Microstructure and Properties of MAG Weld Overlay Deposits on Grade E Steel
1. Definition and Technical Context
Grade E steel, commonly designated under ASTM A514 Grade E (a quenched-and-tempered high-strength low-alloy structural steel with minimum yield strength of 950 MPa / 137 ksi), represents one of the most demanding substrate materials for weld overlay applications. The MAG (Metal Active Gas) weld overlay process applied to Grade E steel involves the sequential deposition of one or more layers of weld metal using a consumable electrode wire in a shielding gas atmosphere, typically CO₂, Ar/CO₂ mixtures, or He/Ar blends, to build up a corrosion-resistant, wear-resistant, or transition layer on the base metal surface.
The study of microstructure and mechanical properties of MAG weld overlay deposits on Grade E steel is a critical metallurgical research activity that bridges the gap between process development and production qualification. It addresses the fundamental challenge of depositing weld metal onto an extremely hard, high-carbon-equivalent substrate where the Heat Affected Zone (HAZ) is inherently susceptible to martensitic transformation, micro-cracking, and hydrogen-induced delayed cracking.
2. Technical Purpose and Value
2.1 Metallurgical Understanding
The primary technical purpose of studying MAG weld overlay microstructure and properties on Grade E steel is to establish a scientifically rigorous foundation for:
- WPS qualification — determining the optimal combination of current, voltage, travel speed, wire feed rate, gas composition, and preheat that produces acceptable microstructure and mechanical performance
- Layer integrity verification — confirming that each deposited layer achieves proper metallurgical bonding with the substrate and with preceding layers without excessive dilution
- Residual stress management — understanding how thermal cycling during multi-pass MAG overlay affects the residual stress state and distortion behavior of the substrate
- Service life prediction — correlating microstructural features (grain size, phase composition, carbide distribution) with long-term mechanical and corrosion performance
2.2 Business and Qualification Value
This technical capability directly supports Cladding Technology Shanxi Co., Ltd.'s qualification portfolio by:
- Enabling WPS/PQR development for high-strength steel substrates under ASME Section IX and GB/T 19243
- Providing documented metallurgical evidence for customer audits and regulatory inspections under NB/T 47013 (China's NDT standard for pressure equipment)
- Reducing trial-and-error costs during production by leveraging pre-established process windows
- Enhancing customer confidence through published metallurgical data and independent test reports
3. Microstructural Analysis of MAG Weld Overlay on Grade E Steel
3.1 Base Metal Microstructure
ASTM A514 Grade E steel is delivered in a quenched-and-tempered condition with a base microstructure consisting of tempered martensite with a fine dispersion of carbides. The carbon equivalent (Ceq) typically ranges from 0.45% to 0.55%, calculated per IIW formula:
Ceq = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15
This high Ceq value means that during welding, the HAZ experiences rapid austenitization followed by extremely fast cooling rates (often exceeding 100°C/s in thin sections), resulting in the formation of untempered or partially tempered martensite with hardness values potentially reaching 450–550 HV in the coarse-grained HAZ.
3.2 Weld Overlay Microstructure
The microstructure of MAG weld overlay deposits on Grade E steel is governed by several interdependent factors:
- Dilution rate — the percentage of base metal melted and incorporated into the weld deposit, which directly affects the alloy composition and phase balance of the overlay
- Cooling rate — determined by preheat temperature, interpass temperature, and travel speed; controls the transformation products (ferrite, pearlite, bainite, martensite)
- Wire composition — the selection of filler metal determines the available phase field for solidification and transformation
- Thermal cycling — multi-pass welding subjects previously deposited layers to repeated heating and cooling, potentially softening or re-hardening the microstructure
3.3 Typical Microstructural Features
| Zone | Typical Microstructure | Hardness (HV) | Key Concern |
|---|---|---|---|
| Base Metal (BM) | Tempered martensite + carbides | 250–350 | High Ceq, crack susceptibility |
| Coarse-Grained HAZ | Untempered/partially tempered martensite | 400–550 | Cracking, excessive hardness |
| Fine-Grained HAZ | Bainite + some martensite | 350–450 | Transition in properties |
| First Weld Pass | Ferrite + martensite (high dilution) | 300–420 | Crack initiation sites |
| Intermediate Passes | Mixed ferrite + bainite + martensite | 250–350 | Thermal cycling effects |
| Surface Passes | Refined ferrite + pearlite (low dilution) | 200–280 | Final surface quality |
4. Key Process Parameters and Implementation
4.1 Critical Process Variables
The MAG weld overlay process on Grade E steel requires precise control of the following parameters to achieve acceptable microstructure and mechanical properties:
| Parameter | Typical Range for Grade E Overlay | Effect on Microstructure |
|---|---|---|
| Preheat Temperature | 150–250°C | Reduces HAZ cooling rate; suppresses martensite formation |
| Interpass Temperature | 200–350°C | Controls thermal cycling; prevents cold cracking |
| Shielding Gas | 80% Ar / 20% CO₂ or 100% CO₂ | Affects arc stability, penetration profile, carbon pickup |
| Wire Diameter | 1.2 mm or 1.6 mm | Larger wire reduces dilution per pass; improves deposition rate |
| Current | 180–320 A | Higher current increases penetration and dilution |
| Voltage | 24–32 V | Controls arc length and bead profile |
| Travel Speed | 200–500 mm/min | Higher speed reduces heat input and dilution |
| Heat Input | 0.8–2.5 kJ/mm | Primary driver of grain size and transformation products |
| Wire Stick-out (ETD) | 10–15 mm | Affects arc force, penetration, and gas coverage |
4.2 Dilution Control Strategy
Dilution is the single most critical factor governing the microstructure and properties of the first weld pass on Grade E steel. The recommended approach includes:
- Substrate preparation — bevel the substrate edge to create a groove that limits base metal melting; a 60° V-groove with 2 mm land is typical
- First pass technique — use short arc length, moderate current, and relatively high travel speed to minimize penetration into the base metal
- Filler metal selection — choose a wire with sufficient alloy content (e.g., 309L for stainless overlay, or specific low-alloy wires for carbon steel overlay) to compensate for expected dilution
- Multi-pass buildup — plan a minimum of 3–5 passes to progressively reduce dilution from ~40% in the first pass to <5% in the final passes
- Post-weld heat treatment (PWHT) — apply a tempering cycle at 550–650°C for 2 hours per 25 mm thickness to relieve residual stresses and reduce HAZ hardness
4.3 Filler Metal Selection Guidelines
| Overlay Objective | Recommended Wire (ER Type) | Typical Composition | Target Application |
|---|---|---|---|
| Corrosion resistance (stainless) | ER309L | 22–25% Cr, 12–15% Ni, ≤0.03% C | Atmospheric/water corrosion on structural steel |
| Wear resistance (high carbon) | ER60C or ER70S-7 with hardfacing wire | 0.6–1.2% C, 0.8–1.5% Mn | Abrasion surfaces on structural components |
| Transition layer (low alloy) | ER80S-D2 or ER90S-D2 | 1.7–2.2% Cr, 0.4–0.6% Mo | Corrosion-resistant build-up before stainless pass |
| General purpose (matching) | ER70S-6 | 0.08–0.18% C, 1.0–1.6% Mn | Repair and build-up of Grade E structural parts |
5. Mechanical Property Requirements and Acceptance Criteria
5.1 Required Testing
The following mechanical tests are essential for qualification of MAG weld overlay on Grade E steel:
- Hardness survey — traverse hardness testing (HV10 or HV5) across the full cross-section from base metal through HAZ to surface of overlay, per GB/T 3894.2 or ASTM E18
- Tensile testing — transverse and longitudinal tensile specimens from the overlay, per GB/T 228.1 or ASTM E8
- Bend testing — face bend, side bend, and root bend tests per GB/T 2651 or ASTM E236
- Impact testing — Charpy V-notch impact at service temperature per GB/T 229 or ASTM E23
- Microstructural examination — metallographic preparation and optical/SEM analysis per ASTM E3 for specimen preparation and ASTM E125 for corrosion
5.2 Acceptance Criteria
| Test | Acceptance Criterion | Reference Standard |
|---|---|---|
| Hardness (HAZ) | ≤ 350 HV (or as specified by design code) | GB/T 19243, ASME Sec. IX |
| Hardness (Overlay surface) | As specified by overlay specification (typically 200–350 HV for structural; >400 HV for wear) | Customer specification / API 650 |
| Tensile strength (overlay) | ≥ 490 MPa (matching or exceeding base metal) | GB/T 228.1, ASTM E8 |
| Bend test | No cracks ≥ 1 mm on the bend face | GB/T 2651, ASTM E236 |
| Impact energy (overlay) | ≥ 27 J at -20°C (or specified temperature) | GB/T 229, ASTM E23 |
| NDT — Surface | MT or PT per NB/T 47013.5 or ASME Sec. V | NB/T 47013.5, ASME Sec. V |
| NDT — Volumetric | UT or RT per NB/T 47013.2/3 or ASME Sec. V | NB/T 47013.2/3, ASME Sec. V |
6. Common Risks and Control Measures
6.1 Hydrogen-Induced Cracking (Cold Cracking)
Grade E steel's high carbon equivalent makes it extremely susceptible to hydrogen-induced delayed cracking in the HAZ. The three essential elements — hydrogen, susceptible microstructure (martensite), and tensile stress — are all present during MAG welding on this substrate.
Control measures:
- Maintain preheat at ≥ 150°C (≥ 200°C for thick sections > 25 mm)
- Use low-hydrogen flux-cored wire or ensure gas shielding integrity with 100% CO₂ or Ar/CO₂ mixtures
- Limit travel speed to ensure adequate heat input and slow cooling
- Apply post-weld bake (200–300°C for 2–4 hours) to diffuse residual hydrogen before PWHT
- Perform delayed NDT (24 hours post-weld) to detect delayed cracks
6.2 Excessive HAZ Hardness
Without adequate preheat and PWHT, the HAZ can develop hardness values exceeding 500 HV, rendering the component brittle and prone to cracking during service or subsequent machining.
Control measures:
- Apply PWHT at 580–650°C with adequate soak time (1 hour per 25 mm of thickness, minimum 2 hours)
- Use controlled cooling rate during PWHT (≤ 200°C/hour below 600°C) to avoid re-hardening
- Verify hardness after PWHT; if HAZ hardness remains > 350 HV, consider additional tempering or process modification
6.3 Excessive Dilution
High dilution in the first pass can result in an overlay layer that does not achieve the intended alloy composition, leading to inadequate corrosion or wear resistance.
Control measures:
- Use a beveled substrate preparation to limit base metal melting
- Adopt a multi-pass strategy with progressive dilution reduction
- Verify dilution by chemical analysis (OES or wet chemistry) of the first pass
- For stainless overlay on carbon steel, include a transition pass (ER80S-D2) before the final ER309L passes
6.4 Porosity and Inclusions
MAG welding on high-strength steels is susceptible to gas porosity from inadequate shielding gas coverage and slag inclusions from wire surface contamination.
Control measures:
- Ensure gas flow rate of 15–25 L/min with proper gas nozzle positioning
- Use wire brush or solvent cleaning before each pass to remove oxide and contamination
- Maintain consistent stick-out (ETD) to ensure stable arc and gas coverage
- Inspect wire for surface defects and moisture before use
7. Application Across Technology Routes
7.1 TIG/MIG Weld Overlay Route
The MAG weld overlay technology studied here is the core process of the company's TIG/MIG weld overlay route. The metallurgical understanding gained from this research directly enables:
- Process optimization — data-driven WPS development for specific substrate/overlay combinations
- Quality assurance — established microstructural benchmarks for in-process inspection and lot acceptance
- Capability expansion — qualification for new substrate materials (high-strength steels, duplex stainless steels, nickel alloys) based on systematic metallurgical evaluation
- Customer engineering support — providing metallurgical reports that demonstrate compliance with design specifications and service requirements
7.2 Hydraulic Explosive Bonding Route
While hydraulic explosive bonding (hydraulic explosion welding) is a solid-state joining process that does not involve melting, the metallurgical knowledge of MAG weld overlay on Grade E steel contributes to this route in the following ways:
- Post-bonding weld repair — when hydraulic explosion bonded clad plates require edge repair or local defect remediation, the MAG overlay process knowledge ensures proper repair welding without damaging the explosion-bonded interface
- Interface characterization comparison — understanding the metallurgical gradients in weld overlays provides baseline data for evaluating the metallurgical compatibility at explosion-welded interfaces
- Substrate preparation — knowledge of Grade E steel's weldability characteristics informs the surface preparation and edge conditioning of substrates before hydraulic explosion bonding
7.3 Explosion Welding Route
Explosion welding of Grade E steel with corrosion-resistant cladding materials (stainless steel, nickel alloys, copper alloys) is a critical capability. The MAG weld overlay metallurgical research supports explosion welding through:
- Post-explosion weld repair qualification — explosion welding may produce local defects (bonding failures, surface damage) that require MAG weld repair; the metallurgical understanding ensures these repairs maintain the integrity of the explosion-welded joint
- Edge cladding of explosion-welded plates — the exposed edges of explosion-welded clad plates often require MAG weld overlay to provide corrosion protection; the process knowledge ensures proper dilution control and metallurgical compatibility
- Process development for hybrid clad structures — combining explosion welding for the main cladding area with MAG weld overlay for edges, corners, and penetrations requires integrated metallurgical understanding across both processes
- Failure analysis and root cause investigation — metallurgical expertise in weld overlays enables accurate diagnosis of field failures in hybrid clad components
8. Standards and Regulatory Framework
| Standard/Code | Scope | Relevance to MAG Overlay on Grade E |
|---|---|---|
| GB/T 19243 | Welding procedure qualification for steel | Primary qualification standard for WPS/PQR development |
| ASME Section IX | Welding, Brazing, and Fusing Qualifications | International qualification framework for production WPS |
| NB/T 47013 | NDT methods for pressure equipment | Acceptance criteria for weld overlay inspection |
| ASTM A514 | Quenched and tempered alloy steel plate | Base material specification for Grade E steel |
| GB/T 8110 | Welding wire specifications | Filler metal specification and acceptance |
| ISO 15614-1 | Qualification testing of welding procedures for steels | International WPS qualification methodology |
| GB/T 985 | Welding groove standard forms | Substrate preparation for overlay welding |
| NACE MR0175 / ISO 15156 | Materials for H₂S-containing environments | Applicable when overlay is used for sour service protection |
| API 650 / API 620 | Welded tanks for oil storage / atmospheric storage | End-use specification governing overlay requirements |
9. Qualification Building and Customer Value
9.1 Qualification Portfolio Enhancement
The systematic study of MAG weld overlay microstructure and properties on Grade E steel directly contributes to the company's qualification portfolio by:
- Extending WPS coverage — enabling qualified procedures for high-strength steel substrates that were previously outside the company's scope of qualification
- Meeting customer specification requirements — many oil and gas, power generation, and heavy machinery customers require demonstrated capability on high-strength steels per their procurement specifications
- Supporting third-party audit readiness — documented metallurgical studies provide evidence of technical competence during customer or regulatory audits
- Enabling design code compliance — qualification under GB/T 19243 and ASME Section IX ensures that overlay procedures meet national and international design code requirements
9.2 Product Delivery Enhancement
From a production standpoint, the metallurgical understanding enables:
- Reduced rework rates — by establishing optimal process windows, the probability of weld defects (cracking, porosity, excessive hardness) is significantly reduced
- Shorter production cycles — validated WPS eliminates the need for extensive trial welding on each new project
- Consistent quality — standardized procedures based on metallurgical evidence ensure uniform product quality across multiple production lots
- Scalability — process knowledge developed on test coupons can be reliably transferred to production-scale components with documented scaling rules
9.3 Customer Value Delivery
The technical capability translates directly to customer value through:
- Technical documentation packages — providing customers with metallurgical reports, hardness maps, and microstructural photographs that demonstrate compliance with their specifications
- Extended service life — properly qualified overlay procedures ensure that the cladding maintains its protective function throughout the design service life of the component
- Reduced total cost of ownership — by minimizing in-service failures and unplanned shutdowns associated with overlay degradation or failure
- Regulatory compliance support — providing the metallurgical evidence required for regulatory inspections and pressure equipment registration
10. Implementation Roadmap
To fully leverage this metallurgical knowledge in production, the following implementation steps are recommended:
- Phase 1 — Laboratory Validation: Conduct systematic parameter studies varying preheat, heat input, and filler metal selection; establish dilution curves and hardness profiles for each configuration
- Phase 2 — WPS Development: Select optimal parameter sets and develop formal WPS documents per GB/T 19243 and ASME Section IX, including complete mechanical test packages
- Phase 3 — Pilot Production: Apply qualified procedures to pilot-scale production components; verify that laboratory results are maintained in production conditions
- Phase 4 — Full Qualification: Complete PQR documentation, obtain third-party witness testing, and register qualifications with relevant certification bodies
- Phase 5 — Production Deployment: Train production welders on qualified procedures; implement in-process monitoring (temperature logging, parameter recording) and final product NDT protocols
11. Conclusion
The systematic study of microstructure and properties of MAG weld overlay deposits on Grade E steel represents a foundational technical capability for Cladding Technology Shanxi Co., Ltd. This metallurgical understanding bridges the gap between empirical welding practice and scientifically rigorous process qualification, enabling the company to deliver high-quality weld overlay products on the most demanding high-strength steel substrates. By integrating this knowledge across all three technology routes — TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding — the company creates a comprehensive, metallurgically-informed capability that supports qualification building, ensures product delivery quality, and delivers measurable value to customers in the oil and gas, power generation, heavy machinery, and infrastructure sectors.