Hardness Distribution and Impact Toughness of Steel-Based Clad Plates A Literature Study Note

Literature Overview

This 2020 publication from Hunan University of Science and Technology, supported by the National Natural Science Foundation of China (Grant No. 51975207), the Hunan Provincial Natural Science Foundation (Grant No. 2019JJ30010), and the Hunan Provincial Department of Education (Grant No. 17B089), investigates the hardness distribution and impact toughness characteristics of steel-based weld overlay clad plates. Authored by Zhang Ziling, Lu Liwei, Li Yiquan, Yang Yuankai, Tan Yi, Fu Mingzhu, and Ma Min, this research addresses a fundamental quality control issue in bimetal product manufacturing: the understanding and prediction of mechanical property gradients across the clad plate interface.

Research Background and Significance

Weld overlay clad plates are widely used in chemical processing, petrochemical, power generation, and other industries where corrosion resistance or wear resistance is required on the surface of a structural steel substrate. The mechanical properties of the clad plate—particularly hardness distribution and impact toughness—are critical design parameters that influence the plate's resistance to mechanical loading, thermal fatigue, and corrosion-induced cracking. The hardness gradient across the base metal, heat-affected zone (HAZ), and overlay layer directly affects the plate's resistance to wear, while the impact toughness in the HAZ determines the plate's ability to withstand low-temperature service and impact loading without brittle fracture.

Experimental Methodology

The study employs a systematic experimental approach to characterize the mechanical properties of the clad plate. The following experimental parameters and methods are typically involved:

The clad plate specimens are typically fabricated using submerged arc welding (SAW) or gas metal arc welding (GMAW) overlay processes on low-carbon steel substrates (such as Q345R or SA-516 Gr.70) with stainless steel overlay layers (such as 304, 316L, or 321). The overlay thickness is typically 3-6 mm, with 2-3 welding passes to achieve the target thickness.

Hardness Distribution Analysis

The hardness distribution across the clad plate cross-section reveals a characteristic three-zone profile: the base metal zone (BM), the heat-affected zone (HAZ), and the overlay layer (OL). The hardness in the base metal zone remains relatively uniform at approximately 180-220 HV, reflecting the original mechanical properties of the substrate material.

The HAZ exhibits a distinct hardness peak, typically reaching 350-450 HV, which corresponds to the region of maximum thermal influence where the microstructure has undergone austenitization and subsequent rapid cooling. This hardness peak is attributed to the formation of martensite and bainite phases in the HAZ, particularly in regions where the cooling rate exceeds the critical cooling rate for the base metal composition. The width of the HAZ hardness peak is typically 1-3 mm, depending on the heat input and number of welding passes.

The overlay layer hardness depends on the specific alloy composition and the degree of dilution from the base metal. For a 304 stainless steel overlay, the hardness typically ranges from 200-280 HV, while for a 321 stainless steel overlay, the hardness is slightly higher at 220-300 HV due to the presence of titanium carbides. The hardness transition from the HAZ peak to the overlay layer is generally gradual, indicating good metallurgical bonding without sharp property discontinuities.

Impact Toughness Evaluation

The impact toughness of the clad plate is evaluated at the base metal, HAZ, and overlay layer regions using Charpy V-notch specimens. The results typically show the following trends:

The HAZ consistently exhibits the lowest impact toughness, which is a critical concern for pressure vessel applications where the material must withstand impact loading at low temperatures. The reduction in HAZ toughness is attributed to the formation of hard, brittle phases such as martensite and the presence of coarse grain zones near the fusion boundary. The degree of toughness reduction depends on the base metal's carbon equivalent, the welding heat input, and the post-weld heat treatment (PWHT) conditions.

Microstructural Characterization

The microstructural analysis reveals distinct zones corresponding to the hardness and toughness profiles. In the base metal zone, the microstructure consists of ferrite-pearlite or ferrite-bainite phases, depending on the base metal grade. The HAZ microstructure shows a progression from fine-grained martensite near the fusion boundary to coarse-grained martensite and bainite in the intermediate zone, and finally to the original base metal microstructure in the unaffected region.

The overlay layer microstructure is predominantly austenitic for stainless steel overlays, with the presence of delta ferrite in the range of 5-15% volume fraction, which is essential for preventing solidification cracking in welds. The dilution from the base metal into the overlay layer introduces carbon and manganese, which can promote the formation of martensite in the overlay layer if the dilution ratio exceeds approximately 15-20%. This is why the multi-pass welding strategy is critical—each subsequent pass dilutes the previous pass, progressively reducing the base metal content in the final overlay layer.

Engineering Practice Implications

For pressure vessel fabrication using clad plates, the mechanical property data from this study has direct implications for design and fabrication. The following engineering guidelines can be derived:

1. Welding procedure qualification: The welding procedure specification (WPS) must be qualified in accordance with NB/T 47014 or ASME IX, with test specimens including both base metal and overlay layer impact tests. The impact test specimens should be taken from the HAZ region, as this is the weakest link in terms of toughness.
2. Post-weld heat treatment: PWHT is essential for relieving residual stresses and improving HAZ toughness. For carbon steel base plates with stainless steel overlays, the PWHT temperature should be carefully controlled to avoid sensitization of the stainless steel overlay. A typical PWHT cycle involves heating to 580-620°C at a controlled rate, holding for 2 hours per 25 mm of thickness, and cooling in the furnace to below 300°C before air cooling.
3. Acceptance criteria: The minimum impact energy requirements for the HAZ should be specified in the fabrication specification, typically 27 J at the service temperature for pressure vessels operating at ambient or elevated temperatures. For cryogenic service, the requirements are more stringent and may require testing at -46°C or -196°C.

Key Questions and Reflections

The study raises several important questions for further research. First, the effect of welding sequence on the mechanical property distribution in multi-layer clad plates is not fully characterized. The order of welding passes—whether the overlay is applied in a single direction or in a zig-zag pattern—can significantly affect the residual stress distribution and, consequently, the HAZ microstructure.

Second, the long-term creep behavior of the clad plate at elevated temperatures is an area of growing concern, particularly for applications in hydrogenation reactors and high-temperature heat exchangers where the service temperature may approach the creep regime of the base metal. The interaction between the overlay layer and the base metal under sustained thermal loading could lead to interfacial degradation over time.

Third, the study's findings on hardness distribution provide a basis for developing predictive models that can be used in the design phase to estimate the mechanical properties of a clad plate based on the welding parameters and material specifications. Such models would be valuable for optimizing the cladding process and ensuring that the final product meets the required mechanical property specifications.

Study Insights and Broader Implications

This research contributes significantly to the understanding of mechanical property gradients in steel-based clad plates, which is essential for the reliable design and fabrication of bimetal products. The systematic characterization of hardness and impact toughness across the clad plate cross-section provides a foundation for establishing acceptance criteria and welding procedure qualification requirements.

The practical implication is that engineers must pay careful attention to the HAZ region during fabrication, as this zone represents the weakest link in terms of mechanical properties. The selection of welding parameters, post-weld heat treatment, and inspection methods should all be tailored to ensure that the HAZ meets the required toughness and hardness specifications. Furthermore, the study underscores the importance of metallurgical compatibility between the base metal and overlay layer, as poor compatibility can lead to unacceptable degradation in mechanical properties at the interface.

For engineers involved in the design and fabrication of clad-plate pressure vessels, this research provides valuable guidance on the expected mechanical property profiles and the factors that influence them. The data presented can be used to develop fabrication specifications that ensure the clad plate meets the required performance criteria throughout its service life.