Hardness Distribution and Impact Toughness Analysis of Steel-Based Weld Overlay Clad Plates
1. Definition and Technical Principles
Weld overlay cladding is a surface engineering process in which a layer of wear-resistant, corrosion-resistant, or high-temperature alloy material is deposited onto a steel substrate through fusion welding techniques. The resulting clad plate exhibits a composite microstructure in which the functional surface layer, the transition (interfacial) zone, and the base metal each possess distinct mechanical properties. Understanding the hardness distribution and impact toughness across these three zones is fundamental to ensuring that the clad plate meets design specifications for both wear/corrosion resistance and structural integrity.
The hardness gradient from base metal to overlay layer is governed by several metallurgical factors:
- Composition mismatch: The overlay alloy (e.g., 309L, 310, 2205, or hardfacing alloys) typically contains higher levels of alloying elements (Cr, Ni, Mo, C) than the carbon or low-alloy steel base, resulting in elevated hardness in the deposited layer.
- Thermal cycling effects: Multiple weld passes produce repeated heating and cooling cycles that refine grain structure in the transition zone and can induce tempering of the base metal near the fusion line.
- Dilution and intermixing: Incomplete fusion or excessive penetration can cause base metal dilution in the overlay, reducing hardness and compromising corrosion resistance. Conversely, overlay material penetration into the base can create brittle phases at the interface.
- Residual stress: Differential thermal contraction between overlay and base metal generates residual tensile stresses at the interface, which can reduce impact toughness and increase susceptibility to cracking.
Impact toughness, measured by Charpy V-notch (CVN) testing, is equally critical because it quantifies the material's ability to absorb energy under dynamic or low-temperature loading conditions. A well-designed weld overlay clad plate must exhibit adequate impact toughness not only in the base metal but also in the transition zone and overlay layer, particularly for applications subject to thermal shock or mechanical impact.
2. Category and Business Positioning
This research capability falls squarely within the company's TIG/MIG weld overlay technology route and serves as a critical knowledge asset for product qualification and process optimization. The study of hardness distribution and impact toughness is not merely academic—it directly informs:
- WPS (Welding Procedure Specification) development and qualification testing
- Welding parameter selection (heat input, travel speed, interpass temperature)
- Layer design (number of passes, transition layer selection)
- Post-weld heat treatment (PWHT) protocols
- Acceptance criteria for customer-specific applications
By systematically characterizing the mechanical property profiles of clad plates produced under various conditions, Cladding Technology Shanxi Co., Ltd. can demonstrate to customers and certification bodies that their products meet or exceed applicable standards, thereby building trust and expanding market access.
3. Technical Purpose and Value
3.1 Ensuring Structural Integrity
The primary engineering concern with weld overlay clad plates is that the transition zone becomes the weakest link. If hardness is too high in the transition zone without corresponding toughness, the material becomes susceptible to brittle fracture. Conversely, if hardness is too low, the overlay fails to provide the intended wear or corrosion protection. The research establishes optimal windows for hardness (typically 200–400 HV for austenitic overlays, 400–600 HV for hardfacing overlays) and impact toughness (typically ≥27 J at test temperature per ASTM A388 or customer specification) that balance these competing requirements.
3.2 Process Optimization and Cost Control
Understanding the relationship between welding parameters and resulting mechanical properties enables the company to:
- Minimize the number of weld passes required to achieve target overlay thickness, reducing labor and consumable costs
- Identify the optimal heat input range that produces acceptable hardness without excessive grain growth or cracking
- Determine whether PWHT is necessary for a given overlay/base combination, potentially eliminating a costly manufacturing step
3.3 Qualification Building and Customer Confidence
Detailed hardness and impact toughness data packages are essential for:
- WPS/PQR qualification per ASME Section IX, AWS D10.9, or NB/T 47014
- Product certification for pressure vessel and piping applications per ASME Section VIII Div. 1 or GB/T 150
- Customer-specific qualification programs in oil & gas (API 660/661), power generation, and mining sectors
- Demonstrating compliance with NACE MR0175/ISO 15156 for sour service applications
4. Key Process and Implementation Points
4.1 Hardness Measurement Protocol
Hardness distribution is typically measured along a cross-sectional line perpendicular to the clad surface, traversing from base metal through the transition zone to the overlay surface. The following table summarizes typical measurement parameters:
| Parameter | Specification | Notes |
|---|---|---|
| Test Method | Vickers (HV) or Rockwell (HRB/HRC) | HV preferred for gradient mapping; per ASTM E92 or ISO 6507 |
| Indentation Spacing | 1–2 mm (HV 5 or HV 10) | Closer spacing in transition zone (0.5 mm) |
| Sampling Depth | Full section thickness | Include base metal ≥5 mm from interface |
| Number of Test Lines | Minimum 3 per plate coupon | Statistical confidence in gradient characterization |
| Temperature | Ambient (20 ± 5°C) | Per ASTM E92 |
4.2 Impact Toughness Testing Protocol
| Parameter | Specification | Notes |
|---|---|---|
| Test Method | Charpy V-Notch (CVN) | ASTM E23 or GB/T 229 |
| Specimen Orientation | T-NL, L-NL, or T-NR per customer spec | T-NL (transverse, notch perpendicular to weld) is most conservative |
| Test Temperatures | 20°C, -10°C, -20°C, -40°C (as applicable) | Low-temperature testing for cryogenic or arctic service |
| Notch Location | In base, at interface, in overlay | Three separate specimen sets per location |
| Acceptance Criteria | ≥27 J (1 ft-lb) average per ASTM A388 | Individual specimen ≥20 J minimum |
4.3 Welding Parameter Optimization
The following table illustrates how key TIG/MIG welding parameters influence hardness and impact toughness:
| Parameter | Effect on Hardness | Effect on Impact Toughness | Optimal Range (Typical) |
|---|---|---|---|
| Heat Input (kJ/mm) | Higher → lower hardness (tempering) | Higher → improved toughness (grain refinement) | 0.8–2.5 kJ/mm for austenitic overlay |
| Travel Speed (mm/min) | Faster → higher hardness | Faster → reduced toughness | 200–400 mm/min (TIG) |
| Interpass Temperature (°C) | Lower → higher hardness | Higher → improved toughness | 100–250°C (austenitic); ≤150°C (hardfacing) |
| Wire Diameter (mm) | Larger → lower hardness per pass | Larger → improved toughness | 1.0–1.6 mm (TIG); 1.2 mm (MIG) |
| Shielding Gas | Ar/CO₂ mix → higher hardness | Pure Ar → better toughness | 100% Ar or Ar + 2% O₂ (TIG); Ar + 5% CO₂ (MIG) |
4.4 Transition Layer Design
For dissimilar metal combinations (e.g., austenitic overlay on low-alloy steel base), a transition layer is often required to:
- Reduce dilution effects on the functional overlay layer
- Accommodate thermal expansion coefficient mismatch
- Reduce residual stress at the interface
- Prevent cracking during subsequent welding or PWHT
Common transition layer alloys include:
- 309L / 309Mo: For overlay of 316L/310 on carbon steel or 15CrMo base
- E309L filler: TIG transition between 316L overlay and P91 base
- 2205 duplex: As transition for 2507 overlay on low-alloy steel
5. Applicable Standards and Acceptance Criteria
5.1 Material and Product Standards
- ASTM A388: Standard Specification for Clad Steel Plate, Sheet, and Strip for Pressure Vessel Applications
- ASTM A403: Standard Specification for Stainless Steel, Alloy Steel, and Composite Steel Plate, Sheet, and Strip for Pressure Vessels and Other Pressed Parts
- GB/T 24511-2017: Steel Clad Plate and Strip (Chinese national standard)
- NB/T 47090-2014: Technical Conditions for Composite Steel Plate (Chinese industry standard for pressure vessels)
- ASME SA-388 / SA-403: Clad and composite steel plate for pressure vessels
- ASTM A563: Standard Specification for Clad Steel Plate, Sheet, and Strip for General Applications
5.2 Welding Procedure Standards
- ASME Section IX: Qualification Rules for Welding, Brazing, and Fusing
- AWS D10.9M/D10.9: Welding Procedure and Performance Qualification for Fusion Welding of Non-Ferrous Metals (adapted for overlay)
- NB/T 47014-2011: Qualification Test for Welding Procedure of Pressure Vessels
- ISO 15614-1: Qualification Test for Welding Procedures for Metals — Arc Welding
5.3 Testing and Acceptance Standards
- ASTM E92: Standard Test Method for Vickers Hardness of Metallic Materials
- ASTM E23: Standard Test Method for Notched Bar Impact Testing of Metallic Materials
- GB/T 229-2020: Charpy Impact Test Method for Metals
- ASTM A388 Section 6: Hardness limits (overlay ≤ specified max; base ≥ specified min)
- ASTM A388 Section 7: Impact test requirements (≥27 J at specified temperature)
- NACE MR0175/ISO 15156: Hardness limits for sour service (≤22 HRC for carbon steel; specific limits for austenitic and duplex alloys)
- API 660/661: Hardness and impact toughness requirements for heat exchanger tubes and bundles
5.4 Typical Acceptance Criteria Summary
| Property | Standard | Typical Acceptance Criterion |
|---|---|---|
| Overlay Hardness (316L) | ASTM A388 | ≤250 HV (≤25 HRC) |
| Overlay Hardness (310) | ASTM A388 | ≤250 HV |
| Overlay Hardness (Hardfacing) | Customer spec | 400–600 HV (varies by alloy) |
| Base Metal Hardness | ASTM A388 | ≥120 HV (≥120 B) |
| Impact Toughness (20°C) | ASTM A388 | ≥27 J average |
| Impact Toughness (-20°C) | Customer spec | ≥27 J average (for cryogenic service) |
| Interface Bond Strength | ASTM A388 | Full fusion (no separation on macrograph) |
6. Common Risks and Controls
6.1 Excessive Hardness in Transition Zone
Risk: If the welding parameters produce a narrow, rapidly solidified transition zone with high hardness (e.g., >400 HV for austenitic overlay), the interface becomes susceptible to cracking under thermal cycling or mechanical loading.
Controls:
- Use a dedicated transition layer alloy with composition intermediate between base and overlay
- Control heat input to ensure adequate melting of the interface (avoid cold lap)
- Apply post-weld heat treatment (PWHT) to soften the transition zone (e.g., 650–750°C for 1 hour for austenitic overlays)
- Verify hardness gradient through cross-sectional testing before shipping
6.2 Reduced Impact Toughness
Risk: High heat input, excessive interpass temperature, or improper grain refiner content in the filler metal can lead to coarse grain structure and reduced Charpy V-notch energy.
Controls:
- Maintain interpass temperature within specified limits (typically ≤250°C for austenitic, ≤150°C for martensitic/hardfacing)
- Use filler metals with appropriate grain refiners (Nb, Ti, Zr additions)
- Perform impact testing at the required temperature and orientation during WPS qualification
- Implement in-process monitoring of welding parameters (CMT, pulsed TIG for heat input control)
6.3 Cracking at Interface
Risk: Dissimilar metals with large thermal expansion coefficient differences can develop interfacial cracking during welding, PWHT, or in-service thermal cycling.
Controls:
- Select compatible base/overlay combinations per ASTM A388 Table 1 or customer specification
- Use low-stress welding sequences (symmetrical pass patterns, back-step welding)
- Apply appropriate PWHT to relieve residual stresses
- Perform magnetic particle inspection (MT) or penetrant testing (PT) of the interface area
6.4 Incomplete Fusion
Risk: Insufficient heat input or improper torch manipulation can result in lack of fusion at the base/overlay interface, creating a discontinuity that severely compromises structural integrity.
Controls:
- Maintain minimum heat input per qualification test results
- Use backing bars or backing gas to ensure full penetration
- Perform ultrasonic testing (UT) or radiographic testing (RT) to detect interface defects
- Train welders on proper torch angle and travel technique for overlay applications
7. Application Scenarios Across Technology Routes
7.1 TIG/MIG Weld Overlay Route
This research directly supports the TIG/MIG weld overlay route, which is the company's primary technology for producing clad plates and pipes with precise control over overlay composition and thickness. Key applications include:
- Oil & Gas Heat Exchangers: 316L or 2205 overlay on carbon steel tubesheets for sour service (NACE MR0175 compliant). Hardness must be ≤22 HRC for base metal and ≤25 HRC for overlay to prevent sulfide stress cracking.
- Power Generation Boiler Tubes: 310 or 625 overlay on P91 or 9Cr-1Mo tubes for high-temperature corrosion resistance. Impact toughness at 20°C must exceed 27 J to withstand thermal shock during startup/shutdown.
- Mining and Cement Industry: Hardfacing overlay (e.g., Ni-based or Cr-C alloy) on steel liners for abrasion resistance. Hardness of 450–600 HV provides wear life extension while maintaining adequate toughness for impact loading.
- Chemical Processing: 2507 or Hastelloy overlay on carbon steel piping for chloride pitting resistance. Transition layer hardness and toughness are critical to prevent intergranular cracking.
7.2 Hydraulic Explosive Bonding Route
While hydraulic explosive bonding produces clad plates through solid-state diffusion bonding (without melting), the research on hardness distribution and impact toughness remains relevant for:
- Post-bonding annealing: Determining optimal annealing temperature and duration to achieve desired hardness in the bonded interface zone while maintaining impact toughness
- Interface characterization: Comparing the mechanical properties of the bonded interface with weld overlay interfaces to validate the bonding quality
- Multi-layer designs: When a bonded plate is subsequently weld-overlaid with an additional functional layer, understanding the existing hardness/toughness profile is essential for WPS development
- Acceptance testing: Applying the same hardness and impact toughness criteria (per ASTM A388) to bonded clad plates as to weld overlay clad plates
7.3 Explosion Welding Route
Explosion welding produces clad plates through high-velocity impact bonding, creating a characteristic wavy interface with metallurgical bonding. The research contributes to:
- Interface hardness mapping: Characterizing the work-hardened zone adjacent to the wavy interface, which typically exhibits elevated hardness due to plastic deformation during impact
- Toughness verification: Ensuring that the impact-toughness of the clad plate (particularly in the base metal adjacent to the interface) meets requirements for structural applications
- Subsequent welding qualification: When explosion-welded clad plates are further processed (e.g., welding of overlay layers for specific corrosion protection), the existing hardness/toughness profile informs the welding procedure
- Product differentiation: Demonstrating that explosion-welded clad plates achieve equivalent or superior mechanical properties compared to weld overlay alternatives, supporting competitive positioning
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
The systematic study of hardness distribution and impact toughness directly supports the company's qualification infrastructure:
- WPS/PQR packages: Each welding procedure qualification requires documented hardness and impact test results demonstrating compliance with ASME Section IX, NB/T 47014, or ISO 15614-1
- Product certifications: Third-party inspection agencies (e.g., ABS, DNV, LR, CCIC) require mechanical property data as part of product certification for pressure vessel and piping applications
- Customer-specific qualifications: Major oil & gas and power generation customers (e.g., Shell, BP, Sinopec, State Grid) require detailed mechanical property data packages as part of their supplier qualification programs
- ISO 9001 / API Q1 quality systems: Documented research and testing data demonstrate the company's commitment to continuous improvement and process control
8.2 Product Delivery
Understanding hardness and impact toughness profiles enables the company to:
- Guarantee product performance: Provide customers with certified mechanical property data packages demonstrating compliance with specifications
- Optimize production parameters: Reduce trial-and-error in new product development by leveraging existing research data
- Minimize rework and scrap: Predictive understanding of property outcomes reduces the likelihood of non-conforming products
- Accelerate delivery timelines: Pre-qualified procedures and established testing protocols reduce qualification lead times
8.3 Customer Value
The research translates directly into customer value through:
- Reduced lifecycle cost: Optimized overlay design extends service life of critical components (heat exchangers, boiler tubes, mining equipment), reducing unplanned shutdowns and replacement costs
- Enhanced safety: Adequate impact toughness ensures structural integrity under dynamic loading, preventing catastrophic failures
- Regulatory compliance: Certified mechanical property data ensures products meet applicable codes and standards, reducing regulatory risk for customers
- Technical support: The company can provide customers with detailed hardness/toughness data to support their own engineering calculations and design verifications
9. Conclusion
The research on hardness distribution and impact toughness of steel-based weld overlay clad plates represents a foundational capability for Cladding Technology Shanxi Co., Ltd. It bridges the gap between metallurgical science and practical manufacturing, enabling the company to deliver clad products that reliably meet the demanding mechanical property requirements of critical industrial applications. By maintaining rigorous testing protocols, adhering to applicable standards (ASTM A388, ASME Section IX, NB/T 47014, NACE MR0175/ISO 15156, and others), and continuously refining process parameters based on research findings, the company positions itself as a technically credible and quality-focused supplier in the global clad plate and pipe market.
This capability is particularly valuable in the current market environment where customers increasingly demand traceable, certified mechanical property data as part of their procurement and qualification processes. The company's investment in understanding and controlling hardness and impact toughness profiles provides a competitive advantage that extends beyond simple product manufacturing into the realm of engineering partnership and long-term customer relationships.