Stainless steel tube sheet: polishing and processing 

Stainless Steel Tubesheet: Polishing and Finishing is the Key to Heat Exchanger Durability

The quality of surface treatment of the stainless steel tube sheet directly determines the service life of all heat exchange equipment. In our practice, we have repeatedly encountered situations where an expensive device failed after 18 months of operation, not due to metal corrosion, but due to microcracks in the welding zone caused by improper grinding or residual stresses after machining. A stainless steel tube sheet, polished and processed in violation of technological regulations, becomes a source of accumulation of deposits and pitting corrosion. This article takes a closer look at the manufacturing steps, critical roughness parameters, and quality control methods to avoid such problems.

We have been working with manufacturers of heat transfer equipment for more than 15 years and know that savings on tube sheet finishing result in multiple repair costs and production line downtime. Below we'll look at the specific standards, tools, and step-by-step procedures that leading factories use to ensure flaring assembly tightness.

Why surface condition is more important than steel grade

Many buyers focus exclusively on the grade of material (AISI 304, 316Ti, 321), believing that this guarantees reliability. However, in real conditions of aggressive environments, it is the surface topography that plays a decisive role. Surface roughness Ra (arithmetic mean deviation of the profile) is a critical parameter. For food production and pharmaceuticals, Ra ≤ 0.4 µm is required, while for the chemical industry Ra ≤ 0.8–1.6 µm is acceptable, but only if there are no directional marks.

Directional marks left by rough grinding act like capillaries, drawing an aggressive environment into the metal. Under the influence of temperature and pressure, the liquid penetrates into microcavities, causing subsurface corrosion that cannot be detected by visual inspection. One of our clients lost a batch of products due to contamination caused by the leaching of corrosion products from precisely such microcracks on the tube sheet. Therefore, polishing should be not just an aesthetic procedure, but a technological step that eliminates defects in casting or mechanical cutting.

It is important to understand the difference between mechanical polishing and electrochemical polishing. Mechanical processing removes irregularities with an abrasive, but can lubricate the surface layer, closing the pores. Electrochemical polishing, on the contrary, dissolves protruding metal particles, leveling the surface at the atomic level and enriching it with a chromium oxide film, which is responsible for the passivity of stainless steel. The choice of method depends on the operating environment: for chlorine-containing environments, the electrochemical method is often preferable.

Processing technology: from rough cutting to mirror polishing

The process of making a tube sheet begins not with drilling holes, but with preparing the surface. Rolled sheets or forgings often have scale after heat treatment or unevenness after gas cutting. Primary processing includes milling the planes to ensure parallelism of the ends. The flatness tolerance is usually no more than 0.1 mm over the entire length of the part. If this step is ignored, subsequent pipe flaring will be uneven: in some places the connection will be too weak, in others there will be excessive stresses leading to cracking.

Drilling the holes for the pipes is the next critical step. Here it is important not only to maintain the diameter (usually with a tolerance of H7 or H8), but also to the quality of the edge. Burrs at the entrances and exits of holes must be removed immediately. We recommend using a 45° or 60° countersink to facilitate pipe insertion and improve conditions for welding or flaring. The burrs left behind can damage the O-rings (if used) or create a gap where crevice corrosion will begin.

The main stage is grinding and polishing. It is divided into several stages depending on the required surface cleanliness class:

• Rough grinding (Grinding):Removing traces of welding studs, supports or technological bosses. The abrasive used is P40–P80 grit. The goal is to level the terrain, not to achieve smoothness.
• Fine Grinding:Removing scratches from the previous stage. Grit size P120–P240. At this stage, a uniform matte surface is formed.
• Pre-polishing:Grit size P320–P600. The surface becomes smooth to the touch, but still has visible structure under certain light angles.
• Final polishing (Mirror Polishing):Using felt wheels with GOI pastes or diamond suspensions. Achieving mirror gloss (Ra < 0.2 µm). This step is required for devices that work with viscous media or require frequent sanitary cleaning (CIP/SIP).

Particular attention should be paid to the areas around the holes. Automatic polishing machines often miss these areas due to the complex geometry. In our practice, we require manual finishing of these areas using special cylindrical heads or flexible shafts. Ignoring the perimeter of the holes is the most common mistake that leads to premature failure of the unit.

Quality control and parameter measurement

How can I make sure the processing is done correctly? Visual inspection under a fluorescent lamp is the first barrier. The surface should not have stains, rainbow stains (a sign of overheating during polishing) or deep scratches. However, for an objective assessment, a profilometer is required. The Ra parameter is measured at several points, especially in the annulus, where access is difficult.

In addition to roughness, control of the thickness of the removed layer is critical. Excessive metal removal can compromise the design strength of the partition, especially if it is operating under high pressure. According to GOST and international standards, thickness deviation should not exceed 5% of the nominal value. We record cases where unqualified contractors “refaced” a part, removing an extra 2-3 mm, which led to deformation of the grille when tightening the body bolts.

Another important test is the Free Iron Test. After machining, microparticles of carbon steel from a tool or abrasive may remain on the surface of stainless steel. These particles rust first, starting the process of corrosion of the base metal. The test is carried out using a ferroxyl solution or special indicator gels. If the test is positive (blue spots appear), re-etching and passivation are necessary.

Problem areas: welding, flaring and thermal influences

The tubesheet rarely operates in isolation. It is part of an assembly unit where complex physical and chemical processes occur. The most vulnerable areas are the contact areas between the pipe and the grate. There are two main joining methods: flaring and welding (or a combination of both). Each method imposes its own requirements for surface preparation.

During expansion, the pipe is pressed into the grid opening, creating tight contact due to plastic deformation. To do this, the surface of the hole must be clean and have a certain roughness (usually Ra 3.2–6.3 µm inside the hole) for better adhesion. A hole that is too smooth can cause the pipe to rotate due to vibration or thermal expansion. However, the outer plane of the grille around the opening must be perfectly polished to prevent dirt accumulation and to ensure a seal-tight seal.

When welding pipes (welding), the requirements for cleanliness increase many times over. Any contaminants, oils or oxides on the edges will cause pores in the weld. Moreover, the heat-affected zone (HAZ) around the weld changes the structure of the metal. If polishing was performed before welding, then after it it is necessary to process the seams and the adjacent area again, removing tarnished colors (blue, yellow, purple shades), which indicate depletion of chromium in the surface layer. We strongly recommend the use of orbital welding for tube sheets as it minimizes human error and produces consistent weld quality that requires minimal post-processing.

Thermal stress is the hidden enemy. Rapid heating or cooling during polishing (if intense friction without cooling is used) can cause warping of fine grids. In one of the projects, we encountered a situation where a 40 mm thick grating, after a series of polishing operations, received a “saddle-shaped” deformation of 1.5 mm. This made it impossible to install the tube bundle into the heat exchanger housing. The fix required expensive machining on a large milling center. To avoid this, the process must include intermediate geometry measurements and the use of coolants.

Comparison of surface treatment methods

The choice of processing method depends on the budget, circulation and customer requirements. Below is a comparative table of the main technologies used in modern production.

The table shows that for critical heat exchanger units operating in aggressive environments, electrochemical polishing is the most preferable option, despite the complexity of organizing the process. It guarantees no contamination and maximum corrosion resistance. Mechanical polishing remains indispensable for local repairs or when perfect specularity is required for aesthetic reasons.

Standards and certification: what to refer to in the specification

When ordering tube sheets or services for their processing, it is necessary to clearly formulate the requirements in the technical specifications. Reference to general phrases such as “high-quality polishing” is unacceptable and leads to disputes during acceptance. Use specific international and national standards.

In Russia and the CIS countries the main document isGOST 9.301-86"Metallic and non-metallic inorganic coatings. General requirements for selection", which regulates roughness classes. Also importantGOST 14192-96for marking and packaging to prevent damage to the finished surface during transportation. For the food industry, compliance is mandatoryTR TS 005/2011“On the safety of packaging”, which describes the requirements for materials in contact with products.

In the international market the de facto standard isASME BPE (Bioprocessing Equipment), which details surface finishing requirements for the biotechnology and pharmaceutical industries. The standard divides surfaces into classes SF1–SF4, where SF4 corresponds to electropolishing with Ra ≤ 0.4 µm. The standard is also widely usedDIN EN ISO 1302to indicate roughness in drawings.

Certification of the process itself often requires the manufacturer to have a quality management systemISO 9001. To work with hazardous production facilities in the Russian Federation, a Rostechnadzor license for the manufacture of pressure equipment may be required. Availability of certificateE.A.C.confirms compliance with the technical regulations of the Customs Union. When importing components from China or Europe, make sure that the Mill Test Certificate includes data not only on chemical composition, but also on mechanical properties and surface quality.

Typical mistakes when ordering and receiving

Analyzing hundreds of projects, we have identified a number of typical mistakes that engineers and buyers make:

1. Lack of protection during transportation.Polished grating is often loaded onto pallets without individual packaging. The result is a network of small scratches across the entire surface. Require shrink wrapping using spacers and padding.
2. Incorrect choice of abrasive.Trying to switch from P120 grain directly to P600 misses deep risks that become visible only after installation. Observe the gradation of sanding steps.
3. Ignoring fiber direction.For sheet metal, it is important to follow the grinding direction along the rolled product to avoid visual stains after installation. For forgings, direction is less critical, but must be uniform.
4. Passivation savings.Many people consider polishing to be the final stage. This is a mistake. After any mechanical treatment, stainless steel must undergo passivation (usually with nitric or citric acid) to restore the protective layer. Without this, even a mirror surface will quickly become covered with rust.

Practical recommendations for extending service life

To ensure that your tube sheet lasts for decades, it is not enough to simply buy a quality part. The equipment must be used correctly. Regular cleaning and inspection is the key to success. Use detergents approved by the equipment manufacturer and avoid chlorine-containing products if temperatures exceed 60°C, as this is a recipe for pitting corrosion on even the best stainless steel.

When repairing a heat exchanger, never use steel brushes to clean the tube sheet. Steel particles will penetrate the surface and become centers of corrosion. Use only nylon brushes or special sponges for stainless steel. If damage to the sealing surfaces is detected, do not attempt to sand them by eye. Restoring flatness requires professional milling followed by polishing of the entire plane to maintain parallelism.

In our company, we have developed internal control regulations that are stricter than GOST requirements. We carry out ultrasonic inspection of the flaring zone of every fifth pipe in a batch to ensure the absence of microcracks. This approach allows us to provide an extended warranty on components. We recommend that you demand similar process transparency from suppliers.

Conclusion and next steps

The stainless steel tube sheet is the heart of the heat exchanger. Its polishing and processing is not a cosmetic procedure, but a complex engineering process that determines the reliability of the entire device. The correct choice of processing method, strict control of roughness and adherence to welding and flaring technologies allow you to avoid emergency stops and expensive repairs. Don't let savings in the finishing stage compromise your production efficiency.

Putting these high standards into practice requires a reliable partner with deep industry experience.Wuxi Kaisheng LLCspecializes in the development and production of advanced heat transfer, power and petrochemical equipment. Наш портфель включает не только готовые решения, такие как титановые кожухотрубные теплообменники, ASME высоконапорные аппараты и гофрированные трубные пучки из различных сплавов (нержавеющая сталь 316, морская латунь C46400, медно-никелевые сплавы, никель N06625), но и высокоточные комплектующие, включая трубные решетки из нержавеющей стали 321 и других коррозионностойких материалов.

Мы понимаем, что качество поверхности решает все. Поэтому наше производство, сертифицированное по стандартамPED и ASME, использует широкий спектр материалов — от углеродистой и легированной стали до титана и специальных сплавов — обеспечивая изделиям исключительную стойкость к высоким давлениям, температурам и агрессивным средам. Наша продукция успешно эксплуатируется в нефтепереработке, химической промышленности, опреснении морской воды и судостроении по всему миру. Мы предоставляем индивидуальные инженерные решения, гарантируя, что каждый компонент, от воздушного охладителя до котла-утилизатора, соответствует самым жестким требованиям надежности.

Если вы планируете модернизацию теплообменного оборудования или заказ новых трубных решеток с высокими требованиями к качеству поверхности, обратитесь к специалистамWuxi Kaisheng LLC. Мы проведем аудит вашей текущей документации, предложим оптимальный технологический маршрут с учетом специфики вашей среды и обеспечим полный цикл контроля качества согласно международным стандартам.

Contact us todayдля получения консультации инженера-технолога и расчета стоимости обработки вашей детали. Мы также рекомендуем ознакомиться с нашим руководством повыбору марок нержавеющей стали для агрессивных сред, чтобы сделать комплексное решение для вашего проекта.