Kinetics of grain growth in the weld heat-affected zone of alloy 718
- PDF / 4,277,643 Bytes
- 13 Pages / 597 x 774 pts Page_size
- 42 Downloads / 189 Views
I.
INTRODUCTION
C O N S T I T U T I O N A L liquation of precipitate particles and the subsequent formation of grain-boundary liquid in the subsolidus portion of the weld heat-affected zone (HAZ) have been observed in several commercial alloy, t~-61 The mechanics of transition from the individual pools of liquid, produced by a constitutional liquation of precipitates, to a continuous grain-boundary film involves the capture of the pools of liquid by the moving grain boundaries and the spreading of the liquid along the grain boundaries assisted by surface tension f o r c e s , tT] These processes go to completion during the heating cycle and a microstructure consisting of a continuous grainboundary liquid film is produced at the peak temperature of the thermal cycle, within a certain distance from the fusion line. tT] At HAZ locations where the peak temperatures just exceed the solidus temperature of the matrix, liquation initiates at the grain boundaries independent of the constitutional liquation mechanism to form a continuous film of grain-boundary liquid. Liquation cracks occur in the HAZ if the grain-boundary liquid film remains stable at a temperature and time when the tensile stresses develop in the HAZ close to the fusion zone. Since tensile stresses in the weld HAZ develop only after the HAZ cools down to a certain temperature, the susceptibility to liquation cracking depends on the temperature range during cooling in which the grain-boundary liquid exists. A predictive model for liquation cracking can be developed if the temporal evolution of the grain-boundary liquid and the thermal stresses can be modeled for any given location in the HAZ. The kinetics of formation and B. RADHAKRISHNAN, formerly Research Associate, with the Department of Materials Science and Engineering, University of Alabama at Birmingham, Birmingham, AL, is now with the Metals and Ceramics Division, Oak Ridge National Laboratory, TN 378316096. R.G. THOMPSON, Professor, is with the Department of Materials Science and Engineering, University of Alabama at Birmingham, Birmingham, AL 35294. Manuscript received November 18, 1992. METALLURGICAL TRANSACTIONS A
solidification of the grain-boundary liquid in the subsolidus HAZ has been modeled for a simple binary assuming plane-front solidification, local equilibrium at the solid-liquid (S-L) interface, and solute diffusion in the solid. ]8] The model also assumed that the grain boundaries were held stationary by the liquid, which completely wet the grain boundaries. Such an assumption was based on the arrest in the grain growth observed for 18Ni maraging steel, Iv] where grain-boundary liquid in the subsolidus portion of the HAZ was produced by constitutional liquation of titanium sulfide particles present in the steel. A. Liquation and Grain Growth The effectiveness of the metastable liquid as an obstacle to grain growth should depend upon the ability of the liquid to wet the grain boundaries, which is a function of the imbalance of the surface tension forces at the triple point between t
Data Loading...
