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Digging into Steel: How does steel quality impact wind turbine reliability

Have you noticed when you talk to your reliability engineers about wind turbine issues how quickly the jargon flows … “martensite”, “irregular white etch crack”, and “retained austenite”.

What does this all mean? They are talking about steels, materials science and the failure mechanisms for the main bearings, gearboxes, generators or perhaps blade bolts. A deeper understanding of these topics helps turbine owners to determine the root cause and find ways to mitigate repeat failures.

At the upcoming 2019 NA Wind Turbine Technical Symposium, our day 1 “Pre-Symposium Training” presenter Cameron Misegadis will be helping to educate the industry in this area with his topic “Digging into Steel”.

In this blog post we provide a window into this upcoming presentation.

Figure 1: Micrograph of secondary fracture initiation points under the head of a bolt. The radius with the three initiations is the radius that transitions from the bolt shank to the base of the head.

Figure 2: Close-up view of a brittle lens fracture initiation point in a black oxidized, bainitically through hardened bearing.  (red arrows indicate the lens feature, black arrows indicate direction of fracture propagation)


Wind turbine drivetrains must be as light as possible to keep installation costs to a minimum yet still able to withstand the operational loads expected during the 20-year life.  Meeting the demands of these two competing goals requires that design engineers select particular steel alloys and heat treatment processes that provide high performance for strength, ductility, and toughness.

To make the material selection even more challenging, the various drivetrain components must face a variety of wear and fatigue mechanisms.  For example, a main bearing will experience low lubrication film thickness, a tight shaft fit, and highly variable loading.  At the other end of the drivetrain,  the generator bearings are subject to higher temperatures, the chance of electrical discharge, and a higher number of fatigue cycles. The same optimized material or heat treatment for a main bearing may not be suited or viable for the generator bearing.

Inside the gearbox, the gears must endure high contact stresses and sliding on their flanks while retaining sufficient ductility and strength in the center of the gear teeth to resist shock loads and brittle fracture. An inclusion in the wrong location, grinding burn that was not detected during initial quality checks, or an area of insufficient case depth may spell disaster for the long-term reliability for the gearbox.

Although the industry is investing in research and high-quality steel suited for each drivetrain component, reliability issues relating to these components continue to hit the bottom line. As an example, we show a fishbone diagram of a recent main bearing with an inner ring axial crack, notice the entire leg dedicated to material selection, heat treat, and quality.  Materials are often critical to the failure root cause.

We hope you will join ONYX InSight at our 4th NA Annual Technical Symposium to learn more from ONYX InSight engineer’s on this fascinating subject.  The topic can be a little overwhelming, so we will present the basic concepts of metallurgy in the session “Digging into Steel” during the pre-symposium workshop and our team will be available for deeper discussion over the 2.5 day event.



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