Segregation During Welding: Friend or Foe to Engineered Weld Properties

Segregation during Welding: Friend or Foe to Engineered Weld Properties

Stephen Liu and David L. Olson

Center for Welding, Joining and Coatings Research

Colorado School of Mines

Golden, Colorado 80401, U.S.A.

Abstract

Beginning with a molten puddle of an alloy, thermodynamics dictates the partitioning of alloying elements during solidification. As a result, regions enriched with one or more of the elements will occur. Under appropriate conditions, cells or dendrites may form. Element enrichment will then occur in the core of the cells/dendrites or in the inter- cellular/dendritic regions. Hot cracking and cold cracking are both good examples of the negative effects of positive segregation of certain deleterious elements.

However, accumulation of an element during solidification may not explain entirely the mechanical behavior of a weld. The fracture behavior of an alloy is often related to the concentration gradient of the segregation element instead. It has been observed that crack propagation would preferentially follow the steeper gradient. Modulating the concentration gradient by controlling the solidification behavior will then affect the crack propagation behavior and performance of an engineering component.

Interstitial elements such as hydrogen, carbon and nitrogen are typically recognized as harmful to steels and steel weldments. Nevertheless, hydrogen in higher concentrations has been shown to affect the plasticity of steels and can be used to assist in forming operations. Due to aging and formation of carbides and nitrides, these two elements are typically maintained at very low levels. However, it is known that higher concentrations of the two (at the order of 0.5 wt%) in steels will result in the formation of superlattices that proportion unique hardness and wear properties to the alloys. This work will address the use of segregation to engineer welds that exhibit unique sets of physical and mechanical properties.