Influence of tempering temperature on the corrosion behaviour of plastic mould steels X38CrMo16

The modified stainless martensitic chromium steel X38CrMo16 offers excellent toughness, corrosion and wear resistance. The typical heat treatment of this steel consists of a hardening treatment followed by a more-step tempering procedure. Tempering causes the precipitation of secondary carbides, nitrides and carbonitrides which are responsible for the improved hardness at elevated temperatures. Experiments have shown that the behaviour in corrosion resistance varies within the temperature range commonly used for tempering this steel. The corrosion resistance reached a minimum at 540 °C until it increases again. For the microstructural investigations the samples have been tempered for shorter times and the minimum moved to 575 °C. At that temperature, significant chromium depletion zones in the size range of approximately 6 nm extent could be visualized by means of atom probe tomography. The chromium concentration decreased to 10 at% within these zones. Depletion zones could be shown for small nitrides as well as for larger carbides which were formed during tempering. The existence of these local anodic zones could be confirmed by TEM investigations.