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

Literature Overview

The study examined the welding behavior of DH36-Z35 steel—a high-strength low-alloy structural steel with exceptional low-temperature toughness—when the root weld is performed without back grinding. This approach is particularly relevant for thick-section weldments where conventional back-chipping introduces residual stress, distortion, and productivity losses. The research investigated how the as-welded root pass microstructure, hydrogen distribution, and mechanical properties evolve through the full weld build-up sequence, offering practical guidance for engineers working on heavy plate fabrication in marine, offshore, and pressure vessel applications.

Core Technical Points

Material Characteristics of DH36-Z35

DH36-Z35 is a carbon equivalent-limited steel designed for sub-zero service conditions. The Z35 designation indicates a minimum Charpy V-notch impact energy of 35 J at −60 °C, achieved through strict control of sulfur, phosphorus, and carbon equivalents. The base metal typically contains 0.12–0.18% C, 1.0–1.6% Mn, and trace amounts of Nb, V, and Ti for grain refinement. The low carbon equivalent (CE ≤ 0.42) provides good weldability, yet the high strength (yield ≥ 355 MPa) combined with the demanding toughness requirement creates a challenging window for weld procedure qualification.

Root Weld Without Back Grinding

The fundamental premise of this technique is that the first (root) pass is deposited from one side only, without access to the back of the joint for chipping, machining, or inspection. This eliminates a significant source of residual stress and avoids the risk of undercut or incomplete penetration at the back surface. However, it demands precise control of the root geometry, heat input, and interpass temperature to ensure full penetration and avoid lack of fusion or excessive solidification cracking.

Microstructural Evolution

The root weld, deposited without back grinding, forms a columnar dendritic microstructure with a high aspect ratio due to the steep thermal gradient in the first pass. As subsequent passes are added, the thermal gradient decreases, and the microstructure transitions to a more equiaxed grain structure. The HAZ adjacent to the root pass exhibits a fine martensitic-ferritic microstructure due to the high cooling rate, while deeper passes show a coarser ferrite-pearlite structure. The key finding is that the root pass HAZ maintains adequate Charpy impact energy (≥ 35 J at −60 °C) when the heat input is kept below 1.5 kJ/mm and the interpass temperature is controlled.

Process Analysis and Defect Prevention

Common Defects and Countermeasures

The most critical defect in no-back-grinding root welds is incomplete penetration, which can occur if the root gap is too narrow or the arc force is insufficient. Other defects include root concavity (excessive penetration), slag inclusion at the root interface, and hydrogen-induced cold cracking in the HAZ.

Heat Input Management

The study demonstrated that a cumulative heat input exceeding 25 kJ/mm leads to a significant drop in impact toughness due to the formation of upper bainite and retained austenite in the HAZ. For thick plates (t > 50 mm), multi-layer multi-pass welding with alternating sequences is recommended to minimize peak temperatures and reduce the number of high-temperature cycles in the HAZ. The welding sequence should follow a skip-weld pattern to distribute heat symmetrically and minimize angular distortion.

Engineering Practice Implications

This technique is particularly valuable in applications where access to the back of the joint is limited, such as in thick-walled pressure vessels, ship hull structures, and offshore platforms. The elimination of back grinding reduces fabrication time by 15–25%, which is economically significant for large-scale projects. However, the technique requires rigorous procedure qualification under NB/T 47014 or ASME IX, with specific attention to the root pass parameters and the acceptance criteria for internal defects.

From a quality assurance perspective, the absence of back grinding means that root weld defects cannot be visually inspected from the back side. Therefore, 100% ultrasonic testing (UT) or phased array ultrasonic testing (PAUT) is mandatory for the root pass, with acceptance criteria aligned to JB/T 4730 or ASME V. The study confirmed that when the root pass is properly controlled, the overall weld quality meets or exceeds the requirements of GB/T 150 for pressure vessel fabrication.

Study Insights and Reflections

The most striking insight from this literature is that the root weld, often considered the most critical pass, can be successfully executed without back access when the process parameters are precisely controlled. This challenges the conventional wisdom that back grinding is essential for ensuring root weld quality. The key lies in the synergy between root geometry preparation, heat input control, and interpass temperature management.

Another important observation is the sensitivity of DH36-Z35 to hydrogen-induced cracking. The low carbon equivalent provides good weldability, but the high strength and demanding toughness requirements leave little margin for error. The study emphasized the importance of using low-hydrogen consumables and maintaining strict control of electrode storage and preheating conditions.

For future work, the integration of real-time welding monitoring systems—such as arc voltage and current tracking, and thermal imaging—could further enhance the reliability of no-back-grinding techniques. Additionally, the application of this technique to even higher strength grades (e.g., DH40-Z35) and thicker sections warrants further investigation. The literature provides a solid foundation for engineers seeking to optimize thick-plate welding procedures in demanding low-temperature service environments.