EQUIVALENT CHARPY-V IMPACT CRITERIA FOR NODULAR CAST IRON Kim R.W. Wallin
VTT Technical Research Centre of Finland, Espoo, Finland Copyright © 2014 American Foundry Society Abstract
The relevance of the Charpy-V test for nodular cast iron is re-examined here and as a result, a new definition of an equivalent energy criterion for nodular cast iron is pro- posed. Attempts to transfer the Charpy requirements devel- oped for steel to nodular cast iron have not been successful. Actually a recent conclusion was that “Charpy impact test- ing is neither an accurate nor acceptable way to measure
Introduction
On occasion, it may be impossible or impractical to perform fracture toughness tests on a material. There may be insufficient material available for testing, or the cost, in relation to the struc- ture, may be too high. In these instances, the fracture toughness may sometimes be estimated indirectly from a simpler test. An example of such a test is the Charpy impact test. Today, the Charpy impact test is still the most commonly used standard test to evaluate a material’s fracture properties. The test is quick and simple to perform and requires only comparatively small specimens. Nearly all materials and manufacturing standards apply requirements based on the Charpy impact test.
The development of the Charpy impact test was led by A. G. A. Charpy at the beginning of the 20th century; thus, the test is more than 100 years old. The most common notch geome- try today is the V-notch, but other geometries are sometimes used. The main parameter determined in the Charpy test is the energy required to fracture the specimen.
The use of notched impact specimens stems from the revela- tion that dynamic loading and stress concentrations promote cleavage fracture, i.e. such a loading was more severe from a structural integrity point of view than for a simple tension test. The choice of notch configuration is usually connected to material type. For structural ferritic steel, which exhibit a ductile-to-brittle transition, the V-notch is commonly ap- plied. Other geometries or even un-notched specimens are used for brittle materials like some cast iron, or tough mate- rials like austenitic stainless steel.
The Charpy-V data is normally used in the context of some tran- sition temperature concept. Transition temperature concepts are generally used with structural steels that undergo a ductile to
International Journal of Metalcasting/Volume 8, Issue 2, 2014
impact toughness in cast iron. It is inappropriate to use it as a means of comparison between cast iron and steel.” The reason for this conclusion lies in the fact that nodular cast iron has different transition behaviour than steel and they experience notches differently.
Keywords: cast iron, Charpy-V, fracture toughness
brittle transition. However, Charpy-V tests may also be used to indirectly estimate the material ductile tearing properties.
The Charpy tests were originally used as quality control tests. After World War II, the analysis of the failures in welded merchant ships changed the nature of these tests to be more of a design tool. With 1,289 out of the 4,694 ships built during the war experiencing serious or potentially seri- ous fractures, a large effort was put into understanding the reason for the failures. Ship plates where cracks had initiated in general showed a lower toughness at the failure tempera- ture than plates where the fracture had arrested. This led to the introduction of the first transition temperature concepts.
Much of the early development work was performed at the United States Naval Research Laboratory (NRL). Their test methods and transition temperature definitions are still being used throughout different industries. The materials that were used in the NRL research were ship steel plates common for the time. This is an important detail that is often forgot- ten when the transition temperature definitions are used for other classes and types of metals.
Attempts to transfer the Charpy requirements developed for steel to nodular cast iron have not been successful. A recent conclusion1
was that “Charpy impact testing is neither an accurate nor acceptable way to measure impact toughness in cast iron. It is inappropriate to use it as a means of compari- son between cast iron and steel.” The reason for this conclu- sion lies in the fact that nodular cast iron has different transi- tion behaviour than steel and they experience notches differ- ently. By accounting for these differences, the relevance of the Charpy-V test for nodular cast iron is re-examined here and, as a result, a new definition of an equivalent energy criterion for nodular cast iron is proposed.
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