904L vs 254SMO – Composition, Heat Treatment, Properties, and Applications

Introduction

904L and 254SMO are two high-performance austenitic stainless steels frequently considered for aggressive chemical and marine environments. Engineers and procurement teams commonly weigh trade-offs between corrosion resistance, cost, weldability, and mechanical performance when selecting between them. Typical decision contexts include chemical processing equipment, heat exchangers, piping in chloride-bearing environments, and high-integrity welded fabrications where long-term resistance to pitting and crevice corrosion is critical.

The principal technical distinction is that 904L is a highly alloyed, copper-bearing, low‑carbon austenitic grade engineered for resistance to reducing acids and general corrosion, while 254SMO is a superaustenitic grade with very high molybdenum and nitrogen levels designed primarily for superior resistance to pitting and crevice corrosion in chloride-bearing media. That difference drives design and cost decisions, particularly where chloride-induced localized corrosion is the limiting factor.

1. Standards and Designations

• 904L
• UNS: N08904
• Common standards: ASTM A240 / ASME SA-240 (plate/sheet), ASTM A276 (bars), EN (sometimes referenced to EN equivalents)

• Classification: Austenitic stainless steel (stainless)

• 254SMO

• UNS: S31254
• EN: 1.4547 (often referred to as 254 SMO)
• Common standards: ASTM A240 / ASME SA-240, ASTM A276 (bars)
• Classification: Superaustenitic stainless steel (stainless)

Both are austenitic stainless steels (not carbon, tool, or HSLA steels) and are specified in stainless product standards for sheets, plates, pipes, and bars.

2. Chemical Composition and Alloying Strategy

The following table lists typical compositional ranges found in manufacturer datasheets and standards for each grade. Values are given as weight percent; exact composition should be checked against mill certificates for each heat.

Element	904L (typical range, wt%)	254SMO (typical range, wt%)
C	≤ 0.02	≤ 0.02
Mn	≤ 2.0	≤ 0.5–1.0
Si	≤ 1.0	≤ 0.8
P	≤ 0.035	≤ 0.03–0.035
S	≤ 0.01	≤ 0.01
Cr	19.0–23.0	20.0–22.0
Ni	23.0–28.0	17.0–19.0
Mo	4.0–5.0	6.0–6.5
V	—	—
Nb	—	—
Ti	—	—
B	—	—
N	≤ 0.1 (typically low)	0.18–0.24 (elevated)

Notes: - 904L contains significant nickel and copper (Cu typically ~1.5–2.5 wt%, not shown in the simplified table above) to enhance resistance to reducing acids (e.g., sulfuric acid) and to preserve ductility and weldability. - 254SMO achieves high localized corrosion resistance through higher molybdenum and purposely elevated nitrogen; copper is minimal or absent. - Both grades are kept low in carbon to minimize sensitization and intermetallic precipitation during welding and service.

Alloying strategy implications: - Chromium forms a passive oxide film providing general corrosion resistance. - Molybdenum markedly improves pitting and crevice corrosion resistance; higher Mo in 254SMO drives much of its superior performance in chlorides. - Nitrogen stabilizes austenite, increases strength, and boosts pitting resistance (multiplicative in PREN). - Nickel stabilizes austenite and improves toughness and formability. - Copper in 904L specifically helps resistance to reducing acids such as sulfuric acid.

3. Microstructure and Heat Treatment Response

Both 904L and 254SMO are fully austenitic in the solution-annealed condition. Typical microstructural characteristics and heat treatment response:

• 904L
• Microstructure: Fully austenitic with low carbide precipitation if properly solution annealed.
• Heat treatment: Not hardened by heat treatment; recommended solution anneal (e.g., 1010–1120 °C / 1850–2050 °F) followed by rapid quench to dissolve intermetallics and restore corrosion resistance. Sensitization risk is low when carbon is controlled.

• Cold work increases strength via work hardening; anneal needed to recover corrosion resistance if intergranular precipitates form.

• 254SMO

• Microstructure: Fully austenitic with a higher austenite stability due to Mo and N.
• Heat treatment: Also solution annealed (typically ~1100–1150 °C / 2010–2100 °F) and fast-cooled. Because of high Mo and Cr, improper thermal cycles can promote sigma phase or other intermetallics; strict control of solution anneal and cooling is important, especially after welding.
• Thermo-mechanical processing and cold work increase strength; nitrogen content helps maintain austenite and primary microstructure stability.

Neither grade is hardenable by conventional quench-and-temper treatments—their properties are set primarily by composition and cold work or by work hardening.

4. Mechanical Properties

Mechanical properties are process- and product‑form dependent (cold‑worked vs. annealed). Typical ranges for solution-annealed product forms:

Property	904L (solution annealed, typical)	254SMO (solution annealed, typical)
Tensile strength (MPa)	~500–700	~500–700
Yield strength (0.2% offset, MPa)	~200–300	~250–350
Elongation (A%, in 50 mm)	~40–60%	~30–50%
Impact toughness (Charpy, room temp)	Good, ductile fracture	Good, slightly higher strength may reduce elongation
Hardness (HB or HRC)	Typically low (soft in annealed)	Typically low but slightly higher than 904L when alloyed with N/Mo

Interpretation: - Both grades are ductile and tough in the annealed condition; 254SMO can exhibit slightly higher yield due to nitrogen strengthening. - Cold working increases tensile and yield strength in both but reduces elongation. - For load-bearing or high-strength structural needs, neither competes with precipitation‑hardening steels; selection centers on corrosion performance balanced with mechanical requirements.

5. Weldability

Weldability is a key selection parameter.

Key factors: - Low carbon content and austenitic structure give both grades excellent general weldability compared with martensitic or ferritic grades. - Nitrogen in 254SMO raises strength and improves pitting resistance but requires welding procedures that control nitrogen loss and avoid porosity. - Copper in 904L can shift solidification behavior but is generally compatible with standard austenitic filler metals; 904L is often considered easier to weld in shop or field conditions.

Useful weldability indices (qualitative use only): - Carbon equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Pitting corrosion index (Pcm) also informs weldability/susceptibility to cracking: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - Both grades typically show low $CE_{IIW}$ values relative to hardenable steels, indicating low risk of martensitic hardening and hydrogen-assisted cold cracking. - 254SMO’s elevated Mo and N increase its $P_{cm}$-related parameters for corrosion resistance but may complicate filler-metal selection and weld cooling to avoid intermetallic sigma phase. - Preheat is usually unnecessary but control of interpass temperature and post-weld solution anneal (or pickling and passivation) may be recommended for critical applications.

6. Corrosion and Surface Protection

Stainless grades differ fundamentally from carbon steels in corrosion behavior.

• Carbon/alloy steels: protection usually requires coatings (hot-dip galvanizing, painting, lining) to prevent uniform corrosion; corrosion-resistant performance depends on coating integrity.

• Stainless steels (904L and 254SMO): corrosion resistance is intrinsic through the passive Cr-oxide film; comparison uses pitting indices.

Pitting Resistance Equivalent Number: - A common index is PREN: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$

Representative PREN interpretation: - 904L (representative composition) yields a PREN in the mid‑30s, offering excellent general corrosion and reasonable pitting resistance, and exceptional performance in reducing acids due to Cu. - 254SMO (representative composition) yields a PREN commonly in the low-to-mid 40s, reflecting superior resistance to pitting and crevice corrosion in chloride-bearing environments and suitability for severe marine and chemical process services.

Clarification: - PREN is a comparative index; actual field performance also depends on temperature, chloride concentration, flow, and crevice geometry. For highly aggressive chloride environments (e.g., warm seawater, high surface area crevices), 254SMO’s higher PREN is often decisive.

7. Fabrication, Machinability, and Formability

• Cutting and machining:
• 904L machines reasonably well for an austenitic stainless, but its high nickel content and work-hardening tendency require rigid setups and appropriate carbide tooling.
• 254SMO is more challenging to machine due to high Mo and N that increase hardness and tool wear; slower cutting speeds and robust tooling are recommended.
• Forming and bending:
• 904L has excellent formability and deep-drawing performance.
• 254SMO is formable but requires larger bend radii and may need annealing after heavy cold forming to recover ductility.
• Surface finishing:
• Both can be polished to high finishes; 254SMO’s high Mo content can make electrochemical polishing more demanding but results in highly passive surfaces.
• Fabrication notes:
• Use appropriate filler metals (e.g., matching superaustenitic fillers for 254SMO in critical applications) to preserve corrosion performance in welded joints.
• Post-weld pickling/passivation is commonly used to restore passive films.

8. Typical Applications

904L — Typical Uses	254SMO — Typical Uses
Chemical process equipment exposed to reducing acids (sulfuric acid service), heat exchangers, evaporators	Seawater components, desalination, scrubbers, chloride-laden process piping, flue-gas desulfurization components
Piping and fittings in petrochemical plants where resistance to sulfuric and phosphoric acids and general corrosion is required	Pressure vessels and piping in aggressive chloride environments, offshore seawater systems, subsea fittings
Tankage and storage for corrosive, reducing media	Components subject to severe pitting/crevice risk and elevated temperatures in chloride solutions
Architectural applications requiring high nickel-based aesthetics and corrosion performance	High-reliability, low-maintenance installations where long service life justifies higher material cost

Selection rationale: - Choose 904L when resistance to reducing acids and excellent formability/weldability are primary and when cost must be moderate relative to superaustenitic alloys. - Choose 254SMO when localized chloride-induced corrosion (pitting/crevice) is the primary risk and lowest long-term maintenance is desired even at higher initial material cost.

9. Cost and Availability

• Relative cost: 254SMO is typically significantly more expensive per kilogram/metre than 904L due to higher Mo and N alloying and more limited production volumes. 904L is expensive relative to standard austenitics (304/316) because of its high nickel and copper content, but it is generally less costly than superaustenitic grades.
• Availability: 904L has broader availability across plate, sheet, pipe, fittings, and bars from multiple mill sources. 254SMO is available but more often stocked in limited product forms and may require longer lead times or special procurement depending on region and required product form (e.g., seamless pipe, large-diameter plate).
• Procurement tip: For large projects in aggressive chloride environments, include lead-time and wastage in cost comparison; lifetime maintenance savings of 254SMO may offset higher initial purchase cost.

10. Summary and Recommendation

Summary table (qualitative):

Criterion	904L	254SMO
Weldability	Excellent (standard austenitic welding practice)	Good to fair (requires controls and matching filler for critical service)
Strength–Toughness	Good ductility; moderate yield	Slightly higher yield (N-strengthened); good toughness
Corrosion (general)	Excellent (reducing acids, general corrosion)	Excellent (superior pitting/crevice resistance in chlorides)
Cost	High (but lower than superaustenitics)	Very high
Availability	Broad	More limited

Recommendation: - Choose 904L if: - The service atmosphere includes reducing acids (e.g., sulfuric) or mixed acid environments where copper benefits and overall good corrosion resistance are required. - Good weldability and formability are priorities and budget/lead-time constraints exist. - The chloride exposure is moderate and localized corrosion risk is manageable with design controls.

• Choose 254SMO if:
• The primary failure mode to prevent is chloride-induced pitting and crevice corrosion (warm seawater, concentrated chloride process streams, long static exposure in crevices).
• Long service life with minimal maintenance and maximum resistance to localized attack justify the higher material cost.
• Application tolerates more stringent welding and fabrication controls and potential longer procurement lead times.

Concluding note: Final material selection should combine corrosion risk assessment (environmental chloride concentration, temperature, crevice geometry), mechanical and fabrication requirements, and life-cycle cost analysis. For critical chloride-exposed systems, laboratory testing (exposure, coupons, or electrochemical tests) and consultation with material suppliers and corrosion engineers are recommended to validate the choice between 904L and 254SMO for the specific service condition.