Degradation of 18·8 Series Stainless Steels at Creep Temperature Range and Prediction of Remaining Life Time

Abstract

For the estimation of remaining life time of structural parts used at high temperatures a lot of systematic data should be collected on the specimens which were subjected the simulated history. Types 304H, 316H, 321H and 347H stainless steels and 18 Cr 10 NiTiNb stainless steel were crept for up to 60000hr at 600-700°C and were also aged for up to 25000hr at 600-800°C. The cavity density was determined in the creep-ruptured specimens. The change of optical micro-structure due to aging was examined and X-ray analyses of extracted residues were performed. The change of mechanical properties, particularly creep rupture properties, were also investigated.
The results show that the creep strain of structural parts after a long service period is usually less than 1%, and that the creep strain of a few per cent is necessary for detecting creep cavities. In addition to well-known M₂₃C₆ carbide, sigma phase and M₇C₃ were newly found in Type 304H aged at 700°C for 25000hr. The creep rupture strength of Types 321H and 347H steels decreased clearly after long time aging. However, the creep rupture strength of these steels for any aging condition was not less than 80% of that of the as-solution treated. When the creep strain and cavitation of materials in service are negligible, its remaining life time is considered to be predictable from synthetic analysis of various properties, i.e. creep rupture, impact value, hardness, etc., as refering the data on aged materials under no load condition.