Multi-Pass GMAW Welding of Q690D High-Strength Steel: Microstructure and Mechanical Property Analysis

1. Definition and Technical Principles

1.1 Q690D Steel Overview

Q690D is a high-strength low-alloy (HSLA) structural steel classified under the Chinese national standard GB/T 1591, with a minimum yield strength of 690 MPa and a minimum tensile strength of 770 MPa. The "D" suffix designates enhanced low-temperature toughness, with mandatory Charpy V-notch impact testing at −20 °C. The typical chemical composition of Q690D includes carbon content of 0.10–0.18%, manganese of 1.2–1.6%, and micro-alloying additions of niobium (0.02–0.05%), vanadium (0.02–0.05%), and titanium to achieve precipitation hardening and grain refinement without excessive carbon equivalence.

1.2 GMAW Multi-Pass Welding Principle

Gas Metal Arc Welding (GMAW), also known as MIG/MAG welding, employs a continuously fed consumable wire electrode and a shielding gas to produce an arc that melts both the electrode and the base metal, forming a weld pool that solidifies into the joint. In multi-pass GMAW welding, successive layers of weld metal are deposited in a systematic sequence to fill thick-section joints. Each subsequent pass re-heats the previous pass, creating a complex thermal history that governs grain structure, phase transformation, and residual stress distribution throughout the weld zone.

The fundamental metallurgical challenge in multi-pass GMAW of Q690D lies in the interaction between the high carbon equivalent (CE typically 0.45–0.55%), the rapid cooling rates inherent to high-strength steels, and the re-heating cycles of subsequent passes. These factors collectively influence the formation of susceptible microstructural features such as martensite, bainite, and Widmanstätten ferrite in the Heat-Affected Zone (HAZ), which directly impact toughness and crack resistance.

1.3 Microstructure Evolution in Multi-Pass Welding

In multi-pass GMAW joints of Q690D, three distinct metallurgical regions develop:

Weld Metal (WM): The solidified filler metal deposits, whose microstructure is governed by solidification cooling rate, filler wire composition, and welding parameters. With appropriate filler selection (e.g., E71T-8 or equivalent low-carbon, low-alloy wires), the weld metal typically exhibits a fine-grained acicular ferrite and ferrite-pearlite microstructure.
Heat-Affected Zone (HAZ): The region subjected to peak temperatures above the Ac3 transformation temperature but not melted. The HAZ undergoes austenitization and subsequent non-equilibrium transformation, potentially forming coarse-grained martensite or bainite in the coarse-grained HAZ (CGHAZ) where peak temperatures exceed 1,300 °C.
Re-Heated HAZ: Subsequent passes re-heat previously solidified weld metal and HAZ regions to intermediate temperatures (400–800 °C), causing grain coarsening, precipitate coarsening, and softening, which can reduce local hardness and strength below acceptable thresholds.

2. Category and Business Positioning

2.1 Positioning Within the Company's Technology Portfolio

This technical study belongs to the Weld Overlay and Structural Welding capability domain, serving as a foundational qualification asset within the company's three core technology routes. While the company's primary business centers on bimetallic cladding and weld overlay manufacturing, mastery of multi-pass GMAW welding of ultra-high-strength steels like Q690D is essential for:

Structural integrity of cladding substrates and support structures
Welded joint qualification for pressure vessels and piping systems requiring high-strength base materials
Transition layer and build-up welding in multi-material joint configurations
Repair and maintenance welding of critical components in energy and infrastructure sectors

2.2 Relationship to Core Technology Routes

The knowledge gained from Q690D multi-pass GMAW welding directly interfaces with all three of the company's technology routes:

TIG/MIG Weld Overlay: Understanding of thermal cycling effects, residual stress management, and microstructural control in multi-pass welding directly informs overlay layer design, particularly when applying corrosion-resistant or wear-resistant cladding layers onto Q690D substrates.
Hydraulic Explosive Bonding: High-strength steels such as Q690D are frequently used as base substrates for explosive bonding with copper, titanium, or stainless steel. Welded joints in the bonded assembly must match or exceed the base metal properties, making GMAW qualification essential.
Explosion Welding: Similar to hydraulic explosive bonding, explosion-welded clad plates and pipes made on Q690D substrates require qualified attachment welds for forming, piping, and structural integration.

3. Technical Purpose and Value

3.1 Primary Technical Objectives

The study of multi-pass GMAW joints in Q690D steel serves several critical technical objectives:

WPS Qualification: Establishing qualified Welding Procedure Specifications (WPS) that ensure consistent mechanical performance across multi-pass joints, enabling certification under applicable codes.
Mechanical Property Assurance: Demonstrating that weld metal, HAZ, and re-heated zones meet or exceed the base metal's yield strength (≥690 MPa), tensile strength (≥770 MPa), and impact energy requirements at −20 °C.
Crack Resistance Optimization: Identifying parameter combinations that minimize cold cracking susceptibility (hydrogen-induced cracking) and hot cracking, which are prevalent concerns in high-CE steels.
Residual Stress Management: Characterizing and mitigating welding residual stresses that could compromise dimensional stability or fatigue performance in service.

3.2 Business Value

From a commercial perspective, qualified multi-pass GMAW procedures for Q690D enable the company to:

Expand product scope into ultra-high-strength structural applications in oil & gas, offshore platforms, and heavy machinery
Provide turnkey cladding solutions where the base substrate is Q690D or equivalent grades
Support customer projects requiring ASME, API, or PED code compliance with high-strength base materials
Reduce dependence on external welding qualification bodies, accelerating project timelines

4. Key Process and Implementation Points

4.1 Welding Parameter Optimization

The following table summarizes recommended GMAW parameters for multi-pass welding of Q690D steel in typical thickness ranges:

Parameter	Root Pass	Filler Passes	Cover Pass	Notes
Welding Current (A)	120–160	200–280	180–240	Current density 18–25 A/mm² for 1.2 mm wire
Welding Voltage (V)	18–22	24–30	22–27	Maintain spray transfer mode for filler passes
Travel Speed (mm/min)	200–300	350–500	300–400	Control heat input to 12–25 kJ/cm
Interpass Temperature (°C)	—	150–250	150–250	Strict control to prevent CGHAZ softening
Shielding Gas	Ar 98% + CO₂ 2%	Ar 95% + CO₂ 5%	Ar 95% + CO₂ 5%	Higher CO₂ for penetration and cost efficiency
Gas Flow Rate (L/min)	15–20	20–25	20–25	Adequate flow with wind shielding in outdoor conditions

4.2 Filler Metal Selection

Proper filler metal selection is critical to matching or exceeding Q690D base metal properties while maintaining weldability:

Filler Wire Grade	Equivalent Classification	Typical Tensile Strength (MPa)	Carbon Content (%)	Application
E71T-8	GB/T 8110 ER70S-G	≥485 (WM)	≤0.06	General structural welding, lower CE
E80T-5	GB/T 8110 ER80S-D2	≥550 (WM)	≤0.08	Higher strength matching, good toughness
E91T-8	GB/T 8110 ER90S-G	≥620 (WM)	≤0.06	Full-strength matching for Q690D
E110T-1	GB/T 8110 ER110S-6	≥760 (WM)	≤0.08	Ultra-high-strength matching, limited ductility

For Q690D multi-pass GMAW, E91T-8 or equivalent filler wire is typically recommended as it provides adequate weld metal strength (≥620 MPa) while maintaining good toughness and low hydrogen diffusible content. In applications requiring full strength matching, E110T-1 may be considered with careful attention to ductility and crack sensitivity.

4.3 Pre-Weld Preparation and Heat Treatment

Preheat Temperature: 80–150 °C for thicknesses ≤25 mm; 150–250 °C for thicknesses >25 mm. Preheat reduces cooling rate in the HAZ, suppressing martensite formation and hydrogen cracking risk.
Joint Design: Single-V groove with 60° included angle and 1–2 mm root gap for thicknesses up to 40 mm. Double-V or double-U groove for thicker sections to minimize total heat input.
Post-Weld Heat Treatment (PWHT): Recommended for critical applications. Temper at 580–620 °C for 2 hours per 25 mm thickness to relieve residual stresses and improve HAZ toughness. However, PWHT must be carefully controlled to avoid excessive softening of the Q690D base metal.
Hydrogen Control: Use low-hydrogen flux-cored or solid wires with diffusible hydrogen content ≤5 mL/100g. Bake electrodes and flux at 300–400 °C for 2 hours prior to use.

4.4 Multi-Pass Sequencing Strategy

Optimal pass sequencing minimizes peak re-heating temperatures and residual stress accumulation:

Root pass: Low heat input, back-gas protection (Ar 99.9%), careful root formation to prevent undercut and incomplete fusion.
Hot pass (if applicable): For thick sections, deposit a second pass with slightly higher current to "hot pass" over the root, relieving high residual stresses and preventing cold cracking.
Filler passes: Alternate sides for multi-V grooves to balance thermal distortion. Maintain stringer beads with adequate overlap (≥30% of previous bead width).
Cover pass: Final pass with lower heat input, wider bead for cosmetic finish and stress relief. May include a "stitch weld" technique to reduce distortion.

4.5 Microstructural Control Measures

The following measures are critical for controlling microstructure in Q690D GMAW joints:

Cooling rate control: Target 8/950 °C cooling time (t₈/₉₅₀) of 3–8 seconds to promote acicular ferrite formation and suppress coarse-grained martensite in the CGHAZ.
Thermal input management: Maintain heat input between 12–25 kJ/cm. Excessive heat input promotes grain coarsening and softening; insufficient heat input leads to high cooling rates and martensite formation.
Interpass temperature: Strictly maintain between 150–250 °C. Below 150 °C increases cold cracking risk; above 250 °C promotes grain coarsening and softening in re-heated zones.
Micro-alloying of weld metal: Nb and V additions in filler wire promote acicular ferrite nucleation and grain refinement in the weld metal.

5. Applicable Standards and Acceptance Criteria

5.1 Material Standards

GB/T 1591-2018: High-strength structural steels — Q690D classification, chemical composition, mechanical properties, and delivery conditions.
ASTM A709: High-strength low-alloy structural steel plate (HR40/HR60/HR70 equivalents for international projects).
EN 10025-6: Technical delivery conditions for structural steels — high-strength structural steels with improved through-thickness properties (S690QL equivalent).

5.2 Welding Procedure and Qualification Standards

GB/T 9858-2008: Welding procedure qualification rules for steels.
GB/T 19866-2005: Qualification of welding procedures and welders for steels.
ASME Section IX: Qualification rules for welding, brazing, and fuse bonding procedures and personnel.
ISO 15614-1: Qualification procedures for welding of metallic materials — General rules for arc welding.
EN ISO 3834-2: Quality requirements for fusion welding of steel — Quality level B.
NB/T 47014: Qualification rules for welding procedures and welders of pressure vessels.

5.3 Mechanical Property Acceptance Criteria

Test Method	Standard	Acceptance Criteria for Q690D Joint	Test Location
Tensile Test	GB/T 228.1	UTS ≥770 MPa; weld metal UTS ≥95% of base metal	Transverse weld metal, HAZ, and base metal
Hardness Test	GB/T 231.1	HAZ hardness ≤350 HV; ≤32 HV per ASTM A370	Full-width traverse across HAZ
Impact Test	GB/T 229	≥34 J at −20 °C (per Q690D requirement)	HAZ (Weld Neck), WM, and Base Metal
Bend Test	GB/T 232	Side bend, face bend, and root bend — no cracks or defects	Full-section transverse specimens
Macro/Micro Etch	GB/T 1955	No cracks, inclusions, or lack of fusion	Cross-section of representative joint

5.4 Non-Destructive Testing (NDT) Acceptance

Visual Testing (VT): Per GB/T 3323.1 or ISO 17637 — no undercut >1 mm, no porosity >1 mm, no slag inclusion >1 mm.
Ultrasonic Testing (UT): Per GB/T 11345 or ISO 17640 — acceptance level per ISO 17635-2 Level B or better.
Penetrant Testing (PT): Per GB/T 18851 or ISO 3452 — no linear indications >2 mm for critical applications.
RT (Radiographic Testing): Per GB/T 3323 or ISO 17636-1 — acceptance per ISO 17635-2 Level B.

6. Common Risks and Controls

6.1 Cold Cracking (Hydrogen-Induced Delayed Cracking)

Risk: Q690D has a carbon equivalent of 0.45–0.55%, placing it in the high crack-sensitivity category. Cold cracking typically manifests as transverse or longitudinal cracks in the HAZ or weld metal, appearing hours to days after welding.

Controls:

Preheat to 150–250 °C and maintain interpass temperature
Use low-hydrogen filler metals (diffusible hydrogen ≤5 mL/100g)
Post-weld slow cooling or immediate PWHT within 6 hours of welding completion
Limit heat input to avoid excessive martensite formation
Implement post-weld bake at 200–300 °C for hydrogen embrittlement relief

6.2 HAZ Softening and Coarse Grain Formation

Risk: In multi-pass welding, re-heating of the CGHAZ during subsequent passes can cause austenite grain coarsening, leading to softening (hardness drop below 250 HV) and loss of impact toughness.

Controls:

Strict interpass temperature control (150–250 °C)
Limit the number of passes to minimize cumulative re-heating cycles
Use multi-layer multi-pass techniques with adequate root preparation to reduce total layers
Consider PWHT at 580–620 °C to restore toughness (with awareness of strength reduction)

6.3 Residual Stress and Distortion

Risk: High-strength steels like Q690D develop significant welding residual stresses (approaching yield strength in the transverse direction), leading to distortion, stress corrosion cracking susceptibility, and fatigue degradation.

Controls:

Optimize pass sequencing to balance thermal input (alternate sides, skip-welding)
Use back-plate clamping and tacking to minimize distortion
Implement stress-relieving PWHT where design permits
Consider post-weld vibration stress relief (VSR) for large structures
Design joints with generous root gaps to reduce拘束 stress

6.4 Incomplete Fusion and Lack of Penetration

Risk: High-strength steels with tight tolerances and potentially low thermal conductivity can suffer from incomplete fusion, particularly at the root pass and in high-restraint joints.

Controls:

Ensure adequate root gap (1–3 mm) and proper joint preparation
Use back-gas protection (Ar 99.9%) for root pass
Maintain proper arc length and travel speed
Implement UT inspection between critical passes for thick sections
Train welders on technique for high-strength steel joints

6.5 Weld Metal Dilution and Property Mismatch

Risk: Excessive dilution of the filler metal by Q690D base metal can alter weld metal composition, potentially leading to hard, brittle microstructure with reduced toughness.

Controls:

Select filler metal with adequate strength margin (E91T-8 or higher)
Control heat input to minimize dilution ratio
Use narrower root gaps and controlled penetration
Verify weld metal composition by optical emission spectroscopy (OES) after coupon welding

7. Application Scenarios Across the Three Technology Routes

7.1 TIG/MIG Weld Overlay Integration

In the company's weld overlay operations, Q690D multi-pass GMAW knowledge is directly applicable in the following scenarios:

Substrate welding for overlay: When Q690D clad plates are fabricated into structures, the attachment welds must be qualified using multi-pass GMAW procedures. Understanding the thermal effects on the overlay layer is critical — excessive heat input from structural welding can damage the overlay bond interface.
Transition layer design: When overlaying austenitic stainless steel (e.g., 309L/310) onto Q690D substrates, the transition layer must be designed to accommodate the large thermal expansion mismatch. Multi-pass GMAW experience informs the selection of intermediate layers (e.g., 309L → 312L → 316L) and the thermal cycling strategy.
Build-up welding: Dimensional repair and build-up of Q690D components prior to overlay requires multi-pass GMAW with controlled interpass temperatures to maintain base metal properties.
Overlay joint qualification: The welding procedure qualification for overlay systems often requires demonstration of multi-pass weldability of the base material, leveraging Q690D GMAW qualification data.

7.2 Hydraulic Explosive Bonding Integration

In hydraulic explosive bonding operations, Q690D welding expertise supports:

Base plate preparation: Q690D base plates for hydraulic explosive bonding require edge welding (typically GMAW) to join individual plates into larger bonded areas. Multi-pass GMAW qualification ensures these welds meet structural requirements.
Post-bond welding: After hydraulic explosive bonding of Q690D with copper or aluminum, the bonded assembly may require structural welds for forming or attachment. Understanding Q690D welding behavior ensures these welds do not compromise the bonded interface.
Welding of bonded pipe ends: Explosion-welded or hydraulic explosive bonded Q690D pipes require end welds for piping installation. Multi-pass GMAW procedures ensure full-penetration butt welds with adequate toughness.
Repair welding: Damage to bonded assemblies during handling or forming requires careful repair welding that avoids disturbing the bonded interface while restoring structural integrity.

7.3 Explosion Welding Integration

In explosion welding operations, Q690D GMAW knowledge contributes to:

Clad plate structural welding: Explosion-welded Q690D clad plates (e.g., Q690D + 316L stainless, Q690D + copper) are routinely welded into structures. Multi-pass GMAW procedures ensure that structural welds match the base metal properties without damaging the clad layer.
Clad pipe fabrication: Explosion-welded Q690D clad pipes require welding of pipe ends, branch connections, and flange attachments. GMAW qualification covers both the structural weld in the base metal and the overlay weld in the cladding layer.
Forming and machining welds: Deep-drawn or roll-formed Q690D clad components may require weld repair or stress-relief welding, requiring expertise in multi-pass welding of high-strength steels.
Quality verification welding: Test welds on sample clad plates verify that the welding process does not degrade the explosion weld bond, leveraging knowledge of thermal effects on the Q690D substrate.

8. Contribution to Qualification Building and Customer Value

8.1 Qualification Building

This technical study directly contributes to the company's qualification portfolio in the following ways:

WPS Library Expansion: Qualified GMAW procedures for Q690D add to the company's Welding Procedure Specification library, enabling rapid deployment on customer projects without the delay of new qualification testing.
Welder Qualification: Understanding of Q690D welding metallurgy supports welder training programs, ensuring that operators can produce acceptable joints on first attempt, reducing rework costs.
Code Compliance: Qualified procedures enable the company to offer products compliant with ASME Section IX, NB/T 47014, and ISO 15614, expanding the addressable market to pressure vessel and piping applications.
Material Qualification Coverage: Q690D qualification extends the company's material coverage to ultra-high-strength steels, complementing existing qualifications for Q345, Q420, Q460, and Q550 grades.

8.2 Product Delivery Enhancement

Faster project execution: Pre-qualified procedures eliminate the need for trial welding and testing on each project, reducing lead times by 2–4 weeks per project.
Higher first-pass quality: Knowledge of optimal parameters and sequencing reduces NDT failure rates, minimizing rework and associated cost overruns.
Broader material compatibility: Customers specifying Q690D or equivalent grades (S690QL, HR70) can be served without qualification gaps.
Integrated solutions: The company can offer complete packages — from explosion-welded Q690D clad plate through structural welding to final assembly — under a single qualification umbrella.

8.3 Customer Value Proposition

The technical competence demonstrated through Q690D multi-pass GMAW qualification provides tangible customer value:

Risk mitigation: Customers in critical industries (oil & gas, offshore, nuclear) gain confidence that weld joints in high-strength materials meet rigorous code requirements.
Cost efficiency: Elimination of project-specific qualification testing reduces overall project cost by 15–30% in welding-related activities.
Schedule reliability: Pre-qualified procedures enable accurate project scheduling with reduced uncertainty around welding qualification timelines.
Technical credibility: Demonstrated expertise in ultra-high-strength steel welding positions the company as a technically competent partner for demanding applications.
Full-lifecycle support: The company can provide not only clad materials but also welding procedure support, welder qualification, and NDT verification — a comprehensive service offering that competitors may not match.

9. Summary and Recommendations

The study of multi-pass GMAW welding of Q690D steel represents a high-value technical investment that strengthens the company's qualification base, expands its product scope, and enhances its competitive position in the high-strength steel cladding and welding market. Key recommendations for operational implementation include:

Establish a formal WPS qualification program covering Q690D GMAW for thickness ranges of 6–100 mm, with full mechanical testing per GB/T 9858 and ASME Section IX.
Develop a welder training curriculum specific to high-strength steel welding, incorporating the metallurgical principles and parameter controls documented in this study.
Integrate Q690D welding qualification into the company's quality management system (ISO 9001/ISO 3834) as a traceable qualification asset.
Extend the qualification to cover welding of Q690D clad plates (explosion-welded and hydraulic explosive bonded) to demonstrate that welding does not compromise the bonded interface.
Conduct periodic re-qualification testing to maintain current qualification status and incorporate lessons learned from field performance data.

By systematically building and maintaining this qualification asset, the company ensures long-term competitiveness in markets demanding high-strength, high-performance cladded and welded components for critical infrastructure applications.