Weld Overlay Repair Process and Performance Study of 45 Steel

1. Definition and Technical Principles

45 steel, designated under the Chinese national standard GB/T 699-2015 (Carbon Structural Steel), is a medium carbon steel with a nominal carbon content of 0.42%–0.50%. It is one of the most widely used steels in heavy industry for manufacturing shafts, gears, crankshafts, connecting rods, and high-strength structural components. Due to its relatively high carbon equivalent (CE ≈ 0.45–0.55), 45 steel exhibits significant susceptibility to cold cracking during welding, making repair welding a technically demanding operation.

Weld overlay repair of 45 steel involves the controlled deposition of one or more layers of filler metal onto worn, damaged, or dimensionally deficient surfaces to restore geometry, improve surface properties (hardness, wear resistance, corrosion resistance), and return the component to serviceable condition. The fundamental principle relies on achieving a metallurgically sound bond between the base metal and deposited layers while managing residual stresses, microstructural transformation, and hydrogen-induced cracking risks inherent to medium carbon steels.

The repair process typically follows a multi-layer strategy:

• Transition layer(s): A low-carbon or low-alloy filler (e.g., E5015, E5016, or AWS A5.1 E7015) is deposited first to dilute carbon content at the fusion boundary and reduce cracking susceptibility.
• Build-up layer(s): Subsequent layers restore the required dimensional geometry.
• Functional surface layer (if required): A hard-facing or specialized alloy layer may be applied to impart enhanced wear or corrosion resistance.

2. Category and Business Positioning

This technology entry falls squarely within the company's TIG/MIG Weld Overlay technology route, which constitutes one of the three core manufacturing capabilities of Cladding Technology Shanxi Co., Ltd. The other two routes—hydraulic explosive bonding and explosion welding—address permanent cladding of dissimilar metals on plate and pipe products, whereas weld overlay repair addresses in-service component restoration and surface engineering.

In the company's business portfolio, 45 steel weld overlay repair serves as a foundational qualification technology that demonstrates process competence in:

• Weldability assessment of carbon steels with elevated carbon equivalent
• Preheat and interpass temperature control protocols
• WPS/PQR development and qualification
• Post-weld heat treatment (PWHT) procedures
• Non-destructive testing (NDT) and acceptance criteria application

This capability positions the company as a qualified service provider for power generation, petrochemical, mining, and heavy equipment OEM customers who require field repair or shop-based restoration of critical rotating equipment and structural components.

3. Technical Purpose and Value

3.1 Primary Technical Objectives

• Dimensional restoration: Return worn shafts, journals, and gear blanks to specified diameters and tolerances per the original engineering drawing.
• Surface property enhancement: Achieve target surface hardness (typically 250–350 HB for structural restoration, or up to 500–600 HB for hard-facing applications) through appropriate filler selection.
• Crack-free weld integrity: Achieve zero transverse or longitudinal cracking in weld deposits and heat-affected zone (HAZ) through rigorous thermal management.
• Mechanical property preservation: Ensure that post-repair tensile strength, impact toughness (per GB/T 229 or ASTM E23), and fatigue performance meet or exceed original specifications.

3.2 Economic and Operational Value

Weld overlay repair of 45 steel components typically reduces replacement costs by 60–80% compared to fabrication of new components. For large-diameter shafts, gear housings, and heavy-duty structural parts, the time savings from repair versus replacement can range from weeks to months, directly impacting production downtime and asset availability.

4. Key Process and Implementation Points

4.1 Pre-Weld Preparation

• Surface cleaning: Grind away all oxide, scale, and contamination to bare metal using flap wheels or angle grinders. The weld area must be visually clean and free of rust, paint, oil, and moisture.
• Defect removal: Any cracks, porosity, or material defects must be fully removed by grinding or machining, with a minimum groove angle of 60° to ensure crack termination.
• Preheat: Apply uniform preheat using induction heating, propane torch, or electric resistance heating. The preheat temperature must be maintained across a zone extending at least 50 mm beyond the weld area.

4.2 Critical Process Parameters

Parameter	TIG (GTAW) Repair	MIG (GMAW) Repair
Preheat Temperature	200–250 °C (minimum 200 °C)	200–300 °C (minimum 200 °C)
Interpass Temperature	150–250 °C (controlled, not to exceed 250 °C)	150–300 °C (controlled, not to exceed 300 °C)
Filler Metal (Structural)	E5015 (AWS A5.1 E7015) or E5016 (E7016)	ER50-6 (AWS A5.18 ER70S-6)
Filler Metal (Hard Facing)	CB-4, CB-5 (Cast Iron type), or Alloy 6 (Ni-based)	ER614, ER615 (Ni-based hard facing)
Welding Current (TIG)	80–180 A (depending on thickness and layer)	—
Welding Current (MIG)	—	120–250 A
Shielding Gas	Ar 99.99% (pure argon)	Ar + 5% CO₂ or Ar 98% + CO₂ 2%
Travel Speed	20–40 mm/min (TIG)	100–250 mm/min (MIG)
Layer Thickness	2–4 mm per pass	2–5 mm per pass
PWHT Requirement	600–650 °C × 2–4 hours (for components >25 mm or critical service)	600–650 °C × 2–4 hours (same criteria)
Post-Weld Cooling	Controlled cooling in furnace or under insulation (≤50 °C/hr)	Controlled cooling in furnace or under insulation (≤50 °C/hr)

4.3 Welding Execution Sequence

1. Step 1 – Preheat: Apply and verify preheat temperature using calibrated infrared thermometer or contact thermocouple. Record temperatures at multiple points across the workpiece.
2. Step 2 – Transition layer: Deposit the first layer using low-carbon, low-hydrogen filler metal (E7015/E7016). Use stringer beads with full interpass grinding to remove all oxide. Maintain interpass temperature within the specified range.
3. Step 3 – Build-up layers: Continue deposition to achieve required geometry. For thick repairs, use multi-pass technique with weave pattern to control heat input and minimize residual stress.
4. Step 4 – Surface finishing layer (if required): Apply hard-facing or specialized alloy layer. For Ni-based hard facing, ensure proper dilution control (target dilution ≤15% for maximum hardness).
5. Step 5 – Post-weld heat treatment: If required per WPS, place component in furnace for stress relief at 600–650 °C. Soak time calculated at minimum 1 hour per 25 mm of thickness. Cool at controlled rate under 50 °C/hour to 300 °C, then air cool.
6. Step 6 – Machining: Machine to final dimensions and tolerances after PWHT. Verify hardness at machined surface.

4.4 Heat Input Control

Heat input is a critical parameter governing microstructure and cracking susceptibility. The recommended heat input range for 45 steel weld overlay repair is:

• TIG: 0.8–1.5 kJ/mm
• MIG: 1.0–2.0 kJ/mm

Excessive heat input leads to coarse grain HAZ, increased retained austenite, and reduced hardness; insufficient heat input results in incomplete fusion and poor mechanical properties. Heat input should be calculated per pass using the formula: Q = (V × I × η) / v, where V = voltage, I = current, η = arc efficiency (0.85 for TIG, 0.8 for MIG), and v = travel speed.

5. Applicable Standards and Acceptance Criteria

5.1 Governing Standards

Standard	Scope of Application
GB/T 699-2015	Base material specification for 45 carbon structural steel
GB/T 5117-2012	Classification and specification of low-alloy steel electrode E5015
GB/T 8110-2008	Classification and specification of solid wire ER50-6
AWS D10.9/D10.9M	Recommendations for repair welding of cast and wrought iron and steel
ASTM A29/A29M	Standard practice for chemical analysis of iron and steel
GB/T 3323-2005	Acceptance criteria for radiographic testing of welds (Level B or C)
GB/T 11345-2013	Ultrasonic testing of welds — acceptance criteria
GB/T 13894-2016	Magnetic particle testing of welds
GB/T 229-2007	Impact testing method (Charpy V-notch)
GB/T 228.1-2021	Tensile testing method for metallic materials
ASTM E10/E10M	Rockwell hardness testing method
NB/T 47014-2011	Qualification of welding procedures for pressure equipment
ASME Section IX	Qualification rules for welding procedures and welders (if applicable to pressure vessels)
API 579-1/ASME FFS-1	Fitting for service — assessment of repairs (fitness-for-service evaluation)

5.2 Acceptance Criteria Summary

• Visual inspection (VT): No surface cracks, undercut exceeding 0.5 mm, porosity, or lack of fusion visible on the finished surface. Weld profile must be smooth and uniform.
• Radiographic testing (RT): Per GB/T 3323-2005 Level B (or Level C for critical applications), no Category 1 defects (cracks, lack of fusion). Porosity and slag inclusions limited per Table 13 of the standard.
• Magnetic particle testing (MT): Per GB/T 13894-2016, no linear indications (cracks) permitted. Round indications limited to 3 mm length with spacing ≥5 mm.
• Ultrasonic testing (UT): Per GB/T 11345-2013 Level B, acceptance per Table 5, no indications exceeding acceptance level for the respective weld thickness.
• Hardness: Surface hardness 250–350 HB (structural repair) or as specified for hard-facing. HAZ hardness must not exceed 350 HB to maintain toughness. Measured per ASTM E10 or GB/T 231.1.
• Mechanical properties (if required): Tensile strength ≥490 MPa per GB/T 228.1. Impact energy ≥27 J at −20 °C (if specified) per GB/T 229.

6. Common Risks and Controls

6.1 Hydrogen-Induced Cold Cracking

Risk: 45 steel has a carbon equivalent sufficient to form hard, brittle martensite in the HAZ during rapid cooling. Combined with diffusible hydrogen from moisture in flux or filler coating, cold cracking can occur within hours of welding.

Controls:

• Maintain preheat at ≥200 °C and interpass temperature within 150–250 °C range.
• Use low-hydrogen electrodes (E7015/E7016) baked at 350 °C for 1 hour immediately before use.
• Use dry shielding gas with dew point ≤ −40 °C.
• Implement post-weld bake-out at 250–300 °C for 2 hours per 25 mm thickness to allow hydrogen diffusion before PWHT.

6.2 Excessive Hardness in HAZ

Risk: Rapid cooling can produce martensitic HAZ with hardness exceeding 400 HB, leading to reduced toughness and increased cracking risk.

Controls:

• Use preheat and interpass temperature control to slow cooling rate.
• Apply PWHT at 600–650 °C to temper HAZ martensite and reduce hardness to ≤350 HB.
• Minimize heat input per pass to avoid excessive grain growth while maintaining adequate dilution.

6.3 Distortion and Residual Stress

Risk: Large-volume weld deposits on asymmetric components (shafts, flanges) cause angular and longitudinal distortion.

Controls:

• Use balanced weld sequence (weld opposite sides alternately on symmetric geometries).
• Apply back-step welding technique for long welds.
• Use clamping fixtures or backing bars to constrain movement.
• Post-weld machining to correct minor distortion (allow 1–2 mm machining allowance in WPS).
• PWHT to relieve residual stresses to acceptable levels.

6.4 Incomplete Fusion and Lack of Penetration

Risk: Insufficient heat input or improper technique leads to lack of fusion at the root or between layers.

Controls:

• Verify proper groove preparation and cleaning before each pass.
• Use adequate current and travel speed per WPS parameters.
• Perform interpass grinding to remove all oxide and scale before the next pass.
• Implement NDT (UT or MT) on intermediate layers for critical repairs.

6.5 Dilution and Property Degradation (Hard-Facing Applications)

Risk: Excessive base metal dilution reduces the effectiveness of hard-facing alloys, lowering surface hardness below required levels.

Controls:

• Use pre-weld hard-facing layer to reduce dilution in subsequent functional layers.
• Control heat input to minimize base metal melting.
• Use smaller diameter filler wire or rod for hard-facing passes.
• Verify hardness by spot testing after each layer.

7. Application Across the Company's Three Technology Routes

7.1 TIG/MIG Weld Overlay (Primary Route for This Technology)

The 45 steel weld overlay repair technology is directly executed under the company's TIG/MIG weld overlay capability. This includes:

• Shop-based repair: Large components (shafts, gears, housings) brought to the facility for controlled environment repair with full NDT and PWHT capability.
• Field repair: Mobile TIG/MIG units deployed to customer sites for in-situ repair of rotating equipment, structural components, and piping.
• Surface engineering: Application of hard-facing and corrosion-resistant overlay layers on 45 steel components for enhanced service life.

7.2 Hydraulic Explosive Bonding

While hydraulic explosive bonding is primarily used for permanent cladding of dissimilar metals on plate and pipe, the 45 steel weld overlay research contributes indirectly by:

• Establishing baseline understanding of 45 steel weldability, which informs the design of transition layers in bonded assemblies where 45 steel is the substrate.
• Providing process parameters (preheat, interpass temperature, filler selection) that can be adapted for post-bonding welding operations (e.g., welding a cladding layer to a 45 steel backing plate after hydraulic bonding).
• Demonstrating metallurgical competence that supports qualification of composite products combining bonded cladding with welded overlays.

7.3 Explosion Welding

Explosion welding creates permanent metallurgical bonds between dissimilar materials. The 45 steel weld overlay study supports this route by:

• Providing reference data on 45 steel's thermal and mechanical response to rapid heating and cooling, which is relevant to the high-strain-rate conditions of explosion welding.
• Enabling development of hybrid products where explosion-welded clad plate (e.g., 316L/45 steel) requires subsequent weld overlay repair or attachment welding.
• Contributing to the company's overall metallurgical knowledge base for carbon steel substrates used in explosion-welded cladding systems.

8. Contribution to Qualification Building, Product Delivery, and Customer Value

8.1 Qualification Building

The 45 steel weld overlay repair study directly supports the company's qualification portfolio:

• WPS/PQR Development: The research generates qualified Welding Procedure Specifications (WPS) and Procedure Qualification Records (PQR) for 45 steel repair welding per NB/T 47014-2011 or ASME Section IX, enabling the company to perform repairs on pressure equipment and critical components.
• Welder Qualification: Trained welders qualified on 45 steel weld overlay procedures can be deployed across multiple customer projects, reducing qualification lead times.
• NDT Competence: The study validates NDT acceptance criteria and techniques for weld overlay repairs, supporting the company's NDT Level II/III certification requirements.
• ISO 3834 / EN ISO 3834 Compliance: The systematic approach to process control, documentation, and quality assurance demonstrated in this study supports the company's quality management system certification.

8.2 Product Delivery

This technology enables the company to deliver:

• Custom repair solutions: Tailored WPS for each customer's specific component geometry, service conditions, and acceptance criteria.
• Accelerated turnaround: Optimized process parameters reduce repair cycle time compared to conventional approaches, delivering faster component return-to-service.
• Full documentation package: Each repair delivery includes complete weld logs, NDT reports, hardness maps, and traceability records meeting customer and regulatory requirements.

8.3 Customer Value

The 45 steel weld overlay repair capability provides direct customer value through:

• Cost reduction: 60–80% savings versus component replacement, with documented payback periods typically under 3 months for critical equipment.
• Downtime minimization: Field repair capability and optimized shop processes reduce unplanned shutdown duration.
• Extended asset life: Hard-facing overlay layers extend component service life by 2–5× compared to unmodified 45 steel surfaces.
• Technical expertise: The research-backed approach ensures reliable, repeatable results with documented performance data, reducing customer risk and building long-term trust.
• Regulatory compliance: Deliverables meet all applicable standards (GB, ASTM, ASME, API, NB), ensuring customer compliance with regulatory and insurance requirements.

9. Conclusion

The study on 45 steel weld overlay repair process and performance represents a core technical capability that underpins the company's TIG/MIG weld overlay service offering. By systematically addressing preheat control, filler metal selection, heat input management, post-weld heat treatment, and NDT acceptance criteria, the company delivers reliable, standards-compliant repair solutions for one of the most widely used carbon steels in heavy industry. This capability not only generates direct revenue through repair services but also strengthens the company's overall qualification portfolio, supports cross-route technology integration, and establishes a foundation for advanced surface engineering applications on carbon steel substrates.