Views: 18 Author: Wang Publish Time: 2024-09-24 Origin: Site
Alloy 904L(UNS N08904) is a superaustenitic stainless steel designed for corrosion and pitting resistance in a wide range of process environments. It is used extensively in the chemical process industry including the production of phosphate-based fertilizers. UNS N08904, commonly known as 904L, is a low carbon high alloy austenitic stainless steel which is widely used in applications where the corrosion properties of AISI 316L and AISI 317L are not adequate.
904L may considered as a high performance austenitic stainless steel when compared with the general purpose austenitics such as 304 and 316. It has the characteristic high ductility and formability and good weldability of the austenitic grades, but its use is primarily a consequence of its corrosion resistance rather than its mechanical properties. Applications for the alloy range from the chemical and mineral processing industries to pollution control and water treatment. For many years it has been a useful upgrade to 316L but in more recent times the cost effectiveness of duplex grades has limited its use.
The addition of copper to stainless steel 904L gives it corrosion resistant properties superior to the conventional chrome nickel stainless steels, in particular to sulphuric, phosphoric and acetic acids. However, there is limited use with hydrochloric acids. It also has a high resistance to pitting in chloride solutions, a high resistance to both crevice and stress corrosion cracking. Alloy 904L performs better than other austenitic stainless steels due to the higher alloying of nickel and molybdenum. The grade is non-magnetic in all conditions and has excellent formability and weldability. The austenitic structure also gives this grade excellent toughness, even down to cryogenic temperatures.
A primary feature of 904L is its very good corrosion resistance in dilute sulphuric acid across the full range of concentrations up to 35° C. It also has useful resistance to a number of other inorganic acids, but its performance may be limited when halide ions are present. The high chromium content promotes and maintains a passive film which protects the material in many corrosive environments.904L has a greater resistance to precipitation of ferrite and sigma phases on cooling and welding than other stainless steels containing molybdenum such as 316L and 317L. There is no risk of intercrystalline corrosion on cooling or welding due to the low carbon content. Its maximum service temperature is at 450°C.
904L stainless steel is one of the best stainless steel grade for corrosion resistance. This grade is written as 1.4539 or X1NiCrMoCu25-20-5 according to EN norm and it is written as UNS S08904 according to UNS norm. 904L stainless grade contains very high level of nickel, molibdenium, chromium and combinations of all these elements makes this stainless steel grade very resistant to corrosion resistance. 904L material is one of the strongest material to corrosion at Bircelik warehouse and if this grade is not enough, then nickel alloys or titanium alloys has to be chosen, because rest of the stainless steel grades will not survive if 904L is not enough. 904L has also a very good anti-magnetic property and for a lot of applications, this is another reason to use 904L stainless steel grade. 1.4539 stainless steel has a very low content of carbon and low carbon also improving this grade's corrosion resistance level.
904L (1.4539) Chemical Composition | |||||||||
Grade | C | Mn | Si | P | S | Cr | Ni | Mo | Cu |
904L (1.4539) | %0.02 max | %2.0 max | %1.0 max | %0.045 max | %~0.01 min | %19-21 | %24-26 | %4-5 | %1.2-2.0 |
Since the carbon content of 904L is very low (0.020% maximum), there is no carbide precipitation under normal heat treatment and welding conditions. This eliminates the risk of intergranular corrosion that occurs after general heat treatment and welding. Due to its high chromium-nickel-molybdenum content and the addition of copper, 904L can be passivated even in reducing environments such as sulfuric acid and formic acid. The high nickel content gives it a lower corrosion rate even in the active state. 904L can be used at temperatures up to 40 degrees Celsius in pure sulfuric acid at concentrations ranging from 0 to 98%. The corrosion resistance is excellent in pure phosphoric acid in the concentration range of 0 to 85%. In the industrial phosphoric acid produced by the wet process, impurities have a strong influence on corrosion resistance.
904L has better corrosion resistance than ordinary stainless steel of all phosphoric acid types. In strongly oxidizing nitric acid, 904L has lower corrosion resistance compared to highly alloyed steels that do not contain molybdenum. In hydrochloric acid, the use of 904L is limited to lower concentrations of 1-2%. In this concentration range, 904L has better corrosion resistance than conventional stainless steel, and 904L steel has a high resistance to point corrosion. The high nickel content of 904L reduces the rate of corrosion in pits and crevices. Ordinary austenitic stainless steels may be sensitive to stress corrosion in a chloride rich environment at temperatures above 60 degrees Celsius, and this sensitivity can be reduced by increasing the nickel content of the stainless steel. Due to its high nickel content, 904L has a high resistance to stress corrosion rupture in chloride solutions, concentrated hydroxide solutions and hydrogen sulfide rich environments.
904L stainless steel can be welded using common fusion and resistance methods. A more highly alloyed filler is recommended to give the optimum corrosion properties in the weld. Alloy 904L can be readily welded by most standard processes. Austenitic plate materials have a homogeneous austenitic structure with an even distribution of alloying elements. Solidification after welding causes the redistribution of certain elements such as molybdenum, chromium and nickel. These segregations remain in the cast structure of the weld and can impair the corrosion resistance in certain environments. Segregation is less evident in 904L, and this material is normally welded using a filler metal of the same composition as the base metal and can even be welded without filler metal.
Like ordinary stainless steel, 904L can be welded by a variety of welding methods. The most common welding method is manual arc welding or inert gas welding. The welding rod or welding wire is up to the composition of the base metal and is of higher purity, but its molybdenum content is required to be higher than that of the base metal. Preheating is generally not required before welding, but in cold outdoor operations joints or adjacent areas can be uniformly heated in order to avoid condensation of vapor. Note that the local temperature should not exceed 100 °C to avoid carbon accumulation and intergranular corrosion. When doing welding, it is suggested to adopt a small line energy, continuous and fast welding rate.
Working temperatures of 1562 – 2102°F (850 –1150°C) are recommended for hot working processes. Normally hot working should be followed by a solution anneal and quench, but for 904L, if hot forming is discontinued at a temperature above 2012°F (1100°C) and the material is quenched directly thereafter, the material may be used without subsequent heat treatment. It is important that the entire workpiece be quenched from temperatures above 2012°F (1100°C). In the event of partial heating or cooling below 2012°F (1100°C), or if the cooling has been too slow, hot working should always be followed by a solution anneal and quenching. 904L should be solution annealed at 1940 – 2084°F (1060 –1140°C).
Stainless steel 904L has a wide range of applications in various industries, including:
Oil and gas industry: pipes, tubes, and components for offshore platforms, pipelines, and refineries.
Chemical processing: tanks, vessels, and valves for handling corrosive chemicals.
Pulp and paper: digester tanks and bleach washers.
Food processing: equipment for handling acidic foods and beverages.
Pharmaceuticals: process equipment for manufacturing drugs and medicines.
316L Stainless Steel: 316L is a low-carbon, molybdenum-bearing austenitic stainless steel; the "L" denotes its low carbon content (typically ≤ 0.030%). 316L forms the standard grade for improved corrosion resistance in a wide range of industrial and marine environments.
904L Stainless Steel: 904L is a high-alloyed, low-carbon, copper-bearing austenitic stainless steel. It offers superior corrosion resistance compared to standard molybdenum-bearing grades like 316L, particularly in aggressive environments containing sulfuric acid and chlorides. The "L" shows low carbon content (typically ≤ 0.020%), although this is inherent to the UNS specification.
| Element | 316L (%) | 904L (%) | Impact of Difference |
|---|---|---|---|
| Chromium (Cr) | 16.0 - 18.0 | 19.0 - 23.0 | ↑904L: Basic corrosion resistance, passivity |
| Nickel (Ni) | 10.0 - 14.0 | 23.0 - 28.0 | ↑904L: Austenite stability, acid resistance (esp. reducing) |
| Molybdenum (Mo) | 2.00 - 3.00 | 4.0 - 5.0 | ↑↑904L: Pitting/crevice corrosion resistance |
| Carbon (C) | ≤ 0.030 | ≤ 0.020 | ↓904L: Minimizes sensitization risk (carbide precip) |
| Manganese (Mn) | ≤ 2.00 | ≤ 2.00 | - |
| Silicon (Si) | ≤ 0.75 | ≤ 1.00 | - |
| Phosphorus (P) | ≤ 0.045 | ≤ 0.045 | - |
| Sulfur (S) | ≤ 0.030 | ≤ 0.035 | - |
| Copper (Cu) | - | 1.00 - 2.00 | ↑↑904L: Critical for sulfuric acid resistance |
| Iron (Fe) | Balance | Balance | - |
316L:
Advantages: Excellent general corrosion resistance better than 304L; Good formability and weldability; Widely available in various product forms such as sheet, plate, tube, bar, fittings; Good mechanical properties; Relatively lower cost compared to highly alloyed grades like 904L; Good resistance to food processing environments and mild chemicals.
Disadvantages: Easy to pitting and crevice corrosion in warm chloride environments; Reduced corrosion resistance in strong acids; Cannot be strengthened by heat treatment.
904L:
Advantages: Outstanding resistance to pitting and crevice corrosion, especially in chlorides—often suitable for seawater immersion; Excellent resistance to general corrosion in a broad range of acids, particularly sulfuric acid at both low and high concentrations due to copper content; Superior resistance to stress-corrosion cracking compared to standard austenitics; Excellent resistance to reducing acids; Generally higher strength than 316L.
Disadvantages: Significantly higher alloy cost; Wider availability but potentially longer lead times for niche forms/sizes; More difficult to machine due to work hardening tendency and higher strength; Requires stricter control during welding; Formability is good but lower than 316L due to higher strength.
316L:
Chemical processing equipment handling milder corrosives.
Pharmaceutical and food processing equipment.
Pulp and paper processing.
Marine fittings and components above waterline.
Architectural applications near coasts.
Medical implants and surgical tools.
Heat exchangers and condensers in less aggressive waters.
Chemical tankers for certain cargos.
904L:
Seawater handling systems for piping, pumps, valves.
Components for salt and sour gas environments in oil/gas.
Phosphoric acid production and handling equipment.
Sulfuric acid coolers, pickling equipment.
Components in flue gas desulfurization (FGD) plants.
Pulp digesters and bleach plant equipment exposed to corrosive liquors.
Very demanding chemical processing involving mixtures of strong acids.
Heat exchangers using seawater or very brackish cooling water.
Structural components in aggressive marine environments.
| Property | 316L | 904L | Notes |
|---|---|---|---|
| Density (g/cm³) | ~7.99 | ~8.05 | 904L is slightly denser due to higher alloying. |
| Melting Range (°C) | 1375 - 1400 | 1300 - 1390 | - |
| Specific Heat (J/kg·K) | ~500 (20°C) | ~500 (20°C) | Similar |
| Thermal Conductivity (W/m·K) | ~15 (20°C) | ~12 (20°C) | 904L has lower thermal conductivity. |
| Electrical Resistivity (μΩ·cm) | ~74 (20°C) | ~95 (20°C) | 904L has higher electrical resistance. |
| Magnetic Permeability | Essentially Non-Magnetic (µ_r ~1.02 max) | Essentially Non-Magnetic (µ_r ~1.02 max) | Both reliably non-magnetic in annealed state. |
| Coeff of Thermal Expansion (μm/m·°C) | ~16.0 (0-100°C) | ~14.5 (0-100°C) | Slightly lower expansion for 904L. |
General Corrosion: 904L offers a much broader corrosive media compared to 316L, especially mineral acids, organic acids, and mixtures. Its copper content is pivotal for sulfuric acid resistance.
Pitting and Crevice Corrosion: This is the most significant differentiator. 904L's much higher molybdenum content gives it vastly superior resistance. Generally, a PREN ≥ 35 is desirable for seawater resistance, placing 904L in this category, while 316L often performs poorly below ~35°C and is unsuitable for warm seawater.
Stress Corrosion Cracking: Both are resistant for most atmospheric and freshwater applications. 904L offers greater resistance to chloride-induced SCC than 316L, making it suitable for hotter, more concentrated chloride environments.
Galvanic Corrosion: Both are cathodic to carbon steel but similar enough to each other to be generally compatible where electrical connection is necessary.
Typical Properties in Annealed Condition:
| Property | 316L | 904L | Significance |
|---|---|---|---|
| Tensile Strength (MPa) | 515 min / 500-700 typical | 490 min / 600-750 typical | ↑904L (Typical): Higher as-annealed strength. |
| Yield Strength (0.2% Offset MPa) | 205 min / 200-350 typical | 220 min / 300-420 typical | ↑↑904L (Typical): Significantly higher proof stress. |
| Elongation (% in 50mm) | 40 min / 40-50 typical | 35 min / 35-45 typical | ↓904L: Slightly reduced ductility (still good) due to higher strength. |
| Hardness (Brinell HBW) | Max 217 / 120-180 typical | Max 230 / 140-200 typical | ↑904L: Harder in annealed condition. |
| Modulus of Elasticity (GPa) | ≈200 | ≈200 | Similar stiffness. |
904L has higher yield and tensile strength than 316L in the standard annealed condition.
Cost: 904L is more expensive than 316L, typically between 2 and 4 times the cost depending on market prices of raw materials, product form, and size.
Availability: 316L is a standard industrial alloy available worldwide in an extensive range of product forms, shapes, and sizes from numerous stockists and mills with short lead times. 904L is broader than exotics like 6% Mo super austenitics or nickel alloys, but specific forms, sizes, or thicknesses may have longer lead times or require specialized suppliers. Thin sheets/plates and tubing are generally more readily stocked.
In architecture, food processing, and fresh water, 316L is a highly durable and cost-effective material, offering excellent long-term life.
In hot chlorides/seawater, concentrated acids, and sour oil/gas, 316L can suffer rapid localized corrosion.
904L, with its superior corrosion resistance, will maintain integrity much longer under these harsh conditions. Its lifespan will often significantly exceed that of 316L.
However, 904L is not invincible and can fail if misapplied in strongly oxidizing conditions or very high chloride combined with high temperature/pH outside design limits.
