L290 vs L360 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
Engineers, procurement managers, and manufacturing planners commonly face the choice between L290 and L360 when specifying structural steel for frames, bridges, offshore structures, and heavy fabrication. The decision often balances higher strength versus formability and weldability: higher-strength alloys can reduce section size and weight but may impose tighter fabrication controls and higher cost.
The fundamental distinction between L290 and L360 is a step up in guaranteed minimum strength: L360 provides a higher yield-strength class than L290. Because that strength increase is usually achieved through alloy design and thermomechanical processing, the two grades are compared routinely for trade-offs in toughness, weldability, fabrication, and cost.
1. Standards and Designations
- Common standards and systems referenced by engineers:
- EN / ISO (European / international structural steel standards)
- ASTM / ASME (U.S. material specifications; different nomenclature)
- JIS (Japanese industrial standards)
- GB (Chinese national standards)
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National shipbuilding or pipeline specifications that use "L" prefixes for linear yield classes
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Classification:
- L290 and L360 are structural low-alloy / high-strength low-alloy (HSLA) steels rather than stainless, tool, or high-alloy steels.
- They are typically specified by minimum yield strength (MPa) and by product form (plate, sheet, section, or hollow section).
- Note: "L" labels denote minimum yield levels in some national/specification systems rather than a single, unified chemical specification; exact composition limits may vary by supplier and standard.
2. Chemical Composition and Alloying Strategy
Below is a representative composition table for steels in the 290–360 MPa yield-strength class. These are typical ranges for modern HSLA/structural steels; exact limits are found in specific standards or mill certificates.
| Element | Typical range (wt.%, representative for L290–L360 HSLA steels) |
|---|---|
| C (Carbon) | 0.06 – 0.18 |
| Mn (Manganese) | 0.3 – 1.5 |
| Si (Silicon) | 0.02 – 0.6 |
| P (Phosphorus) | ≤ 0.035 (controlled to low ppm) |
| S (Sulfur) | ≤ 0.035 (often ≤ 0.010 in low-sulfur grades) |
| Cr (Chromium) | 0 – 0.5 |
| Ni (Nickel) | 0 – 0.5 |
| Mo (Molybdenum) | 0 – 0.5 |
| V (Vanadium) | 0 – 0.12 |
| Nb (Niobium) | 0 – 0.08 |
| Ti (Titanium) | 0 – 0.02 |
| B (Boron) | 0 – 0.003 |
| N (Nitrogen) | 0.005 – 0.020 |
How alloying affects properties - Carbon and manganese primarily control strength and hardenability; higher C increases strength but reduces weldability and ductility. - Microalloying elements (V, Nb, Ti) produce precipitation strengthening and refine grain size, enabling higher yield at low carbon levels and improving toughness. - Small additions of Cr, Ni, and Mo can increase hardenability and strength without large carbon increases; they also affect tempering behavior. - Boron in very low ppm improves hardenability by segregating to austenite grain boundaries when carefully controlled. - Control of P, S, and N is crucial for toughness and weldability.
3. Microstructure and Heat Treatment Response
Typical microstructures - L290: produced to achieve a balance of ductility and strength. Typical as-rolled/normalized microstructure is a ferrite–pearlite or fine ferrite with dispersed bainite depending on cooling rate and alloy content. - L360: to reach the higher minimum yield, microalloying (Nb, V) and controlled rolling or thermomechanical treatment are commonly used to produce finer ferrite, bainite, or a mixed ferrite–bainite microstructure. Increased hardenability can lead to a larger proportion of bainitic microconstituents.
Response to processing routes - Normalizing: raises toughness by producing a fine, uniform grain structure; both grades benefit, but L360 often requires tighter control of cooling rates to avoid excessive hardness. - Quenching & tempering (Q&T): not typical for basic structural product forms, but possible if higher strength and toughness combinations are required—Q&T produces martensitic-tempered structures and higher strength at the cost of more processing. - Thermo-mechanical control processing (TMCP): widely used for L360 to obtain higher yield strength via grain refinement and precipitation strengthening without large carbon increases — improves toughness and weldability relative to carbon-strengthened steels.
4. Mechanical Properties
The essential, guaranteed mechanical difference is minimum yield strength. Absolute numbers depend on the specific standard, product thickness, and heat treatment.
| Property | L290 (typical/nominal) | L360 (typical/nominal) |
|---|---|---|
| Minimum yield strength (MPa) | ~290 MPa (design class) | ~360 MPa (design class) |
| Tensile strength (MPa) | Varies with thickness/process; typical lower-to-mid range for structural steels | Higher typical tensile range than L290 for same product form |
| Elongation (%) | Generally higher than L360 at equal thickness | Typically lower than L290 due to higher strength |
| Charpy impact toughness | Good at room and subzero temperatures when specified/controlled | Can match L290 if specified; requires controlled chemistry and processing |
| Hardness (HB) | Lower on average | Higher on average |
Interpretation - L360 is stronger by design; that strength increase is often achieved with microalloying and thermomechanical processing rather than large carbon increases. Therefore, L360 can provide higher strength with reasonable toughness, but ductility and formability are generally reduced compared with L290. - For applications where deformation capacity and forming are primary concerns, L290 is often preferred. For weight-sensitive designs or higher load-carrying capacity, L360 enables thinner sections or reduced material usage.
5. Weldability
Key factors: carbon content, carbon equivalent, and microalloying.
Common carbon-equivalent formula (useful for qualitative weldability assessment): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
A 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 - Because manufacturers commonly keep carbon low in both classes and use microalloying to raise yield strength, both grades are generally weldable with proper preheat, interpass, and filler selection. - L360, having higher hardenability (from Mn, microalloying, or small alloy additions), is more sensitive to hydrogen-assisted cold cracking and may require higher preheat or controlled cooling to avoid martensite formation in the heat-affected zone (HAZ). - Use of low-hydrogen consumables, appropriate preheat/interpass temperatures, and post-weld heat treatment (as required by contract/spec) addresses cracking risk. - Always calculate or estimate $CE_{IIW}$ or $P_{cm}$ for the specific mill certificate composition to determine allowable welding procedures.
6. Corrosion and Surface Protection
- These grades are not stainless steels; corrosion performance is dependent on environment and surface protection.
- Typical protection strategies:
- Hot-dip galvanizing for atmospheric corrosion resistance.
- Paint systems (zinc primers, epoxies, polyurethanes) for long-term protection.
- Metallurgical coatings (thermal spray) for abrasion plus corrosion.
- PREN (pitting resistance equivalent number) is not applicable to carbon/HSLA steels, as it is used for stainless alloys: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
- For L290 and L360, corrosion allowance or protective coatings are the standard approach; selection depends on service environment (marine, industrial, chemical exposure).
7. Fabrication, Machinability, and Formability
- Cutting: Plasma, oxy-fuel, and laser cutting are routine for both grades, with thicker L360 requiring more attention to edge hardening when cutting fast.
- Machinability: Lower carbon and microalloyed compositions yield moderate machinability; L360 (higher strength) typically machines slightly harder than L290—tool wear and cutting forces increase.
- Formability and bending: L290 exhibits better bendability and cold-forming capacity at equal thickness. L360 demands larger bend radii, bending force, and sometimes intermediate annealing for severe forming.
- Surface finishing and welding preparations are similar; L360 may need more stringent control of fit-up to avoid localized strain concentrations.
8. Typical Applications
| L290 — Typical uses | L360 — Typical uses |
|---|---|
| General structural steelwork (light to moderate loads) | Heavier structural members where higher load capacity or reduced section thickness is required |
| Building construction members, subframes, and secondary steelwork | Bridges, heavy cranes, main girders, and structural members for offshore platforms |
| Agricultural equipment, general fabrication | Ship hull plating, pressure-retaining structural sections where higher strength-to-weight is needed |
| Applications where formability and low-cost fabrication are prioritized | Fabrications where weight saving, higher allowable stress, or improved span capability are critical |
Selection rationale - Choose L290 when fabrication speed, forming, and cost sensitivity outweigh the benefit of smaller cross-sections. - Choose L360 when structural efficiency, weight reduction, or higher allowable stresses are prioritized and the fabrication team can manage the tighter welding and forming controls.
9. Cost and Availability
- Cost: L360 is generally more expensive than L290 due to additional alloying control, thermomechanical processing, and tighter quality controls. The price difference varies with market conditions and product form.
- Availability: Both grades are widely produced in plates, coils, and sections, but local supply depends on mill capabilities. L290 is often more common in commodity structural markets; L360 may be more available from mills targeting heavy construction, bridge, and offshore markets.
- Lead times can increase for L360 in large-quantity or thick-plate orders, particularly when specific toughness or chemical controls are required.
10. Summary and Recommendation
| Criterion | L290 | L360 |
|---|---|---|
| Weldability | Good—more forgiving due to lower hardenability | Good with controls—higher HAZ hardenability requires stricter procedures |
| Strength–Toughness balance | Moderate strength with relatively higher ductility | Higher strength with good toughness if properly processed |
| Cost | Lower (general structural grade) | Higher (HSLA processing and controls) |
Recommendation - Choose L290 if: you need a cost-effective, easily formed and welded structural steel for moderate load-bearing members where maximizing ductility and ease of fabrication are priorities. - Choose L360 if: you need higher guaranteed yield strength to reduce section size or weight, and you can implement controlled welding, forming practices, and possibly slightly higher material cost to gain structural efficiency.
Final note: Always review the supplier’s mill certificate and the applicable standard or specification for the precise chemical limits, mechanical guarantees, thickness-dependent data, and welding recommendations. When in doubt, request specific composition and heat-treatment records and perform application-specific weldability and toughness assessments.