304L vs 347 – Composition, Heat Treatment, Properties, and Applications

Introduction

Stainless steels 304L and 347 are two widely used austenitic grades that often compete for the same applications. Engineers, procurement managers, and manufacturing planners frequently weigh corrosion resistance, weldability, and lifecycle cost when deciding between them. Typical decision contexts include welded assemblies where intergranular corrosion is a concern, food and pharmaceutical equipment where cleanability is critical, and fabricated components exposed to cyclic or elevated-temperature service.

The primary metallurgical distinction between the two is their strategy for avoiding carbide precipitation at grain boundaries during welding or thermal exposure: one uses a deliberately low carbon content to limit carbide formation, while the other uses microalloy stabilization (niobium) to tie up carbon as more stable carbides. This difference drives how each behaves after welding, how it resists grain-boundary attack, and how it is specified in fabrication.

1. Standards and Designations

Common standards and designations for these grades include:

• ASTM/ASME: 304L — UNS S30403 (ASTM A240, A276, A312); 347 — UNS S34700 (ASTM A240, A276, A312).
• EN: 304L — X2CrNi18-9 / 1.4306 (approx.); 347 — X6CrNiNb18-10 / 1.4550 (approx.).
• JIS: 304L — SUS304L; 347 — SUS347.
• GB: 304L — 06Cr19Ni10; 347 comparable in stabilized variant.

Both are stainless (austenitic) stainless steels. They are not carbon steels, tool steels, or HSLA grades.

2. Chemical Composition and Alloying Strategy

The following table summarizes typical composition ranges used for comparison. Values are representative ranges from common specifications; consult the specific standard or mill certificate for exact composition for a given batch.

Element	304L (typical range, wt%)	347 (typical range, wt%)
C	≤ 0.03	≤ 0.08
Mn	≤ 2.0	≤ 2.0
Si	≤ 1.0	≤ 1.0
P	≤ 0.045	≤ 0.045
S	≤ 0.03	≤ 0.03
Cr	17.5–19.5	17.0–19.0
Ni	8.0–12.0	9.0–13.0
Mo	— (minor/trace)	— (minor/trace)
Nb (Nb+Ta)	— (trace)	0.10–1.0
Ti	—	— (some stabilized variants use Ti in other grades, but 347 is Nb-stabilized)
B	trace	trace
N	≤ 0.10	≤ 0.10

How alloying affects properties: - Chromium (Cr) provides general corrosion resistance through a passive Cr-oxide film. - Nickel (Ni) stabilizes the austenitic phase, provides toughness and formability. - Low carbon (304L) reduces the tendency for chromium carbide ($\text{Cr}_{23}\text{C}_6$) precipitation at grain boundaries during thermal exposure, preserving corrosion resistance after welding. - Niobium (347) forms stable niobium carbides ($\text{NbC}$) that preferentially consume carbon, preventing chromium carbide precipitation and maintaining intergranular corrosion resistance even if the carbon level is higher than in low-carbon grades.

3. Microstructure and Heat Treatment Response

Both 304L and 347 are fully austenitic in the annealed condition. They are not hardened by conventional heat treatment (they are non-heat-treatable by quench and temper methods). Key microstructural considerations:

• 304L: annealed microstructure is single-phase austenite with very low carbide precipitation when cooled from annealing/welding temperatures due to low carbon. On prolonged exposure in the sensitization range (~425–850 °C), some carbide precipitation can still occur but at a much-reduced rate.
• 347: annealed microstructure is also single-phase austenite; Nb exists in solid solution or as fine $\text{NbC}$ particles that act as carbon traps. During welding, niobium promotes formation of stable niobium carbides rather than chromium carbides, reducing sensitization.

Processing routes: - Normalizing is not conventional or necessary for austenitic grades — solution annealing (typically 1010–1150 °C) followed by rapid cooling is used to restore corrosion resistance and dissolve undesired precipitates. - Cold working increases strength by strain hardening for both grades and can influence corrosion behavior (cold work can raise susceptibility to stress corrosion cracking in chloride environments). - There is no meaningful quench-and-temper response; any strengthening is by work hardening or alloy selection.

4. Mechanical Properties

Typical mechanical properties are controlled by product form (plate, sheet, bar), cold work condition, and specification. The following table gives representative annealed values for common product forms (e.g., plate/sheet), to illustrate relative behavior. Always refer to the relevant standard for guaranteed values.

Property (annealed)	304L (typical)	347 (typical)
Tensile strength (MPa)	485–620	485–620
Yield strength 0.2% offset (MPa)	170–310	170–310
Elongation (A%)	40–60%	40–60%
Impact toughness (J, room temp)	Generally high; notch toughness excellent	Generally high; similar to 304L
Hardness (HRB)	≤ 95 (annealed)	≤ 95 (annealed)

Interpretation: - In the annealed condition, both grades have very similar strength, ductility, and toughness because they share the austenitic matrix. Differences in mechanical properties are negligible for most structural applications. - Any strength difference is normally achieved by cold work rather than heat treatment.

5. Weldability

Both 304L and 347 are considered highly weldable when standard practices are followed. Key welding considerations:

• 304L’s low carbon content minimizes the risk of chromium carbide precipitation in the heat-affected zone (HAZ) during welding; this makes 304L a preferred choice when post-weld sensitization must be avoided without special procedures.
• 347’s niobium stabilization makes it robust against sensitization even if carbon content is higher — niobium ties up carbon as $\text{NbC}$, preventing formation of chromium carbides.
• Both grades can be welded by common processes (GMAW, GTAW, SMAW, etc.) with suitable filler metals (e.g., 308L/309 for 304L; 316L/347-compatible fillers depending on service).

Useful weldability indices (qualitative interpretation only):

• Carbon equivalent (IIW): $$ CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15} $$ Lower $CE_{IIW}$ correlates with lower hardenability and reduced risk of cold cracking in ferritic steels; for austenitic stainless steels this indicator is less critical but still used for mixed-alloy assessments.

• Pcm (decarburization and weld-cracking risk indicator): $$ P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000} $$ For stainless grades, niobium increases $P_{cm}$ slightly, but in stabilized grades this is compensated by the chemical locking of carbon. Interpret these formulas qualitatively: 304L has inherently low C and therefore lower sensitization risk; 347’s stabilization provides similar or superior resistance to grain boundary carbide-induced corrosion after welding.

6. Corrosion and Surface Protection

As austenitic stainless steels, both grades depend on a continuous chromium-rich passive film for corrosion resistance.

• PREN (Pitting Resistance Equivalent Number) is commonly used for assessing pitting resistance where Mo and N are present: $$ \text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N} $$ For 304L and 347, which have negligible Mo content and low N, PREN values are modest; PREN is therefore of limited use in differentiating these two grades versus Mo-bearing grades (e.g., 316L).

Intergranular corrosion: - 304L: low carbon minimizes formation of $\text{Cr}_{23}\text{C}_6$ at grain boundaries, reducing susceptibility to intergranular corrosion after welding without requiring post-weld heat treatment. - 347: niobium stabilization prevents chromium carbide formation by preferentially forming $\text{NbC}$, delivering robust resistance to intergranular attack even when carbon content is higher or cooling is slow.

Other protections: - If a non-stainless protective strategy is used (not typical here), coatings such as painting or galvanizing are outside the norm for these grades; stainless steels are typically protected by passivation treatments (nitric or citric acid) or mechanical polishing to restore the passive film.

7. Fabrication, Machinability, and Formability

• Formability: Both grades are highly formable in the annealed condition due to their austenitic structure. Deep drawing and complex bending are common.
• Machinability: Austenitic stainless steels are generally more difficult to machine than carbon steels (tendency to work-harden and lower thermal conductivity). 304L and 347 have similar machinability; 347 can be somewhat tougher on tooling due to Nb carbides, but differences are modest.
• Surface finish: Both polish well; 347 can develop slightly different carbide-related surface features after aggressive thermal exposure.
• Post-fabrication treatments: Passivation or pickling after fabrication/welding is recommended to restore surface chromium oxide and remove foreign contamination.

8. Typical Applications

304L – Typical Uses	347 – Typical Uses
Food processing equipment, dairy, brewing, and kitchenware (where welds are common and cleanliness critical)	Chemical processing equipment and heat exchangers where service includes cycling or elevated temperatures and welded assemblies
Pharmaceutical and medical components requiring easy cleaning and corrosion resistance	Exhaust systems, aircraft ducting, and industrial oven components where stabilization improves performance after thermal cycles
Architectural trim, tanks, and piping in mild corrosive environments	Pressure vessels, high-temperature steam lines, and welded assemblies exposed to sensitization risk

Selection rationale: - Choose 304L when minimizing initial material cost and maximizing post-weld corrosion resistance without special stabilization is preferred (e.g., food, pharma). - Choose 347 where welded components will see prolonged thermal exposure or where material will be exposed to sensitization risk and stabilization (niobium) gives more predictable long-term performance.

9. Cost and Availability

• Cost: 347 is typically priced slightly higher than 304L due to the addition of niobium and sometimes tighter specification controls. However, premium may be modest and depends on market conditions and product form.
• Availability: Both are widely available worldwide in sheet, plate, tube, and bar forms. 304/304L is more common and stocked in greater variety and volumes, which can reduce lead times for specialized sizes. 347 is commonly stocked for pressure and high-temperature applications but may have longer lead times for certain forms or finishes.

10. Summary and Recommendation

Attribute	304L	347
Weldability	Excellent (low C minimizes sensitization)	Excellent (Nb stabilization minimizes sensitization)
Strength–Toughness	Similar; both austenitic and ductile	Similar; marginal benefits at elevated temperatures
Cost	Lower (generally)	Higher (niobium alloying cost)

Choose 304L if: - You need a low-carbon austenitic stainless with reliable post-weld corrosion resistance for food, pharmaceutical, or general-purpose applications. - Cost and broad availability are primary concerns and service temperatures are not high enough to cause sensitization after welding.

Choose 347 if: - The design includes significant welded assemblies that will see prolonged thermal exposure, cycling, or elevated temperatures where stabilization against intergranular corrosion is critical. - You prefer a stabilization strategy (niobium) rather than relying solely on low carbon, or when material procurement can accommodate a slightly higher-cost stabilized alloy.

Concluding note: both 304L and 347 are proven austenitic stainless steels with overlapping mechanical properties but different anti-sensitization approaches. Final selection should consider welding procedure, service temperature history, corrosion environment (chloride, nitric, sulfide), and procurement constraints. For critical applications, request mill certificates, and if necessary, perform qualification welds and corrosion testing in representative service conditions.