Throughout the past several months, our technology experts at TimkenSteel have discussed various topics in Materials Matter around steel used in the gear industry — clean steel and why it matters, standards for cleanness measurement, gear failure analysis, the importance of modeling, power densification, lessened distortion, and improved machinability. Collectively, these topics highlight how far industry has come in terms of understanding the impact of steels on gear design, manufacturing, and performance. Here’s a recap of what we believe are the most significant factors to consider when it comes to gear performance:
Affordable clean steel: It’s been proven many times over that highly loaded components like gears benefit from clean steels. Gear designers worked for decades to improve manufacturing and assembly practices to optimize designs. However, the impact steel choices have on the design and performance were often overlooked. Now, as demands on gears continue to grow, including performance requirements such as increasing torque and/or power density, one should investigate the impact that steel design and steel cleanness can have.
Importance of measurement: With improved processing practices that make cleaner air-melt steels available, gear designers have new options to aid in achieving the higher performance levels. Measurement is key to achieving this: knowing what attribute of the material is actually being measured, how it’s being measured, and whether this attribute actually correlates to the desired steel cleanness performance characteristic all contribute to understanding how steel processing practices need to be controlled. Measurement methods like high-resolution ultrasonic tests are good predictors of fatigue performance because they analyze volume as well as area. And, in cases when gears falter, conducting a full failure analysis — which includes understanding the design and function of the gear as well as gathering information about the processing history and service life of the part — can identify a root cause for gear failure, which provides invaluable information for designers in correcting the issue.
Updated standards and adoption of steel manufacturing/product advancements: Today’s industry standards for measuring steel cleanness are limited. Steel cleanness has improved over the years, but measurement practices and advancements often lagged behind these shifts. The level of discrimination in measurement or the requirements defined in the standards often have not improved to meet increasing demands. Accurate measurement now depends on using the right mix of methods rather than relying on just one technique. Ultrasonic testing evaluates volume as well as area, allowing much more material to be inspected. Scanning electron microscopy together with simultaneous energy-dispersive X-ray spectroscopy combines optical imaging of relatively large areas and the chemical analysis of each inclusion detected. While traditional measurement techniques and cleanness standards provide for timely and cost-effective order fulfillment, they lack the sensitivity necessary to fully characterize inclusion population.
In addition, existing clean steel standards are not effective in advancing material performance. Often, designers do not analyze material choices because existing materials meet the current standards. By implementing a new set of standards that pays attention to more aspects of steel cleanness measurement, designers will have a tool to define materials that allow them to capture desired performance.
Affordable higher-strength, higher-toughness steels: Increased strength means higher loads or better endurance at existing loads. Increased fatigue strength, wear resistance and resistance to bending overload damage have driven steel designers to develop more capable steels. The classical trade-off in all materials is that increasing strength nearly always results in reduced toughness, but with careful design and processing, steels can achieve significant strength improvement and still display excellent toughness properties.
Lower system cost via modeling and material selection: Cost is king, and understanding and managing system-level costs drives overall success. Today’s steel solutions can save costs over the long term with reduced processing costs and achieving first time quality requirements. Advanced modeling tools may be used to decrease the time and money spent designing and implementing new steel solutions. Material models become even more powerful when used in conjunction with refined measurement methods.
Another area for savings is in machining and gear-tooth cutting, where material modifications can make a significant impact. By optimizing the gear blank material, designers can maximize the life of machine tooling and increase options for manufacturing routes.
Reducing distortion effects — or unplanned dimensional changes of the final part — can have a major impact on manufacturing costs. Material manufacturing methods can introduce numerous potential direct or indirect sources for distortion. With in-depth material knowledge, designers can identify and select the right chemistry for the application. Having a steel supplier that can maintain precise control of the steel’s manufacturing processes reduces the likelihood of distortion and allows for easier achievement of dimensional requirements, resulting in less scrap and overall reduced cost.
Finally, manufacturing advancements in gear design may also help to optimize quality and minimize costs. Selecting the right steel grade and corresponding heat-treat process is a foundational step for achieving the highest quality. For example, vacuum carburizing may offer many benefits for some designs, including consistent hardenability for each application, robust material response to varying quench rates, high-temperature grain coarsening resistance, and a cost-effective, lean alloy design. All of these aspects can help achieve cost-reduction and performance-enhancement opportunities.
In the next installment of the column we’ll look at what’s next for the gear industry, and where technology needs to go to ensure continued optimum performance.
About the author E. Buddy Damm is a scientist–advanced steel solutions at TimkenSteel Corporation. He is responsible for developing new or improved products for the company’s customers and new or improved processes for its manufacturing operations. His responsibilities include technical leadership for TimkenSteel’s high-strength, high-toughness steels innovation platform and corrosion-resistant steels innovation platform. Damm holds a bachelor’s degree in metallurgical engineering from Michigan Technological University and a master’s and a doctorate degree in material science and engineering from Colorado School of Mines. He can be reached at firstname.lastname@example.org. Learn more at www.timkensteel.com.