SHT-3523-2020: Welding Code for Chromium-Nickel Stainless Steel, Iron-Nickel Alloys, Nickel-Based Alloys, and Clad Steel in Petrochemical Service
1. Definition and Scope of SHT-3523-2020
SHT-3523-2020, formally titled "Welding Specification for Chromium-Nickel Stainless Steel, Iron-Nickel Alloys, Nickel-Based Alloys, and Stainless Steel Clad Steel in Petrochemical Industry", is an enterprise-level technical standard developed under the Shenhua Group's Shenhua Technology (SHT) series of welding codes. This specification establishes the governing framework for the design, fabrication, qualification, and inspection of welded joints involving austenitic stainless steels (e.g., 304, 316, 321), iron-nickel alloys (e.g., Monel 400, Inconel 600), nickel-based superalloys (e.g., Inconel 625, Hastelloy C-276), and multi-layer clad steel assemblies used in petrochemical processing environments.
The standard encompasses the full welding lifecycle: from Welding Procedure Specification (WPS) development and qualification testing through to production execution, non-destructive examination (NDE), and final acceptance. It is specifically tailored to address the unique metallurgical challenges associated with dissimilar metal welds, high-alloy weld overlays, and the integrity of cladding layers in aggressive chemical service.
2. Category and Business Positioning
SHT-3523-2020 occupies a critical position within the company's qualification and standards architecture. Its role can be categorized as follows:
- Qualification Foundation: The standard serves as the primary reference for WPS qualification and welder performance qualification (WPQ) when the end-user or project owner mandates adherence to SHT-series codes. This is particularly common in petrochemical projects developed under Shenhua Group or affiliated EPC contractors.
- Technical Governance: It provides a structured methodology for selecting filler metals, preheat parameters, interpass temperatures, and post-weld heat treatment (PWHT) regimes for dissimilar and high-alloy weldments.
- Customer Value Delivery: Demonstrated proficiency with SHT-3523-2020 signals to petrochemical clients that the company can deliver clad and overlay products meeting the most stringent domestic petrochemical welding requirements, reducing the risk of rework, rejection, and project delay.
3. Technical Purpose and Value
The core technical purpose of SHT-3523-2020 is to ensure that welded joints involving high-alloy materials maintain:
- Corrosion resistance integrity across the weld zone, including the heat-affected zone (HAZ) and weld metal;
- Mechanical integrity under cyclic thermal and pressure loading conditions typical of petrochemical service;
- Cladding continuity without cracking, delamination, or excessive dilution that would compromise the protective overlay layer;
- Long-term service reliability in environments containing hydrogen sulfide (H₂S), chlorides, sulfuric acid, and other aggressive species.
For Cladding Technology Shanxi Co., Ltd., mastery of this standard directly translates into the ability to qualify and deliver clad plates, clad pipes, weld overlay components, and dissimilar weld assemblies for high-value petrochemical equipment such as heat exchangers, reactor internals, distillation columns, and storage vessels.
4. Key Process and Implementation Points
4.1 Filler Metal Selection Philosophy
SHT-3523-2020 prescribes a systematic approach to filler metal selection based on the base metal combination, service environment, and mechanical requirements. The following table summarizes common filler metal selections for typical dissimilar weld combinations covered by the standard:
| Base Metal Combination | Recommended Filler Metal | Welding Process | Typical Application |
|---|---|---|---|
| Carbon Steel + 304/316L SS | E309L (AWS) / ER309L | TIG / MIG | Clad pipe root weld, transition weld |
| 304/316L SS + Inconel 625 | ERNiCrMo-3 / ERNiCr-3 | TIG (GTAW) | High-temperature overlay, reactor internals |
| Carbon Steel + Monel 400 | ERNiCu-7 / ERNiCu-2 | TIG / MIG | H₂S-resistant overlay, acid service |
| 316L SS + Hastelloy C-276 | ERNiCrMo-16 / ERNiCrMo-14 | TIG (GTAW) | Severe corrosion overlay, chemical processing |
| Carbon Steel + 321 SS (stabilized) | E309L / ER309L | MIG (GMAW) | General petrochemical clad plate |
| Inconel 625 + Inconel 625 | ERNiCr-3 / ERNiCr-11 | TIG (GTAW) | High-temperature alloy weldment |
4.2 Preheat and Interpass Temperature Control
Control of thermal input is paramount in high-alloy welding. SHT-3523-2020 establishes the following general guidelines:
| Material Group | Preheat Temperature | Maximum Interpass Temperature | Rationale |
|---|---|---|---|
| 304/316L Austenitic SS | Generally not required (0–50°C) | ≤ 150°C | Prevent sensitization and grain boundary carbide precipitation |
| 321/347 Stabilized SS | 0–50°C | ≤ 200°C | Manage residual stress; avoid excessive grain growth |
| Iron-Nickel Alloys (Monel, Inconel 600) | 50–150°C (thickness-dependent) | ≤ 250°C | Reduce cracking susceptibility; manage thermal expansion mismatch |
| Nickel-Based Superalloys (Inconel 625, Hastelloy) | 50–100°C | ≤ 250°C | Minimize solidification cracking; control HAZ microstructure |
| Carbon Steel (clad base) | 100–250°C (thickness-dependent) | ≤ 250°C | Reduce hydrogen-induced cracking; control HAZ hardness |
4.3 Welding Sequence for Clad Assemblies
The standard mandates a specific welding sequence for clad steel assemblies to protect the overlay layer from excessive dilution and thermal damage:
- Base metal weld: Complete all welds on the carbon steel substrate first, using appropriate low-dilution filler metals (e.g., E7018 or equivalent).
- Transition/root weld: Weld through the cladding interface using a high-alloy transition filler (e.g., ER309L) to ensure a compatible metallurgical transition.
- Overlay welds: Cap the weld with the cladding alloy filler metal to restore the corrosion-resistant surface layer.
- Overlay repair (if needed): If the overlay is damaged during fabrication, re-overlay using the same alloy, ensuring full penetration of the damaged zone.
4.4 Post-Weld Heat Treatment (PWHT)
SHT-3523-2020 addresses PWHT requirements with the following key provisions:
- Carbon steel base with SS cladding: PWHT of the base metal is permitted but must be performed before overlay welding or with strict control to avoid sensitization of the cladding layer. If PWHT is applied after overlay, the temperature must not exceed 425°C for 304/316L cladding or 480°C for 321/347 cladding.
- Pure austenitic SS weldments: Solution heat treatment (1010–1150°C) may be required for critical applications, followed by rapid cooling to prevent carbide precipitation.
- Nickel-based alloy weldments: Solution annealing (1050–1150°C) is recommended for full mechanical property restoration, particularly for Inconel 625 and Hastelloy C-276 weldments.
5. Applicable Standards and Acceptance Criteria
5.1 Cross-Reference Standards
SHT-3523-2020 is developed in alignment with and references the following international and national standards:
- GB/T 985.1 – Welding Procedure Qualification Test
- GB/T 150 – Pressure Vessels (design and fabrication)
- GB/T 12466 – Welding Procedure Specification
- GB 150 – Fusion-welded steel pressure vessels
- NB/T 47014 – Qualification test of welding procedure for pressure vessel
- ASME Section IX – Qualification Rules for Welding, Brazing, and Fusing
- ASME Section VIII Div. 1/2 – Rules for Construction of Pressure Vessels
- ASTM A377 – Clad Steel Plate, Sheet, and Strip
- ASTM A270 – Corrosion-Resistant Clad Steel Pipe
- ASTM A393 – Clad Steel Pipe
- API 5L – Specification for Line Pipe
- ISO 15614 – Qualification testing of welding procedures for metallic materials
- NACE MR0175/ISO 15156 – Materials for use in H₂S-containing environments
5.2 NDE and Acceptance Criteria
The standard specifies the following NDE methods and acceptance levels for different weld types:
| Weld Type | Required NDE Methods | Acceptance Criteria | Reference Standard |
|---|---|---|---|
| Base metal butt weld | RT (100%) + MT/PT (100%) | Level II (GB/T 3323) / Level 1 (GB/T 18851) | GB 150 / NB/T 47013 |
| Transition weld (through clad) | RT (100%) + PT (100%) + UT (overlay thickness) | No cracks, no lack of fusion; dilution ≤ specified limit | SHT-3523-2020 |
| Overlay weld (cap) | PT/MT (100%) + UT (thickness verification) | No surface discontinuities; minimum overlay thickness verified | GB/T 11345 / GB/T 18851 |
| Clad pipe circumferential weld | RT (100%) + PT (100%) + UT (clad thickness) | Level II RT; no overlay defects; minimum clad thickness | ASTM A270 / API 5L |
| Weld overlay (TIG multi-pass) | PT (100%) + UT (100% thickness mapping) | No cracks, pores ≤ 1 mm; overlay thickness within tolerance | GB/T 11345 |
5.3 Mechanical Testing Requirements
For WPS qualification under SHT-3523-2020, the following mechanical tests are typically required:
- Tensile testing: Transverse and longitudinal tensile specimens; minimum yield strength and tensile strength per material specification.
- Bend testing: Face bend, root bend, and side bend (for clad welds); minimum 5T bend diameter unless otherwise specified.
- Hardness testing: HAZ hardness survey to ensure maximum hardness does not exceed 350 HV for carbon steel base (per NACE MR0175/ISO 15156 for sour service).
- Corrosion testing: Intergranular corrosion testing per ASTM A262 Practice E (for austenitic SS); pitting resistance testing (PREN ≥ 24 for 316L; PREN ≥ 32 for 625; PREN ≥ 40 for C-276).
- Impact testing: Charpy V-notch at service temperature or -29°C (as applicable), minimum 27 J per specimen.
6. Common Risks and Controls
6.1 Solidification Cracking in Nickel-Based Alloys
Nickel-based superalloys (Inconel 625, Hastelloy C-276) are highly susceptible to solidification cracking due to their low solidification range and susceptibility to low-melting-point inclusions. Controls include:
- Use of narrow groove geometry to promote rapid cooling and reduce hot cracking susceptibility.
- Strict interpass temperature control (≤ 250°C) to minimize grain boundary liquation.
- Selection of appropriate filler metal with controlled sulfur and phosphorus content.
- Use of backing gas (argon or argon/helium mixture) to prevent oxidation and porosity.
- Avoidance of excessive travel speed or heat input that creates wide, flat weld beads.
6.2 Sensitization of Austenitic Stainless Steel HAZ
Prolonged exposure to the 450–850°C range during welding can cause chromium carbide precipitation at grain boundaries, leading to intergranular corrosion susceptibility. Controls include:
- Use of low-carbon (L-grade) filler metals (304L, 316L) with carbon content ≤ 0.03%.
- Minimization of total heat input per pass.
- Strict interpass temperature control (≤ 150°C for L-grade materials).
- Consideration of solution heat treatment for critical applications.
6.3 Excessive Dilution in Clad Welds
Excessive dilution of the base metal into the overlay weld reduces the corrosion resistance of the final surface layer. Controls include:
- Use of multi-pass overlay with the first pass (transition) using a highly alloyed filler (e.g., ER309L or ERNiCr-3) to reduce subsequent dilution.
- Limiting the number of overlay passes and ensuring adequate penetration of each pass.
- Verification of overlay composition by optical emission spectroscopy (OES) or XRF after welding.
- Adherence to SHT-3523-2020 dilution limits (typically ≤ 30% for single-pass overlay; ≤ 15% for multi-pass overlay).
6.4 Hydrogen-Induced Cracking in Carbon Steel Base
When welding dissimilar joints involving carbon steel, hydrogen-induced cracking (HIC) can occur in the carbon steel HAZ. Controls include:
- Appropriate preheat (100–250°C depending on carbon equivalent and thickness).
- Use of low-hydrogen electrode or wire (diffusible hydrogen ≤ 8 mL/100g for E7018).
- Post-weld drying at 100–150°C for 2–4 hours to allow hydrogen diffusion.
- Hardness control of the HAZ to remain below 350 HV (NACE MR0175/ISO 15156 requirement for sour service).
7. Application Across the Company's Three Technology Routes
7.1 TIG (GTAW) / MIG (GMAW) Weld Overlay
SHT-3523-2020 provides the governing qualification framework for TIG and MIG weld overlay operations. The standard's provisions on filler metal selection, interpass temperature, and NDE acceptance criteria are directly applicable to multi-pass overlay welds performed on equipment surfaces. Key applications include:
- TIG overlay of Inconel 625 on carbon steel for reactor internals and heat exchanger tubesheets, with qualification performed per SHT-3523-2020 Section 6.
- MIG overlay of 316L on clad plate edges for repair and finishing operations on large-format clad assemblies.
- TIG transition and overlay welds on clad pipes (per ASTM A270/A393) where the welding procedure must comply with SHT-3523-2020 when specified by the project owner.
7.2 Hydraulic Explosive Bonding (Hydrosplit / Hydraulic Cladding)
While hydraulic bonding produces the initial clad assembly through a solid-state process, SHT-3523-2020 becomes relevant during the fabrication of welded joints in the resulting clad product. The standard governs:
- Welding procedures for joining hydrosplit clad plates in pressure vessel fabrication.
- Acceptance criteria for welds that penetrate through the bonded interface, ensuring no degradation of the bonded layer's integrity.
- Qualification requirements for welders performing repair welds on hydrosplit clad surfaces where the bond layer has been locally damaged.
The company's hydrosplit products (e.g., SS316L/CS, 625/CS, Monel 400/CS clad plates) must be welded in accordance with SHT-3523-2020 when specified, requiring qualified WPS that address the unique challenge of welding through a cold-bonded interface without causing delamination or bond degradation.
7.3 Explosion Welding
Explosion welding produces clad assemblies with a metallurgically strong bond interface. SHT-3523-2020 applies to:
- Welding of explosion-welded clad plates and pipes during downstream fabrication (cutting, forming, welding of equipment components).
- Qualification of welding procedures for dissimilar metal welds where one side is an explosion-welded clad surface.
- NDE acceptance criteria for welds adjacent to explosion-welded interfaces, ensuring that the thermal effects of welding do not compromise the explosion bond.
For explosion-welded clad pipe (e.g., 625/CS per ASTM A270), the circumferential and longitudinal welds must be performed and inspected per SHT-3523-2020 when the project specification requires it. This includes the critical transition weld that must be qualified to ensure full penetration through the cladding layer with acceptable dilution.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
Systematic study and implementation of SHT-3523-2020 enables the company to:
- Develop and qualify a comprehensive library of WPS covering all common dissimilar weld combinations in petrochemical service.
- Qualify welder performance (WPQ) under SHT-3523-2020 requirements, expanding the pool of certified welders for high-alloy and clad welding operations.
- Establish qualification records that are directly acceptable to Shenhua Group, Sinopec, Sinochem, and other petrochemical clients who reference SHT standards in their procurement specifications.
- Integrate SHT-3523-2020 requirements into the company's quality management system (ISO 9001) and welding procedure management documentation.
8.2 Product Delivery
Mastery of SHT-3523-2020 directly enhances product delivery capability by:
- Reducing the risk of WPS rejection by client inspectors, thereby accelerating project timelines.
- Providing a standardized methodology for addressing welding queries and deviation requests during fabrication.
- Enabling the company to bid on projects that require SHT-qualified welding procedures, expanding the addressable market.
- Ensuring consistent quality across multiple production sites and shifts through standardized procedures.
8.3 Customer Value
For the end customer, SHT-3523-2020 compliance delivers:
- Assured long-term service life of clad and overlay components in aggressive petrochemical environments.
- Reduced lifecycle cost through minimized rework, unplanned shutdowns, and premature failure.
- Regulatory and insurance compliance with domestic petrochemical industry standards.
- Traceable quality documentation including WPS, WPQ, NDE reports, and mechanical test certificates that meet the standard's documentation requirements.
9. Implementation Recommendations
- Conduct a gap analysis between existing WPS library and SHT-3523-2020 requirements, identifying qualification gaps for each material combination.
- Develop and qualify WPS for the top 10 most frequently requested material combinations (e.g., CS+316L, CS+625, CS+Monel 400, 316L+625, CS+C-276) in priority order.
- Train and certify welders on the specific techniques required for each material group, with emphasis on heat input control, backing gas management, and overlay bead geometry.
- Establish a dedicated NDE protocol aligned with SHT-3523-2020 acceptance criteria, including dilution analysis procedures for clad welds.
- Integrate SHT-3523-2020 into the company's digital welding management system to ensure real-time monitoring of welding parameters and automatic generation of compliance documentation.
- Pursue third-party verification of SHT-3523-2020 qualifications through recognized testing laboratories to enhance credibility with major petrochemical clients.
10. Conclusion
SHT-3523-2020 represents a critical technical standard for any organization engaged in the fabrication of clad and overlay components for petrochemical service. Its comprehensive coverage of welding procedure qualification, filler metal selection, NDE requirements, and acceptance criteria for chromium-nickel stainless steels, iron-nickel alloys, nickel-based superalloys, and clad steel assemblies provides a robust framework for delivering high-quality, code-compliant products. For Cladding Technology Shanxi Co., Ltd., systematic implementation of this standard across all three technology routes—TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding—strengthens qualification credentials, accelerates product delivery, and delivers measurable value to petrochemical customers who demand the highest standards of welding integrity and corrosion resistance.