Fertigungsprozess für Wärmetauscherrohre mit DLC-Innenbeschichtung

In plant engineering, especially in heat exchanger construction, fouling on heat transfer surfaces usually means a considerable reduction in the performance of the system. A measure that is often helpful is coating. However, many variants of coatings are not abrasion resistant. In this work, a manufacturing process for tubes with DLC coating (inside as well as outside) is to be developed, because DLC is effective against certain forms of fouling and proves to be very abrasion resistant. Two variants (process P01 and process P10) were proposed. Fabrication with slitted tubes (process P01) shows some difficulties because welding because the required longitudinal weld caused significant distortion. In process P10, a larger number (compared to process P01) of unslitted DLC-coated tubes are joined by orbital welding to form a 6-8 m product tube. However, the possible coating length of the DLC coating on the inside of the tube is limited in process P10.
In this work, the focus was set on orbital welding, since it is necessary for both processes. The company Polysoude Austria GmbH from Admont specializes in the manufacture of automated welding equipment for orbital welding and was the project partner. The task was to develop a test setup, the sample preparation and the test series. Tubes DA25 mm, 2 mm wall thickness made of 1.4571 were used. The test pieces were uncoated or DLC-coated (a-C:H:Si: 30% Si doped, approx. 8 μm thick), each approx. 100 mm long and face-turned at both ends.
The objective was to show whether removal of the DLC coating is necessary as a welding preparation. The welding program should result in as little removal of the DLC layer as possible, especially on the inside.
The test welds showed good results in terms of straightness, degree of automation, microstructure and carbon insertion. However, it was surprising that the welding program for uncoated tubes could not be made suitable for coated tubes by minor adaptations, but an additional heat-treatable seam was required, which, however, is of little importance in economic terms.
The analyses using the GDOES method found that the input from welding increases the carbon content by 0.0188% by mass. In the material specification of 1.4571, the permissible carbon maximum is 0.08 mass %. No higher values were measured in any of the weld specimens. The carbon can diffuse in the microstructure and forms carbides with chromium, which entail an increased risk of corrosion.
The tensile tests according to EN ISO 4136 showed that the weld is the breaking point. In the material specification 1.4571, the tensile strength is given as 500 - 700 MPa. The tensile strength of the welds of both the uncoated and the coated pipes is well above 500 MPa. The elongation at break was similar for all weld measurements. However, there were differences from the elongation at break of the original material 1.4571 and those of the welds. Regardless of whether coated or uncoated, minor embrittlement resulted from the welding, as was to be expected.
For use in heat exchanger construction, this indicates that only an insignificant weakening of the strength occurs when welding tubes with DLC coating in the edge area without special weld preparation - butt to butt. The microstructural changes due to welding are different and so is the carbon content in the weld. It can be assumed that the risk of corrosion increases due to carbide formation - because of the increased carbon content, in the presence of chromium. The results from the tensile test show no loss of strength from the welds between coated and uncoated pipes. The pickling test showed no increased DLC coating damage. Diffusion of carbon is present in the DLC coated pipes. Due to the presence of chromium, embrittlement is realistic and also an increased susceptibility to corrosion. For the final answer whether a removal of the DLC layer in the edge area before welding is necessary, the corrosion behavior has to be investigated.