Method of calculating the cooling rate - IDC

1 Method of calculating the cooling rate This chapter describes Method of calculating the cooling rate in HAZ during welding of thick and thin plates besides that of critical cooling rate for steel under welding conditions. Further, significance of peak temperature in heat affected zone and solidification time of weld metal for development of sound weld joint has also been presented. Keywords: Peak temperature, solidification time, width of HAZ, weld structure Calculations of cooling rate Thickness of the plate to be welded directly affects the cross sectional area available for the heat flow from the weld which in turn governs cooling rate of a specific location. Accordingly, two different empirical equations are used for calculating the cooling rate in HAZ for a) thin plates and b) thick plates, depending upon the thickness of plate and welding conditions.

2 There is no clear demarcating thickness limit to define a plate thick or thin. However, two methods have been proposed to take decision whether to use thick or thin plate equation for calculating the cooling rates and these are based on 1) number of passes required for completing the weld 2) relative plate thickness According to first Method , if number of passes required for welding of two plates is less than 6 then it is considered as thin plate else thick plate for selection of suitable equation to calculate cooling rate. Since this Method is not very clear as number of passes required for completing the weld can vary with diameter of electrode and groove geometry being used for welding, therefore a more logical second Method based on relative plate thickness criterion is commonly used.

3 The relative plate thickness criteria is more logical as it considers all the relevant factors which can affect the cooling rate such as thickness of the plate (h), heat input (Hnet), initial plate temperature (To), temperature of interest at which cooling rate is desired (Ti) and physical properties of plate like (specific heat C, density ). Relative plate thickness ( ) can be calculated using following equation: h{ C(Ti To)/Hnet}1/2 Thin plate cooling rate equation is used when relative plate thickness < and thick plate cooling rate equation is used when > If value of is in range of to then is used as a limit value to decide the cooling rate equation to be used.

4 cooling rate (R) equation for thin plates: {2 k C (h/ Hnet)(Ti To)3}0 (1) cooling rate (R) equation for thick plates: {2 k(Ti T0)2}/Hnet0C/sec ..(2) Where h is the plate thickness (mm), k is thermal conductivity, is the density (g/cm3), C is specific heat ( ), Ti is the temperature of interest (0C), and To is the initial plate temperature (0C). cooling rate equations can be used to a) calculate the critical cooling rate (CCR) under a given set of welding conditions and b) determine the preheat temperature requirement for the plate in order to avoid the CCR. Critical cooling rate (CCR) under welding conditions To determine the critical cooling rate for a steel plate under welding conditions, bead on plate welds are made with varying heat input.

5 On the basis of thickness of the plate (5 mm) to be welded suitable electrode diameter is chosen first and then accordingly welding current and arc voltage are selected (20V, 200A, To=300C) for bead-on-plate (BOP) welding. Number of BOP welds is deposited using increasing welding speed (8, 9, 10, 11, ). Once the BOP weld is completed at different welding speed, transverse section of weld is cut to measure the hardness. Thereafter, hardness vs. welding speed plot is made to identify the welding speed above which abrupt increase in hardness of the weld and HAZ takes place. This welding speed is identified as critical welding speed (say 10mm/min in this case) above which cooling rate of the weld & HAZ becomes greater than critical cooling rate.

6 This abrupt increase in hardness of the weld and HAZ is attributed to martensitic transformation during welding as cooling rate becomes greater than critical cooling rate owing to the reduction in heat input (Hnet) with increase of welding speed. Using welding conditions corresponding to this critical welding speed for a given steel plate, critical cooling rate can be calculate using appropriate cooling rate equation. Corresponding Hnet = f X VI/S = X 20 X 200 /10 = 360 J/mm or kJ/mm. Calculate relative plate thickness (RPT) parameter for these conditions: h [(Ti-T0)C/Hnet]1/2 : RPT suggests use of thin plate equation for calculating the cooling rate: 2 k c(h/Q) (tc-to)3 cooling Rate (R): 0C/s and it will be safer to consider CCR: 6 0C/s Similarlyidentifyingiven PThe welrate as fPeak temweld poand so mVariationdistributinput byincreasedistributIncreasedistributFig.

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