Orbital TIG welding of thin-walled stainless steel/titanium pipes for cleanroom applications:
how to achieve a repeatable weld and how to confirm quality Automatic translate

Automatic orbital TIG welding (TIG) has become the standard for industries where the cost of error is measured not in the cost of a pipe, but in the downtime of entire production lines. The pharmaceutical, semiconductor, and aerospace industries place demands on pipelines that are physically impossible to achieve manually with the required consistency. Human error, hand tremor, or eye strain are unacceptable. The internal surface of the weld must be so smooth that bacterial colonies cannot establish themselves or corrosive agents accumulate.

Physics of the process in a closed chamber

The core technology for small- and medium-diameter pipes (typically from 3 to 170 mm) is enclosed welding heads. Unlike open systems, here the tungsten electrode rotates around a stationary pipe inside a sealed chamber filled with shielding gas. This creates a unique micro-atmosphere around the melting zone. The absence of oxygen inside the chamber prevents oxidation of the weld from the outside, but the main battle for quality occurs inside the pipe.

A distinctive feature of closed-die welding is the absence of filler wire. The connection is made by melting the edges of the pipes themselves. This is called autogenous welding. This method eliminates the risk of introducing external contaminants, but imposes extremely stringent assembly requirements. While in manual welding, the welder can "bridge" the gap by adding more filler wire, any gap here will lead to a collapse of the weld pool or a thinning of the weld wall.

Process engineers often underestimate the effect of gravity on the weld pool at thin wall thicknesses. Even at a thickness of 1.6 mm, the liquid metal behaves differently at the 12 o’clock position (top) and the 6 o’clock position (bottom). At the top, gravity aids penetration by pushing the pool inward. At the bottom, it tries to pull the metal outward, creating sag. The automation compensates for these forces, but only if the program is correctly configured.

The operator must control not only the end face geometry but also all input parameters. Even the highest-quality welding materials and expensive gases will not save the joint if the gap between the parts exceeds the permissible hundredths of a millimeter. The joint fit must be perfect: no gaps are allowed, and edge misalignment should not exceed 10-15% of the wall thickness.

Orbital Welding of Stainless Steel and Titanium

When working with austenitic stainless steels (the most popular grade is 316L), overheating is the main problem. Stainless steel has low thermal conductivity. Heat is not transferred into the pipe body, but accumulates in the weld area. If heat input is not controlled, discoloration occurs. These iridescent streaks are not just a visual defect, but a layer of chromium oxide depleted of metal underneath. In an aggressive environment, this is where corrosion will begin.

For "clean" production (UHP - Ultra High Purity), standards generally prohibit any discoloration inside the pipe. Only a light straw hue is permitted, but the ideal weld should be silvery. This is achieved not only by current settings but also by the residual oxygen content in the shielding gas.

Welding titanium requires even more rigorous discipline. Titanium is an extremely reactive metal. When heated above 400°C, it begins to greedily absorb oxygen, nitrogen, and hydrogen from the atmosphere. While steel is forgiving of a brief breach in protection, titanium instantly becomes brittle. A gas-saturated weld may appear normal, but it will crack at the first vibration load. When orbital welding titanium pipelines, purging is performed for a longer period, and the metal temperature is monitored at the exit of the protection zone.

The influence of sulfur in the steel composition also alters the hydrodynamics of the weld pool. A difference in sulfur content between two pipes being welded (for example, one pipe has 0.005% sulfur and a fitting has 0.015%) can cause arc displacement and asymmetrical penetration. This phenomenon is known as the Marangoni effect: liquid metal flows from areas of low surface tension to areas of high surface tension. The process engineer is required to check the heat numbers of the welds before beginning work.

Sectoral energy management

Orbital welding isn’t a monotonous process where the current is applied evenly from start to finish. The process is broken down into sectors. Typically, the circle is divided into 4 to 12 segments, and each has its own parameters.

At the start (usually in the "side" or "bottom" position), a high current is required to quickly form a weld pool. As the electrode moves upward, the heat input should be reduced, as the pipe is already heated and gravity facilitates penetration. During the descent (after 12 o’clock), the current is adjusted again to prevent the weld pool from leaking out. The process is completed by overlapping the weld — the arc passes the starting point by 5–10 mm, gradually reducing the current (slope out) to weld the crater.

Pulsed Current mode is the primary tool for controlling the weld pool on thin walls. High-frequency pulsating current compresses the arc, making it needle-like, ensuring deep penetration with lower overall heating. Low-frequency pulsating allows the metal to crystallize in "flakes," step by step. During the pause between pulses, the weld pool partially cools, preventing burn-through.

Gas protection and purging

Back purging is a critical step. An inert gas (99.998% purity argon or a mixture with hydrogen/helium) is injected into the pipe. Mixtures with 2–5% hydrogen are often used for stainless steel: the hydrogen binds residual oxygen and increases the arc temperature, producing a narrower and smoother weld. However, hydrogen is strictly prohibited for titanium due to the risk of hydrogen embrittlement.

The gas pressure inside the pipe must be balanced. Too much pressure will "blow" the molten metal outward, forming a concave root. Insufficient pressure will lead to excessive inward sagging, which will narrow the pipe’s cross-sectional area. Technologists use special plugs with calibrated holes or systems with automatic internal pressure control.

Residual oxygen analyzers are essential. Welding should not begin until the sensor reads below 10–20 ppm (parts per million). Attempting to begin welding "by eye," relying solely on the purge time, often results in the failure of expensive components. The gas must displace all air, including microscopic volumes in the metal pores and on the surface of the plugs.

Tungsten electrode as a precision variable

The electrode’s sharpening geometry directly affects the arc shape and penetration depth. Orbital welding heads use pre-cut electrodes of a fixed length. The sharpening angle (usually 15–30 degrees) determines the weld width: the sharper the angle, the wider the arc and the shallower the penetration depth. An obtuse angle concentrates the energy.

The electrode surface must be polished. Grinding marks on tungsten can cause arc instability — electrons will be ejected from the sharp edges of the scratches, causing the arc to wander. Alloyed electrodes (cerium or lanthanum) are used in closed welding heads, as pure tungsten cannot withstand thermal stress. Thorium electrodes, once popular, are now avoided due to the low radioactivity of the dust during sharpening, which conflicts with the safety standards of many cleanroom industries.

The distance from the electrode to the workpiece (the arc gap) is set mechanically and does not change during welding with a closed head. This simplifies the system, as an automatic arc voltage control (AVC) unit is not required, but it does require the pipe to be perfectly oval. If the pipe is oval, the arc gap will change during rotation, resulting in uneven penetration.

Objective control and validation

In mass production, it’s impossible to inspect every joint with an X-ray or endoscope, especially when thousands of connections are being made in a single plant. Therefore, the emphasis shifts to process validation. If the welding parameters (current, voltage, rotation speed, gas flow) remain within a narrow tolerance throughout the entire cycle, the joint is considered acceptable.

Modern orbital welding power sources act as data loggers. They record actual parameter values up to several times per second. At the end of a shift, the technologist receives a digital log. Any deviation, such as a power surge or a brief gas outage, is recorded. The system can automatically flag a weld as suspicious.

Visual inspection (VII) and endoscopy remain the primary inspection methods. An endoscope is inserted into the pipe to inspect the weld root. The operator looks for signs of incomplete fusion, oxidation, or tungsten inclusions. Temper colors are classified according to specific standard tables (e.g., ASME BPE). For pharmaceutical applications, the presence of purple or blue discolorations inside the pipe is a definite defect, requiring the section to be cut out and rewelded.

Surface preparation factor

Pre-weld machining of the ends is more important than the welding itself. Cutting with an abrasive wheel is unacceptable: it overheats the metal, altering its structure, and leaves abrasive particles. Only orbital pipe cutters with high-speed steel or carbide tips are used. These ensure perpendicular cuts and a burr-free finish.

After cutting, the surface is trimmed to achieve a perfectly flat surface. Then comes thorough cleaning. The use of chlorinated solvents is prohibited, as the arc produces phosgene, which causes stress corrosion cracking of stainless steel. High-purity alcohol or acetone and lint-free wipes are used. Do not touch the cleaned joint with bare hands, as oily fingerprints will turn into carbon inclusions and carbides during welding, reducing corrosion resistance.

Orbital welding is a disciplined technology. The machine will perform the task perfectly only as many times as the operator ensures ideal conditions. Repeatability is the result of strict adherence to a protocol, where every movement is regulated, and every variable is accounted for and documented.