The influence of plasma nitriding and post oxidising treatment on the resistance ofaisi h11 to cycling immersion in molten aluminium alloy

Abstract

This paper is based on an experimental tests plan, which aims at strengthening the knowledge on the influence on die life of plasma nitriding and post oxidising surface modification treatment.

An AISI H11 steel obtained through modern vacuum melting and remelting processes was used as base material.
Un-notched impact test specimens were fabricated from a core vacuum heat treated block.
Some specimens were plasma nitrided and oxidised. Impact toughness was evaluated both on some of the heat treated and surface modified specimens.
The other specimens were subjected to a program of cyclic immersion in molten aluminium alloy and cooling in water.
The heat checking and soldering phenomena were analysed through periodic inspections and, after the test through, the evaluation of cracks and cavities length and density.
At the end of thermal fatigue test the impact test was also performed to assess the toughness modification in comparison with the initial condition.
Finally, a metallographic investigation was performed through optical and electron microscopy on all the specimens considered.
Both soldering and heat checking were found to be active damaging mechanisms on both simply heat treated and nitrided and oxidised specimens.
However, all the revealed damages resulted to be limited to the surface and sub-surface layers.
The clearer effect which was detected was the softening, clearly visible through impact toughness increases and microhardness decreases along with the number of immersion cycles.
The amount of softening was found to be limited by the surface modification treatment.
The plasma nitriding and post oxidising treatment had also prevented the metallic surface from corrosion attack (acting as a barrier to the aluminium aggression) at least for the first 8,500 immersion cycles.
The developed test rig for cyclic immersion in molten aluminium alloy provided results similar to other experiences currently present in the open literature.