SA387 Gr11 vs Gr22 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
SA387 Grade 11 and Grade 22 are two closely related chromium–molybdenum (Cr–Mo) pressure‑vessel steel grades widely specified for elevated‑temperature service in boilers, pressure vessels, and petrochemical equipment. Engineers, procurement managers, and fabricators often weigh tradeoffs between material cost, weldability, toughness, and elevated‑temperature strength when selecting between these grades. Typical decision contexts include piping and vessel components exposed to high steam temperatures, heat exchangers, and components where thermal creep resistance and hardenability are important.
The principal difference between the two grades is the amount of chromium and molybdenum alloying: Grade 11 is a lower‑Cr/Mo variant intended for moderate elevated temperatures, while Grade 22 contains higher Cr and Mo to provide greater strength and creep resistance at higher temperatures. Because of that, Gr22 is specified where higher long‑term strength and hardenability are required; Gr11 is selected where slightly lower cost and easier welding are priorities, while still providing good elevated‑temperature properties.
1. Standards and Designations
- Major standards and designations:
- ASTM/ASME: SA387 (plate) — Grade 11 (often abbreviated Gr11) and Grade 22 (Gr22). Related piping specs: ASTM A335/ASME SA335 P11 and P22 (pipes).
- EN: Comparable alloys exist in EN standards for alloy steels for pressure equipment (selection requires cross‑reference because direct one‑to‑one mapping is not exact).
- JIS / GB: Japanese and Chinese standards have similar 1.25Cr–0.5Mo and 2.25Cr–1Mo families, but check national designations for equivalence.
- Classification: Both SA387 Gr11 and Gr22 are alloy steels (Cr–Mo ferritic steels) used for elevated temperature service; they are not stainless steels, carbon steels, nor HSLA in the modern sense.
2. Chemical Composition and Alloying Strategy
The following table lists typical composition ranges commonly associated with SA387 Gr11 and Gr22. These are representative ranges from industry practice and standard cross‑references; always verify with the mill test certificate for exact values.
| Element | Typical range — Gr11 (approx.) | Typical range — Gr22 (approx.) |
|---|---|---|
| C | 0.08 – 0.15 wt% | 0.08 – 0.15 wt% |
| Mn | 0.30 – 0.70 wt% | 0.30 – 0.70 wt% |
| Si | 0.10 – 0.50 wt% | 0.10 – 0.50 wt% |
| P (max) | ≤ 0.035 wt% | ≤ 0.035 wt% |
| S (max) | ≤ 0.035 wt% | ≤ 0.035 wt% |
| Cr | ~1.0 – 1.4 wt% (nominal ~1.25%) | ~2.0 – 2.5 wt% (nominal ~2.25%) |
| Mo | ~0.40 – 0.65 wt% (nominal ~0.50%) | ~0.85 – 1.15 wt% (nominal ~1.00%) |
| V, Nb, Ti, B | Typically present only as residual/trace microalloying (≤0.05 wt% each) | Typically present only as residual/trace microalloying (≤0.05 wt% each) |
| N | Trace | Trace |
How the alloying strategy affects behavior: - Chromium increases hardenability, strength at temperature, and oxidation resistance; higher Cr in Gr22 gives greater high‑temperature strength and creep resistance than Gr11. - Molybdenum contributes to creep strength and hardenability; the higher Mo in Gr22 amplifies the effect. - Carbon, manganese, and silicon set base strength and hardenability; carbon raises strength but can reduce weldability and toughness if excessive. - Minor microalloying (V, Nb, Ti) refines grain size and can improve creep strength when present intentionally, but typical SA387 plates generally rely on Cr–Mo balance rather than heavy microalloying.
3. Microstructure and Heat Treatment Response
Typical microstructures: - In normalized and tempered (or quenched‑and‑tempered) condition both grades are ferritic matrix with tempered martensite or bainitic constituents depending on the cooling rate and alloy content. Gr11, with lower hardenability, tends to form finer tempered martensite/bainite with appropriate heat treatment; Gr22, being more highly alloyed, achieves higher hardenability and retains more hardenable microstructure after similar cooling.
Heat treatment routes and effects: - Normalizing: Refines grain size and homogenizes microstructure; both grades benefit from normalization to improve toughness prior to tempering. - Quenching & tempering (Q&T): For SA387 plates intended for high strength, a Q&T route increases yield and tensile strength; tempering reduces residual stresses and improves toughness. Gr22 achieves higher tempered strength at comparable tempering temperatures due to higher Cr–Mo content. - Thermo‑mechanical processing: Controlled rolling and accelerated cooling can further refine grain size and improve toughness; effect is similar conceptually for both grades but Gr22’s higher hardenability makes microstructure control more sensitive to cooling rate.
Practical notes: - Tempering temperature selection is critical to balance high‑temperature strength and toughness. Over‑tempering reduces strength; under‑tempering can leave brittle martensite. Gr22 often requires careful tempering to meet both creep and toughness targets.
4. Mechanical Properties
Mechanical performance depends on heat treatment, thickness, and testing temperature. The following table gives qualitative comparisons and typical directional behavior rather than precise guaranteed values; project specifications and mill certificates govern contractual values.
| Property | SA387 Gr11 (directional) | SA387 Gr22 (directional) |
|---|---|---|
| Tensile strength | Moderate; adequate for moderate elevated temperatures | Higher; improved tensile strength due to higher Cr–Mo |
| Yield strength | Moderate | Higher |
| Elongation (ductility) | Typically slightly higher (more ductile) | Slightly lower than Gr11 at equivalent temper |
| Impact toughness (RT and low T) | Good when normalized/tempered; toughness easier to achieve due to lower hardenability | Good when properly heat treated, but more sensitive to heat input and cooling rate |
| Hardness (HRC/Brinell) | Moderate | Higher potential hardness after hardening/tempering |
Why these differences occur: - Higher Cr and Mo in Gr22 increase hardenability and temper‑harden response, producing higher strength and hardness. However, increased hardenability tends to reduce ductility and can make achieving uniform toughness more demanding, particularly in thick sections or with improper heat treatment.
5. Weldability
Weldability considerations include carbon content, hardenability, and the presence of alloying elements that promote martensite formation in the heat‑affected zone (HAZ).
Key concepts and predictive formulas: - Use carbon equivalent formulas to estimate preheating and post‑weld heat treatment (PWHT) needs. A commonly used carbon‑equivalent formula is: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - A more conservative parameter for structural steels is the weldability parameter $P_{cm}$: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation for SA387: - Gr22 has higher Cr and Mo, increasing $CE_{IIW}$ and $P_{cm}$ relative to Gr11. This means Gr22 is more susceptible to HAZ hardening and potential cold cracking if welded without appropriate precautions. - Practical implications: - Preheating often required for both grades on thicker sections or low ambient temperatures, but Gr22 typically requires higher preheat and stricter control. - PWHT (stress‑relief tempering) is commonly mandated for Cr–Mo steels used at elevated temperatures to temper the HAZ and restore creep resistance; the PWHT temperature and hold time are normally defined by code (e.g., ASME) and service requirements. - Use low‑hydrogen welding consumables and qualified procedures; control interpass temperature and heat input to avoid excessive HAZ hardness.
6. Corrosion and Surface Protection
- Both Gr11 and Gr22 are non‑stainless alloy steels and therefore require surface protection for atmospheric, chemical, or marine corrosion environments.
- Typical protection strategies:
- Painting/coating systems (epoxy, polyurethane, intumescent for fire protection).
- Hot‑dip galvanizing is possible for some product forms but may not be compatible with high‑temperature service requirements or code‑mandated PWHT.
- Cathodic protection if used in buried or marine environments.
- Stainless indices: PREN is not applicable to Cr–Mo ferritic alloy steels because PREN applies to corrosion resistance of austenitic stainless steels: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
- Clarification: Although Gr22 has higher Cr and Mo than Gr11, the absolute Cr level (~2–2.25%) is far below stainless grade levels; it improves oxidation and creep resistance at elevated temperatures but does not confer significant general corrosion resistance compared to stainless steels.
7. Fabrication, Machinability, and Formability
- Machinability: Both grades machine similarly to other medium‑alloy steels; Gr22 may be slightly harder and less machinable if delivered in higher‑strength temper. Tooling and cutting parameters should be adjusted based on hardness.
- Formability/bending: Ductility is adequate for forming in normalized/tempered condition; Gr11 tends to be more forgiving due to somewhat higher ductility. For cold forming, check hardness and temper; hot forming may be required for complex shapes.
- Surface finish: Grinding and finishing operations are straightforward but caution is required with heat generation to avoid tempering or surface hardening.
- Heat treatment after fabrication: Components welded or heavily cold worked typically require PWHT to meet mechanical and creep requirements for high‑temperature service.
8. Typical Applications
| SA387 Grade 11 (Gr11) | SA387 Grade 22 (Gr22) |
|---|---|
| Boiler headers, drums, and piping operating at moderate elevated temperatures where cost and weldability are considerations | High‑temperature piping, such as superheater and reheater tubes, and pressure vessels requiring higher long‑term strength and creep resistance |
| Heat exchangers and pressure parts where moderate steam temperatures are expected | Components in petrochemical and power plants operating at higher steam temperatures or pressures where enhanced creep strength is necessary |
| Flanges, fittings, and plates where easier welding and lower alloy content are advantageous | Thick sections or higher stress components where hardenability and elevated temperature mechanical properties are critical |
Selection rationale: - Choose Gr11 when the design temperature and required creep/rupture strength are within Gr11’s capability, when welding ease and cost control matter, and when moderate elevated‑temperature performance suffices. - Choose Gr22 when long‑term high‑temperature strength, resistance to creep, and higher allowable stresses at temperature justify higher alloy cost and stricter fabrication controls.
9. Cost and Availability
- Relative cost: Gr22 is generally more expensive than Gr11 because of higher Cr and Mo content (Mo is a costly alloying element). The price premium depends on market Mo and Cr prices and form (plate, pipe, forging).
- Availability: Both grades are commonly produced by plate and pipe mills; Gr22 can be slightly less available in some product sizes and specialized forms, but both are standard alloy categories with broad industrial supply. Procurement should confirm lead times and check mill certifications for heat treatments and mechanical test results.
10. Summary and Recommendation
Summary table (qualitative):
| Criterion | SA387 Gr11 | SA387 Gr22 |
|---|---|---|
| Weldability | Better (lower CE) | More demanding (higher CE) |
| Strength–Toughness balance | Good; easier to achieve toughness | Higher strength; toughness achievable with controlled HT |
| Cost | Lower | Higher |
Recommendations: - Choose SA387 Gr11 if you need a cost‑effective Cr–Mo alloy for moderate elevated‑temperature service, prefer easier welding and fabrication, and your design temperature and creep requirements are within Gr11’s capability. - Choose SA387 Gr22 if the application demands higher long‑term tensile and creep strength at elevated temperature, higher hardenability for thick sections, or if code/temperature limits specify the higher alloy content despite the higher material and fabrication cost.
Final practical advice: - Always specify the exact required heat‑treatment condition, PWHT parameters, and acceptance criteria (mechanical tests, impact energy at required temperatures) in purchase documents. - Require mill test reports and ensure welding procedures (preheat, interpass, consumables, PWHT) are qualified for the chosen grade and thickness to achieve the intended in‑service performance.