Microstructural characterization of INCOLOY 903 weldments
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I.
INTRODUCTION
T H E microstructure o f the fusion zone o f superalloy welds typically consists o f dendritic gamma matrix and phases formed during the terminal solidification o f the interdendritic liquid. The nature o f the constituents formed during interdendritic terminal solidification is o f fundamental importance in determining the properties o f w e l d ments o f superalloys. The formation o f terminal solidification constituents accompanies an extension o f the temperature range over w h i c h solidification takes place. The presence of a liquid phase from w h i c h terminal solidification constituents form at a l o w e r temperature has been shown by several investigators [Z-Sl to be responsible for hot cracking o f weldments. Even a small volume fraction o f terminal solidification constituent involving topologically closed-packed phases can be detrimental to an alloy's resistance to hot cracking, t51 In several Nb-containing iron-base and nickel-base austenitic superalloys, the presence o f a Laves phase in the terminal solidification constituents has been observed t6,v] to be detrimental to weld metal hot cracking resistance. During solidification o f the welded regions, the gamma dendrites selectively reject alloying elements, such as Nb and Ti, to the interdendritic liquid. In nickel-base superalloys, the interdendritic liquid is depleted in nickel while being enriched in Nb and Ti.t41
R. NAKKALIL, Graduate Student, and M.C. CHATURVEDI, Professor, are with the Department of Mechanical and Industrial Engineering, University of Manitoba, Winnipeg, M B R3T 2N2, Canada. N.L. RICHARDS, Manager, is with the Test and Development Laboratory, Bristol Aerospace Ltd., Winnipeg, MB, Canada. Manuscript submitted August 2 6 , 1992. METALLURGICAL TRANSACTIONS A
The 900 series superalloys, based on the Fe-Ni-Co system, have been designed for low coefficients o f thermal expansion as well as ambient and elevated-temperature strength, m'9'l°l The 900 series alloys are primarily strengthened by Ni3(A1, Ti, Nb) y ' phase. Depending on the carbon content, Nb-rich cubic M X carbides/ carbonitrides can form both during ingot solidification and in the solid state during therrnomechanical processing and heat treatment. It has been s h o w n ILIA2] that in INCOLOY 903,* w h i c h is the most widely used alloy *INCOLOY is a trademark of Inco Alloys International Inc., Huntington, WV.
o f the 900 series, under certain processing conditions, an orthorhombic MNP-type phosphide (E phase), along with fine carbides, can also precipitate on the grain boundaries. It was also suggested that some boron substitution f o r phosphorus may occur in M N P phosphides. In I N C O L O Y 903, overaging a n d / o r high-temperature solution treatments in the temperature range o f 875 °C to 900 °C lead to the formation o f needle-shaped, orthorhombic Ni3(Nb, Ti) 6 phase,t131 However, like many other nickel and nickel-iron-base superalloys, H A Z cracking susceptibility is a p r o b l e m that plagues the 900 series superalloys. Heat-affected
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