304L vs 304H – Composition, Heat Treatment, Properties, and Applications

Introduction

Type 304 variants are among the most widely used austenitic stainless steels in industry. Engineers, procurement managers, and manufacturing planners frequently need to decide between low-carbon 304L and higher-carbon 304H when specifying materials for pressure equipment, piping, heat exchangers, or fabricated components. The decision typically balances corrosion resistance and weldability against elevated-temperature strength and creep resistance.

The core practical distinction is that 304L is optimized to minimize carbide precipitation during welding and service (improving intergranular corrosion resistance and weldability), while 304H contains deliberately higher carbon to retain greater strength at elevated temperatures. Because both grades share the same fundamental chromium–nickel austenitic matrix, they are often compared in designs where temperature exposure, fabrication route, and post-weld performance are the deciding factors.

1. Standards and Designations

• ASTM/ASME: 304L — ASTM A240/A240M (sheet/plate), A312 (pipes) as UNS S30403; 304H — ASTM A240 (A240M) as UNS S30409 or equivalent.
• EN (European): EN 1.4306 (304L), EN 1.4948 sometimes used for 304H-equivalent or other high-C austenitic stainless; national EN variants reference composition bands.
• JIS (Japan): SUS304L and SUS304H nomenclature in JIS G4303/G4312-type standards.
• GB (China): 06Cr19Ni10/06Cr19Ni10-2L equivalents for 304/304L; local designations exist for 304H.
• Classification: Both are stainless steels (austenitic). They are not carbon steels, tool steels, or HSLA.

2. Chemical Composition and Alloying Strategy

How alloying affects performance: - Chromium (Cr) provides the passive oxide responsible for corrosion resistance. Both grades have similar Cr, so base corrosion behavior is similar. - Nickel (Ni) stabilizes the austenitic phase and improves toughness and corrosion resistance; similar contents mean similar ductility. - Carbon (C) influences carbide formation: higher C increases strength (especially at elevated temperature) but promotes chromium carbide precipitation and possible intergranular corrosion if not properly controlled. - Minor elements (Mn, Si, N) influence mechanical strength and work-hardening; nitrogen boosts strength and pitting resistance, Mo would enhance pitting resistance but is not present.

3. Microstructure and Heat Treatment Response

Typical microstructure for both 304L and 304H is fully austenitic (face-centered cubic) in the annealed condition. Because austenite is stable at room temperature in these compositions, there is no martensitic transformation during cooling for standard processing.

• 304L: Low carbon minimizes chromium carbide ($\text{Cr}_{23}\text{C}_6$) precipitation at grain boundaries during weld cooling or sensitizing heat exposure (approximately 450–850 °C). As a result, microstructure remains free of significant grain-boundary carbides after common fabrication, preserving intergranular corrosion resistance.
• 304H: Higher carbon increases the driving force for carbide precipitation during thermal exposure. At elevated temperatures, some $\text{Cr}_{23}\text{C}_6$ can form at grain boundaries, which can locally reduce corrosion resistance unless stabilizers or post‑weld heat treatments are applied. However, the higher carbon content also increases solid-solution strengthening and creep resistance at temperatures typically above 500–600 °C.

Heat-treatment response: - Annealing (full solution anneal followed by rapid quench) restores ductility and dissolves most carbides in both grades. For 304H, the dissolution temperature and kinetics are similar but re-precipitation upon slow cooling is more likely. - Normalizing is not typically used for austenitic stainless steels because the austenitic phase is stable; mechanical properties are primarily controlled by cold work and solution annealing. - Thermo-mechanical processing (cold work followed by anneal) changes yield and tensile behavior similarly in both grades, but 304H will retain somewhat higher yield/tensile at elevated temperatures.

4. Mechanical Properties

Notes: Exact numeric values depend on product form (sheet, plate, pipe), heat treatment, and cold work. The key takeaway: 304H typically offers higher strength at elevated temperature at the expense of somewhat reduced resistance to carbide precipitation and slightly lower fabricability margins.

5. Weldability

Weldability of austenitic stainless steels is generally excellent due to their austenitic matrix and low propensity to form martensite.

Key welding considerations: - Carbon content matters: lower carbon in 304L reduces the risk of sensitization (intergranular corrosion) after welding and allows omission of post-weld solution annealing in many applications. - 304H’s higher carbon increases sensitization risk; weld procedure controls (filler selection, fast cooling, or post-weld solution anneal) may be required for corrosive environments or code compliance. - Hardenability is low for both; cracking susceptibility from hard microstructures is limited.

Useful empirical formulas for assessing weldability/hardenability: - Carbon equivalent (IIW): $$ CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15} $$ - Decreasing weldability correlates with higher $CE_{IIW}$. - Chromium equivalent or Pcm for stainless steels: $$ P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000} $$ - Higher $P_{cm}$ suggests a greater tendency to form ferrite/austenite balance issues and may inform preheating/post‑weld requirements.

Interpretation: - 304L typically scores lower on carbon-sensitive indices and is preferred where weld integrity without post-weld heat treatment is required. - 304H may require more stringent welding control in corrosive or code-bound applications but offers better strength for high‑temperature welded assemblies.

6. Corrosion and Surface Protection

• Both 304L and 304H are stainless (contain ~18% Cr) and rely on a passive Cr2O3 film for general corrosion resistance in many environments.
• Pitting and crevice corrosion resistance are moderate because Mo is not present. For pitting resistance evaluation, PREN is a common index: $$ \text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N} $$
• For 304 variants (Mo ≈ 0), PREN is driven by Cr and N; with similar Cr and low N, both grades have comparable general and pitting resistance.
• Sensitization risk: 304H’s higher carbon promotes chromium carbide formation at grain boundaries when exposed to sensitizing temperature ranges, which can locally lower corrosion resistance (intergranular attack). 304L is chosen to mitigate that risk.
• Surface protection for non-stainless steels (not applicable here) would include galvanizing or coatings; for these stainless grades, cleaning, passivation, and avoidance of chloride-containing environments are primary measures.

7. Fabrication, Machinability, and Formability

• Formability/bending: 304L has excellent formability and deep-drawing characteristics due to lower yield and higher ductility in the annealed state. 304H is still workable but may show marginally reduced forming limits.
• Machinability: Austenitic stainless steels are work-hardening and have lower machinability than carbon steels. 304H’s higher carbon and potential increased strength can slightly reduce tool life and require more robust tooling or slow feeds; free‑cut versions or added sulfur improve machinability but reduce corrosion resistance.
• Surface finishing: Both grades polish and passivate well; however, grinding or aggressive finishing that heats the surface could locally sensitize 304H more readily than 304L.
• Welding fabrication: 304L is generally the preferred choice for welded structures unless elevated-temp strength is required.

8. Typical Applications

Selection rationale: - Choose 304L where welding simplicity, resistance to intergranular corrosion, and forming are higher priorities. - Choose 304H where sustained elevated-temperature strength and creep resistance are required, and where post-weld or fabrication measures can control sensitization risk.

9. Cost and Availability

• Cost: 304L is broadly produced and stocked; relative cost is similar to standard 304 but slightly higher due to controlled low-carbon processing. 304H is a more specialized grade—material cost can be comparable or slightly higher due to tighter carbon specification and possibly lower production volumes.
• Availability: 304L is widely available in many product forms (sheet, plate, coil, tube, bar, forgings). 304H is available but less common in some markets and product forms; lead times for specialty shapes or large quantities may be longer.
• Procurement note: When specifying, include the correct ASTM/EN/JIS designation and desired product form to avoid substitution of standard 304 or stabilized grades.

10. Summary and Recommendation

Choose 304L if: - The component will be welded extensively and post‑weld heat treatment is impractical. - Intergranular corrosion resistance (e.g., piping for food, pharmaceutical, or potable water) is a priority. - Good formability and deep drawing are required.

Choose 304H if: - The application involves sustained elevated-temperature service where higher tensile strength or creep resistance is required (e.g., heat exchangers, boilers, furnace components). - The procurement and fabrication plan allows for welding controls, selection of compatible filler metals, and, if necessary, post-weld solution annealing or alternative mitigation to manage sensitization.

Final note: Both 304L and 304H are valid choices within their design envelopes. Specify the intended service temperature, corrosive environment (chloride exposure, acidity), fabrication sequence, and applicable codes/standards when selecting between them to ensure the correct balance of weldability, corrosion resistance, and high‑temperature performance.