|Some forging companies have resorted to placing coal dust on the upset billet (left), which results in a forceful ejection process (right), seen here.
In December 2009, a project was initiated by the Forging Industry Educational and Research Foundation — FIERF — to study the elimination of coal dust from ring rolling preforms. Most ring rollers use similar processes to produce the blanks, including an upset, open-die piercing, and trim operation.
Coal dust is used by many forgers to remove the perform of a rolled ring from the indenting punch. Since this is essentially an open-die process, removing the punch can be a challenge. Thus, the producers have resorted to placing coal dust on the upset billet, which results in a forceful ejection process.
Apart from that function, there are no redeeming factors to this method. The coal dust is an operational and environmental nightmare. Forging producers have worked on other solutions for decades, however the majority of them still regularly use the coal dust to remove the preforms.
A team consisting of FIA/FIERF, Scientific Forming Technologies Corporation, and three forging producers — Ringmasters, Frisa, and McInnes Rolled Rings — was assembled to tackle the project. (Other contributors included consultant Bob Fullerton, A. Finkl & Sons, Bhler-Uddeholm, and Case Western Reserve University.)
The team proceeded with its research, documenting the ‘as is’ process for two parts in each of the partner plants and running process simulations to identify the root cause of the problem. Following multiple in-person and online team meetings to review process results and conclusions, new processes were developed, with redesigned punches.
|Face design and corner radii of punches cover a wide range. However, a combination of thermal fatigue, wear, and plastic deformation can cause the punches to fail, regardless of their design and radii.
The punches are generally produced from alloy steel, such as 4340, and are initially heat-treated to a hardness of Rc 40-45. Punch design varies by manufacturer, but all include a small to moderate taper on the major diameter. Face designs and corner radii cover a wide range. When punches fail, it is due to a combination of thermal fatigue, wear, and plastic deformation.
Computer simulations were run using DEFORMTM2D to model the flow, temperature profile in the punch, and thermal cycle over a complete production run. Additionally, an elasto-plastic coupled simulation was run on the first and last production part to evaluate tool wear, stress on the die, and plastic deformation. Following the research, the team concluded that:
1. The punches are overheating due to extended contact time with the workpiece. Corner temperatures range from 1,200 – 1,600°F as the punch is being retracted. This leads to severe tempering, resulting in a hardness of Rc 27-30.
2. The punches are plastically deforming (yielding), as the strength at temperature of a fully tempered 4340 material is inadequate to withstand the forging pressure.
3. Die wear is high due to the low hardness, punch material and high surface temperature.
4. Tapered punches result in an enforced sticking condition. The punch nose starts the hole with its smallest diameter, to then, on its way into the part, getting bigger along the tapered punch. This starts a thermal shrink-fit process, where the workpiece is shrinking (as it cools) and the punch is expanding (as it heats).
A range of changes were recommended by the team and contributors to improve the known issues.
Die material — While the 4340 is inexpensive and readily available, it is not an excellent die material. Both A. Finkl & Sons and Bhler Uddeholm recommended alternative die materials, which will be more temper-resistant. Unfortunately, the temperatures in excess of 1,000° F are fairly routine due to the extended contact time. Thus, the initial trial was run with Inconel 718 material. Follow-up trials will be run with tooling material supplied by that purpose, with an overlay of 718.
Cooling — When first evaluating the current processes of cooling within the project partners, it was found that the punch cooling practice was not universally adopted. It was clear that the temperature build-up had to be controlled for lubricant adherence. A dip cooling tank was fabricated for the trial.
|FIERF researchers used DEFORM-2D software to calculate what caused the die wear, including hardness, punch mateiral, and
surface temperature. The screen capture above illustrates tapered punches that result in an enforced sticking condition.
|Following research, a trial run was initiated in June at McInnes Rolled Rings in Erie, PA.
Lubrication — Walter Zepf Spezialschmierstoffe developed a set of lubricants to test in this process. RZ 10, a water-based graphite lube containing 5% graphite, and RZ 20, also water-based containing 20% graphite, were sprayed only on the bottom face of the punch, not even on its side walls.
Punch design — A significantly different punch design was proposed. The nose design was developed to maximize radial flow during piercing. Additionally, sharp corners were eliminated to minimize heat buildup, die wear and plastic deformation. Most importantly, the taper was eliminated. Multiple simulations run during the development phase showed that the primary sticking mechanism on the top of the punch was eliminated.
On June 16, 2010, an initial trial was run at McInnes Rolled Rings in Erie, PA. Two of the worst cases were tested to see how the new process would perform. It was reported that some of these worst-case scenarios could result in sticking on a new punch during the first forging operation. It was further reported that, historically, a partial hole was punched, then the punch was retracted and additional coal dust was applied in the pre-punched hole.
The trial, implementing new punch design, cooling, and lubrication without coal dust, was successful. No sticking or tendency to stick was observed. The trial was conducted on stainless steel blank material, with a punching depth of 7.5 to 9.25 inches, and a blank weight range of 400 to 900 lbs.
The initial trial was viewed as a success, and preparations are in process to implement the findings of this project into production, especially the automation of cooling and lubricant application.
John Walters is a vice president at Scientific Forming Technologies, responsible for sales, marketing, technical support, business development, consulting, and project management. With over 30 years in the metalworking industry, he has published more than 30 technical articles. Carola Sekreter is the technical director for the Forging Industry Association, responsible for the development of new training seminars, industry technical support, and serving as a liaison to academia and national laboratories. She has over 16 years of forging industry experience.