304 vs 304H – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
Type 304 and type 304H stainless steels are two austenitic stainless grades widely used across process, pressure-vessel, and general fabrication industries. Engineers and procurement professionals commonly weigh corrosion resistance, weldability, formability, high-temperature performance, and cost when choosing between them. Typical decision contexts include specifying material for a welded pressure vessel, selecting tubing for heat exchangers, or choosing sheet for general fabrication.
The principal distinguishing feature between the two grades is the carbon content: 304H has an elevated carbon composition relative to standard 304. That single change shifts performance in predictable ways—most importantly increasing strength and creep resistance at elevated temperatures while increasing the risk of carbide precipitation and related sensitization during certain thermal cycles. Because 304 and 304H are otherwise very similar (same austenitic matrix stabilized by chromium and nickel), they are compared when designs require a balance of high-temperature mechanical performance versus corrosion resistance, weldability, and formability.
1. Standards and Designations
Major specification standards that cover 304 and 304H include: - ASTM / ASME: ASTM A240 / ASME SA-240 (plates, sheets), ASTM A312 (seamless and welded tubing), ASTM A269 (wrought tubing), etc. - EN: EN 10088 series for stainless steels (EN 1.4301 corresponds to 304). - JIS: JIS G4303 / JIS G4305 (stainless steels; equivalents). - GB: GB/T 1220 and related Chinese standards for stainless steels.
Classification: - Both 304 and 304H are stainless steels (austenitic). They are not carbon steels, tool steels, or HSLA grades. - They are specified and used as stainless (corrosion-resistant) alloys rather than structural carbon steels.
2. Chemical Composition and Alloying Strategy
The following table summarizes typical composition ranges from common standards (values are given as weight percent, and are intended as typical standardized ranges rather than specific mill certificates).
| Element | 304 (typical range) | 304H (typical range) |
|---|---|---|
| C | ≤ 0.08 wt% | 0.04 – 0.10 wt% |
| Mn | ≤ 2.0 wt% | ≤ 2.0 wt% |
| Si | ≤ 1.0 wt% | ≤ 1.0 wt% |
| P | ≤ 0.045 wt% | ≤ 0.045 wt% |
| S | ≤ 0.03 wt% | ≤ 0.03 wt% |
| Cr | ~18.0 – 20.0 wt% | ~18.0 – 20.0 wt% |
| Ni | ~8.0 – 10.5 wt% | ~8.0 – 11.0 wt% |
| Mo | Not specified (usually ≈ 0) | Not specified (usually ≈ 0) |
| V, Nb, Ti, B | Not specified / trace only | Not specified / trace only |
| N | Trace (typically ≤ 0.11) | Trace (typically ≤ 0.11) |
How alloying affects behavior: - Chromium (Cr) supplies corrosion resistance by forming a protective oxide film and is the principal alloying element for stainless behavior. - Nickel (Ni) stabilizes the austenitic phase and enhances toughness and formability. - Carbon (C) strengthens austenite by solid-solution strengthening and can form chromium carbides (Cr23C6) during exposure to sensitizing temperatures; increased C in 304H boosts high-temperature strength and creep resistance but raises sensitization risk. - Manganese (Mn) and silicon (Si) are minor austenite stabilizers and deoxidizers; sulfur and phosphorus are impurity elements controlled to low levels to preserve toughness and corrosion resistance.
3. Microstructure and Heat Treatment Response
Both 304 and 304H are fully austenitic (face-centered cubic) in the solution-annealed condition at ambient temperature. Under standard processing (hot rolling, solution anneal, air cooling), microstructure is homogeneous austenite with possible twin boundaries and some annealing twins.
Key microstructural differences and heat-treatment responses: - 304 (lower C) is less prone to forming chromium carbides during slow cooling or intermediate-temperature exposures; solution annealing above ~1,040–1,100 °C followed by rapid cooling restores a carbide-free austenitic matrix. - 304H (higher C) has greater driving force for chromium carbide precipitation when exposed in the sensitization range (~450–850 °C). Carbide precipitation occurs at grain boundaries and can locally deplete chromium, reducing intergranular corrosion resistance. - Neither grade hardens by quench-and-temper like martensitic steels; they are not heat-treatable for strength via conventional transformations. Strength adjustments are achieved by cold work or by specifying higher carbon (304H) for elevated-temperature strength. - Thermo-mechanical processing (cold work, annealing schedules) affects dislocation density, grain size, and texture similarly for both grades. Annealing at solutionizing temperatures will dissolve carbides if held and quenched appropriately; slow cooling after welding or prolonged service at intermediate temperatures promotes carbide precipitation in 304H more readily than in 304.
4. Mechanical Properties
Both steels provide good ductility and toughness in the annealed condition; 304H generally offers slightly higher strength, especially at elevated temperatures, attributable to its higher carbon.
| Property | 304 | 304H |
|---|---|---|
| Tensile Strength (relative) | Standard austenitic level | Slightly higher (notable at elevated T) |
| Yield Strength (relative) | Baseline for 300-series austenitic | Slightly higher than 304 |
| Elongation / Ductility | High ductility; good formability | Slightly reduced ductility vs 304 |
| Impact Toughness | Excellent at ambient temperature | Comparable at ambient if not sensitized; reduced if carbide precipitation occurs |
| Hardness | Typical annealed austenitic hardness | Slightly higher in annealed condition due to C |
Explanation: - At room temperature, differences are modest—both grades exhibit similar tensile and yield strengths with high elongation. 304H typically yields marginally higher tensile and yield values because carbon is a solid-solution strengthener. - At elevated temperatures or under creep conditions, 304H retains higher strength than 304 and is therefore specified for pressure-vessel service at higher allowable temperatures. - If 304H is exposed to sensitizing thermal cycles (e.g., welding without proper post-weld heat treatment or prolonged service in the 450–850 °C range), intergranular corrosion and reduced toughness can result from chromium carbide precipitation.
5. Weldability
Weldability of 304 and 304H is generally good; both are readily welded by common processes (GMAW/MIG, GTAW/TIG, SMAW). However, carbon level affects the risk of sensitization and HAZ properties.
Relevant carbon-equivalent/weldability indices: - Interpreted qualitatively using the IIW carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - For stainless steels a more complex compositional influence can be expressed with $P_{cm}$: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - 304 (lower C) has a lower $CE_{IIW}$ and $P_{cm}$ contribution from carbon than 304H, which means 304 is less likely to form hard, brittle microstructures in the HAZ and is less sensitive to intergranular corrosion from welding if proper filler and procedures are used. - 304H’s higher carbon increases hardening potential during rapid thermal cycles and increases the risk of sensitization in and near the heat-affected zone (HAZ) if austenite grain boundary carbides form. For welded pressure-vessel work at elevated temperatures, 304H is often specified to meet allowable stress requirements; filler selection and welding practice (e.g., use of stabilized grades or post-weld solution anneal where practicable) mitigate risks. - Preheat is generally not required for these austenitic stainless steels, but control of heat input and selection of appropriate filler metal (e.g., matching or low-carbon/stabilized grades) is an important consideration for 304H to avoid embrittlement or intergranular corrosion.
6. Corrosion and Surface Protection
- Both 304 and 304H are corrosion-resistant in a wide variety of environments due to chromium passivation. Neither contains molybdenum, so they are less resistant to chloride pitting than Mo-bearing grades (e.g., 316).
- Carbon increase in 304H makes sensitization and intergranular corrosion a practical concern if material is exposed to sensitizing temperatures without adequate control. For applications where intergranular corrosion resistance is critical after welding, low-carbon 304L or stabilized grades (321, 347) may be preferred.
- PREN (Pitting Resistance Equivalent Number) is a useful metric for pitting resistance where Mo and N are present: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
- For 304/304H, Mo ≈ 0 and N is low, so PREN is relatively low; PREN is more meaningful for duplex or Mo-bearing austenitics.
- Surface protection for non-stainless substrates (not applicable here) would include galvanizing or coatings; for stainless, surface finish and passivation treatments are often used to maximize service life.
7. Fabrication, Machinability, and Formability
- Formability: 304 (lower carbon) is marginally better for deep drawing and forming due to slightly greater ductility and lower work hardening sensitivity with regard to sensitization upon subsequent heating. Both are good for forming operations if appropriate tooling and incremental forming practices are used.
- Machinability: Austenitic stainless steels are generally more difficult to machine than carbon steels because of work-hardening and low thermal conductivity. 304H can be slightly more challenging to machine than 304 because its higher carbon and resultant higher strength lead to increased cutting forces and tool wear. Using sharp tooling, rigid setups, and appropriate lubricants reduces problems.
- Surface finishing: Both polish and electropolish similarly; however 304H may require more careful control of thermal exposure during finishing to avoid carbide precipitation if material is heated.
8. Typical Applications
| 304 (common uses) | 304H (common uses) |
|---|---|
| Food processing equipment, sinks, kitchenware, architectural trim | Pressure vessels and boilers where higher allowable stresses at elevated temperature are required |
| Chemical process piping and tanks in mild environments | Superheater and reheater tubing in boiler applications (where higher creep strength is needed) |
| Heat exchangers, sanitary tubing, fasteners | Furnace and high-temperature fabricated components where operating temperature is elevated |
| General fabrication, decorative building components | Applications requiring the mechanical properties of a higher-carbon austenitic at elevated T |
Selection rationale: - Choose 304 when general corrosion resistance, formability, and weldability at normal service temperatures are the dominant requirements and when minimizing sensitization risk after welding is important. - Choose 304H when design calls for higher allowable stress or improved strength at elevated temperature (e.g., pressure vessels operating above typical 300 °C thresholds), and when the project specifies appropriate fabrication controls to manage sensitization and corrosion risk.
9. Cost and Availability
- 304 is one of the most common stainless grades worldwide and is broadly available in plate, sheet, coil, tube, and bar. Its cost is typically competitive within the 300-series family.
- 304H is a recognized variant and is available in product forms commonly used for high-temperature service (plate, tubing for boilers, and pressure components). It is less commonly stocked in generic commodity markets than 304 and can carry a modest premium depending on regional stocking practices and the need for specific mill certification of carbon content.
- Lead times and availability depend on product form (sheet/plate versus seamless tubing) and required certifications for pressure-vessel or elevated-temperature service.
10. Summary and Recommendation
| Aspect | 304 | 304H |
|---|---|---|
| Weldability | Excellent; lower sensitization risk | Good, but higher sensitization risk; requires procedural controls |
| Strength–Toughness | Good balance; excellent ductility | Higher strength at elevated T; slightly lower ductility when compared directly |
| Cost & Availability | Widely available; generally lower cost | Readily available for pressure-temp applications; may carry premium |
Conclusions: - Choose 304 if you need excellent general corrosion resistance, superior fabricability and formability, and minimal risk of sensitization from normal welding processes. 304 is the practical default for sanitary equipment, architectural applications, and many chemical service cases at ambient to moderate temperatures. - Choose 304H if the design requires higher allowable stress or improved resistance to deformation at elevated service temperatures (e.g., pressure vessels, boilers, heat exchangers operating at higher temperatures), and you can accept and manage the greater risk of carbide precipitation through appropriate welding procedures, post-weld treatments if feasible, or by selecting compatible filler metals and fabrication practices.
If elevated-temperature creep resistance and allowable stress tables are governing material selection (ASME codes, pressure-vessel specifications), consult the applicable code for the required grade and temperatures; in many cases 304H will appear where 304’s allowable stress limits are insufficient for the intended service temperature.