Spring Steel Research Articles

WARM DEFORMATION BEHAVIOR OF A 65MN SPRING STEEL

The warm deformation behavior of 65Mn spring steel has been carried out by a thermomechanical simulator. The deformation temperatures are in the range of 550 ~ 700℃ and strain rates are in the range of 0.001 ~ 1 s-1. The deformation activation energy is calculated to be 486.829 KJ•mol-1. The strain compensated Arrhenius-type constitutive model was established. The relationship materials constants and strain were fitted with an 8th order polynomial. It was found that the strain has a significant influence on the instability map. At the strain is 0.3, the optimum flow zone may take place with the deformation temperatures higher than 626 ℃ and strain rate in the range of 0.001 ~ 1 s-1.

Corrosion Behavior of Corrosion-Resistant Spring Steel Used in High Speed Railway

Corrosion behavior of 60Si2Mn-A and 60Si2Mn-B in simulated industrial atmospheric environment was investigated by alternate immersion corrosion test and electrochemical method. The phase, morphology, characteristics of corrosion products, and the distribution of Cr, Cu, and Ni in the corrosion products of experimental steel were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe microanalyzer (EPMA). The results show that the phase of rust layer is Fe3O4 and γ-FeOOH in the early stage and then changes to α-FeOOH and γ-FeOOH in the later stage; the size of the rust layer with corrosion resistance of 60Si2Mn is less than 60Si2Mn; the Cr element accumulates in the rust layer of the experimental steel in the early stage of corrosion resistance; and Cu, Ni, and Cr in the corrosion resistance 60Si2Mn are concentrated in the rust layer near the substrate In the later stage of corrosion. As the corrosion cycle is prolonged, the corrosion potential and the resistance of the rust layer of the experimental steel increases, and the corrosion current decreases; in the same corrosion cycle, the corrosion potential and corrosion resistance of 60Si2Mn-B are greater than 60Si2Mn, and the corrosion current is less than 60Si2Mn.

Environmentally assisted fracture behavior of edge and corner notched spring steel

The components made out of spring steel are usually subjected to different environmental conditions when in service. The hydrogen embrittlement is one such process which is inevitable for spring steel. The embrittlement will make the material brittle and induce cracks at the edges and corners. In the presentation experimental investigation has been focused on the analysis of the behavior of embrittled spring steel with edge and corner cracks, under impact and bending loads. The specimen preparations and experimentations have been carried out according to ASTM standards. The results have revealed that the embrittlement induced side and corner cracks play a vital role in the fracture process of the spring steel. The mechanism of fracture in edge and corner notched samples has been analyzed with the aid of scanning electron microscopy. The microscopic observations have shown that the failure in the edge and corner cracked samples is almost catastrophic, and increase in the rate of crack growth in these embrittled samples is highly influenced by the embrittlement induced cracks.

Effects of quenching and tempering heat treatment on microstructure, mechanical properties, and fatigue crack growth behavior of 51CrV4 spring steel

Herein, the microstructure, mechanical properties, and fatigue crack growth (FCG) behavior of 51CrV4 spring steels under different quenching and tempering heat treatment were studied. Results show that tempering temperature would not affect the prior austenite grain (PAG) size, while quenching temperature has a great influence on it; the volume fraction of carbide precipitates decreased with increasing quenching temperature. The width of martensite lath has little relationship with the prior austenite grain size. The mechanical properties of the investigated steels decreased with an increasing tempering temperature. Tempering temperature had a greater influence on the fatigue crack growth behavior compared with quenching temperature. The FCG of all the investigated steels were well interpreted by Paris model. When the quenching temperature was 840 °C, the stable expanding stage of the fatigue fracture was representative fatigue striations, fatigue steps and secondary cracks; multiple failure mechanisms coexisted in the rapid expanding stage; the fracture morphology of unstable failure stage was mainly dimples. When the quenching temperature increased to 880 °C and 920 °C, quasi-cleavage was observed in all the stages compared with the samples quenched at 840 °C. Secondary cracks, fatigue steps and crack closure could retard the FCG rate by consuming the driving force near the tip of fatigue crack.

Tensile behavior of pre-stress corroded and post hydrogen embrittled spring steel

Stress Corrosion Cracking (SCC) and Hydrogen Embrittlement (HE) are two major degradation phenomena experienced by spring steel. The consequences of these phenomena have an impact both on the mechanic response, sufficiency, and life of the mechanical system. The susceptibility to premature failure is never the same and is unique for different environment combinations. There will be a combined effect of these phenomena. Here in this experimental investigation, an attempt is made to understand the tensile behavior of combined treatment. The pre-stressed specimens were corroded and post-treated with the HE process. Traditional tensile testing was used to investigate the combined effect of corrosion and post-treated HE. From the test results, it can interfere that the post hydrogen charged specimen exhibits ductility loss, but recover in the ultimate strength, and increase in yield strength. The dominance of recovery was also shown by an increase in fracture strength and toughness. The scanning electron microscope was used to assess the mechanism and to characterize it.

A Study on the Surface Integrity of 50CrMnMoVNb Spring Steels with Different Matrix Strengths Processed by Shot Peening

The surface integrity of structural materials after surface mechanical strengthening has a significant effect on service performance. Herein, the 50CrMnMoVNb spring steels with different microstructures and strengths are surface strengthened by shot peening (SP) with the same parameter, and the effect of matrix strength on surface integrity, including the roughness, residual stress, and gradient structure, is systematically studied. The experimental results show that both the line roughness and surface roughness decrease with the increase in matrix strength; the microhardness enhancement at the topmost surface layer and thickness of gradient layer decrease with the increase in matrix strength, whereas the surface residual stress and the maximum compressive residual stress display an increasing trend with the increase in matrix strength, and both the depth at the maximum compressive residual stress and the thickness of compressive residual stress zone are stable with a value of ≈75 and ≈300 μm, respectively. Inspired by these results and discussions, the surface integrity of the shot peened (SPed) metallic materials would be improved greatly by composition adjustment and microstructure optimization to regulate the matrix strength, as well as the SP process optimization.

Influence of boron addition on the hot ductility of medium carbon spring steel

In this article, the influence of boron addition (0.0005%, 0.0020% and 0.0047%) on the hot ductility of the medium carbon spring steel 60Si2Mn is studied. Hot tensile experiments in the temperature range of 873–1573 K (600-1300℃) are conducted using Gleeble 1500D. Stress-elongation curves are measured to describe the dynamic recrystallization (DRX) behavior. Reduction of area (R.A.) until fracture is taken to characterize the steel hot ductility. Sample microstructures and fracture morphologies are analyzed using optical microscopy (OM), scanning electron microscopy (SEM) and electron probe microanalyser (EPMA). Besides, the evolution of the secondary phase precipitations are studied by CALPHAD modelling. Results show that the influence of boron addition on steel hot ductility varies with test temperatures: i) in the temperature range of 1323–1573 K (1050-1300℃), boron segregation at austenite grain boundaries leads to BN precipitation which would pin the grain boundaries and deteriorate hot ductility, ii) in the temperature range of 1073–1273 K (800-1000℃), increase of boron addition lowers the DRX onset temperature thus refining grain size and improving the steel hot ductility, iii) in the temperature range of 873–1023 K (600-750℃), the precipitation and distribution of BN along austenite grain boundaries inhibits the formation of grain boundary ferrite, hence the toughness of the austenite grain boundaries and the steel hot ductility are improved.

Development and comprehensive evaluation of a new spring-steel-driven glove for grasping assistance during activities of daily living.

Millions of people suffer from a decline in grip strength and hand function due to conditions such as chronic disease, injuries, and aging. Hand function decline results in difficulties with performing activities of daily living, where grasping, lifting, and releasing objects are essential. There is an increasing demand for assistive gloves to enhance users' hand function and improve their independence. This paper presents the design of a new bidirectional lightweight assistive glove and demonstrates its capabilities through comprehensive experiments using human subjects. The developed glove can provide adequate power augmentation for grasping and releasing objects due to its simple yet effective design using spring steel strips and linear actuators. The glove directly transfers assistive forces to users' fingertips without any complex intermediate mechanism, and its low weight of 196 g promotes its usability. The rigorous experiment design provided a thorough assessment of the developed glove by accounting for both parameters of size and weight of objects and by including subjects with different hand sizes. To quantify the glove's performance, the subjects' muscle activity, their finger and thumb joints' trajectories, and their grasping forces while using the glove were investigated. The glove could generate the necessary grasping forces to assist with lifting common-household objects. The subjects' muscle activity significantly decreased when using the glove for object manipulation. The trajectories of the index finger and thumb joints when using the glove were dependent on the size of objects similar to natural unassisted grasping. The obtained results demonstrate the glove's ability for grip power augmentation of individuals with declining hand strength.

Modelling and simulation of isothermal and continuous-heating surface decarburization behaviour of Fe-0.6C-1.8Si-0.8Mn spring steel

Steel surface decarburization leads to insufficient hardness in the surface area due to carbon depletion, reducing its fatigue life significantly. Various models have been proposed to simulate the decarburization behaviour. However, the influence of steel oxidation is either completely ignored or calculated separately, and the concentration-dependent carbon diffusivity in austenite is seldom considered. In this research, a numerical decarburization-oxidation integrated model based on the control volume method considering concentration-dependent carbon diffusivity was proposed. Isothermal and continuous-heating decarburization tests were conducted for a Fe-0.6C-1.8Si-0.8Mn spring steel in the air. Under isothermal holding between 700–1000 °C for 30–60min, the decarburization ferrite thickness rises with the increase of temperature, levelling off around 800–825 °C, and then starts to decrease. During continuous heating, the decarburization ferrite thickness increases at first to 76.1 μm at 840 °C and then starts to decrease to 61.6 μm after holding at 900 °C for 300s, finally rising to 65.8 μm after holding at 900 °C for 900s. Predictions of the proposed decarburization model were compared with experimental results, and a good match can be found between them. The significance of considering oxidation and concentration-dependent carbon diffusivity during modelling was emphasised as well.

Characterizing the Fatigue Damage in a Martensitic Spring Steel

Herein, a short crack propagation in a martensitic spring steel by means of in situ fatigue tests in a confocal laser microscope and additional electron back‐scattered diffraction analyses to link the local crystallographic orientation of the hierarchical martensitic microstructure with the short crack propagation behavior are investigated. For this purpose, different stress levels are imposed at a constant R ratio of −1 and a frequency of 10 Hz applying a sinusoidal command signal. It is found that the early fatigue damage evolution is characterized by the formation of slip bands, which subsequently serve as crack initiation sites. Most of the slip bands and correspondingly most of the short fatigue cracks initiate at or close to prior austenite grain boundaries. A strong dependence of the crack density, used as a parameter for fatigue damage, on the applied stress amplitude could be observed. The prior austenite grain boundaries can be identified to act as obstacles to short crack propagation because of the corresponding strong change in the crystallographic orientation, leading to an oscillating short crack propagation rate.

Enhanced strength and plasticity in a novel 55Si2MnMoV spring steel via austempering

A series of austempering heat treatments were carried out on the designed new spring steel to study the influence of various parameters on the microstructure and mechanical properties of 55Si2MnMoV steel. Experimental results indicate that excellent comprehensive mechanical properties can be obtained at 275 °C for 30 min, with ultimate tensile strength (UTS) and product of strength and elongation (PSE) being 2144 MPa and 25.26 GPa%, respectively. Transmission electron microscope (TEM) shows that the multiphase microstructure is composed of lath bainite ferrite (BF), martensite, and retained austenite (RA), and the RA with film-like structure is mainly distributed between bainite laths, while the fine carbides are embedded in the matrix. As the austempering temperature increases, the grain size increases gradually, while the strength decreases slowly. Austempering at 325 °C will produce a long-term transformation induced plasticity (TRIP) effect during tensile strain, making total elongation (TE) reach the maximum. While having ultra-high strength and good plasticity, the average rotating bending fatigue strength of the sample at 275 °C is 952 MPa and the fatigue ratio is 0.444, which is mainly determined by the reasonable partition of the multiphase microstructure at low temperature.

Simulation and experimental investigation of performance characteristics of dry and near dry EDM process

ABSTRACT In many industrial applications, necessity of new age materials; however some typical alloys like spring steel material are used in critical sector such as defence, aerospace, and automobile. To work on spring steel material, there are several difficulties such as hard to cut, low material removal rate, and high energy consumption. The EDM is critical thermal evacuation process. This paper presents the result of experimentation and simulation carried out on a spring steel material by varying current, gap voltage, cycle time and pressure. The experimental results are verified by developing a thermal-electrical model of workpiece material using ANSYS workbench. Transient thermal analysis is used to simulate the electric discharge machining. The finite element method (FEM) analysis includes spark radius variation and temperature profile formation by electric discharge. It is found that the dielectric fluid pressure is closely related to the discharge environment, such as debris concentration and fluid viscosity. Further, it is found that distilled water provides more stability to plasma channel in near dry electric discharge machining (NDEDM) compared to dry electric discharge machining (DEDM). From the results obtained, it is concluded that the material removal rate is affected by dielectric medium properties viz. dielectric strength, dielectric constant and thermal conductivity.

New approach to heat treatment of spring steel

In the 54SiCr6 spring steel, a spheroidized structure was achieved by two processing methods: conventional soft annealing above the temperature A1 in a furnace and accelerated ASR annealing by an induction heating equipment. The ASR technology is much faster, even for materials with a worse tendency to the spheroidisation, such as steels with higher silicon content. Thanks to this, a finer structure is obtained. This is the basis of other advantages for the subsequent hardening. Quenching can be done at lower temperatures, higher yield and tensile strength is obtained while maintaining same or improved ductility and contraction. This also results in influencing of fatigue behavior.

Quantifying deep cryogenic treatment extent and its effect on steel properties

Deep cryogenic treatment (DCT) has been known since the 1930s to improve hardness, fatigue resistance, and wear resistance of steel. While the effect of DCT on wear properties has been well documented, there is no consensus regarding the causal mechanisms, nor a widely accepted quantitative description of them. DCT transforms retained austenite into martensite and triggers the precipitation of fine carbides, among other things. We observed that DCT had a negligible effect on Young's modulus and the yield limit of high carbon spring steel. The observed microstructural changes (presence of specific dendritic inhomogeneities typical for inclusions of austenitic phase in non-treated specimens and homogeneous microstructure of treated ones) can serve for qualitative purposes only. However, we found that DCT led to a decrease in steel electrical resistivity which can be explained by noticeable differences between the resistivities of the martensitic and austenitic phases. We propose a micromechanical model for electrical resistivity which allows monitoring of the content of retained austenite and postulate that it can be used for other materials as well. We also observed increased resistivity after mechanical loading of the specimens, correlating with increased dislocation density caused by loading. This quantity can be used to assess the average dislocation density.

Influence of carbides on the strain hardening behavior of 60Si2CrVAT spring steel treated by a Q&T process

The strain hardening behavior was studied in ultra-high strength 60Si2CrVAT spring steel subjected to quenching and tempering (Q&T) treatments via tailoring the carbides precipitated during tempering at 400–530 °C. Results showed that the strain hardening characteristic of the spring steel changed from a single-n upwardly convex (tempering temperatures below 470 °C) to a double-n downwardly convex (tempering temperatures equal to or higher than 470 °C), which is more suitable for cold coiling process. For the double-n state, the value of the strain hardening exponent(n) in the low-strain region of uniform plastic strain (n1) gradually decreased while the value of n in the high-strain region of uniform plastic strain (n2) gradually increased with tempering temperature increasing. It was attributed to the continuous decrease in initial dislocation density of matrix with tempering temperature increasing and the continuous increase in geometric necessary dislocations (GNDs) during cold deformation, which was significantly affected by the quantity, size, distribution and related microscopic characteristic parameters of carbides. The strengthening effect corresponding to the high-strain region is proportional to the value of n2 and the carbide particle size parameter (f/d).

Design, Fabrication, and Performance Analysis of Intelligent Mesoscale Capacitive Accelerometer for Vibration Measurement

Abstract This paper portrays an accelerometer as a motion sensor used to measure acceleration, vibration, and tilt angles which plays a vital role in the constructional field, inertial measurements, automotive safety system, biomedical, and yet other safety applications. A small packaged accelerometer can be made larger in size by acquiring an external power source or battery. The designed micro-accelerometer is fabricated using clean room technology, which is not quite cost effective, and even the silicon base is replaced by steel because of steel’s high yield strength and low cost to fabricate a mesoscale accelerometer. Design parameters are optimized by a Decision Tree Algorithm where the optimized dimensions are used to model the mesoscale accelerometer in INTELLISUITE 8.6 software. A Wire-cut discharger is used for the fabrication of mechanical elements of an accelerometer with a spring steel foil to reduce fabrication cost. Finally, the fabricated accelerometer with a mesoscale was made to undergo experimentation with a hammer drill and compared with simulation results to evaluate its performance in a harsh construction environment. 0.576 (μm/g) of mechanical sensitivity and 0.629 (pF/g) of electrical sensitivity is achieved.

Modelling and optimization of sulfur addition during 70MnVS4 steelmaking: An industrial case study

Štore Steel Ltd. is one of the major flat spring steel producers in Europe. Among several hundred steel grades, 70MnVS4 steel is also produced. In the paper optimization of steelmaking of 70MnVS4 steel is presented. 70MnVS4 is a high-strength microalloyed steel which is used for forging of connecting rods in the automotive industry. During 70MnVS4 ladle treatment, the sulfur addition in the melt should be conducted only once. For several reasons the sulfur is repeatedly added and therefore threatening clogging during continuous casting and as such influencing surface defects occurrence and steel cleanliness. Accordingly, the additional sulfur addition was predicted using linear regression and genetic programming. Following parameters were collected within the period from January 2018 to December 2018 (78 consequently cast batches): sulfur and carbon cored wire addition after chemical analysis after tapping, carbon, manganese and sulfur content after tapping, time between chemical analysis after tapping and starting of the casting, ferromanganese and ferrosilicon addition and additional sulfur cored wire addition. Based on modelling results it was found out that the ferromanganese is the most influential parameter. Accordingly, 12 consequently cast batches (from February 2019 to October 2019) were produced with as lower as possible addition of ferromanganese. The additional sulfur addition in all 12 cases was not needed. Also, the melt processing time, surface quality of rolled material and sulfur cored wire consumption did not change statistically significantly after reduction of ferromanganese addition. The steel cleanliness was statistically significantly better.

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.

Optimization of the Continuous Casting Process of Hypoeutectoid Steel Grades Using Multiple Linear Regression and Genetic Programming—An Industrial Study

Štore Steel Ltd. is one of the major flat spring steel producers in Europe. Until 2016 the company used a three-strand continuous casting machine with 6 m radius, when it was replaced by a completely new two-strand continuous caster with 9 m radius. For the comparison of the tensile strength of 41 hypoeutectoid steel grades, we conducted 1847 tensile strength tests during the first period of testing using the old continuous caster, and 713 tensile strength tests during the second period of testing using the new continuous caster. It was found that for 11 steel grades the tensile strength of the rolled material was statistically significantly lower (t-test method) in the period of using the new continuous caster, whereas all other steel grades remained the same. To improve the new continuous casting process, we decided to study the process in more detail using the Multiple Linear Regression method and the Genetic Programming approach based on 713 items of empirical data obtained on the new continuous casting machine. Based on the obtained models of the new continuous casting process, we determined the most influential parameters on the tensile strength of a product. According to the model’s analysis, the secondary cooling at the new continuous caster was improved with the installation of a self-cleaning filter in 2019. After implementing this modification, we performed an additional 794 tensile tests during the third period of testing. It was found out that, after installation of the self-cleaning filter, in 6 steel grades out of 19, the tensile strength in rolled condition improved statistically significantly, whereas all the other steel grades remained the same.

A simultaneous improvement of the strength and plasticity of spring steels by replacing Mo with Si

A synchronous improvement of the strength and plasticity (SISP) is a significantly important subject attracted by many researchers. In this study, the key factors affecting the plasticity and strength of spring steels were analyzed and discussed in detail. Meanwhile, the differences in the microstructures and fractographies between two Cr–Mn steels were compared. It is found that the 50CrMnSiVNb spring steel has higher strength as a result of the dislocation strengthening, the solution strengthening caused by the Si solid solution as well as higher work-hardening capacity by comparing four strengthening mechanisms and work-hardening capacity. Furthermore, the better plasticity of the 50CrMnSiVNb steel may be mainly attributed to the lighter banded segregation and hydrogen embrittlement compared with the 50CrMnMoVNb steel. This study may enrich the fundamental knowledge regarding how to optimize the composition of spring steel and provide some new ideas for synchronous improvement of the strength and plasticity of spring steels.