L245 vs L290 – Composition, Heat Treatment, Properties, and Applications

Introduction

L245 and L290 are two commonly referenced low-alloy structural steel grades used across construction, bridgework, shipbuilding, heavy fabrication, and general structural applications. Engineers, procurement managers, and manufacturing planners weighing these two grades typically balance competing priorities such as minimum yield requirement versus weldability, toughness at low temperature versus material cost, and the need for higher hardenability versus ease of fabrication.

The primary practical distinction between the two is their specified minimum yield strength: L290 requires a higher guaranteed yield level than L245. That difference is usually achieved by alloying strategy and processing choices (microalloying, controlled carbon and manganese, and thermo-mechanical processing), which in turn influence hardenability, toughness, and fabrication behavior. Because both grades are used for similar structural roles, designers commonly compare them when specifying plates, rolled sections, and welded components where the trade-off between strength and fabrication friendliness matters.

1. Standards and Designations

• Typical standards where L-style designations appear: national and regional standards for structural steels and pressure equipment. The exact designation and chemical/mechanical requirements should be verified against the applicable standard or mill certificate for the supply region.
• Classification: Both L245 and L290 are low-alloy or carbon structural steels (not stainless, not tool steels). They are often grouped with hot-rolled structural steels intended for general welded and riveted construction.
• Commonly relevant standards and documents to consult for specific requirements:
• EN/European norms for structural steels (verify local normative designation)
• National standards (e.g., GB, JIS, ASTM/ASME may provide functional equivalents but different names)
• Supplier mill sheets and purchaser specifications (PSL, API, etc.)

2. Chemical Composition and Alloying Strategy

The L245 and L290 grades are not defined by a single unique chemistry but by a permitted chemistry window and mechanical property targets. The following table gives indicative, representative composition ranges and the typical role of each element. These numbers are guideline-level; consult the governing specification and mill certificate for exact composition.

How alloying affects properties: - Raising the carbon and manganese content increases yield and tensile strengths and hardenability but can reduce weldability and toughness unless offset by microalloying or controlled processing. - Microalloying (Nb, V, Ti) allows higher strength at lower carbon by precipitation strengthening and grain refinement—beneficial to keep weldability and toughness better than equivalent C–Mn steels strengthened by carbon alone. - L290 is generally achieved by a slightly stronger alloying and/or thermo-mechanical processing route compared with L245, producing a higher minimum yield without excessively increasing carbon.

3. Microstructure and Heat Treatment Response

Typical microstructures and processing responses for both grades: - As-rolled/normalized: ferrite–pearlite with possible bainitic fractions depending on cooling and alloying. Normalizing refines ferrite grain size, improving toughness. - Thermo-mechanically controlled processing (TMCP): produces fine-grained ferrite and locally transformed bainite/tempered martensite pockets which increase yield strength and toughness simultaneously—this route is commonly used to meet higher yield grades like L290 without high carbon. - Quenching & tempering (Q&T): not typical for standard L-series structural steels unless special mechanical properties are required; Q&T will increase strength but at the cost of increased processing complexity and potential reduction in ductility if over-tempered or if carbon is high. - Heat-affected zone (HAZ): in welded structures, HAZ properties are sensitive to carbon equivalent and microalloying content; microalloyed TMCP steels tend to have more benign HAZ behavior than high-carbon steels of equivalent nominal strength.

Comparative notes: - L245, with its lower strength target, is often achievable by conventional rolling or light TMCP, resulting in predominantly ferrite–pearlite with good ductility. - L290 more frequently relies on TMCP and microalloying to achieve higher yield while maintaining toughness; microstructure will have finer grains and a higher fraction of strengthening constituents.

4. Mechanical Properties

The definitive mechanical requirements must be read from the applicable standard or mill certificate. The single reliable differentiator in designation is minimum yield strength.

Interpretation: - L290 delivers a higher guaranteed yield strength; that is the basis for selection when design requires higher allowable stress or thinner sections for the same load. - Toughness and ductility can be comparable if L290 is produced via modern TMCP and microalloying; if higher strength is achieved by increasing carbon, toughness and weldability will suffer.

5. Weldability

Weldability depends primarily on carbon equivalent (CE) and the presence of alloying elements that promote hardenability.

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

Qualitative interpretation: - Lower $CE_{IIW}$ and lower $P_{cm}$ improve preheat/consumable selection flexibility and reduce HAZ cracking risk. - L245, with its lower yield target, often has a lower carbon equivalent and therefore tends to be easier to weld with less preheat than L290 when the latter achieves higher strength by higher alloy content. - If L290 is produced by microalloying and TMCP rather than higher carbon, weldability can remain acceptable; however, a slightly higher preheat or controlled welding procedure may still be recommended depending on thickness. - Always consult qualified welding procedure specifications (WPS) and run HAZ and PWHT assessments for critical fabrications.

6. Corrosion and Surface Protection

• These two grades are non-stainless carbon/low-alloy steels. They do not provide intrinsic corrosion resistance beyond that of ordinary structural carbon steels.
• Standard protection strategies: coatings (epoxy, polyurethane), hot-dip galvanizing, metallizing, or sacrificial coatings depending on environment and service life.
• Stainless indices such as PREN are not applicable to L-series structural carbon steels: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ Use of PREN is relevant only when assessing stainless alloys; for L245/L290, corrosion resistance is a function of external protection and environmental control.
• When specifying for aggressive environments (marine splash, chemical), consider corrosion allowances, protective coatings, or selection of stainless or corrosion-resistant alloys instead.

7. Fabrication, Machinability, and Formability

• Machinability: Generally comparable to other low-alloy steels. Higher yield grades can be slightly more difficult to machine due to increased strength and potential for work-hardening; tooling and feeds should be adjusted.
• Formability and bending: L245 will typically allow slightly easier cold forming and tighter bend radii for the same thickness compared with L290. For L290, limit bending strains according to supplier guidance and use proper mandrels/annealing if needed.
• Cutting and thermal processing: Oxy-fuel, plasma, and laser cutting are common; higher alloy content or thicker sections can affect cutting settings and dross.
• Surface preparation and welding consumables: For both grades, follow supplier recommendations for preheat, interpass temperature, and filler metal selection to maintain toughness and avoid HAZ issues.

8. Typical Applications

Selection rationale: - Choose L245 for lower cost, easier fabrication, and where the design loads are met with lower yield. - Choose L290 when design requirements need higher yield to reduce section size or weight, or when a higher safety margin is required—provided welding procedures and toughness targets can be met.

9. Cost and Availability

• Cost: L290 is generally costlier than L245 per unit mass due to tighter processing controls, microalloying, or additional heat treatments needed to guarantee higher yield levels. However, cost per functional performance (e.g., cost per unit load capacity) can be favorable for L290 if section reduction offsets material cost.
• Availability: Both grades are commercially available from major steel mills and service centers, especially in plate and rolled form. Lead times and forms offered (plates, coils, structural shapes) depend on regional mill production and demand; L245 is usually more common in general structural supply chains.

10. Summary and Recommendation

Recommendation: - Choose L245 if you prioritize fabrication ease, lower material cost, and your structural design can meet load requirements with a 245 MPa yield floor. L245 is a solid choice for general construction and for components where extensive welding and forming are required without aggressive strength needs. - Choose L290 if you need a higher guaranteed yield to reduce section size or weight, or to increase allowable stresses in structural calculations. L290 is appropriate where higher strength is required while still maintaining good toughness through modern processing (TMCP and microalloying). Ensure appropriate welding procedures, preheat, and testing are specified for thick sections or critical applications.

Final note: Always specify the governing standard, required impact test temperatures, material thickness limits, and welding procedure qualifications in the procurement documents. Verify chemistry and mechanical properties against the mill certificate and order-specific requirements before production or critical design acceptance.