Creep Behavior of High Temperature Cast Materials for Exhaust Applications
Time: Fri 2020-12-18 09.00
Subject area: Materials Science and Engineering
Doctoral student: Christian Öberg , Skolan för industriell teknik och management (ITM)
Opponent: Professor John Hald, Technical University of Denmark
Supervisor: Professor Stefan Jonsson, Skolan för industriell teknik och management (ITM); PhD Baohua Zhu, Scania
This thesis focuses on creep of four cast materials intended for exhaust manifolds in heavy-duty truck engines. Two of the materials are ferritic ductile cast irons, SiMo51 and SiMo1000, one is an austenitic ductile cast iron, D5S, and another one is an austenitic cast steel, HK30. The ductile cast irons, rich in carbon, have a microstructure with graphite nodules and precipitates, mainly carbides and intermetallics. The cast steel, on the other hand, being meagre in carbon, has precipitates but lacks graphite nodules. During service, the exhaust components are thermally cycled up to 800 °C in a locked stated, bolted to an engine block. This gives rise to creep deformation, fatigue, oxidation and microstructural changes. Driven by the development of environmental friendly engines of lower emissions, the exhaust gas temperature is increasing, continuously leading to higher demands on the materials.
The main aim was to investigate the creep behavior and related phenomena of the included materials. A secondary aim was to compare results from three types of tests, i) SRTC (stress relaxations with thermal cycling), provoking stress relaxations in a locked specimen subjected to thermal cycling, ii) STT (sequential tensile test), changing the strain rate at selected strain levels during a tensile test at a selected temperature, iii) CL (constant-load creep test), i.e. traditional creep testing, applying a constant load at a given temperature. SRTC and STT are intended as quick and cheap methods while CL is generally considered slow and associated with high costs. Results of the three methods were regularly compared in Norton plots, i.e. double logarithmic plots of stress and strain rate.
Results of i) SRTC (in compression) and ii) STT (in tension) were generally in very close agreement which indicates that creep of the included materials is independent of loading direction. In addition, the creep rates obtained by SRTC were also constant with number of cycles. Both findings facilitate modeling of cyclic creep, although this was not in the scope of the present thesis. There were discrepancies between data sets of CL and SRTC/STT which could not be explained, although several reasons were discussed. In addition, the time-dependent creep damage which develops during a slow CL test is always missed in quick stress relaxation tests or tensile tests.
The microstructural events taking place during creep were documented using LOM, SEM and EBSD microscopy techniques, with various etching and sample preparation procedures.
When CL tested at 700 °C, SiMo51 showed primary creep, more or less directly followed by tertiary creep. The tertiary creep regime was in turn divided into two stages of which the first was associated with the formation of typical creep cavities around the graphite nodules and at the grain boundaries, and the second associated with larger cracks between the graphite nodules. Oxidation was significant but not enough to be held responsible for the tertiary creep stages. The oxidation on the surface and around the graphite nodules was explicitly studied. Layered oxides were identified by combining EDX data with thermodynamic calculations.
Both D5S and HK30 were CL tested at 750 °C, reflecting a higher service temperature of these materials compared with SiMo51. After prolonged creep exposure, HK30 exhibited typical creep cavitation at the grain boundaries, precipitation of sigma phase and G phase, oxide intrusions and recrystallization in a thin layer at the specimen surface. D5S exhibited various types of cavities/voids around the graphite nodules (like SiMo51 at 700 °C) and fracture occurred by shear cracks growing nodule-to-nodule. Various precipitates developed during creep.