August 2024 Volume 6

MATERIALS

PREMIUM TOOL STEELS IN FORGING APPLICATIONS By J. P. Heil and D. Livree

D riven by stronger competition, higher economic and ecolog ical tasks, the forging shops are forced to produce higher quantities with lower unit costs. Requirements in light weighting led to new designs for passenger cars, LCVs and commer cial vehicles. Lower weight goes often along with smaller parts in size and weight and die forging processes have to bear higher loads during production. More near net forged parts are required to compete with other material options. Especially in forging of steel there are high demands on dies and forged parts. The dies must withstand high forging forces and abrasive wear in order to maintain the tolerances of the produced parts. Furthermore, the “as forged” surface of the produced parts must be as smooth as possible in order to prevent unnecessary and cost intensive machining of the parts. Every crack or heat checking in a die will leave a mark on the as-forged part and also weaken the die, causing premature die failure or rejected parts. Tool steel producers can help dealing with these tasks by providing hot-work tool steels, tailored to the needed properties. During the last years, Kind&Co has accompanied several forging shops all over the world in order to provide customized premium hot work tool steels tailored to the individual demands. In the following sections, the requirements with regard to cracks or wear are discussed using case studies as example. Task 1: Cracks in a Crankshaft Die Crankshafts are usually forged in two steps. The defect pattern differs between the blocker die and the finisher die. While the blocker dies tend to exhibit thermal cracks in the cavity in addition to abrasive wear, the finisher die exhibits thermal cracks as well as cracks in the shaping radius. The cracks are caused by mechanical or thermal stresses that affect the material during the process. As an example, a crack in a blocker die for crankshafts is displayed below. The present crack is highlighted by using magnetic particle testing and lighting up under UV-light. The commonly used hot work-tool steels AISI H11, H13, L6 or 1.2367 are often facing difficulties in order to meet the demands described before. Kind&Co has developed premium hot-work tool steels as non-remelted steels such as LMF, Cr7V-L or DFA and remelted grades such as TQ1 and CS1 to meet high demands in die forging. The international forging industry faces multiple technological chal lenges. Die forging has been and will be an important way of producing parts with high complexity and increasing demands in “as forged” product quality. Tighter tolerances and cleaner forging surfaces are just two examples.

Figure 1: Radial crack in a crankshaft die, highlighted by magnetic particle testing.

Cracking of a die cannot be avoided totally. The main target for Kind&Co together with the forging shops, is choosing a hot-work tool steel with higher toughness to avoid cracking as good as possible, while maintaining wear resistance.

Figure 2: Possible premium hot-work tool steels with higher toughness. The premium hot-work tool steels LMF and Q10 are characterized by a higher toughness compared to H11. By adding molybdenum and reducing the carbon and vanadium content compared to H13, the carbide formation changes from type M23C6 formation to type M2C. Because of the smaller carbide type, the toughness increases. The premium steel CS1 offers a higher alloy content of carbon and

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