Study of subsurface initiation mechanism for white etching crack formation

Abstract

The present paper describes a novel way to detect early stages in the gradual transformation of SAE 52100 bearing steel material to white etching cracks (WEC). The underlying transformation is recorded and investigated by a complementary use of Barkhausen noise measurements, ultrasonic measurements and scanning electron microscopy. While ultrasonic measurements can only be used to detect cracks in a failed component, the recently improved Barkhausen noise measurement technique can be used to detect possible early stages of microstructural transformation. A cross section from a region without ultrasonic signal but with a Barkhausen signal has been investigated by the use of scanning electron microscopes in order to reveal possible pre-stages of WEC formation. These findings support that WEC are locally initiated in subsurface regions. Within those modified regions, carbides start to dissolve in consequence of deformation accumulation, which has been identified as an early state of WEA microstructure formation. This paper is part of a Themed Issue on Recent developments in bearing steels.

The Evolution of White Etching Cracks (WECs) in Rolling Contact Fatigue-Tested 100Cr6 Steel

“…This is proposed to be due to the steel experiencing a failure-free period (no WECs detected), in which energy is absorbed (explained by Barkhausen noise (BN) measurements [54]), a limit being reached with a sudden release of WECs. This failure-free period could be local subsurface transformations that have been observed as 'crack-free' dark etching regions suggested to lead to the formation of WEA and subsequently WEC [39]. This investigation shows that WECs do exist during this period before a sudden rupture occurs, ~ 20% outstanding RCF time corresponding to 14.4 h, WECs being recorded between 4 and 12 h.…”

“…The energy generated during crack rubbing leading to amorphisation is also proposed to be sufficient to dissolve large amounts of carbides in the WEA [16]. A counter argument to crack rubbing comes through subsurface inspection using Barkhausen noise measurements, where subsurface changes are investigated without the presence of cracks [39]. A local transformation in the microstructure is observed as 'crack-free' irregular dark etching regions and is suggested to lead to the formation of WEAs.…”

Accelerated White Etch Cracking (WEC) FE8 type tests of different bearing steels using ceramic rollers

“…This can include using hydrogen pre-charging of the specimens [26][27][28], using specific lubricants [29][30][31], introducing excessive slip [32][33][34] or external electrical currents [35,36]. One of the most common accelerated WEC tests is the FE8 type test rigs with axial cylindrical roller bearings, which have been extensively used for studying WEC [7,14,15,17,24,29,30,[37][38][39][40][41][42]. It relies on the inherent slip in 81212 type bearings, as well as using WEC inducing lubricants.…”

FE8 type laboratory testing of white etching crack (WEC) bearing failure mode in 100Cr6

“…Replicating WEC using laboratory scale bearing test rigs under pure mechanical load have only been successful in a few setups without resorting to highly aggressive conditions such as hydrogen pre-charging [18,19] or stray currents [20,21]. Currently the most established method of reproducing WEC under pure mechanical load is by using cylindrical roller thrust bearings (axial bearings) under constant loading conditions with FE8 type test rigs [22][23][24][25][26][27][28][29] originally intended to be used for testing lubricants standardized in DIN 51819-1:2016-12 [30]. By using lubricants with a chemical composition which promotes WEC formation, this test setup using constant load is a reliable way of forming WEC.…”