Optimizing Stainless Steel Bearings: The Impact of Low-Temperature Forming on Bearing Life

1. Introduction

Due to the global economic and ecological change, the usage of innovative environmentally friendly and resource-saving technologies is in the focus of current research. In addition to the application of light metals, ultra-high-strength steel alloys and load-adapted components structures with improved material properties are increasingly being used. By adding alloying elements and targeted heat treatment, properties such as hardness, residual stresses, yield strength, tensile strength, elongation at break, toughness, fatigue strength, heat strength, machinability and corrosion resistance of the steel can be varied in many ways and adapted to the respective requirements. To manufacture highly resilient components, manufacturing processes of forming technology are often used. Besides densification sub surface compressive residual stresses can be achieved by forging processes. For example, in terms of achieving a beneficial sub surface structure forging processes had been in the focus also in the project tailored forming. Here a forging process is performed after a laser cladding or plasma welding process on bearings made of a AISI 52100 raceway on AISI 1022M structure material or martensitic chromium silicon steel (1.4718) on a base substrate of S235 (1.0038) steel [1, 2]. These forming processes could serve for an improved surface quality, the temperature had to be adjusted to enable an adequate forging process. The positive effects of residual compressive stresses on the service life in component areas subject to the highest rolling stresses have been known for a long time [3-5]. Previous studies could prove a beneficial effect of preinduced compressive residual stresses on bearing fatigue life for ALSI52100 bearings. These residual stresses were induced by deep rolling and turn-rolling [6-8]. In the present study the optimization of bearings made of stainless steel is in the focus. Such bearings are used for application in food industry, paper industry or for chemical processes as mass product [9]. But such bearings can be also found as high strength stainless steel bridge roller bearings [10]. Also, to withstand high temperature oxidation and corrosion aircraft bearings are made of stainless steels [11]. Thus, the idea is to achieve a beneficial residual stress state subsurface the bearing by improved forging processes. In this case a special focus must be set on a low temperature forging process. In general, forming at low temperatures, due to work hardening, sets higher material strengths in the component. During the forming of metastable austenitic steels, the resulting hardening is significantly increased by an additional deformation-induced martensitic phase transformation (see Figure 1 on the left). The phase transformation is accompanied by the introduction of compressive stresses due to the increase in volume of the martensitic phase.

Martensitic Phase Transformation in Forging

In the cold forming of metastable austenitic steels, in addition to work hardening, an additional strength-enhancing phase transformation occurs, which is induced by externally introduced stresses and strains. Most stainless austenitic CrNi steels with approx. 18% Cr by mass and 10% Ni by mass are metastable austenitic in the non-deformed state. During forming, a partial conversion of the cubic face-centered (kfz) phase of the austenite into a cubic space-centered (krz) martensitic phase can take place [12]

The metastable austenitic stainless steels are mainly used in the chemical and food industries, e.g. for condensers, heat exchangers, coolers and pipelines. These are mainly sheet metal components. On the other hand, the effect of deformation-induced martensite formation in the field of solid forming has so far been little used industrially. These stainless steels are well known for applications for excavator bucket teeth and forklift tines, which harden due to mechanical loads in use.

Figure 1 shows the basic deformation-induced transformation process and the resulting solidification. The formed martensite consists of ferritic α’-martensite, which has a tetragonal-distorted fcc structure, and unstable ε-martensite with an hcp structure. This can be converted into α’-martensite (bcc) with further stress [13] The phase transformation from austenite to martensite caused by deformation is generally referred to as deformation-induced martensite formation.

Furthermore, there is a dependence between the deformation-induced martensite formation and the forming temperature. By cooling the samples to T = -5 °C, an increase in the martensite content is possible compared to the experiments at room temperature. An increased forming temperature, on the other hand, reduces the effect of martensitic phase transformation (Figure 2). At a temperature of T = 60 °C, martensite formation is almost completely suppressed. This temperature dependence has already been described by [15], among others.

The deformation-induced martensite formation and the associated expression of the yield stress of a metastable austenitic CrNi steel are complex mechanisms that are largely determined by alloy components and the parameters temperature as well as stress and strain state. There is a lot of research describing the fundamental effects of phase transformation in metastable austenitic steels [12,15,16]. Specifically, in the field of forging, only a few research papers are known in which the effect of deformation-induced phase transformation and its applications are investigated [17,18].

In the past, several doctoral theses have been written at the Bergakademie Freiberg, the subject of which is the occurrence of martensitic phases due to mechanical stress on the components close to the operation [19,20]. Currently, within the framework of the CRC 799 "TRIP-Matrix Composite", metastable austenitic CrMnNi steel casting materials with TRIP effect are being investigated [17].

At the University of Kaiserslautern, the influence of near-operational isothermal and thermal-mechanical stresses on the deformation behavior of metastable austenitic steels is investigated in the project "Influence of low temperatures on the cyclic deformation and transformation behavior of differently processed metastable austenites" as part of the Research Group "Engineering Materials on Different Scales: Experiment, Modelling and Simulation" [23].

Roller Bearings

Bearings enable a relative movement of machine parts and thus represent the concrete shape of joints in the sense of gear theory. Bearings take on both load-bearing and guiding tasks. The "carrying" function focuses on transmitting forces and moments between parts that are moving relative to each other. Depending on the direction of the forces to be transmitted, a distinction can be made between radial, axial and angular contact bearings. The "Guide" function determines the relative position of the parts that are moving relative to each other as precisely as possible. Non-locating bearings are used for guidance in the radial direction, fixed bearings additionally or exclusively for axial determination [23].

The relative motion between the active surfaces of a bearing can be sliding or drilling (tangential), rolling (radial) or rolling. Rolling is a combination of sliding and rolling motion. Sliding movements of solid-state surfaces in direct contact require a comparatively high amount of force and energy. At the same time, particles can detach, leading to surface wear during operation. This results in an essential requirement for a machine element: the function must be ensured or fulfilled for as long as possible. This means that destruction or progressive damage resulting in the eventual failure of the active surfaces or active bodies and an immediate or gradual change in the geometry with impairment of the guiding properties must be avoided or delayed for as long as possible. Typical mechanisms that can lead to functional impairment are breakage, plastic deformation, melting, seizure, fatigue, surface breakdown and wear [23].

In addition to adhesive and abrasive wear in conjunction with tangential relative movements and friction under unfavorable operating and lubrication conditions, surface fatigue is a common cause of failure of rolling bearing systems. Increasing the reliability of rolling bearing assemblies by avoiding early failures as well as increasing bearing life are therefore the main concerns in the development of rolling bearings. As a result of improved bearing designs, they can not only achieve a longer bearing life, but can also be used in smaller, lighter and low-friction designs with the same service life. Kloos and Broszeit define surface fatigue as "the separation of microscopic and macroscopic material particles (…) caused by fatigue cracking, crack progression and residual fracture during the rolling of two force-bound surfaces (with and without slippage) under certain mechanical, thermal and chemical stress conditions" [24]. In contrast to wear, surface fatigue is a typical form of damage caused by material chipping instead of the removal of the material. Wear and surface fatigue are system properties of a component pairing, while general fatigue is a component property. In this work angular contact ball bearings were used, see Figure 3. These bearings allow radial and axial loading, but in addition to rolling also experience spinning motion with additional tangential shear forces [25]. Most damages occur at the inner ring, as it is the part of a bearing that sees the highest loads. Therefore, the inner ring was the part that was manufactured with the aforementioned process. For the testing the inner ring (as seen in Figure 3) was combined with a bought-in outer ring and balls.

2. Test Rig Development and Bearing Performance Tests

At the IMKT a novel test rig has been constructed on the foundational structure of a pre-existing SKF-manufactured machine designed for radial loading (as seen in Figure 4). This upgraded test rig boasts the capability to concurrently test four bearings across two test heads. With a total of four machines featuring this configuration, a maximum capacity of testing up to 16 bearings simultaneously has been achieved. The two test heads are both connected to a central main shaft, interconnecting each unit’s secondary shaft via hydraulic couplings.

The support mechanism for the test heads involves radial suspension utilizing a rod, with provision for attachment of a force sensor to measure the Friction Torque exhibited by a pair of bearings. Furthermore, the axial load is applied through a set of flat springs situated on the outermost periphery of the test head. This applied force is monitored using a load sensor, ensuring precise control and measurement during testing procedures. A cross section of the test head is shown in Figure 5.

Each individual bearing within this setup is equipped with a temperature sensor positioned on its outer ring, enabling monitoring of thermal variations. Regrettably, due to spatial constraints, the installation of a temperature sensor on the inner ring of the bearing was deemed impractical and therefore not implemented.

The implementation of a free-floating support system for the test head was chosen for its simplicity and capabilities in regard to the measurement of friction torque, constituting a pivotal advantage. However, this advantageous setup introduces its own set of challenges, primarily revolving around the need for precise alignment during the installation phase. The precise alignment of the secondary shafts within the hydraulic couplings has shown to be a critical factor, influencing especially the baseline vibrations recorded throughout the tests.

Moreover, the design decision for the free-floating support structure inherently subjects the bearings to radial loading, a factor demanding careful consideration. Evaluating the mass of the test head to approximately 38 kg, allows for an estimation of the radial loading exerted on each bearing, which can be conservatively estimated to reach 200N or below. While this magnitude of radial loading warrants attention, preliminary calculations suggest that its impact on the anticipated life expectancy of the bearings appears negligible, with no discernible substantial influence identified through initial assessments.

In addition to the measured friction torque and temperature values, the condition monitoring of the bearings in the test is ensured by a vibration sensor system attached to the front side of the test heads. With this, it is possible to detect any damage that forms in a timely manner.

Experimental Setup

When selecting the operating parameters for life expectancy tests, a critical consideration revolves around the distinctive properties of the utilized materials. Given the limited knowledge regarding the fatigue limits of bearings made from austenitic steels, a pragmatic approach was applied to derive the operational parameters. Consequently, an outlined solution was proposed to address this challenge. The initial step involved identifying the limit for a standard bearing possessing identical outer dimensions, readily accessible through manufacturers’ catalogs. A comparison of the material properties between bearing steel and austenitic steel revealed a notable reduction in the latter’s capabilities. In the literature the topic of bearing life for stainless-steel bearings has not seen a broad attention. So, for an initial estimate the technical manual of the bearing manufacturer has been taken, where it was proposed that a reduction to 80% of the load applied on a bearing of same dimensions, but made from standard bearing steel, has shown to be sufficient to ensure the stainless-steel bearings can endure a similar runtime under load.

Consequently, a reduction in load was implemented to achieve a comparable fatigue life using the following approach:

For the standard 7206 bearings, operating within a Hertzian Pressure limit of approximately 2.2 GPa and subjected to an axial load of 17500N, the maximum capacity of Stainless-Steel Bearings is conservatively estimated at 80% of that of the standard 100Cr6. Given the recognized lower hardness of 1.4301 in its austenitic state, a further reduction in the load by an additional 5% was undertaken. Therefore, the axial load for the series can be written as:

$$F_a=17500 \ast \mathrm{0,75}\cong 13 kN$$

Another important point for the operating conditions is the height of the lubrication film and the state of lubrication. The parameter that has been considered is the Specific height of lubricating film $\Lambda$ which can be calculated from the roughness of the two contacting surfaces and the calculated minimal film height.

$$\Lambda =\frac{h_{min}}{\sqrt{R_{q,2}^2+R_{q,1}^2}}$$

However, it is important to add, that the measurement for the initial surface state of the Bought-in variants could only be determined on a small number of exemplary units. It was not possible to reliably disassemble the bearings without damaging them. Therefore, a small number of randomly chosen bearings were pulled from the test pool and investigated for their surface structure beforehand. As those numbers were showing a small deviation from each other, the then obtained values were taken as the initial state of all. An overview on the test parameters is given in Table 1.

In the test cycle, the bearings were lubricated using a grease for high-speed applications. This specialized grease was selected to accommodate the relatively low load used in this test regimen. (DATA!!!).