At the time of the project, there had not been a new nuclear power plant brought online in the United States for 17 years. These reactors had developed cracks with time, and they had to be repaired with weld overlays.* Nickel based wire is used to weld the overlays to repair reactors. Nickel alloy is used for its high temperature and corrosion resistance. Nickel based weld wire has a tendency to have pure fluid flow and wetting, but also oxidation and cracking problems. This makes welding with Nickel based wire difficult for the operator.
Ductility Dip Cracking (DDC) is a problem common to these alloys. DDC is an intergranular form of cracking that typically asserts itself in the GCHAZ of multipass welds or in weldments that have been heat treated within the temperature range where aging occurs. Ductility dip cracking occurs with the presence of two main factors. The first factor is intragranular precipitation hardening. When this happens, the intergranular region is significantly weakened locally. The second factor is stress. Stress can come from many sources, but usually exerts itself in strains due to the thermal cycle of a typical welding process.
HAZ microfissuring is another form of cracking found in these alloys. It is important to realize that microfissuring is a form of liquation cracking and is not analogous of ductility dip cracking. Microfissures form on cooling in the PMZ of the HAZ. Low melting point constituents, such as sulfur, promote local melting in these regions. This liquid phase will then liquate along the boundaries of still-solid grains. Once solidified, these high sulfur regions will separate in the presence of stress and reveal a crack. Base materials containing higher levels of sulfur are much more susceptible to localized liquation cracking. Therefore, filler materials with a high sulfur content, such as Alloy 52, will also crack in the PMZ when welding over a previous pass (such as in weld cladding or overlays).
Alloys 52, 52M, and 52MSS are nickel based alloys that are currently being used in the power generation industry for their superior corrosion resistance and toughness at elevated temperatures. These alloys are precipitation hardenable, and strengthened by controlled heating. Controlled heating, such as in heat treatment, precipitates a second phase known as gamma prime. Gamma prime is the most important phase from a strengthening standpoint and is ordered in a face-centered-cubic configuration. This phase is based on the compound Ni3Al, which has a high solubility for Ti and Nb. Therefore, gamma prime formation will vary significantly depending on the composition of the base material, as well as the thermal profile.
Developing different parameters to use with this nickel based weld wire could improve the operator’s ability to weld with it. This would lower the time needed inside the reactor to make repairs and make it safer for the operator. Optimizing the parameters would result in a higher quality overlay.
The purpose of the project was to correlate chemistry to wettability, weld penetration, oxide formation, and overall cracking susceptibility. Two separate experiment methods were used to determine the individual effects of alloying elements and oxygen in the GTAW shielding gas.
The first was a surface tension analyzing test. In this test, a specific amount of weld wire (usually 0.75 grams) was weighed out and placed on a pure copper block. A welding arc was then passed over the wire to melt it into a “button.” As this was being accomplished, a thermal imaging camera was recording the shape of the button as it solidified and cooled. Image analyzing software was then used to record the contact angle between the pure copper block and the button. These angles were used to compare the surface tension and wettability of each weld wire of differing alloy compositions. A gas analyzer was used to evaluate the oxygen and nitrogen levels of each weld button after testing. The gas analyzer provided a method of evaluating oxidation potential of each weld wire in different ambient atmospheres.
A second experimental method was used to provide a more practical comparison, as well as to validate the first experimental method. The second experimental method was a controlled weld test. In this test, each wire was used to create a bead-on-plate, which was later analyzed for depth/width ratio, dilution, and cracking susceptibility. Both single pass and double pass welds were made in order to induce cracking in the double pass welds. The double pass welds were examined in the SEM for cracking. All weld wire chemistries were compared with data procured from the two experiment methods and analyzed for correlations. These correlations were compared with previously known effects of alloying elements for confirmation of experiment accuracy.
Trends in alloy/element behavior were observed, evaluated, and recorded. The information was given to Areva, and direction for further research was recommended.
Areva NR, Lynchburg, VA: $55,000 / 1 year
*Areva NR is the U.S. affiliate for Nuclear Repair of Areva LTD of France, the third largest energy company in the world.