African Fusion

Dominic gridweld bohler pipe welding celulose"Today we have 41 branch offices in 28 countries worldwide and a global network of 12 production plants. In addition, we support over 1 000 selected distribution partners in 65 countries and have customers in 120 countries," he says, adding that the company has been a Voestalpine Group company since 2007.

On the pipeline side, its first bare welding rods were produced in 1926. "These were used in a very primitive joining process associated with large amounts of spatter and porosity," he relates. "In order to improve the process at that time, however, a prototype flux-cored wire was produced, called Seelendraht. This development was not taken to market for a further 30 years, however," Höfer says.

Böhler produced its first cellulosic electrode in the 1950s; its first solid gas metal arc welding (GMAW) wire in the 1960s; and began to manufacture flux-cored wire and its BVD range of low hydrogen vertical down pipeline electrodes in the 1970s. "Low hydrogen technology is now over 40 years old and it remains a sensitive topic for pipeline welding," Höfer relates.

Global pipeline welding trends

Key to the ongoing improvements in pipeline construction is the introduction of new welding equipment and pipe materials, which offer better productivity, quality and reduced construction costs. Summarising the key trends, Höfer says that mechanised welding, and in particular, narrow gap welding with solid GMAW wire, offers significantly better productivity. "The use of solid, gas-shielded and self-shielded flux-cored wires and metal-cored wires continue to grow, while the use of cellulosic electrodes is expected to reduce in developed markets in favour of processes that offer higher productivity," he predicts.

The higher yield and tensile strengths associated with new pipe materials such X70 and X80 enable reduced wall thickness and/or increased pipeline pressures, compared to the X60 and X65 grades. "Also, in the offshore industry, we are seeing increased demand for corrosion resistant alloy (CRA) pipe materials of types 625 and 825, for example, to combat the aggressive corrosive environment offshore.

Displaying a bar chart showing the deposition rate comparisons for different pipe welding options, Höfer says that if a productivity index of one is allocated to downhand pipe welding using cellulose (6010) electrodes, then low hydrogen vertical down electrodes offer 30% better productivity and have an index of 1.3. "When using self-shielded flux-cored wire we go to an index of 1.6, ie, a further 30% improvement, with gas-shielded cored wires offering productivity of a few percentage points better.

"But if adopting mechanised solutions with flux-cored wires, this productivity index increases to 2.5 and, by fully adopting automatic GMAW welding, then factors of 4.4 and 7.5 are achievable using single torch and dual torch options respectively. Imagine how much faster a project can be finished, and how much money can be saved by adopting an automated welding process that offers 7.5 times better productivity than traditional cellulose electrode welding," he says.

Developments in pipe steel grades

The original steel pipe grades in the sixties were produced in the normalised condition while today most of the grades are produced with micro-alloying concepts and rely on thermo-mechanical treatment processes for their strength. These grades are much less sensitive to work hardening than normalised pipe. "But there are very long lead times to using these new materials. For example, the first X80 project was completed in Germany in the nineties.

"Globally, we are only now moving towards the use of X80, with X100 and X120 still a long way off. From a consumable point of view though, basic coated electrodes for shielded metal arc welding (SMAW); solid wires for gas metal arc welding (GMAW); and rods for gas tungsten arc welding (GTAW) are ready. Submerged arc (SAW) wire and flux is also available; and, for flux-cored welding (FCAW), gas shielded metal-cored wire is available for grades up to X120," Höfer informs.

Processes and consumables in pipeline girth welding

Although there are differences between the countries of the world, the use of cellulosic stick electrodes (SMAW) in the vertical down mode is still the Number 1 process used in the pipeline industry. Basic electrodes welded in either the vertical down or vertical up positions are also widely used. "The SMAW process is cheap, reliable and people are used to it. But its dominance is falling," he reports.

Cellulosic electrodes such as those in the BÖHLER FOX CEL range include consumables from E6010 to E9010 in a number of different alloy options. "Cellulosics are associated with high hydrogen content, though, so precautions such as proper pre-heating and inter pass temperature control, according to wall thickness and the type of electrodes being used, have to be applied," Höfer points out. The SMAW process is suitable for use in ambient temperatures from -40 to +50 °C and the process speed is relatively high for root pass welding with cellulosic electrodes.

Low hydrogen basic electrodes are found in the BÖHLER FOX BVD and FOX EV classifications. "In terms of mechanical properties, maximum tensile strength for cellulosics goes up to 650 MPa. Basic electrodes can usually accommodate higher tensile strengths, up to 850 and 900 MPa, and these low hydrogen electrodes also achieve higher Charpy impact values.

Basic systems give less than 5.0 mg of hydrogen per 100 g of weld metal, but for cellulosic electrodes, hydrogen levels have to be much higher. There is, therefore, a risk of hydrogen cracking occurring in the heat-affected zone and/or in the weld metal. "But if preheating and interpass temperatures are correctly controlled, hydrogen cracking need never be a problem," Höfer asserts, displaying a slide relating wall thickness to interpass temperature for the E6010 to E9010 range of BÖHLER FOX CEL electrodes. Giving an example of a wall thickness of 8.0 mm, he says: "cracks will be avoided if the interpass temperature is at a minimum of 80 °C, for example, when using E8010 electrode. And if the thickness increases, to say 15 mm, the interpass temperature should be raised to a minimum of 110 °C. So as long as you apply proper preheating and inter pass temperatures for the electrode being used, hydrogen induced cracking will not be a problem," he says.

In terms of speed, while cellulosics offer higher welding speeds for root passes, basic electrodes are faster for fill and cap passes. It is therefore common to mix cellulosic with basic electrodes on pipeline projects, with cellulosic electrodes being used for the root and hot pass, and basic vertical down electrodes for increasing the deposition rates of the fill and capping passes. "Globally, this principle has been successfully applied for over 20 years," Höfer says, showing a number of completed pipeline references that have used BÖHLER SMAW electrodes.

"At the other extreme in terms of productivity, we have the mechanised GMAW solid wire process used in narrow gap joint preparation," he continues.

In general, apart from submerged arc welding, the GMAW process offers the highest productivity and can result in clean, high quality welds with low hydrogen content. On the down side, gas shielding is required, along with protective tents against wind. The equipment investments costs have to be seen as rather high.

"A disadvantage: if the fit-up, positioning accuracy and welding process parameters are not well controlled, lack of fusion may result, which will be costly to repair," Höfer points out.

GMAW root welding can be performed from inside or outside and, depending on pipe diameters, "up to eight torches can be used for internal root welding, which makes this critical pipewelding operation very economical".

On the selection of the welding wire, Höfer says that the mechanical properties published by consumables' manufacturers are usually based on all weld-metal tests. "For an ES70-S6 wire, we will typically get yield strengths of between 460 and 530 MPa and tensile strength from 530 to 680 MPA during as standard all weld metal test.

"But the values achieved for a pipe weld joints are dependent on cooling rates. Yield strengths of 650 to 700 MPa are typical, depending on the actual cooling rate," he says pointing to a graph showing how yield and tensile stress falls off with increasing t8/5 cooling time. "What does it mean?" he asks: "For an X70 pipe, minimum yield strength of 482 MPa is required. To achieve this value in a weld joint, though, GMAW wire with a lower all weld metal yield strength can be used, because the cooling effect of a narrow gap weld preparation gives rapid t8/5 times. The achieved weld strengths are, therefore, significantly higher than the published all weld metal consumable strengths," he explains." For a stick electrode or a flux-cored wire, typical t8/5 times are above 10 seconds, but with mechanised narrow gap welding, the cooling rate is in the 4.0 to 5.0 second range, and you can see how much the yield and tensile strengths increase as a result," he adds.

Also widely used for pipe welding are flux-cored wires, and "here we need to differentiate between gas-shielded flux-cored wires and self-shielded wires". Self-shielded wires are applied semi-automatically in the vertical down position and are very widely used in China," Höfer continues, adding that mechanised systems can also be used with gas shielded flux-cored wires.

The use of gas-shielded wires such as BÖHLER Ti 70 Pipe T-FD, offers low hydrogen content; low investment costs compared to solid wires; and good weldability in spray mode at currents as low as 180 A. "Flux-cored wires are not usually used for root welding but are ideal for fill and cap passes – and the slag is usually self-releasing.

A key issue with flux-cored welding is the influence of the layer sequence, governed by the heat input of individual passes. Describing two tests done with BÖHLER Ti 70 Pipe T-FD on a 910 mm API 5L X70 pipe with a 14 mm wall thickness, Höfer says that typical all weld metal yield strength is 620 MPa, with Charpy impact values of 90 J at -40 °C. Both tests were done using a GMAW root using Fronius' CMT process, followed by fill and cap passes using Ti 70 Pipe T-FD. A constant interpass temperature of 140 °C was maintained.

"The first test was done by weaving, while the second was completed using two beads for the upper layers instead of weaving, to reduce the heat input." Showing a comparison of the yield and tensile results taken from the 12:00, 3:00, and 6:00 o'clock positions, he points out that the yield strength varied from 520 MPa for Test 1 (higher heat input) at the 6:00 position, to 590 MPa for Test 2 (lower heat input) at the 6:00 position. "Higher heat input associated with weaving also reduces the impact properties. In the 3:00 position at -40 °C, for example, from nearly 60 J for Test 2 to 40 J for Test 1," he points out. "In summary, it is important two remember that when using shielded flux-cored wires, the mechanical properties are influenced by heat input; preheat and interpass temperatures; cooling rates; and layer sequence."

Self shielded flux cored wires, such at Böhler Pipeshield are "very interesting for the African market", as a direct replacement for coated SMAW electrodes. Available for pipe grades up to X80, these wires do not require gas shielding or curtaining. They are easy to handle and give excellent impact properties. "Self-shielded flux cored wire (SSFCW) are used in the vertical down position for manual (semi automatic) pipe welding for fill and cap layers. They are usually basic, low-alloy type wires with some nickel and depend on a micro-alloyed microstructure. The mechanical properties of these modern consumables are excellent, with Charpy toughness values as high at 150 at -27 °C," says Höfer.

The equipment required is also much cheaper than GMAW equivalents and the process is almost as easy to handle as SMAW welding.

Conclusion

Voestalpine Böhler Welding has a long history in pipeline welding and can offer solutions involving any combination of consumables and processes. "We offer full support for any chosen application and our global engineering division is on hand to give expert advice with respect to the correct consumables and welding process

Contact African Fusion

Title: Editor
Name: Peter Middleton
Email: peterm@crown.co.za
Phone: (011) 622 4770
Fax: (011) 615 6108

Title: Advertising Manager
Name: Helen Couvaras
Email: helencou@crown.co.za
Phone: (011) 622 4770
Fax: (011) 615 6108

 
Full Name*
Invalid Input

Company Name*
Invalid Input

Your Email*
Invalid Input

Phone*
Invalid Input

Postal Address 1*
Invalid Input

Postal Address 2*
Invalid Input

Postal Code*
Invalid Input

Street Address 1
Invalid Input

Street Address 2
Invalid Input

Postal Code
Invalid Input

Town / City*
Invalid Input

Country*
Invalid Input

Magazine

Invalid Input

Invalid Input