High Strength Spring Steel Research Articles

Effect of austenitisation and tempering treatments on the mechanical properties of advanced high strength spring steel SAE 9254

In response to increasingly stringent global regulations within the automotive industry, there is a growing demand for lightweight yet high-strength steel suspension coil springs. This study undertakes a comprehensive examination of the critical factors influencing the mechanical properties of SAE 9254, a well-established spring material, in its quenched and tempered (QT) state. The primary objective is to optimise its strength and ductility, especially in high-performance applications with an emphasis on lightweight design. The investigation encompasses a detailed exploration of the effects of varying austenitisation temperature (AT) and austenitisation time (At) on the microstructure and mechanical properties of the steel. Of particular interest is the study of tempering temperatures ranging between 300°C and 350°C to elucidate the mechanisms responsible for enhancing the steel's strength. Notably, the findings highlight dislocation density as the primary factor driving the strengthening of martensitic spring steel. Furthermore, the pivotal role of austenite grain size before quenching becomes evident, influencing toughness, ductility, martensite block size, and the percentage of retained austenite (γR). This study provides valuable insights into optimising the mechanical properties of SAE 9254 spring steel for high-performance applications, achieving ultimate tensile strengths (UTS) exceeding 2100 MPa and a reduction in area (RoA) exceeding 30%. Notably, the properties attained for this grade are comparable to those achieved with microalloying additions. Consequently, this research underscores the significance of controlling austenite grain size and dislocation density as critical factors for tailoring the properties of martensitic steels, positioning them as competitive materials, particularly in the pursuit of lightweight design.

A high volumetric flow rate infrasonic cantilever fan

Cantilever fans exhibit the desirable properties of low noise and long lifetime. However, current cantilever fan designs have low volumetric flow rates compared to other fan types. The study was focused on improving the volumetric flow rate, while keeping its desirable properties. Here we show a cantilever fan system that achieves a volumetric flow rate of 58.9 L/s. This was achieved by increasing the size of the fan blades, using high strength spring steel fan blades at their endurance limit, driving the fans with a custom linear motor and suppressing edge vortexes with shrouding. The flow rate reported here is 118 times higher than has been previously reported.

Experimental validation and thermodynamics analysis of (Nb, V)(C, N) coupling precipitation in high-strength spring steel FAS3550

Based on the thermodynamic coupling model of multiple microelements in steels, the solid solution and coupling precipitation behavior of Nb-V-N-C system quaternary secondary phase was further verified in microalloyed steels, and combining with the engineering practice in high-strength spring steel FAS3550 containing nanoparticles. According to the thermodynamic calculations, the dissolved V (or Nb) content increased with the increasing addition V (or Nb), while decreased with the increase of addition C, N or Nb (or V). The complete dissolution temperature was 1314 °C, and the effective precipitation temperature of carbonitride was 1060 °C in 0.49%C-0.0060%N-0.03%Nb-0.12%V system spring steel FAS3550, the grains in steel began to coarsen above 1100 °C during heat treatment, and the (Nb, V)(C, N) nanoprecipitates were smaller than 100 nm. The strength and plasticity have been improved simultaneously, and its fatigue life has been increased by 13% after optimizing the heat treatment process.

Influence of Structural State prior Quenching in Spring Steel

Various structure states before quenching significantly influence final mechanical properties as well as the dispersion of chemical composition at the same steel grade. Therefore heat treatment with mechanical properties is specified in the technical delivery conditions. Even if the heat treatment is determined, different mechanical properties can be achieved. These differences are increasing in im-portance in high-strength applications like springs, cutting tools, safety, and load-bearing parts of automotive design et al. Because it has a direct impact on their lifetime. The structure consists of ferrite and cementite after spheroidization annealing prior quenching process. The cementite could be observed in various shapes, e.g. fine and large globular particles or the rest of the disintegrated lamellar shape. This article shows how these cementite morphologies affect the quenching behavior and final mechanical properties in high-strength spring steel 54SiCr6.

Enhancing the hydrogen embrittlement resistance with cementite/VC multiple precipitates in high-strength steel

The superior hydrogen embrittlement (HE) resistance and satisfied work hardening capacity are achieved in high-strength spring steel after hydrogen pre-charging with dispersed multiple precipitates via the multi-microalloying of 1.04 wt% Cr and 0.14 wt% V. The precipitates are determined to be the triplex microstructures, i.e., the isolated (Mn, Cr, V)-enriched cementite, isolated V-enriched VC, and the co-precipitation of cementite and VC. These multiple precipitates could act as the effective hydrogen traps to achieve more potent irreversible hydrogen trapping capacity, thus inhibiting the HE susceptibility by ∼23%. Hence, the strategy of cementite/VC multiple precipitates is beneficial in designing the high-strength and HE-resistant automotive steels.

On the low temperature creep controlling mechanism in a high strength spring steel

Despite of the well-known fact that high strength steels are exhibiting low temperature creep deformation, its rate controlling mechanism is yet to be understood. The strain hardening theory and the exhaustion creep model were proposed almost seven decades ago to unravel the low temperature creep mechanism in a single phase homogeneous isotropic material. However, their applicability to low temperature creep deformation in a technical material like a modern high strength steel is still a matter of investigation owing to their nature of multi-phase, in-homogeneous, and an-isotropic behavior. The authors have grabbed this chance to experimentally validate the exhaustion creep model based on low temperature creep tests of the SAE 9254 spring steel. The ultimate tensile strength and yield strength σe of SAE 9254 were determined in a temperature range of 298K≤T≤353K at a constant strain rate of 2.5∙10−4s−1. Low temperature creep deformation behavior was studied at the above-mentioned temperatures T at a constant stress σ=1634MPa, and at stresses 1071MPa≤σ≤1634MPa at a constant temperature T=353K for a duration of 1hr, respectively, at each condition. The retained austenite quantity was determined prior and post low temperature creep testing by means of X-ray diffraction. A reworked exhaustion creep model well describes the stress σ and temperature T dependency of low temperature creep deformation behavior in SAE 9254. Consequently, our hypothesis of a low temperature creep rate controlling mechanism reads: Low temperature creep strain is predominantly contributed by dislocation glide in the retained austenite phase. The reduction in creep rate with time can then be attributed to the exhaustion of this specific deformation mechanism by strain induced martensitic transformation of the retained austenite.

Effects of Non-Metallic Inclusions and Mean Stress on Axial and Torsion Very High Cycle Fatigue of SWOSC-V Spring Steel

Inclusion-initiated fracture in high-strength spring steel is studied for axial and torsion very high cycle fatigue (VHCF) loading at load ratios of R = −1, 0.1 and 0.35. Ultrasonic S-N tests are performed with SWOSC-V steel featuring intentionally increased numbers and sizes of non-metallic inclusions. The fatigue limit for axial and torsion loading is considered the threshold for mode I cracks starting at internal inclusions. The influence of inclusion size and Vickers hardness on cyclic strength is well predicted with Murakami and Endo’s area parameter model. In the presence of similarly sized inclusions, stress biaxiality is considered by a ratio of torsion to axial fatigue strength of 0.86. Load ratio sensitivity is accounted for by the factor ((1 − R)/2)α, with α being 0.41 for axial and 0.55 for torsion loading. VHCF properties under torsion loading cannot appropriately be deduced from axial data. In contrast to axial loading, the defect sensitivity for torsion loading increases significantly with superimposed static mean load, and no inclusion-initiated fracture is found at R = −1. Size effects and the stress gradient effective under torsion loading are considered to explain smaller crack initiating inclusions found in torsion ultrasonic fatigue tests.

Improving the high-cycle fatigue life of a high-strength spring steel for automobiles by suitable shot peening and heat treatment

Shot peening (SP) treatment is a common and useful method that can affect the fatigue life of metallic components. In this work, the high-cycle fatigue (HCF) lives of 50CrMnMoVNb spring steels with different SP intensities and conditions were investigated. Meanwhile, the differences in the microstructures and crack initiation sites were compared. The fatigue life of the investigated steels is greatly improved via suitable heat treatment. Furthermore, SP treatment is beneficial to the improvement of the fatigue life of heat-treated specimens with higher strength, and the optimal SP intensity is 0.15 ∼ 0.25 mmA. However, SP deteriorates the HCF life of the as-received specimens with lower strength. This study can enrich the fundamental knowledge on how to improve the HCF life of spring steels and provide new ideas about the influence of different SP intensities on the surface layer and fatigue properties.

Determination of Critical Transformation Temperatures for the Optimisation of Spring Steel Heat Treatment Processes

Bogie spring performance can be improved by using the exact heat treatment process parameters. The purpose of the study is to determine the critical transformation temperatures and investigate the effect of the cooling rates on microstructural and mechanical properties. The precise determination of the required cooling rates for the particular grade of steel is important in order to optimise the heat treatment process of heavy-duty compression helical spring manufacturing. A traditional heat treatment system for the manufacture of hot coiled springs requires heating the steel to homogenize austenite; then, it is decomposed to martensite by rapid cooling. By analyzing the transition properties by heating and differing cooling rates, this analysis examines the thermal behaviour of high strength spring steel. Using the dilatometer and differential scanning calorimeter, scanning electron microscope, optical microscope, and hardness checking, critical transition temperatures and cooling rates of three springs steels were measured. Although the thermal transformation of materials has been researched for decades using dilatometers, not all materials have been characterized. The research offers insights into the critical transformation temperatures for the defined grades of spring steel and the role of cooling rates in the material’s properties. Mechanical properties are influenced by the transition data obtained from the dilatometric analysis.

Prediction of Fatigue Limit of Spring Steel Considering Surface Defect Size and Stress Ratio

Surface defects decrease the fatigue limit of metals. In this study, the effects of surface defects on the fatigue limit of high-strength spring steel specimens were investigated. Several equations to predict the fatigue limit of specimens with surface defects were evaluated. Specimens with a semicircular slit with depths ranging from 30 to 400 µm were prepared. The Vickers hardness of the specimen was approximately 470 HV. Bending fatigue tests were performed at stress ratios ranging from −2 to 0.4. The fatigue test results showed that the fatigue limit decreased with an increase in the slit depth. Moreover, the maximum defect size that resulted in a decrease in the fatigue limit was dependent on the stress ratio. On comparing the predicted fatigue limits with the experimental results, it was confirmed that the predictions made based on the modified El-Haddad model were in good agreement with the experimentally obtained data. Thus, this model can be used to evaluate the fatigue limit of high-strength steels containing small surface defects with different stress ratios.

Cyclic Behavior of Beam-Column Connections in Precast Structures

The performance of precast concrete structures is greatly influenced by the response of beam–to-column connections. In this study, a new moment resisting precast concrete beam-column connection detail with post-tensioning bolts, made out of high yield strength spring steel, has been experimentally investigated as an alternative to the conventional precast moment resisting connections. Precast specimens and an aseismic monolithic reference specimen have been tested under reverse cycling loads. The contribution of the mild steel in the connection region to the flexural moment capacity, and to the initial pre-stressing force on post-tensioning bolt have been the main test variables. The moment capacity, stiffness, energy dissipation capacity and the residual displacement performance of the precast connections have been compared with those of the aseismically detailed monolithic connection. The conducted tests reveal that the connection detail with steel corbel along with the post-tensioning bolt and mild steel has superior performance properties as compared to the companion precast specimens.

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.

On the effects of heat and surface treatment on the fatigue performance of high-strength leaf springs

Leaf springs constitute the most effective suspension way of commercial vehicle axles from the cost and maintainability point of view. Especially in case of front axles, they overtake both the guidance and suspension functions, which consequently designates them as safety components, whose pre-mature failure is explicitly prohibited. The present paper deals with the fatigue performance of downsized parabolic leaf specimens made of the high-strength spring steel 51CrV4 under serial manufacturing conditions. It focuses on the influence of the major manufacturing steps, i.e. the heat treatment and the subsequently applied stress shot peening. The effectiveness of the applied heat treatment on the microstructure transformation and the extent of surface decarburization is determined by means of optical microscopy and corresponding microstructural analyses. Comprehensive series of constant amplitude fatigue tests are executed before and after the applied stress shot peening to quantify its effectiveness on the fatigue performance. The tests cover two characteristic stress ratios of operational significance with the complete range of interest being experimentally investigated. Additionally, surface residual stresses measurements together with micro- and macro-hardness and roughness values before and after stress shot peening are executed to expose the influence of each individual technological effect on the overall fatigue performance.

Inclusion initiated fracture in spring steel under axial and torsion very high cycle fatigue loading at different load ratios

Inclusion initiated fracture under axial and torsion very high cycle fatigue loading at load ratios R = −1, 0.1 and 0.35 is compared. High strength spring steel with intentionally increased density of aluminate inclusions was tested at ultrasonic frequency. Failures under axial loading started at internal aluminate inclusions at all load ratios, and at aluminate and MnS inclusions in cyclic torsion at positive load ratios. Considering crack initiation at same size aluminate inclusions, a cyclic torsion stress amplitude at the inclusion that is a factor 0.86 ± 0.05 smaller than the cyclic tension stress amplitude has an equally detrimental effect on cyclic strength.

Design and Fatigue Analysis of Shot Peened Leaf Spring

The paper handles the fatigue and failing analysis of serial shot-peened leaf springs of cumbersome vehicles emphasizing on the impact of shot peening on fatigue life, coping with automotive leaf springs, the shot peening method turns into an important step in production.In the situation of leaf spring suspensions, however, asystematic research of the effect of shot peening about fatigue life isstill required. Experimental stress-life curves are determined with the aid of the usage of investigating clean specimen subjected to shot peening. those test consequences are as compared to corresponding ones identified from cyclic three-point test on shot peened serial leaf springs in order to show the influence of applied heat treatment and shot peening approach on fatigue existence of high-strength used to get leaf spring manufacturing, reliant on the load level. Analyses are performed to explain the effects resulting from shot peening practice on the surface features of the high-strength spring steel under examination. The evaluation of fatigue results shows that almost no life improvement due to production highlighting the importance for mutual variation in parameters of shot peening and thermal treatment so that there is sufficient progress in life

Effects of mean shear stress on the torsional fatigue strength of a spring steel with small scratches

The torsional fatigue behavior of a high-strength spring steel, SWOSC-V, was investigated. Of particular interest were the influences of small scratches and superimposed static shear stress in the very high cycle fatigue regime. To achieve the research objectives, an ultrasonic torsional fatigue testing machine was developed. S-Ncurves showed the fatigue limit for the tested material condition (i.e. with no residual stresses at specimen surface). This study revealed that a notable influence of the stress ratio, R, on the torsional fatigue behavior was not explicitly recognized. Concerning the influence of small scratches, it was found that the [Formula: see text]-parameter model can predict the lower bound of the torsional fatigue limit, irrespective of the value of R.

Inclusion initiated fracture under cyclic torsion very high cycle fatigue at different load ratios

The influence of inclusions on cyclic torsion very high cycle fatigue (VHCF) is studied at different load ratios. Testing material is a high-strength spring steel, which was prepared based on the SWOSC–V composition with intentionally increased number and size of inclusions. Ultrasonic torsion fatigue tests were performed at load ratios R = –1, R = 0.1 and R = 0.35 with shot peened specimens. All specimens that were cycled at R = 0.1 or R = 0.35 and failed above 108 cycles showed crack initiation at internal aluminate or MnS inclusions. Interface failures between particles and metallic matrix generate cracks at aluminate inclusions, which predominantly grow in mode I. MnS inclusions have an elongated shape oriented in the specimens' length direction. Shearing of the MnS inclusions generates initial mode II/mode III cracks, which subsequently branch to mode I cracks. All specimens cycled at R = –1 showed surface crack initiation without inclusions, irrespective of lifetime. S–N curves are shifted towards lower shear stress amplitudes with increasing load ratio, indicating a pronounced mean stress influence on cyclic torsion strength.

Enhanced springback prediction for bending of high-strength spring steel using material data from an inverse modelling approach

Abstract Springs for electrical components made from strip materials and flat wires are produced using stamping and forming technologies. The process design of multi-stage stamping and bending processes is still based on expert knowledge and experimental tests. The complexity of the forming processes and the high-strength materials used for springs make the FE simulation of those processes a challenging task. For the springback prediction in FEM, however, an accurate description of the elasto-plastic material behaviour is mandatory. Under process conditions such as bending of spring steel, the locally occurring strains are many times higher than the ultimate elongation of approx. 2 % that can be determined in tensile test. For those materials, standard tests fail to receive enhanced material data such as the influence of plastic deformation on the Young’s modulus. The goal of this work is to determine the elasto-plastic material parameters of high-strength spring steel (X10CrNi18-8) using an inverse modelling approach by means of a 3-point bending test. The inverse approach is used for determining the Young’s modulus and hardening parameters of Ludwik-Hollomon’s law for bending of high-strength spring steel. Furthermore, this work focuses on the unloading at the end of the bending experiment and its effect on the elastic material parameters. The results demonstrate that the inversely determined material data show lower scattering compared to material data from tensile test. Furthermore, material data of high-strength spring steel from the inverse modelling approach significantly enhances the springback prediction under bending conditions compared to FE simulations using material data from tensile test.

Dynamic Recrystallization Behavior and Critical Strain of 51CrV4 High-Strength Spring Steel During Hot Deformation

Single-pass compression experiments have been performed on 51CrV4 spring steel using a Gleeble 3800 thermomechanical simulator at temperatures in the range of 800–1000°C and strain rate of 0.01 s−1 or 0.1 s−1; the maximum deformation degree was 50%. By considering the inflection of the ln θ–e curve and minimum value of the − ∂(ln θ)/∂e–e curve, the relationship between the critical strain (ec) of dynamic recrystallization (DRX) and the deformation temperature was determined. The results showed that steady flow behavior could be observed during low-temperature (800°C, 850°C) deformation, and dynamic recovery (DRV) regulated the trend of the stress–strain curve. Dislocation cell structures and polygonization were formed during the DRV stage. DRX of the alloy occurred when the deformation temperature reached a higher value (900°C, 1000°C). The amount of ec required for DRX decreased with increase in the deformation temperature, and the relationship between ec and the peak strain (ep) was determined as ec = 0.49ep. Discontinuous DRX was clearly favored when the strain was lower than the critical value.

Corrosion behavior of high-strength spring steel for high-speed railway

The corrosion resistance and evolution of corrosion products in medium-carbon high-strength spring steels were investigated in a neutral salt spray (5wt% NaCl solution). A formation model of γ-FeOOH and a transformation model describing the conversion of γ-FeOOH to α-FeOOH were constructed. The results indicated that, at the initial corrosion stage, the corrosion resistance was gradually improved with the addition of Cr; however, with the addition of alloying element V, the corrosion resistance decreased. These results were attributed mainly to the initial corrosion stage being closely related to the matrix microstructure parameters such as grain-boundary character and dislocation density. After the rust layer was formed at a later corrosion stage, the corrosion resistance was reinforced with the addition of Cr and V because Cr strongly influenced the composition, structure, and morphology of the corrosion products. The results presented herein show that Cr was conducive to the transformation of γ-FeOOH into α-FeOOH. Moreover, V and Cr exhibited obvious synergy and were enriched in the inner layer of the corrosion products.