Microstructure and Mechanical Properties of Multi-Pass GMAW Joints in Q690D Steel

Literature Overview and Research Context

Q690D is a high-strength low-alloy (HSLA) steel with a minimum yield strength of 690 MPa, widely used in heavy-load structural applications including offshore platforms, bridge construction, and heavy industrial equipment. The "D" designation indicates low-temperature toughness qualification down to -20°C. Multi-pass Gas Metal Arc Welding (GMAW) is the predominant welding process for thick-section Q690D joints, and understanding the microstructural evolution and mechanical property distribution throughout the multi-pass weld is critical for ensuring structural integrity.

This study systematically examines the microstructure, hardness distribution, tensile strength, and impact toughness of multi-pass GMAW joints in Q690D steel, addressing the interpass temperature effects, heat input management, and post-weld heat treatment (PWHT) requirements that are essential for achieving acceptable joint performance.

Core Technical Content

Welding Process Parameters

The study typically employs solid wire GMAW with ER80S-D2 or equivalent filler metal to match the high strength of the Q690D base metal. Typical welding parameters for multi-pass deposition include:

Parameter	Root Pass	Fill Passes	Cap Pass
Wire Diameter	1.2 mm	1.2-1.6 mm	1.2 mm
Current (A)	120-150	250-350	120-150
Voltage (V)	18-22	28-34	18-22
Travel Speed (mm/s)	3-5	8-12	3-5
Heat Input (kJ/mm)	0.8-1.2	1.5-3.0	0.8-1.2
Interpass Temperature	100-200°C	100-250°C	100-250°C

Microstructural Characteristics

The microstructure of the multi-pass GMAW joint exhibits significant variation between different regions:

Weld Metal: The weld metal typically consists of acicular ferrite (AF), ferrite-pearlite (F-P), and granular bainite (GB) phases. In the earlier deposited passes that undergo multiple thermal cycles from subsequent passes, the microstructure tends to be finer with higher acicular ferrite content due to the tempering effect. The last deposited pass typically shows coarser microstructure with more bainitic features due to slower cooling rates.

Heat-Affected Zone (HAZ): The HAZ in Q690D exhibits a gradient from the base metal to the fusion line, with distinct sub-zones:

Coarse grain HAZ (CGHAZ): 100-300°C below the melting point, characterized by coarse acicular ferrite or upper bainite
Fine grain HAZ (FGHAZ): 500-800°C peak temperature, showing fine acicular ferrite
Inter-critical HAZ: Partial austenitization with retained ferrite

Mechanical Property Distribution

The hardness profile across the multi-pass joint typically shows the highest values in the CGHAZ (350-420 HV) and weld metal (300-380 HV), with the base metal at approximately 250-290 HV. The tensile strength of the weld metal generally exceeds 690 MPa, meeting or exceeding the base metal requirement. Impact toughness (Charpy V-notch at -20°C) shows the most significant variation, with the CGHAZ being the weakest link, typically achieving 40-80 J depending on heat input and microstructure.

Interpass Temperature Effects and Heat Input Control

Critical Process Variables

The interpass temperature is one of the most critical parameters affecting multi-pass GMAW joint quality in Q690D. The study demonstrates that:

Interpass temperatures below 100°C risk hydrogen-induced cracking in the HAZ
Interpass temperatures above 250°C promote grain growth and reduce impact toughness
The optimal interpass temperature range for Q690D GMAW is 150-200°C

Heat input management follows a similar criticality. Excessive heat input (>4 kJ/mm) leads to coarse grain formation in the HAZ and reduced toughness, while insufficient heat input (<0.5 kJ/mm) increases the risk of cold cracking and incomplete fusion.

Multi-Pass Thermal Cycle Effects

An important finding from multi-pass welding studies is the tempering effect of subsequent passes on previously deposited weld metal. Earlier passes experience multiple reheat cycles, which can:

Temper martensitic structures to tempered martensite
Reduce hardness by 30-80 HV in previously deposited passes
Improve toughness through microstructural refinement
Create property gradients within the weld metal itself

This tempering effect is beneficial for overall joint toughness but complicates qualification testing, as the properties of a single test specimen may not represent the entire weld.

Engineering Practice Integration

Welding Procedure Qualification

For Q690D multi-pass GMAW qualification under NB/T 47014 or ASME IX, the following considerations are essential:

Test coupon thickness should cover the production thickness range
Heat input limits should be established based on toughness requirements
Interpass temperature limits should be specified and monitored
Post-weld heat treatment may be required for thicknesses above 25-30 mm or when toughness requirements are stringent

Defect Prevention Strategy

Using FMEA methodology, the primary failure modes for Q690D GMAW joints include:

Defect Type	Root Cause	Prevention Measure
Cold cracking	High hardness in CGHAZ, hydrogen presence	Preheat 150-200°C, low-H filler, post-weld bake
Excessive HAZ hardness	High heat input	Limit heat input, use low-alloy filler
Poor impact toughness	Coarse grain HAZ	Control interpass temp, optimize heat input
Incomplete fusion	Insufficient heat input	Increase heat input, improve technique
Porosity	Moisture in flux/wire	Dry flux, proper storage

Post-Weld Heat Treatment

PWHT is often required for Q690D welded structures, particularly for:

Thicknesses above 25 mm
High-stress applications
Requirements for specific toughness levels
Seismic applications

Typical PWHT parameters include: temperature of 580-620°C, hold time of 1 hour per 25 mm thickness (minimum 2 hours), and controlled cooling rate below 200°C.

Study Insights and Reflections

The multi-pass GMAW process for Q690D steel presents a complex interplay between thermal cycles, microstructural evolution, and mechanical properties that demands careful process control. The study reinforces the fundamental principle that in high-strength steel welding, the HAZ is invariably the weakest link, and process parameters must be optimized primarily to protect HAZ toughness rather than to maximize weld metal strength. For engineering practice, the key takeaway is that welding procedure qualification for Q690D must include multi-pass testing that accurately represents production conditions, with particular attention to interpass temperature control and heat input management. The tempering effect of subsequent passes, while beneficial, introduces variability that must be accounted for in qualification and acceptance criteria. Engineers should recognize that achieving the required -20°C toughness in Q690D joints is achievable but requires disciplined process control, proper preheat and interpass temperature management, and potentially PWHT for thicker sections.