Monotonic and Cyclic Creep of Cast Materials for Exhaust Manifolds

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Monotonic and Cyclic Creep of Cast Materials for Exhaust Manifolds

Zhu, Baohua / Öberg, Christian / Rablbauer, Ralf / Jonsson, Stefan

Cast materials are creep tested between 600 and 900°C using three methods: (i) tensile testing at different strain rates, (ii) stress relaxation during thermal cycling and (iii) traditional creep tests at constant load. Comparisons are made between fast and slow methods and between monotonic and cyclic deformation modes. The tested materials, SiMo51, SiMo1000, Ni-resist D5S and HK30, are used for exhaust manifolds in heavy-duty diesel engines. The fast and cheap methods, (i) and (ii), were used on all materials, while the tedious and costly method, (iii), was used on SiMo51 only. The creep rates from monotonic tensile tests and stress relaxations during thermal cycling agree well. There is no difference between monotonic and cyclic creep rates, and cyclic rates are practically unchanged with the number of thermal cycles. No or small differences in creep rates are observed when comparing tension and compression, although three of the materials include large graphite nodules. At 700°C, a Norton plot for SiMo51 shows coinciding results for tensile test and compressive stress relaxations, whereas the minimum creep rates from constant load tests fall one order of magnitude lower, but with the same slope. For all materials, the Norton creep parameters are evaluated with accurate reproduction of the experimental data. For HK30, two sets of parameters are needed because of deformation hardening.