Hamilton Sundstrand presents the company's economic justification, implementation, and ongoing development of low-pressure/vacuum carburizing at their Singapore Gear Center of Excellence. A review of economic advantages and process controls of vacuum vs. endothermic gas carburizing of aerospace gears is presented with an emphasis on reduced cost of quality. Implementation of statistical process control (SPC) and certification of the vacuum carburizing process have been defined as critical in their aerospace gear manufacturing operations.

Fig. 1. Ipsen's AvaCTM system at Hamilton Sundstrand Singapore


Hamilton Sundstrand (HS), a United Technologies Company, is a leading supplier of aircraft systems and equipment to the aerospace industry. These systems include a multitude of mechanical components requiring precision gears designed and manufactured to aerospace requirements. The company's Singapore facility has been established as a Gear Center of Excellence for the complete manufacturing cycle of gear parts, including heat treatment. For years, HS Singapore utilized the traditional atmosphere gas carburizing process for heat treatment. In the 2001-2002 time frame, the conversion to a low-pressure carburizing process was evaluated.

Hamilton Sundstrand subsequently decided to implement an automated low-pressure carburizing line from Ipsen International that includes Ipsen's proprietary AvaCTM process. This low-pressure carburizing line has been operational since 2003, processing the carburizing production requirements for more than 18 individual gear cells within the HS Singapore operation, and is now their only source for carburizing gears. This article outlines the economic and safety justifications related to purchasing this line, the implementation process and production results.

Fig. 2. Examples of HS Singapore manufactured gears
Before converting to a low-pressure carburizing system, the company underwent an extensive review of economic and operational benefits to justify the significant investment of converting to this technology. This approach produced real advantages from a manufacturing operations standpoint and produced gears with at least equal metallurgical/engineering properties. The following economic and operational benefits were identified:

Reduced Cost of Quality
  • Improved manufacturability of gears
  • Apply SPC and population control logic vs. destructive lab testing

Reduced Direct Labor
  • Automated system; available 24/7, as required
  • No start-up or shutdown time required

Reduced Maintenance Cost
  • Reduced total maintenance expense savings per year vs. endo atmosphere carburizing
  • WIP collaboration with equipment manufacturer to improve reliability

Reduced Utilities Cost
  • Include endo atmosphere cost and continuous heating demand for endo carb furnaces
  • Vacuum carburizing only consumes high energy levels when actually performing work vs. 24/7


Fig. 3. Typical recipe process capability verification graph
Reduced Inventory (Improved Throughput)
  • Supports 18 small factories (18 separate supply chain gear cells)
  • Assigned process times (time of day by "customer")
  • Typical three-day turnaround for carburizing and Gleason hardening (was seven days)

The primary economic advantage planned as a result of changing from endothermic gas carburizing to vacuum carburizing was the reduced cost of quality; i.e. to be able to ensure part-to-part process repeatability and subsequently reduce the amount of costly, destructive lab inspections.

The equipment supplier's patented acetylene-based technology minimizes sooting using acetylene for carburizing under low-pressure conditions. This technology offers extremely predictable carburizing rates, higher case hardness, and optimum case uniformity, especially in difficult to carburize features including gear roots and small diameter or blind holes. Another advantage is predictable performance of physical, mechanical and metallurgical properties yielding consistent end results.

By switching from a traditional endothermic process to a vacuum carburizing process, the company gained environmental health and safety advantages. As a United Technologies Company, Hamilton Sundstrand places great importance on manufacturing processes that are more environmentally friendly. For example, the equipment supplier's patented process reduces greenhouse gas emissions by more than 90%. Secondly, this process improved health and working conditions by reducing the ambient temperature in the workshop, which is a non-air-conditioned shop located in Singapore (on the equator!). Lastly, it greatly reduced many fire and safety concerns by lessening the risk of catastrophic loss of life or personal injury and the risk of business loss in case of explosion or fire.

Fig. 4. "Traditional" heating and rapid 2-bar quench

Implementation of Vacuum Carburizing to HS Aerospace Gears

To achieve the benefits identified, HS Singapore initiated a program to implement standard practices both within gear manufacturing and heat treating. The key process engineering parameter needed was additional grind stock to improve gear manufacturability. A complete review and reprocess engineering activity resulted for all of the company's manufactured aerospace gears.

Standard Practices Established for Gear Machining
  • Normalize, harden and draw after rough machining
  • Machine to maintain symmetry as much as possible (gear webs)
  • Control gear webs to maximum allowable thickness (machine after heat treat if possible)
  • Control lightening holes to optimum size (pilot runs for each part number required)
  • Control optional carb areas recognizing impact on stress and resultant distortion
  • Plan grind stock allowance to 33% max of minimum effective case (HRC 50); e.g.:
    • 009" max grind stock for a .028"-.032" planned case depth (was .007" max)
    • Additional .002" grind stock improved gear manufacturability (key result to justify equipment purchase)


Fig. 5. Two-stage controlled heating & two-stage controlled cooling
Standard Practices Established at the Heat-Treating Center

To achieve the target of .002" additional grind stock, the heat-treat process needed to demonstrate and control full-hard case depth as well as effective case depth. The Figure 3 shows a typical recipe process capability verification.
  • Apply pilot runs to verify process prior to production use
  • Apply control plan logic to every part: i.e. carburize is a "critical feature"
  • Maintain manufacturing engineering control over all recipes
  • Utilize preheat (250°F in air prior to vacuum carb) to minimize pump-down time
  • Document racking methods as part of control plan
  • Utilize metallurgical-grade acetylene
  • Minimize temperature differential through varying part cross-section
    • Preheat before reaching austenitic temperature
    • Controlled heating through austenitic range
    • Controlled cooling through austenitic range
  • Develop and certify each vacuum carb recipe
  • Track results statistically by recipe (by case depth) and feature


Develop Heating and Cooling Rates

The heating and cooling rates were optimized to minimize gear distortion recognizing the fact that non-symmetric gear features are prone to irregular and uncontrollable feature distortion that can not be corrected with gear hardening using die quenching. Pilot-run testing of the most dimensionally critical gears found that the original low-pressure carburizing furnace recipes, using 100% heating rate and 2-bar nitrogen gas quenching (Fig. 4), resulted in greater (unacceptable) amounts of gear web distortion than the previous endothermic process.

Using test load thermocouples, new controlled heating carburize recipes were developed to minimize internal part temperature variation through the austenitic start temperature to the carburizing temperature, followed by static cooling after carburizing and negative pressure cooling to a safe part-handling temperature (Fig. 5). These new standard recipe heating and cooling practices provided dimensionally acceptable results while not compromising metallurgical or economic requirements.

Fig. 6. Traditional flat racking method results
  • Gear web "oil-canned" up to .035"
  • Gear face runout to .018" ~ .038"

New vertical standard practice racking results
  • Gear web flat within .003"
  • Gear face runout within .010"
Carburize Racking Methods

Coincidental to implementing the new vacuum carburizing process, the company also had to begin manufacturing the newest generation HS gears for new, larger, commercial jet engine gearboxes. These larger gears had unacceptable distortion results, even with the improved heating and cooling furnace recipes as noted above, until new racking methods were also incorporated. Again, pilot-run studies were performed, and as a result of these studies new standard practices to include vertical gear web-racking orientation for large diameter gears were incorporated (Fig. 6).

Fig. 7. Hardness profiles for gear pitch diameter and root
Lab Analysis Results

It is critical to understand that every part carburized feature must be controlled when attempting to verify carburizing process capability and to certify the part process. Once the furnace process recipe capability has been achieved for all required part features, every feature must be inspected as part of certifying the process. As depicted in Fig. 7, the company achieved metallurgical results for every part feature consistent with requirements. This includes achieving effective case depth within control limits and full-hard case depth of more than 50% of the minimum of the effective case depth. The charts show results for a standard vacuum carburize recipe that requires .028"-.032" effective case.

Fig. 8. Process (recipe) hardness capability for all features Business Justifications to Convert to Low-Pressure Carburizing Technology

Process Certification

Once the overall process was defined as capable, HS undertook to certify the process at the individual part level. This certification process consisted of checking every feature on every part to ensure certification was met (Fig. 8). Once a predetermined number of samples was inspected and found to be in control, the sampling frequency was reduced to include in some cases the ability to eliminate final destructive testing. This reduction in lab destructive testing cost is on track to the original company plan, and as mentioned earlier is the ongoing primary economic benefit that justified the purchase of this new technology.

Summary

After an extensive review of economic and operational benefits, Hamilton Sundstrand implemented a state-of-the-art capabil-ity at their Singapore Gear Center of Excellence. Using an Ipsen furnace with AvaCTM as a standard for process plan-ning/gear machining and heat treating, HS saw positive economic benefits to their business plan (cost, quality and delivery). They experienced many economic benefits, such as reduced cost of quality, reduced direct labor, reduced maintenance and utility costs, and improved throughput. In addition, significant environmental advantages resulted. Greenhouse gas emissions were reduced by more than 90%, and health and working conditions improved due to a decrease in ambient temperature in the shop. The supply chain production requirements of the 18 individual gear cells were also maintained.

Increasing costs and competitive pricing are forcing companies to look at better efficiencies. New technologies, including the adoption of advanced processes and materials, can have a direct positive effect on economics and environmental health and safety.

For more information: Wally Fortner is Principal Engr - Associate Fellow at Hamilton Sundstrand, Mechanical Operations Engineering, Rockford, IL 61125; ph: 815-394-4237; e-mail: wally.fortner@hs.utc.com


Additional related information may be found by searching for these (and other) key words/terms via BNP Media LINX at www.industrialheating.com: vacuum carburizing, greenhouse gas emissions, low-pressure carburizing, effective case depth, gas quenching, austenitic