High pressure die casting (HDPC) allows to produce aluminum parts for car industry of complicated shapes in long series. Dies used in this process must be robust enough to withstand long term injection cycling with liquid aluminum alloys, as otherwise their defects are imprinted on the product making them unacceptable. It is expected that nitriding followed by coating deposition (duplex treatment) should protect them in best way and increase intervals between the cleaning/repairing operations. The present experiment covered investigations of the microstructure of the as nitride and deposited with CrAlN coating as well as its shape after foundry tests. The observations were performed with the scanning and transmission electron microscopy (SEM/TEM) method. They showed that the bottom part of this bi-layer is formed by roughly equi-axed Cr2N crystallites, while the upper one with the fine columnar (CrAl)N crystallites. This bi-layers were matched with a set of 7x nano-layers of CrN/(CrAl)N, while at the coating bottom a CrN buffer layer was placed. The foundry run for up to 19 500 cycles denuded most of coated area exposed to fast liquid flow (40 m/s) but left most of bottom part of the coating in the areas exposed to slower flow (7 m/s). The acquired data indicated that the main weakness of this coating was in its porosity present both at the columnar grain boundaries (upper layer) as well as at the bottom of droplets imbedded in it (both layers). They nucleate cracks propagating perpendicularly and the latter at an angle or even parallel to the substrate. The most crack resistant part of the coating turned-out the bottom layer built of roughly equiaxed fine Cr2N crystallites. Even application of this relatively simple duplex protection in the form of CrAlN coating deposited on the nitride substrate helped to extend the die run in the foundry by more than three times.

The new cast steel with a chemical composition of Fe-(0.85-0.95)C-(1.50-1.60)Si-(2.40-2.60)Mn-(1.0-1.2)Al-(0.30-0.40)­Mo-(0.10-0.15)V-(1.0-1.1)Ni (all in wt.%) was investigated in aspect of formation of the multiphase microstructure leading to high strength and ductility. Two types of heat treatment technologies were developed. The first one involves softening annealing at a temperature of 650°C for 4 hours, heating up to 950°C and holding for 2 hours, and then fast cooling down to 200°C and isothermally treated for 2 hours. The second one involves homogenizing annealing at 1100°C for 6 hours, then cooling with furnace down to 950°C and holding for 2 hours, then fast cooling down to 200°C and isothermally treated for 2 hours. A unique microstructure of cast steel consisting of martensite and retained austenite plates of various thicknesses and volume fractions was obtained. Additionally, nanometric transition carbides were noticed after the above-mentioned heat treatments. This microstructure ensures high hardness, strength and plasticity ( Rm = 1426 MPa and A = 9.5%), respectively, due to the fact that TWIP/TRIP processes occur during deformation related to the high volume fraction of retained austenite, which the stacking fault energy is above 15 mJ/m ^–2 resulting from the chemical composition of the investigated cast steel.