The eec HotTap web tool assesses the suitability of hot tap welding procedures (or repair welds) by evaluating the risk of burn-through and cold cracking due to rapid cooling. A transient thermal FEA solver is used to predict the peak wall temperature, maximum cooling rate, and shortest cooling time (from 800 C to 500 C) at various locations surrounding the weld region (especially within the HAZ). The cooling effect of the contained fluid (either stagnant or flowing) is of particular importance in hot tapping, since this may contribute to higher cooling rates, which increases the risk of cold cracking due to the harder material that typically results. In order to account for this, empirical heat transfer correlations are used to estimate the cooling effect of the fluid for both liquid and vapor fluid phases. A selection of common fluids can be chosen, but custom fluid mixtures can also be created by specifying the type and composition of the constituents. For liquids, nucleate boiling is accounted for if the wall temperature exceeds the saturation temperature. For high enough wall heat fluxes, film boiling may occur, which can raise the temperature of the wall considerably.

These cooling rates and cooling times are then used to estimate the hardness of any material that has undergone a microstructural change. The maximum predicted hardness must be less than an upper-bound value that is deemed relatively safe. The carbon equivalent approach is based off of the original work done by Graville and Read (1974) which was later modified by Battelle in their 1991 report. In order to be safe, the carbon equivalent of the material must be less than some maximum carbon equivalent that is thought to keep the hardness less than 350 HV, depending on the cooling rate. When a material composition for the pipe material is specified, a more robust method is used. The Yurioka (1995) model is used to predict the resulting hardness, and as long as that hardness is less than critical hardness values determined experimentally (Bruce 2012), or specified by the user, the risk of cold cracking is assumed to be low. The default critical hardness values depend on the hydrogen content of the electrode.

To assess the risk of burn-through, the maximum inside wall temperature predicted by the thermal model is compared to either the value 1800 F for low-hydrogen electrodes or 1400 F for high-hydrogen electrodes. An inner wall temperature that never exceeds these values is deemed to be at a low-risk of burn-through. This is the generally accepted, albeit somewhat conservative, approach discussed in Bruce 2012 and elsewhere.

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[1] Graville, B.A. and Read, J.A., “Optimization of Fillet Weld Sizes,” Welding Journal Supplement, April, 1974, pp. 161s-169s.

[2] Battelle Report “Investigation and Prediction of Cooling Rates During Pipeline Maintenance Welding”, Battelle Report to the American Petroleum Institute, December, 1991.

[3] Kasuya, T., Yurioka, N. and Okumura, M. “Methods for Predicting Maximum Hardness of Heat-Affected Zone and Selecting Necessary Preheat Temperature for Steel Welding,” Nippon Steel Technical Report No. 65, April 1995.

[4] Bruce and Etheridge “Further Development of Heat-Affected Zone Hardness Limits for In-Service Welding” ASME, 9th International Pipeline Conference, Volume 3: Materials and Joining, Calgary, Alberta, Canada, September 24–28, 2012, Paper No. IPC2012-90095, pp. 71-81.


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