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Eternal
904L forged tube sheets are high-performance parts that hold pressure. They are made from 904L super austenitic stainless steel (UNS N08904 / EN 1.4539) by forging. They are designed for shell-and-tube heat exchangers, reactors, and pressure vessels in very corrosive environments, such as sulfuric acid, phosphoric acid, seawater, and high chloride conditions. The alloy has high chromium, high nickel, high molybdenum, and added copper. This makes its corrosion resistance much better than normal 316L/317L stainless steel. Jiangyin Eternal Heavy Industry Co., Ltd. improved the forging process. This gives the material a dense inner structure and reliable mechanical properties. It helps ensure the tube sheet runs safely for a long time under high pressure, high temperature, and highly corrosive conditions.
The material composition and grade system: The chemical composition of 904L has been precisely balanced, with the synergistic effect of various elements, enabling the material to possess excellent corrosion resistance in both oxidizing and reducing media:
Element | wt% | Function |
C | ≤ 0.020 | Ultra-low carbon design eliminates the risk of intergranular corrosion after welding and heat treatment. |
Cr | 19.0 – 23.0 | Form a dense passivation film, providing a basic medium for anti-oxidation corrosion resistance |
Ni | 23.0 – 28.0 | Stabilize the austenitic structure and significantly enhance the resistance to chloride stress corrosion cracking |
Mo | 4.0 – 5.0 | Enhance the resistance to pitting and crevice corrosion |
Cu | 1.0 – 2.0 | Key element, specifically designed to provide excellent corrosion resistance against reducing acids such as sulfuric acid and phosphoric acid. |
Mn | ≤ 2.00 | Auxiliary deoxygenation, stabilizing the tissue |
Si | ≤ 1.00 | Auxiliary deoxygenation |
P | ≤ 0.045 | Impurity control |
S | ≤ 0.035 | Impurity control |
N | ≤ 0.10 | Solid solution strengthening enhances the resistance to pitting corrosion. |
The 904L tube sheet has corresponding grades in all major global standard systems. During procurement, the choice can be made based on the project's standard system.
Standard | Grade |
UNS | N08904 |
ASTM/ASME Forgings | ASTM A182 F904L |
ASTM/ASME Plate | ASTM A240 Type 904L |
EN | 1.4539 / X1NiCrMoCu25-20-5 |
GB | 015Cr21Ni26Mo5Cu2（S31782） |
JIS | SUS890L |
W.-Nr. | 1.4539 |
904L forgings mainly follow the ASTM A182/A182M standard. This standard covers forged flanges, fittings, and valve parts for high-temperature pressure service. It is the most commonly used international standard basis for forged tube sheets. The European standard system refers to EN 10088-3. In addition, the 904L tube sheets forged by Jiangyin Eternal Heavy Industry Co., Ltd. also meet several international code requirements, including PED 2014/68/EU (Pressure Equipment Directive) and NACE MR0175 / ISO 15156-3 (materials for sour oil and gas environments).
After solution treatment, the mechanical properties of the 904L forged tube plate meet the following requirements:
Item | Value |
Tensile strength（Rm） | ≥ 490 MPa |
Yield strength（Rp0.2） | ≥ 220 MPa |
Elongation（A） | ≥ 35% |
Hardness | ≤ 90 HRB |
904L is a high-alloy super austenitic stainless steel. During casting, it easily develops internal defects such as dendritic segregation, shrinkage cavities, and gas porosity. These defects become weak points that corrosive media can penetrate first. Forging uses repeated plastic deformation to effectively compact the internal structure, break up coarse grains, and produce a dense, uniform microstructure. This greatly improves mechanical properties and fatigue resistance, making it more suitable for pressure-bearing, load-bearing, and high-reliability applications.
Parameters | Control |
Maximum heating temperature | 1180 ℃ |
Minimum forging cessation temperature | ≥ 900 ℃ |
Thermal forming temperature range | 1000 – 1150 ℃ |
Recommended heat processing range | 1150 – 850 ℃ |
Single deformation amount | Keep it within 15% to avoid cracking. |
Forging method | slow static pressure forging ensure the continuity of grain flow lines. |
904L has poor hot ductility at high temperatures and is easy to crack during hot working. Therefore, the amount of deformation per pass must be strictly controlled, and a multi-pass forging process should be used. Based on many years of forging experience, Jiangyin Eternal Heavy Industry Co., Ltd. has found that dynamic recrystallization of 904L begins at about 1050°C. To avoid overloading the rolling mill, the recommended forging temperature is around 1100°C.
Ingot surface inspection → Heating (≤1180 °C) → Multi-directional forging (final forging temperature ≥900 °C) → Rapid cooling after forging to below 538 °C → Solution treatment → Performance testing → Precision machining of tube holes → Ultrasonic testing → Final product inspection
The standard heat treatment for 904L forged tube sheets is solution treatment:
Heating temperature: 1090–1175 °C (recommended 1100–1150 °C)
Holding time: determined by effective thickness, usually 1–2 hours
Cooling method: rapid water cooling or rapid air cooling
The purpose of solution treatment is to fully dissolve carbides and intermetallic phases that may precipitate during forging and high-temperature processing, and to obtain a uniform, single-phase austenitic structure, thereby maximizing the material's intergranular corrosion resistance. 904L cannot be hardened by heat treatment.
The core value of 904L lies in its comprehensive and outstanding corrosion resistance, making it particularly suitable for a variety of harsh media environments.
Resistance to Pitting and Crevice Corrosion
904L has a pitting resistance equivalent number (PREN = %Cr + 3.3×%Mo + 16×%N) of no less than 34, far higher than that of 316L (PREN ≈ 25). It offers excellent resistance to pitting and crevice corrosion in chloride-containing environments. In a chloride environment at 50°C, 904L had only 2 crevice corrosion initiation sites, compared with 8 for 304L and 9 for 316L—a significant difference.
Resistance to Reducing Acid Corrosion
The addition of copper gives 904L outstanding performance in reducing acids:
Sulfuric acid: in pure sulfuric acid at concentrations of 0–98%, the service temperature can reach 40°C.
Phosphoric acid: excellent corrosion resistance in pure phosphoric acid at concentrations of 0–85%, superior to ordinary stainless steels.
Organic acids: it also has good corrosion resistance in organic acids such as acetic acid and formic acid.
The high nickel content (25%) of 904L significantly reduces its susceptibility to stress corrosion cracking in chloride solutions. In most common chloride environments, 904L can effectively resist SCC, whereas 316L is more susceptible in such environments.
Item | 316L | 904L |
PREN Value | ≈ 25 | ≥ 34 |
Sulfuric acid (medium concentration) | Limited corrosion resistance | excellent |
phosphoric acid | common | excellent |
Chloride pitting/corrosion of crevices | medium | excellent |
stress corrosion cracking | sensitive | High resistance |
Applicable temperature upper limit (chloride environment) | ~60℃ | Higher temperatures can still be manageable. |
904L forged tube sheets are widely used in critical industrial fields that demand extremely high material reliability:
Chemical and Petrochemical
Tube sheets for reactors and heat exchangers in sulfuric acid and phosphoric acid production
Chloride-containing reaction vessels and organic acid handling systems
Tube sheets for heat exchangers in petrochemical plants
Environmental Engineering — Flue Gas Desulfurization (FGD)
Tube sheets for FGD absorber towers and flue sections in power plants
Spray systems, mist eliminators, and related internals, resisting acidic flue gas and high chloride corrosion
Marine and Seawater Engineering
Tube sheets for heat exchangers in seawater desalination units
Tube sheets for seawater cooling systems and seawater pipelines
Pressure-bearing equipment related to offshore platforms
Pulp and Paper, and Pharmaceutical
Tube sheets for pulp bleaching equipment
Tube sheets for pharmaceutical reactors, centrifuges, and other equipment
Oil and Gas
Tube sheets for sour oil and gas environments containing H₂S, CO₂, and chlorides
Tube sheet components related to downhole tools and wellhead equipment
Typical Application Case
The shell-and-tube heat exchanger at the a Plant of Shandong Aluminum used a 904L tube sheet (50 mm thick), with 32% caustic soda as the tube-side medium at 90 °C. This fully verified the reliability of 904L in a high-temperature, strongly alkaline environment.
The following quality inspections are required for 904L forged tube sheets:
Chemical composition analysis: Verify alloying element content by spectral analysis.
Mechanical property tests: Tensile and hardness tests.
Intergranular corrosion test: Conducted in accordance with ASTM A262 Practice E.
Ultrasonic testing (UT): Perform once after heat treatment and once after finish machining to ensure that internal defect equivalent values meet standard requirements.
Precision machining of tube holes: Positioning accuracy can reach 0.01 mm.
How should one choose between 904L tube sheets and duplex stainless steel 2205 tube sheets?
Duplex stainless steel 2205 has higher strength than 904L and lower cost, and performs well in most chloride-containing environments. However, 904L is significantly superior to 2205 in corrosion resistance in reducing acids such as sulfuric acid and phosphoric acid, and it is more welding- and fabrication-friendly. If the medium is mainly reducing acids or involves a complex mixed-acid environment, 904L should be preferred; if it is a general neutral chloride-containing medium, 2205 can be an economical choice.
How are 904L tube sheets accepted? What quality documents are required?
During acceptance, the supplier should be required to provide: chemical composition analysis report, mechanical property test report (tensile, hardness), intergranular corrosion test report (ASTM A262 Practice E), ultrasonic testing report (UT), and heat treatment records. If necessary, third-party witness inspection can be added.
Can 904L tube sheets be used to replace 316L tube sheets?
Yes. The corrosion resistance of 904L is comprehensively superior to that of 316L, especially in reducing acid environments such as sulfuric acid and phosphoric acid, as well as in high-chloride environments, where the difference between the two is even more pronounced. However, the raw material cost of 904L is much higher than that of 316L, so it is usually used only in critical locations where 316L cannot perform adequately. For general service conditions, duplex stainless steel 2205 can be considered first as an economical alternative.
What is the applicable temperature range for 904L forged tube sheets?
The allowable design stress values for 904L in the ASME Boiler and Pressure Vessel Code cover up to 371°C. However, it should be noted that above about 550°C, the material becomes microstructurally unstable due to the precipitation of brittle phases such as sigma phase; therefore, long-term use above 550°C is not recommended. At low temperatures, 904L has excellent toughness and can be used in low-temperature services such as liquefied gases.
