Avoiding Burning Through: Control the Inside Surface Temperature, Not the Pressure

Transcription

1 Originally published in the Canadian Welding Association Journal, Spring 2013, pp Welding on In service Pipelines: Dispelling Popular Myths and Misconceptions Bill Amend Sr. Principal Engineer, Welding & Materials Technology Det Norske Veritas (USA), Inc Condor Ridge Rd Yorba Linda, CA USA William A. Bruce Director, Welding & Materials Technology Det Norske Veritas (USA), Inc Frantz Rd Dublin, OH, USA 614 B Introduction Successful welding on in service pipelines and piping systems requires that two separate vulnerabilities be considered; burning through the pipe wall during welding, and forming hydrogen cracks. Burning through (illustrated in Figure 1) occurs when the inside surface temperature of the pipe reaches temperatures at which the pipe wall has little remaining strength. This situation typically occurs when the pipe wall is relatively thin and the heat input exceeds the ability of the pipe wall and the pressurized fluid to quickly conduct the heat away from the weld. Hydrogen cracking (illustrated in Figure 2) can occur either during the welding process or some time after welding is completed. It most often occurs in hard heat affected zones, but it can also occur in weld metal deposits. It requires the simultaneous presence of a susceptible microstructure (martensite), and sufficient amounts of both stress and hydrogen in the weld zone. Avoiding hydrogen cracking involves minimizing one or more of those three factors. For example a low hydrogen welding process is typically preferred over the use of cellulosic flux covered SMAW electrodes and heat inputs are selected to produce relatively slow weld cooling rates that prevent the formation of martensite. Problems with welding on in service piping typically occur as a result of various myths and misconceptions about how to avoid burning through and hydrogen cracking. Some of those misconceptions are addressed below. Avoiding Burning Through: Control the Inside Surface Temperature, Not the Pressure A common misconception is that the susceptibility to burning through is highly related to the pressure of the fluid in the pipe. It has been common to use simple calculations to determine the safe pressure that would avoid burning through by keeping the hoop stress low relative to the pipe yield stress. More recently, researchers have demonstrated that the susceptibility to burning through is only weakly related to pressure. It is much more strongly related to the maximum temperature reached at the inside surface of the pipeiine the same way that a small diameter, deep corrosion pit can withstand very high internal pressure without leaking, the relatively small surface area of the weld puddle can withstand high internal pressure even though the hottest portion of the pipe wall under the puddle has little strength. Empirical testing has shown that for welding with low hydrogen SMAW electrodes, burning through is highly unlikely if the inside surface temperature of the pipe is no greater than about 1800 F (982 C) [ref1 3]. For cellulosic electrodes the recommended surface temperature is lower, about 1400 F

2 (760 C). Thermal analysis models enable a user to quickly and easily predict what the inside surface temperature will be if details of the pipe thickness, fluid type, pressure and velocity, weld heat input, and joint design are known [ref4 5]. An example of model output is shown in Table 1. The models show that reducing the pressure in gas pipelines can increase the susceptibility to burning through because thermal conductivity of gas is lower at lower pressure. As a result, the heat is conducted away from the weld zone more slowly, so maximum inside surface temperatures rise. Testing has shown that at a minimum wall thickness of 0.25 in (6.5 mm) or greater it is very difficult to burn through using low hydrogen welding processes typically used on in service pipelines and normal welding practice [ref6]. Using carefully controlled heat inputs it is possible to weld on wall thicknesses as low as bout in (3.2 mm) without burning through. Consider Amperage and Not Just Heat Input Heat input (HI) is influenced by welding current, voltage and travel speed in accordance with the following equation: HI (kj/mm) =0.06 x Amps x Volts / Travel speed (mm/min) Researchers found that there was increased resistance to burning through when desired heat inputs were obtained by using slow travel speed and low welding current, rather than by using faster travel speed and higher current [ref7]. While SMAW electrodes can be operated at a range of current, welding current can easily be limited to low values by specifying the use of small diameter electrodes (e.g., 2.4 mm diameter maximum for 6.4 mm wall thickness and less). Take Care in Selection and Storage of Low Hydrogen Electrodes The best resistance to cracking occurs when the hydrogen content of the weld zone is minimized. Today, most pipeline operators and contractors are aware of the benefits of producing welds having a low hydrogen content when making hot tap welds or in service repairs. For example, CSA Standard Z [ref8] contains several references to the use of low hydrogen welding practices. The statements range from low hydrogen welding practices shall be used to low hydrogen welding practices should be considered, depending upon the specific application. What is less commonly recognized is the influence of electrode selection and care on the hydrogen content of deposited weld metal. The designation H4R following a SMAW electrode classification indicates an electrode with a moistureresistant flux coating capable of producing hydrogen contents of 4 ml/100 gm of deposited weld metal in the as received condition. In comparison, cellulosic flux covered electrodes, for example AWS EXX10 type electrodes, deposit weld metal with up to 10 times as much hydrogen. Maintaining the designated hydrogen content is dependent upon atmospheric conditions, time of exposure to the atmosphere, and storage conditions. Electrodes are typically rated for a certain number of hours of exposure to standard atmospheric conditions before the electrode moisture content increases to unacceptable levels. More severe (hot and humid) atmospheric conditions can reduce the recommended exposure time. Exposure time limits also relate to weld metal tensile strength, with higher strengths having shorter exposure time limits.

3 Storage in a holding oven heated to about C after removal from hermetically sealed cans effectively prevents moisture absorption. As a result, the atmospheric exposure clock does not start until the electrodes are removed from the holding oven. However, atmospheric exposure time is cumulative. Two separate exposures of six hours separated by a day of storage in a heated storage oven are the same as a single exposure of 12 hours. An electrode should either be discarded or re dried (baked) at high temperature after the specified exposure time limit is exceeded. Note that the temperature required to re dry an electrode is much higher than the temperature required to simply maintain low hydrogen characteristics. Electrode holding ovens do not reach the temperature range of about C typically required to re dry electrodes. For small in service welding applications, it may be advantageous to purchase and use lowhydrogen electrodes in smaller quantities (e.g., 10 lb [4.5 kg] cans) so that the challenge of managing exposure time for unused electrodes is minimized. Note that not all low hydrogen electrodes are delivered in the same degree of dryness. Electrodes delivered in hermetically sealed cans will typically achieve the designated weld metal hydrogen contents when used right out of the can. However, electrodes delivered in some plastic wrapped cardboard boxes can require re drying prior to use in order to achieve the rated weld metal hydrogen contents. It is important to check the manufacturer s recommendations prior to use. A Hardness of HV 350 is Not the Magic Number to Avoid Cracking Traditionally, it has been common to specify that hardness of the heat affected zone not exceed HV 350 when qualifying welding procedures for use on in service piping. CSA Z662 states that for fillet welds and branch connections made onto piping containing flowing fluid, weld metal or HAZ hardness values in excess of 350 HV shall require an evaluation of the welding procedure specifications to determine that they are suitable for the avoidance of hydrogen induced cracking. However, it is not the hardness, but rather the presence of a crack susceptible microstructure that influences the likelihood of cracking. Hardness limits are merely used as indicators of detrimental amounts of martensite. The fact that is seldom recognized is that the hardness associated with martensite is different for steels of various compositions that are commonly found in pipelines, and, the measured hardness will be different depending upon how much of the martensite is present. Furthermore, the amount of martensite that represents a significant susceptibility to hydrogen cracking is dependent upon the amount of hydrogen present in the weld zone. Weld zones with lower weld metal hydrogen contents can tolerate more martensite without cracking. Another factor that affects crack susceptibility is the steel thickness. After a weld solidifies, it can be supersaturated with hydrogen. Hydrogen then diffuses to free surfaces, where it can escape. When the material being welded is thick, diffusion distances are greater, which results in more hydrogen remaining in the weld. Thickness also affects restraint levels with thicker materials developing higher levels of residual stress.

4 Therefore, the specified maximum hardness limit needs to consider the composition of the steel being welded, the thickness of the steel, as well as the likely amount of hydrogen in the weld zone [ref9]. As illustrated in Figure 3, a hardness of HV 350 can be non conservative for low carbon equivalent steels thicker than in (9.5 mm). On the other hand, for relatively high carbon equivalent steel, for example higher than 0.42 CE IIW, a hardness higher than HV 350 can be tolerated when low hydrogen weld practices are used that produce hydrogen contents no higher than 4 ml/100 gm deposited weld metal. For thinner steel, the hardness limits can also be higher, as illustrated in Figure 4. Conclusions Welds of good structural integrity can be made safely onto in service pipelines and piping systems by selecting welding procedures that balance the susceptibility to hydrogen cracking with the susceptibility to burning through. Adjusting heat inputs to maintain acceptably low inside surface temperatures while still minimizing the likelihood of forming martensite and using low hydrogen welding practices are keys to success. Table 1 Effect of Heat Input on Inside Surface Temperature and Critical Cooling Rate of Fillet Weld, Calculated Using Model from Ref.4 kj/in (kj/mm) Inside surface temperature, F ( C) Cooling time ( C, seconds) 25 (0.98) 1248 (676) (1.18) 1355 (735) (1.38) 1485 (807) Conditions: 5.56 mm pipe with matching thickness full encirclement sleeve. Pipe carrying 400 psig (2758 kpa) natural gas at 13 C, flowing at 10 ft/sec (3 m/sec)

5 Figure 1 Typical appearance of a pinhole resulting from burning through on pressurized thin wall piping.

6 Figure 2 Typical appearance of hydrogen cracking in the hard heat affected zone at the toe of a fillet weld made onto pipe subjected to accelerated cooling of the weld zone Figure 3 Hardness criterion for thick materials (>9.5 mm) having >0.1%C, based on CE IIW Figure 4 Hardness criterion for thin materials ( 9.5 mm) having >0.1%C, based on CE IIW

7 References 1. Howden, D. G., "Welding on Pressurized Pipeline," Loss Prevention, Vol. 9 (New York, NY: American Institute of Chemical Engineers, 1975), pp Wade, J. B., "Hot Tapping of Pipelines," Australian Welding Research Association Symposium, Paper No. 14 (Melbourne, Australia, 1973). 3. Cassie, B. A., "The Welding of Hot Tap Connections to High Pressure Gas Pipelines," J. W. Jones Memorial Lecture (Pipe Line Industries Guild, October 1974). 4. J. F. Kiefner, R. D. Fischer, and H. W. Mishler, "Development of Guidelines for Repair and Hot Tap Welding on Pressurized Pipelines," Final Report, Phase 1, to Repair and Hot Tap Welding Group, Battelle Columbus Division, Columbus, OH, September Bruce, W. A., Li, V., Citterberg, R., Wang, Y. Y., and Chen, Y., "Improved Cooling Rate Model for Welding on In Service Pipelines," PRCI Contract No. PR , EWI Project No CAP, Edison Welding Institute, Columbus, OH, July Kiefner, J. F. and Fischer, R. D., "Repair and Hot Tap Welding on Pressurized Pipelines," Symposium during 11th Annual Energy Sources Technology Conference and Exhibition, New Orleans, LA, January 10 13, 1988, (New York, NY: American Society of Mechanical Engineers, PD Vol. 14., 1987) pp Bruce, W. A., Welding onto In Service Thin Wall Pipelines, International Conference on Pipeline Repairs, Welding Technology Institute of Australia, Wollongong, Australia, March Standard Z Oil & Gas Pipeline Systems, Canadian Standards Association, Bruce, W. A., and Etheridge, B. C., Further Development of Heat Affected Zone Hardness Limits for In Service Welding, Proceedings of IPC 2012, 9th International Pipeline Conference, September 24 28, 2012, Calgary, Alberta, Canada.