Cause analysis and control of black spots on the surface of titanium welded pipe
In the production process of titanium welded pipe, some irregular black spots appear on the surface of titanium welded pipe, which not only reduces the yield, but also increases the production cost. In order to find out the cause of black spot formation, the black spot area was characterized and analyzed by scanning electron microscope, energy dispersive spectrometer, Raman spectrometer and metallographic microscope. The results show that the carbon content in the black spot area is high, and the Raman spectrum has sharp D and G peaks, which are carbides attached to the surface. Combined with the production process, it is believed that the black spots are formed by the carbonization of the lubricating grease at the moving parts of the production line when it is mixed with the emulsion and brought to the surface of titanium welded pipe by the emulsion during high-temperature annealing. By controlling the amount of grease used in moving parts, increasing the frequency of cleaning up spilled grease, setting a filter device for emulsion and increasing the pressure of cleaning water, the problem of black spots on the surface of titanium welded pipe is effectively solved.
Titanium pipes are divided into titanium welded pipes and titanium seamless pipes, and the mechanical properties of titanium welded pipes are basically the same as those of titanium seamless pipes, but with uniform wall thickness, good concentricity, high finish, and obvious cost and environmental advantages . In recent years, with the rapid development of China’s titanium industry and the increasing maturity of the welding process, titanium welded pipes are used in large numbers in various types of heat exchangers, which further improve the service life of heat exchangers and reduce equipment maintenance costs while ensuring heat exchange efficiency, resulting in good economic benefits .
The highly passivated titanium metal surface is extremely easy to generate a firmly adhered dense protective film and thus has excellent corrosion resistance. A new oxide film is immediately formed on the removed titanium surface as soon as it is exposed to air or aqueous solutions . Therefore, dressing the surface of titanium welded pipes does not affect their corrosion resistance.
During the production of titanium welded pipes, irregularly shaped black spots appeared on the surface of some titanium welded pipes, and the black spots mainly appeared at the edges of the weld seam. According to ASTM B338-17 standard , the surface of the titanium welded pipe can be trimmed. However, the amount of trimming cannot be controlled, and it is easy to cause the wall thickness to be too thin and scrap, while it will increase the production cost. For this reason, the causes of the formation of black spots on the surface of titanium welded pipes are analyzed and improvement measures are developed based on the analysis results to solve the problem of black spots on the surface of titanium welded pipes.
Table of Contents
- 1. Experiment
- 2. Results and analysis
- 2.1 Morphological and compositional analysis of the black spot area
- 2.2 Analysis and control of black spot causes
- 3. Conclusion
1.1 Experimental material
The experimental material is Gr.2 titanium welded pipe with the specification of 26mm×0.711mm and the chemical composition as shown in Table 1. The surface of the titanium welded pipe has black spots, and the typical shape is shown in Figure 1.
Table 1 Impurities content of Gr.2 titanium welded tube
Fig.1 Typical black spots on the surface of titanium welded tube
The production process of titanium welded tube is: strip → cleaning → shear end welding → cleaning → cold bending → welding → primary sizing → online annealing → secondary sizing/straightening → eddy current flaw detection → laser sizing → sizing and cutting → ultrasonic flaw detection → fine cutting → end deburring → underwater airtight test → surface air drying → surface inspection.
1.2 Method and equipment
Randomly selected titanium welded pipe and cut three typical specimens with black spots, numbered 1#, 2#, 3#, as shown in Figure 2. The black spot and non-black spot areas were observed using a Zeiss Merlin Compact field emission scanning electron microscope (SEM), and the composition was analyzed using an Oxford AZtec X-Max 50 energy spectrometer. Raman spectroscopy was performed using a Renishaw Invia laser confocal Raman spectrometer. The XJA-6A metallurgical microscope was used to observe the metallographic organization of the black spot area (after grinding and polishing) and the non-black spot area of the sample.
Fig.2 Photos of test specimens
2. Results and analysis
2.1 Morphological and compositional analysis of the black spot area
2.1.1 SEM analysis
Fig. 3 and Fig. 4 show the SEM photos of the black spot area and the non-black spot area of the three specimens, respectively. From Fig. 3 and Fig. 4, it can be seen that the morphology of the black spot area of the three specimens is mainly in the form of micro-nano-scale lamellae, while the morphology of the non-black spot area is in the form of cold-rolled titanium strip after cold bending.
The results are shown in Table 2 and Table 3 for the black spot region and non-black spot region of the three specimens. The results are shown in Table 2 and Table 3, respectively. From Table 2, it can be seen that the black spot area of the three specimens contains more C and Ti elements, a certain amount of O elements and a small amount of Cu, Al, Si, K and Ca elements. From Table 3, it can be seen that the composition of the non-black spot area of all three specimens is mainly Ti elements, a certain amount of C, O elements and a small amount of other elements. Comparing Table 2 and Table 3, it can be seen that the difference between the composition of black spot and non-black spot area is mainly the content of C elements, and the content of C elements in the black spot area is obviously higher than that in the non-black spot area, and there are more kinds of other elements. More types of elements in the black spot than in the non-black spot area should be due to the residual of foreign substances on the surface of the titanium welded pipe.
Fig.3 SEM morphologies of black spot areas:(a) 1#specime;(b) 2#specime;(c) 3#specime
Fig.4 SEM morphologies of non-black spot areas:(a) 1#specime;(b) 2#specime;(c) 3#specime
Table 2 EDS analysis results of black areas
Table 3 EDS analysis results of non-black areas
2.1.2 Raman spectral analysis
Figure 5 shows the Raman spectra of the black spot areas of the specimens. From Fig. 5, it can be seen that the Raman spectra of the three specimens have sharp D and G peaks, and the height of the peaks corresponds to the C element content detected by the energy spectrum. The morphology of the D and G peaks and the absence of other spurious peaks in the figure can indicate that the black spot is a graphitized carbon material .
2.1.3 Metallographic organization analysis
Figure 6 shows the metallographic photos of the black spot area of the specimen after 5 times light grinding and corrosion by 2000# metallographic sandpaper. From Fig. 6, it can be seen that the microstructure of the three specimens is basically the same, showing the typical microstructure of cold-rolled titanium strip after cold bending process, which confirms that the metallographic organization at the black spot has not changed due to the presence of the black spot.
Fig.5 Raman spectra of black spot areas
Fig.6 OM micrographs of black spot areas after polishing: (a) 1#specime; (b) 2#specime; (c) 3#specime
2.2 Analysis and control of black spot causes
2.2.1 Analysis of the causes of black spots
Comprehensive analysis of the above experimental analysis and production process tracking analysis, the formation of black spots is believed to be the reason: the production line bearings and other moving parts of the grease following the emulsion cycle residue in the titanium welded tube towards the upper weld position, after the roller extrusion closely combined in the titanium welded tube surface, cleaning process failed to clean off, the subsequent annealing process in the chemical reaction formed black spots.
2.2.2 Control of black spot generation
According to the above analysis, the black spot generated mainly with moving parts lubricating grease, emulsion circulation and cleaning capacity. Therefore, to eliminate black spots, the main consideration is to avoid mixing grease into the emulsion and enhance the cleaning ability of the titanium welded tube surface.
The purpose of using grease is to reduce the friction of moving parts such as bearings and extend the service life of equipment. During the movement of the bearing parts, grease will spill out from the gap of the parts and mix into the emulsion. In order to avoid grease overflow, through many practical attempts, the use of special cups in the moving parts coated with a thin layer of grease, which can effectively play the role of lubrication, but also reduce the grease overflow during the movement of the parts. At the same time, each shift inspection 4 times grease overflow situation, timely clean up the overflow of grease.
The role of emulsion in the production process of titanium welded pipe is lubrication and cooling, the main role is lubrication, if the lubrication performance is not good will make the surface of the titanium welded pipe scratches and other defects, affecting the surface quality. In order to avoid the emulsion into the moving parts at the overflow of grease, and then residual in the titanium welded pipe surface, the emulsion cycle at the entrance to set up an additional filter device, with 282μm (50 mesh) filter grease filter, and regularly clean the filter.
To ensure the cleaning effect, the cleaning water pressure increased by 0.1MPa, in the titanium welded tube into the annealing furnace before the surface of the grease clean.
2.2.3 Control effect
After taking the above improvement measures, tracking statistics for 3 months of titanium welded tube production, titanium welded tube surface no longer found black spots, completely solved the problem of black spots on the surface of titanium welded pipes.
- (1) The black spot on the surface of the titanium welded tube is due to the production line moving parts overflow grease mixed into the emulsion, the emulsion to the surface of the titanium welded tube, while being rolled combined with the surface of the titanium welded tube failed to be cleaned and removed, the formation of carbonization in the high temperature annealing.
- (2) Through the control of bearings and other moving parts of the amount of grease, increase the frequency of cleaning overflow grease, emulsion set filtering device and increase the pressure of the cleaning water and other measures, so that the surface of the titanium welded tube does not appear again black spots, completely solve the problem of black spots on the surface of the titanium welded tube.
Authors: Zhang Wangcheng, Li Haiming, Tang Shuang, Zeng Xianshan
Source: Network Arrangement – China Titanium Welded Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.titaniuminfogroup.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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