Microstructure and Performance of Weld Overlay Deposits on Q345E Steel Substrate

Literature Overview and Background

This study investigates the microstructural evolution and mechanical performance of weld overlay (cladding) layers deposited on Q345E structural steel, a widely used low-carbon low-alloy steel in pressure vessel fabrication and heavy equipment manufacturing. Q345E, characterized by a yield strength of at least 345 MPa and good low-temperature toughness (the "E" suffix denoting -40 °C impact resistance), serves as a common substrate in hydrogenation reactors, storage vessels, and structural components that require localized corrosion or wear resistance enhancement. The literature examines how the welding process parameters, filler metal composition, and interfacial heat-affected zone (HAZ) conditions influence the final cladding quality. Understanding this system is critical for engineers specifying weld-overlay clad pressure vessels per GB/T 150 and NB/T 47002.

Core Technical Points

The study identifies several key metallurgical phenomena governing the overlay on Q345E steel. First, the carbon equivalent (Ceq) of Q345E is approximately 0.42–0.45%, which places the material in a moderate preheat and interpass temperature regime. Insufficient preheat can lead to martensitic transformation in the HAZ, increasing the risk of cold cracking due to hydrogen diffusion into the hard and brittle microstructure. The literature recommends a minimum preheat of 80–120 °C for typical plate thicknesses up to 40 mm, with higher values for thicker sections or high-restraint joints.

Second, the dilution between the Q345E base metal and the overlay alloy is a decisive factor. When depositing stainless steel or nickel-based cladding, the first pass inevitably experiences significant base metal dilution, often exceeding 40% by weight. This dilution can compromise the corrosion resistance of the resulting deposit. The study highlights that a transition layer or "buffer pass" using a filler with intermediate composition (such as a 309L-type stainless steel before applying 316L cladding) effectively reduces the dilution effect on the functional overlay. This practice is consistent with ASME IX welding procedure requirements and industry best practices for bimetallic joints.

Microstructural Analysis

The study employs metallographic examination to map the dilution gradient across the overlay. It is noted that achieving a uniform microstructure throughout the cladding requires careful control of the number of passes, travel speed, and arc length. A typical submerged arc welding (SAW) overlay on Q345E with a 316L filler requires at least 3–4 passes to achieve less than 10% base metal dilution in the top layer.

Process Parameters and Engineering Practice

The literature discusses several welding processes applicable to overlaying on Q345E steel, including SAW, GMAW, and flux-cored arc welding (FCAW). Each process offers distinct advantages: SAW provides high deposition rates and excellent weld quality with minimal spatter, making it ideal for thick cladding layers on large pressure vessel shells. GMAW offers superior flexibility and portability for field repair work. FCAW combines high productivity with good all-position capability.

For engineering practice in pressure vessel fabrication, the following process window is recommended based on the study findings:

The study emphasizes that the interpass temperature should not exceed 250 °C to avoid grain coarsening and excessive dilution. Post-weld heat treatment (PWHT) at 580–620 °C for 2 hours per 25 mm of thickness is recommended when the overlay contains carbon or alloying elements that promote martensitic formation, as this relieves residual stresses and improves ductility of the dilution zone.

Common Defects and Countermeasures

The literature catalogs several defects observed in weld overlay on Q345E steel and proposes systematic countermeasures. Hydrogen-induced cracking (HIC) in the HAZ is the most critical defect, typically occurring within 24 hours of welding. The countermeasure involves strict control of hydrogen content in the filler metal (using low-hydrogen fluxes with moisture content below 0.1%), adequate preheat, and post-weld baking at 200–250 °C for hydrogen diffusion. Lack of bond between the overlay and base metal is another concern, often caused by inadequate cleaning of the base metal surface prior to welding or excessive travel speed resulting in incomplete fusion. The study recommends thorough grinding and wire-brushing of the substrate to a bright metallic finish, followed by immediate welding without prolonged exposure to the atmosphere.

Study Insights and Reflections

Reflecting on this literature, I am particularly struck by the interplay between metallurgical compatibility and process control in achieving a reliable weld overlay on Q345E steel. The study underscores that the HAZ is often the weakest link in the clad system, not the overlay itself. In my own engineering experience with hydrogenation reactor fabrication, we have encountered cases where the overlay layer passed all corrosion and hardness tests, yet the joint failed during hydrostatic testing due to HAZ cracking. This reinforces the importance of treating the base metal-overlay interface with the same rigor as the functional layer.

Furthermore, the study's emphasis on dilution management resonates with practical experience in specifying multi-pass overlay sequences. The concept of using a transition layer is not merely an academic recommendation but a practical necessity when the composition mismatch between substrate and overlay is significant. Engineers must carefully evaluate the dilution rate for each specific application, considering plate thickness, welding process, and required overlay thickness, to design an appropriate pass sequence that ensures both metallurgical integrity and functional performance.