Many automobile components — e.g. for engine, powertrain and chassis — are high-performance parts with special requirements concerning mechanical properties and quality. They are commonly produced by forging. Quality standards for these parts are constantly increasing.
| Figure 1: Forging process chain |
Thus, forged products and forging processes have to fulfill rising requirements. Additionally, the globalization of economic life has intensified the suppliers’ competition. The increasing prices for energy and steel cause a rising need to reduce these cost factors. Besides savings, another target is to meet the public concern for ecologic compliance, e. g. reduction of energy and material.
Forgings are produced in several process steps. This process chain begins with cutting and heating of the billet, followed by the forming operations, and it ends with final treatments like flash removal, punching and cooling (see Figure 1). The usual forming operations are preforming and die forging. The preforming can be done by rolling or die forging.
Forging is a shaping process. The two major technologies are conventional forging in open dies – with flash – and flashless forging in closed dies. A hybrid of these technologies is the forging in partially closed dies, where the material flow is restricted in some directions and free in other ones. Intermediate goods produced with this technology are most often flashless [Beh08, Mue08].
The majority of parts made of steels are hot forged at temperatures between 1,000°C and 1,300°C. The main disadvantage of this temperature level is the development of scale. It leads to bad surface qualities and a waste of valuable material. Additionally, scale exerts a negative influence on the durability of the tools. On the other side of the temperature range is cold forging.
Cold forging is forming of massive steel parts without heating of the workpiece. The main advantages are a good surface quality, due to an absence of scale, closer tolerances, and a decreased energy demand. The main disadvantages are that press forces are necessarily high and that complex geometries like long flat pieces cannot be produced due to high stresses and the resulting die wear. This can be avoided by raising the temperature of the workpiece, up to the maximum point where scale develops. This is called warm forging [ICF01].
Therefore, warm forging is an economical alternative to conventional hot forging technology. There are numerous definitions for the warm forging process of steel. They have in common that the temperature range is considered to be approximately 600°C to 900°C. The lower limit of the temperature range is given by a significant increase of flow stress and a decrease of formability of material [Gei03, Hus03]. Warm forging processes offer the following advantages:
• Closer tolerances, improving the material utilization and decreasing the allowances for subsequent machining operations;
• Reduced surface roughness;
• no scale and reduced decarburization improving the product quality; and,
• Reduced energy demand.
| Figure 2: Forged steering links and connecting rods were used as sample parts for the research. |
None of the three competing forming technologies (cold, warm and hot forging) is generally superior to the others. Depending on desired part geometry, material, production quantity and accuracy, the most suitable technology has to be chosen. The major benefit of warm forging compared to hot forging is higher accuracy. By combining a warm forging process with a final cold coining operation, IT classes of IT 9 (and in special cases of IT 8) can be achieved [Klo10].
To describe the shape range that can be produced, it needs to be added that most of the geometries that are currently warm forged are rotational symmetric. Until the European research project “DeSProCh – Design of a Semi-hot Process Chain” was finished, other more complex part geometries like long flat pieces had not been manufactured with this technology [Beh08]. Steering links and connecting rods are two representative long flat pieces that are commonly hot forged.
The conventional hot forging sequence of the steering link starts with a cross wedge rolling operation that provides the necessary mass distribution. The rolled intermediate workpiece is transferred to the forging press and a cross-section is preformed in a closed die, followed by a closed-die final forming operation. As a first step, only the forging in dies was developed. Within the DeSProCh project, production lines were set up for the two mentioned long flat pieces (see Figure 2), the forging of these parts was investigated, and a guideline for the introduction of the warm forging technology has been derived [Beh08, Rei07, Rei06].
The material utilization achieved with the steering link forging process is 86%. This is higher than for common hot forging processes, which range from 60 to 80% material utilization. The material utilization obtained for the connecting rod forging process is about 80% [Beh08]. Although the project’s achievements exceeded previous expectations, potential for further improvement was identified:
• comparably low production output due to the high number of operations in the press, and thus a low production rate;
• High development effort for the preforming operations in the press;
• Limitation of the mass deviation along the longitudinal axis; and, therefore,
• Limitation of producible workpiece geometries by the developed warm die forging processes.
The more the mass distribution varies along the longitudinal axis, the more forging operations are necessary, as the tool material can only withstand limited loads. Also, additional forming operations cause a negative impact on the achievable tolerances. Here, rolling operations are a suitable alternative, because they allow a wider mass distribution. Therefore, the rolling as preforming operation in the warm temperature field is investigated as a next step.