Alloy steels including elements like Ni, Cr and Mo are well known and widely used for their excellent mechanical properties. These additional elements are used to improve the chemical-physical properties of the steel, transforming it into a material suitable to withstand high loads and to resist fatigue.
For example, large axle shafts and aircraft parts are made of NiCrMo steel. From quality control to research and development, the measurement of the resilience and the absorbed energy during an impact test enable to evaluate the suitability of the material for specific applications and the effect of composition on the mechanical behavior of the material. Impact testing on steel are usually requiring high capacity impact testing instrument.
We have tested some NiCrMo steel specimens by using a CEAST 9350 drop tower equipped with a strain gage tup, 90kN force range and including a Charpy insert, and related specimen vice and shoulders. All accessories selected to impact specimens are in accordance to ASTM E23 standard. DAS64k data acquisition system and VisualIMPACT software complete the equipment to perform the instrumented impact test.
The specimen were prepared with dimensions and notch according to the specification of the ASTM E23 standard (Type A). They were tested at a constant temperature of 20°C, and the impact energy of the instrument was set to 302 J, with an initial velocity of 5.5 m/s. Acquisition parameters were accordingly set, to record an impact event of about 10ms.
Five metallic specimens were tested to determine the absorbed energy by Charpy test method. The software allowed to display detailed impact curves and results, showing for example the force acquired during the impact as a function of the calculated deformation of the specimen. All impacted specimens have shown a similar type of brittle failure. The absorbed energy was evaluated for any tested specimen from the acquired signal registered from the instrumented tup. The average absorbed energy for such a material is in the range of 25 J. It was also interesting to notice that some specimens reached higher deformation at break respect to others of the same lot. This behavior is also consistent with the appearance of the specimens after test, revealing some variability in the response to the test. A similar behavior could be also explained in terms of variation in the contents of alloying elements of the steel under analysis.
Note: It is necessary to mention that this way to perform Charpy impact test, by means of the drop tower and according to ASTM E23 standard, has been done for research purposes only. Please, refer to ASTM E23 international standard for more precise apparatus general requirements and exact specifications.