Publish Time: 2026-08-01 Origin: Site
Large quantities of stainless steel valves and fittings are used throughout the coolant circulation pipelines of liquid-cooled data center systems. Thanks to their corrosion resistance, cleanliness, mechanical strength, and long service life, stainless steel components have become an important part of cooling distribution units, manifolds, main pipelines, branch lines, and other liquid-cooling infrastructure.
However, stainless steel valves and fittings used in liquid-cooled data center projects are often subject to a specific sulfur-content requirement. In many projects, the sulfur content of the stainless steel is required to be controlled within the range of:
S = 0.005%–0.030%
This requirement is not intended to improve cooling performance, nor does sulfur itself improve the corrosion resistance of stainless steel.
The main purpose is to ensure stable and repeatable automatic welding performance for stainless steel pipelines, manifolds, and prefabricated piping assemblies.
This is particularly important when autogenous orbital GTAW, also known as automatic orbital TIG welding without filler metal, is used. In this welding process, sulfur content can significantly affect weld-pool flow, weld penetration, bead geometry, and welding consistency.
Liquid-cooling systems for data centers, including cooling distribution units, in-rack manifolds, main pipelines, branch pipelines, and precision thin-wall tubing, require a large number of highly consistent welded connections with an extremely low risk of leakage. For this reason, automatic orbital welding is increasingly used in the fabrication and installation of these systems.
Sulfur is a surface-active element in the molten weld pool of stainless steel.
It can change the relationship between surface tension and temperature in the molten metal, thereby changing the direction of fluid movement inside the weld pool. This phenomenon is known as the Marangoni effect.
When sulfur content falls below approximately 0.005%, the following conditions may occur:
Molten metal tends to flow outward from the center of the weld pool.
Welding heat spreads toward both sides of the joint.
The weld bead becomes wider and shallower.
Insufficient root penetration or localized lack of fusion may occur.
The same welding program may produce different results on materials from different heats.
According to orbital welding literature, when the sulfur content of 316L stainless steel is below approximately 0.005%, achieving sufficient penetration becomes more difficult.
The external weld-bead width may increase by approximately 50%, and in some cases the welding heat input may need to be increased by approximately 40% to achieve the same level of penetration. These effects have been discussed in technical materials published by Swagelok and other orbital welding specialists.
When sulfur content is maintained within an appropriate range:
Molten metal tends to flow toward the center of the weld pool.
Welding heat becomes more concentrated at the weld center and root.
The weld bead becomes narrower and deeper.
Full penetration in a single pass is easier to achieve.
Automatic welding programs become more repeatable.
Experimental studies have also shown that weld penetration in austenitic stainless steel generally increases as sulfur content rises within a suitable range. The main reason is that sulfur changes the direction and intensity of Marangoni convection inside the weld pool.
The effect can be summarized as follows:
Sulfur Condition | Weld-Pool Flow | Typical Weld Shape | Main Risk |
|---|---|---|---|
Below 0.005% | Outward from the center | Wide and shallow | Incomplete penetration and unstable welding parameters |
Properly controlled | Toward the center | Narrow and deep | Easier and more stable full penetration |
Excessively high | Increased penetration but more inclusions | Potentially deep but less stable | Hot cracking, inclusions, and reduced corrosion resistance |
A sulfur content of 0.005% is equivalent to 50 ppm.
The main purpose of this lower limit is to prevent the material from entering an ultra-low-sulfur range. Contrary to a common assumption, lower sulfur does not always mean better weldability.
This distinction is important:
From the perspective of material purity and corrosion resistance, lower sulfur is generally considered beneficial.
From the perspective of autogenous orbital welding, excessively low sulfur may make it more difficult to achieve sufficient weld penetration.
Therefore, when a customer specifies a minimum sulfur content of 0.005%, the likely objective is to ensure that tubing, fittings, valves, and other weld-end components can be welded using a stable and standardized orbital welding program.
Without this lower limit, welding current, pulse settings, travel speed, and other parameters may need to be adjusted whenever materials from different heats are introduced.
Two separate issues must be distinguished.
Under ASTM material specifications, the maximum sulfur content for many commonly used grades, including TP304, TP304L, TP316, and TP316L, is typically 0.030%.
Therefore, when a customer specifies:
S = 0.005%–0.030%
the requirement may mean:
Sulfur must not be below 0.005%, in order to avoid orbital welding difficulties caused by ultra-low sulfur.
Sulfur must not exceed the normal maximum limit of 0.030% for 304L or 316L stainless steel.
For highly controlled autogenous orbital welding applications, some engineering guidelines recommend that:
Sulfur content should preferably remain below approximately 0.017%.
The sulfur-content difference between two components being welded should preferably not exceed approximately 0.007%.
When the sulfur-content difference exceeds approximately 0.010%, the weld pool may shift toward the lower-sulfur material, potentially causing localized incomplete penetration.
Therefore, from a welding-process-control perspective, the range of 0.005%–0.030% is relatively broad.
It does not mean that all materials within this range will provide identical orbital welding performance.
For example:
Stainless steel tubing has a sulfur content of 0.006%.
A stainless steel fitting has a sulfur content of 0.028%.
Both components comply individually with the specified range of 0.005%–0.030%. However, their sulfur-content difference is 0.022%.
When these two components are joined by autogenous orbital welding, the sulfur mismatch may cause asymmetric weld-pool movement, localized insufficient penetration, or inconsistent root-bead formation.
This is a critical issue for manufacturers of stainless steel valves, fittings, manifolds, and prefabricated piping assemblies.
Although increasing sulfur content within a controlled range can improve weld penetration, higher sulfur content also has disadvantages.
Sulfur can combine with manganese to form manganese sulfide inclusions. Excessive sulfur may result in:
Increased MnS and other sulfide inclusions.
A higher risk of weld solidification cracking.
Reduced surface cleanliness and smoothness.
More initiation points for localized corrosion.
Greater difficulty in achieving a clean, high-purity internal weld surface.
Research associated with the U.S. Department of Energy has indicated that sulfur may increase weld penetration, but low-melting-point sulfide phases can also increase the risk of grain-boundary solidification cracking.
The sulfur requirement is therefore a balance:
Sulfur must not be too low, otherwise stable full penetration may be difficult to achieve.
Sulfur must not be too high, otherwise inclusions, hot cracking, surface-quality problems, and corrosion risks may increase.
Liquid-cooling pipelines in data centers are different from ordinary building water systems or conventional industrial piping.
Several characteristics make weld quality especially important.
A minor leak in an ordinary industrial pipeline may primarily result in maintenance work or production interruption.
However, a leak near a server rack, cooling distribution unit, manifold, GPU system, or electrical component may directly affect computing equipment and data center operation.
Welded joints must therefore provide:
Full penetration.
No incomplete fusion.
No significant internal oxidation.
High repeatability.
Full traceability.
Liquid-cooling systems have limited installation space and often contain large numbers of customized pipelines, branch connections, compact manifolds, and small welded assemblies.
Automatic orbital welding can provide stable weld geometry and controlled heat input, including in compact or difficult-to-access locations.
A single liquid-cooling project may require hundreds, thousands, or even more identical valves, fittings, spool pieces, and manifold components.
Customers and system integrators do not want to readjust welding parameters every time a new stainless steel heat is introduced.
Controlling both the sulfur-content range and the sulfur-content difference between mating components can improve:
Compatibility with standardized welding programs.
Batch-to-batch production consistency.
First-pass welding acceptance rates.
Welding procedure qualification.
Quality control and material traceability.
Factories should not interpret the requirement simply as:
The sulfur value shown on the material certificate is between 0.005% and 0.030%, so the material is acceptable.
Several additional factors need to be controlled.
Manufacturers should clarify the following requirements with the customer:
What is the maximum permitted sulfur-content difference between the tubing and fittings?
Must all components to be welded come from heats with similar sulfur levels?
Does the sulfur requirement apply only to each individual base material, or must mating components also be sulfur matched?
Will the final components be joined using autogenous orbital welding, robotic TIG welding, or manual welding with filler metal?
When autogenous orbital welding is used, the sulfur-content difference between two mating components may be more important than whether each individual component simply falls within the broad range of 0.005%–0.030%.
For long-term, high-volume production, manufacturers may consider adopting a narrower internal purchasing and production target, such as:
S = 0.008%–0.017%
This is only a practical engineering-control approach. It should not replace the customer’s technical specification.
The final acceptable range should be determined according to the customer’s welding procedure qualification, project specifications, and actual welding trials.
Manufacturers should record and control:
Stainless steel grade.
Heat number.
Actual sulfur content.
Production batch.
The tubing, fitting, valve, manifold, or assembly with which the component will be welded.
Products from different heats should not be mixed together without control simply because they are all marked as 304L or 316L.
Handheld XRF analyzers are useful for identifying 304, 316, and other stainless steel grades by measuring major alloying elements.
However, standard handheld XRF equipment generally cannot reliably measure low concentrations of sulfur and phosphorus in stainless steel.
Technical information from Thermo Fisher and other material-analysis specialists indicates that handheld XRF is not suitable for accurately verifying low sulfur and phosphorus levels in stainless steel.
For sulfur-content verification, more appropriate methods include:
Mill material test reports showing the actual sulfur value.
Spark optical emission spectroscopy, or spark OES.
Laboratory combustion analysis using a carbon-sulfur analyzer.
Manufacturers should therefore:
Require the steel mill’s MTR to state the actual sulfur content.
Use spark OES for verification when necessary.
Use laboratory combustion testing for disputed or critical batches.
Avoid using handheld XRF results alone as proof that the material meets the 0.005% minimum sulfur requirement.
The requirement to control sulfur content between 0.005% and 0.030% in stainless steel valves and fittings for liquid-cooled data centers is mainly related to welding performance rather than cooling efficiency.
Proper sulfur control helps manufacturers and system integrators achieve:
Stable orbital welding penetration.
Consistent weld geometry.
Higher first-pass acceptance rates.
Reduced leakage risk.
Better batch-to-batch repeatability.
Improved material and welding traceability.
However, compliance with the specified range alone may not be sufficient.
For autogenous orbital welding applications, sulfur matching between mating components, heat-number management, reliable sulfur testing, and actual welding qualification are also essential.
SANSUN Fluid Equipment Co., Ltd. has more than 15 years of experience in manufacturing sanitary stainless steel valves, fittings, and fluid-handling equipment.
Our products are widely used in the food, beverage, pharmaceutical, biotechnology, and other hygienic-processing industries.
In recent years, SANSUN has also manufactured large quantities of stainless steel valves and fittings for liquid-cooled data center projects, including products manufactured with specially controlled sulfur content according to customer and project requirements.
For inquiries about stainless steel valves, fittings, manifolds, and customized components for liquid-cooling systems, please contact us:
Website: www.sansunstainless.com
Email: sansun@sansunstainless.com