PSI - Issue 42

Margot Pinson / Structural Integrity Procedia 00 (2019) 000–000

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472 © 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of 23 European Conference on Fracture - ECF23 Keywords: Bearing steel; Hydrogen embrittlement; In-situ bending test; Martensitic steel; Sustainability Margot Pinson et al. / Procedia Structural Integrity 42 (2022) 471–479

1

Introduction

Since the transport sector is responsible for about 25% of the total CO 2 emissions (Solaymani, 2019), the development of new high strength and low density materials is imperative to meet the increasingly stringent environmental standards. Al containing steels are a valid starting point since the molar mass of Al is about half of that of Fe. Recently, a new steel alloy, i.e. Fe-8Al-1.1C, has been engineered which has a high weight fraction (8.09 wt%) of aluminum thereby lowering the density substantially when compared to traditional bearing steels such as 100Cr6 (Li et al., 2016). Another advantage of this innovative steel alloy is that it contains fewer alloying elements than traditional steels effecting a reduction in economic and ecological cost. Bearings, for example, are indispensable components within the transport sector because their main function is to transfer forces between different machine parts. Since bearing steels are subjected to high loads, they need to possess a high enough abrasive resistance, fatigue limit, strength and hardness level which can be obtained by the martensitic microstructure. In addition, bearings are constantly lubricated with oil-based lubricants that can decompose into atomic H due to friction and associated heat generation (Stopher & Rivera-Diaz-del-Castillo, 2016). Owing to its small dimensions, H can diffuse very easily trough the metallic crystal lattice, where it can cause damage in the steel structure, i.e. hydrogen embrittlement (HE). In literature can be found that the susceptibility to HE is proportional to the hardness of the steel (Depover et al., 2014; Pérez Escobar et al., 2012; Pinson et al., 2020). Therefore, it can be expected that bearing materials will suffer to a high extent from HE. The presence of inclusions also plays an important role since they are considered as initiation sites for the so-called white etching cracks (WECs) that are considered to be the preliminary microstructural damage mode in bearing steels (Evans, 2012). Diffusible H even further enhances the formation of WECs (Evans et al., 2013). However, there are also studies that indicate that the presence of nanosized carbides reduces the HE susceptibility by trapping diffusible H (Bhadeshia, 2012; Szost et al., 2013). Nonetheless, the efficiency of carbides as H traps depends to a high extent on the carbide-matrix interface coherency and the carbide size (Man Lee & Young Lee, 1987). In bearing steels, small carbides ( ∼ 10 nm) are favored due to their large number density and strengthening effect. Large incoherent carbides (>20 µm), on the other hand, have a detrimental effect (Bhadeshia, 2012). However, the effective failure mechanisms also depend on additional factors such as the chemical composition, microstructure, and H quantity which make it hard to predict failure and to implement appropriate design considerations. Therefore, the goal of this paper is to compare the HE behavior of two bearing steels; an industrial (100Cr6) and a lightweight alternative (Fe-8Al-1.1C). The obtained conclusions may contribute to the further development of lightweight bearing materials with a better resistance to H induced failure. The chemical composition of the two bearing steels is found in Table 1. Both materials have a high C content which contributes to an increased wear resistance and hardenability. The high Al content of the Fe-8Al-1.1C steel results in a very good atmospheric corrosion resistance and a lower density (  Fe-8Al-1.1C = 6.95 g/cm 3 ) compared to the industrial 100Cr6 material (  100Cr6 = 7.61 g/cm 3 ). Therefore, the Fe-8Al-1.1C steel will hereafter be referred to as ‘lightweight bearing steel’ and the 100Cr6 steel will be named ‘industrial bearing steel’. The chemical composition of the industrial bearing steel is in agreement with the ASTM 52100 standard for high carbon bearings (He, n.d.). 2 Materials and methods

Table 1: Chemical composition in wt.% of the industrial 100Cr6 and lightweight Fe-8Al-1.1C bearing steel

Material

C

Al

Si

Mn

Cr

Ni

Mo

P

S

100Cr6

0.94

0.03

0.30

0.33

1.48

0.20

0.09

0.01

90 ppm

Fe-8Al-1.1C

1.11

8.09

-

-

-

-

-

-

<10 ppm

Both alloys are hot rolled at 1150°C and afterwards they undergo several heat treatment stages based on literature (Li et al., 2016; Springer et al., 2019). Due to the high amount of C, both materials be subjected to a soft annealing