Requirements for Marine Thick-Wall Seamless Stainless Steel Pipes

Marine piping systems are often referred to as the "vascular system" of a vessel, responsible for transporting fuel oil, lubricating oil, steam, seawater, and freshwater. Within this complex network, thick-wall seamless stainless steel pipes play a critical role in high-pressure pipelines, boiler systems, and heat exchangers due to their superior corrosion resistance, high-temperature strength, and pressure-bearing capacity.

What are the materials used for marine stainless steel pipes?

304L Stainless Steel

With strong versatility and an austenitic microstructure, it possesses excellent comprehensive corrosion resistance and formability. It is suitable for ordinary water systems and air pipelines in ships, as well as other scenarios without strong corrosive media. It is the basic choice for marine stainless steel pipes. Its carbon content is ≤0.03%, which can effectively avoid intergranular corrosion and is suitable for medium and low pressure piping systems.

316L Stainless Steel

The most widely used marine stainless steel grade, 304L, adds molybdenum (Mo), significantly improving its resistance to chloride corrosion. It is particularly suitable for applications involving direct contact with high-salt, corrosive media, such as seawater cooling systems and fuel lines. It exhibits excellent resistance to salt spray and seawater immersion。

2205 Duplex Stainless Steel

Composed of 22% chromium, 2.5% molybdenum and 4.5% nickel-nitrogen alloy, it belongs to the category of duplex stainless steel. Its yield strength is more than twice that of 304L and 316L. It also has high strength, good impact toughness and excellent resistance to stress corrosion. It is suitable for high-pressure piping systems of ships and key pipelines of deep-sea vessels. It can withstand higher pressure and more complex corrosive environments, making it the preferred material for high-end ships and special-purpose ships.

International Standards and Certifications

Marine stainless steel pipes must comply with global standards and classification society requirements.

Common Standards:

Classification Society Certifications:

CCS (China Classification Society)

DNV (Det Norske Veritas)

LR (Lloyd's Register)

ABS (American Bureau of Shipping)

BV (Bureau Veritas)

Suppliers must obtain factory approval certificates and pass product inspections.

Wall Thickness Classification and Application

Marine pipes are generally classified into primary (Class I) and secondary (Class II) pipes based on pressure and service conditions.

According to GB/T 31929-2015, the piping system classification for marine thick-walled stainless steel pipes is as follows:

Toxic and corrosive media, flammable media heated above their flash point, media with flash points below 60°C, and liquefied gases are generally classified as Class I piping systems. If safety protection measures are in place to prevent leaks and their consequences, they can also be classified as Class II piping systems, except for toxic media. Cargo oil piping systems are generally Class III. Unpressurized open piping systems such as drain pipes, overflow pipes, vent pipes, breather pipes, and boiler vent pipes are also Class III piping systems.

Performance Advantages of Marine Seamless Stainless Steel Pipes

Selection Scenarios for Thick-Walled Tubes and Thin-Walled Tubes

Thick-walled tubes (wall thickness ≥ 10mm) are suitable for: main steam pipelines, high-pressure fuel injection pipelines, deep-sea submersible pressure hull penetrations, boiler header tube supports, and other high-pressure, high-stress areas.

Thin-walled tubes are suitable for: heat exchanger tubes, instrument pressure taps, sanitary water systems, and other scenarios where heat exchange efficiency or lightweight design is prioritized.

Marine Seamless Stainless Steel Tube Forming Process

The process employs an integrated "hot piercing + cold rolling/cold drawing" forming process. Welding followed by grinding to imitate seamless tubes is strictly prohibited (this is an industry red line):

- Hot Piercing: The stainless steel billet is heated to 1050℃~1150℃ and pierced through a piercing machine to form a hollow tube, ensuring that the tube is free of shrinkage cavities and cracks, and has uniform wall thickness.

- Cold Rolling/Cold Drawing: The tube undergoes multiple cold rolling or cold drawing passes to precisely control the wall thickness and outer diameter dimensions, while simultaneously improving the mechanical properties of the tube. After cold rolling, solution treatment (water cooling at 1000-1100℃) is required to eliminate processing stress, restore the overall performance of the tube, and prevent embrittlement caused by work hardening.

Surface Treatment Options

Acid Pickling and Passivation: The pipe surface undergoes professional acid pickling and passivation treatment to remove surface oxide scale, oil, and impurities, forming a dense passivation film (mainly composed of Al₂O₃ and Cr₂O₃), improving corrosion resistance. The passivation film thickness is ≥8μm, and the surface is free of oxide spots, color differences, and scratches.

Bright Annealing: Heat treatment is performed in a protective atmosphere, resulting in a silvery-white metallic luster. No secondary acid pickling is required.

Mechanical Polishing: Surface roughness can reach Ra≤0.8μm or even higher precision, suitable for media systems with high cleanliness requirements.

Comparison Table of Specifications for Grade I and Grade II Steel Pipes for Ships

This comparison table strictly follows GB/T 5312-2025 "Seamless Steel Pipes for Ships" and the China Classification Society (CCS) standard. Grade I pipes are high-precision wall thickness pipes (suitable for Grade I piping systems, high-pressure/critical pipelines), while Grade II pipes are ordinary precision wall thickness pipes (suitable for Grade II and III piping systems, general low-pressure pipelines). Both have the same outer diameter and length specifications, differing only in wall thickness tolerance and curvature.