Weld thermal cycles and precipitation effects in Ti-V-containing HSLA steels
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I.
INTRODUCTION
M A N Y high-strength low-alloy (HSLA) steels contain two or more of the c o m m o n microalloying additions (Nb, Ti, V), so that the benefits of grain size control during rolling or welding operations and effective precipitation strengthening can be realized in a single alloy composition. The mutual solubility of the microalloying elements in carbonitrides having the rock-salt structure complicates any thermodynamic and kinetic analysis of precipitation effects on these steels. II-5] This problem is one that must be considered during welding, when the rapid thermal cycle of the weld can lead to dissolution followed by reprecipitation of the same or a different precipitate species close to the peak temperature of the thermal cycle. 15j Under these conditions, the extent to which an equilibrium thermodynamic analysis can be used to predict the composition and volume fraction of precipitates formed at any point in the thermal cycle is questionable. Most previous studies of precipitation phenomena during welding have used thermal simulation techniques to study the response of carbonitride particles to a rapid thermal pulse. In these techniques, the peak temperature is usually chosen to match that in the coarse-gained heataffected-zone (CGHAZ) adjacent to the fusion line ( - 1 4 0 0 ~ to 1450 ~ with the cooling rate from the peak temperature taken to match that expected for the particular heat input being simulated. The advantage of this technique is that it leads to a large CGHAZ of uniform properties that is amenable to further testing. One disadvantage of the technique is that the residual stress pattern of an actual weld is not duplicated, and this might
G.R. W A N G , Professor, is with the South China Institute of Science and Technology, Guangzhon, People's Republic of China. T . W . LAU, Research Associate, is with O R T E C H , Sheridan Park, Oakville, ON, Canada. G.C. W E A T H E R L Y , Professor, and T.H. NORTH, W l C / N S E R C Professor, are with the Department of Metallurgy and Materials Science, University of Toronto, Toronto, ON, Canada. Manuscript submitted September 27, 1988. METALLURGICAL TRANSACTIONS A
affect precipitation behavior during cooling, which is known to be sensitive to deformation effectsJ 6,71 To address this problem and provide a clearer picture of precipitation effects in the C G H A Z , a technique to study extraction replicas taken from the fusion line region of weldments has been developed. This technique is described in Section III. Before describing this method, the thermal cycle close to the fusion line and particle dissolution effects are discussed, as these provide useful guidelines to the area of interest in the weldment for further study.
II. T H E R M A L C Y C L E A N D PARTICLE DISSOLUTION EFFECTS In the experiments described below (Section III), beadon-plate welds were made on a number of HSLA steels with a heat input varying from 3 to 6 k J / m m . Figure 1 shows the thermal cycle measured for 3 k J / r a m and 6 k J / m m heat-input welds at the r
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