72 Recent Patents on Mechanical Engineering 2010, 3, 72-81 1874-477X/10 $100.00+.00 © 2010 Bentham Science Publishers Ltd. The Effect of Welding Parameters on the Burn-Through During In-Service Welding of 316 Stainless Steel T Joint Branch Connections Farid Vakili-Tahami*, Mohammad Zehsaz and Mohammad-Ali Saeimi-Sadigh Department of Mechanical Engineering, University of Tabriz, Tabriz, Iran Received: July 28, 2009; Accepted: December 1, 2009; Revised: December 4, 2009 Abstract: In this paper, the effect of three main parameters: a) welding speed, b) cooling rate of fluid flow through the main pipe; and c) number of welding passes, have been studied to obtain an effective method to reduce the burn-through risk during the in-service welding of AISI-316 pipe branch connection to perform hot-tapping. In addition, important patents regarding the new methods of hot-tapping have been reviewed. To carry out numerical simulation, a 3D Finite Element (FE) based thermo-mechanical model has been developed. Using this model, thermo-mechanical stresses and temperature distribution along the main-pipe wall-thickness have been obtained with maximum and minimum allowable welding speeds; and with two high and low level of steam flow rate through the main pipe. The Von-Mises yield criterion using the temperature dependent yield stress has been used to check the main pipe failure during the welding process. The results show that current techniques, including API recommendations, which only rely on the observation of the main-pipe inner wall temperature, do not take into account the effect of mechanical or thermal stresses due to the inline pressure or other working parameters which have significant role in burn-through. In addition, the results show that the increase of welding speed reduces the risk of burn-through but it increases the risk of hot cracking. On the other hand, decreasing the steam flow rate has the opposite effect. It has also been shown that using smaller electrode size is the most effective way to decrease burn-through risk. Keywords: In-service welding, burn-through, 3D FE model, branch connection & welding parameters. 1. INTRODUCTION 1.1. Definition of Burn-Through Nowadays in-service welding of branch connections on pipelines while they are operating at full line-pressure is becoming a necessity in industrial plants. In addition, branch connections to perform hot-tapping or repairing defects in pipelines are becoming common industrial problems. Although, welding at full line pressure is a preferred technique, but it requires careful selection of the welding parameters; otherwise burn-through may cause severe human damages or financial losses. Therefore, the mechanism of burn-through or failure during in-service welding and its affecting parameters need to be examined carefully. To carry out these operations safely, weld parameters must be selected so that heat inputs remain low enough to avoid burn-through yet not so low that hot cracking occurs [1]. When the heat input is too low, hot cracking of the heat-affected zone (HAZ) may occur [2]. On the other hand, when the amount of heat input is high, although the main pipe wall may not melt through completely, but it could soften locally, leak or rupture, which is called burn-through. According to the description of the American Petroleum Institute (API) burn-through will occur if the un-melted area beneath the weld pool can no longer contain the pressure within the pipe. Figure 1 shows the type of wall failure *Address correspondence to this author at the Department of Mechanical Engineering, University of Tabriz, Tabriz, Iran; Tel: +98-411-3392463; Fax: +98-411-3354153; E-mail: f_vakili@tabrizu.ac.ir (burn-through) due to the in-service welding. In industry, current practice is to follow empirical guidelines to prevent burn-through which imply that “burn-through does not occur as long as the temperature level on the inside surface never exceeds a critical level of 980°C [1]”. While the empirical guidelines highlight the principal role of the welding heat input, they neglect the influence of the existing thermal stresses or the mechanical stresses due to the internal pressure. Rupture of the main pipe can occur even when the fusion zone only penetrates partially through the main-pipe wall. This is mostly because of the internal pressure and existing thermal or mechanical stresses. Series of experiments have been carried out with short welds on water filled, pressurized vessels. In these tests, slight thinning of the vessel wall has been observed with a fusion zone penetration of only 1/3 of the main pipe wall-thickness [2]. In another case, partial rupture and incipient failure has been observed with a penetration of fusion zone in half of the wall thickness [2]. 1.2. Analysis Methods Due to the enormous expenses of experimental tests, there is a general trend to develop and use numerical methods to model welding processes and also the strength or mechanical behavior of the weldments. These models can be divided in three different fields: a) models which study the welding process itself; b) models which study the mechanical behavior of the weldments; and c) models which study the mechanical behavior of the surrounding parts during the welding process. First and second fields have