August 2024 Volume 6

MATERIALS

molybdenum to increase the hardness and wear resistance. All three discussed premium tool steels have also a lower silicon content in order to achieve elevated toughness levels. In the following, we describe cases and the effects of upgrading the material for dies. • A die for a 2000t mechanical press for hot forging devel oped cracks. By changing the material from AISI H13 to the premium steel Q10, the service life was increased by 52%. • In another case of cracks and wear in a die, changing from AISI H 3 to LMF doubled the service life without cracking of the die. By changing to Q10 in the same case, the service life could even be increased by a factor of 2.3 without the die cracking. • The service life could also be increased by 62 % in the case of radial cracks by changing from 1.2367 to the tougher premium steel CS1. Task 2: Wear In addition to cracks, abrasive wear is the main reason for replacing a die. Positive radii are the most common areas of wear. In these elevated areas, usually an intensive material flow is observed. As it contains oxide scaling, especially in steel forging, intensive abra sion takes place. This grinding leads to increasing wear and leaves a unique wear pattern. An example of this pattern is displayed in the picture of a forging die for connection rods below.

Figure 4: Possible premium hot-work tool steels with higher wear resistance. The premium grades Cr7V-L and DFA are totally different in their alloying design compared to H11 or H13. Cr7V-L offer a good wear resistance by the formation of larger, homogenously spreaded type M23C6- chromium- and M2C- chromium-vanadium carbides. The slightly higher carbon content furthermore helps increasing the martensitic hardness of the steel. The even further increased carbide content by higher alloying of vanadium and molybdenum of the premium steel DFA improves the hardness and resulting wear resistance. Again, some case examples: • Replacing the AISI H11 with the premium steel Cr7V-L for a blocker die for truck parts leads to a 50% higher wear resistance. • Compared to the steel 8Cr3, the service life could even be tripled by using the Cr7V-L for punches in chain link forging. In addition to hardening, the hardness can also be increased by nitriding the surface. The premium steels from Kind&Co have good hardenability as well as good nitridability. In combination, surface hardness values of up to 1260 HV0,3 can be achieved. The nitriding layer also has a very smooth surface that reduces friction. Because of this surface, the forged material can move easier over radii or edges of the die. But nitriding of forging dies makes a resinking of the surface more difficult due to the hard and brittle surface layer. Such dies have to be cut slower and a new nitriding has to be performed. Also, over-nitriding has to be avoided in order to keep the tough ness up. For further advanced wear resistance, different steels could be selected, such as DFA. This premium steel was for example used for the pre-forging step for axle components for trucks, forged out of steel. It was possible to achieve the forging of 30,000 parts with a DFA die compared to H43 die which could only forge 20,000 parts.

Figure 3: Typical wear in a die for forging connecting rods. Abrasion can be reduced by increasing the hardness of the material until a certain point. Further increase in wear resistance can only be achieved by changing the alloying content of the steel. Due to the increased carbon concentration and precipitation of useful carbides, the resulting hardness of the premium steel Cr7V-L offers high wear resistance.

FIA MAGAZINE | AUGUST 2024 31