SA387 11CL2 vs 22CL2 – Zusammensetzung, Wärmebehandlung, Eigenschaften und Anwendungen

Introduction

SA‑387 (also referenced as ASTM A387) is a family of chromium‑molybdenum low‑alloy steels for pressure‑containing parts in elevated‑temperature service. Engineers and procurement teams frequently weigh the tradeoffs between two commonly specified members: 11CL2 (often called P11 type) and 22CL2 (often called P22 type). Typical decision contexts include selecting the minimum alloying for acceptable creep strength at temperature, balancing weldability and post‑weld heat treatment demands, or optimizing procurement cost versus lifecycle performance.

The primary practical contrast between these grades is their alloy content targeted at high‑temperature strength and creep resistance: the higher‑chromium, higher‑molybdenum composition of the 22CL2 grade delivers greater strength and creep capability at elevated temperatures, while 11CL2 offers lower alloy content that improves weldability and reduces material cost. Because both are designed for elevated‑temperature pressure applications, they are commonly compared when designers choose materials for boilers, heat exchangers, piping, and pressure vessels operating across similar temperature ranges.

1. Standards and Designations

• Major standards:
• ASTM/ASME: SA‑387 / A387 (Grades 11, 22; Classes 1, 2, etc.)
• EN: Equivalent designations often fall into P‑grades (e.g., P11 / P22) or EN 10222/10028 equivalents depending on product form.
• JIS/GB: National standards may specify comparable Cr‑Mo steels with different grade codes.
• Material type:
• Both SA387 11CL2 and 22CL2 are low‑alloy, chromium‑molybdenum steels intended for elevated‑temperature service. They are not stainless steels and are not HSLA in the sense of microalloyed high‑strength structural steels; they are heat‑resisting pressure‑vessel alloys with intentional Cr and Mo additions.

2. Chemical Composition and Alloying Strategy

The table below gives representative typical ranges (wt%) encountered in SA‑387/A387 Grade 11 Class 2 and Grade 22 Class 2 specifications or common mill practice. Exact contractual chemistry should always be taken from the mill certificate or the applicable edition of the standard.

How the alloying affects performance: - Chromium (Cr) increases oxidation resistance and high‑temperature strength and contributes to hardenability. - Molybdenum (Mo) increases creep strength and stabilizes carbides at elevated temperature; Mo also promotes hardenability. - Carbon raises strength but reduces weldability and toughness if excessive; both grades maintain low carbon to balance room‑temperature toughness and weldability. - Manganese and silicon are deoxidizers and contribute to strength. - Trace microalloying elements (V, Nb, Ti) when present in small amounts influence grain size control and precipitation strengthening, but SA‑387 grades are principally strengthened by Cr/Mo chemistry and heat treatment.

3. Microstructure and Heat Treatment Response

Typical processing and microstructure: - As‑rolled or normalized and tempered, both grades develop a tempered bainitic/tempered martensitic microstructure depending on cooling rate and alloy content. - 11CL2 (lower Cr/Mo): tends to transform at higher temperatures and displays somewhat coarser carbides after tempering compared with 22CL2; microstructure is sufficient for moderate creep service. - 22CL2 (higher Cr/Mo): shows higher hardenability and forms a finer distribution of tempered carbides that are more effective at resisting creep and softening at elevated temperature.

Influence of heat treatments: - Normalizing and tempering (common route): refines grain size and produces a tempered martensite/bainite mix with improved toughness and strength balance. - Quenching and tempering: may be used for higher strength requirements, but both grades are typically used in normalized/tempered condition for pressure vessel service. - Thermo‑mechanical processing (controlled rolling): can improve yield strength and toughness by grain refinement and precipitation control; more effective in 22CL2 due to alloying but requires tight process control. - Post‑weld heat treatment (PWHT): required for many pressure applications to temper the heat‑affected zone and restore toughness; PWHT schedules depend on thickness, design code, and alloy content.

4. Mechanical Properties

The following table provides a qualitative comparison of common mechanical performance characteristics under standard heat‑treated conditions (normalized and tempered). Absolute values depend on thickness, exact chemistry, and heat treatment; consult the material certificate and design code for minimum guaranteed numbers.

Why these differences occur: - The higher Cr and Mo in 22CL2 increase hardenability and precipitation of stable carbides that maintain strength at temperature. This increases tensile and yield strength, particularly in the creep regime, at the cost of modest reductions in ductility unless tempering and heat treatment are optimized. - 11CL2's lower alloy content results in slightly better weldability and often higher measured elongation at room temperature.

5. Weldability

Key weldability factors: - Carbon equivalent and hardenability govern susceptibility to cold cracking and the need for preheat and PWHT. - Alloy content (Cr, Mo) increases hardenability; thus 22CL2 usually requires more conservative preheat and PWHT practices than 11CL2 for equivalent thickness.

Useful empirical formulas (interpret qualitatively; no numeric calculation provided): - International Institute of Welding Carbon Equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Dearden & O'Neill or Pcm (expresses weld cracking susceptibility): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Interpretation: - 22CL2 will generally produce a higher $CE_{IIW}$ and $P_{cm}$ than 11CL2 because of its higher Cr and Mo, implying more stringent welding controls (preheat, interpass temperature, PWHT). - Both grades are weldable with appropriate procedures; common practice is to apply preheat and mandatory PWHT per ASME Boiler & Pressure Vessel Code for pressure parts, with PWHT temperatures and durations selected to relieve hydrogen‑induced and residual stresses and to temper the HAZ.

6. Corrosion and Surface Protection

• Neither grade is stainless; both are susceptible to general corrosion and localized corrosion in aggressive environments.
• Protection strategies: painting, epoxy linings, polymer coatings, and galvanizing (depending on design temperature and service) are typical surface protection methods. For high‑temperature oxidation/scale resistance, the Cr content in 22CL2 offers somewhat improved scale resistance compared with 11CL2, but neither replaces stainless steels for corrosive service.
• PREN (pitting resistance equivalent number) is not applicable to these non‑stainless steels, however for completeness: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• Use corrosion‑resistant alloys or cladding/lining when chemical attack or high local corrosion risk exists; for many boiler and steam applications, corrosion concerns are addressed by water chemistry control rather than relying on base‑metal corrosion resistance.

7. Fabrication, Machinability, and Formability

• Machinability: both grades machine readily in normalized/tempered conditions. 11CL2’s slightly lower alloy content can offer marginally better ease of machining; 22CL2 may require more frequent tool changes when machining to the same hardness.
• Formability/bending: both can be formed when supplied in the normalized condition; minimum bend radii and forming temperatures should follow standard practices for tempered Cr‑Mo steels. Higher hardenability in 22CL2 can increase the risk of cracking during cold forming of thicker sections.
• Surface finishing: both accept common finishing methods (grinding, shot blasting, coating). Heat treatment after forming or welding is often required to meet toughness and stress requirements.

8. Typical Applications

Selection rationale: - Choose 11CL2 when operating temperatures and stresses fall within its safe design envelope, and when minimizing alloy cost and easing fabrication/welding are desired. - Choose 22CL2 when anticipated service demands (higher temperature, higher sustained stress, thicker sections) require improved creep and elevated‑temperature strength.

9. Cost and Availability

• Cost: 22CL2 is typically higher in material cost than 11CL2 due to larger Cr and Mo content. The delta varies with market prices for alloying elements.
• Availability: Both grades are widely produced in plate and forgings for pressure equipment; availability by product form (plate, forgings, pipe) and lead time depends on mill schedules and regional demand. 11CL2 is often more commonly available in a broad range of sizes due to its wider use in cost‑sensitive applications.

10. Summary and Recommendation

Conclusions and practical recommendations: - Choose 11CL2 if you need a cost‑effective Cr‑Mo pressure steel for moderate elevated‑temperature service where easier welding and lower alloy cost are priorities, and where design stresses and temperatures are within the allowable limits for Grade 11 material. - Choose 22CL2 if the application requires enhanced creep resistance and higher sustained strength at elevated temperatures (or thicker sections where hardenability is needed), and you can accommodate more rigorous welding procedures and a higher material cost.

Final notes: - Always verify the exact chemistry and the guaranteed mechanical properties on the mill certificate for the lot being procured. - Follow applicable design codes (ASME Section II/Code Case, ASME BPVC, EN standards) for allowable stresses, required PWHT, and testing. Selection should be made in the context of operating temperature, pressure, expected life, welding procedure specifications (WPS/PQR), and in‑service inspection plans.