Creo que ya lo he puesto otras veces, pero el 904L no aporta ninguna ventaja frente al 316L para la aplicación de la que estamos hablando. Es más, lo único que aporta es una mayor dificultad de mecanización y una mayor propensión a provocar alergias cutáneas, lo que no hace ese acero mejor, sino indudablemente peor que el acero 316L para la fabricación de cajas y brazaletes de relojes.
Ni siquiera aporta más dureza de la base (en 70 y 90 HRB frente a los 95 HRB del 316L).
Aquí pego unos datos del 904L:
Corrosion Resistance
Although originally developed for its resistance to sulphuric acid it also has a very high resistance to a wide range of environments. A PRE of 35 indicates that the material has good resistance to warm sea water and other high chloride environments. High nickel content results in a much better resistance to stress corrosion cracking than the standard austenitic grades. Copper adds resistance to sulphuric and other reducing acids, particularly in the very aggressive "mid concentration" range.
In most environments 904L has a corrosion performance intermediate between the standard austenitic grade 316L and the very highly alloyed 6% molybdenum and similar "super austenitic" grades.
In aggressive nitric acid 904L has less resistance than molybdenum-free grades such as 304L and 310L.
For maximum stress corrosion cracking resistance in critical environments the steel should be solution treated after cold work.
Heat Resistance
Good resistance to oxidation, but like other highly alloyed grades suffers from structural instability (precipitation of brittle phases such as sigma) at elevated temperatures. 904L should not be used above about 400°C.
Heat Treatment
Solution Treatment (Annealing) - heat to 1090-1175°C and cool rapidly. This grade cannot be hardened by thermal treatment.
Welding
904L can be successfully welded by all standard methods. Care needs to be taken as this grade solidifies fully austenitic, so is susceptible to hot cracking, particularly in constrained weldments. No pre-heat should be used and in most cases post weld heat treatment is also not required. AS 1554.6 pre-qualifies Grade 904L rods and electrodes for welding of 904L.
Fabrication
904L is a high purity, low sulphur grade, and as such will not machine well. Despite this the grade can be machined using standard techniques.
Bending to a small radius is readily carried out. In most cases this is performed cold. Subsequent annealing is generally not required, although it should be considered if the fabrication is to be used in an environment where severe stress corrosion cracking conditions are anticipated.
Applications
Typical applications include:
· Processing plant for sulphuric, phosphoric and acetic acids
· Pulp and paper processing
· Components in gas scrubbing plants
· Seawater cooling equipment
· Oil refinery components
· Wires in electrostatic precipitators
Y aquí las mismas del 316L:
Corrosion Resistance
Excellent in a range of atmospheric environments and many corrosive media - generally more resistant than 304. Subject to pitting and crevice corrosion in warm chloride environments, and to stress corrosion cracking above about 60°C. Considered resistant to potable water with up to about 1000mg/L chlorides at ambient temperatures, reducing to about 500mg/L at 60°C.
316 is usually regarded as the standard “marine grade stainless steel”, but it is not resistant to warm sea water. In many marine environments 316 does exhibit surface corrosion, usually visible as brown staining. This is particularly associated with crevices and rough surface finish.
Heat Resistance
Good oxidation resistance in intermittent service to 870°C and in continuous service to 925°C. Continuous use of 316 in the 425-860°C range is not recommended if subsequent aqueous corrosion resistance is important. Grade 316L is more resistant to carbide precipitation and can be used in the above temperature range. Grade 316H has higher strength at elevated temperatures and is sometimes used for structural and pressure-containing applications at temperatures above about 500°C.
Heat Treatment
Solution Treatment (Annealing) - Heat to 1010-1120°C and cool rapidly. These grades cannot be hardened by thermal treatment.
Welding
Excellent weldability by all standard fusion and resistance methods, both with and without filler metals. Heavy welded sections in Grade 316 require post-weld annealing for maximum corrosion resistance. This is not required for 316L.
316L stainless steel is not generally weldable using oxyacetylene welding methods.
Machining
316L stainless steel tends to work harden if machined too quickly. For this reason low speeds and constant feed rates are recommended.
316L stainless steel is also easier to machine compared to 316 stainless steel due its lower carbon content.
Hot and Cold Working
316L stainless steel can be hot worked using most common hot working techniques. Optimal hot working temperatures should be in the range 1150-1260°C, and certainly should not be less than 930°C. Post work annealing should be carried out to induce maximum corrosion resistance.
Most common cold working operations such as shearing, drawing and stamping can be performed on 316L stainless steel. Post work annealing should be carried out to remove internal stresses.
Hardening and Work Hardening
316L stainless steel does not harden in response to heat treatments. It can be hardened by cold working, which can also result in increased strength.
Applications
Typical applications include:
· Food preparation equipment particularly in chloride environments.
· Pharmaceuticals
· Marine applications
· Architectural applications
· Medical implants, including pins, screws and orthopaedic implants like total hip and knee replacements
· Fasteners
En ambos casos la fuente es Atlas Steel.
En mi opinión, una mayor resistencia al agua marina a 60º o a ambientes ricos en ácido sulfúrico que el 316L no hacen al 904L mejor que el 316L.
Además en conducciones de ácido sulfúrico, se utiliza más acero al carbono (no inoxidable) que 904L debido a que es totalmente inerte al 904L si no está también en contacto con oxígeno. Tan solo las terminales de las conducciones se hacen en 904L porque ahí va a intervenir el oxígeno. Y es más en las dos únicas unidades de Ácido Sulfúrico que yo he montado no he metido 904L, sino Hastelloy C debido a su mayor resistencia a ambientes ricos en oxígeno y sulfúrico.
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