L360 vs L390 – Composition, Heat Treatment, Properties, and Applications

Introduction

L360 and L390 are closely related high‑strength structural steels commonly specified where designers balance strength, toughness, weldability, and cost. Engineers, procurement managers, and manufacturing planners frequently face the decision whether to use the slightly lower‑strength, more forgiving grade (L360) or the incrementally stronger L390 when designing load‑bearing components, welded assemblies, or fabricated structures.

The principal technical difference is a modest, deliberate increase in yield (and often tensile) strength from L360 to L390 achieved primarily by thermomechanical processing and microalloying rather than dramatic changes in bulk chemistry. Because both grades target structural applications, they are often compared when optimizing member weight, plate thickness, forming behaviour, and fabrication procedures.

1. Standards and Designations

• Common standards where analogous grade families appear: EN (e.g., EN 10025 family), ISO, ASTM/ASME (structural designations), JIS, and national standards (GB for China). Exact designation strings vary by standards body and supplier.
• Classification: Both L360 and L390 are high‑strength low‑alloy (HSLA) structural steels (not stainless, not tool steels). They are intended for welded and formed structural components.

Note: Specific standard numbers and mill‑certified compositions differ by region; always use the exact certified grade/standard in procurement documents.

2. Chemical Composition and Alloying Strategy

Notes: - These ranges are representative of HSLA structural steels and illustrate typical alloying strategies. Exact compositions are mill‑specific and governed by the chosen standard or specification; always verify with mill certificates. - Microalloying elements (V, Nb, Ti, and sometimes B) are used in small amounts to refine grain size, promote precipitation hardening, and raise yield strength with minimal increase in carbon content—important for maintaining weldability.

How alloying affects properties: - Carbon increases strength but degrades weldability and toughness when elevated. - Manganese and silicon assist deoxidation and contribute to hardenability. - Microalloying (V, Nb, Ti) allows strength increases via precipitation and grain refinement without high carbon—this is why L390 can be stronger with only minor chemical differences from L360. - Low phosphorus and sulfur control improves toughness and reduces weld defects.

3. Microstructure and Heat Treatment Response

• Typical microstructure for both grades in delivered (thermomechanically rolled or normalized) condition: ferrite matrix with fine, dispersed bainitic or tempered martensite islands and microalloy precipitates. Grain size is refined by controlled rolling and accelerated cooling.
• L360: Processed to achieve a balance of ductile ferrite and fine bainite; microalloy precipitates (NbC, V(C,N), TiN) strengthen the matrix.
• L390: Tends to use slightly more aggressive thermomechanical control (lower finish rolling temperature and faster cooling) and targeted precipitation strengthening to raise yield strength while maintaining similar ductile microstructure.

Heat treatment response: - Normalizing: Restores uniform microstructure and can improve toughness; both grades respond predictably. - Quench & temper: Not typical or necessary for routine structural supply; when applied, higher tempering control is needed to avoid over‑tempering microalloy precipitates. - Thermomechanical control processing (TMCP): Primary industrial route to produce these grades — controlled rolling plus accelerated cooling gives the desired strength/toughness without post‑weld heat treatment in most cases.

4. Mechanical Properties

Interpretation: - L390 provides a modest but useful increase in yield strength over L360; tensile strength typically increases proportionally. - Ductility and toughness can remain similar between grades if L390 is produced with appropriate TMCP and microalloy balance. However, designers should expect marginally reduced elongation and slightly higher hardness for L390, making forming limits tighter. - Always reference specific supply condition (plate thickness, processing route, testing temperature) for exact values.

5. Weldability

Weldability assessment centers on carbon equivalent and processing controls. Microalloying helps keep carbon equivalents low for a target strength.

Common weldability indices: - IIW carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - The more comprehensive parameter: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - Both L360 and L390 are engineered to have relatively low $CE_{IIW}$ and $P_{cm}$ compared with higher carbon alloys. Microalloyed grades typically show good weldability when standard precautions are followed. - L390 may require slightly more attention for thicker sections (preheat, controlled interpass temperature) because higher hardenability and strength can raise the risk of cold‑crack susceptibility in heavy sections or poorly prepared joints. - Welding consumables: choose low‑hydrogen electrodes/fluxes and matching toughness filler metals; follow supplier preheat and interpass recommendations. - Post‑weld heat treatment is rarely required for ordinary structural applications, but it may be specified for critical low‑temperature or large/thick structures.

6. Corrosion and Surface Protection

• These grades are carbon/HSLA steels—not stainless. The corrosion resistance is that of ordinary carbon steels.
• Standard protection options: hot‑dip galvanizing, zinc metallizing, painting/coating systems, epoxy/organic coatings, or cathodic protection for buried or submerged service.
• PREN (pitting resistance equivalent number) and similar stainless indices do not apply for L360/L390 because they are not stainless alloys. For reference, stainless selections use: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• For atmospheric environments, galvanized L390 will provide similar protection to galvanized L360; selection should be driven by mechanical requirements and coating life targets.

7. Fabrication, Machinability, and Formability

• Cutting (flame, plasma, laser): both grades behave similarly; L390's slightly higher strength may require modestly more power or slower cutting speeds.
• Forming and bending: L360 offers slightly better cold formability because of lower yield; L390 may require larger bend radii or warm forming for tight bends, particularly in thicker sections.
• Machinability: Both are typical of low‑carbon HSLA steels—good machinability but not as free‑cutting as leaded steels. L390’s higher strength can slightly reduce tool life or require more cutting force.
• Surface finish and grinding: Both respond well to standard finishing practices; note higher hardness areas (e.g., heat‑affected zones) may need dressing or specific grinding parameters.

8. Typical Applications

Selection rationale: - Choose L360 where forming, bending, and ease of welding are priorities and small sacrifices in weight are acceptable. - Choose L390 when the incremental strength enables thinner or lighter designs and when manufacturers can meet more stringent processing controls to maintain toughness.

9. Cost and Availability

• Relative cost: L390 is typically slightly more expensive than L360 due to tighter process control (TMCP), and sometimes higher microalloy content and processing yield losses. The unit material cost difference is modest compared to overall fabrication savings from reduced thickness.
• Availability: Both are commonly available in plate and coil from major mills in many regions, but availability depends on local mills’ product ranges. L360 variants are often more ubiquitous; L390 may be a specialty offering in some markets or require minimum order quantities.
• Product forms: plate, coil, hot‑rolled sections. Lead times and mill testing (charpy, tensile) should be specified in purchase orders.

10. Summary and Recommendation

Recommendations: - Choose L360 if: - The design prioritizes ease of forming and welding, and tight bend radii or high elongation are required. - Project supply chains favor readily available, lower‑cost plate and coil. - Weight savings are not a primary driver.

• Choose L390 if:
• A modest increase in allowable stress or reduced plate thickness will create cost or weight savings in the assembly.
• Fabrication shops can maintain recommended preheat/interpass controls for thicker weldments.
• The project demands higher nominal yield strength while retaining acceptable weldability and toughness.

Final note: Because both grades are part of the HSLA family and differ primarily by processing and microalloy optimization rather than radically different chemistries, the practical choice often depends on structural calculations, forming constraints, and supply considerations. For critical applications (low‑temperature service, heavy welded structures), always specify required Charpy temperatures, thickness effects, and request mill test certificates to confirm the delivered chemistry and mechanical data.