1 Introduction
Disc springs are an essential part of mechanical engineering, where high spring force is required in a small installation space [
1]. In applications dominated by dynamic cyclic loading, discs should have high fatigue strength and show little stress relaxation to avoid degradation and failure during service life. Due to their good combination of high strength, low stress relaxation tendency, high corrosion resistance and ductility, metastable austenitic stainless steels (MASS) are a popular choice for disc springs. MASS are also identified as transformation-induced plasticity (TRIP) steels because either during forming or service life a strain-induced austenite-martensite transformation takes place under deformation. Two variants are usually found in austenitic steels, i.e., austenite phase (
\(\gamma )\rightarrow \varepsilon\)-martensite phase and austenite phase (
\(\gamma )\rightarrow \alpha '\)-martensite. In essence, the martensite transformation depends on the material’s critical free energy. The austenite grains are stable at a higher temperature. At a lower temperature, the martensite exists usually in meta-stable state [
2]. The formation of the
\(\alpha '\)-martensite phase is crucial as it improves the material’s work hardening. This enhancement in work hardening increases the resistance to further plastic deformation at room temperature and is accompanied by expansion in
\(\gamma \rightarrow \alpha '\) phase transformation [
3,
4].
However, the possible use of disc springs is constrained by the buildup of residual tensile stresses during the manufacturing process. In order to create compressive stresses in regions that may show tensile stresses after forming, shot peening operations are typically used. During the shot peening process, tiny metallic balls impact the surface and create compressive residual stresses near the surface. Shot peening of disc springs impedes the nucleation of fatigue cracks [
5]. Furthermore, shot peening operations also induce martensite (deformation-induced martensite—DIMT) in MASS disc springs. Scherpereel et al. [
6] reported that larger residual compressive stresses are developed in the area where the magnitude of martensite content is highest. Fu et al. [
7] studied the properties of 18CrNiMo7‑6 steel after shot peening. He found the maximum compressive stress at a depth of 20
\(\mu m\) for the optimized shot-peening process. The effect of shot peening of the AISI 301 LN steel in an annealed and cooled rolled condition is studied by Fragas et al. [
8]. They found more extensive plastic deformation and martensite formation in the annealed specimen compared to cold rolled samples. Furthermore, they also reported the drawbacks of the traditional shot peening method due to its stochastic nature. The result of the process is highly dependent on the selected parameter values [
8]. The shot peening operation on one hand increases the residual stress but causes adverse effects on the surface topology i.e. superficial micro cracks, which decreases fatigue life [
9,
10]. Another major drawback of this process is that it is difficult to control the residual stress distribution. Radial residual stresses change the height of the spring [
11]. The residual stress in radial direction should hence be minimal and is not possible to control the stress components individually in shot peening. Moreover, shot peening is postproduction process and suitable for the bulk production. For short lot it appeared as an expensive procedure and has limitation especially once this process is required for complex geometries.
Incremental sheet forming (ISF) has been introduced to replace the shot peening process. It is appropriate for economical batch production, in order to promote tangential residual compressive stresses selectively in the disc springs and enhance the industrial manufacturing process. A small hemispherical tool forms a sheet blank locally across a specified path resulting final formed shape during operation. The ISF process is characterized by its localized deformation mechanism, allowing sheet metal to be stretched far beyond the conventional forming limits [
12,
13]. The ISF process has two main variants, i.e., single point incremental forming (SPIF) and two-point incremental forming (TPIF). The localized forming process results in high residual stresses. The evolution of this residual stress in ISF is investigated in several studies [
14,
15]. Due to its adverse effect on geometrical accuracy, techniques like stress relief annealing are used to minimize the residual stress in ISF [
16,
17]. Maaß et al. [
18] have investigated the aluminum alloy 5083 and formed linear grooves by the multiple-stage process. They measured the residual stress state via X‑ray diffraction in a complex shape component and concluded that the effect of tool step down is unincisive. They detect a relationship between the parameters of the forming process, the residual stresses, and mechanical characteristics of an incrementally formed part [
19]. Katajarinne et al. [
20] established a progressive method for adjusting material properties, including ductility and strength for the parts produced by incrementally shaping metal sheets. They controlled properties by controlling the martensitic transformation induced by deformation. They also investigated the dependence of the martensitic transformation in MASS on the ISF process parameters. Turski et al. [
21] adopted different surface treatment process parameters and studied the residual stresses evolution in the sub-surface of AISI 301LN and AISI 316L steel components. They investigated that 301LN showed 100% martensite transformation at a maximum 60° forming angle. For the 316L grade, only 20% transformation is measured for the respective conditions. Further, the higher rates of formation are converted into an increase in temperature. This increase in temperature suppresses the transformed fraction.
Two strategies for the targeted generation of residual stresses in MASS disc springs by ISF were proposed in previous studies [
12,
22,
23]. The effect of the ISF process parameters, i.e., the forming tool diameter, feed rate, and step-down, on the generated residual stresses are investigated in detail using X‑ray diffraction. The role of residual stresses present in the material prior to ISF was not studied in the past. The present study aims at elucidating the role of residual stresses present in MASS after the rolling operation, i.e., whether the capability of controlling the residual stresses using ISF is affected by prior residual stresses.
4 Conclusions
Previous research indicates that ISF can regulate radial and tangential stresses in the springs. However, no study on the impact of residual stress conditions in the rolled sheet strips was done with elevated temperatures. The aim of the analysis is to examine residual stress during rolling and its consequent effects on the forming of disc springs. Disc springs were carried out with a negative die on a TPIF set-up. The results of the process parameters on the stresses caused by ISF were observed. The findings demonstrate the significance of close regulation of the rolling temperature, and that ISF is able to yield strong compressive residual stresses and better spring characteristics when smaller tool diameters and step-down values are used. The following conclusions can be drawn from the investigations:
-
A considerable build-up of martensite is observed in the rolled sheet of EN 1.4310 produced at room temperature. Rolling at elevated temperatures decrease the martensite contents and residual stresses.
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EN 1.4401 does not develop martensite after rolling and ISF. Even though the generation of residual stresses depends on plastic deformation of the austenite, the same tendency is observed as with EN 1.4310, i.e., the largest residual stresses are observed when the steel is cold rolled. This shows that temperature control in rolling (active cooling to counteract the effects of dissipation and friction) is vital in order to achieve reproducible residual stress states.
-
The highest compressive residual stresses are found in EN 1.4310 and EN 1.4401, when cold rolled sheet is incrementally formed. Residual stresses in formed disc springs are less by rolling at elevated temperature in both materials EN 1.4310 and EN 1.4401.
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A smaller tool diameter could induce a more residual stresses compared to a larger tool diameter. Increasing the tool step down resulted in a decrease in residual stress induction. The variation of the feed rate had no significant effect on the residual stress properties of the springs.
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The residual stress analyses and force-deflection tests indicate that incrementally formed springs have better spring characteristics than the conventionally formed ones and ISF process improves the magnitude of the induced residual compressive stresses.
However, it is essential to analyze the stability of the residual stresses under service conditions. The cyclic testing will be conducted on the manufactured disc springs to describe the effect of the prior forming process, i.e., rolling, on the fatigue life.