Microstructure and Hardness Gradient Analysis of the Fusion Zone in Gradient Weld Overlay on 45 Steel Substrate

Overview of the Study

This paper by Zhu Chengjun and Li Sicheng from Henan Polytechnic University, published in the journal "Hot Working Technology" in 2015, addresses a fundamental yet often underappreciated challenge in cladding technology: the formation of a controlled hardness and microstructure gradient across the fusion zone when performing gradient weld overlay on 45 steel (a medium-carbon structural steel with approximately 0.45 wt% C). The research was supported by the President's Fund of Henan Polytechnic University (Project No. GYYJ20140021), reflecting its significance as a pedagogically valuable investigation into the metallurgical behavior at the interface between dissimilar materials during multi-layer overlay welding.

The core objective of gradient weld overlay is to achieve a smooth transition in mechanical properties between the base material and the final overlay layer, thereby mitigating the risk of cracking, delamination, and premature failure at the base-overlay interface. For 45 steel, which possesses a relatively high carbon content among structural steels, the carbon activity and hardenability of the base are significant concerns that directly influence dilution behavior and the resulting hardness profile in the fusion zone.

Core Technical Content and Interpretation

Dilution Behavior and Carbon Activity

The dilution ratio in the fusion zone is the single most critical parameter governing the microstructure and hardness of the overlay. For 45 steel, the carbon content of 0.42–0.50 wt% is substantially higher than that of most overlay consumables used for corrosion or wear resistance applications. During the first overlay pass, the molten weld pool entrains a significant quantity of base metal, resulting in a dilution ratio that can exceed 30–40% depending on the welding process, current density, and travel speed.

The authors systematically analyzed the relationship between dilution and the resulting hardness profile. The key finding is that the hardness in the fusion zone follows a non-linear gradient that is strongly correlated with the local carbon equivalent and the cooling rate. In regions where dilution exceeds 25%, the formation of martensite and bainite becomes increasingly probable, leading to localized hardness peaks that can reach 400–500 HV, far exceeding the base material hardness of approximately 200–250 HV.

Multi-Layer Gradient Strategy

The study advocates a multi-layer gradient approach in which the composition of each successive layer is progressively modified to reduce the dilution effect. A typical gradient sequence might proceed as follows:

This layered approach ensures that each subsequent layer dilutes the previous layer rather than the base directly, progressively reducing the carbon activity at the final interface.

Microstructural Observations

Metallographic examination of the fusion zone reveals a distinct band structure. In the base metal adjacent to the weld, a fine-grained martensite-ferrite structure is observed, resulting from the rapid cooling imposed by the weld pool. Moving toward the overlay side, the microstructure transitions through a mixed ferrite-austenite regime into the target overlay structure. The grain size in the fusion zone is typically refined compared to the base due to the high thermal gradient, which can be beneficial for toughness if controlled properly.

The hardness gradient, as measured by Vickers microhardness traversals, typically shows a peak hardness zone located 0.5–2.0 mm from the base-overlay interface. This "hardness island" represents the most vulnerable region for crack initiation under cyclic or impact loading. The authors emphasize that the width and magnitude of this hardness peak are directly controllable through welding parameter optimization and preheating.

Process Parameters and Engineering Considerations

Welding Process Selection

For gradient overlay on 45 steel, the choice of welding process is critical. Submerged arc welding (SAW) and electroslag welding (ESW) are preferred for thick sections due to their high deposition rates and deep penetration characteristics. However, the deep penetration of these processes can increase dilution in the first layer, necessitating careful parameter control.

Heat Input and Cooling Rate Management

The heat input per unit length (q = UI/v) is a primary lever for controlling the cooling rate at the fusion zone. For 45 steel, a heat input in the range of 15–25 kJ/mm is generally recommended for the transition layers to ensure adequate austenitization without excessive grain growth. Post-weld heat treatment (PWHT) at 550–650 °C for tempering is strongly advised to reduce residual stresses and soften the hard martensitic regions in the fusion zone.

Post-Weld Heat Treatment

The study recommends a two-stage PWHT: an initial tempering at 620–650 °C for 2 hours per 25 mm of thickness, followed by a slower cooling rate (not exceeding 100 °C/h) to prevent thermal shock cracking. This treatment effectively reduces the peak hardness in the fusion zone by 30–50%, transforming the brittle martensite into tempered martensite or sorbite, which provides a more ductile and crack-resistant microstructure.

Common Defects and Countermeasures

Based on the metallurgical analysis presented, the following defect modes are most relevant to gradient overlay on 45 steel:

• Cold cracking at the fusion zone: Caused by high carbon activity, rapid cooling, and hydrogen accumulation. Countermeasures include preheating, low-hydrogen consumables, and controlled cooling.
• Hot cracking in the overlay layer: Particularly in austenitic overlay layers due to solidification cracking susceptibility. Countermeasures include grain refiners (Ti, Zr) in the consumable and controlled heat input.
• Delamination at the base-overlay interface: Resulting from excessive hardness mismatch or residual tensile stress. Countermeasures include multi-layer gradient strategy and PWHT.
• Porosity in the transition layers: Due to incomplete deoxidation or gas entrapment. Countermeasures include flux selection (for SAW) and surface cleaning between layers.

Integration with Engineering Practice

In practical engineering applications, such as the fabrication of wear-resistant liners on 45 steel equipment, the gradient overlay approach described in this study directly addresses the challenge of achieving both the toughness of the base and the hardness or corrosion resistance of the overlay. For instance, in coal-handling equipment or mining machinery where 45 steel components are subject to severe abrasion, a gradient overlay of high-chromium cast iron or carbide-containing alloy can extend service life by 3–5 times compared to the base material alone.

The key engineering insight is that the hardness gradient must be designed intentionally rather than left to chance. A well-designed gradient not only prevents cracking but also provides a beneficial residual stress profile, with compressive stresses at the surface that improve fatigue life. The study's emphasis on systematic dilution control and multi-layer composition design provides a replicable methodology for engineers working on similar cladding applications.

Key Questions and Reflections

Several questions arise from this study that merit further investigation. First, the optimal number of gradient layers and their composition transitions for specific service conditions (e.g., high-temperature oxidation versus room-temperature wear) require further systematic study. Second, the interaction between the hardness gradient and residual stress distribution is complex, and the current analysis does not fully quantify the residual stress field. Third, the applicability of this approach to thicker sections (beyond 50 mm) with higher cooling rates remains to be validated.

From a standards perspective, the gradient overlay approach aligns with the philosophy of NB/T 47014 and ASME IX, which require qualified weld procedures that account for base metal dilution. However, the specific qualification of multi-layer gradient procedures is not explicitly addressed in most standards, representing a gap that engineers must bridge through careful procedure development and qualification testing.

Study Insights and Implications

The fundamental contribution of this work lies in its systematic quantification of the dilution-hardness-microstructure relationship in gradient overlay on a high-carbon substrate. The methodology of progressive dilution reduction through layer composition design is a powerful concept that can be generalized to other base-overlay combinations, including low-alloy steels, stainless steels, and even nickel-based substrates.

For practicing engineers, the practical takeaway is clear: when performing overlay welding on high-carbon steels such as 45 steel, always design the first transition layer with a composition that is intermediate between the base and the target overlay, rather than attempting to deposit the target material directly. This simple but critical principle can prevent costly failures and rework. The study also reinforces the importance of post-weld heat treatment as an indispensable step in ensuring the long-term reliability of gradient overlays.

In conclusion, this research provides a valuable metallurgical foundation for the design of gradient weld overlay systems on medium-carbon steels, and its principles remain highly relevant to contemporary cladding engineering practice where the demands for performance, reliability, and cost-effectiveness continue to intensify.