TECHNICAL ARTICLE:

Introduction

Laser Beam Welding and the hybrid process (Laser + MIG) are increasingly used for the series production of longitudinally welded pipes in chromium steels and chromium-nickel steels. Owing to the stringent requirements that longitudinally welded pipes in stainless materials are subject to, the raw material quality prerequisites must be ensured when using these welding processes, including geometry of the cut edges, weld control, avoidance of welding spatters, heat treatment, non-destructive testing and quality assurance. Much experience is needed to fulfil these requirements. However, once implemented properly, the laser welding and hybrid processes are suitable for the industrial production, both from a technical and economical point of view.

Longitudinally Welded Pipes Enables Large Size Range

Pipes in high-alloy stainless steels are available in seamless and longitudinally welded construction. Taking into account technical and economical considerations, there are typical size ranges for each type of pipe. As regards welded pipes, these can be made either from coil or from plate. Illustration 1 shows the different size ranges. Generally raw materials in the form of coils are used for the continuous series production of longitudinally welded pipes having outside diameters from 20.0 up to 600mm and wall thicknesses of 0.5–12.0mm.

Illustration 1: Size ranges – technical-economical size ranges for seamless and welded stainless steel pipes

The Production of Pipes is Subject to Continual Improvement

The welding of the longitudinal pipe weld in series production is carried out using only fully automatic welding processes for economical reasons. The following processes are used: TIG welding, plasma welding, plasma-TIG welding, laser beam welding and the hybrid process (laser + MIG).

During the last few years, laser beam welding has increasingly replaced the shielding gas processes. Since the introduction of laser beam welding at Butting, Germany in 1991, a lot of experience has been gained through relevant R+D relating to the production of longitudinally welded pipes in high-alloy steels. This has led to hence the possibility to optimize the continuous series production of pipes having wall thicknesses of up to 4mm. Illustration 2 shows a table of high-alloy stainless steels used to date.

Illustration 2: Materials – overview and chemistry of materials used

Quality throughout the Entire Pipe Production

A coil is fed continuously into the welding line and is formed into a pipe in several steps. The cold forming in a pipe production line is shown in illustration 3. A diagram of the entire production process is shown in illustration 4. Some details of the welding using a laser beam can be taken from illustrations 5 and 6. After welding, the pipes are subjected to induction annealing in-line (optional), calibration, measurement, eddy-current testing and cutting to the required length.

Illustration 3: Pipe welding line – continuous cold forming
of the coil in the pipe production line

Illustration 4: Series production – series production of longitudinally welded pipes in high-alloy steels

Different Laser Welding Processes

At this moment in time, Butting uses different welding lines manufactured by Rofin Sinar. The Laser welding equipment type DC860HF is used for the series production of longitudinally welded pipes. With this 6.0 kW CO2 laser, approximately 1 million metres of pipe are produced per annum, especially for the paper and chemical industry, but also for the food industry, pharmaceutical applications and for the oil and gas industry.

Illustration 6: Hybrid process

For the hybrid process (see illustration 6), which combines the energies of a CO2 laser and a MIG equipment, a welding head was developed by the “Bremer Institut für Angewandte Strahltechnik“ (BIAS). The hybrid process was used for the production of longitudinal welds of pipes in supermartensitic materials. This was carried out within the scope of the JOTSUP project (Advanced JOining Technologies for SUPermartensitic Stainless Steel Line Pipes), a European research project (Project no: GDR1-1999-10278 of the European Commission).

Precision Required during Pipe Production

Owing to the relatively high welding speed of 5–10m/min of laser welding, high precision is required during the pipe production, with regard to the raw material and all welding parameters and testing of the finished product. In order to ensure the necessary quality, the main concerns at Butting during the entire production process are related to the geometry of the cut edges of the raw material, the gap-free and exact guiding of the weld edges, minimization of welding spatters, heat treatment and non-destructive testing.

Illustration 7: Weld edge criteria – criteria for cut edges of coils in high-alloy steels

Precise Geometry of the Cut Edges of the Raw Material

The raw material is ordered in a condition that is “suitable for laser beam welding”, i.e. the weld edges must be weldable without any further treatment. The criteria for the coil edges are shown in illustration 7. However, should the cut edges not comply with the requirements, then additional machining (milling operation) has to be added, which is very time consuming and very costly, due to the short service life of the cutting tools.

Illustration 8: Undercuts – undercuts in the root of a Laser weld

Root undercuts that have an impact on the quality are very often caused by inadequate edge preparation, as shown in illustration 8. The cross section shows an undercut with a depth of 0.07mm maximum, in case of a pipe OD 88.9 x 2.60mm wall, in material type 1.4541.

Eddy-Current Testing is a Suitable Weld Control System

The coil formed into a slit pipe is guided underneath the laser beam. There should not be a gap and the edges must be parallel and at the same level, with a tolerance of approx. +/-0.1mm maximum. In order to ensure proper guiding, the operator disposes of a weld control system with sensor and camera. In case there is a deviation from the axis of the laser beam and the welding gap, then lack of fusion might occur. Such a defect is documented by a cross section of a longitudinal weld of a pipe OD 60.3 x 3.0mm wall in the material type 1.4541.

Illustration 9: Lack of fusion – lack of fusion in a laser weld (Cross section)

Control systems based on eddy current are appropriate tools to control the weld of high-alloy steel pipes. As regards the optical systems generally used, these very often failed due to the bright surface finish of the raw materials.

Minimization of Welding Spatters

The specifications of the food and pharmaceutical industry, for example, ask for pipes free from undercuts and welding spatters. As already pointed out before, it is of utmost importance to keep the prescribed tolerances of the coil edges. The steel mills must provide suitable materials in order to guarantee the absence of undercuts.

Illustration 10: Welding spatters – welding spatters on the inside surface of a
pipe OD 60.3 x 3.0mm in 1.4541. Diameter of spatter between 0.1 and 0.5mm

The welding spatters that are inevitably caused during laser welding can be removed by grinding from the outside surface of the pipe. However, the removal of welding spatters from the inside (see illustration 10) would be very time consuming and costly. The decrease in the number of spatters is achieved by using appropriate purging gases.

Induction Annealing Reduces Hardness and Strength Values

After welding the pipe is subjected to heat treatment (induction annealing) in-line. With the exception of a few material grades, all stainless chromium-nickel materials used for the production of pipes at Butting are subjected to solution annealing at temperatures between 1,020°C and 1,100°C, followed by quenching in water. The ferritic Cr-steels are heat treated at temperatures between 700°C and 900°C. For the supermartensitic steels, temperatures of 620°C-640°C are used.

As this material loses strength during annealing, the pipe cannot be kept indefinitely at a certain temperature without heavy deformation occurring. Investigations have shown that relatively short holding times suffice to achieve the required annealing result. The comparison (see illustration 11) of the hardness results of two laser welds, one in the as-welded condition and one in the heat treated condition, clearly shows that the hardness prevailing after welding was reduced considerably.

Illustration 11: Hardness comparison – two Laser welds of a pipe OD 35.0 x2.0mm in the ferritic material 1.4016; above: hardness values HV 5 of the as-welded condition, below: values of the heat treated condition

Should the heat treatment temperature be higher than the recrystallization temperature of the material being used, recrystallization of the structure of the weld can be achieved by selected deformation during the rolling of the weld area directly after welding. As regards mechanical-technological properties, as well as corrosion resistance, the structure of the weld is nearly identical to that of the pipe body.

Non-Destructive Testing Proves the Quality

After annealing, the pipe is subjected to calibration and testing. Owing to the considerable welding speeds, very fast detection of defects and their evaluation is required of the non-destructive testing equipment. This is ensured by an automatic testing process and an appropriate sorting system.

As non-destructive testing in-line, Butting uses the eddy-current testing process. The testing consists of three steps: By a segment spool, which controls the welding gap, only the beginning and the end of a defect (gaps, undercuts, lack of fusion) are recorded and marked on the pipe with a paint mark.

Testing of the entire pipe body using a spool surrounding the pipe body covers all possible defects in the pipe body. According to SEP 1925 it replaces hydrostatic pressure testing. A weld test using a segment spool refers to pores, cracks, lack of fusion etc. This test also comprises up to 10mm of parent material at either side of the weld. All data relating to the production and testing is stored in the computerized data base and available for evaluation purposes and automatic issuance of mill inspection certificates required by the customers.

Obvious Metallurgical Advantages by the Use of Laser Beam Welding

Owing to the relatively high welding speeds, laser beam welding and hybrid welding are not only very economical processes, but offer metallurgical advantages at the same time. Illustrations 12 and 13 show macrographs of a TIG-weld and a laser weld (CO2-laser) of pipes in material type 1.4435. The difference of both welding processes is clearly demonstrated by the cross sections of the welds.

Illustration 12: Longitudinal pipe weld in grade 1.4435 – cross section
of a longitudinal TIG weld in a pipe (OD 63.5 x 1.65mm)

The weld depth/width ratio of Laser welds is 3:1, while it is approximately 2:1 with TIG welds. Analogous to the cross section of the weld, also the heat affected zone (HAZ) adjacent to the weld is very narrow. As this zone has a considerable influence on the quality of the welded joint, the laser weld offers more advantages than those produced by conventional welding processes, due to the very narrow heat affected zone.

Illustration 13: Longitudinal pipe weld – cross section of a longitudinal
pipe weld using CO2 Laser Beam welding (pipe OD 76.2x1.65mm)

Conclusion

As the use of laser beam welding processes offers more technical and economical advantages than the conventional shielding gas processes, Butting plan to utilize it also for the production of pipes in nickel alloys in the not too distant future. However, depending on the material grade, the use of filler metal will be required. Welding trials using the laser cold wire process have proved successful and the integration of a wire feed equipment into a pipe welding line is currently under way. The corrosion properties of the weld can be improved by using filler metal. It will also be possible to reduce the risk of hot cracking of certain chromium-nickel alloys and nickel alloys using appropriate filler metals.

Finally it can be stated that the laser beam welding processes and the hybrid process can be used for the series production of pipes in conventional chromium and chromium-nickel steels, duplex steels and nickel alloys. The required welding parameters, such as focal distance, focus position, purging gases and welding speed must be adjusted with respect of the material grade to be used.