of metals in corrosive media [29]. The strain caused by stress in the test steel will increase the number of vacancies and the dislocation density, resulting in an increase in the number of active sites on the surface, thereby intensifying the corrosion reaction. Under continuous stress, the passive film on the surface of the test steel ruptures and microcracks emerge. Once microcracks generate, they expose the fresh surface of the experimental steel to corrosive media. The Cr and Mo elements in the steel can promote repassivation of these fresh surfaces. This “crack–repassivation” cycle phenomenon causes SCC cracks to expand into the interior of the steel, accelerating the fracture of the specimen.

The variation in AT leads to changes in the distribution and morphology of bainite laths, dislocation density, and M/A island area [18], which affect the stress corrosion resistance of spring steel. In the presence of tensile stress, for SCC, stress concentration in the microstructure plays a more important role than corrosion [30]. The specimen austempered at 270 °C achieves a refined microstructure with elevated dislocation densities and disordered bainite laths. This microstructural characteristic multiplies electrochemically active sites on the surface, which is highly favorable for oxidation reactions. The concurrent application of stress and corrosive media accelerate corrosion degradation. Once the AT exceeds 320 °C, the bainite laths become coarser as AT increases, making it is easier for stress concentration to occur [31], which results in a decline in stress corrosion resistance. At the same time, as AT increases, the area of M/A islands also increases. Due to phases differences between the bainite and M/A, a potential difference exists, making microelectrolyte coupling corrosion likely. In the corrosive medium, the ferrite slats around M/A islands are preferentially dissolved as the anode to form etch pits [32], and the bottom of the etch pit will continue to dissolve, which will lead to stress concentration, promote crack initiation, and eventual propagation, resulting in plasticity degradation.

When AT increases from 310 °C to 320 °C, the synergistic effect of stress and corrosion amplifies the slight coarsening of the bainite laths and the increase in small M/A islands, reducing properties [33]. This increases the possibility of stress concentration and further decreases stress corrosion resistance. The lower SCS coefficient of the test steel obtained after austempering at 400 °C may be due to the smaller residual stress in the microstructure, which reduces stress concentration and improves SCC resistance [34]. Another factor may be the reduction in subgrain boundary in the microstructure after 400 °C AT, while higher AT causes redistribution of carbides as well as enhancement of residual austenite content [35]. In spite of the increase in the area of M/A island after austempering at 400 °C [36], the SCC resistance of the test steel is still improved by the combination of the above factors.

Although the fracture surface exhibits ductile dimples, the effect of hydrogen cannot be completely ruled out due to the harsh chloride environment. Hydrogen may accelerate the final ductile fracture process by affecting dislocation motion or promoting microvoid nucleation.

5. Conclusions

(1)

The main type of cracking that occurs in the SCC process of the test steel is anodic dissolution-type stress cracking. Under the action of tensile stress, stress concentration plays a more important role in SCC initiation than corrosion.

(2)

With the rise in AT, the ability of test steel to resist stress corrosion first rises and then declines, rising again at 400 °C. The test steel austempered at 310 °C has the best stress corrosion resistance, while the test steel austempered at 270 °C has poor resistance due to fine and disordered bainitic laths with more active sites in the microstructure.

(3)

When the AT exceeds 310 °C, the coarsening of bainitic laths and the increase in M/A island area in the microstructure increase the possibility of stress concentration. Under the combined action of stress and corrosive media, the plasticity of the test steel decreases when subjected to tensile stress, resulting in reduced resistance to stress corrosion cracking.

(4)

The reduction in subgrain boundaries and redistribution of carbides enhance stress corrosion resistance when the steel is austempered at 400 °C. The test steel austempered at 300 °C has a better stress corrosion resistance than that of the test steel treated by the traditional process.