G. A. Young, M. J. Hackett, J. D. Tucker, and T. E. Capobianco

Knolls Atomic Power Laboratory, Schenectady, NY, USA

© 2012 Elsevier Ltd. All rights reserved.

Crack-like defects can degrade component lifetime by eliminating the initiation stage of phenomena such as fatigue or stress corrosion. Similarly, other flaws (e. g., lack of fusion defects, gas porosity, and inclusions) can act to magnify global stresses, pro­duce locally aggressive environments via their occluded geometry or composition, and initiate cracking. In order to mitigate these flaws, it is critical to differentiate between defect types.

The first step in the prevention of cracking is understanding the temperature range over which the cracking occurs. The primary measure is to determine whether defects are ‘hot cracks’ or ‘cold cracks,’ that is, whether they form above or below the solidus temperature of the alloy. Secondly, the loca­tion of the crack in the weld (composite region, unmixed zone, partially melted zone, heat-affected zone) and in the microstructure (solidification boundary, crystallographic boundary, etc.) must be determined.1 Once these distinctions are made,
strategies to eliminate cracking can be developed via changes to the welding process, weld parameters, filler metal, joint design, fixturing, and/or postweld heat treatment.

The supersolidus/subsolidus distinction, com­bined with the unifying concept of homologous tem­perature, illustrates the commonality of welding defects and degradation mechanisms across alloy sys­tems as shown in Figure 1. Supersolidus ‘hot crack­ing’ defects include solidification cracking, liquation cracking, and hot tearing. Subsolidus ‘cold cracking’ includes precipitation, transformation, and segregation — induced cracking (SIC) mechanisms.