Abstract
Abstract In cylinder heads of compacted graphite iron (CGI), the heating and cooling cycles can lead to localized cracking due to thermomechanical fatigue (TMF). Traditionally, TMF behavior is studied by thermal cycling of smooth specimens. The resulting number of cycles to failure (N f) constitutes a single parameter that can be used to predict actual service failures. Nevertheless, there are also some drawbacks of the conventional testing procedures, most noticeably the prolonged testing times and a considerable scatter in test results. To address these drawbacks, TMF tests were performed using notched specimens, resulting in shorter testing times with less scatter. In the case of cast iron, artificial notches do not necessarily change the TMF behavior since the inherent graphite particles behave as internal notches. Using a notch depth of 0.2 mm, the effect of prolonged holding times (HT) on TMF lifetime was studied and a clear effect was found. Extended holding times were also found to be accompanied by relaxation of compressive stresses, causing higher tensile stresses to develop in the subsequent low temperature stages of the TMF cycles. The lifetimes in notched CGI specimens can be predicted by the Paris’ fatigue crack growth model. This model was used to differentiate between the individual effects of stress level and holding times on TMF lifetime. Microstructural changes were evaluated by analyzing quantitative data sets obtained by orientation contrast microscopy based on electron backscattered diffraction (EBSD).
Highlights
CAST iron components in engines, such as cylinder blocks and heads, are exposed for long periods of time to elevated temperatures and are subjected to large numbers of heating and cooling cycles
With respect to the thermomechanical fatigue (TMF) tests under total constraint between 323 K and 693 K (50 °C and 420 °C) performed on pearlitic compacted graphite iron (CGI) samples, the following is concluded:
TMF lifetime was found to decrease by approximately 60 pct, when holding times at 693 K (420 °C) were increased from 30 seconds to 1800 seconds
Summary
CAST iron components in engines, such as cylinder blocks and heads, are exposed for long periods of time to elevated temperatures and are subjected to large numbers of heating and cooling cycles. The influence of the holding time at maximum operating temperature has been evaluated by many researchers and was first reported by Gundlach.[8] In order to study the response of cast iron undergoing thermal cycling, he performed TMF tests on different types of cast irons His results revealed that during thermal cycling, the compressive stress at maximum temperature decreased with the number of cycles, while the tensile stress at minimum temperature increased. The effect of holding time on the TMF behavior of CGI is studied using notched specimens In this way, the total testing time is kept reasonably short in spite of the extended holding times per cycle. Orientation contrast microscopy allows observing minute orientation changes (of the order of less than 1 deg), which are reminiscent and characteristic of the phenomena under consideration
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