Using tensile and compressive forces to transform the shape of a steel block
April 2011 - In fabrication and forming, workers use a variety of steel products and different manufacturing processes on them, from reforming, forging, swaging and extrusions to deep drawing and stripping. Depending on the project, the prints and parts list require specific steel half stock of different shapes and specifications, because many industrial projects start as a block of steel.
Steel is cast in blocks, called ingot molds, in desired sizes for the production of rolling stock. The ingots are placed in vertical furnaces for heat equalization or soaking. The uniformly soaked ingots are heated to approximately 1,200 degrees Celsius or 2,200 degrees Fahrenheit and are processed further into sheets, sections, wires and rods. (Figure 1)
Reforming is production by the plastic transformation of the shape of a solid body. This can be carried out by tensile compressive or binding forces either after heating the body (ingots) or without heating.
Pressure reforming by rolling
In rolling, the workpiece is reformed either continuously or step-by-step by the application of pressure with revolving tools. In the production of sections and plates, the duo rolling mill has two rollers driven in opposite directions. These rollers may be cylindrical for the production of sheets and plates or profiled for the production of sections.
The cooling times are long for the duo mill because the rollers must reverse directions after each passage. The three-high rolling mill does away with the need to reverse direction by using a third roller. (Figure 2)
For the production of tubes in a skew-rolling mill, the ingot is made to revolve by two double-cone rollers, which squeeze it and break down the core. Once formed, the cavity is pierced by a plug. The raw piece is then finished to the desired size by a roll-mandrel, also known as a reciprocating rolling process. (Figure 3)
Changes in material structure
Forging is a production process in which the workpiece is pre-heated and reformed by compressive forces. (Figure 4) Forged workpieces have more favorable grain flows than workpieces produced by chip-forming machining and can tolerate higher loads. The forge ability of steel decreases with rising carbon contents. A high content of sulphur results in red shortness, and a high content of phosphorus makes the steel cold short.
The elastic limit of the material must be exceeded in forging. The grains are deformed and displaced, a process in which they slide along the grain boundaries without losing cohesiveness. The material undergoes minimal grain deformation in Zone 1-1, radical grain deformation and displacement in Zone 2 and slight grain deformation in Zone 3-3.
The forging temperature depends on how the steel is worked. The workpiece must be brought to its finished form with as little heating as possible. Overheating will make the steel structure coarse-grained, brittle and weak. When the steel is white hot, it begins to throw off sparks and it scales. Burnt steel becomes unusable. In the blue-hot region between 290 degrees and 350 degrees Celsius or 554 degrees and 653 degrees Fahrenheit, the extensibility of steel is particularly low.
Stresses build up in the crystallites during cold-working, and the effects range from strengthening the material to crack formation. Stress cracks are a result of over-rapid heating because the temperature difference between the inner and outer layers of the workpiece becomes very great.
The steel is heated gradually to 700 degrees Celsius and rapidly thereafter to prevent decarburization of the outer layer and coarse grain formation. The forged workpieces should be cooled gradually and uniformly. The stresses can be reduced by annealing. FFJ
Udo O.J. Huff is an independent consultant with project experience in machine building, welding engineering, training and development. He holds Master of Education and Bachelor of Science in Technology degrees from Bowling Green State University. Questions or comments? E-mail uhuff@sbcglobal.net.
