SA-387 (Cr-Mo) Chromium-Molybdenum Steel Plate as High-Temperature Pressure Vessel Base Material

1. Definition and Fundamental Principles

SA-387 is a family of chromium-molybdenum (Cr-Mo) alloy steel plates defined under ASTM SA-387/SA-387M, covering Grades 11 through 22. These grades are characterized by their controlled chromium and molybdenum content, which confers exceptional resistance to high-temperature oxidation, hydrogen attack, and creep deformation. The most widely specified grades for cladding base applications are SA-387 Grade 11 (1.25Cr-0.5Mo) and SA-387 Grade 22 (2.25Cr-1Mo), which are the industry standard substrates for hydrogenation reactors, reformers, and high-temperature heat exchangers in the petroleum refining and petrochemical sectors.

The metallurgical mechanism underlying the performance of Cr-Mo steels rests on solid solution strengthening from molybdenum and precipitation hardening from chromium carbide phases (primarily M₂₃C₆ and M₇C₃). At operating temperatures between 400°C and 650°C, these carbide precipitates impede dislocation motion and grain boundary sliding, thereby maintaining mechanical integrity under sustained mechanical stress. The chromium content simultaneously forms a stable, self-healing Cr₂O₃ oxide scale that protects the surface from hot gas corrosion and reduces hydrogen permeation at elevated temperatures.

When used as a base layer in bimetallic cladding, SA-387 plates provide the mechanical strength and pressure containment capability, while the overlay layer (typically austenitic stainless steel such as 304, 309, or 321) provides corrosion resistance. This metallurgical pairing exploits the complementary strengths of each material system to achieve a composite component that would be economically or technically impossible from a single homogeneous material.

2. Category and Business Positioning

Within the cladding materials supply chain, SA-387 Cr-Mo steel plates occupy the critical base material tier. They represent the structural foundation upon which all overlay and bonding processes are applied. The positioning of this material within Cladding Technology Shanxi Co., Ltd.'s capability portfolio is strategic for several reasons:

• High-Value Market Access: Hydrogenation reactors and high-temperature heat exchangers represent capital-intensive equipment with unit values frequently exceeding USD 5 million. Supply qualification in this segment provides long-term revenue streams and positions the company within the premium end of the cladding market.
• Technical Differentiation: The mandatory post-weld heat treatment (PWHT) requirement and the stringent quality controls associated with Cr-Mo base materials create significant technical barriers to entry. Mastery of these requirements distinguishes qualified suppliers from commodity-level competitors.
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• Cross-Process Applicability: SA-387 plates serve as the substrate material across all three company technology routes—TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding—making them a universal platform material that maximizes process versatility.

3. Technical Purpose and Value Creation

The primary technical purpose of SA-387 Cr-Mo steel plates is to serve as a high-temperature pressure-bearing substrate that maintains structural integrity under combined loads of internal pressure, mechanical stress, and elevated temperature. The specific value propositions include:

3.1 Hydrogenation Reactor Applications

In catalytic hydrogenation reactors operating at 350–500°C with partial hydrogen pressures exceeding 15 MPa, SA-387 Grade 11 or Grade 22 base plates provide resistance to hydrogen blistering and high-temperature hydrogen attack (HTHA) as defined by API RP 941. The overlay layer protects against corrosion from process media (hydrocarbons, amines, and sulfides), while the Cr-Mo base ensures long-term mechanical reliability and creep resistance.

3.2 High-Temperature Heat Exchanger Applications

In reformers, superheaters, and high-temperature heat exchangers operating at 400–650°C, SA-387 plates provide the necessary tensile strength and creep rupture life. The overlay protects against oxidation and sulfur corrosion from process gases, while the Cr-Mo base maintains dimensional stability and pressure containment capability over extended service intervals.

3.3 Economic Value

By using a Cr-Mo steel base with a thin corrosion-resistant overlay (typically 3–12 mm), manufacturers achieve a material cost reduction of 40–60% compared to monolithic nickel alloy or high-alloy stainless steel construction, while maintaining equivalent or superior performance. This cost optimization is a primary driver for customer adoption.

4. Key Process and Implementation Points

4.1 Material Specification and Selection

Grade	Cr (%)	Mo (%)	Max Operating Temp (°C)	Typical Application
SA-387 Gr. 11	0.90–1.20	0.40–0.60	593	Hydrogenation reactors, reformer tubes
SA-387 Gr. 22	2.00–2.50	0.85–1.10	650	High-pressure hydrogen reactors, superheaters
SA-387 Gr. 122	2.00–2.50	0.85–1.10	650	Enhanced toughness applications
SA-387 Gr. 91	8.00–9.50	0.85–1.10	650	Ultra-high-temperature service

4.2 Plate Preparation Requirements

• Surface Condition: Plates must be delivered in normalized or normalized-and-tempered condition. Surface scale must be removed by grinding or shot blasting to achieve a surface finish suitable for bonding or welding. The surface roughness should not exceed Ra 6.3 μm for hydraulic explosive bonding and Ra 12.5 μm for weld overlay applications.
• Flatness and Dimensional Tolerance: Plate flatness must comply with ASTM A6, with maximum deviation of 0.15% of width (not exceeding 3 mm/m). Thickness tolerance per ASTM A6, Class A (±0.125 mm per 25 mm of thickness).
• Chemical Verification: Each heat lot must be accompanied by a certified chemical analysis report verifying compliance with ASTM SA-387 compositional limits. Spectroscopic verification (PMI) is required at receipt to confirm grade identity and detect potential mix-ups.

4.3 Post-Weld Heat Treatment (PWHT)

PWHT is a mandatory and non-negotiable process step for all SA-387 Cr-Mo steel components. The purpose of PWHT is to relieve residual stresses introduced during welding, to temper the heat-affected zone (HAZ), and to reduce the susceptibility of the HAZ to delayed cracking and hydrogen-induced cracking.

Parameter	SA-387 Gr. 11	SA-387 Gr. 22	SA-387 Gr. 91
PWHT Temperature	720–760°C	760–790°C	760–790°C
Soak Time	1 hour per 25 mm thickness (min 2 hr)	1 hour per 25 mm thickness (min 2 hr)	1.5 hours per 25 mm thickness
Heating Rate	170°C/hr (or 20°C/hr per 25 mm, whichever is less)	170°C/hr	170°C/hr
Cooling Rate	≤170°C/hr below 540°C	≤170°C/hr below 540°C	≤170°C/hr below 540°C
Hardness Limit (post-PWHT)	≤22 HB (230 HV)	≤22 HB (230 HV)	≤22 HB (230 HV)

The PWHT cycle must be executed in a controlled atmosphere furnace with uniform temperature distribution (±15°C across the entire component). Thermocouple instrumentation must be placed at the thickest section and at least three locations across the component surface. The entire cycle must be documented with time-temperature charts for quality traceability.

4.4 Welding Parameter Considerations for Cr-Mo Base Materials

• Preheat Temperature: Minimum 100–150°C for SA-387 Gr. 11; 150–200°C for SA-387 Gr. 22, depending on plate thickness and carbon equivalent (CE). The carbon equivalent per IIW formula: CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15.
• Interpass Temperature: Maintain between 150°C and 300°C to prevent low-temperature cracking while avoiding excessive grain growth.
• Filler Metal Selection: Transition layers typically use E810T1-A or ER810T1 (309L equivalent) for weld overlay, or E910T1-2/ER910T1-2 (310L equivalent) for thick sections. The filler must provide adequate ductility in the transition zone to accommodate differential thermal expansion between the Cr-Mo base and austenitic overlay.
• Deposition Rate Control: Limit heat input to 1.5–2.5 kJ/mm for the transition layer to minimize dilution into the Cr-Mo base. Excessive dilution can produce martensitic phases in the transition zone, leading to cracking during cooling or PWHT.

5. Applicable Standards and Acceptance Criteria

5.1 Material Standards

• ASTM SA-387/SA-387M: Specification for Chromium-Molybdenum Steels for Pressure Vessels, for Elevated Temperature Service. Defines chemical composition, mechanical properties, heat treatment requirements, and testing protocols.
• ASME BPV Code Section II, Part D: Materials for pressure vessel construction, incorporating SA-387 grades with additional Code stamp requirements.
• NB/T 4701.2: Chinese national standard for steel plates for pressure vessels, with Cr-Mo grades designated as 12Cr1MoV and 14Cr1MoR equivalents.
• GB/T 5310: Steel tubes for high-temperature pressure applications (relevant for pipe applications).

5.2 Welding and Overlay Standards

• ASME BPV Code Section IX: Qualification of welding procedures and welders for pressure vessel construction.
• ASME BPV Code Section II, Part C: Welding consumable specifications (E810T1-A, E910T1-2, etc.).
• ASTM A240: Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip (for overlay material selection).
• NB/T 47014: Chinese standard for qualification of welding procedures for pressure vessels.
• API 579-1/ASME FFS-1: Fitness-for-service assessment of clad components.

5.3 Non-Destructive Testing Standards

• ASME BPV Code Section V, Article 2: Radiographic testing (RT) for weld joints.
• ASME BPV Code Section V, Article 7: Magnetic particle testing (MT) for surface discontinuities.
• ASME BPV Code Section V, Article 8: Ultrasonic testing (UT) for weld inspection.
• ASTM E1659: Ultrasonic thickness measurement of overlay deposits.
• ISO 17638: Ultrasonic testing of welds.
• ASTM E2715: Ultrasonic testing for detection of disbondment in bonded joints.

5.4 Acceptance Criteria

• Mechanical Properties: Tensile strength per SA-387 grade requirements (Gr. 11: 415–550 MPa; Gr. 22: 450–620 MPa). Hardness post-PWHT not exceeding 22 HB (230 HV) at any location on the base plate or HAZ.
• Weld Overlay: Minimum 2.5 mm effective overlay thickness (net of weld reinforcement). No cracks, porosity exceeding 1 mm diameter, or lack of fusion permitted. Transition layer must be fully austenitic or duplex (ferrite content <10%) per ASTM A240.
• Bond Strength: For explosion-welded joints, peel test per ASTM F2469 with minimum bond strength of 90 MPa (or per design specification). For hydraulic explosive bonding, adhesion test per ASTM E2715 with no disbondment exceeding 25 mm² per meter of joint length.
• Microstructural: No tempered martensite or brittle phases in the HAZ. Overlay microstructure shall be fully austenitic with no delta ferrite exceeding 10% for 300-series overlay.

6. Common Risks and Controls

6.1 Hydrogen-Induced Delayed Cracking (HIDC)

Risk: Cr-Mo steels are highly susceptible to hydrogen-induced delayed cracking, particularly in the HAZ and weld metal. Hydrogen can accumulate from welding processes, moisture in flux, or contaminated base metal, leading to microcracking hours or days after welding.

Controls:

• Maintain preheat and interpass temperatures above the minimum specified values.
• Use low-hydrogen welding consumables (diffusible hydrogen content <5 ml/100g).
• Implement post-weld bake cycles (250–300°C for 1–2 hours) before PWHT to allow hydrogen diffusion.
• Ensure strict control of welding environment humidity (<70% RH).
• Conduct magnetic particle or dye penetrant inspection 24–48 hours after welding to detect delayed cracking.

6.2 Inadequate PWHT

Risk: Insufficient PWHT temperature or soak time results in retained high hardness in the HAZ, creating susceptibility to stress corrosion cracking and reduced fatigue life. Overheating during PWHT can cause excessive grain growth and loss of toughness.

Controls:

• Calibrate furnace thermocouples annually per ASME BPV Code Section IV.
• Implement independent verification of PWHT cycles using calibrated witness coupons.
• Monitor hardness at multiple locations (HAZ, weld metal, and base) post-PWHT to confirm cycle effectiveness.
• Limit PWHT temperature to avoid exceeding the upper critical temperature (Ac3) by more than 50°C.

6.3 Excessive Dilution in Transition Layer

Risk: High dilution of Cr-Mo base metal into the transition weld layer can produce hard, brittle martensitic phases, leading to cracking during cooling or PWHT.

Controls:

• Use multi-layer transition sequences (typically 309L → 304L or 310L → 304L).
• Limit heat input per pass to 1.5–2.5 kJ/mm.
• Perform hardness traversals across the transition zone to verify no locations exceed 22 HB.
• Conduct metallographic examination of transition layers to confirm absence of martensite.

6.4 Hydrogen Attack (HTHA) in Service

Risk: During service exposure to hydrogen at elevated temperatures, Cr-Mo steels can experience carburization and decarburization at grain boundaries, leading to intergranular cracking.

Controls:

• Verify compliance with API RP 941 Nelson Curve limits for the specific Cr-Mo grade.
• Ensure overlay integrity through comprehensive NDT to prevent hydrogen ingress to the base material.
• Implement periodic in-service inspection programs including UT for overlay disbondment and RT for internal cracking.
• Specify low-carbon grades (SA-387 Gr. 11C or Gr. 22C) for high-hydrogen service to reduce HTHA susceptibility.

7. Application Across Company Technology Routes

7.1 TIG/MIG Weld Overlay Route

SA-387 Cr-Mo steel plates are the most common substrate for weld overlay applications in high-temperature pressure equipment. The typical process sequence involves:

1. Base Preparation: Plate surface is ground to remove scale and achieve Ra < 6.3 μm. Preheat to 150°C minimum.
2. Transition Layer: 2–3 passes of E810T1-A (309L) or E910T1-2 (310L) deposited using GMAW or GTAW. Heat input controlled at 1.5–2.5 kJ/mm. Interpass temperature maintained at 200–250°C.
3. Overlay Layer: 2–4 passes of E309L (309L) or E308L (304L) deposited to achieve minimum 3 mm net overlay thickness. For high-corrosion applications, overlay may include E347 (321) or E309Mo (316L) top layers.
4. PWHT: Full component PWHT at 720–760°C (Gr. 11) or 760–790°C (Gr. 22) for the specified soak time.
5. Final Inspection: UT for overlay thickness, MT for surface defects, hardness survey, and dye penetrant testing.

Key Advantages for SA-387: The TIG/MIG route offers precise control over dilution rates, enabling optimization of the transition zone microstructure. It is particularly suitable for complex geometries (flanges, nozzles, curved surfaces) where hydraulic bonding or explosion welding is impractical. The process is well-qualified under ASME Section IX, facilitating Code stamp certification.

7.2 Hydraulic Explosive Bonding Route

Hydraulic explosive bonding (also known as hydraulic shock bonding) uses high-pressure water jets to generate shock waves that create metallurgical bonds between the SA-387 base plate and the overlay plate (typically 304L or 316L stainless steel). The SA-387 plate serves as the stationary anvil, while the overlay plate is accelerated by hydraulic shock to impact velocities of 100–200 m/s.

1. Plate Pair Preparation: SA-387 base plate (typically 25–100 mm thick) is cleaned and positioned as the stationary target. The overlay plate (3–12 mm thick) is mounted on the accelerator.
2. Alignment and Clamping: Plates are aligned with precision (±0.1 mm) and clamped to the bonding frame. Critical for achieving uniform bonding across large plate dimensions (up to 3000 × 2000 mm).
3. Bonding Execution: High-pressure water jets (150–350 MPa) generate shock waves that propagate through the accelerator and into the plate pair, creating the metallurgical bond at the interface.
4. PWHT: Post-bonding PWHT is mandatory for SA-387 components to relieve residual stresses introduced during the shock event.
5. Inspection: UT disbondment testing per ASTM E2715 across the entire bonded area. Bond strength verification by peel testing.

Key Advantages for SA-387: Hydraulic bonding provides 100% bond coverage with no dilution at the interface, preserving the full metallurgical properties of both base and overlay materials. It is ideal for large-format plates where weld overlay would be prohibitively expensive. The process is particularly suited for hydrogenation reactor shells and heat exchanger channel plates.

7.3 Explosion Welding Route

Explosion welding uses detonation of a shaped explosive charge to accelerate the overlay plate toward the SA-387 base plate at supersonic velocities (500–1000 m/s), creating a high-strain-rate collision that produces a metallurgical bond through plastic instability (typified by the characteristic wavy interface). SA-387 plates are frequently used as base materials in explosion welding due to their favorable ductility-to-strength ratio at elevated temperatures.

1. Plate Pair Setup: SA-387 base plate (25–80 mm) is positioned on the anvil. Overlay plate (3–12 mm) is mounted on the explosive charge with precise stand-off distance (typically 2–5 mm) and angle (10°–20°).
2. Explosive Configuration: Detonating cord and shaped explosive charges are arranged to ensure uniform acceleration of the overlay plate. For large plates, multiple charges with synchronized initiation are used.
3. Impact and Bonding: The detonation accelerates the overlay plate to impact the SA-387 base at the designed velocity and angle, creating a jet-removed, metallurgically bonded interface with characteristic wavy morphology.
4. Post-Bonding Processing: The bonded assembly is trimmed, machined to final dimensions, and subjected to PWHT at the specified Cr-Mo temperature range.
5. Quality Verification: Peel testing per ASTM F2469, UT for disbondment, and metallographic examination of the bond interface.

Key Advantages for SA-387: Explosion welding achieves the highest bond strengths among all cladding processes and is capable of bonding dissimilar materials with vastly different thermal expansion coefficients. The process is well-suited for large-area cladding of hydrogenation reactor shells, where the mechanical demands are highest. The characteristic wavy interface provides mechanical interlocking in addition to metallurgical bonding, offering superior resistance to delamination under cyclic loading.

7.4 Comparative Summary

Parameter	TIG/MIG Weld Overlay	Hydraulic Explosive Bonding	Explosion Welding
Bond/Interface Type	Metallurgical weld fusion	Metallurgical shock bond	Metallurgical impact bond
Dilution	Yes (transition layer required)	None	Minimal (jet removal)
Maximum Plate Size	Unlimited (weldable)	~3000 × 2000 mm	~1500 × 1000 mm
Overlay Thickness	3–12 mm (multi-pass)	3–12 mm (pre-cut plate)	3–12 mm (pre-cut plate)
PWHT Requirement	Mandatory	Mandatory	Mandatory
Geometric Flexibility	High (complex shapes)	Low (flat plates only)	Low (flat plates only)
Typical Application	Flanges, nozzles, small reactors	Large reactor shells, channel plates	High-strength cladding, dissimilar pairs
Cost per m²	Medium	Low-Medium	High

8. Contribution to Qualification Building, Product Delivery, and Customer Value

8.1 Qualification Building

SA-387 Cr-Mo steel plate applications represent the highest technical qualification tier in the cladding industry. Successfully delivering components with SA-387 base materials requires:

• ASME Code Stamp Qualification: Demonstrating capability to meet ASME BPV Code requirements for Cr-Mo pressure vessel materials, including PWHT procedures, NDT protocols, and material traceability systems.
• WPS/PQR Qualification: Developing and qualifying welding procedure specifications (WPS) and performance qualification records (PQR) specifically for Cr-Mo base materials with austenitic overlay, per ASME Section IX and NB/T 47014.
• Third-Party Inspection (TPI): Accepting independent inspection by ASME Authorized Inspection Agencies (AIAs) or equivalent third-party organizations throughout the manufacturing process.
• Customer-Specific Qualification: Many hydrogenation reactor OEMs (Lurgi, KBR, Technip, etc.) require supplier qualification through witnessing of the entire manufacturing sequence, including PWHT and NDT.

8.2 Product Delivery Excellence

Cladding Technology Shanxi Co., Ltd.'s capability in SA-387 Cr-Mo applications enables the company to deliver:

• Full-Spectrum Component Supply: From raw plate to finished cladded components ready for pressure vessel fabrication, including hydraulic bonding, explosion welding, and weld overlay options tailored to specific component geometries.
• Traceability and Documentation: Complete material traceability from mill certificate through to final component, including heat treatment records, NDT reports, and dimensional inspection documentation. This is critical for Code stamp certification and customer quality assurance requirements.
• Integrated Process Solutions: The ability to combine multiple cladding technologies on a single component (e.g., explosion-welded shell with TIG weld-overlaid nozzles and flanges) provides customers with a single-source solution that reduces interface management and delivery risk.

8.3 Customer Value Proposition

The strategic importance of SA-387 Cr-Mo capability to end customers includes:

• Extended Service Life: Properly cladded SA-387 components achieve design lives of 20–40 years in hydrogenation service, compared to 5–10 years for uncladded carbon steel. This represents a 4–8x life extension with only 15–25% additional material cost.
• Reduced Maintenance Intervals: Cladded components eliminate the need for periodic replacement of corroded internals, reducing unplanned shutdown costs by USD 500,000–2,000,000 per avoided shutdown in a typical hydrogenation unit.
• Regulatory Compliance: Code-stamped cladded components meet regulatory requirements for pressure equipment in all major jurisdictions, enabling global project deployment.
• Technology Risk Mitigation: By offering multiple cladding routes for the same base material, the company provides customers with flexibility to select the optimal process for their specific application, reducing technology risk and enabling contingency planning.

9. Conclusion

SA-387 (Cr-Mo) chromium-molybdenum steel plates represent a cornerstone material in the high-temperature pressure equipment cladding industry. Their unique combination of creep resistance, hydrogen attack resistance, and mechanical strength at elevated temperatures makes them indispensable for hydrogenation reactors, reformers, and high-temperature heat exchangers. The mandatory PWHT requirement and stringent quality controls associated with Cr-Mo materials create significant technical barriers that distinguish qualified suppliers from commodity competitors.

For Cladding Technology Shanxi Co., Ltd., mastery of SA-387 base material applications across all three technology routes—TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding—positions the company as a comprehensive solutions provider capable of addressing the full spectrum of high-temperature cladding requirements. This capability directly supports qualification advancement, enables premium product delivery, and delivers substantial economic value to customers through extended asset life and reduced lifecycle costs.