Microstructure and Mechanical Properties of Weld Overlay Deposited Metal on AISI 1045 (45 Steel) Substrates
1. Definition and Technical Context
Weld overlay on 45 steel (Chinese standard designation, equivalent to AISI 1045 / GB 699) refers to the deliberate deposition of a functionally distinct metallurgical layer onto a medium-carbon steel substrate using fusion welding techniques. The deposited metal (weld metal) acquires its own microstructure, phase composition, hardness profile, and corrosion or wear resistance characteristics that differ fundamentally from the base material. This entry represents a systematic technical learning and analytical exercise conducted by Cladding Technology Shanxi Co., Ltd. engineers to deepen metallurgical understanding of how weld overlay deposits behave on carbon-steel substrates—a foundational competency for qualified WPS development, NDT interpretation, and customer-facing technical consultation.
2. Base Material Characteristics: 45 Steel
45 steel is a normalized medium-carbon steel with a nominal composition of 0.42–0.50 wt% carbon, 0.50–0.80 wt% manganese, and trace amounts of silicon, sulfur, and phosphorus. Its as-received microstructure typically consists of pearlite (50–70%) and ferrite (30–50%), with a hardness range of 197–241 HBW (normalized condition). The key metallurgical challenges associated with weld overlay on this substrate include:
- Carbon content sensitivity: The 0.45% carbon level places 45 steel in a moderately high-hardenable category, making the heat-affected zone (HAZ) susceptible to martensitic transformation and cracking under rapid cooling.
- Preheating requirements: Welding procedures typically mandate preheat temperatures of 150–300°C to control cooling rates and reduce hydrogen-induced cracking risk.
- Dilution management: Base metal dilution into the overlay deposit directly affects the final hardness, phase balance, and service performance of the cladding layer.
3. Microstructure of Weld Overlay Deposited Metal
3.1 Phase Formation Mechanisms
The microstructure of deposited metal on 45 steel substrates is governed by the interplay between the filler wire composition, welding heat input, cooling rate, and interpass temperature. Depending on the filler metal system selected, the following microstructural outcomes are typical:
| Filler System | Primary Microstructure | Typical Hardness (HV) | Key Phase Constituents |
|---|---|---|---|
| Low-alloy steels (e.g., E7018 equivalent) | Ferrite + Pearlite | 200–280 | α-Fe, cementite (Fe₃C) |
| High-carbon martensitic (e.g., E6015Mo) | Martensite + Tempered carbides | 450–650 | Martensite, M₇C₃, M₂₃C₆ |
| Austenitic stainless (e.g., ER309L) | δ-ferrite + Austenite | 200–250 | γ-Fe, δ-ferrite, Cr₂₃C₆ |
| Hardfacing (e.g., Ni-Cr-C-B) | Carbide network + Matrix | 600–1200 | Cr₇C₃, Ni₃B, Cr₇C₃ + austenitic matrix |
| Stellite-type (Co-Cr-W) | Carbides + Solid solution | 400–550 | Co solid solution, Cr₂₃C₆, W₆C |
3.2 Dilution Effects on Microstructure
Dilution—the proportion of base metal melted and incorporated into the weld deposit—is the single most influential factor in determining overlay microstructure. For 45 steel substrates, dilution rates typically range from 10% to 35% depending on the welding process and parameters:
- Low dilution (10–15%): Achieved with low heat input, short arc length, and high travel speed. The deposited microstructure closely resembles the filler metal's as-cast structure.
- Moderate dilution (15–25%): Typical of standard TIG/MIG parameters. Carbon from the substrate enriches the deposit, potentially promoting carbide precipitation or increasing hardness.
- High dilution (25–35%): Occurs with high heat input, low travel speed, and wide groove preparations. Excessive dilution can compromise the intended overlay properties entirely.
3.3 Heat-Affected Zone (HAZ) Metallurgy
The HAZ adjacent to the overlay weld on 45 steel undergoes significant microstructural transformation. Depending on peak temperature and cooling rate:
- Above A₃ (~850°C): Full austenitization followed by rapid cooling produces fine-to-coarse martensite, with hardness potentially reaching 350–450 HBW.
- A₁–A₃ range (~727–850°C): Partial austenitization yields a mixed microstructure of retained ferrite and transformed products (martensite, bainite, or pearlite).
- Below A₁ (~727°C): Grain growth and spheroidization of carbides may occur, generally softening the local area.
4. Mechanical Properties of Deposited Metal
4.1 Hardness Distribution
Hardness measurement across the weld cross-section reveals characteristic gradients. A typical profile from the overlay surface to the base metal includes:
- Overlay surface layer: Highest hardness zone, representing the intended functional property (e.g., 500–1200 HV for hardfacing overlays).
- Overlay-to-HAZ transition: Gradual hardness decrease over 0.5–2 mm as dilution effects and microstructural changes occur.
- HAZ: Often exhibits peak hardness exceeding both base metal and weld metal due to martensitic transformation (up to 400 HBW on 45 steel).
- Base metal: Returns to nominal 45 steel hardness (197–241 HBW normalized).
4.2 Tensile and Toughness Properties
Weld overlay deposited metal on 45 steel substrates must be evaluated for:
- Tensile strength: Typically ranges from 500 MPa (austenitic overlays) to 1200+ MPa (martensitic/hardfacing overlays). Testing per ASTM E8/E8M is standard.
- Impact toughness: Critical for overlays in low-temperature or cyclic loading applications. Charpy V-notch testing per ASTM E23/E23M at service-relevant temperatures.
- Fatigue resistance: Surface finish, residual stress, and microstructural homogeneity collectively determine fatigue life. Residual stresses in overlay welds can reach 300–500 MPa in the tensile range.
4.3 Corrosion and Wear Resistance
The functional value of weld overlay on 45 steel is typically realized through enhanced corrosion resistance (stainless overlays), wear resistance (hardfacing overlays), or thermal barrier properties (ceramic-reinforced overlays). Key evaluation methods include:
- Electrochemical polarization testing per ASTM G5/G5-02
- Pin-on-disk wear testing per ASTM G99/G99-02
- Corrosion rate measurement per ASTM G15/G15-02
- High-temperature oxidation testing per ASTM G22/G22-04
5. Key Process Parameters and Implementation Points
5.1 TIG Weld Overlay Parameters
| Parameter | Typical Range | Effect on Deposited Metal |
|---|---|---|
| Preheat temperature | 150–300°C | Controls HAZ cooling rate; reduces cracking susceptibility |
| Welding current (DCEN) | 100–250 A | Higher current increases dilution and penetration |
| Travel speed | 5–25 cm/min | Lower speed increases heat input and dilution |
| Shielding gas (Ar) | 10–20 L/min | Prevents oxidation; critical for stainless overlay purity |
| Interpass temperature | ≤300°C (max) | Prevents interpass softening and grain coarsening |
| Wire feed rate | 1.0–3.0 m/min | Controls deposit thickness per pass |
5.2 MIG Weld Overlay Parameters
| Parameter | Typical Range | Effect on Deposited Metal |
|---|---|---|
| Welding current | 150–350 A | Higher current increases dilution; short-circuit transfer at lower current |
| Voltage | 18–28 V | Controls arc length and bead profile |
| Wire feed speed | 3–8 m/min | Directly correlated with current; affects deposit composition |
| Shielding gas | Ar 100% or Ar/CO₂ mix | Pure Ar for stainless; mixed gas for carbon steel overlays |
| Travel speed | 10–40 cm/min | Higher speed reduces dilution and heat input |
5.3 Multi-Layer Build Strategy
For thick overlay deposits (≥3 mm), multi-layer build strategies are employed to manage dilution and achieve target properties:
- Transition layer: First layer deposited with a filler composition designed to bridge the 45 steel base to the final overlay composition (e.g., ER309L as transition between carbon steel and ER316L overlay).
- Build-up layers: Subsequent passes progressively approach the target overlay composition, with dilution decreasing as each new layer is deposited on the previous weld metal rather than base steel.
- Final surface layer: The topmost layer achieves near-zero dilution from base metal, delivering the full functional properties of the selected filler system.
6. Applicable Standards and Acceptance Criteria
6.1 Welding Procedure Standards
- GB/T 985.1–985.6: Welding procedures for steel—general and specific requirements for TIG and MIG processes.
- GB/T 19866–19870: Classification and qualification of welding procedures for steel.
- ASME BPV Section IX: Qualification of welding procedures, welders, and welding operators (particularly QW-400 series for weld overlay).
- ASTM A5.1: Specification for welding procedure and performance qualification.
- NB/T 47014: Qualification test of welding procedure for pressure vessels (Chinese national standard).
6.2 Material and Performance Standards
- GB 699–2015: Technical conditions for hot-rolled steel bars (45 steel specification).
- ASTM A29/A29M: Standard specification for bar and shapes for mechanical and structural purposes (AISI 1045 equivalent).
- GB/T 11345: Ultrasonic testing of welds in steel.
- GB/T 3323: Radiographic testing of welds.
- ASTM A388: Specification for low-alloy steel pressure vessel plates.
- API 579-1/ASME FFS-1: Fitness-for-service assessment criteria for overlay repairs.
6.3 Acceptance Criteria for Weld Overlay Deposits
| Inspection Method | Acceptance Criteria | Standard Reference |
|---|---|---|
| Visual inspection | No cracks, porosity, undercut, or incomplete fusion visible | GB/T 3375; AWS D1.1 |
| Magnetic particle testing (MT) | No linear indications; round indications ≤3 mm | GB/T 26952; ASTM E709 |
| Ultrasonic testing (UT) | No indications exceeding acceptance threshold | GB/T 11345; ISO 17640 |
| Hardness testing | Within specified range per overlay specification | ASTM E18; GB/T 231.1 |
| Macrographic examination | No centerline cracking; uniform microstructure | ASTM E3; GB/T 1954 |
| Overlay thickness | ≥ specified minimum (typically 2–10 mm) | Project specification; API 570 |
7. Common Risks and Control Measures
7.1 Cracking Risks
- Hot cracking: Occurs in the solidification zone due to sulfur and phosphorus segregation. Controlled by low-sulfur filler selection, restricted sulfur/phosphorus in base metal, and adequate preheat.
- Cold cracking (hydrogen-induced): The most critical risk for weld overlay on 45 steel. Mitigated by: preheat ≥150°C, low-hydrogen filler metals (≤5 mL/100g H₂), post-weld heat treatment (PWHT) at 550–650°C for 2–4 hours per 25 mm thickness, and hydrogen bake-out.
- Overlay cracking: Hardfacing overlays may develop microcracks due to high residual stress and brittle carbide networks. Controlled by stress-relief annealing and proper interpass temperature management.
7.2 Dilution-Related Risks
- Property degradation: Excessive dilution compromises the functional overlay layer (e.g., stainless overlay becomes sensitized or loses corrosion resistance due to carbon pickup from 45 steel base).
- Control measures: Low heat input, short arc length, high travel speed, narrow groove preparation, and use of transition layers.
7.3 Residual Stress
- Weld overlay deposits on 45 steel develop significant tensile residual stresses (300–500 MPa) due to differential thermal contraction between overlay and substrate.
- Consequences include reduced fatigue life, increased susceptibility to stress corrosion cracking, and potential dimensional distortion.
- Control: Post-weld stress relief at 550–650°C, peening of overlay surface, or design of weld sequence to balance stresses.
8. Application Scenarios Across Technology Routes
8.1 TIG/MIG Weld Overlay Route
The knowledge of deposited metal microstructure and properties on 45 steel is directly applicable to the company's TIG and MIG weld overlay operations:
- Wear-resistant overlay on rotating equipment: 45 steel shafts, rollers, and gears receiving hardfacing overlays (Ni-Cr-C-B, Co-based) for extended service life in mining, cement, and power generation applications.
- Corrosion-resistant overlay on process vessels: 45 steel pressure vessels and heat exchangers receiving austenitic stainless (309L/316L) transition layers followed by functional overlay layers for chemical processing environments.
- Repair and restoration: Dimensional restoration of worn 45 steel components (crane hooks, gear teeth, valve seats) with matched or enhanced mechanical properties.
- High-temperature overlay: Stellite-type or Ni-base overlays on 45 steel furnace components and exhaust system parts requiring thermal barrier and oxidation resistance.
8.2 Hydraulic Explosive Bonding Route
While hydraulic explosive bonding produces mechanically bonded clad plates without melting, the understanding of weld overlay metallurgy on 45 steel provides critical context for:
- Hybrid cladding solutions: When hydraulic bonding produces a clad plate with 45 steel as the structural backing, subsequent TIG weld overlay may be applied to specific areas requiring additional functional properties.
- Weldability assessment: Engineers must understand how the explosive bond interface and its microstructure interact with subsequent welding operations on the clad assembly.
- Qualification of welded joints in bonded plates: NDE and mechanical testing of welds joining explosive-bonded 45 steel clad plates requires metallurgical knowledge of the base material's welding behavior.
8.3 Explosion Welding Route
Explosion welding of 45 steel with various cladding materials (copper, stainless steel, aluminum alloys, nickel alloys) produces a bond interface with unique metallurgical characteristics:
- Interface metallurgy correlation: Understanding weld overlay deposited metal microstructure helps engineers interpret and predict the deformation and diffusion characteristics at explosion weld interfaces.
- Post-weld treatment planning: When explosion-welded clad plates require machining, drilling, or welding operations, the knowledge of HAZ behavior in 45 steel under welding heat input is essential for WPS development.
- Performance qualification: Peel testing, shear testing, and hardness traverses across explosion weld interfaces benefit from metallurgical knowledge of both the base material and the cladding material's response to thermal cycling.
9. Contribution to Qualification Building and Customer Value
9.1 WPS Qualification Enhancement
Deep understanding of deposited metal microstructure and properties on 45 steel substrates directly strengthens the company's welding procedure qualification program:
- Procedure optimization: Enables data-driven selection of heat input ranges, travel speeds, and wire feed rates that produce target microstructures and properties.
- Essential variable control: Informs which welding parameters are "essential" (requiring requalification upon change) versus "non-essential" for specific overlay applications.
- WPS documentation quality: Produces more technically rigorous and defensible Welding Procedure Specifications that satisfy customer and third-party inspection requirements.
9.2 Product Delivery Quality
- Reduced rework rates: Predictive understanding of cracking susceptibility, dilution effects, and residual stress enables proactive process control rather than reactive inspection-driven corrections.
- Property consistency: Hardness, microstructure, and mechanical property uniformity across production batches improves when process parameters are optimized based on metallurgical understanding.
- NDT interpretation: Engineers can distinguish between benign indications (e.g., carbide segregation in hardfacing deposits) and critical defects (e.g., true cracks) with greater accuracy.
9.3 Customer Value and Technical Consultation
- Application engineering support: Ability to explain microstructure-property relationships enables the company to provide technical justification for material and process selections, building customer confidence.
- Failure analysis capability: When overlay deposits fail in service, metallurgical knowledge allows root cause identification (e.g., excessive dilution, inadequate preheat, improper filler selection) and corrective action development.
- Custom solution development: Understanding of how filler composition, dilution, and process parameters interact enables the design of tailored overlay solutions for specific customer applications not covered by standard specifications.
- Compliance documentation: Metallurgical test reports with microstructure documentation (metallographic examination per ASTM E3, hardness traverses per ASTM E18) provide objective evidence of performance for regulatory and customer qualification purposes.
10. Technical Recommendations for Implementation
- Establish a dilution control matrix: Systematically document dilution rates for each WPS under different parameter combinations, enabling rapid procedure selection for new jobs.
- Maintain microstructure reference library: Compile metallographic micrographs of all qualified overlay systems on 45 steel substrates for quick visual comparison during production NDE.
- Implement hardness profile mapping: Conduct systematic hardness traverses on qualification coupons and production samples to verify property gradients match design expectations.
- Develop preheat and PWHT protocols: Formalize temperature control procedures for 45 steel overlay welding to minimize cracking risk, with documented thermocouple monitoring.
- Cross-reference with NDT data: Correlate NDE results (UT, MT) with macrographic and micrographic findings to build a predictive database linking inspection signals to metallurgical quality.
- Train welding operators: Ensure field personnel understand the metallurgical consequences of parameter deviations (excessive heat input, inadequate shielding, high interpass temperature) and the importance of procedural discipline.
11. Conclusion
The systematic study of weld overlay deposited metal microstructure and properties on 45 steel substrates represents a fundamental metallurgical competency for Cladding Technology Shanxi Co., Ltd. This knowledge underpins every aspect of the company's weld overlay operations—from WPS qualification and parameter optimization to NDT interpretation, failure analysis, and customer technical consultation. By integrating this metallurgical understanding across all three technology routes (TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding), the company positions itself to deliver technically superior cladding solutions with documented performance, regulatory compliance, and measurable customer value. The continued refinement of this metallurgical knowledge base—through production data collection, qualification testing, and cross-functional knowledge sharing—will sustain and enhance the company's competitive position in the industrial cladding and overlay market.