August 2024 Volume 6

MATERIALS

Within forging four main failure mechanisms are: hot wear, cracking, plastic deformation, and thermal fatigue. The most dominating for high-demand forging is hot wear.

than most tool steels for forging. It exhibits excellent weldability due to its high homogeneity and controlled carbide distribution which contribute to stable welding performance. Materials and Methods Three ESR re-melted tool steels were selected; Uddeholm Skolvar, Uddeholm Unimax and AISI H13 ESR. Uddeholm Skolvar is a 5% chromium martensitic tool steel with vanadium content to create vanadium hard phase carbides which are very stable at elevated temperatures. The maximum potential hardness is 61 HRC. Uddeholm Unimax is also a 5% chromium martensitic tool steel but does not contain any bigger vanadium carbides but still have a potential to reach 57 HRC. AISI H13 ESR is selected as a standard material which is very often used. From Figure 3, the carbides in Uddeholm Skolvar are evenly distrib uted in the matrix, the electro slag remelting process lowers the segre gation levels in the material hence making it high homogeneity and weldable.

Figure 2: Illustration of failure mechanism from left: abrasive wear, cracking, plastic deformation and thermal fatigue. Many tool steels, such as AISI H13 and H11 types, are commonly used but do not have the properties required to resist hot wear. Higher alloyed grades, such as AISI D2 types, improve hardness and wear resistance but lack high-temperature stability, making them non-durable and not resistant to hot wear for long production cycles. Therefore, it’s crucial to choose a material that is highly hot wear resistant and can withstand these harsh conditions. Temper back is a condition that occurs when the die surface is exposed to high temperatures over time. This causes a drop in hard ness, strength, wear resistance, and the ability to maintain its shape. To avoid this, it’s important to use alloying elements that give high temper back resistance. Hot strength is another important factor to consider. The die mate rial must be strong enough to handle the significant load and forces exerted during the hot forming process without deforming. Yield and proof strength of a material normally has a tendency to significantly drop at elevated temperatures. Weldability is also a crucial factor in the tool steel selection to make repair of cracks or worn surfaces. Uddeholm Skolvar, with its properties, resists this significant drop making it less vulnerable for plastic deformation during the hot forming process. For superior hot forming tools these properties lead to increased efficiency and decreased maintenance costs. When it comes to weldability, Uddeholm Skolvar is a higher alloyed grade

Figure 3: Microstructure of the three materials, from left: Uddeholm Skolvar with vanadium carbides seen as with dots, Uddeholm Unimax and AISI H13 ESR all in hardened and tempered condition, magnification 500x. From Table 1, it can be seen based on the chemical composition that the capability of reaching a high wear resistance hardness varies between the grades. Uddeholm Skolvar was not heat treated to max hardness of 61 HRC but 58 HRC, the reason for this is to keep a higher level of toughness to have a better resistance against cracking in the forging application.

Table 1: Chemical composition and capability hardness for the selection steel grades.

FIA MAGAZINE | AUGUST 2024 39