304 vs 304L – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
Stainless steels 304 and 304L are among the most widely specified austenitic grades in engineering, manufacturing, and procurement. Engineers and procurement managers typically weigh corrosion resistance, weldability, mechanical strength, and cost when choosing between them. Manufacturing planners must also consider downstream fabrication: heavy welding, post-weld treatments, forming, and surface finish requirements.
The principal practical difference is carbon content: 304L is the low‑carbon variant of 304, formulated to reduce the risk of chromium carbide precipitation (sensitization) during welding and high‑temperature exposure. That distinction is why 304 and 304L are commonly compared — they provide nearly the same corrosion performance and microstructure, but the lower carbon in 304L improves performance in welded assemblies and components that cannot be solution‑annealed after fabrication.
1. Standards and Designations
Major standards and common designations: - ASTM / ASME: ASTM A240 / ASME SA240 (plate, sheet, strip for pressure vessels and general use). - EN: EN 10088 series (stainless steels — various product standards). - JIS: SUS304 and SUS304L. - GB: 0Cr18Ni9 (304) and 0Cr18Ni9L (304L) equivalents in Chinese standards.
Material classification: - Both 304 and 304L are austenitic stainless steels (stainless, non‑ferromagnetic when fully austenitic). - They are not carbon steels, tool steels, or HSLA; they are corrosion‑resistant stainless alloys.
2. Chemical Composition and Alloying Strategy
Typical composition ranges (wt%) used in specifications; actual permitted maxima/minima vary slightly by standard:
| Element | 304 (typical/spec range) | 304L (typical/spec range) |
|---|---|---|
| Carbon (C) | ≤ 0.08% | ≤ 0.03% |
| Manganese (Mn) | ≤ 2.0% | ≤ 2.0% |
| Silicon (Si) | ≤ 1.0% | ≤ 1.0% |
| Phosphorus (P) | ≤ 0.045% | ≤ 0.045% |
| Sulfur (S) | ≤ 0.03% | ≤ 0.03% |
| Chromium (Cr) | 18.0–20.0% | 18.0–20.0% |
| Nickel (Ni) | 8.0–10.5% | 8.0–12.0% |
| Molybdenum (Mo) | ≤ 0.10% (generally none) | ≤ 0.10% |
| Nitrogen (N) | ≤ 0.10% (trace) | ≤ 0.10% |
| Nb, Ti, V, B | trace / typically not added | trace / typically not added |
Alloying strategy and effects: - Chromium provides the passive oxide film that gives stainless steels corrosion resistance. Typical Cr content (≈18–20%) defines “18‑8” stainless steel. - Nickel stabilizes the austenitic (FCC) structure, enhances toughness and ductility, and improves formability. - Carbon increases strength and hardness slightly but at the cost of sensitization risk in the 425–850°C range; carbon reacts with chromium to form chromium carbides at grain boundaries, locally depleting Cr and reducing corrosion resistance. - 304L reduces carbon to control carbide precipitation during welding; nickel range may be adjusted slightly to preserve austenite stability. - Manganese and silicon are deoxidizers and minor strength contributors; nitrogen, when present, can incrementally increase strength and pitting resistance.
3. Microstructure and Heat Treatment Response
Microstructure: - Both grades are essentially fully austenitic (face‑centered cubic) in the annealed condition. Austenite provides excellent ductility and toughness at cryogenic to elevated temperatures. - Neither 304 nor 304L responds to conventional quench‑and‑temper hardening because they are austenitic and do not transform to martensite with heat treatment. Strengthening is achieved primarily by cold work (strain hardening).
Heat treatment and microstructure evolution: - Solution annealing (typical range: $1010^\circ\text{C}$–$1150^\circ\text{C}$) dissolves any precipitates, restores ductility, and returns corrosion resistance; rapid cooling (quenching to water or air) is required to avoid carbide precipitation. - Sensitization: exposure in the approximate range $425^\circ\text{C}$–$850^\circ\text{C}$ can precipitate chromium carbides at grain boundaries. 304 is more susceptible than 304L because of higher carbon; 304L is specified when post‑weld solution annealing is impractical. - Long exposure above ~600°C can also promote sigma phase or other intermetallics in heavily cold‑worked or alloyed variants; these are uncommon in standard 304/304L service but should be considered for high‑temperature service.
Processing routes: - Normalizing is not a meaningful strengthening operation for these austenitic grades. - Thermo‑mechanical routes (cold rolling, annealing) control grain size and texture for sheet or strip products; final anneal fixes the austenitic microstructure.
4. Mechanical Properties
Typical mechanical property minimums (annealed condition), commonly specified in product standards:
| Property | 304 (annealed, typical) | 304L (annealed, typical) |
|---|---|---|
| Tensile strength (Rm) | ≈ 515 MPa (min) | ≈ 485 MPa (min) |
| 0.2% Yield strength (Rp0.2) | ≈ 205 MPa (min) | ≈ 170 MPa (min) |
| Elongation (A) | ≥ 40% (in 50 mm) | ≥ 40% (in 50 mm) |
| Impact toughness (room temp) | High ductile toughness | High ductile toughness |
| Hardness (HB / HRB) | Moderate, strain‑hardening behavior | Slightly lower initial hardness |
Interpretation: - 304 typically shows slightly higher minimum yield and tensile strength than 304L because of higher carbon. In practice, the difference is modest and both are ductile and tough. - Both grades maintain excellent toughness down to cryogenic temperatures; neither is brittle at service temperatures normally encountered in industry. - Strength differences are most relevant where design is close to allowable stress limits or where post‑weld strength reduction is a factor.
5. Weldability
Weldability factors: - Low carbon reduces the tendency to form chromium carbides at grain boundaries during cooling through the sensitization range. Therefore, lower carbon lowers the susceptibility to intergranular corrosion after welding. - Hardenability in austenitic stainless steels is low; they do not form hard martensitic structures on cooling, so cracking from hard phases is not a primary concern. However, cold working and thermal cycles can create strain‑induced martensite in 304 under certain conditions; 304L, with slightly different composition, may be marginally less prone to strain‑induced martensite.
Useful weldability indices (qualitative interpretation): - Carbon equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Pitting corrosion equivalent (Pcm) for weldability assessment: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Interpretation: - Lower $C$ reduces both $CE_{IIW}$ and $P_{cm}$, indicating reduced hot‑cracking sensitivity and lower risk of segregation/sensitization during welding. - In practice, 304L typically welds more readily in large, heat‑intensive weldments because the lower carbon content reduces intergranular corrosion risk without requiring post‑weld solution annealing. - When welding 304 (higher carbon), designers often control heat input, use filler metals with stabilizers (e.g., Ti or Nb in some fillers), or perform post‑weld solution anneal when corrosion resistance of the weld zone must equal that of the base metal.
Welding practice notes: - Use matching or low‑carbon filler metals depending on service and corrosion requirements. - Minimize hold times in the sensitization range and use rapid cooling or local heating control. - For critical corrosive environments, 304L or stabilized grades (e.g., 321, 347) are preferred when post‑weld solution annealing is impractical.
6. Corrosion and Surface Protection
Corrosion behavior: - Both 304 and 304L rely on the chromium‑rich passive film for general corrosion resistance in atmospheric, mildly acidic, and alkaline environments. - Pitting and crevice corrosion resistance are limited because both lack molybdenum; therefore, in chlorides or aggressive marine environments, higher alloyed grades (316, duplex, etc.) are preferred.
PREN relevance: - PREN is used for assessing resistance to chloride pitting; for these grades: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ - For 304/304L, Mo is essentially zero or very low and N is low; PREN values are modest, so neither grade is recommended for severe chloride environments.
Sensitization and intergranular corrosion: - The primary corrosion concern where 304 can be disadvantaged is intergranular corrosion after welding or prolonged exposure in the critical temperature range where chromium carbides precipitate. - 304L’s low carbon reduces carbide formation and thereby lowers the risk of intergranular corrosion in welded zones.
Surface protection for non‑stainless steels: - Not applicable here; for non‑stainless carbon steels, galvanizing or coatings are common. For 304/304L, surface passivation (nitric acid or citric acid passivation) and pickling are common to restore or improve passive film after fabrication.
7. Fabrication, Machinability, and Formability
Forming and bending: - Both grades exhibit excellent cold formability; 304L may be marginally easier to form due to slightly lower yield strength. - All austenitic stainless steels work‑harden rapidly; forming operations often require intermediate anneals for severe deformation.
Machinability: - Austenitic stainless steels are more difficult to machine than carbon steels because of high ductility and work‑hardening. 304 and 304L have similar machinability; process controls (rigid tooling, sharp inserts, adequate chip control, and lubricant/coolant) are critical. - 304L’s slightly lower strength can marginally ease cutting forces on some operations.
Surface finishing: - Both grades can be polished, passivated, electrochemically treated, and bead blasted to achieve required surface finishes. 304L is frequently used for welded assemblies where a consistent finish across welds is needed without solution annealing.
8. Typical Applications
| 304 — Typical Uses | 304L — Typical Uses |
|---|---|
| Kitchen equipment, food processing, beverage plants | Heavy welded vessels and piping in chemical plants |
| Architectural trim, decorative panels | Welded storage tanks and vessels where post‑weld anneal is impractical |
| Heat exchangers (mild environments) | Sewage and wastewater piping and tanks with extensive welding |
| Fasteners, springs (where corrosion resistance and strength needed) | Pharmaceutical and biotech welded systems requiring corrosion resistance at welds |
| Automotive trim, consumer goods | Pressure vessels and piping with large weld volumes where sensitization risk is a concern |
Selection rationale: - Choose 304 where higher minimum strength and slightly lower material cost are acceptable and where welding is limited or post‑weld anneal is possible. - Choose 304L where heavy welding, inability to solution‑anneal after fabrication, or service in slightly more corrosive post‑weld conditions is expected.
9. Cost and Availability
- Both grades are widely available worldwide in plate, sheet, strip, tube, and bar forms.
- 304 is typically the most common and, in many markets, marginally less expensive due to broader production volumes and less stringent carbon control.
- 304L can carry a small premium because of tighter carbon control during melting and processing, but the premium is often small compared to the total fabrication cost when eliminating costly post‑weld heat treatment.
- Lead times and availability are generally excellent for both in standard product forms; for very large fabricated items or special mill certifications, schedule impacts should be verified with suppliers.
10. Summary and Recommendation
| Criterion | 304 | 304L |
|---|---|---|
| Weldability (resistance to sensitization) | Good; requires care for heavy welds | Better for heavy weldments and where PWHT is not done |
| Strength–Toughness | Slightly higher minimum strength; equally high toughness | Slightly lower minimum strength; equally high toughness |
| Cost | Typically marginally lower | Typically marginally higher but often cost‑effective for welded systems |
Recommendation: - Choose 304 if: your design benefits from slightly higher minimum strength and you can control welding practices (low heat input, use of appropriate filler metals) or perform solution annealing after welding to restore corrosion resistance. - Choose 304L if: the component or piping will have extensive welding, post‑weld solution anneal is impractical, or there is concern about intergranular corrosion at welded joints. 304L is often the safer, lower‑risk choice for welded pressure vessels, storage tanks, and heavy piping where maintaining corrosion resistance at and near weld heat‑affected zones is critical.
Concluding note: Both 304 and 304L are robust, widely specified austenitic stainless steels. The design decision usually turns on welding practice and the acceptability of the small tradeoff in minimum strength for improved as‑welded corrosion resistance. For critical or chloride‑exposed services, consider higher‑alloyed (Mo‑bearing) stainless grades or duplex alternatives.