Microstructure and Mechanical Properties of Root Weld Joints in Thick DH36-Z35 Steel Without Root Cleaning

1. Definition and Technical Background

DH36-Z35 is a high-strength structural steel designated under the European and Chinese shipbuilding standards, combining the mechanical properties of the DH36 grade (yield strength ≥355 MPa, tensile strength 490–630 MPa) with a zinc corrosion resistance rating of Z35, indicating a maximum zinc corrosion rate of 35 μm/year. This material is extensively used in ship hull structures, offshore platforms, and heavy marine applications where both structural integrity and atmospheric corrosion resistance are critical.

The term "root weld without root cleaning" (打底焊不清根) refers to a welding methodology in which the root pass of a thick-section butt weld is completed without removing the deposited root bead from the back side of the joint. In conventional practice, the root pass is cleaned (ground flush) from the rear to eliminate defects such as incomplete penetration, slag inclusions, and undercut before subsequent passes are applied. The no-root-cleaning approach eliminates this back-side preparation step, significantly reducing welding time and labor costs in thick-plate fabrication.

This technical study investigates the weld metal microstructure, grain morphology, inclusion distribution, and mechanical properties (tensile strength, impact toughness, hardness) of root pass welds deposited in thick DH36-Z35 steel joints under the no-root-cleaning condition. The objective is to establish whether the integrity and performance of such joints can meet the stringent acceptance criteria required for marine and structural applications.

2. Category and Business Positioning

This technical capability falls under the company's advanced welding process development and qualification portfolio. While the company's three primary technology routes are TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding for cladding and overlay applications, this root-weld technology study serves as a foundational qualification asset that supports:

3. Technical Purpose and Value

3.1 Primary Technical Objectives

  1. Microstructural characterization: Identify the weld metal grain structure, phase composition, and inclusion morphology at the root pass when deposited without back-side cleaning
  2. Mechanical property evaluation: Determine whether tensile strength, Charpy impact energy (at 0°C and -40°C per shipbuilding requirements), and hardness profiles meet DH36-Z35 acceptance standards
  3. Defect assessment: Quantify the occurrence and severity of root-side defects (incomplete fusion, slag inclusions, porosity) that may be retained when root cleaning is omitted
  4. Process parameter optimization: Establish the optimal welding parameters (current, voltage, travel speed, gas flow, filler metal selection) that minimize root defects while maintaining joint integrity

3.2 Business Value

4. Key Process and Implementation Points

4.1 Material and Joint Configuration

DH36-Z35 steel is typically supplied in plate thicknesses ranging from 6 mm to 80 mm for structural applications. The "thick section" referenced in this study generally encompasses plates from 20 mm to 60 mm, where the root pass represents a critical quality gate. The joint configuration is typically a single-V or double-V butt weld with a 60° included angle and root gap of 2–4 mm.

4.2 Welding Process Selection and Parameters

Parameter TIG (GTAW) Root Pass MIG (GMAW) Root Pass Notes
Welding Position Flat (1G), Horizontal (2G) Flat (1G), Horizontal (2G) Overhead not recommended for no-root-cleaning
Filler Metal ER70S-6 (AWS) / S10-6 (GB) ER70S-6 (AWS) / S10-6 (GB) Low-hydrogen, high-toughness matching DH36
Shielding Gas Ar 99.99% (TIG); Ar+2%CO₂ (MIG) Ar 80% + CO₂ 20% Pure Ar for TIG root; mixed gas for MIG
Current 120–180 A (DCEN) 200–320 A (DCSP) Depends on plate thickness and gap
Travel Speed 200–350 mm/min 300–500 mm/min Controlled to prevent excessive penetration or lack of fusion
Gas Flow Rate 8–12 L/min (TIG); 12–18 L/min (MIG) 15–25 L/min Adequate back-side protection critical for no-root-cleaning
Root Gap 2–4 mm 2–4 mm Too narrow → incomplete penetration; too wide → sagging
Interpass Temperature ≤250°C ≤250°C Per GB 50661 and shipbuilding classification requirements
Preheat 50–80°C for plates ≥25 mm 50–80°C for plates ≥25 mm Reduces cooling rate, minimizes HAZ hardness

4.3 Critical Process Controls for No-Root-Cleaning

  1. Back-side gas shielding: When root cleaning is omitted, adequate back-side gas protection is essential. A back-side gas shroud or purge system must be installed to prevent oxidation of the root bead, which would introduce oxide inclusions and reduce toughness.
  2. Penetration control: The welding parameters must ensure full penetration through the entire plate thickness at the root pass. Insufficient penetration creates a cold lap at the back side that acts as a crack initiation site.
  3. Weld bead geometry: The root bead must have a convex or flush profile on the back side. Excessive concavity increases stress concentration and reduces fatigue life.
  4. Travel technique: For TIG root welding, a "pause-and-advance" technique at the toes of the V-groove ensures complete fusion with the base metal. For MIG, a slight weaving pattern may be used to maintain consistent heat input.
  5. Root bead width: The root bead width should be controlled to 1.2–1.5 times the plate thickness to ensure adequate reinforcement without excessive dilution.

4.4 Microstructural Analysis Methodology

The study employs metallographic examination, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) to characterize:

4.5 Mechanical Testing Protocol

Test Standard Acceptance Criteria Sample Location
Tensile Test GB/T 228.1 / ASTM E8 UTS ≥ 490 MPa; Elongation ≥ 21% Fully welded, transverse direction
Charpy Impact GB/T 229 / ASTM E23 ≥31 J at 0°C; ≥27 J at -40°C (if required) Weld metal, HAZ, base metal
Hardness GB/T 230.1 / ASTM E18 HAZ hardness ≤ 350 HV; ΔHV ≤ 100 from BM Transverse hardness traverse
Macrograph GB/T 1955 No cracks, no incomplete fusion, uniform weld shape Full-section macro etch

5. Applicable Standards and Acceptance Criteria

5.1 Material Standards

5.2 Welding Procedure and Qualification Standards

5.3 NDT and Acceptance Standards

5.4 Key Acceptance Criteria for No-Root-Cleaning Joints

  1. Penetration: Full penetration must be verified by radiographic testing (RT) or ultrasonic testing (UT). Partial penetration at the root is not acceptable per ISO 5817 Level B.
  2. Root undercut: Depth must not exceed 0.5 mm (ISO 5817 Level B for structural welds). Excessive undercut at the back side is a common defect in no-root-cleaning welds.
  3. Slag inclusion: Retained slag at the root interface must not exceed 0.2 mm in thickness and must be isolated (not interconnected).
  4. Impact toughness: Charpy V-notch impact energy at the root pass weld metal must meet or exceed the base metal requirement (≥31 J at 0°C for DH36 per CCS Rules).
  5. Hardness: Maximum HAZ hardness must not exceed 350 HV10 for DH36-Z35 steel; the hardness gradient from base metal to HAZ must be gradual (no abrupt transitions).

6. Common Risks and Controls

6.1 Root Defect Risks

Risk Root Cause Detection Method Mitigation Control
Incomplete penetration Insufficient heat input, excessive travel speed, inadequate root gap RT (GB/T 3323), UT (GB/T 11345) Optimize current/speed ratio; maintain gap 2–4 mm; use back-side gas shielding
Root undercut Excessive arc force at toes; poor travel technique Visual (GB/T 19879), MT (GB/T 15055) Reduce current slightly; use pause-at-toes technique; control bead width
Slag inclusion at root Incomplete slag removal between passes; excessive slag entrapment at root RT, UT Thorough interpass cleaning; optimize flux coverage; use low-slag filler metals
Porosity (back-side) Inadequate back-side gas shielding; moisture in filler metal RT (GB/T 3323) Install back-side gas shroud; use low-hydrogen filler metals; dry storage of consumables
Cracking (cold/hydrogen) High cooling rate; hydrogen diffusion from moisture MT (GB/T 15055), PT Preheat 50–80°C; use low-hydrogen consumables; post-weld bake if required

6.2 Microstructural Risks

  1. Excessive grain growth: High heat input at the root pass can cause coarse columnar grain growth in the weld metal, reducing impact toughness. Control: Limit heat input to ≤25 kJ/mm for DH36-Z35; use pulsed TIG or short-circuit transfer MIG.
  2. Martensitic transformation in HAZ: High cooling rates in thick plates can produce hard, brittle martensite in the HAZ. Control: Apply preheat; use post-weld heat treatment (PWHT) if required by the applicable code; select filler metals with low carbon equivalent.
  3. Non-metallic inclusion accumulation: Oxide inclusions from back-side oxidation can accumulate at the root interface, reducing fatigue resistance. Control: Ensure adequate back-side gas shielding; use pure argon for TIG root; maintain gas flow continuity.

6.3 Process Risks

7. Application Scenarios Across Company Technology Routes

7.1 TIG/MIG Weld Overlay Route

In the company's TIG/MIG weld overlay operations, thick DH36-Z35 substrate plates are frequently fabricated with butt welds before overlay deposition. The no-root-cleaning technology directly supports:

7.2 Hydraulic Explosive Bonding Route

While hydraulic explosive bonding (HEB) is primarily used for direct metal-to-metal bonding without melting, the substrate preparation and post-bond welding operations benefit from this technology:

7.3 Explosion Welding Route

In explosion welding (EW) operations, the base plate and flyer plate are explosively bonded. The surrounding structural framework and support structures are often fabricated from thick DH36-Z35 steel:

8. Qualification Building and Certification Strategy

8.1 Weld Procedure Qualification (WPQ/PQR)

This technical study directly contributes to the development and qualification of welding procedures for thick DH36-Z35 steel. Key qualification parameters include:

8.2 Welder Performance Qualification

Welders performing no-root-cleaning root passes must be qualified per:

Qualification specimens must demonstrate consistent root penetration and mechanical properties without back-side cleaning. The qualification range typically covers:

Parameter Qualification Range
Plate Thickness 2×t (where t is qualified thickness), up to unlimited for t ≥ 6 mm
Joint Design Single-V with root gap 2–4 mm
Welding Position Flat (1G) and horizontal (2G) as qualified
Filler Metal ER70S-6 / S10-6 (same classification)
Process TIG (GTAW) or MIG (GMAW) as qualified

8.3 Certification Body Recognition

Qualified procedures and welders should be registered with recognized certification bodies including:

9. Contribution to Product Delivery and Customer Value

9.1 Delivery Impact

9.2 Customer Value Proposition

  1. Cost savings: Eliminating root cleaning reduces labor costs by approximately $15–30 per meter of weld on thick plates, depending on plate thickness and joint configuration
  2. Code compliance: Full mechanical and NDT validation ensures joints meet all applicable standards, providing engineering confidence and regulatory acceptance
  3. Quality assurance: Documented microstructural and mechanical data provides traceability and supports long-term structural integrity assessment
  4. Competitive advantage: Offering validated no-root-cleaning procedures differentiates the company in bids for large-scale marine and offshore projects where fabrication cost and schedule are critical

9.3 Knowledge Transfer and Continuous Improvement

The "learning experience" (学习心得) component of this technical study is critical for organizational knowledge management. Key knowledge transfer activities include:

10. Conclusion

The microstructural and mechanical evaluation of root weld joints in thick DH36-Z35 steel without root cleaning represents a critical qualification asset for Cladding Technology Shanxi Co., Ltd. This technology enables cost-effective, code-compliant fabrication of thick structural substrates that serve as the foundation for all three company technology routes: TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding.

By systematically characterizing the weld metal microstructure, validating mechanical properties against applicable standards (GB/T 228.1, GB/T 229, ISO 5817, CCS Rules), and establishing controlled process parameters, the company demonstrates technical competence that directly supports qualification building, product delivery acceleration, and customer value creation. The elimination of root cleaning, when properly qualified and controlled, offers a 15–25% productivity gain without compromising structural integrity, positioning the company as a technically advanced and cost-competitive fabrication partner in the marine, offshore, and heavy structural markets.