Like austenitic stainless steels, single-phase nickel — based alloys are susceptible to solidification and liquation cracking (Chapter 2.08, Nickel Alloys: Properties and Characteristics). Common melting point suppressants that must be controlled to avoid cracking include tramp elements such as sulfur and intentional alloying additions such as boron, zirconium, niobium, and molybdenum.7,10-15,18,109,110 Addition­ally, both low-strength and precipitation hardenable grades can be susceptible to PIC mechanisms (ductility dip and strain-age cracking) as discussed previously.

4.09.4.2 Zirconium Alloys

Zirconium alloys are readily weldable but as with all reactive metals, need special precautions to prevent pickup of interstitial elements such as oxygen, carbon, and nitrogen that can degrade both the mechanical

properties and the corrosion performance of the weld (Chapter 2.07, Zirconium Alloys: Properties and Characteristics).9 , , 2 Vacuum or inert gases

(argon, helium, or Ar-He mixtures) can be used to shield zirconium, but again, care needs to be taken to ensure sufficient vacuum level or gas purity to pre-

92,111

vent contamination.

Zirconium alloys can be susceptible to both super­solidus and subsolidus (i. e., hydride-type) cracking. Supersolidus cracking is typically solidification-type and many common alloying elements and/or poten­tial contaminants promote susceptibility. For exam­ple, iron, nickel, chromium, and copper all stabilize low-temperature eutectic reactions, and small con­centrations can greatly increase the solidification temperature range. An example of this is shown in Figure 22, which compares the maximum crack length in transvarestraint tests for a Zr-Cr alloy, Zircaloy-4 (Zr-4), and Zr-2.5Nb welded under iden­tical conditions. As shown, the Zr-Cr alloy exhibits the most susceptibility to solidification cracking.

Acknowledgments

The Authors are indebted to the welders, technicians, specialists, and engineers of the Welding & Materials Process Development Unit at Knolls Atomic Power

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Laboratory, whose dedication and expertise made this work possible. They are also grateful to Dr. David

S. Knorr of General Electric for his important con­tributions to the manuscript.