COR-TEN B vs COR-TEN C – Composition, Heat Treatment, Properties, and Applications

Introduction

COR-TEN B and COR-TEN C are commercially recognized weathering steels used for structural and architectural applications where atmospheric corrosion resistance is required without continuous painting. Engineers, procurement managers, and manufacturing planners commonly face the trade-off between corrosion resistance, mechanical performance, and fabrication/workability when selecting between these two grades. Typical decision contexts include specifying material for long-life outdoor structures (balancing initial cost vs. maintenance), selecting plate for welded structural components (balancing weldability vs. strength), and choosing sheet for forming (balancing ductility vs. surface performance).

The principal practical difference between COR-TEN B and COR-TEN C is that COR-TEN C is formulated and processed to provide higher through-thickness strength and enhanced load-bearing capability (a higher-strength variant), while COR-TEN B is aimed at a balance of atmospheric corrosion resistance and excellent fabrication properties. Because both are weathering steels, they are frequently compared when designers require both durable patination and elevated mechanical performance in structural service.

1. Standards and Designations

Major standards that cover weathering and low-alloy structural steels include:

• ASTM/ASME:
• ASTM A242 (historical COR-TEN A)
• ASTM A588 (high-strength low-alloy, often associated with COR-TEN B characteristics)
• ASTM A606 (thin-gauge weathering sheet)
• EN:
• EN 10025 series for structural steels (some weathering steels specified as “Corten-type” in national annexes)
• JIS: Japanese standards include weathering steels with different trade names and classifications.
• GB: Chinese national standards include weathering steels with similar property classes.

Classification by metallurgical type: - COR-TEN B and C: HSLA (high-strength low-alloy) carbon steels with alloying additions for atmospheric corrosion resistance. - They are not stainless steels; they rely on alloying and patina formation rather than continuous passive films of chromium-rich stainless grades.

2. Chemical Composition and Alloying Strategy

The alloying strategy for weathering steels is to combine modest amounts of Cu, Cr, P, and other elements to promote a tightly adherent, stable surface patina while maintaining good fabrication behavior. COR-TEN C is generally engineered with a compositional and processing approach that elevates strength (for example, via higher microalloying or controlled carbon/hardenability) compared with COR-TEN B.

Table: qualitative chemistry indicators (presence/relative level)

Element	COR-TEN B (typical role)	COR-TEN C (typical role)
C (carbon)	Low–moderate (basic strength/ductility balance)	Moderate (slightly increased to raise strength/hardenability)
Mn (manganese)	Moderate (strength and deoxidation)	Moderate–elevated (strength, work-hardening)
Si (silicon)	Low–moderate (deoxidation, improves patina formation)	Low–moderate
P (phosphorus)	Low (sometimes intentionally present in small amounts to aid patina)	Low (controlled)
S (sulfur)	Very low (low sulfides for toughness)	Very low
Cr (chromium)	Trace–low (promotes patina stability)	Low (may be slightly higher for corrosion/strength synergy)
Ni (nickel)	Often low or absent	Low (not a defining alloying element)
Mo (molybdenum)	Typically absent or very low	Typically absent or very low
V (vanadium)	Absent or trace	Possible microalloying (to raise strength)
Nb (niobium)	Absent or trace	Possible microalloying (grain control, strength)
Ti (titanium)	Trace (deoxidation/stabilization)	Trace/microalloying possible
B (boron)	Not typical	Occasionally used in trace amounts in higher-strength variants
N (nitrogen)	Trace	Trace (if microalloyed, N interacts with Ti/V)

Explanation: Alloying elements such as Cu, Cr, and small amounts of P are central to the weathering behavior—promoting a protective, adherent oxide layer. Microalloying elements (V, Nb, Ti, B) and slightly higher carbon or manganese are the typical routes to raise yield and tensile strength in higher-strength variants like COR-TEN C without moving to stainless or heavy-alloy steels.

3. Microstructure and Heat Treatment Response

Microstructure in both grades is controlled primarily by hot-rolling and cooling practice rather than extensive heat treatment.

• COR-TEN B:
• Typical microstructure after conventional rolling/air cooling: ferrite with dispersed pearlite and fine carbides, plus microstructural refinement aimed at toughness and ductility.

• Responds well to normalizing and stress-relief; limited hardenability means standard quench-and-temper routes are not commonly used for weathering applications.

• COR-TEN C:

• Designed to achieve higher strength—microstructure may include finer ferrite-pearlite or controlled amounts of bainitic constituents if thermo-mechanical processing is used.
• Microalloying elements (Nb, V, Ti) act as precipitation-strengthening agents and grain refiners, so thermo-mechanical control (controlled rolling, accelerated cooling) produces improved strength–toughness balance.
• Quenching and tempering is generally unnecessary for typical weathering-steel service but can alter properties if needed; beware of losing corrosion behavior if high-temperature treatments change surface chemistry.

How processing routes affect properties: - Normalizing (reheating and air cooling) can homogenize and slightly refine grain size, improving toughness. - Thermo-mechanical rolling with controlled cooling increases yield strength and toughness synergy for COR-TEN C by refining grain size and precipitating microalloy carbides/nitrides. - Excessive quench-and-temper to push strength can reduce atmospheric corrosion performance if surface alloy distribution changes.

4. Mechanical Properties

Because specific numeric values depend on standard, product form, and heat treatment, the table below compares typical relative mechanical behavior.

Table: qualitative mechanical comparison

Property	COR-TEN B	COR-TEN C
Tensile Strength	Moderate	Higher
Yield Strength	Moderate	Higher (primary design advantage)
Elongation (ductility)	Higher (more ductile)	Moderate–lower (trade-off for strength)
Impact Toughness	Good (especially at ambient and subambient if specified)	Good but can be slightly reduced if strength increased
Hardness	Lower–moderate	Moderate–higher

Interpretation: COR-TEN C is intended as the higher-strength alternative; the increased strength is achieved through compositional adjustments and thermo-mechanical control. COR-TEN B typically offers greater ductility and often easier forming and consistent impact toughness across thicknesses, making it preferable when deformation or energy-absorbing capacity is a primary requirement.

5. Weldability

Weldability is a key consideration in structural applications. Factors include carbon equivalent, hardenability from microalloying, and residual elements that influence hydrogen cracking susceptibility.

Useful weldability indices: - Carbon equivalent IIW: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Pcm for assessing cold cracking tendency: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - COR-TEN B: Lower carbon and limited microalloying typically give a lower $CE_{IIW}$ and $P_{cm}$, translating to excellent general weldability with standard filler metals and common procedures. Preheat and interpass control are normally modest. - COR-TEN C: Slightly higher carbon and possible microalloy additions can increase hardenability. This raises $CE_{IIW}$ and $P_{cm}$ relative to COR-TEN B, implying that preheat, controlled heat input, and hydrogen control should be used more conscientiously — especially in thicker sections — to avoid hard, brittle heat-affected zones and cold-cracking risks.

Practical guidance: - Use low-hydrogen consumables, control heat input, and apply preheat when warranted by thickness and local welding codes. - Match filler chemistry to ensure compatible corrosion behavior in the welded zone (filler metals with adequate Cu/Cr content for weathering performance are often recommended).

6. Corrosion and Surface Protection

Both COR-TEN B and C rely on forming a protective patina (oxide layer) when exposed to alternate wetting and drying in atmospheres containing oxygen and pollutants. They are not stainless steels; therefore, surface preparation and environmental conditions determine patina development.

• Surface protection options for non-stainless weathering steels:
• Allow natural patination in suitable environments (rural, urban, industrial variations affect rate and quality).
• Protective coatings (painting) or galvanizing can be applied when immediate protection is required, but coating adherence to the patina should be considered.
• Cathodic protection or sacrificial coatings are alternatives in aggressive marine or chloride-rich environments.

PREN (Pitting Resistance Equivalent Number) applies to stainless alloys where chromium, molybdenum, and nitrogen dominate pitting resistance: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ This index is not applicable to COR-TEN B/C because they are not stainless steels and do not rely on passive chromium-rich films. Use PREN only when evaluating stainless materials.

When indices are not applicable: - For weathering steels, the relevant metrics are long-term atmospheric exposure tests, salt-spray results for comparative assessment, and empirical service histories rather than PREN.

7. Fabrication, Machinability, and Formability

• Forming and bending:
• COR-TEN B: Generally easier to form due to lower yield and higher elongation; suitable for complex shapes in thinner gauges.
• COR-TEN C: Higher yield and slightly reduced ductility mean forming limits are reduced; springback can be greater and may require higher forming forces.
• Machinability:
• Both steels machine similarly to other low-alloy carbon steels; COR-TEN C's higher strength may slightly increase cutting forces and tool wear.
• Surface finishing:
• Avoid surface contamination that could alter patina performance (e.g., grease, oils, galvanic couples).
• Machining chips and burrs should be removed to ensure consistent patination.

8. Typical Applications

COR-TEN B (typical uses)	COR-TEN C (typical uses)
Architectural façades, sculptures, and cladding where patina and formability are priorities	Bridges, heavy structural members, and load-bearing plates where higher yield strength is required
Light structural components, guardrails, and signage	Crane rails, high-load structural sections, and primary framing in civil structures
Thin-gauge weathering sheet for enclosures and facades	Heavy plates and rolled sections where improved strength-to-weight is desired

Selection rationale: Choose COR-TEN B where ease of fabrication, forming, and consistent patina are priorities and loads are moderate. Choose COR-TEN C where higher structural capacity per unit area is needed and fabrication plans account for the slightly higher demands on welding and forming.

9. Cost and Availability

• Relative cost:
• COR-TEN B: Typically lower cost in many markets because it is closer to conventional weathering grades and uses fewer microalloying elements.
• COR-TEN C: Slightly higher cost due to alloying/processing and the value assigned to higher-strength product forms.
• Availability:
• Both grades are commonly available in plate, sheet, and structural sections, but specific availability depends on mill production, regional demand, and product thicknesses. Higher-strength COR-TEN C in heavy plate forms may be more specialized and have longer lead times in some markets.

Procurement tip: Specify product form (plate vs. sheet), required mechanical properties, and welding/fabrication constraints early in procurement to get accurate lead times and pricing.

10. Summary and Recommendation

Table: quick comparison

Characteristic	COR-TEN B	COR-TEN C
Weldability	Very good (lower preheat needs)	Good, but greater attention to preheat/heat input
Strength–Toughness balance	Good toughness, moderate strength	Higher strength, good toughness if processed correctly
Cost	Lower	Moderate–higher

Conclusion and recommendations: - Choose COR-TEN B if you need a weathering steel with excellent formability, easier welding procedures, consistent patination, and lower material cost — suitable for façades, thin-gauge architectural elements, and moderately loaded structural applications. - Choose COR-TEN C if the primary requirement is higher yield and tensile strength in a weathering steel—suitable for heavy structural plates, bridges, and components where reduced section size or improved load capacity is required and where fabrication procedures accommodate slightly higher hardenability and preheat needs.

Final practical notes: - Always specify the intended environment and required performance (mechanical limits, impact toughness, and corrosion exposure class) in procurement documents. - Work with the steel supplier to confirm mill processing (i.e., thermo-mechanical control, normalizing) because processing choices significantly affect the final strength–toughness–corrosion balance. - For welded structures, include welding procedure specifications that account for $CE_{IIW}$ and $P_{cm}$ implications and select filler metals compatible with weathering performance.