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.
