Nanostructured Surface Layer via Pre-Pressure Roll Rolling on Weld Overlay Cladding

1. Definition and Fundamental Principles

1.1 Technical Definition

Pre-pressure roll rolling (also known as pre-stress rolling or cold rolling of weld overlay surfaces) is a solid-state plastic deformation process applied to the as-welded surface of a cladding layer. The technique subjects the weld overlay cladding to controlled compressive plastic deformation through high-force rollers, inducing severe plastic deformation (SPD) at the surface. This process transforms the coarse, columnar, or equiaxed microstructure of the as-deposited weld metal into a refined nanostructured layer—typically achieving grain sizes in the range of 10–100 nm at the deformed surface—while introducing beneficial residual compressive stresses in the subsurface region.

1.2 Microstructural Transformation Mechanisms

The formation of the nanostructured layer follows a well-established hierarchy of microstructural evolution:

• Dislocation accumulation: At low rolling reductions, dislocation density increases dramatically (from ~10¹⁴ m^-2 to ~10¹⁵ m^-2), creating dense dislocation networks.
• Dislocation cell formation: Further deformation causes dislocations to arrange into cells with sub-micron to nanometer-scale subgrains bounded by low-angle boundaries.
• Nanograin refinement: With increasing strain (typically ε > 2–3), low-angle boundaries evolve into high-angle boundaries, producing fully nanostructured grains below 100 nm.
• Grain boundary engineering: The nanostructured layer exhibits an increased fraction of special grain boundaries (low-Σ CSL boundaries), enhancing resistance to crack initiation and propagation.

1.3 Pre-Pressure Concept

The "pre-pressure" designation distinguishes this technique from conventional surface rolling. It refers to the application of an initial, controlled pre-load before the main rolling pass. This pre-pressure serves to:

• Ensure uniform roller-to-surface contact without slippage
• Establish a baseline elastic deformation zone that prevents surface tearing
• Create a gradual strain gradient from the surface into the bulk, avoiding abrupt strain discontinuities that could cause cracking

2. Category and Business Positioning

2.1 Technology Classification

This technique belongs to the category of post-weld surface modification within the broader cladding technology value chain. It is classified as:

• A solid-state processing method (no melting, no thermal cycle)
• A surface engineering technology (localized modification, limited depth of influence typically 100–500 μm)
• A work-hardening process that enhances surface mechanical properties without altering bulk composition

2.2 Positioning Within Cladding Technology Shanxi Co., Ltd.

This nanostructuring capability serves as a value-added finishing process that complements all three primary technology routes (TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding). It is not a standalone cladding method but rather a critical enhancement step that elevates the performance envelope of welded cladding products, particularly in applications demanding extreme surface durability.

3. Technical Purpose and Value

3.1 Primary Technical Objectives

• Surface hardness enhancement: Achieve 30–100% increase in surface microhardness compared to the as-welded condition
• Wear resistance improvement: Extend service life in abrasive and erosive environments by 2–5×
• Corrosion resistance enhancement: Reduce localized corrosion susceptibility through grain refinement and residual compressive stress
• Fatigue life extension: Introduce beneficial surface compressive residual stresses that inhibit crack initiation
• Elimination of surface defects: Flatten and close micro-porosity and surface roughness through plastic flow

3.2 Quantifiable Performance Improvements

Property	As-Welded Condition	After Nano-Rolling	Improvement Factor
Surface Microhardness (HV0.3)	250–350 HV	400–600 HV	1.5–2.0×
Surface Roughness Ra (μm)	3.2–6.3 μm	0.2–0.8 μm	5–10× reduction
Wear Life (pin-on-disk)	Baseline	2–5× baseline	2–5×
Corrosion Potential Shift (mV)	Baseline	+20 to +80 mV (noble)	Improved passivity
Surface Residual Stress (MPa)	+50 to +150 (tensile)	−300 to −800 (compressive)	Stress reversal

3.3 Customer Value Proposition

The nanostructured surface layer technology provides a performance multiplier for existing cladding products without requiring requalification of the base weld procedure. This means:

• Existing WPS/PQR qualified weld overlay procedures remain valid
• Enhanced performance is achieved through a separate, independently qualified surface treatment process
• Customers receive superior surface properties without the cost and complexity of alternative cladding materials

4. Key Process and Implementation Points

4.1 Process Parameters

Parameter	Typical Range	Critical Influence
Rolling Force (kN)	50–500	Determines strain level; must exceed yield threshold
Rolling Speed (m/min)	0.5–10	Affects strain rate and heat generation
Number of Passes	3–12	Controls depth and degree of nanostructuring
Roll Diameter (mm)	50–200	Smaller diameter = higher strain concentration
Roll Material	Hardened steel (HRC 60+) or carbide	Must resist wear and transfer deformation uniformly
Surface Reduction per Pass (%)	0.5–3.0	Higher reduction = faster nanostructuring but risk of cracking
Temperature Control	Ambient to 200°C max	Avoid dynamic recovery that reverses nanostructure
Interpass Inspection	After every 2–3 passes	Detect micro-cracking or delamination

4.2 Implementation Sequence

1. Surface preparation: Grind or brush the as-welded surface to Ra ≤ 6.3 μm; remove spatter, slag, and obvious defects. Ensure surface is clean and free of contaminants.
2. Baseline characterization: Record as-welded surface hardness profile (HV0.3), roughness, and residual stress state for comparison.
3. Pre-pressure application: Apply initial light contact force (10–20% of maximum rolling force) to establish roller alignment and contact uniformity.
4. Incremental rolling passes: Execute rolling passes with progressively increasing force, monitoring surface temperature and visual appearance between passes.
5. Post-process inspection: Perform hardness profiling (depth-wise), roughness measurement, optical/SEM microstructural examination, and residual stress measurement.
6. Final surface conditioning: If required, perform light polishing or chemical polishing to achieve specified surface finish without disturbing the nanostructured layer.

4.3 Microstructural Characterization Protocol

The study and qualification of the nanostructured layer requires systematic microstructural characterization:

• Optical microscopy (OM): Depth profiling of hardness gradient, identification of deformed layer thickness
• Scanning electron microscopy (SEM): Grain size measurement, dislocation structure observation, grain boundary character analysis
• Transmission electron microscopy (TEM): Confirmation of nanograin structure, dislocation density quantification, identification of deformation twins
• Electron backscatter diffraction (EBSD): Grain size distribution, texture analysis, grain boundary character distribution
• X-ray diffraction (XRD): Residual stress measurement, texture quantification, phase identification

5. Applicable Standards and Acceptance Criteria

5.1 Relevant Standards

Standard	Applicability	Key Requirements
GB/T 11345	Ultrasonic testing of weld overlay	VT1-2 level inspection for volumetric defects
GB/T 3323	Radiographic testing	Verification of cladding integrity post-rolling
ASTM E92	Knoop and Vickers hardness	Microhardness measurement methodology
ASTM E10	Rockwell and Brinell hardness	Macro hardness verification
ASTM E918	Residual stress by X-ray diffraction	Compressive stress verification
ASTM B557	Surface roughness measurement	Ra determination on nanostructured surface
ASME BPV Section V	Nondestructive examination	Acceptance criteria for final cladding inspection
NACE SP0169	Corrosion control verification	Corrosion performance of treated surfaces
ISO 14409	Welding procedures for overlay	WPS validity with post-weld treatment
GB/T 20548	Welding procedure qualification	Chinese standard for overlay welding procedures

5.2 Acceptance Criteria for Nanostructured Surface Layer

• Hardness: Surface microhardness ≥ 1.3× the as-welded surface hardness; hardness gradient from surface to bulk must be continuous without abrupt transitions
• Integrity: No cracks, delamination, or spalling in the nanostructured layer; verified by MT/PT of surface
• Roughness: Ra ≤ 1.6 μm (or as specified by customer/engineering); uniform across the treated area
• Residual stress: Surface compressive stress ≥ −200 MPa (longitudinal direction); no tensile stress in the nanostructured zone
• Microstructure: Nanostructured layer thickness ≥ 100 μm; grain size ≤ 100 nm confirmed by TEM/EBSD
• Base metal integrity: No distortion, cracking, or mechanical property degradation in the base metal or weld interface

6. Common Risks and Controls

6.1 Technical Risks

Risk	Mechanism	Control Measure
Surface micro-cracking	Excessive strain in brittle phases or hard carbides	Limit single-pass reduction; use intermediate annealing; select appropriate rolling parameters for material
Delamination at weld interface	High shear stress at cladding-base metal interface	Control rolling force to avoid full-thickness deformation; verify interface integrity pre- and post-rolling
Dynamic recovery/reversal	Excessive temperature during rolling causes recrystallization	Monitor surface temperature; use water or air cooling between passes; limit rolling speed
Surface contamination	Roller wear debris or lubricant transfer	Use clean, dedicated rollers; avoid lubricants unless specified; inspect surface cleanliness
Non-uniform deformation	Edge effects, roller misalignment, or surface irregularities	Use multiple overlapping passes; employ CNC-controlled rolling; verify uniformity by hardness mapping
Stress relaxation	Subsequent heat treatment or service temperature exceeds stress relief threshold	Document maximum service temperature; perform stress verification at elevated temperature if needed

6.2 Quality Control Measures

1. Pre-rolling verification: Confirm weld overlay quality (VT, MT, PT, hardness) meets WPS requirements before applying surface treatment
2. In-process monitoring: Real-time force monitoring, temperature measurement, and visual inspection between passes
3. Post-rolling NDT: Full-surface MT or PT to detect any cracks introduced during rolling
4. Microstructural verification: Representative cross-sections examined by OM/SEM for nanostructured layer confirmation
5. Performance testing: Wear testing, corrosion testing, and fatigue testing on witness coupons to validate improvement

7. Application Scenarios Across Technology Routes

7.1 TIG/MIG Weld Overlay Cladding

Primary application context: The nanostructured surface treatment is most directly applicable to TIG and MIG weld overlay cladding, as these processes produce weld surfaces with inherent roughness, porosity, and residual tensile stresses that benefit most from post-weld surface modification.

• Stainless steel overlay (309L/310/625): Nano-rolling enhances surface hardness of austenitic weld metal, improving resistance to abrasive wear in slurry service
• Hardfacing overlay (Co-Cr, Ni-Cr): Further refinement of the dendritic microstructure improves tribological performance in high-wear applications
• Transition layer treatment: Nanostructuring the transition zone surface can improve bonding characteristics for subsequent layers or coatings
• Repair applications: Surface nano-rolling of repaired welds restores and exceeds original surface properties

7.2 Hydraulic Explosive Bonding

Application context: For hydraulically bonded clad plates and pipes, the nanostructured surface treatment can be applied to the exposed cladding surface to enhance surface properties while maintaining the integrity of the hydraulic bond interface.

• Clad surface finishing: Replace conventional machining/polishing with nano-rolling to achieve superior surface properties and finish simultaneously
• Post-bond surface conditioning: Eliminate surface residual stresses introduced during hydraulic bonding
• Functional enhancement: Improve the surface of bonded pipes for subsequent machining or coating operations

7.3 Explosion Welding

Application context: Explosion-welded clad plates often exhibit a wavy bond interface with localized plastic deformation zones. Surface nano-rolling provides an additional layer of surface engineering without disturbing the bond interface.

• Surface property enhancement: Add nanostructured surface layer to explosion-welded clad plates for applications requiring both corrosion resistance (from cladding) and wear resistance (from nanostructured surface)
• Surface defect elimination: Close surface micro-porosity and micro-cracks on explosion-welded surfaces
• Pre-machining treatment: Improve machining performance and tool life when finishing explosion-welded surfaces

8. Contribution to Qualification Building and Customer Value

8.1 Qualification and Certification Impact

• WPS/PQR extension: The nanostructured surface treatment can be incorporated as a post-weld treatment step in existing welding procedure specifications without requiring full requalification of the base welding procedure
• Independent process qualification: Develop a standalone process specification (PS) for the nano-rolling treatment, including parameter ranges, acceptance criteria, and performance verification
• Third-party certification: Submit nano-rolling process for qualification under relevant industry standards (e.g., ASME Section IX for post-weld treatments, or NACE MR0175 for sour service verification)
• Research-qualified status: The microstructural study provides the scientific foundation for process optimization and code acceptance

8.2 Product Delivery Enhancement

• Performance differentiation: Offer nanostructured surface finish as a premium option that provides quantifiable performance advantages over standard cladding
• Reduced downstream processing: Nano-rolled surfaces may eliminate the need for additional surface treatments (shot peening, polishing, coating), reducing total cost of ownership
• Extended service life: Documented wear and corrosion life improvements provide customers with reduced maintenance frequency and extended asset life
• Design flexibility: Enhanced surface properties allow engineers to specify less aggressive materials while achieving required performance, enabling cost optimization

8.3 Customer Value Summary

The nanostructured surface layer technology transforms standard weld overlay cladding from a corrosion/wear protection solution into a multi-functional surface engineering system. By combining the compositional advantages of the cladding alloy with the mechanical advantages of severe plastic deformation, Cladding Technology Shanxi Co., Ltd. delivers products that exceed conventional performance benchmarks while maintaining established qualification frameworks. This represents a significant competitive advantage in demanding applications such as nuclear components, petrochemical equipment, mining machinery, and aerospace structures where surface durability is critical to safety and availability.

9. Future Development Directions

• Multi-pass rolling optimization: Develop predictive models correlating rolling parameters with nanostructured layer thickness and grain size distribution
• In-situ monitoring: Implement real-time acoustic emission or strain gauging during rolling to detect incipient cracking
• Combined treatments: Develop hybrid processes combining nano-rolling with thermal treatment (e.g., low-temperature tempering) to optimize the balance between hardness and toughness
• Scale-up qualification: Qualify the process for large-diameter pipes and thick plates with rigorous performance verification
• Digital twin integration: Create process simulation models for virtual qualification and parameter optimization before physical testing