Relevance of Drop-Weight Testing in the Determination of the Reference Nil-Ductility Temperature

The drop-weight test has become commonplace in the testing of ferritic steel and weld metal used in several types of components in nuclear reactor pressure vessels. But is it now obsolete? Is there another predictive tool that should be used to determine the RTNDT of weld deposits?

Commercial light-water nuclear reactors built in the U.S. are required by Title 10 Code of Federal Regulations Part 50 (10CFR50) to be designed to incorporate fracture toughness considerations. Presented in Appendix G of 10CFR50, the concept of a reference nil-ductility temperature (RTNDT) was created roughly three decades ago to ensure that a minimum level of toughness is present in a ferritic steel and weld metal, especially after being irradiated.

The ASME Boiler and Pressure Vessel Code, Section III, outlines the determination of RTNDT at a temperature above the ductile to brittle transition temperature, as formulated through the use of the nil-ductility transition temperature (NDTT) and Charpy V-notch (CVN) toughness tests.

In order to begin a discussion of the determination of the RTNDT, it is necessary to discuss just exactly what the NDTT is and the manner in which it is established. The NDTT is defined as the temperature above which a steel will fracture in a ductile mode and exhibit plastic deformation at nominal stresses beyond its yield strength. Below this temperature, the steel will fail in a brittle fashion when loaded to its yield strength.

A value for the NDTT in ferritic steel at least 5/8 in thick is generated through the ASTM E208 drop-weight test. This test was developed in the early 1950s by the U.S. Naval Research Laboratory and was used to examine the conditions that lead to brittle fractures in structural steels. Since then, the drop-weight test has become commonplace in the testing of ferritic steel and weld metal used in several types of components in nuclear reactor pressure vessels.

For those readers who are not familiar with the drop-weight test, a brief summary will now be provided. The test is conducted upon a steel specimen (more like a block) fabricated from the material to be used in service. As shown in Figure 1, a 1in thick test specimen is to be 14 in long by 3.5 in wide, and a one-pass crack starter weld a few inches in length is deposited on one side. If it is weld metal for which the NDTT is to be ascertained, the starter weld is oriented perpendicular to the direction of the weld joint. Into this weld is machined a groove, and the bottom surface of the groove is 0.07 in to 0.08 in above the surface of the block of material being tested.

Now the “cracked” side of the block is placed face down on top of an “anvil” that supports the end of the specimen in place. Also included as part of the anvil assembly is a “stop” that limits the deflection of the specimen.

And finally, as the name of the test implies, a guided, free-falling weight is dropped onto the side of the block opposite from the crack starter weld from a predetermined height. The weight striking the surface possesses a cylindrical shape, and can vary from 50 lb to 300 lb (and is selected based upon the yield strength of the material being tested). The mass of the weight and the distance of the drop used produce an impact energy, which is selected based upon the yield strength of the material.

The qualitative nature of the crack resulting from the drop-weight test is the criterion that provides an estimate of the NDTT. For example, in order for a sample to be deemed as showing a “break” condition, the crack produced from the test generally must touch at least one edge of the specimen. If it does not, the test may be considered exhibiting a “no-break” condition.

The selection of the temperature at which the drop-weight test must be conducted can appropriately be described as a trial-and-error process. As one would expect, a testing operator would naturally want to conduct the test in as few tests as possible – ideally in as few as three tests. A NDTT would be confirmed when one specimen at a lower temperature would exhibit a “break” condition, while two tests at the same temperature above the first would both show “no-break” result. Upon achievement of two no-breaks at this higher testing temperature, an estimate of the nil-ductility transition temperature defined as TNDT has now been attained.

On several occasions, we have received a considerable number of requests from nuclear constructors to utilize a TNDT acquired in the drop-weight test in the “determination” of an RTNDT for our filler metals. To make this connection between TNDT and RTNDT, a set of three CVN tests are conducted at a temperature of TNDT + 60 deg F. If, at this temperature, the average impact toughness obtained from three CVN breaks is greater than 50 ft-lb and the average lateral expansion is greater than 35 mils (.035 in, not to be confused with 35 mm), then the RTNDT is determined to be equal to or greater than the TNDT.