SA-387 Cr-Mo Steel Plate as High-Temperature Base Material for Bimetallic Cladding
1. Definition and Technical Overview
SA-387 is a family of chromium-molybdenum (Cr-Mo) alloy steel plates specified under ASTM/ASME standards for high-temperature pressure vessel and piping applications. The designation "SA" indicates a pressure vessel-grade material (ASME Section II, Part A), distinguishing it from the general-purpose A-387 grade. Within the SA-387 family, the most commonly deployed grades for cladding base-layer applications include:
- SA-387 Gr. 22 (1¼Cr-½Mo): Maximum service temperature approximately 593°C (1100°F); the workhorse grade for hydrogen service and moderate-temperature creep resistance.
- SA-387 Gr. 11 (¼Cr-½Mo): Maximum service temperature approximately 538°C (1000°F); widely used in superheater tubes and moderate-temperature hydrogen reactors.
- SA-387 Gr. 2 (½Cr-½Mo): Maximum service temperature approximately 482°C (900°F); legacy but still in service for lower-temperature hydrogen applications.
- SA-387 Gr. 91 (9Cr-1Mo): Maximum service temperature approximately 593°C (1100°F) in normalized-and-tempered condition; superior creep strength for advanced high-temperature applications.
As a base material (基层) in bimetallic cladding systems, SA-387 Cr-Mo plates provide the structural integrity, creep resistance, and hydrogen-attack resistance required for high-temperature pressure-retaining components. The overlay layer (typically austenitic stainless steel such as 304, 304L, 309, or 321) provides corrosion resistance against process media, while the SA-387 base carries the mechanical load at elevated temperatures.
2. Category and Business Positioning
Within the raw material classification of Cladding Technology Shanxi Co., Ltd., SA-387 Cr-Mo steel plate falls under the Base Material – Low Alloy Steel category. This positioning reflects its role as the structural foundation upon which corrosion-resistant overlays are deposited. The technical purpose is explicitly defined as high-temperature pressure-bearing substrate (高温承压基材), targeting applications in:
- Hydrogenation reactors and hydrogen-rich environments
- High-temperature heat exchangers and superheater bundles
- Ammonia synthesis loops and methanol synthesis reactors
- Refinery FCC units and catalytic cracker regenerator components
- Power generation boiler sections operating above 425°C
The mandatory post-weld heat treatment (PWHT) requirement noted in the entry is a critical differentiator. Unlike carbon steel bases that may be exempt from PWHT below certain thickness thresholds, SA-387 Cr-Mo materials require PWHT to relieve welding residual stresses, promote tempering of the heat-affected zone (HAZ), and prevent delayed cracking. This requirement directly influences process planning, cost estimation, and delivery timelines.
3. Technical Purpose and Value Proposition
3.1 Mechanical Performance at Elevated Temperatures
SA-387 Cr-Mo steels derive their high-temperature capability from the synergistic effects of chromium and molybdenum:
- Chromium promotes the formation of stable carbides (Cr₇C₃, Cr₂₃C₆) that pin grain boundaries and retard creep deformation. It also enhances oxidation resistance at elevated temperatures.
- Molybdenum raises the recrystallization temperature, improves creep strength through solid-solution strengthening, and promotes the formation of fine M₂₃C₆ carbides that contribute to secondary hardening during tempering.
3.2 Hydrogen Attack Resistance
Per NACE MR0175/ISO 15156 and API 941 guidelines, Cr-Mo steels of the SA-387 type provide inherent resistance to high-temperature hydrogen attack (HTHA) when properly tempered. The carbon activity in the microstructure is sufficiently low to prevent the formation of methane bubbles at grain boundaries—a failure mode that would be catastrophic in carbon steel under hydrogen partial pressures above 1.0 MPa at temperatures exceeding 200°C.
3.3 Value in Cladding Systems
Using SA-387 as the base material in a cladding system delivers:
- Material cost optimization: Replaces the need for full-thickness austenitic stainless steel construction, reducing material costs by 40–60% for large-diameter vessels.
- Weight reduction: The lower density of Cr-Mo steel compared to austenitic stainless steel contributes to structural weight savings in overhead-mounted equipment.
- Compliance with design codes: ASME Section VIII Division 1 and Division 2 permit Cr-Mo base materials with corrosion-resistant overlays under specific qualification requirements.
4. Key Process and Implementation Points
4.1 Material Preparation and Pre-Weld Treatment
SA-387 plates supplied in the normalized and tempered (N&T) condition must be verified for compliance with ASTM A-387/SA-387 chemical and mechanical requirements. Key acceptance parameters include:
| Parameter | SA-387 Gr. 22 (Typical) | SA-387 Gr. 91 (Typical) | Acceptance Reference |
|---|---|---|---|
| Carbon (C) | ≤0.40% | 0.05–0.12% | ASTM A-387 |
| Chromium (Cr) | 1.00–1.50% | 8.00–9.50% | ASTM A-387 |
| Molybdenum (Mo) | 0.40–0.60% | 0.85–1.05% | ASTM A-387 |
| Yield Strength (RT) | ≥205 MPa | ≥415 MPa | ASTM A-387 |
| Tensile Strength (RT) | 415–620 MPa | 585–795 MPa | ASTM A-387 |
| Impact Energy (350°C) | ≥34 J (Charpy V) | ≥47 J (Charpy V) | ASME VIII Div.1 UG-83 |
| Hardness | ≤22 HB | ≤250 HB | ASME VIII Div.1 UG-91 |
4.2 Welding Considerations for Cr-Mo Base Materials
The welding of SA-387 Cr-Mo base materials requires careful control of heat input, interpass temperature, and preheat to minimize HAZ hardening and avoid the "temper-rolled" zone phenomenon.
| Process Parameter | SA-387 Gr. 22 (1¼Cr-½Mo) | SA-387 Gr. 91 (9Cr-1Mo) | Rationale |
|---|---|---|---|
| Preheat Temperature | 150–250°C | 200–300°C | Reduce cooling rate; prevent HAZ embrittlement |
| Interpass Temperature | ≤300°C | ≤300°C | Avoid temper-rolled zone softening |
| Heat Input | 0.5–2.5 kJ/mm | 0.3–1.5 kJ/mm | Limit grain growth; control dilution |
| Welding Consumable (base weld) | SAE-430 / E8010 / E8018 | E905-T1 / E905-T1L / ER90S-T1 | Match Cr-Mo chemistry for HAZ compatibility |
| PWHT Temperature | 704–760°C | 760–790°C | Temper HAZ; relieve residual stress |
| PWHT Hold Time | 1 hour per 25 mm thickness (min. 2 hr) | 1 hour per 25 mm thickness (min. 2 hr) | ASME VIII Div.1 UCS-56 |
4.3 Post-Weld Heat Treatment (PWHT) – Critical Control Point
The mandatory PWHT requirement for SA-387 Cr-Mo base materials is non-negotiable and represents the single most critical process variable in the fabrication sequence. The PWHT serves multiple purposes:
- HAZ Tempering: The welding thermal cycle partially tempers or over-tempers the base material HAZ. Re-tempering at the prescribed temperature restores toughness and prevents brittle fracture at service temperature.
- Residual Stress Relief: Reduces longitudinal and transverse residual stresses to below 50% of yield strength, mitigating stress-corrosion cracking susceptibility in the overlay layer.
- Microstructural Stabilization: Promotes uniform carbide distribution in the HAZ, improving creep resistance and long-term dimensional stability.
- Overlay Layer Compatibility: Ensures that the base metal hardness is compatible with the deposited overlay, preventing differential thermal expansion issues during subsequent service.
PWHT parameters shall comply with ASME Section VIII Division 1, UCS-56, or ASME Section IX QW-406 as applicable. The heating and cooling rates during PWHT shall not exceed the following limits (ASME VIII Div.1 UCS-56(f)):
| Plate Thickness | Maximum Heating Rate | Maximum Cooling Rate (below 400°C) |
|---|---|---|
| ≤13 mm (½ in.) | 230°C/hr | 230°C/hr |
| 13–38 mm (½–1½ in.) | 260°C/hr | 260°C/hr |
| 38–76 mm (1½–3 in.) | 200°C/hr | 200°C/hr |
| >76 mm (>3 in.) | 140°C/hr | 140°C/hr |
4.4 Dilution Control During Overlay Application
When applying a corrosion-resistant overlay (e.g., 309L or 304L) onto an SA-387 Cr-Mo base, the dilution of base material into the first overlay pass is a critical concern. Excessive dilution can:
- Introduce Cr-Mo into the overlay, forming brittle carbide networks susceptible to intergranular corrosion
- Alter the overlay's weldability and reduce its corrosion resistance
- Create a metallurgical mismatch zone prone to cracking under thermal cycling
Industry practice limits base metal dilution in the first overlay pass to ≤15% for austenitic overlays on Cr-Mo bases, per AWS D10.9 and EN 1653 guidelines.
5. Applicable Standards and Acceptance Criteria
5.1 Material Standards
- ASTM A-387 / ASME SA-387: Standard specification for chromium-molybdenum-vanadium alloy steel plates for pressure vessels at elevated temperatures.
- ASTM A-204 / ASME SA-204: For SA-387 Gr. 91 (9Cr-1Mo) plates in some specifications.
- GB/T 12228: Chinese standard for Cr-Mo steel plates for pressure vessels (equivalent to ASTM A-387 for domestic applications).
- NB/T 20320: Chinese power industry standard for materials used in nuclear island components (where applicable).
5.2 Welding and Fabrication Standards
- ASME Section IX: Qualification of welding procedures and welders; QW-406 for PWHT requirements.
- ASME Section VIII Division 1: UCS-56 (PWHT), UG-91 (hardness limitations), UCS-53 (impact testing).
- ASME Section VIII Division 2: Part 3 (material requirements), Part 4 (design by analysis), Part 5 (fabrication).
- ASME BPVC Section I: Power piping; PWHT requirements for Cr-Mo alloys.
- API 578: AWS D1.1 welding qualification for Cr-Mo base materials.
- EN 1561: European welding procedure specification for Cr-Mo steels.
- NB/T 20321: Chinese nuclear industry welding procedure specification.
5.3 Overlay and Cladding Standards
- AWS D10.9: Specification for weld overlay cladding.
- EN 1653: Technical delivery inspection conditions for weld overlay cladding.
- ISO 15614-1: Qualification of welding procedures for metallic materials.
- ASME Section IX QW-451.3: Qualification of weld overlay procedures.
- GB/T 23837: Chinese standard for weld overlay cladding of pressure vessels.
5.4 NDT and Acceptance Standards
- ASME Section V: Non-destructive examination; Article 4 (RT), Article 7 (MT), Article 12 (PT), Article 16 (ET).
- ASME Section VIII Div.1 UW-51: Acceptance criteria for radiographic examination.
- ASME Section VIII Div.2 Part 5: NDE requirements for Division 2 vessels.
- API 510: Inspection code for pressure vessels; PWHT documentation requirements.
- NB/T 47013: Chinese standard for NDE of pressure vessels and components.
6. Common Risks and Controls
6.1 HAZ Embrittlement and Cracking
Risk: SA-387 Cr-Mo steels are susceptible to temper embrittlement in the 370–540°C range and to cold cracking if cooling rates exceed acceptable limits. The high carbon equivalent of some grades (particularly Gr. 22 with C > 0.35%) increases susceptibility to hydrogen-induced cracking.
Controls:
- Strict preheat and interpass temperature control per WPS
- Limit heat input to prevent excessive grain growth
- Mandatory PWHT immediately after welding (or within the same heat cycle)
- Post-PWHT hardness testing (≤22 HB for Gr. 22; ≤250 HB for Gr. 91) per ASME UG-91
6.2 Temper-Rolled Zone Softening
Risk: If interpass temperatures exceed 300°C during multi-pass welding, a partially tempered zone can form in the previously deposited weld metal, resulting in unacceptable softening (loss of creep strength) and potential Type IV cracking at service temperature.
Controls:
- Enforce interpass temperature ≤300°C (≤572°F) with IR pyrometer monitoring
- Document interpass temperatures in the weld log
- Include Type IV cracking resistance testing in WPS qualification for critical applications
6.3 Overlay Dilution and Corrosion Performance Degradation
Risk: Excessive base metal dilution into the overlay layer can compromise the corrosion resistance of the deposited material, particularly in chloride-containing or high-temperature sulfuric acid environments.
Controls:
- Limit first-pass dilution to ≤15% (verified by spectrographic analysis of overlay cross-section)
- Use appropriate backing or "buttering" layer if dilution cannot be controlled
- Apply minimum of 3 overlay passes with total thickness ≥3 mm for critical service
- Perform intergranular corrosion testing (ASTM A-262 Practice E or F) on overlay qualification coupons
6.4 PWHT-Induced Distortion
Risk: Large SA-387 base plates or formed components may experience significant distortion during the PWHT cycle, particularly if heating/cooling rates are not properly controlled or if the component geometry creates thermal gradients.
Controls:
- Use controlled-atmosphere furnaces with instrumented thermocouples at multiple locations
- Apply mechanical strapping or support fixtures during PWHT for thin-walled or formed components
- Plan PWHT prior to final machining to allow post-PWHT correction
- Limit PWHT temperature to the minimum effective value (704°C for Gr. 22; 760°C for Gr. 91)
6.5 Hydrogen-Induced Delayed Cracking
Risk: Diffusible hydrogen trapped in the HAZ during welding can cause delayed cracking hours to days after welding, particularly in SA-387 Gr. 22 with higher carbon content.
Controls:
- Use low-hydrogen consumables (E8010, E8018 with controlled moisture)
- Apply post-weld bake at 200–250°C for 2 hours per 25 mm thickness to diffuse hydrogen
- Inspect welds after a 48-hour delay period before proceeding to PWHT
- Control electrode storage and baking per AWS D1.1 requirements
7. Application Across the Three Technology Routes
7.1 TIG/MIG Weld Overlay Route
SA-387 Cr-Mo steel plate serves as the base substrate for TIG (GTAW) and MIG (GMAW) weld overlay cladding. This is the most versatile and widely deployed route for SA-387 base materials.
Typical Configuration:
- Base: SA-387 Gr. 22, 16–60 mm thickness, N&T condition
- Overlay: 309L (E309L/ER309L) or 304L (E308L/ER308L), 3–10 mm total overlay thickness
- Application: Hydrogenation reactor shells, high-temperature heat exchanger tubesheets, superheater headers
Process Sequence:
- Base plate preparation: grinding to smooth, clean surface; verify hardness ≤22 HB (Gr. 22)
- Preheat base to 150–250°C per WPS
- Apply transition layer (optional): 1–2 passes of 309L to reduce dilution
- Apply build-up passes: 2–4 passes of 309L or 304L with controlled heat input
- Post-weld bake at 200–250°C for hydrogen removal (if applicable)
- PWHT at 704–760°C for specified hold time
- Post-PWHT hardness verification and NDT
Qualification Requirements: WPS qualification per ASME Section IX QW-451.3 or AWS D10.9, including:
- Base metal dilution analysis (spectrographic cross-section)
- Hardness survey of overlay and HAZ (≤22 HB for base; overlay hardness per AWS D10.9)
- Intergranular corrosion testing of overlay (ASTM A-262 Practice E)
- Impact testing of HAZ at service temperature (ASME UG-83)
- PWHT qualification per QW-406
7.2 Hydraulic Explosive Bonding (Hydroforming/Explosive Forming) Route
While hydraulic explosive bonding is more commonly associated with dissimilar metal bonding (e.g., copper-aluminum), the SA-387 Cr-Mo base plate can serve as a structural substrate in hybrid bonding systems where a Cr-Mo base is bonded to a thin corrosion-resistant layer under controlled plastic deformation conditions.
Relevant Applications:
- SA-387 Gr. 22 base plate with 304L stainless overlay bonded via explosive welding for reactor internals
- SA-387 base with nickel-alloy overlay (Inconel 625, Hastelloy C-276) for highly corrosive high-temperature service
Process Considerations for Cr-Mo Base:
- SA-387 plates must be in the fully tempered condition prior to explosive bonding; the N&T microstructure provides the ductility required for plastic flow during bonding
- The bonding interface quality depends on achieving sufficient jet velocity (≥200 m/s) at the collision point; Cr-Mo steels require higher explosive loading than carbon steels due to higher flow stress
- PWHT is still required post-bonding to relieve residual stresses introduced during the explosive forming cycle
- Interface strength testing per ASTM E1394 or AWS D10.9 Annex D
Technical Advantage: Explosive bonding of SA-387 base with thin overlay layers eliminates the dilution concerns inherent to weld overlay, producing a metallurgically clean interface with no intermetallic compound formation when properly controlled. This is particularly valuable for applications requiring both high-temperature mechanical strength and superior corrosion resistance without the risk of overlay degradation.
7.3 Explosion Welding Route
Explosion welding (explosive cladding) is a well-established process for bonding SA-387 Cr-Mo base plates with austenitic stainless steel or nickel-alloy overlay layers. This route is particularly suited for large-format production of clad plates for reactor shells and heat exchanger components.
Typical Configurations:
| Base Material | Overlay Material | Overlay Thickness | Application |
|---|---|---|---|
| SA-387 Gr. 22, 20–80 mm | 304L / 304H, 3–12 mm | 5–10 mm typical | Hydrogenation reactor shells |
| SA-387 Gr. 22, 25–60 mm | 321H, 3–10 mm | 5–8 mm typical | High-temperature heat exchangers |
| SA-387 Gr. 91, 20–50 mm | Inconel 625, 3–8 mm | 3–6 mm typical | Advanced hydrogen service components |
| SA-387 Gr. 22, 30–100 mm | 309, 5–15 mm | 8–12 mm typical | Large-diameter reactor pressure vessels |
Explosion Welding Process Parameters for SA-387 Base:
- Standoff distance: 15–25 mm (controlled by gap fixtures)
- Explosive charge: TNT or equivalent; charge weight optimized per base thickness
- Collision velocity: 250–400 m/s for Cr-Mo base / austenitic overlay combinations
- Plate flatness: ≤1.5 mm/m (critical for uniform bonding)
- Surface preparation: Grinding to remove scale; acid cleaning; no oil or moisture contamination
Post-Bonding Requirements:
- Trimming and edge machining of bonded plate
- Non-destructive examination of bond quality (ultrasonic testing per AWS D10.9 or EN 1653; magnetic particle testing of edges)
- PWHT at 704–760°C (for SA-387 Gr. 22) or 760–790°C (for SA-387 Gr. 91) to relieve explosive-bonding residual stresses
- Post-PWHT bond strength verification (shear testing per ASTM E1394)
- Hardness survey of base material (verify ≤22 HB for Gr. 22)
Key Advantage of Explosion Welding for SA-387: The explosion welding process produces a metallurgically pure interface without dilution, ensuring that both the base material's high-temperature mechanical properties and the overlay's corrosion resistance are fully preserved. This is a significant advantage over weld overlay methods where dilution is an unavoidable concern.
8. Contribution to Qualification Building and Customer Value
8.1 Qualification Portfolio Enhancement
SA-387 Cr-Mo steel plate represents a high-value qualification asset for Cladding Technology Shanxi Co., Ltd. for the following reasons:
- Code compliance demonstration: Successful fabrication and qualification of SA-387 base cladding components demonstrates capability to meet ASME Section VIII Division 1/2, API 620/625, and NB/T 47013 requirements—essential for entry into the pressure vessel and refinery markets.
- WPS library expansion: Qualified welding procedures for SA-387 Cr-Mo base materials with various overlay combinations create a reusable IP asset that accelerates future project execution.
- PWHT capability validation: The mandatory PWHT requirement validates the company's heat treatment infrastructure, instrumentation, and procedural control—capabilities that are prerequisites for all Cr-Mo alloy work.
- NDT proficiency: The NDE requirements for Cr-Mo base materials (particularly UT for bond quality and RT for weld integrity) build NDT team capability that is transferable across the product portfolio.
8.2 Product Delivery Capability
The SA-387 Cr-Mo base material capability enables delivery of complete clad plate and pipe products for:
- Hydrogenation reactors: Full-scale reactor shells with SA-387 Gr. 22 base and 304L/309L overlay, ready for fabrication into pressure vessels per ASME VIII Div.1
- Heat exchanger tubesheets: SA-387 Gr. 22 or Gr. 11 tubesheets with austenitic overlay for high-temperature service
- Reactor internals: SA-387 Gr. 91 base with nickel-alloy overlay for advanced hydrogen service
- Boiler components: SA-387 Gr. 11/22 headers and spools with corrosion-resistant overlay for superheater circuits
8.3 Customer Value Proposition
For end customers in the oil, gas, petrochemical, and power generation industries, the availability of SA-387 Cr-Mo base cladding capability provides:
- Single-source procurement: Eliminates the need to separately procure Cr-Mo base plates and arrange overlay cladding, reducing interface risk and schedule uncertainty.
- Integrated PWHT capability: On-site PWHT of clad components eliminates the risk of damage or distortion during transportation to external heat treatment facilities.
- Full traceability: Material certification, welding records, PWHT charts, NDT reports, and hardness surveys all integrated into a single quality package per API 510 / ASME Section VIII requirements.
- Cost competitiveness: In-house capability for the complete cladding system (base + overlay + PWHT + NDT) reduces total delivered cost compared to outsourced fabrication.
9. Conclusion
SA-387 Cr-Mo steel plate is a cornerstone base material in the high-temperature cladding portfolio of Cladding Technology Shanxi Co., Ltd. Its deployment across TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding routes demonstrates the company's technical versatility and commitment to meeting the demanding requirements of the energy and chemical processing industries. The mandatory PWHT requirement, while adding process complexity, serves as a quality gate that ensures the metallurgical integrity of the final product. By maintaining rigorous control over material selection, welding procedures, heat treatment, and non-destructive examination, the company delivers clad products that meet the highest international code standards and provide reliable long-term service in the most demanding high-temperature pressure environments.