Sometimes the size of the piece of metal being machined presents equally large challenges.
Ladish Forging, a firm with over a century of experience forging big parts for demanding applications, had to do some serious planning when it took on the task of ring rolling and forging a 21.5-ft diameter kiln tire out of a 12-ft-wide, 225,000-lb carbon-steel donut. A kiln tire is a large, seamless ring used to support an enormous kiln and ensure its weight is properly positioned on rollers when the kiln rotates. This new ring, a record setter for Ladish, was destined for an iron-orepelletizing kiln. Just getting the initial raw material to the factory in Wisconsin proved difficult. The railroad company, for example, forced Ladish to keep the steel “donut” below 12 ft in diameter. Otherwise, it could not be moved by rail in the U.S.
Still, despite efforts to anticipate every contingency, the rail car that showed up to deliver the donut was 90-ft long rather than the standard 50-ft rail car that was expected. Even using every available inch of track, the rail car would not fit far enough into the building to place the donut directly under the crane. Ladish ended up removing the permanent rail stop and running the rail-car wheels 10 ft beyond the end of the tracks to get the donut positioned.
In all, five pieces of major equipment helped create the finished kiln tire. The ring mill reduced the wall thickness while increasing the diameter. The size press “rounded up” the piece, bringing it to its final shape. A forging furnace heated the piece to the proper temperature before and during each forging operation. And after each operation, the ring went into a heat treating furnace. A vertical boring mill machined the ring to its final dimensions.
Ladish held several meetings to establish forging instructions based on material and size, as well as the mechanical and metallurgical properties needed in the finished piece. Forging instructions are tightly specified directions that guide operators through every phase of forging. In this case, the instructions covered 23 distinct operations, included three major rolling procedures and two sizing operations, as well as normalizing operations in between. Other steps included a hardness check before machining, magnetic and sonic inspections which, due to the ring’s size, had to be done manually, a second hardness check, and final dimensional inspections.
Heat was a major concern to those in charge of the furnaces. The ring would be heated to about 2,350°F before each operation. And a ring this size, which started out approximately twice the weight of average forged rings, was expected to radiate much greater heat than was typical.
And as soon as forging began, operators knew the concern was valid. They also knew that the ring would get larger with each forging step, bringing it closer and closer, perhaps dangerously so, to the forge’s control station. So they moved the station eight feet back from its original location, which was 20 feet from ring at the start of forging operations. Facility engineers also built a moveable heat shield to protect the station. Sidewalls on the shield deflected heat and made the shield more stable. During forging, engineers twice thickened up the shield and the Plexiglas window in it when thinner windows showed signs of melting in the intense heat.
When metal is heated in a furnace, its surface becomes brittle, and this layer, called scale, falls off the work piece. Ladish anticipated that since this ring weighed about twice as much as average, it could generate twice as much hot scale. To deal with it, fire-retardant foam went on the lower portion of the sizing press before each sizing to protect the press’s hydraulics and working mechanisms from the falling scale and heat.
For forging and heat treating, the furnaces were maintained to aerospace standards, including AMS 2750, Rev. D and those of the National Aerospace & Defense Contractors Accreditation Program (Nadcap). This means every thermocouple for each forging or heat-treating operation was used only once to maintain traceability and operational-repeatability standards. In fact, even the thermocouple wire Ladish uses is rated and must be traceable to its source, so it is used only once.
Postforging machining also presented obstacles. For example, when the piece had to be turned over, it took close coordination between machinists, forge-shop personnel, crane operator, and maintenance personnel. The ring had to be removed from the machine tool, moved to an area with enough clearance, flipped, and returned to the machine. This took approximately 5 hr and also required realignment of the workpiece on the machine without causing any “dings” or creating blemishes on the machined surface. And because it took 5 hr, the move was often split between shifts. So workers on one shift had to closely coordinate with their counterparts on the next shift.
When the ring was finally finished, an 11-week journey, Ladish applied a rust inhibitor to keep the ring pristine. The customer also designed handling devices that attached to the ring, making it easier to load and unload. The ring even had a police escort on its trip from the Ladish facility to the Port of Milwaukee.