In this installment we talk about the variables that affect the transition temperature and look at the future of impact testing.
Variables Affecting Transition Temperature
The transition temperature in steel can be affected by a number of factors, the most interesting of which is the microstructure (Fig. 3). If we compare a quenched-and-tempered microstructure with that of a normalized-and-tempered microstructure, we can see the effect of these two different heat treatments on the low-temperature impact strength of a 4340 medium-carbon alloy steel with high hardenability. While both heat treatments produced the identical hardness, the microstructure of the quenched-and-tempered material consisted of spheroidized carbides while the normalized structure consists of pearlite. As one quickly observes, the transition temperature is approximately 150ºC (300ºF) lower for the quenched-and-tempered microstructure.
The chemistry of the steel also plays a prominent role. For example, the transition temperature for a 1% manganese, 0.30% silicon mild-carbon steel rises rapidly with increasing carbon content. Thus, for all other factors being equal, the higher the carbon content, the more prone a material is to brittle fracture, especially near room temperature. Phosphorous has an even stronger effect, while increasing manganese contents tends to lower it.
Trends
In some ways impact testing has changed very little over the years, and both Charpy and Izod impact tests are often performed today in exactly the same way that they have been in the past. The key difference today is the way in which the data that comes from the test equipment is handled.
ASTM E23 (Standard Test Method for Notched Bar Impact Testing of Metallic Materials) is perhaps the most widely adopted standard on metals impact testing established guidelines for test specimens (e.g., configuration, orientation, machining), preparation of the apparatus, test temperature conditions and actual test procedures.
While mechanical pointers are still in use, many equipment manufacturers also include a rotary encoder on the machine axle to collect data. This data can then be used not only to determine the height to which the hammer rose but also to calculate the impact speed of the hammer. Further manipulation can yield impact stress data, which can then be instantly displayed on the machine.
One new trend currently being driven by the nuclear industry is toward a fully instrumented impact tester. In these machines, the striking tup is equipped with strain gauges that are in turn hooked up to high-speed data-capture equipment. A typical impact test is over in approximately 10 milliseconds or, quite literally, the blink of an eye. This sophisticated equipment can capture up to 20,000 data points during this test. This data can be gathered to generate a graph, which represents the striking tup position as it travels through the specimen.
This new technology won’t change the way in which the test is performed or the equipment required to perform it. Instead, we now have access to highly repeatable and accurate data with the capability of greater resolution and interpretation of results.
References
1. Dieter Jr., George E., Mechanical Metallurgy, McGraw-Hill Book Company, 1961
2. Wilby, A. J. and D. P. Neale, “Defects Introduced into Metals During Fabrication and Service,” Vol. III, Materials Science and Engineering, Encyclopedia of Life Support Systems
3. Reed-Hill, Robert E., Physical Metallurgy Principles, D. Van Nostrand Company, Inc., 1964
4. Herring, Daniel H., “Understanding Component Failures Parts 1 & 2,” Industrial Heating, July/August 2013
5. Herring, Daniel H., Atmosphere Heat Treating, Volume II, BNP Media, 2015
6. ASTM E23 (Standard Methods for Notched Bar Impact Testing of Metallic Materials)
