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WELDING RESEARCH -s149WELDING JOURNAL ABSTRACT. An innovative method for estimating the actual cooling rate in a welded secti

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Content	WELDING RESEARCH -s149WELDING JOURNAL ABSTRACT. An innovative method for estimating the actual cooling rate in a welded section is presented. The method is based on applying a weighting factor to the Rosenthal analytical solutions for thick and thin plates. The factor is deter- mined from the heat-affected zone (HAZ) width, obtained from etched sec- tions, and reflects the actual response of the plate to the heat flow condition. Previ- ous formulations in the literature are based on the assumption of either thin- plate or thick-plate conditions, while most actual conditions lie somewhere between the two extremes. Limited experimental measurements of cooling rate, carried out by instrumented welding, showed good agreement with predicted values. The model was further used to predict the peak temperature profile across the HAZ. Introduction A typical arc weld thermal cycle con- sists of very rapid heating (several hun- dreds of degrees per second) to a peak temperature, followed by relatively fast cooling (a few tens or hundreds of degrees per second) to ambient temperature. The microstructural changes in the weld zone, as well as the weld heat-affected zone (HAZ), are greatly dependent on the heating and cooling rates, which in turn depend on the weld heat input (a function of arc energy, travel speed, and the ther- mal efficiency of the process), the plate thickness/geometry, and the initial or in- terpass temperature. The microstructural changes will directly affect the property changes (whether mechanical or corro- sion related) in the weld zone and the HAZ. Therefore, it is important to be able to predict the actual thermal cycle charac- teristics such as peak temperature and cooling rate if microstructure is to be char- acterized and correlation with the proper- ties is sought. This becomes more signifi- cant if the effect of heat input on the microstructural changes in the HAZ is to be examined for a given material, as the heat input is only a rough, simplified pa- rameter specific to a welding process. Moreover, knowledge of the cooling rate is required for simulation approaches. The most widely used and the best known analytical solutions to predict weld thermal history and cooling rate are those of Rosenthal (Refs. 1, 2). His approach was based on the assumption of a moving point heat source on the plate surface, ne- glecting any heat transfer from the sur- face. It was also assumed that physical co- efficients were constant (i.e., independent of temperature), and all the energy from the welding equipment was transferred to the arc. There have been many other rig- orous approaches with more realistic as- sumptions (e.g., Refs. 3–14) that take into account surface heat transfer, an extended or diffuse Gaussian heat source, depen- dence of thermal properties on tempera- ture, etc. All these modifications are rather complicated. One of the findings from these investigations (e.g., Ref. 8) was that the cooling rate (or cooling time) in the HAZ did not depend on the location of the point heat source and, therefore, could be found from Rosenthal’s solution. Ashby and Easterling (Ref. 4) simpli- fied the two limiting solutions derived by Rosenthal to obtain temperature/time profiles in the HAZ. One set of solutions was derived for thick plates (assuming 3- D heat flow) and the other for thin plates (assuming 2-D heat flow). There is also an equation to determine a critical thickness for a given heat input (or a critical heat input for a given plate thickness, if used in reverse) at which the 2-D condition changes to 3-D (Ref. 15). There have also been some attempts to define dimension- less parameters that define the transition point between 2-D and 3-D conditions (Refs. 16, 17). However, these criteria may be too simplistic. Real welds are more likely to lie between the two limiting solu- tions: a situation classified by some re- searchers as 2.5-D (Ref. 16),for which there is no simple solution (Ref. 1). The question then is where a particular case lies with respect to the 2-D and 3-D con- ditions. The objective of this paper is to in- troduce a simple method to answer the above question. The goal is to find the ac- tual values of parameters (as opposed to the lower-bound or upper-bound values), such as cooling rate and peak temperature profile, for a given weld section. This can be very useful as experimental measure- ment of weld thermal cycles, e.g., by em- bedding thermocouples in the HAZ, is in some cases impractical considering the small size of the HAZ. The approach, ex- plained in the next section, is based on de- riving a parameter (i.e., HAZ width) that can be measured by sectioning a sample, and using this parameter to calculate a SUPPLEMENT TO THE WELDING JOURNAL,OCTOBER2005 Sponsored by the American Welding Society and the Welding Research Council Estimation of Cooling Rate in the Welding of Plates with Intermediate Thickness Cooling rates estimated by Rosenthal’s thick- and thin-plate solutions can be modified by a weighting factor to account for intermediate values of plate thickness BY K. POORHAYDARI, B. M. PATCHETT,ANDD. G. IVEY KEYWORDS Cooling Rate Heat-Affected Zone Weld Thermal Cycle Instrumented Welding Transformable Steels Rosenthal’s Analytical Solutions Gas Tungsten Arc Welding Heat Input K. POORHAYDARI (kioumars@ualberta.ca), D. G. IVEY,and B. M. PATCHETTare with the Department of Chemical and Materials Engineer- ing, University of Alberta, Alberta, Canada. Poorhaydari Supplement 10/05cor 9/9/05 11:59 AM Page 149
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