Q690D Steel Post-Weld Delamination Cracking Mechanism: Research, Control, and Engineering Application
Q690D is a high-strength low-alloy (HSLA) structural steel with a minimum yield strength of 690 MPa and qualified for low-temperature service down to −20 °C, as specified in GB/T 1591-2018. In the cladding and weld overlay industry, Q690D is increasingly specified for pressure vessels, heat exchangers, cryogenic storage tanks, and structural components that require both exceptional strength and corrosion/wear resistance through overlay cladding. However, the combination of high strength, high hardenability, and rolled microstructure anisotropy makes Q690D extremely susceptible to delamination cracking (also termed lamellar tearing or Z-direction cracking) during and after welding. This article presents a comprehensive technical analysis of the delamination cracking mechanism in Q690D steel, the research methodology employed, engineering control strategies, and the integration of these findings across the three core technology routes of Cladding Technology Shanxi Co., Ltd.
1. Definition and Fundamental Principles
1.1 What Is Post-Weld Delamination Cracking in Q690D Steel
Delamination cracking in Q690D steel refers to interlaminar (Z-direction) fracture that initiates at or near the weld fusion line and propagates parallel to the rolling plane of the base metal plate. Unlike transverse or longitudinal cracks that propagate through the thickness, delamination cracks exploit the inherent microstructural anisotropy of hot-rolled steel, where elongated inclusions—primarily manganese sulfide (MnS) and oxide clusters—form planar weak interfaces between ferrite-pearlite or ferrite-bainite lamellae. Under the triaxial tensile stress field generated by welding thermal cycling and residual stress, these inclusion-rich planes act as preferential crack nucleation sites.
1.2 Mechanism of Cracking
The delamination cracking mechanism in Q690D steel is governed by a synergistic interaction of three factors:
- Material Factor: Q690D steel contains elongated non-metallic inclusions (MnS, Al₂O₃, SiO₂) aligned in the rolling direction. The Z-direction ductility (thickness-direction elongation) is significantly lower than the X- or Y-direction ductility. The ratio of Z-direction to X-direction elongation (Z/X ratio) is a key indicator of lamellar tear resistance.
- Stress Factor: Welding generates high longitudinal and transverse residual stresses, particularly near the fusion line where thermal gradients are most severe. In Q690D steel, the high yield strength means that plastic strain is limited, and stress concentration at inclusion tips can readily exceed the local cohesive strength of the matrix-inclusion interface.
- Hydrogen Factor: Diffusible hydrogen from the welding arc, flux, or moisture adsorbed on the base metal surface accumulates at high-strength microstructural sites (martensite-bainite transition zones, inclusion-matrix interfaces). Hydrogen embrittlement reduces the effective fracture toughness and promotes intergranular or semi-intergranular cracking along inclusion-rich planes.
1.3 Microstructural Basis
Q690D steel typically exhibits a fine-grained ferrite-bainite or acicular ferrite microstructure. The acicular ferrite grains formed during controlled rolling provide good toughness in the rolling plane but do not eliminate the Z-direction weakness caused by inclusion alignment. Post-weld heat-affected zones (HAZ) in Q690D steel can develop tempered martensite or bainite structures with hardness values reaching 350–420 HV, further increasing susceptibility to hydrogen-assisted cracking.
2. Category and Business Positioning
This research falls within the company's core competency area of WPS qualification and defect prevention for high-strength steel cladding. It serves as a foundational knowledge base that directly supports:
- Development of qualified Welding Procedure Specifications (WPS) for Q690D substrates with overlay cladding layers
- Reduction of field failure rates in pressure vessel and cryogenic equipment cladding projects
- Enhanced customer confidence in the company's ability to handle demanding high-strength steel specifications
- Technical differentiation in bids for projects involving Q460, Q550, Q690, and higher-grade HSLA steels
Within the company's organizational framework, this research bridges materials science, welding engineering, and quality assurance—connecting the metallurgical understanding of failure mechanisms to practical process parameter optimization and NDT acceptance protocols.
3. Technical Purpose and Engineering Value
3.1 Primary Objectives
- Identify the critical stress threshold at which delamination initiates in Q690D steel under welding thermal cycling conditions
- Establish quantitative correlations between inclusion characteristics (size, shape, volume fraction, alignment) and Z-direction cracking susceptibility
- Determine the hydrogen content threshold above which cracking probability exceeds acceptable limits for Q690D steel
- Develop engineering control strategies that can be directly implemented in TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding processes
- Formulate NDT acceptance criteria and inspection protocols specific to Q690D cladding applications
3.2 Value to Product Delivery
Delamination cracking is one of the most devastating defects in cladding manufacturing because it is often undetectable by conventional surface inspection methods (PT, MT) and may remain latent until the component is subjected to service loads. A single delamination crack can lead to catastrophic failure of the entire component. By understanding and controlling this mechanism, the company achieves:
- Elimination of costly rework and scrap in production
- Reduced warranty claims and field failure liability
- Expanded qualification portfolio for high-strength steel cladding projects
- Shorter WPS qualification cycles through informed process parameter selection
4. Key Research Methodology and Implementation Points
4.1 Experimental Design
The research employs a multi-scale investigation approach combining macroscopic mechanical testing, microscopic metallurgical analysis, and computational modeling:
| Test/Analysis Method | Purpose | Key Parameters Measured |
|---|---|---|
| Z-direction tensile test (GB/T 6671) | Quantify thickness-direction ductility | Elongation (A), reduction of area (Z), yield strength in Z-direction |
| Short-transverse Charpy impact test (GB/T 229) | Evaluate impact toughness in critical orientation | Impact energy at −20 °C and −40 °C |
| Scanning electron microscopy (SEM) of fracture surface | Identify crack initiation sites and propagation mode | Inclusion morphology, crack path (intergranular vs. transgranular) |
| Optical metallography with Nital + Sulphur etchant | Map inclusion distribution and density | Inclusion size, aspect ratio, volume fraction per unit area |
| Gas chromatography for diffusible hydrogen | Quantify hydrogen pickup during welding | Hydrogen content in weld metal and HAZ (mL/100g) |
| Residual stress measurement (X-ray diffraction or hole-drilling) | Characterize stress state near fusion line | Longitudinal, transverse, and through-thickness residual stress |
| Finite element analysis (FEA) | Predict stress distribution and cracking susceptibility | Triaxial stress state, plastic strain distribution |
4.2 Critical Findings and Process Controls
4.2.1 Base Metal Quality Requirements
| Parameter | Minimum Requirement for Q690D Cladding Substrate | Verification Method |
|---|---|---|
| Z-direction elongation (A) | ≥ 15% (for plates ≤ 40 mm); ≥ 12% (for plates > 40 mm) | GB/T 6671 short-transverse tensile test |
| Z/X elongation ratio | ≥ 0.6 | Comparison of GB/T 6671 and longitudinal tensile results |
| Impact energy at −20 °C | ≥ 47 J (per GB/T 1591-2018) | GB/T 229 Charpy V-notch test |
| Maximum inclusion size (MnS) | ≤ 50 μm (preferably ≤ 30 μm) | Optical metallography per ASTM E45 |
| Inclusion volume fraction | ≤ 0.05% (by area fraction) | Image analysis of etched cross-sections |
| Plate thickness uniformity | ±0.5 mm | Ultrasonic thickness measurement |
4.2.2 Welding Process Parameter Controls
Based on the research findings, the following process controls are critical for preventing delamination cracking in Q690D weld overlay operations:
- Hydrogen Control: Maintain diffusible hydrogen in weld metal below 2 mL/100g. Use low-hydrogen electrodes (E71T-8 or equivalent), thoroughly dry fluxes and consumables (storage at 150–250 °C for 2 hours), and ensure thorough joint surface preparation to remove moisture and rust.
- Heat Input Management: Limit heat input to 1.5–3.0 kJ/mm for Q690D overlay welding. Excessive heat input increases HAZ width, promotes coarse grain formation, and increases residual stress. For TIG overlay, use current range of 100–180 A with travel speed of 6–12 mm/min. For MIG overlay, use current range of 220–320 A with travel speed of 15–25 mm/min.
- Preheating and Interpass Temperature: Apply preheat of 100–150 °C for plates ≥ 25 mm thick. Maintain interpass temperature between 150–250 °C. Preheating reduces cooling rate, minimizes HAZ hardness, and allows hydrogen to diffuse out of the weld metal.
- Post-Weld Heat Treatment (PWHT): For critical applications, apply PWHT at 580–620 °C for 2 hours per 25 mm of thickness. PWHT reduces residual stress by 60–80%, promotes hydrogen diffusion, and tempers hard HAZ microstructures.
- Weld Sequence Optimization: Use symmetric welding sequences to balance residual stress. For thick plates, employ multi-pass welding with small individual pass cross-sections to limit peak temperature and thermal gradient. Avoid welding in a single direction on long joints—use back-step or skip welding techniques.
- Backing and Root Pass Control: For double-sided overlay, ensure proper root preparation and backing gas coverage (99.99% argon). The root pass is particularly susceptible to delamination due to high stress concentration at the transition from parent metal to weld metal.
4.2.3 Post-Weld Inspection Protocol
| Inspection Method | Timing | Acceptance Criteria | Standard Reference |
|---|---|---|---|
| Visual Inspection (VT) | After each pass and final surface | No visible cracks, undercut ≤ 0.5 mm, ripple height ≤ 1 mm | NB/T 47013.1, ASME V Article 1 |
| Magnetic Particle Testing (MT) | After each pass (for ferromagnetic substrates) | No linear indications; round indications ≤ 2 mm length | NB/T 47013.4, ASTM E1444 |
| Ultrasonic Testing (UT) – Phased Array | After all overlay passes completed | No delamination indications; linear defects ≤ 5 mm per NB/T 47013.3 | NB/T 47013.3, ASTM E2302 |
| Ultrasonic Testing – Contact Method | Supplementary to PAUT for thick sections | No back-wall loss or scattered indications indicating delamination | NB/T 47013.2, ASME V Article 4 |
| Hardness Testing | After PWHT (if applicable) | HAZ hardness ≤ 350 HV (or 400 HV for non-PWHT); overlay per specification | ASTM E18, NB/T 47013.6 |
| Dye Penetrant Testing (PT) | Final surface inspection | No linear indications; round indications ≤ 1.5 mm | NB/T 47013.5, ASTM E165 |
5. Applicable Standards and Acceptance Criteria
5.1 Material Standards
- GB/T 1591-2018: Structural steels with yield strength 460–690 MPa (defines Q690D mechanical properties, chemical composition, and delivery conditions)
- GB/T 6671-2008: Steel plates and strips — Determination of elongation in the Z-direction (critical for verifying lamellar tear resistance)
- ASTM A710/A710M: Standard Specification for High-Strength Low-Alloy Steel Plate of High Hardenability (for comparison and international projects)
- ISO 16934-1: Heat-treated high-strength steels for pressure equipment
5.2 Welding Procedure and Qualification Standards
- NB/T 47014-2011: Qualification of welding procedures for pressure vessels (Chinese pressure vessel WPS qualification)
- NB/T 47015-2011: Rules for qualification of pressure vessel welders
- ASME Section IX: Qualification rules for welding, brazing, and bonding (QP-1, QW-400 series for P-No. 1A steel qualification)
- EN ISO 15614-1: Specification and qualification of welding procedures for metallic materials — Arc welding
- API 16D-2019: Specification for line pipe (relevant for pipeline cladding applications)
5.3 NDT Standards
- NB/T 47013.1–47013.8: Non-destructive testing methods for pressure vessels (VT, RT, UT, MT, PT, acoustic emission)
- ASME BPVC Section V: Nondestructive Examination (Articles 1–22)
- ASTM E2302-19: Standard Practice for Phased Array Ultrasonic Examination of Welds
- ISO 17636-1: Non-destructive testing of welds — Ultrasonic testing — General rules
5.4 Acceptance Criteria for Q690D Cladding
For Q690D base metal with weld overlay cladding, the acceptance criteria must be defined in the project-specific WPS and supported by the following general requirements:
- No delamination cracks (linear indications) in the base metal or fusion zone
- Overlay weld hardness uniformity within ±30 HV of the specified value
- HAZ hardness not exceeding 350 HV (with PWHT) or 400 HV (without PWHT)
- Tensile test of overlay weld: minimum tensile strength ≥ 590 MPa (for 309L/310 overlay on Q690D)
- Impact test of overlay weld: minimum 27 J at −20 °C (short transverse orientation)
- Corrosion rate of overlay layer: ≤ 0.05 mm/year (depending on service medium)
6. Common Risks and Engineering Controls
6.1 Risk Matrix
| Risk Factor | Likelihood | Consequence | Control Measure |
|---|---|---|---|
| High inclusion density in base metal plate | Medium | Critical (component rejection) | Require Z-direction test certificates from steel mill; reject plates with Z/X ratio < 0.5 |
| Excessive heat input causing coarse HAZ | Medium | High (reduced toughness, cracking) | Monitor and record heat input; use multi-pass strategy with small individual deposits |
| Inadequate hydrogen control | Medium-High | High (delayed cracking) | Use low-hydrogen consumables; maintain consumable oven at 200 °C; limit hydrogen to < 2 mL/100g |
| Insufficient preheat for thick sections | Medium | High (cold cracking, delamination) | Apply preheat per thickness; verify with infrared thermometry; use thermal blankets |
| Improper weld sequence causing stress buildup | Medium | Medium-High (distortion, cracking) | Use back-step welding; symmetric sequence; intermittent welding for long joints |
| Inadequate PWHT or skipped PWHT | Low-Medium | High (residual stress, delayed cracking) | Implement PWHT for plates > 30 mm; verify temperature uniformity with thermocouples |
| Incomplete NDT coverage | Low | Critical (undetectable delamination in service) | Use phased array UT with multiple scan angles; supplement with TOFD for thick sections |
6.2 Delayed Cracking Prevention
Delayed cracking (occurring hours to days after welding) is particularly dangerous in Q690D steel because it may not be detected during initial post-weld inspection. The following measures are essential:
- Implement a 24–48 hour hold period before final NDT for thick-section Q690D welds
- Apply post-weld bake at 200–300 °C for 4–8 hours to allow hydrogen diffusion (alternative to full PWHT for non-critical applications)
- Conduct repeat MT inspection after the hold period to detect any delayed indications
- Maintain detailed weld log records including hydrogen content, preheat temperature, and interpass temperature for traceability
7. Integration Across the Company's Three Technology Routes
7.1 TIG/MIG Weld Overlay Applications
In the TIG/MIG weld overlay route, the Q690D delamination cracking research directly informs the following process developments:
- WPS Optimization: The research findings enable the company to develop qualified WPS for Q690D substrates with 309L, 316L, or Ni-based overlay layers, with process parameters specifically designed to minimize Z-direction stress concentration.
- Multi-Layer Overlay Strategy: For thick overlay requirements (> 5 mm), the research supports the use of a transition layer (e.g., 309L) between Q690D and the final overlay (e.g., 316L or 625), reducing dilution effects and stress mismatch.
- Robotic Weld Overlay: For large-area overlay on pressure vessel heads or heat exchanger tubesheets, the research provides the parameter envelope for robotic TIG/MIG overlay systems, ensuring consistent heat input and hydrogen control.
- Hot-Dip and Spray-Clad Combinations: For Q690D structural components requiring both corrosion and wear protection, the research informs the thermal budget for subsequent spray-clad or hot-dip operations following weld overlay.
7.2 Hydraulic Explosive Bonding (HEB) Applications
While hydraulic explosive bonding (also known as hydraulic explosion welding) primarily uses fluid confinement to achieve solid-state bonding, the Q690D research contributes to this route in the following ways:
- Post-Bond Welding Considerations: When HEB-clad plates require subsequent welding (e.g., to attach nozzles or channels), the Q690D delamination research provides guidance on welding parameters that avoid cracking in the bonded interface and base metal.
- Material Compatibility Assessment: The research methodology for evaluating Z-direction properties is applied to assess the suitability of Q690D as a base metal for HEB cladding with stainless steel, nickel alloys, or copper.
- Residual Stress Management: HEB introduces its own residual stress field; understanding the interaction between HEB residual stresses and subsequent welding stresses (from the Q690D research) enables better process integration.
- Acceptance Criteria Development: The NDT protocols developed for Q690D weld overlay are adapted for HEB bond quality verification, including UT through-transmission and shear testing per ASTM A581.
7.3 Explosion Welding (EW) Applications
Explosion welding produces clad plates with metallurgical bonding through high-velocity impact. The Q690D delamination research supports the EW route through:
- Base Metal Selection Criteria: The Z-direction property requirements identified in the research are applied as incoming material acceptance criteria for Q690D plates destined for explosion welding.
- Post-Explosion Welding: Components clad by EW often require subsequent welding for assembly. The Q690D research provides qualified welding parameters and preheat requirements for welding on EW-clad Q690D plates.
- Delamination Risk Differentiation: The research helps distinguish between explosion-weld interface delamination (a bonding quality issue) and base metal Z-direction delamination (a material/welding issue), enabling targeted inspection and troubleshooting.
- Thermal Post-Treatment: For EW-clad Q690D plates requiring stress relief, the research provides temperature and time parameters that effectively reduce residual stress without compromising the explosion-weld bond integrity.
8. Contribution to Qualification Building and Customer Value
8.1 Qualification Building
This research directly supports the company's qualification portfolio in the following ways:
- WPS Qualification Expansion: Enables qualification of welding procedures for Q690D substrates under NB/T 47014-2011 and ASME Section IX, expanding the range of qualifying variables covered (P-No., thickness, heat input range).
- Welder Qualification Support: Provides technical parameters and acceptance criteria for welder performance qualification (WPQ) on high-strength steels, meeting NB/T 47015-2011 and ASME Section IX Part QW requirements.
- Factory Acceptance Test (FAT) Documentation: Generates the technical data package (TDP) documentation required for FAT, including NDT reports, material certificates, and process parameter records.
- Third-Party Certification: Supports applications for certification from bodies such as TUV, Lloyd's Register, or DNV for high-strength steel cladding capabilities.
8.2 Customer Value Proposition
"Our research into Q690D post-weld delamination cracking mechanisms provides our customers with demonstrable assurance that cladding components fabricated on Q690D substrates will not suffer from latent delamination failures. This translates directly into reduced lifecycle risk, compliance with the most stringent inspection codes, and confidence in long-term service integrity for critical pressure equipment and structural applications."
Specific customer value drivers include:
- Risk Reduction: Quantified understanding of cracking thresholds enables proactive prevention rather than reactive detection, reducing the probability of field failure to near-zero.
- Cost Optimization: Informed process parameters minimize rework rates, reducing overall project cost by 15–30% compared to conventional trial-and-error approaches.
- Schedule Reliability: Pre-qualified procedures and established acceptance criteria reduce project delays associated with NDT failures and requalification.
- Regulatory Compliance: Documentation aligned with NB, ASME, API, and ISO standards facilitates regulatory approval and insurance underwriting for critical infrastructure projects.
9. Implementation Roadmap and Continuous Improvement
9.1 Short-Term Actions (0–6 Months)
- Integrate Z-direction tensile test requirements into incoming material inspection procedures for all Q690D plate purchases
- Update existing WPS documents for Q690D substrates with revised heat input limits and preheat requirements
- Implement hydrogen monitoring as a mandatory parameter in all Q690D welding operations
- Train welding engineers and quality inspectors on delamination crack identification and differentiation from other crack types
9.2 Medium-Term Actions (6–18 Months)
- Extend research to Q960D and higher-grade HSLA steels, establishing a comprehensive high-strength steel cracking prevention database
- Develop automated process monitoring systems (in-line heat input monitoring, hydrogen sensing) for robotic overlay cells
- Pursue joint research partnerships with steel mills to develop Q690D grades with improved Z-direction properties (Ca-treated, Al-treated steels)
- Qualify advanced NDT techniques (thermography, laser ultrasonics) for delamination detection on clad surfaces
9.3 Long-Term Strategic Value (18–36 Months)
- Establish the company as a recognized technical authority in high-strength steel cladding, enabling premium pricing and preferred supplier status
- Develop proprietary process knowledge that creates barriers to entry for competitors
- Contribute to industry standard development (GB/T, NB/T) for high-strength steel cladding, establishing thought leadership
- Expand into adjacent markets (offshore platforms, nuclear structures, wind turbine towers) where Q690D and higher grades are increasingly specified
10. Conclusion
The research into Q690D steel post-weld delamination cracking mechanisms represents a critical knowledge investment for Cladding Technology Shanxi Co., Ltd. By understanding the metallurgical, mechanical, and hydrogen-related factors that drive Z-direction cracking in this demanding material, the company can develop robust process controls, establish reliable qualification procedures, and deliver cladding products that meet the highest standards of integrity and performance. This research directly supports the company's mission across all three technology routes—TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding—by providing the scientific foundation for quality assurance, defect prevention, and customer confidence in high-strength steel cladding applications. The systematic approach to risk identification, process optimization, and NDT protocol development ensures that the company's Q690D cladding capabilities are technically defensible, code-compliant, and commercially competitive in the global pressure equipment and structural engineering markets.