Carburizing Wind-Turbine Gears

An inside glimpse into the development of the first integral-quench batch furnace in the United States approved for carburizing critical gears for wind-turbine applications.

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Flame Metals Processing Corp. is a 23 year-old company that heat treats ferrous and nonferrous components in atmosphere and vacuum furnaces at two locations in the Twin Cities area. Today the company is also a certified supplier of carburized gears for wind-turbine applications.

In 2006 Flame Metals was approached by Columbia Gear, a gear manufacturing company servicing a number of OEMs. One of its customers was a large international manufacturer of wind turbines looking for a commercial heat treater capable of meeting its performance standards. Flame Metals saw this as an opportunity to become involved in a new market. To demonstrate its capabilities, a process-development and certification program was undertaken.

Application Requirements
Wind-turbine gears, shafts, and splines must meet tough performance standards because equipment is 250 feet or more off the ground and usually located in remote areas to accommodate rotating blades that can have a 300-foot span. Because wind turbines are difficult and expensive to service, companies and individuals investing in this technology expect a service life of up to 40 years and are pushing for warranties of five years or longer on critical components. These requirements have made heat treating an increasingly important step in the manufacturing process.

The gears Flame Metals was asked to carburize range in size from 24-36 inches in diameter, weigh from 600-3,000 pounds, and require long heat-treat cycles. The gears are made of high chromium, high molybdenum steels like 4320, 4820, 9310 and 18CrNiMo7-6.

Furnace Selection
The first challenge facing Flame Metals was to determine the type of furnace it would use for the gear-carburizing program. Conventional thinking dictated a pit furnace since they are tight, simple to operate, and can handle large loads. The downside of pit furnaces is that loads have to be pulled from the protective atmosphere for quenching. This can contribute to decarburization on part surfaces, something Flame Metals was determined to avoid. Jim Caton, the president at the time—now retired—decided to go in a totally different direction and develop its carburizing process around an Integral Quench (IQ) batch furnace. IQ furnaces are easier to automate and eliminate the need to remove loads from atmosphere for quenching. Figure 1.

The next challenge was to quickly acquire an IQ furnace that could be dedicated to the wind-turbine project. Caton found what he needed on a visit to the AFC-Holcroft plant in Wixom, Michigan. AFC-Holcroft had IQ model Universal Batch Quench (UBQ) furnaces on the floor. Other than being larger than the 36-inch x 48-inch x 36-inch UBQ furnaces AFC-Holcroft builds for stock, the Flame Metals furnace was a standard product. One of the primary selling points of the furnace was its unique inner-door design. The design of the inner door ensures excellent temperature uniformity, ± 5°F out-of-the-box. It also heats up quickly, comes with an excellent atmosphere control system, and offers processing flexibility.

Flame Metals purchased the AFC-Holcroft furnace and over the next year worked with Bill Disler, vice president of sales and technology, and the controls supplier, Super Systems, Inc. (SSi), to develop the carburizing process. An integral part of the effort was the successful development of a method to purge the heat chamber of the furnace prior to loading and the use of a modified atmosphere gas to minimize intergranular oxidation (IGO).

Process Controls
The new furnace was equipped with an AFC-Holcroft BatchMaster furnace management system based on the ProcessMaster® heat-treating supervisory system. AFC-Holcroft worked with SSi Vice President of Engineering Scott Johnstone and President Steve Thompson to modify and expand the capabilities of the BatchMaster system. Flame Metals developed the recipe to carburize wind-turbine gears. SSi and AFC-Holcroft developed the controls hardware and software to make the recipes work to achieve the desired metallurgical results and make the process repeatable. Figure 2

The type of controls and atmosphere monitoring equipment needed to accurately control the carburizing process enabled experimentation, development, and control of the low-temperature ferritic nitrocarburizing (FNC) processing using a combination of endothermic gas and ammonia. This involves calculating the nitriding potential and adjusting the atmosphere during processing.

“Meeting the requirements of the wind-turbine manufacturer was a bit different than traditional processes,” says Thompson. “Some of our software and controls were standard, like installing four IR gas analyzers, mass flow meters, and redundant probe packages to monitor and control endothermic gas, hydrogen, CO, CO2, and CH2. Our engineering was required to create a system that could automate the FNC process using additional algorithms, sensors, and I/Os. The required process needed to account for gas flow rates, atmosphere, temperature, time, and the precise measurements for FNC, including nitriding potential (Kn) and ammonia flow. We were able to provide a recipe structure to give AFC-Holcroft and Flame Metals the ability to control all aspects of the process and a data log of all the information for historical review. On top of that we needed to provide a process controller that would allow the furnace to switch back and forth from atmosphere carburizing to FNC.”

Process Certification
In 2007 the wind-turbine manufacturers sent an independent thermal-processing consultant to evaluate the Flame Metals process. In January 2008, based upon metallurgical examination of more than 20 sample gears, the process was certified as meeting all metallurgical requirements. In his report the consultant said the process developed by Flame Metals was one of the best he had ever seen and, to the best of his knowledge, the first time a batch IQ furnace had been used in the United States to carburize wind-turbine gears. Today, five of the 19 heat-treating furnaces at Flame Metals are used to carburize wind-turbine components. This represents about 50 percent of its carburizing capacity. This segment of the business is expected to expand significantly.

Because its carburizing recipe and controls modifications are proprietary, Flame Metals cannot reveal details of the technology. However, there have been discussions among the involved parties that some of the new technology may eventually be commercialized. Disler gives Flame Metals a lot of credit for its entry into a new market for carburizing.

“They have taken existing processes and pushed them to the next level, hence taking further benefit of some unique AFC-Holcroft batch-furnace design features,” he says. “It has been great seeing them prosper by applying our equipment and raising the line in premium process capabilities in IQ furnaces.”

Innovation and Teamwork
When word of the inroads Flame Metals was making into the wind-turbine market began to circulate, there were people who suggested the company was lucky to be in the right place at the right time when the Minnesota wind-turbine industry took off. Caton disagrees.

Over the past 23 years Flame Metals has purchased three heat-treating companies, an engineering laboratory, expanded its Rogers, Minnesota, plant by 22,500 square feet, and quadrupled sales. Employment has increased to 100 people, including two staff metallurgists. In just the past five years Flame Metals has invested more than $5 million in new equipment and is in the process of a third plant expansion. “It wasn’t luck or coincidence that allowed us to become involved in the wind-turbine industry,” Caton says. “We have been preparing for this opportunity for a long time.”

“Finding the right furnace supplier and controls company was critical to the success of our wind-energy project,” he continues. “We did not own an AFC-Holcroft furnace until 2004. Today we have four, with a fifth on order. The performance of these furnaces, and the willingness of AFC-Holcroft and SSi to get involved in a risky project, is why we selected them for the wind-turbine gear program.”

Something to Think About
Caton believes wind energy represents a tremendous opportunity for North American commercial heat treaters. The in-house heat-treating capacity of turbine manufacturers worldwide already is taxed and, based on the value of the dollar, it now is less expensive to process parts in the U.S. than in Europe. The bad news is surprisingly few heat treaters are in a position to take advantage of these opportunities.

“Commercial heat treaters need to invest in better equipment and more skilled personnel,” says Caton. “More engineers, including metallurgists, are needed on the plant floor to design and manage the processes necessary to meet wind-turbine specifications. Shops acquiring these capabilities are less likely to see work outsourced overseas based only on costs, even with a stronger dollar.”