Effect of Manganese on Nitriding and Softening Behaviour of Steel AISI H10 Under Cyclic Thermal Loads

Hot forging dies are exposed to high mechanical, thermal, chemical and tribological loads during hot forming processes. Near surface tool layers are prone to wear and have to withstand high loads. To reduce forging dies temperature as well as friction due to contact with heated workpieces, cooling lubricants are commonly used. Considering the advantageous hardness of martensite, a material inherent surface rehardening under these conditions would be beneficial to reduce tool wear. This study aims for an optimisation of surface layer properties to improve the service lifetime of tools by both alloying the hot-working tool steel AISI H10 with manganese and by plasma nitriding. Manganese alloying decreases the austenitising temperature (Ac1b temperature) and thereby facilitates rehardening effects. In addition, surface layer thickness and hardness gradients were adjusted by varying nitriding parameters to further increase the surface rehardening effect. Dilatometer tests were carried out to physically model cyclic thermal loading of forging dies. Finally, near surface tool layers were characterized by Vickers hardness measurements in different depths. Cyclic thermal loading resulted in hardness decreases for AISI H10 hot-working tool steel. Both alloying with manganese as well as nitriding using a comparatively thick nitride layer featuring a low hardness gradient resulted in a hardness increase of 50–150 HV0.05 up to 0.2 mm below the surface compared to common AISI H10 steel. In a distance from surface of 100–200 µm the hardness increased by 100 HV0.05 compared to the unloaded state and can be explained by the formation of martensite.