1CrMo Weld Overlay on 9CrMoV Steel: Performance Analysis and Process Optimization

1. Definition and Technical Principles

The weld overlay of 1CrMo (1% Chromium, 1% Molybdenum) low-alloy steel onto 9CrMoV (9% Chromium, Molybdenum, Vanadium) base steel represents a critical metallurgical engineering challenge in the fabrication of high-pressure piping systems, boiler components, and pressure vessels operating at elevated temperatures and pressures. This overlay process creates a graded transition zone between the austenitic/ferritic 9CrMoV substrate and the ferritic 1CrMo overlay layer, fundamentally addressing issues of thermal fatigue resistance, high-temperature creep strength, and corrosion performance at dissimilar material joints.

The fundamental metallurgical principle underlying this overlay involves the controlled dilution and microstructural evolution at the fusion boundary. 9CrMoV steel exhibits a tempered martensitic microstructure with high chromium content providing exceptional resistance to steam oxidation and sulfidation at temperatures exceeding 550°C. The 1CrMo overlay material, while offering lower alloy content, provides superior weldability, reduced susceptibility to hydrogen-induced cracking, and improved mechanical properties in the heat-affected zone (HAZ). The overlay process establishes a diffusion gradient that mitigates the risk of brittle phase formation (such as sigma phase or intermetallic compounds) that would otherwise develop at a direct 9CrMoV-to-carbon steel weld interface.

2. Category and Business Positioning

This technology falls squarely within the company's TIG/MIG Weld Overlay technology route, representing a core capability in dissimilar material welding and surface engineering for critical power generation and petrochemical components. Within the company's three primary technology routes—TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding—the 1CrMo overlay on 9CrMoV steel occupies a specialized niche where precise metallurgical control, thermal management, and post-weld heat treatment are paramount.

From a business positioning perspective, this capability serves the following market segments:

• Power Generation: Main steam piping, hot reheat piping, superheater tubes, and steam drum internals in ultra-supercritical (USC) coal-fired power plants operating at 600–650°C and 25–35 MPa.
• Petrochemical: High-pressure hydrogen piping, reformer tubes, and heat exchanger components requiring resistance to high-temperature hydrogen attack (HTHA).
• Nuclear: Steam generator tubes and secondary system piping where graded transition layers enhance fatigue life and reduce stress corrosion cracking susceptibility.

3. Technical Purpose and Value

The primary technical objectives of applying a 1CrMo overlay layer on 9CrMoV steel include:

1. Thermal Fatigue Mitigation: The graded transition layer reduces thermal stress concentrations at material dissimilarities during thermal cycling, extending component service life by 2–3 times compared to direct dissimilar welds.
2. Weldability Enhancement: 1CrMo serves as an intermediate alloy level that bridges the gap between 9CrMoV (Ceq ≈ 0.65) and carbon steel (Ceq ≈ 0.40), reducing the risk of cold cracking in subsequent weld passes.
3. Creep Strength Preservation: The overlay maintains adequate high-temperature creep resistance while providing a more ductile matrix for stress relaxation during operation.
4. Corrosion Resistance Maintenance: The 1CrMo layer retains sufficient chromium content for resistance to internal corrosion in high-pressure steam environments while avoiding the brittleness associated with higher chromium alloys.
5. Cost Optimization: Compared to using 9CrMoV overlay material throughout, 1CrMo reduces material costs by 30–45% while maintaining acceptable performance characteristics for non-critical overlay zones.

4. Key Process and Implementation Points

4.1 Base Material Preparation

Preparation of the 9CrMoV substrate is critical to overlay quality. The base material must be preheated to a minimum of 200°C (preferably 250–300°C) to reduce residual stress and minimize the risk of hydrogen-induced cracking. Surface preparation includes grinding to bare metal with a minimum 6mm bevel width, ensuring removal of all oxide scale, rust, and contaminant layers. The fusion boundary surface should exhibit a matte finish without sharp edges or undercut from prior operations.

4.2 Weld Overlay Parameters

Parameter	Specification	Rationale
Welding Process	GTAW (TIG) for first pass; GMAW (MIG) for subsequent passes	TIG provides superior control for critical first-layer fusion; MIG enables efficient deposition for build-up layers
Shielding Gas (TIG)	100% Argon or Ar + 2% H₂	Pure Ar minimizes hydrogen pickup; small H₂ addition improves wetting and bead profile
Shielding Gas (MIG)	Ar + 2% CO₂ or Ar + 5% CO₂	Provides arc stability and adequate penetration with controlled carbon pickup
Preheat Temperature	200–300°C (maintained throughout welding)	Reduces cooling rate below 150°C/s to prevent martensitic transformation in HAZ
Interpass Temperature	≤250°C (TIG); ≤200°C (MIG)	Prevents excessive grain growth and maintains controlled microstructure
Deposition Rate (TIG)	3–5 kg/h	Allows adequate heat input control for thin first-layer fusion
Deposition Rate (MIG)	8–15 kg/h	Efficient build-up while maintaining interpass temperature control
Weld Wire (1CrMo)	GB/T 8110 S1Cr1Mo or AWS A5.15 ER80S-D4 equivalent	Composition matched to overlay design requirements
Travel Speed	50–80 mm/min (TIG); 100–150 mm/min (MIG)	Controls heat input at 1.5–3.0 kJ/mm (TIG); 2.0–4.0 kJ/mm (MIG)
Current (TIG)	100–180 A (DC+)	DC+ provides deeper penetration with tungsten electrode stability
Current (MIG)	200–350 A	Adequate penetration for multi-pass build-up
Overlay Thickness	3–6 mm (minimum 2 passes)	Ensures complete dilution of base metal influence; minimum 25% base metal dilution in first pass
Post-Weld Heat Treatment	720–760°C × 2h (air cool or furnace cool)	Tempering treatment to relieve residual stress and refine microstructure

4.3 Microstructural Control

The microstructural evolution at the fusion boundary is the primary determinant of overlay performance. Key metallurgical considerations include:

• First-Pass Dilution Control: The first TIG pass should achieve 20–30% base metal dilution. Excessive dilution (>35%) introduces chromium-enriched zones susceptible to sigma phase precipitation during subsequent PWHT.
• Grain Boundary Character: The overlay should exhibit predominantly high-angle grain boundaries with limited planar eutectoid carbide formation along boundaries.
• Carbide Distribution: M₇C₃ and M₂₃C₆ carbides should be dispersed within the matrix rather than forming continuous network structures, which would compromise toughness.
• Phase Composition: The overlay microstructure should consist of tempered martensite with 5–10% retained austenite, providing an optimal balance of strength and ductility.

4.4 Post-Weld Heat Treatment Protocol

The post-weld heat treatment (PWHT) cycle is non-negotiable for 1CrMo overlay on 9CrMoV steel. The recommended cycle includes:

1. Heat at 50–75°C/h to 720–760°C
2. Hold for 2 hours per 25mm of thickness (minimum 2 hours)
3. Cool at 50°C/h to 500°C, then air cool
4. Maximum cooling rate during austenite range (850–650°C): 30°C/h

This PWHT cycle achieves tempering of the overlay microstructure, relief of welding residual stresses to below 50 MPa, and stabilization of the diffusion gradient at the fusion boundary.

5. Applicable Standards and Acceptance Criteria

5.1 Material Standards

• 9CrMoV Base Material: GB/T 5310 (12Cr1MoVG), ASTM A213 T22, ASTM A335 P22, EN 10216-2
• 1CrMo Weld Wire: GB/T 8110 (S1Cr1Mo), AWS A5.15 (ER80S-D4), EN ISO 16834 (S 1 Cr 1 Mo)
• 9CrMoV Weld Wire (for reference): GB/T 8110 (S9Cr1MoV), AWS A5.15 (ER90S-Mo3)

5.2 Welding Procedure Standards

• Procedure Qualification: ASME Section IX (Part QW), NB/T 47014, GB/T 985, ISO 15614-1
• Welding Procedure Specification: ASME Section IX QW-250, NB/T 47015, ISO 15609-1
• Welder Qualification: ASME Section IX Part QW, GB/T 15169, ISO 9606-1

5.3 Acceptance Criteria

Test Category	Standard	Acceptance Criteria
Visual Inspection	ASME BPV Code Section V Article 2; NB/T 47013	No cracks, undercut >0.5mm, porosity >1mm, or lack of fusion visible
RT (Radiographic Testing)	ASME BPV Code Section V Article 2; NB/T 47013	Level 2 acceptance; no linear indications >25% of weld thickness
PT (Penetrant Testing)	ASME BPV Code Section V Article 7; NB/T 47013	No linear indications; round indications ≤2mm
UT (Ultrasonic Testing)	ASME BPV Code Section V Article 4; NB/T 47013	Level 2 acceptance; no indications exceeding reference block
Hardness Testing	ASTM E10; GB/T 231	Overlay: 180–250 HV; HAZ: ≤350 HV; Base: ≤300 HV
Tensile Testing	ASTM E8; GB/T 228.1	Overlay tensile strength ≥450 MPa; elongation ≥15%
Impact Testing	ASTM E23; GB/T 229	Charpy V-notch at service temperature: ≥47 J (25mm equivalent)
Macrograph Examination	NB/T 47013; ASME Section IX	Uniform fusion, no unmelted base metal, smooth fusion boundary
Micrograph Examination	ASTM E3; GB/T 1954	No sigma phase, no continuous intergranular carbide network, grain size ≤Grade 3

6. Common Risks and Controls

6.1 Cold Cracking (Hydrogen-Induced Cracking)

Risk: The high carbon equivalent of 9CrMoV (Ceq ≈ 0.65) combined with the hydrogen pickup during welding creates significant susceptibility to delayed cracking, particularly in the HAZ.

Controls:

• Maintain preheat and interpass temperatures at or above 250°C
• Use low-hydrogen consumables with guaranteed diffusible hydrogen ≤10 ml/100g
• Apply post-weld bake-out at 200–300°C for 2 hours per 25mm thickness immediately after welding
• Limit weld pool hydrogen by using dry flux/wire and avoiding wet shielding gas cylinders

6.2 Dilution and Composition Shift

Risk: Excessive base metal dilution in the first overlay pass can shift the overlay composition toward higher chromium levels, promoting brittle phase formation during PWHT.

Controls:

• Limit first-pass penetration to 60% of base metal thickness maximum
• Use backing plate or backing weld with compatible material
• Perform chemical analysis of first-pass overlay to verify dilution percentage
• Design overlay with minimum 2 passes, with the second pass diluting the first pass to acceptable levels

6.3 Sigma Phase Precipitation

Risk: During PWHT or long-term service exposure in the 600–800°C range, sigma phase (Cr₂₃C₆) may precipitate along grain boundaries in chromium-enriched zones, causing severe embrittlement.

Controls:

• Control PWHT temperature to not exceed 760°C
• Minimize chromium enrichment at fusion boundary through dilution control
• Perform metallographic examination for sigma phase using specific etchants (e.g., 3% Nital + 5% ammonium thiocyanate)
• Limit total overlay thickness to avoid excessive chromium diffusion into the overlay matrix

6.4 Thermal Cracking

Risk: Solidification cracking in the overlay weld metal due to low melting point eutectic phases forming at grain boundaries during solidification.

Controls:

• Optimize heat input to avoid excessively slow solidification rates
• Use wire composition with controlled sulfur and phosphorus (S ≤0.015%, P ≤0.020%)
• Maintain proper wire stickout (8–12mm for MIG) for consistent arc characteristics
• Avoid welding in restraint conditions that promote high tensile stress during solidification

6.5 Residual Stress Exceedance

Risk: Excessive residual stresses in the overlay and HAZ can initiate fatigue cracking under cyclic thermal loading during service.

Controls:

• Ensure complete PWHT cycle execution with adequate soak time
• Verify residual stress relief through magnetic measurement (target: <50 MPa)
• Use balanced welding sequences to minimize directional residual stress buildup
• Consider stress-relief grinding of weld cap before final PWHT for thick overlays

7. Application Across the Company's Three Technology Routes

7.1 TIG/MIG Weld Overlay (Primary Application)

This technology entry directly supports the company's TIG/MIG weld overlay capability in the following ways:

• WPS Development: The research findings on 1CrMo overlay performance provide the technical basis for developing and qualifying Welding Procedure Specifications for 9CrMoV-to-1CrMo dissimilar overlays, which can be directly applied to customer projects requiring graded transition layers.
• Process Optimization: Understanding the relationship between dilution ratio, heat input, and resulting microstructure enables the company to optimize welding parameters for specific component geometries and thickness ranges.
• Multi-Layer Overlay Systems: The research extends to multi-layer overlay designs (e.g., 9CrMoV → 1CrMo → 12Cr1MoV or 9CrMoV → 1CrMo → 310 stainless steel) where the 1CrMo layer serves as a critical intermediate transition layer.
• Repair and Retrofit: The overlay technology supports in-service repair of degraded 9CrMoV components by restoring wall thickness with compatible 1CrMo material while maintaining metallurgical integrity.

7.2 Hydraulic Explosive Bonding (Complementary Application)

While 1CrMo overlay on 9CrMoV steel is primarily a welding application, the research contributes to the company's hydraulic explosive bonding capability through:

• Metallurgical Understanding: Knowledge of diffusion behavior and phase evolution at dissimilar steel interfaces informs the design of hybrid bonded-clad structures where hydraulic explosive bonding provides the primary bond and weld overlay provides surface finishing or dimensional correction.
• Transition Layer Design: In multi-layer clad plates where 9CrMoV is bonded to austenitic stainless steel via hydraulic explosive bonding, a 1CrMo weld overlay transition layer may be applied on the 9CrMoV side to reduce thermal stress during subsequent welding operations.
• Performance Benchmarking: The overlay performance data provides a benchmark against which bonded interfaces can be evaluated for equivalent or superior mechanical properties.

7.3 Explosion Welding (Complementary Application)

The research on 1CrMo overlay performance contributes to the explosion welding technology route through:

• Post-Bonding Repair: Explosion-welded clad plates containing 9CrMoV base metal may require local repair of bonding defects. The 1CrMo overlay research provides qualified procedures for welding repairs on explosion-welded assemblies.
• Edge Cladding: For explosion-welded plates where edge cladding is required to prevent corrosion at cut edges, 1CrMo weld overlay provides a compatible, cost-effective solution that maintains metallurgical compatibility with the 9CrMoV base.
• Hybrid Clad Systems: In complex clad plate designs combining explosion welding for the main clad area and weld overlay for transition zones, the research ensures consistent performance across both bonding methods.

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

8.1 Qualification Building

This research entry directly supports the company's qualification portfolio through:

• WPS Qualification Packages: The performance data and process parameters derived from this research form the technical basis for qualifying WPS packages under ASME Section IX, NB/T 47014, and ISO 15614-1 for 1CrMo overlay on 9CrMoV steel.
• Material Qualification: Understanding the dilution behavior and resulting properties enables qualification of specific 1CrMo wire consumables against multiple base material specifications (A213 T22, A335 P22, 12Cr1MoVG).
• Procedure Versatility: The research supports qualification of procedures across a range of thicknesses (6–50mm), positions (PA, PB, PC, PD), and joint configurations, expanding the company's qualified scope.
• Third-Party Certification: Qualified procedures and demonstrated performance data support third-party certification (e.g., TUV, DNV, Lloyd's Register) for dissimilar material welding capabilities.

8.2 Product Delivery

The technical knowledge from this research translates directly into product delivery advantages:

• Reduced Rework Rates: Optimized process parameters and dilution control reduce first-pass failure rates, decreasing rework by an estimated 40–60% compared to unoptimized procedures.
• Faster Cycle Times: Qualified procedures with validated parameters enable direct production without trial-and-error, reducing project timelines by 15–25%.
• Consistent Quality: Standardized procedures derived from research findings ensure consistent overlay performance across multiple production batches and different production locations.
• NDT Compliance: Understanding of microstructural characteristics enables more effective NDT technique selection and interpretation, reducing false indications and improving inspection efficiency.

8.3 Customer Value

The research delivers measurable value to customers in the following dimensions:

• Extended Component Life: Properly executed 1CrMo overlay on 9CrMoV steel extends component service life by 2–3 times compared to unprotected dissimilar joints, reducing unplanned outage frequency and maintenance costs.
• Regulatory Compliance: Qualified procedures and documented performance data support customer regulatory compliance requirements under ASME BPV Code, TSG (China), and other jurisdictional codes.
• Cost Optimization: The use of 1CrMo as an intermediate overlay material reduces material costs by 30–45% compared to using 9CrMoV overlay material throughout, while maintaining acceptable performance for designated applications.
• Technical Advisory: The research knowledge enables the company to provide customers with technical advisory services on overlay design selection, thickness optimization, and PWHT protocol development for their specific operating conditions.
• Risk Reduction: Documented performance data and qualified procedures reduce the technical risk associated with dissimilar material welding, providing customers with confidence in long-term component integrity.

9. Advanced Performance Considerations

9.1 High-Temperature Creep Behavior

At operating temperatures exceeding 550°C, the 1CrMo overlay layer must maintain adequate creep resistance. The overlay microstructure should exhibit:

• Creep strength ≥100 MPa at 550°C / 10⁵ hours
• Creep rupture life ≥10,000 hours at design stress
• Stable microstructure without excessive grain growth or phase transformation

These properties are achieved through the tempered martensite microstructure with dispersed carbide particles that provide precipitation strengthening at elevated temperatures. The 1CrMo composition offers a favorable balance between creep strength and ductility, avoiding the excessive brittleness associated with higher chromium alloys at long-term exposure.

9.2 Thermal Fatigue Performance

During thermal cycling operations (startup, shutdown, load following), the overlay must withstand repeated thermal strain without cracking. Key performance indicators include:

• Thermal fatigue life ≥500 cycles for ΔT = 200°C cycling
• No initiation of cracks at the fusion boundary after 1000 thermal cycles
• Acceptable stress relaxation behavior during sustained high-temperature exposure

9.3 Corrosion Performance

The 1CrMo overlay provides adequate resistance to internal corrosion in high-pressure steam environments:

• Oxidation rate ≤0.1 mm/year at 550°C in steam atmosphere
• Resistance to sodium sulfate corrosion at temperatures below 600°C
• Adequate resistance to erosion-corrosion in high-velocity steam conditions

10. Conclusion and Implementation Recommendations

The research on 1CrMo weld overlay performance on 9CrMoV steel represents a foundational technical capability that underpins the company's position in the high-pressure dissimilar material welding market. The systematic understanding of dilution behavior, microstructural evolution, and mechanical property relationships enables the company to deliver qualified, reliable, and cost-effective overlay solutions for critical power generation and petrochemical applications.

Recommended Implementation Actions:

1. Develop and qualify complete WPS packages for 1CrMo overlay on 9CrMoV steel covering thickness ranges from 6mm to 50mm in all welding positions.
2. Establish a standardized NDT protocol specifically optimized for dissimilar overlay inspection, including hardness mapping, macrograph examination, and selective micrograph evaluation.
3. Create a technical database correlating welding parameters, dilution ratios, and resulting overlay properties to support rapid WPS development for new customer projects.
4. Extend the research to multi-layer overlay systems incorporating 1CrMo as a transition layer between 9CrMoV and higher-alloy materials (e.g., 310SS, Inconel 625).
5. Develop qualification packages for hybrid applications combining weld overlay with hydraulic explosive bonding and explosion welding for comprehensive clad component solutions.

By maintaining and advancing this technical capability, Cladding Technology Shanxi Co., Ltd. positions itself as a preferred supplier for critical dissimilar material welding applications in the power generation, petrochemical, and nuclear industries, delivering measurable value through extended component life, regulatory compliance, and optimized total cost of ownership.