Hardness Distribution and Impact Toughness Study of Steel-Based Cladding Composite Plates

Overview and Background

Steel-based cladding composite plates, where a corrosion-resistant or wear-resistant alloy layer is deposited on a structural steel substrate, are widely used in pressure vessels, heat exchangers, and storage tanks. The mechanical properties of these composite plates, particularly the hardness distribution and impact toughness across the cladding-substrate interface, are critical for ensuring structural integrity and service performance. The literature reviewed examines the hardness gradient and impact toughness characteristics of steel-based cladding composite plates fabricated by various welding processes, including submerged arc welding (SAW), gas metal arc welding (GMAW), and electroslag welding (ESW), with particular attention to the effects of welding parameters, heat treatment, and microstructure on these properties.

Hardness Distribution Analysis

The hardness distribution across the cross-section of a steel-based cladding composite plate typically exhibits three distinct regions: the cladding layer (high hardness), the heat-affected zone (HAZ) with a hardness gradient, and the base substrate (uniform hardness). The width of the HAZ and the steepness of the hardness gradient are influenced by the welding process, heat input, and number of passes.

Region	Typical Hardness (HV)	Width (mm)	Microstructure
Cladding Layer (316L)	200–250	3–8	Ferrite-austenite or austenite
Cladding Layer (Inconel 625)	350–450	2–5	γ + Laves phase
HAZ (Weld Side)	250–350	2–5	Fine-grained martensite or bainite
HAZ (Substrate Side)	180–250	1–3	Coarse-grained ferrite-pearlite
Base Substrate (Q345R)	180–220	Bulk	Ferrite-pearlite
Base Substrate (16MnR)	170–210	Bulk	Ferrite-pearlite

The hardness gradient in the HAZ is primarily caused by the variation in cooling rate and microstructural transformation. Near the cladding layer, the high heat input from the weld produces a coarse-grained HAZ with relatively low hardness, while further from the weld, the cooling rate increases and finer microstructures develop, resulting in higher hardness. This gradient can create stress concentrations under cyclic loading, particularly if the hardness difference exceeds a critical threshold (typically 100 HV or more).

Impact Toughness Characteristics

The impact toughness of steel-based cladding composite plates is evaluated using Charpy V-notch (CVN) impact tests at various temperatures, typically from −20°C to 20°C. The impact energy is measured at different positions across the plate thickness to characterize the toughness distribution. The cladding layer generally exhibits higher impact toughness than the base steel due to the ductile austenitic or nickel-based microstructure, while the HAZ may show reduced toughness due to the formation of brittle phases such as martensite or coarse carbides.

Test Position	Impact Energy at 20°C (J)	Impact Energy at −20°C (J)	Ductile-to-Brittle Transition (°C)
Cladding Layer (316L)	80–120	60–90	< −60
Cladding Layer (Inconel 625)	50–80	30–50	< −40
HAZ (Weld Side)	30–60	10–30	−20 to 0
HAZ (Substrate Side)	40–70	20–40	−10 to 10
Base Substrate (Q345R)	50–80	30–50	−20 to 0

The impact toughness of the HAZ is the most critical region for structural integrity, as it represents the weakest link in the composite plate under impact or low-temperature loading conditions. The ductile-to-brittle transition temperature (DBTT) of the HAZ should be well below the minimum service temperature of the pressure vessel to ensure adequate fracture resistance.

Heat Treatment Effects on Hardness and Toughness

Post-weld heat treatment (PWHT) is commonly applied to steel-based cladding composite plates to reduce residual stresses, refine the microstructure, and improve the mechanical properties. The most common PWHT conditions involve heating to 600–650°C for 2–4 hours followed by furnace cooling. However, the PWHT temperature must be carefully controlled to avoid sensitization of the cladding layer (particularly for stainless steel cladding, where temperatures above 450°C can cause chromium carbide precipitation at grain boundaries).

Heat Treatment Condition	Cladding Hardness (HV)	HAZ Hardness (HV)	Impact Energy at 20°C (J)	Notes
As-welded (no PWHT)	220–250	300–380	30–50	High residual stress
PWHT at 600°C/2h	210–240	250–320	50–80	Reduced stress, improved toughness
PWHT at 650°C/4h	200–230	230–300	60–90	Maximum toughness improvement
PWHT at 700°C/4h	200–230	220–280	55–80	Risk of sensitization for SS cladding
Solution treatment at 1050°C/1h	200–220	180–220	70–100	Homogenization, but high cost

Defect Analysis and Quality Control

The quality of steel-based cladding composite plates is assessed through a combination of non-destructive testing (NDT) and destructive testing methods. Common defects include lack of fusion at the cladding-substrate interface, porosity in the cladding layer, cracks in the HAZ, and delamination during forming operations.

Defect Type	Detection Method	Acceptance Criteria	Prevention Measures
Lack of fusion	RT / UT	< 5 mm equivalent, < 20% of plate thickness	Proper surface preparation, adequate heat input
Porosity	RT / UT	Single pore < 3 mm, cluster < 15 mm	Clean filler metal, proper shielding
Cracking	MT / PT / UT	No cracks permitted	Preheating, controlled cooling rate
Delamination	UT (tandem scan)	No delamination permitted	Proper bond strength testing, PWHT
Hardness exceedance	Hardness test	HAZ < 350 HV (per NB/T 47014)	PWHT, controlled heat input

Study Insights and Implications

The hardness distribution and impact toughness of steel-based cladding composite plates are governed by the interaction between the welding process, microstructural evolution, and post-weld heat treatment. The HAZ represents the critical region for both hardness and toughness, and its properties must be carefully controlled to meet the requirements of pressure vessel design codes such as GB/T 150, ASME VIII Div.1, and NB/T 47014. Engineers should pay particular attention to the hardness gradient in the HAZ, as excessive hardness (above 350 HV) can lead to hydrogen-induced cracking and reduced impact toughness, particularly at low temperatures. The selection of welding process parameters, including heat input, interpass temperature, and number of passes, should be optimized to minimize the HAZ width and reduce the hardness gradient. Additionally, the impact toughness of the composite plate should be evaluated at the minimum service temperature to ensure adequate fracture resistance, and the ductile-to-brittle transition temperature of the HAZ should be well below the design temperature to provide a sufficient safety margin. The integration of these findings into welding procedure specifications and quality control plans is essential for ensuring the reliability and safety of steel-based cladding composite plates in pressure vessel applications.