Microstructure and Properties of Weld Overlay Metal on Q345E Steel Substrate

Study Overview and Research Background

This study investigates the microstructural evolution and mechanical performance of weld overlay metal deposited on Q345E low-alloy structural steel substrates. Q345E is one of the most widely used low-alloy steels in pressure vessel and structural fabrication in China, characterized by a minimum yield strength of 345 MPa and excellent low-temperature toughness. The research addresses a fundamental question that every cladding engineer encounters: how does the dissimilar metal interface between a carbon-manganese substrate and a deposited overlay alloy influence the final properties of the cladding system? Understanding this interface is not merely an academic exercise; it directly determines whether a clad component will survive its intended service life under cyclic loading, thermal cycling, or corrosive environments.

The study employs multiple overlay techniques, including submerged arc welding (SAW) and gas metal arc welding (GMAW), to deposit various alloy compositions onto Q345E plates. The deposited alloys include austenitic stainless steels, martensitic stainless steels, and nickel-based superalloys, representing the three major categories encountered in industrial cladding practice. The research methodology follows a systematic approach: substrate preparation, overlay deposition under controlled parameters, heat treatment, and comprehensive characterization through metallography, X-ray diffraction, hardness mapping, and tensile testing.

Substrate Characterization and Dilution Analysis

The base metal characterization establishes the starting point for all subsequent analysis. Q345E steel typically exhibits a ferrite-pearlite microstructure with occasional bainitic features in thicker sections, with a hardness range of 130 to 180 HV. The chemical composition, particularly the carbon equivalent (Ceq), is critical for assessing weldability and hydrogen cracking susceptibility. Q345E has a Ceq of approximately 0.45 to 0.50%, placing it in a moderately sensitive category for cold cracking under certain welding conditions.

Dilution represents the single most important variable governing the final composition of the overlay layer. In the first pass of multi-layer cladding, dilution rates typically range from 25% to 50%, depending on the process, wire diameter, and deposition geometry. As the number of overlay passes increases, dilution drops significantly; by the third or fourth pass, dilution generally falls below 10%. The study demonstrates that dilution profoundly affects the phase composition of the overlay. For example, an austenitic overlay alloy deposited with 40% dilution may exhibit a significant ferrite fraction, while the same alloy with 8% dilution remains fully austenitic. This has direct implications for corrosion resistance, as ferrite phases in austenitic cladding can be preferentially attacked in chloride-containing environments.

Microstructural Evolution at the Interface

The weld interface between Q345E and the overlay metal is the most critical region from a metallurgical standpoint. Metallographic examination reveals a complex transition zone that typically includes a diffusion zone, a partially melted zone, and a heat-affected zone extending into the substrate. The diffusion zone, which may be only 50 to 200 micrometers thick, shows interdiffusion of carbon, chromium, and other alloying elements across the interface. This zone often exhibits a mixed microstructure with retained austenite, martensite, and carbide precipitation depending on the overlay alloy system.

In austenitic overlay systems, the interface microstructure is dominated by a delta-ferrite band formed due to the high cooling rates and the dilution effect. The delta-ferrite band can be continuous or discontinuous, and its morphology depends on the solidification rate and the chromium-to-nitrogen ratio of the overlay alloy. In martensitic overlay systems, the interface presents a more complex picture, with a tempered martensite layer adjacent to the substrate transitioning into a fine-grained martensite in the overlay. The hardness profile across this interface often shows a peak at the boundary, reaching values 30 to 50 HV higher than the surrounding regions, due to the formation of a fine-grained martensitic structure with high dislocation density.

The heat-affected zone in the Q345E substrate can extend 2 to 5 mm from the interface, depending on the heat input. In this zone, the original ferrite-pearlite microstructure transforms into a mixture of tempered martensite and bainite. The hardness in the HAZ typically increases by 30 to 60 HV compared to the base metal. This hardness increase, while beneficial for wear resistance, can reduce the ductility and toughness of the substrate in the vicinity of the cladding. The study emphasizes that the HAZ toughness must be evaluated by Charpy V-notch testing, particularly for components operating at sub-zero temperatures where Q345E's impact energy can drop significantly.

Mechanical Properties and Performance Evaluation

The mechanical properties of the weld overlay system are evaluated through hardness mapping, tensile testing of coupon specimens, and bend testing. Hardness profiles across the cladding show a characteristic gradient: high hardness near the interface, decreasing toward the surface of the overlay, and gradually returning to base metal values in the substrate. This gradient is a direct consequence of the varying dilution levels across the overlay thickness.

Tensile testing of the overlay/substrate system reveals that failure typically occurs in the base metal rather than in the overlay or at the interface, provided that the overlay alloy has adequate ductility. This is an important finding because it confirms that the bond strength of a properly executed overlay exceeds the strength of the weaker material in the system. However, the study also documents cases where excessive dilution led to a brittle martensitic interface, resulting in interfacial fracture during tensile testing. This highlights the importance of process control in maintaining acceptable dilution levels.

Bend testing according to relevant standards provides a qualitative assessment of interface integrity. Full-face bend tests with the overlay facing inward (testing the interface) and outward (testing the overlay surface) are both conducted. Acceptance criteria typically require no cracks exceeding 1 mm in length at the interface. The study reports that overlay systems with controlled dilution below 20% consistently pass bend testing, while systems with uncontrolled dilution above 30% frequently exhibit interfacial cracking.

Engineering Implications and Practical Recommendations

The findings of this study carry significant implications for cladding engineering practice. First, dilution control must be treated as a primary process parameter, not an afterthought. Process variables such as current, voltage, travel speed, and wire stick-out should be systematically optimized to achieve target dilution levels. Second, the heat input for each pass must be carefully managed to avoid excessive HAZ hardening in the Q345E substrate. For Q345E, a heat input range of 15 to 30 kJ/mm is generally recommended for overlay welding, with lower values preferred for thicker sections to minimize the HAZ width.

Third, post-weld heat treatment should be considered for martensitic overlay systems to temper the as-welded martensite and reduce residual stresses. A PWHT at 600 to 650 degrees Celsius for 1 hour per 25 mm of thickness is typical, but the temperature must be carefully controlled to avoid sensitization of any austenitic overlay layers present in the system. Fourth, the interface region warrants special attention during non-destructive testing. Ultrasonic testing with specific probe angles designed for interface detection, combined with magnetic particle testing for surface-breaking defects at the interface, provides the most reliable inspection coverage.

In summary, this study provides a comprehensive understanding of how the microstructure and properties of weld overlay metal on Q345E steel are governed by dilution, heat input, and alloy selection. The key takeaway for practicing engineers is that the quality of a cladding system is determined not by the properties of the overlay alloy alone, but by the interplay between the substrate, the overlay, and the interface that forms between them. Process discipline in controlling dilution and heat input is the single most effective lever for ensuring long-term service reliability of clad components.