Heat treatment and materials Countermeasures

 

Heat treatment and new materials as remedies against white etching areas / white etching cracks

White Etching Areas (WEA) and White Etching Cracks (WEC) have been identified as significant links in the causal chain of premature rolling bearing damage between 1 and 20% of the calculated life (L10). The macroscopic damage pattern of the affected rolling bearings is characterized by axial cracks and breakouts on the inner and outer ring raceway as well as the rolling elements.

The formation of WEA/WEC is dependent on various drivers. These include contact conditions, stress state, lubricant, material, hydrogen, and electric current and temperature. In the FVA 707 project series, the damage mechanism of WEA/WEC was investigated in terms of which reproducible conditions and drivers favor the formation of WEA/WEC. However, no validated economic measures for the prevention of WEA/WEC could be derived for practical applications so far. The focus of current research is the description of the material influence on WEA/WEC development. The field of materials and heat treatments offers a wide range of scientifically possible solutions, but these are severely limited by economic and competitive constraints.

The objective of this study was to identify pre-competitive trends and issues related to materials engineering WEA/WEC countermeasures in rolling bearings. Based on this, materials and material-technological countermeasures were to be evaluated and defined in order to enable targeted research in follow-up projects.

In order to achieve this goal, the literature was first reviewed. In addition to the various materials, heat treatments and coatings were also considered. From over 1200 literature references searched, 73 sources reporting on potential WEA/WEC countermeasures were included in the results. The classification scheme used (Figure 1) to rank the countermeasures linked:

• A driver of WEA/WEC.
• A detailing of the driver
• A mechanism for suppressing the driver effect.

The next step was to conduct expert interviews and a survey. The purpose of the expert interviews was a systematic reduction of the hete-rogenic results of the literature review as well as a first potential assessment of countermeasures in terms of practical relevance and need for research. Finally, the potential countermeasures were evaluated in a feasibility study in the form of a corresponding half-day industry workshop.

As a result, carbonitriding, case hardening, inductive hardening, burnishing and the use of alloys with chromium and nitrogen were identified as suitable measures. What remained unclear was the importance of retained austenite content in WEA/WEC avoidance, which has also not been systematically investigated in the literature. Testing of the identified measures should take place in follow-up projects, taking into account hydrogen influence, current passage, slip, and mechanical loading. Finally, the feasibility study elaborated the recommendation that "evaluation and optimization of carbonitriding with respect to the avoidance of WT/WEC" should be prioritized for future research.

The project 707 VII of the Forschungsvereinigung Antriebstechnik e.V. (FVA) was financed by own funds.