# Study of the Rotary Bending Fatigue Resistance of 30 MnB5, 41 CrS4 and 30 MnVS6 Steels
## 1 Introduction
The starting microstructure of this steel is, fundamentally, pearlitic, with the presence of some particles of ferrite and cementite [14; 15; 16], obtaining a microstructure formed mainly by tempered martensite [15; 16] after heat treatment. One of the advantages of 30 MnB5 steels is that quenching and tempering makes the steel stronger and harder, thus increasing the durability of the final components that usually have a useful life that is double or triple that of items made from 65 G grade steels [17]. 41 CrS4 (1.7039) steel is also a quenched and tempered steel, and the fundamental difference to the previous one is that it is a chromium steel (between 0.9 and 1.2%) and does not contain boron. This type of steel is also used in the automotive industry [18]. It has mechanical properties that can be comparable to those of the referenced boron steel [19]. Both boron steel (30 MnB5) and chromium steel (41 CrS4) are steels that require heat treatment for quenching and tempering to provide the required characteristics, so they can be replaced by other materials that avoid this process, thus saving energy costs and manufacturing time associated with it [20], which constitutes a search of notable interest. In this regard, microalloyed steels are an excellent alternative since they can have equivalent mechanical characteristics without the need to undergo any additional heat treatment. These steels can be presented in different structural forms, which are obtained by varying the composition of the microalloys [21; 22]. However, it is necessary to ensure that, in addition to the savings benefits achieved by avoiding heat treatment, the required mechanical properties are maintained and have adequate resistance to fatigue [23]. A proposal to replace the previously mentioned reference quench and tempered steel (30 MnB5) in the manufacture of commercial vehicle axles is 30 MnVS6 steel (1.7225). Thissteel is a vanadium microalloyed steel with a high sulfur content, and that can be hardened by precipitation. During heating in the manufacturing process, vanadium dissolves in austenite, subsequently precipitating in the form of fine V (C, N) particles, which favor the appearance of proeutectoid and pearlitic ferrite [20; 24]. The sulfur content improves the machinability properties of steel [25; 26]. The steel currently used in the manufacture of this type of element, vehicle axles, does not present a specific problem that makes its substitution mandatory. However, if it were possible to find a material that improves the response to fatigue of the reference, this would imply an increase in the useful life of the parts. In this way, a clear economic benefit would be obtained. The alternatives proposed for this purpose must ensure, at least, mechanical properties (strength) similar to commercial steel. The main objective of this research is twofold. On the one hand, evaluating whether a minimal modification in the mechanical properties of the reference steel, 30 MnB5, that represents an improvement also implies a benefit in fatigue behavior. On the other hand, to determine if the two steels proposed as alternatives (the second for quenching and tempering, 41 CrS4, and the microalloyed steel, 30 MnVS6) can adequately replace conventional steel in the manufacture of axles in heavy vehicles. For this, in addition to complying with the requirements regarding mechanical properties, the fatigue behavior of both steels will be evaluated to ensure that the behavior of the proposed alternative is, at least, similar to the reference one. For this purpose, rotary-bending fatigue tests will be carried out according to ISO 1143: 2021 Metallic materials--Rotating bar bending fatigue testing [27] on the four types of steels, and the observed fracture mechanisms will be evaluated, analyzing the fracture surface with a Scanning Electron Microscope (SEM).