Creep behaviour, creep damage and precipitation in the austenitic cast steel HK30 at 750 °C

The austenitic cast steel HK30, used in exhaust manifolds of heavy-duty truck engines, was studied with respect to creep, creep damage, oxidation and precipitation at 750 degrees C in air. Even though creep clearly affects the lifetime of the manifolds, there are very few studies available on the topic. The results of the present study provide a basis for materials selection as well as creep data input to component lifetime simulations. In the present study, five specimens were subjected to constant loads resulting in creep rupture times between 1 and 94 days. The minimum creep rate follows Norton's creep law with a Norton parameter (n) of 8.9 and the logarithmic creep rupture time increases linearly with reduced stress. At low stresses secondary creep is predominant observing oxide intrusions from the specimen surface, precipitation of sigma and G-phases, creep cavitation at grain boundaries and weak dislocation networks. At high stresses direct transitions from primary to tertiary creep are observed as well as stronger dislocation networks, but no oxide intrusions, no precipitation, or cavitation are observed. The material has grains of cm size and intergranular fracture is predominant at all stress levels. The segregation of elements during solidification was studied by Scheil/Gulliver simulations and precipitations of sigma and G-phases in the last solidified regions were predicted using Thermo-Cale.