ASTM A993 - Dynamic Tear Testing of Cast Irons to Establish Transition Temperature
ASTM A993 Dynamic Tear Testing for Graphitic Cast Irons: A Comprehensive Guide
Introduction
Dynamic Tear Testing is a crucial method used to determine the transition temperature of graphitic cast irons. This testing process involves the examination of fracture appearance to identify the transition temperature accurately. The transition temperature is the point at which the fracture surface exhibits an equal mix of ductile and brittle fracture behavior. This article discusses the test method for Dynamic Tear Testing, including the test procedure and equipment used. It also highlights the importance of safety considerations when conducting these tests.
Test Method and Referenced Documents
The test method for Dynamic Tear Testing of graphitic cast irons is standardized by ASTM (American Society for Testing and Materials). The details of the test procedure, apparatus, and specimens are specified in this method.
Referenced Documents:
ASTM A 644: Terminology Related to Iron Castings
ASTM E 208: Test Method for Conducting Drop-Weight Test to Determine Nil-Ductility Transition Temperature of Ferritic Steels
ASTM E 604: Test Method for Dynamic Tear Testing of Metallic Materials
Summary of Test Method
The Dynamic Tear (DT) test involves subjecting a single-edge notched beam of graphitic cast iron to impact loading in three-point bending. The DT specimens are fractured using a drop-weight machine at various temperatures to cover a range from -73°C (-100°F) to room temperature 22°C (72°F). The fracture surface of each specimen is then examined to determine the ratio of ductile to brittle fracture at different temperatures.
By plotting the values of ductile to brittle fracture ratio against the corresponding temperatures, the transition temperature can be identified. The temperature at which the fracture surface shows 50% ductile and 50% brittle fracture is considered the transition temperature.
Test Specimen
For the Dynamic Tear Test, the DT specimens are prepared with specific dimensions and can be cast in green sand or dried, baked, or chemically bonded molds made with siliceous sand and appropriate binders. The specimens are cooled until they turn completely black before being removed for testing.
The test specimens must be machined to create a notch on one side and an impact surface on the other, following the specifications provided in the standard.
Apparatus
A typical fracture test drop tower is used for the Dynamic Tear Testing. It consists of a drop weight with a non-instrumented tup, and the combined weight of these two items is 22.7 kg (50 lb). The tup design follows the guidelines in ASTM E 604.
The weight is allowed to fall through guide rails to cause DT specimen fracture, and frictional effects that slow the velocity of the falling weight are minimized. The standard distance between the bottom of the tup and the top (impact) surface of the specimen is 760, 6 7.6 mm (30 6 0.30 in.).
The fixture on which the specimen rests at the time of impact consists of anvils with a radius of 12.7 6 0.25 mm (0.500 6 0.01 in.), and the distance between anvil centers is 165.0 6 1.65 mm (6.500 6 0.403 in.).
Procedure
Test Temperature Range: Test temperatures typically fall within the range of -73°C (-100°F) to room temperature 22°C (72°F). To determine the transition temperature accurately, samples must be tested at a minimum of two temperatures. The two test temperatures should bracket the transition temperature and be no more than 14, 6 1°C (25, 6 2°F) apart.
Number of Specimens: A minimum of three specimens are tested at each temperature, but if statistical analysis is required, a minimum of six specimens is necessary.
Pre-Testing Preparation: Before testing, the DT specimens are heated or cooled to the desired test temperature and held at that temperature for a minimum of 45 minutes. A thermocouple is used to monitor and ensure the specimens are at the desired temperature.
Testing Procedure: Each DT specimen of known temperature is placed on the anvil, with the notch side facing down. The drop weight is released within 15 seconds from the time of specimen removal from the heating or cooling medium.
Fracture Surface Examination: After testing, the percent ductile and percent brittle fracture areas on the fractured surface of each specimen are rated. This can be done by visual estimation or preferably by a quantitative method such as photographing the fracture surface and using a transparent film grid to count the number of intersections on each type of fracture.
Determination of Transition Temperature: The percent of ductile to brittle fracture area for each specimen is plotted against the test temperature, and a best fit curve is drawn through the data points. The temperature corresponding to 50% of brittle fracture area is considered the transition temperature.
Confirmation of Transition Temperature: To confirm that the transition temperature is not higher than a specified value, the mean value of the percent brittle (cleavage) fracture for all samples tested at the specified temperature is calculated. The mean value must be less than or equal to 50% for conformance.
Reporting
The test report should include essential information such as material identification, cast date, sample identification, and the transition temperature results. Additionally, any non-standard drop heights, drop weights, or sample thicknesses used during testing must be reported alongside the results.
Safety Considerations
It is crucial to acknowledge that this test method does not address all safety concerns associated with its use. Users must establish appropriate safety and health practices and evaluate the applicability of regulatory limitations before conducting Dynamic Tear Testing.
Conclusion
Dynamic Tear Testing is a valuable method for determining the transition temperature of graphitic cast irons. By carefully following the test procedure and using the specified apparatus, accurate results can be obtained. This test method provides essential information for assessing the fracture behavior of graphitic cast irons under different temperature conditions, aiding in material selection and engineering design processes. However, researchers and engineers must prioritize safety precautions when performing these tests to ensure a secure testing environment.