Publication Date

5-2019

Date of Final Oral Examination (Defense)

4-8-2019

Type of Culminating Activity

Dissertation

Degree Title

Doctor of Philosophy in Materials Science and Engineering

Department

Materials Science and Engineering

Major Advisor

Micheal Hurley, Ph.D.

Advisor

Brian J. Jaques, Ph.D.

Advisor

Hui (Claire) Xiong, Ph.D.

Advisor

Hitesh Trivedi

Abstract

Ceaseless demand for lighter, faster, and more efficient aircraft has been one of the greatest driving forces behind bearing steel innovations. Recent studies demonstrate that corrosion is one of the leading causes of bearing failure in both military and commercial aircraft. High-performing bearing steels are available but are not being used in US military applications due to high cost and security issues when steels are produced outside of the continental United States. One approach to address this issue is to engineer steels that are cost-efficient and heat treated for corrosion resistance, long wear life, etc.

This dissertation presents information on the effects of heat treatment on bearing steels, specifically UNS 42670 (Pyrowear 675, or simply P675). P675 is a martensitic stainless steel (MSS) engineered for use in the aerospace industry. Through proprietary heat treatments, P675 can be transformed from a mediocre performing steel to one which can withstand fatigue more than all other steels in its class, while maintaining acceptable corrosion resistance. Here we demonstrate the effects of heat treatments on the new generation of bearing steels to inform and aid steel developers in designing cost-efficient steels that can provide superior corrosion resistance while maintaining required tribological performance.

Samples studied were heat treated using three different methods; High temperature tempering (HTT), Low temperature tempering (LTT), and Carbo-Nitriding (CN). This study was initiated to test the following hypotheses:

  • Electrochemical techniques (i.e. anodic polarization (AP), electrochemical impedance spectroscopy (EIS)) will yield faster and more accurate results than conventional corrosion testing methods for screening bearing steels for corrosion behavior.
  • HTT samples will have the lowest corrosion resistance due to a larger depletion of chromium from the matrix experienced at the highest tempering temperature which will lead to the highest microgalvanic couple between the carbides and matrix.
  • CN will have the highest corrosion resistance from the steels tested due to the addition of nitrogen and encouraged passivation at the oxide/metal interface.

The objective of this dissertation is to understand and explain the implications of heat treatments on the newest and upcoming generation of MSS. A combination of accelerated corrosion testing, modeling, and nanoscale surface analysis was used to determine corrosion mechanism and provide recommendations.

Key results from this study include the following:

  • Corrosion performance of P675 is highly dependent on heat treatment where CN outperforms all three heat treatments for corrosion testing, while HTT has the lowest corrosion resistance.
  • EIS data was fitted to an equivalent circuit and a mechanism of corrosion attack was proposed for each of the bearing steels studied where HTT experienced general corrosion attack while LTT and CN pitting corrosion.
  • SKPFM Volta potential difference (VPD) measurements in an inert environment showed HTT as the thermodynamically most favorable to experience microgalvanic corrosion between the chromium-rich precipitated carbides and the surrounding martensitic matrix, with a measured carbide-matrix VPD of 200 mV, while LTT (150 mV) and CN (90 mV) were less.
  • Corrosion propagation was also monitored in real time via in situ AFM and revealed that HTT underwent the most rapid spread of corrosion attack across the sample, while LTT and CN were less affected and showed much more localized, intergranular attack and adjacent to carbides.
  • Bulk electrochemical testing results agreed with in situ AFM results, with LTT and CN showing distinct passive regions as compared to HTT, confirming the nanoscale differences in corrosion behavior observed between the steel heat treatments investigated.
  • Corrosion rate measurements alone are not adequate to be a predicting factor of bearing performance. The mechanism of corrosion initiation and propagation must be investigated to properly design new bearing steels.
  • Based on this work, HTT would be recommended over the other two tempering procedures for use in aerospace bearings where corrosion is not a primary concern. However, when the bearing assembly is prone to corrosion attack, CN is recommended for bearing use due to its high resistance to both corrosion onset and propagation.

In conclusion, this study will allow the United States Armed Forces a new tool (electrochemistry coupled with surface analysis via SPM) to screen candidate bearing steels for gas turbine engine applications and will give steel developers insight into the effects of heat treatment on the corrosion performance of MSS (i.e P675). This work is a quintessential application of the materials engineering triangle; By varying the heat treatment (processing) of the steel, the microstructure (structure) of the surface of the steels were changed, thus altering the corrosion behavior (properties) and affecting the overall performance.

DOI

10.18122/td/1535/boisestate

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