Welding Stainless Steel Pipes and Tubes
Kilometers of success, welded centimetre by centimetre, in the field as well as in the shop.
Vincent van der Mee, Lincoln Smitweld BV
INTRODUCTION
In order to continuously meet stringent industrial requirements Lincoln Smitweld's pipewelding program has been extended over the years. Lincoln Electric is active in the whole pipewelding industry and a major supplier of welding consumables, equipment and technical support. Lincoln's pipewelding program provides a total solution for every pipewelding application. From longitudinal welds at the pipemill to circumferrential welds in the field, in mild steel, stainless steel and nickel. This article, is limited to welding of stainless steel.
ADVANTAGE OF THE GMAW-STT PROCESS
One-side welding of stainless tubes and pipes, is widely applied in a variety of stainless steel grades, in the (petro)chemical industry, on- and offshore in oil- and gas production and storage, and food industry. Especially the root passes are very important, as this is often the exposed side and thus determines the quality of the welded joint. Apart from quality, the production costs are important as well. In conventional pipewelding, GTAW is a common, but relatively slow process. A welding process that significantly could improve the welding speed is the GMAW-STT process. This process has gained recognition for welding the root pass in the pipewelding industry, because of the high level of quality as well as the high productivity that can be obtained. When integrated in a mechanized orbital pipewelding system, an even higher productivity can be reached for those projects, where a multitude of identical welds with uniform quality is required.
WHAT MAKES THIS PROCESS SUITABLE FOR ROOT PASSES IN STAINLESS STEEL?
Fig.1: Illustration of the GMAW-STT process
The GMAW-STT process (STT= Surface Tension Transfer), is a modification of the conventional short arc mode. It uses a new generation of inverter-based power sources without a constant current (CC) or constant voltage (CV) setting. Using this process, the heat input can be controlled, independent from the wire feeding. Figure 1 illustrates the arc behaviour and the related current/voltage level in repeating cycles. The current at the initial short is reduced immediately. The low current is maintained for a short period of time, allowing surface tension forces to transfer the droplet to the puddle. A high level of pinch current is applied to speed up the transfer. The "necking down" of the shorted electrode is monitored. When a specific value is reached, the pinch current is quickly reduced to a low value, before the fuse separates. When the sort breaks, it does so at a low current, producing very limited spatter. Next the arc is re-established and a high (peak) current is applied. The formed plasma ia boosted and the momentary pulse of current causes the arc to broaden, melting a wide surface area. This action eliminates cold lapping and promotes good fusion. The time of maintaining high currents is short enough to prevent boiling of the metal at the wire tip, which limits the fume emission substantially compared to open arc GMAW, and is close to the fume level produced with GTAW. The perfect control of heat with the STT process guarantees a superior joint.
Advantages compared to conventional processes, such as increased welding speed, controlled heat input, low fume emission, low spatter and improved weld microstructure, resulting in better corrosion properties, are important arguments in the welding process selection. Some target values The heat input ranges from 0.5 - 0.8 kJ/mm, with a welding speed of approximately 9-21 cm/min. Although not the most important issue for root passes, the deposition rate with a 1.0 mm wire could be 1.0-1.2 kg/hr, whereas with a 1.2mm wire up to 1.3-2.2 kg/hr. Fig.1: Illustration of the GMAW-STT process For wall thickness in pipe of 3mm, the conventional GTAW process would require a minimum of two passes, whereas with GMAW-STT, one pass would suffice. A reduction of the welding time with 70% is feasible with thin wall pipes. Also for heavier wall pipes, for instance 12 mm, the STT process could be used. A root pass of 5 mm can be applied, which is sufficient to fill the joint in one run using the SAW process. The GTAW process would require a minimum of three passes to give sufficient support for filling with SAW.
Fig. 2: GMAW-STT root pass in X80-12Cr4,5Ni1,5Mo pipe.
Preferred shielding gas for "300" series stainless steel is Ar+2%CO2, and Ar as backing gas. For (super)duplex or supermartensitic stainless steel, Ar+38%He+2%CO2 is the most suitable shielding gas, with Ar as backing gas.
Advantages, obtained with 3 component gases are:
• More heat in the arc
• Better arc stability,
• Improved effect on surface tension base metal
• Better wetting of weld metal
• Increased welding speed
An often-asked question is whether, using this low heat input process, welds could be made without backing gas. Although in practice welds are made without backing gas, it is not recommended for those applications where strict corrosion requirements are set.
WELDING CONSUMABLES
A full range of consumables (SMAW, GTAW, GMAW, FCAW, SAW) is available for the major stainless grades. The table below lists recommended consumables for major stainless steel grades.
Fig. 3: Reeling of flowline
Additionally, consumables are developed for specific applications. An example is Arosta 316LP, which is a coated electrode, specific designed for thin wall pipe. It is available in diameters 2.0 and 2.5 mm, and can be applied in wall thickness of 2 mm and up. In most cases, the use of filler material for root passes is mandatory. When the thickness of the first pass is limited to 2 mm, there is a risk for overheating when the second pass is deposited. This overheating may affect the corrosion properties. In order to avoid this tendency for overheating, the heat input for the second pass should be limited to 0,8-1,2kJ/mm.
APPLICATIONS
Welding of flow lines is just one example. Figure 3 shows the reeling of supermartensitic pipe.
Pipe lengths of 12 m in X80-12Cr4.5Ni1.5M are welded to lengths of 930 m, and reeled.
Girth welds are made with Superduplex GMAW wire LNM Zeron 100X.
Weldability, mechanical properties and corrosion properties are passed extended testing.
An example for a welding procedure is shown for a 10" (254 mm) duplex pipe with a wall thickness of 15mm.
The root is welded using the GMAW-STT process with a 22% Cr duplex stainless steel wire (LNM4462) and a heat input of 0.5-1.0 kJ/mm. Shielding gas was Ar+2%CO2, backing gas was Ar.
By using the GMAW-STT process for the root, a welding speed of 3 to 4 times the speed of the GTAW process was obtained.
Layer thickness was such, that already the second pass could be performed with the SAW process.
Fill passes were with a flux / wire combination of LNS4462 / P2000, and a heat input of 0.9-1.4 kJ/mm.
Charpy impact test results at -20°C were approximately 75J.
Regarding corrosion properties, the measured CPT (critical pitting temperature) according to ASTM G48A was over 30°C.
Fig4: WPAR GMAW -STT and SAW: Duplex pipe
Vincent van der Mee, Lincoln Smitweld BV
INTRODUCTION
In order to continuously meet stringent industrial requirements Lincoln Smitweld's pipewelding program has been extended over the years. Lincoln Electric is active in the whole pipewelding industry and a major supplier of welding consumables, equipment and technical support. Lincoln's pipewelding program provides a total solution for every pipewelding application. From longitudinal welds at the pipemill to circumferrential welds in the field, in mild steel, stainless steel and nickel. This article, is limited to welding of stainless steel.
ADVANTAGE OF THE GMAW-STT PROCESS
One-side welding of stainless tubes and pipes, is widely applied in a variety of stainless steel grades, in the (petro)chemical industry, on- and offshore in oil- and gas production and storage, and food industry. Especially the root passes are very important, as this is often the exposed side and thus determines the quality of the welded joint. Apart from quality, the production costs are important as well. In conventional pipewelding, GTAW is a common, but relatively slow process. A welding process that significantly could improve the welding speed is the GMAW-STT process. This process has gained recognition for welding the root pass in the pipewelding industry, because of the high level of quality as well as the high productivity that can be obtained. When integrated in a mechanized orbital pipewelding system, an even higher productivity can be reached for those projects, where a multitude of identical welds with uniform quality is required.
WHAT MAKES THIS PROCESS SUITABLE FOR ROOT PASSES IN STAINLESS STEEL?
Fig.1: Illustration of the GMAW-STT process
The GMAW-STT process (STT= Surface Tension Transfer), is a modification of the conventional short arc mode. It uses a new generation of inverter-based power sources without a constant current (CC) or constant voltage (CV) setting. Using this process, the heat input can be controlled, independent from the wire feeding. Figure 1 illustrates the arc behaviour and the related current/voltage level in repeating cycles. The current at the initial short is reduced immediately. The low current is maintained for a short period of time, allowing surface tension forces to transfer the droplet to the puddle. A high level of pinch current is applied to speed up the transfer. The "necking down" of the shorted electrode is monitored. When a specific value is reached, the pinch current is quickly reduced to a low value, before the fuse separates. When the sort breaks, it does so at a low current, producing very limited spatter. Next the arc is re-established and a high (peak) current is applied. The formed plasma ia boosted and the momentary pulse of current causes the arc to broaden, melting a wide surface area. This action eliminates cold lapping and promotes good fusion. The time of maintaining high currents is short enough to prevent boiling of the metal at the wire tip, which limits the fume emission substantially compared to open arc GMAW, and is close to the fume level produced with GTAW. The perfect control of heat with the STT process guarantees a superior joint.
Advantages compared to conventional processes, such as increased welding speed, controlled heat input, low fume emission, low spatter and improved weld microstructure, resulting in better corrosion properties, are important arguments in the welding process selection. Some target values The heat input ranges from 0.5 - 0.8 kJ/mm, with a welding speed of approximately 9-21 cm/min. Although not the most important issue for root passes, the deposition rate with a 1.0 mm wire could be 1.0-1.2 kg/hr, whereas with a 1.2mm wire up to 1.3-2.2 kg/hr. Fig.1: Illustration of the GMAW-STT process For wall thickness in pipe of 3mm, the conventional GTAW process would require a minimum of two passes, whereas with GMAW-STT, one pass would suffice. A reduction of the welding time with 70% is feasible with thin wall pipes. Also for heavier wall pipes, for instance 12 mm, the STT process could be used. A root pass of 5 mm can be applied, which is sufficient to fill the joint in one run using the SAW process. The GTAW process would require a minimum of three passes to give sufficient support for filling with SAW.
Fig. 2: GMAW-STT root pass in X80-12Cr4,5Ni1,5Mo pipe.
Preferred shielding gas for "300" series stainless steel is Ar+2%CO2, and Ar as backing gas. For (super)duplex or supermartensitic stainless steel, Ar+38%He+2%CO2 is the most suitable shielding gas, with Ar as backing gas.
Advantages, obtained with 3 component gases are:
• More heat in the arc
• Better arc stability,
• Improved effect on surface tension base metal
• Better wetting of weld metal
• Increased welding speed
An often-asked question is whether, using this low heat input process, welds could be made without backing gas. Although in practice welds are made without backing gas, it is not recommended for those applications where strict corrosion requirements are set.
WELDING CONSUMABLES
A full range of consumables (SMAW, GTAW, GMAW, FCAW, SAW) is available for the major stainless grades. The table below lists recommended consumables for major stainless steel grades.
Fig. 3: Reeling of flowline
Additionally, consumables are developed for specific applications. An example is Arosta 316LP, which is a coated electrode, specific designed for thin wall pipe. It is available in diameters 2.0 and 2.5 mm, and can be applied in wall thickness of 2 mm and up. In most cases, the use of filler material for root passes is mandatory. When the thickness of the first pass is limited to 2 mm, there is a risk for overheating when the second pass is deposited. This overheating may affect the corrosion properties. In order to avoid this tendency for overheating, the heat input for the second pass should be limited to 0,8-1,2kJ/mm.
APPLICATIONS
Welding of flow lines is just one example. Figure 3 shows the reeling of supermartensitic pipe.
Pipe lengths of 12 m in X80-12Cr4.5Ni1.5M are welded to lengths of 930 m, and reeled.
Girth welds are made with Superduplex GMAW wire LNM Zeron 100X.
Weldability, mechanical properties and corrosion properties are passed extended testing.
An example for a welding procedure is shown for a 10" (254 mm) duplex pipe with a wall thickness of 15mm.
The root is welded using the GMAW-STT process with a 22% Cr duplex stainless steel wire (LNM4462) and a heat input of 0.5-1.0 kJ/mm. Shielding gas was Ar+2%CO2, backing gas was Ar.
By using the GMAW-STT process for the root, a welding speed of 3 to 4 times the speed of the GTAW process was obtained.
Layer thickness was such, that already the second pass could be performed with the SAW process.
Fill passes were with a flux / wire combination of LNS4462 / P2000, and a heat input of 0.9-1.4 kJ/mm.
Charpy impact test results at -20°C were approximately 75J.
Regarding corrosion properties, the measured CPT (critical pitting temperature) according to ASTM G48A was over 30°C.
Fig4: WPAR GMAW -STT and SAW: Duplex pipe
