Welding of stainless steel tube sheets: technologies 

Technologies for welding stainless steel tube sheets: selection of method and quality control

Welding of stainless steel tube sheets is a critical stage in the production of heat exchange equipment, where an error in the choice of technology or violation of process parameters leads to immediate rejection of the entire expensive part. In our practice, we have repeatedly encountered situations where the use of universal modes for different grades of steel led to intergranular corrosion after just 6 months of operation, which forced customers to stop entire production lines. The correct technology for welding stainless steel tube sheets requires not just compliance with GOST or ASME, but a deep understanding of the metallurgical processes occurring in the heat-affected zone at temperatures above 500°C.

This article was written by the company's process engineersWuxi Kaisheng Electric Power and Petrochemical Equipment Co., Ltd., specializing in the development and production of complex heat exchange equipment for oil refining, chemical industry and energy. Our experience includes titanium shell-and-tube heat exchangers, ASME high-pressure units, and exotic alloy products (N06625, C70600). We will not retell textbooks on materials science; Instead, we will analyze real cases from our workshop, show comparison tables of methods and give specific recommendations for setting up equipment to achieve a seam tightness that can withstand pressure up to 25 MPa. If you're looking for an answer to how to avoid burning through thin pipe walls or warping grids during mass production, this material will save you months of experimentation.

Why standard approaches often don’t work with stainless steel

Stainless steel is a capricious material. Unlike carbon steels, it has a low thermal conductivity coefficient (about 3 times lower than that of conventional steel) and a high linear expansion coefficient. This means that heat does not dissipate quickly away from the welding area, but accumulates, causing local overheating. One of our clients faced the problem of warping of tube sheets with a diameter of 800 mm precisely because of ignoring this factor: they applied the same currents as for black steel, which led to the fact that the holes for the pipes shifted by 1.5 mm, making it impossible to install the tube bundle without mechanical modification.

In addition, preventing the precipitation of chromium carbides is critical. When heated between 450 and 850°C, chromium combines with carbon, depleting the grain boundaries and leaving the weld vulnerable to corrosion. Stainless steel tubesheet welding technologies must strictly control the amount of time the metal remains within this temperature window. We recommend using steels with low carbon content (markings 304L, 316L or their analogues 03Х17Н14М3), where the carbon content does not exceed 0.03%, which virtually eliminates the risk of sensitization even under non-ideal welding conditions.

Comparative analysis of methods: TIG, Plasma and Laser welding

The choice of welding method determines not only the speed of production, but also the final cost of the product. Three main technologies dominate the market today: TIG/GTAW, plasma welding and laser welding. Each method has its own physical limitations and economic niches. Below we provide a detailed comparison based on real-world performance measurements from our shop where we produce 321 stainless steel, C46400 brass and copper-nickel components.

TIG welding (GTAW)remains the “gold standard” for one-off production and repairs. It forgives small errors in assembly and allows you to visually control the formation of the bath. However, when mass-producing heat exchangers with thousands of pipe connections, manual labor becomes a bottleneck. Automated TIG stations increase speed but still suffer from a relatively wide heat affected zone.

Plasma weldingoffers the perfect balance for medium series. The compressed arc provides deep penetration at high speed. In our practice, the transition to plasma allowed us to increase the production of gratings by 40% without increasing the number of welders. The main nuance here is the need to use specialized devices to center the burner relative to each pipe. The slightest displacement of the axis leads to uneven penetration.

Laser weldingis the choice for gigantic volumes. The speed of the process is phenomenal, and deformations are minimal due to the low calorific value. But there is a significant limitation: the laser requires ideal assembly geometry. If the pipe fits into the grid opening with a gap of more than 0.1 mm, the laser beam will simply pass through the gap without melting the edges. We only use laser after introducing precision machining of holes to H7/g6 tolerance, which is especially important when working with expensive alloys such as titanium or nickel.

Surface Preparation and Assembly: Hidden Success Factors

Even the most advanced welding setup will not save the connection if the preparation is done carelessly. The oxide film on stainless steel has a melting point significantly higher than that of the base metal. If it is not removed, it will remain in the seam in the form of inclusions, creating pockets of corrosion and reducing the tightness. Before starting work, we require mandatory mechanical cleaning of the welding zone to a width of at least 20 mm from the edge using stainless steel brushes. The use of brushes after processing carbon steel is strictly prohibited - this will lead to hydrogenation and rust.

Degreasing is the second critical stage. Residues from grease, marking paint or fingerprints decompose at high temperatures, releasing hydrogen and carbon. In our practice, there was a case when a batch of 50 gratings was rejected due to the porosity of the seams. The investigation found that workers used a common solvent, which left a greasy film, instead of specialized degreasers for the food industry. Learn the rule: the cleanliness of the surface directly affects the absence of pores.

The gap between the pipe and the wall of the hole in the grate should be minimal, but sufficient for the penetration of the melt. The optimal range is 0.05–0.15 mm for thin-walled pipes (up to 2 mm). With large gaps, the risk of burning through the inside of the pipe or lack of fusion at the root of the seam increases. To fix the pipes before welding, we use the expansion method only partially to ensure tight contact, but leaving room for gases to escape from the gap to avoid the formation of air pockets.

Root protection and backwash

When welding stainless steels, protecting the back of the weld is just as important as protecting the front. Contact of hot metal with atmospheric oxygen instantly leads to oxidation (“sugar” coating), which cannot be removed mechanically without damaging the channel geometry. The technology for welding stainless steel tube sheets necessarily includes the organization of a gas chamber or local purging of the internal cavity of the pipes with argon.

We recommend the use of indicator pastes or oxygen meters to monitor the O2 content in the protective environment. Oxygen levels should not exceed 50 ppm (0.005%). In real conditions, workshops often neglect this, limiting themselves to visual assessment of the color of the seam. Remember: a light straw color is acceptable, a blue or gray tint indicates overheating and oxidation, and a black color is an absolute defect. Action: Install flow meters in the backflush line and monitor argon flow (typically 5-8 L/min) throughout the cooldown cycle.

Step-by-step regulations for automatic welding (TIG/Plasma)

To ensure repeatable results, we have developed a strict workflow that meets our internal PED and ASME quality standards. Following these steps allows you to minimize the influence of the human factor. This regulation is applicable to semi-automatic and automatic systems used in our production.

1. Equipment calibration and gas testing.Before changing, it is necessary to check the purity of the argon (class 4.8 or 5.0) and the absence of moisture in the lines. Wet gas is the main cause of hydrogen porosity. Check the condition of the tungsten electrode: it should be sharpened with a 30-45 degree cone for TIG or properly shaped for plasma. A blunt electrode destabilizes the arc and expands the heating spot.
2. Setting up starting parameters.Set the welding current according to the pipe wall thickness. For a pipe with a diameter of 25 mm with a wall of 2 mm, the optimal current is 60–75 A. It is important to set the mode for a smooth increase in current (upslope) and decrease (downslope). A sharp start of the arc can cause a burn at the ignition point, and a sharp break can cause a crater at the end of the weld, which will become a crack under load. The downslope time should be at least 1.5 seconds to gradually crystallize the bath.
3. Positioning and beginning of the process.The automatic head must approach the joint strictly perpendicularly. It is better to ignite the arc on a copper lining or away from the joint with subsequent exit to the seam (for complex systems), or use high-frequency ignition without touching. Begin welding by holding the arc in place for 0.5-1 second to warm up the edges without additive.
4. Main process and additive supply.The burner movement should be uniform. When using filler wire (if gap filling is required), insert it into the front of the weld pool without violating the gas shield. A common mistake made by beginners is to insert the wire directly into the arc column, which causes spattering and disrupts the stability of the process. The head rotation speed (if orbital welding is used) is selected experimentally to obtain flakes 1.5–2 mm wide.
5. Completion and post-purging.After completing a circle or finishing a seam, do not remove the torch immediately. Continue argon flow for 5–10 seconds (post-flow) until the tungsten and weld are completely cooled below 400°C. This prevents the hot metal from oxidizing. Only after this can the head be moved to the next joint.

Important note: never interrupt the welding process in the middle of a joint without special crater filling technology. If an accident occurs (voltage surge, wire break), the stopping point must be completely cleaned with a flap wheel to healthy metal and welding must be started again, overlapping the previous section by 10 mm.

Quality control and flaw detection

The finished tube sheet cannot be sent to the customer without comprehensive inspection. Visual inspection (VT) is the first line, but it does not detect internal defects. In accordance with our internal standards, which strengthen the requirements of GOST R 52630 and ASME Sec VIII Div 1, we use the following non-destructive testing (NDT) methods.

Penetrant testing (PT).Mandatory procedure for all welded joints of stainless steels. The application of penetrant allows you to identify microcracks, pores and lack of fusion that appear on the surface. We use contrast dyes of sensitivity class 2. We pay special attention to the transition zones from the seam to the base metal. Any indication longer than 1.5 mm must be repaired.

Hydraulic tests.This is an integral test for the tightness of the entire assembly. The grate is plugged on one side and water is supplied under pressure exceeding the operating pressure by 1.25–1.5 times (usually 2.0–2.5 MPa for standard heat exchangers). Pressure exposure is at least 30 minutes. A drop in pressure or the appearance of water drops at the ends of the pipes indicates through-hole defects. In our practice, about 3% of seams that have passed visual inspection leak hydraulically due to hidden pores.

Ultrasound testing (UT) and radiography (RT).They are used selectively or for critical high-pressure units. Ultrasound allows you to assess the depth of lack of fusion, and x-ray gives a complete picture of the structure of the seam. However, for thin-walled pipes (less than 3 mm), radiography is often uninformative due to the small difference in radiation absorption between the defect and the base metal, so we prefer the PT + Hydraulics combination for mass production.

Typical defects and methods for eliminating them

• Undercuts.Occurs when the current is too high or the welding speed is too fast. Weaken the cross-section of the pipe. Eliminated by stripping and re-welding with a decrease in speed.
• Porosity.Consequence of poor gas protection or surface contamination. Requires complete cutting down of the defective area. Local welding of pores is not allowed, since the gas will remain in the metal.
• Lack of root penetration.Most often encountered during laser welding with a large gap. The part must be scrapped or rebuilt using TIG to fill the gap.

Influence of alloying elements on process technology

Not all grades of stainless steel are welded the same. Understanding the chemical composition helps predict the behavior of a material. For example, austenitic steels (304, 316) are prone to hot cracking due to the formation of low-melting eutectics at grain boundaries. To combat this, ferritizing elements (silicon, niobium) are added to filler materials, which change the structure of the weld and increase its resistance to cracking.

Duplex steels (2205, 2507) require even tighter control of heat input. Their two-phase structure (austenite + ferrite) can be disrupted when overheated, which will lead to a decrease in corrosion resistance and impact toughness. When welding duplexes, we strictly monitor the interpass temperature, not allowing it to exceed 100–150°C. Forced water cooling between passes is often required, which is not typical for conventional stainless steels.

Stabilized steels (321, 347) containing titanium or niobium are more resistant to intergranular corrosion, but titanium is prone to active interaction with the atmosphere. Therefore, argon protection for such brands should be as effective as possible, possibly using additional screens or chambers with a controlled atmosphere. These are the requirements we take into account when producing 321 steel tube sheets for our customers in the oil refining industry.

Economic aspects and production optimization

The implementation of advanced stainless steel tube sheet welding technologies always involves finding a balance between capital expenditure and operational efficiency. The purchase of a laser complex pays off only if the workload is 2-3 shifts and there is a large portfolio of orders for standard products. For small-scale production with frequent product changes, flexible TIG cells with rotary positioners remain the most cost-effective solution.

We analyzed the cost of one welded joint. With manual TIG, the cost consists primarily of the wage fund (up to 70%). With automation, the share of salaries drops to 20%, but depreciation charges and the cost of servicing complex electronics increase. However, reducing the scrap rate from 5% to 0.5% provides enormous savings on rework and loss of expensive metal. One defective 316Ti steel or nickel alloy grate can cost more than a month's rental of a welding robot.

It's also worth considering consumables. High-quality tungsten, pure argon and calibrated filler wire are more expensive than their analogues, but their use reduces the number of stops for re-welding. In our practice, switching to certified consumables has reduced equipment downtime by 15%.

Frequently Asked Questions

What brand of filler wire should I choose for welding AISI 304?

For welding steel AISI 304 (08Х18Н10), the de facto standard is ER308L wire (Sv-04Х19Н11). The letter “L” indicates low carbon content, which is critical for preventing intergranular corrosion. The use of conventional ER308 is only permissible for structures that do not operate in aggressive environments at elevated temperatures. If you are welding a structure that will be operated at temperatures above 400°C, choosing a low-carbon modification is mandatory.

Is it possible to weld a stainless pipe with a carbon grid?

Direct welding of dissimilar metals (stainless steel + carbon steel) is highly discouraged due to the risk of brittle structures and galvanic corrosion. The standard solution is to use bimetallic adapter bushings or apply an austenitic surfacing layer to the edges of the hole in the carbon grid before installing the pipes. In this case, a layer of ER309L type wire is first welded on, which serves as a buffer, and only then a stainless pipe is welded to this layer.

Which gas is better to use: pure argon or a mixture?

For root passes and thin-walled pipes (up to 3 mm), use only pure argon (99.998%). Adding helium can increase heat input and welding speed, but makes arc ignition more difficult and requires more expensive equipment. Adding a small amount of hydrogen (up to 2-5%) is permissible only for austenitic steels during automatic welding to improve the cleanliness of the weld surface, but is strictly prohibited for ferritic and martensitic steels, as well as for root passes due to the risk of hydrogen cracking.

Conclusion

Stainless steel tube sheet welding technologies continue to evolve towards full automation and real-time digital control of parameters. However, the fundamental principles of metallurgy remain unchanged: cleanliness, assembly accuracy and correct thermal conditions are the three pillars of a quality connection. Ignoring any of these factors turns an expensive heat exchanger into a source of constant problems and emergency stops.

Our experience at Wuxi Kaisheng LLC shows that investments in qualified personnel and modern equipment pay off in product reliability. We provide custom solutions for projects ranging from 316 stainless steel corrugated tube bundles to waste heat boilers and air coolers for shipbuilding and seawater desalination. Don't skimp on quality control and edge preparation - this is the foundation on which the integrity of your equipment rests.

If you are faced with difficulties in selecting welding modes for specific steel grades (titanium, copper-nickel alloys, alloy steels) or need an audit of the existing technological process, our team is ready to provide expert advice. We specialize in developing individual welding cards and training personnel to work with modern equipment.Contact us todayto discuss your challenges and receive technical support from practicing engineers. We also recommend that you read our sectionproduction of tube sheetsfor more information on materials and processing standards.