The cast austenitic stainless steels such as HK-40 (25 Cr/20 Ni/0.40 C/Fe) and HP-45 (25 Cr/35 Ni/0.45 C/Fe) are widely used in the petrochemical industry for elevated temperature components. A major use is centrifugally cast fired heater tubes in the standard box furnace, including steam reformers and steam cracking units. Operating conditions range from 871 - 1093°C (1600-2000°F) tube metal temperature and 6.9 -20.7 MPa (1-3 Ksi) internal pressure. The performance of these alloys has generally been excellent; the high alloy and carbon content imparts adequate resistance to oxidation and creep, and tube lives well in excess of ten years have been obtained. However, for other components such as outlet headers and furnace manifolds, there have been numerous cases of premature failures with the Cr/Ni/C/Fe stainless steels. Analysis of these failures has generally found that the failures occurred during shutdown or startup of a unit and were caused by the extremely brittle nature of these alloys at low temperatures following elevated temperature service. The loss of ambient temperature ductility after exposure to higher temperatures, shown in Figure 1, is known as aging. The rapid loss of ductility, which will eventually reach 0-3% elongation (in comparison to an as-cast ductility of 10-20%), is caused by the high carbon content of these materials and results from an increase and redistribution of the carbides present in the steel. The as-cast microstructure of these alloys (Figure 2a) contains austenitic sub-grains surrounded by interdendritic arms of eutectic M7C3 carbides. After initial elevated temperature exposure (Figure 2b), finer secondary carbides precipitate within the austenitic subgrains, resulting in a slight decrease in ductility. Following long-term exposure (Figure 2c), a continuous network of primary carbides has set up by the process of carbide precipitation and coalescence. This continuous network provides an easy crack path and results in extremely low ductility.
