Behavior of oxide scales on alloy 800 H and HK 40 during thermal cycling

Abstract

Cyclic oxidation causes degradation of alloys used in the chemical and petrochemical industries. In this paper, the behavior of the protective oxide scales formed on Alloy 800 H and HK 40 was investigated under thermalcycling conditions with upper-hold temperatures of 900°C (Alloy 800 H) and 950°C (HK 40). The atmospheres in the tests were air, air+0.5% SO ₂ and Ar-5%H ₂ -50%H ₂ O. Tests were accompanied by acoustic-emission measurements in order to detect scale failure in situ during the experiments. During cooling the scales were under compression which led to spalling when critical stress values were reached in the scales. The outer-spinel partial layers are more prone to spallation, and the presence of SO ₂ increases the amount of acoustic-emission activity (scale damage). In the case of HK 40 the oxide scales on the as-cast surfaces showed better spallation resistance than those on the ground surfaces. Quantitative model considerations were able to describe the spallation behavior of the protective scales investigated, and critical-temperature-drop diagrams for scale failure are given. The model approach was supported by results from the acoustic-emission measurements.