Steel-Based Weld Overlay Composite Plate Hardness Distribution and Impact Toughness Study A Literature Study Note

Literature Overview and Background

This paper investigates the hardness distribution and impact toughness of steel-based weld overlay composite plates, which are fabricated by depositing a wear-resistant or corrosion-resistant overlay layer onto a carbon steel or low-alloy steel substrate. These composite plates are widely used in mining, construction, and material handling equipment where the combination of toughness and wear resistance is required. The study employs a systematic approach to characterize the hardness profile through the thickness of the composite plate and to evaluate the impact toughness of the overlay layer, the heat-affected zone (HAZ), and the base metal under different welding conditions.

Core Technical Content

The steel-based weld overlay composite plate typically consists of a carbon steel or low-alloy steel substrate (such as Q345 or 16Mn) with a multi-pass weld overlay of a hardfacing alloy (such as a carbide-containing alloy or a martensitic alloy). The overlay thickness is typically in the range of 3–10 mm, with the hardness of the overlay layer being significantly higher than that of the base metal.

The following table summarizes the typical hardness distribution through the thickness of the composite plate:

The hardness distribution is characterized by a steep gradient from the surface to the base metal, with the highest hardness at the surface and a gradual decrease with depth. This gradient is attributed to the varying degrees of dilution between the overlay alloy and the base metal, as well as the different cooling rates experienced by the different layers during welding.

The impact toughness is evaluated using Charpy V-notch tests at room temperature and at sub-zero temperatures (typically -20 °C and -40 °C). The results show that the overlay layer exhibits low impact toughness due to its high hardness and brittle microstructure, while the base metal exhibits high impact toughness due to its ductile ferrite-pearlite microstructure. The HAZ exhibits intermediate toughness, with the toughness decreasing as the hardness increases.

Hardness Distribution and Microstructure Analysis

The hardness distribution through the thickness of the composite plate is directly related to the microstructure, which is in turn determined by the welding parameters and the alloy composition of the overlay. The surface layer, which has the highest hardness, typically consists of a fine martensitic microstructure with a high volume fraction of hard carbide particles (such as Cr7C3, Cr3C2, or VC, depending on the overlay alloy). The fine martensite and the hard carbides contribute to the high hardness but also reduce the toughness.

The transition zone between the overlay and the base metal is the most critical region in terms of mechanical properties. This zone exhibits a mixed microstructure of martensite, ferrite, and pearlite, with a hardness that gradually decreases from the overlay hardness to the base metal hardness. The transition zone is also the most susceptible to cracking under cyclic loading or impact loading, as the high hardness gradient creates stress concentrations that can initiate cracks.

The study also discusses the effect of the number of overlay passes on the hardness distribution. More passes result in a thicker overlay layer with a more gradual hardness transition, which improves the overall toughness of the composite plate. However, too many passes can lead to excessive heat input, which can cause grain growth in the HAZ and reduce the toughness.

Impact Toughness and Fracture Behavior

The impact toughness of the composite plate is evaluated at different depths from the surface, revealing a clear correlation between the hardness and the toughness. The surface layer, with a hardness of 700–900 HV, exhibits an impact energy of less than 10 J, indicating a brittle fracture behavior. The transition zone, with a hardness of 400–550 HV, exhibits an impact energy of 20–40 J, indicating a mixed ductile-brittle fracture behavior. The base metal, with a hardness of 200–250 HV, exhibits an impact energy of 80–100 J, indicating a ductile fracture behavior.

The fracture surfaces are examined using scanning electron microscopy (SEM), revealing that the surface layer exhibits a cleavage fracture mode with river patterns and cleavage facets, while the base metal exhibits a ductile fracture mode with dimples and microvoids. The transition zone exhibits a mixed fracture mode with both cleavage facets and dimples, indicating a transition from brittle to ductile fracture behavior.

The study also discusses the effect of temperature on the impact toughness. At lower temperatures, the impact toughness decreases significantly, particularly in the overlay layer and the transition zone. This is because the martensitic microstructure in the overlay layer becomes more brittle at lower temperatures, and the retained austenite may transform to martensite during impact loading, further reducing the toughness.

Engineering Applications and Design Guidelines

The findings of this study have direct implications for the design and fabrication of steel-based weld overlay composite plates used in mining, construction, and material handling equipment. The key design considerations include the selection of an appropriate overlay alloy, the control of the welding parameters to achieve the desired hardness and toughness balance, and the optimization of the overlay thickness to minimize the transition zone.

The study provides design guidelines for the overlay thickness as a function of the service conditions. For applications involving high wear but low impact loading, a thicker overlay layer (5–10 mm) with a high hardness (800–900 HV) is recommended. For applications involving both wear and impact loading, a thinner overlay layer (3–5 mm) with a lower hardness (600–700 HV) is recommended, as this provides a better balance between wear resistance and impact toughness.

The study also discusses the importance of post-weld heat treatment (PWHT) in improving the impact toughness of the composite plate. A tempering treatment at 550–650 °C can reduce the hardness of the overlay layer by 100–200 HV while significantly improving the impact toughness. However, the tempering treatment must be carefully controlled to avoid excessive softening of the overlay layer, which would reduce the wear resistance.

Key Reflections and Study Insights

The most important insight from this literature is the recognition that the hardness distribution and the impact toughness of a steel-based weld overlay composite plate are intrinsically linked and must be considered as a coupled system. The pursuit of maximum hardness at the surface inevitably leads to a reduction in impact toughness, and the design must strike a balance between these two competing requirements based on the specific service conditions.

Another significant finding is the critical role of the transition zone in the overall performance of the composite plate. The transition zone, which exhibits a mixed microstructure and intermediate mechanical properties, is the most susceptible to cracking under cyclic or impact loading. The design must minimize the thickness of the transition zone and ensure that it is well-bonded to both the overlay and the base metal to prevent delamination.

The study also highlights the importance of the welding parameters in controlling the hardness distribution and the impact toughness. The heat input, the welding speed, and the interpass temperature all have a significant effect on the microstructure and the mechanical properties of the overlay and the HAZ. The welding parameters must be carefully optimized to achieve the desired balance between hardness and toughness.

In conclusion, this literature provides a comprehensive characterization of the hardness distribution and impact toughness of steel-based weld overlay composite plates, offering valuable design guidelines and engineering insights for the fabrication of wear-resistant and impact-resistant composite plates for mining, construction, and material handling applications.