English Views: 0 Author: Eternal Tech Team Publish Time: 2026-08-26 Origin: Site
In the selection of austenitic stainless steel forgings, AISI 304 and AISI 304L are the two most common and most easily confused grades. Both belong to the 18-8 austenitic stainless steel family (containing 18% chromium and 8% nickel), and their chemical compositions are very similar. However, in actual engineering applications, they perform quite differently—especially in welding, corrosion resistance, and high-temperature service conditions.
For purchasing engineers, forging plant technical staff, and project managers, understanding the real differences between AISI 304 and 304L forgings directly affects equipment safety, service life, and total life-cycle cost. Based on AISI (American Iron and Steel Institute) and ASTM (American Society for Testing and Materials) standards, we will systematically explain the differences and selection logic between the two grades from the aspects of chemical composition, mechanical properties, corrosion resistance, weldability, and forging standards.
The most basic difference between AISI 304 and 304L is carbon content.
AISI 304: carbon content ≤ 0.08%
AISI 304L: carbon content ≤ 0.03% (the "L" stands for Low Carbon)
This small 0.05% difference is the real reason why these two materials behave so differently in welding, corrosion resistance, and heat treatment.
Why is carbon content so important? In austenitic stainless steel, at high temperatures, carbon combines with chromium to form chromium carbide (Cr₂₃C₆). This collects at grain boundaries. As a result, the chromium near these boundaries gets "used up," leaving areas with too little chromium. These areas can then suffer from intergranular corrosion (also called sensitization) when exposed to corrosive environments. 304L lowers carbon to below 0.03%, which basically stops chromium carbide from forming, and so avoids the sensitization problem.
Other than carbon content, both grades have 18–20% chromium, and nickel is 8–10.5% for 304 and 8–12% for 304L — all other differences in composition are very small.
Element | AISI 304 | AISI 304L |
C | ≤ 0.08% | ≤ 0.03% |
Cr | 18.0–20.0% | 18.0–20.0% |
Ni | 8.0–10.5% | 8.0–12.0% |
Mn | ≤ 2.0% | ≤ 2.0% |
Si | ≤ 1.0% | ≤ 1.0% |
P | ≤ 0.045% | ≤ 0.045% |
S | ≤ 0.030% | ≤ 0.030% |
N | ≤ 0.10% | ≤ 0.10% |
Due to its lower carbon content, the strength of 304L is slightly lower than that of 304, but the difference is not significant.
mechanical property | AISI 304 | AISI 304L |
tensile strength | ≥ 515 MPa | ≥ 485 MPa |
yield strength | ≥ 205 MPa | ≥ 170 MPa |
elongation | ≥ 40% | ≥ 40% |
Hardness (HRB) | ≤ 92 | ≤ 92 |
This is the most important engineering difference between 304 and 304L.
AISI 304, when heated to 425–815°C (for example, during welding or when used at medium temperatures), can form chromium carbides at grain boundaries. This causes intergranular corrosion (sensitization). After welding, 304 usually needs a solution heat treatment (annealing) to restore its corrosion resistance.
AISI 304L has a very low carbon content. So even during welding or short times in the sensitization temperature range, very little chromium carbide forms. After welding, 304L does not need solution treatment and can keep good resistance to intergranular corrosion.
In a real case, a chemical plant used ordinary 304 flanges for an acetic acid pipeline. Within two years, knife-line corrosion appeared along the welds. After they replaced the flanges with 304L forged flanges, the pipeline ran for five years with no corrosion or leakage found.
welding characteristic | AISI 304 | AISI 304L |
Is heat treatment required after welding? | need | need not |
Thick section / multi-pass welding | not recommend | recommend |
Intergranular corrosion risk | high | low |
304L is a low-carbon variant specifically developed for welding applications. The lower carbon content minimizes the formation of carbides in the heat-affected zone of the weld seam, making it widely used in welding components of chemical equipment, heat exchanger pipes, and other structures. When welding 304, it is also recommended to use 304L filler wire to reduce the risk of sensitization.
For stainless steel forgings, the most important ASTM standard is ASTM A965/A965M, Standard Specification for Steel Forgings, Austenitic, for Pressure and High Temperature Parts.
This standard clearly covers forging grades including F304, F304H, F304L, F304N, and F304LN. Among them:
F304 is the forging grade for AISI 304
F304L is the forging grade for AISI 304L
The standard requires that forgings be made by melting, forging, and rough machining, and be delivered in the solution-treated condition (solution annealed then quenched in water, oil, or polymer water solution). Also, forgings must be tested for chemical composition (heat analysis and product analysis) and mechanical properties (tensile strength, yield strength, elongation, and reduction of area).
In addition, ASTM A314 and ASTM A473 also cover requirements for stainless steel forging bars and billets.
The working conditions are at normal temperature and pressure, in an environment with normal corrosion or no corrosion.
It does not involve welding, or after welding, a solution treatment can be carried out.
It has high requirements for strength, and there is no need to consider the risk of intergranular corrosion.
Typical applications: countertop of food processing equipment, storage tanks (non-welded structure), architectural decorative parts, kitchen appliances, medical equipment surfaces.
Structural components involving welding, especially those with thick sections or multiple welds
Situations where post-welding annealing cannot be carried out
Operating environments with corrosive media, or equipment operating continuously within the temperature range of 425–815°C
Chemical, petroleum, and marine engineering equipment with clear requirements for resisting intergranular corrosion
Typical applications: welded pipelines, heat exchangers, reactors, chemical storage tanks, flange forgings, fermentation vessels, and components of offshore platforms
The fundamental difference between AISI 304 and 304L forged parts boils down to one factor: carbon content. This 0.05% difference determines the completely different performances of the two materials in terms of welding, corrosion resistance, and heat treatment. In engineering selection, whether welding is involved or not is the primary criterion - for any scenario involving welding and where post-weld solution treatment cannot be performed, 304L is the obvious choice; while for pure strength components that do not require welding, 304 offers greater cost advantages.
Understanding and making good use of this difference is the key to ensuring that forged parts can safely, reliably, and economically operate in the target conditions.
