Like any advanced manufacturing process or enterprise, process controls are critical in meeting the demanding customer requirements of forged products. The fifth installment in our series explores select elements of the forging supply chain as they relate to process control.
One characteristic of advanced manufacturing is the need for and exploitation of information technologies to:
- Create sophisticated designs and development processes
- Utilize precision measurement systems
- Automate processes
- Employ control systems with real-time process monitoring and feedback loops
- Control elaborate, complex and capital-intensive equipment
Recognizing there is so much going on with respect to process controls across the forging supply chain from feedstock to finished part, we decided to focus on three steps of the forging supply chain to illustrate process controls. These are feedstock preparation, forging design and application (as influenced by process modeling), and process control during forging.
Forging feedstock preparation has improved considerably over decades, resulting in the highest-quality alloys the world has ever experienced. In some cases, feedstock quality is so great that forges no longer need a staff metallurgist. The forge instead becomes a converter of barstock to a shaped geometry with guaranteed properties. This is especially true for smaller forge shops with lean workforces. For example, one can readily see how one high-end producer uses process controls to provide world-class feedstock. That producer is TimkenSteel.
Reaching back to October 2013, TimkenSteel’s Patrick Anderson summarized the company’s integration of modeling tools, inspection tools and capital equipment to create high-quality, large-diameter, forged-rolled bar with sound centers and shape control.
Actually, TimkenSteel’s high-quality steel starts with the melting and refining of premium scrap in its electric-arc furnaces. The melt shop controls melt temperature, chemistry and refining for all of its steel, with special attention paid to sulfur (0.025% maximum) and phosphorous (0.025% maximum). By carefully producing the melt, subsequent process steps impart more consistency and quality into the company’s products.
To optimize and control its rolled-forged process, TimkenSteel invokes Scientific Forming Technology’s DEFORM-3D in two ways. First, they created their own Design of Experiments (DOE) to assess a multitude of process parameters affecting center soundness. In parallel, they developed the sets of specific process conditions (or recipes) that were actually used on the shop floor. With simulations in hand, they validated the process models through actual production runs and ultrasonic testing of the centers.
This rigorous and disciplined approach to manufacturing some of the world’s best steel provides forgers with unparalleled quality. These steels are used in tough applications such as bearings, crankshafts, gears, oil drilling, wind power and aerospace components, which classically depend on forgings.
Design and Application
Jet-engine components are among the most critical of forging applications. Twin-engine aircraft fly around the world with hours of flight time to their designated airports. During these hours of flight, the titanium- and nickel-alloy components providing thrust are rotating at 10,000 rpm or greater at temperatures in excess of 1400°F, with failure not being an option. Slight changes in chemistry, microstructure, forging temperature, furnace time, preform geometry and other parameters can change the strength of a component by 10% or more. For critical components, this can spell the difference between a reasonable safety factor in service and none at all.
Years ago, process controls were instituted to maintain forging process consistency after an initial design and qualification. Since the qualification cutup was extremely expensive, engine manufacturers worked with forgers to optimize the process prior to production, followed by freezing it from unwanted changes. Since expected variation was significant, each forging had a sacrificial part included as a test ring. This was cut off the forging and tested to ensure that the forging met the demanding requirements. The costs were high, but it was a form of insurance.
A design was developed over the years using process modeling to control the process window for strain, temperature and rate of deformation. Forging process design evolved from a rough draft followed by adjustments on the floor into a highly engineered design sequence. Optimum process windows were developed for critical aerospace alloys. Forging designs today are far more sophisticated than in the past.
Figure 1 shows a computer simulation of a turbine disk hot-die forged from IN-718. A JMAK* grain-size model in DEFORM is used to predict the grain size of two manufacturing methods. Based on the finer average grain size, the simulation on the right predicts higher yield strength due to the finer grain size.
Automobile components are both critical and extremely competitive. CV joints and drivetrain and suspension components are forged to ensure a trouble-free life cycle, in spite of imperfect roads and driver impatience. Forgings were machined, sorted and inspected years ago to ensure compliance with requirements. To make lighter components at an aggressive market price, highly automated lines produce hundreds of parts per hour with astounding accuracy and repeatability.
To avoid machining operations, cold and warm coining/sizing operations are used to produce splines, gear teeth or simply tighter-tolerance regions that do not require machining. The tight tolerances are achieved with tooling designed using stress, wear and deflection analysis.
Hot forging with a sizing operation has been successful at reducing tolerances. Weight and energy reduction has been achieved through the use of rolled preforms to efficiently distribute volume in connecting rods and suspension components. Preform and forging designs are developed on a computer using state-of-the-art process models rather than slow and expensive shop trials.
Courtesy of LC Forge, Figure 2 shows a progression (top to bottom) with the rolled preform, bending operation, blocker and finish forging. The billet is shown on top of the left image (simulation). This represents a very efficient utilization of material and energy without compromising the final part.
Forging Process Control
Aerospace forgings are produced from many different challenging alloys, ranging from high-strength aluminum to nickel alloys and titanium. Each has an optimum process window to produce the desired microstructure, which controls mechanical properties. Hot-die forging technology reduced thermal variation from “cold” tooling and the expected skin effect that was machined off prior to heat treatment. Computer-controlled presses were used to ensure repeatable forging cycles without operator-induced thermal or strain-rate variation. Furnace designs and controls were refined for tighter tolerances while using less fuel. Heating and reheating times have been studied in many companies, resulting in better temperature control in any forging operation. Statistical process control (SPC) was instituted to ensure no shifts in the forging process from run to run.
When a problem does occur, it is possible to review records that were not even considered 30 years ago. With that, process models are used to test a theory about the root cause of a problem that slips through the system. The result is that these components today are tested statistically rather than 100% physically. The process controls result in more consistent parts with less costly testing. The execution is no longer an art form but a science.
Aircraft-engine components are only cut up and tested at the rate of one part per 10, 20 or even 50. That said, the mean time between failures continues to grow, with engine failures in flight being extremely rare.
Automobile forgings are generally made from common carbon and alloyed steels. With consistency, cost and weight as drivers, the controls are immensely important. Front-wheel-drive systems save precious weight and money. The CV joint and races are forged to near-net shape with very little machining required. Many components use microalloy steels to achieve the required mechanical properties with a post-forging controlled cooling. The temperature controls throughout the forging process are designed to cool through a critical temperature range, resulting in strength and toughness without separate austenitizing, quenching and tempering operations.
Induction heating is fast, efficient and reproducible. Automated material handling and press operation result in far less part-to-part variation. Further refinement occurs with automated lube and cooling application between forging operations. These components are frequently forged in cells with increasing automation and process control. The reduction in part-to-part variation allows designers to develop forgings that have less weight and stock to remove.
To meet the demands of critical applications, especially the assurance or requirement for guaranteed mechanical properties of strength, toughness and fatigue resistance, elements of the forging supply chain invoke tight process controls. This helps define forging as an advanced manufacturing process.
We are considering writing articles regarding the business of forging and workforce development in 2016. Let us know what you are most interested in reading about this year.
Preparing this series of articles is clearly a team effort between SCRA Applied R&D, Scientific Forming Technologies Corporation (SFTC) and BNP. Jon Tirpak, the executive director of the Forging Industry Association – Department of Defense Manufacturing Consortium, and John Walters of SFTC appreciate the support received from the Defense Logistics Agency (DLA) through its Manufacturing Technology Program – PRO-FAST.
Co-author John Walters is vice president of Scientific Forming Technologies Corporation, Columbus, Ohio. He may be reached at 614-451-8330, or firstname.lastname@example.org. Co-author Jon D. Tirpak is the executive director of FDMC and FAST program manager. He is also president of ASM International. He may be reached at 843-760-4346 or email@example.com.