CLADDING TECHNOLOGY SHANXI CO., LTDJoint Microstructure and Hardness Analysis of Stainless Steel Cladded on Q235 Steel
Literature Overview
This study examines the metallurgical characteristics of a dissimilar metal joint formed by welding overlay cladding of stainless steel onto Q235 carbon steel. Q235 steel is one of the most widely used carbon structural steels in China, specified under GB/T 700, and is extensively employed in pressure vessels, storage tanks, and structural components. When such carbon steel components require corrosion resistance in specific zones, weld overlay cladding with austenitic stainless steel (such as 304 or 316L) provides an economical solution. The study focuses on the microstructural evolution at the weld interface, the hardness distribution across the joint, and the factors governing bond quality.
Microstructural Analysis
The microstructure of the stainless steel cladding layer deposited on Q235 steel was analyzed using optical microscopy and SEM-EDS. The cladding layer exhibits a typical austenitic microstructure with some delta ferrite, while the Q235 base metal consists of ferrite and pearlite. The heat-affected zone (HAZ) on the carbon steel side shows a transition from the original ferrite-pearlite structure to a tempered martensite or bainite structure, depending on the cooling rate achieved during welding.
A critical observation is the formation of a diffusion zone at the interface between the cladding layer and the base metal. In multi-pass cladding operations, this diffusion zone can extend 50–150 micrometers into the base metal, characterized by a gradual transition in carbon and alloy element concentration. The presence of this diffusion zone is generally beneficial, as it provides a metallurgical bond rather than a purely mechanical one, but excessive interdiffusion can compromise the corrosion resistance of the cladding layer by diluting the chromium and nickel content near the interface.
The following table summarizes the microstructural characteristics observed in different zones of the joint:
| Zone | Microstructure | Approximate Hardness (HV) | Key Features |
|---|---|---|---|
| Cladding Layer (surface) | Austenite + delta ferrite | 180–220 | Fine grain, uniform composition |
| Cladding Layer (near interface) | Austenite + delta ferrite + carbides | 220–280 | Slight carbon enrichment from base metal |
| Diffusion Zone | Gradient austenite to martensite | 280–350 | Transition region, 50–150 μm wide |
| HAZ (carbon steel side) | Tempered martensite / bainite | 250–350 | Widmanstatten structure possible |
| Base Metal (Q235) | Ferrite + pearlite | 160–200 | Unaffected original structure |
Hardness Distribution and Gradient
The hardness profile across the cladding joint was measured using a Vickers hardness tester with 500 g load at 0.2 mm intervals from the cladding surface to the base metal. The results reveal a characteristic hardness gradient that is critical for evaluating the quality of the cladding operation.
The maximum hardness typically occurs in the diffusion zone or the near-interface region of the cladding layer, reaching values of 280–350 HV. This hardness peak is attributed to the combination of carbon enrichment from the base metal and the formation of fine carbides at the interface. The hardness decreases gradually toward both the cladding surface and the base metal, with the base metal hardness remaining at 160–200 HV.
A hardness ratio analysis between the cladding layer and the base metal is important for assessing the joint's resistance to mechanical damage. A ratio exceeding 2.5:1 may indicate a risk of cracking under cyclic loading, while a ratio below 1.5:1 suggests good compatibility. The study reports hardness ratios in the range of 1.2:1 to 1.8:1, indicating acceptable compatibility for most structural applications.
Bond Quality and Interface Integrity
The bond quality at the cladding interface is the most critical factor determining the service life of the cladded component. The study evaluated bond quality through several methods:
- Macroscopic examination: The full section of the cladding joint was examined for lack of fusion, porosity, and slag inclusions at the interface.
- Microscopic examination: High-magnification optical microscopy was used to identify microcracks, microvoids, and segregation at the bond line.
- Shear testing: The bond strength was measured according to NB/T 47014, with results ranging from 200 to 280 MPa.
- Chemical analysis: EDS line scanning was performed across the interface to verify the composition gradient and detect any anomalous segregation.
The study found that the bond strength was strongly influenced by the welding current and travel speed. Excessive welding current increased the dilution ratio, which can lead to the formation of brittle intermetallic phases at the interface. Conversely, excessively low current and high travel speed resulted in incomplete melting of the base metal surface, creating a weak bond zone with reduced fusion.
Engineering Practice Guidelines
Based on the findings of this study, the following engineering guidelines are recommended for stainless steel cladding on Q235 steel:
- Preheating to 100–150°C is recommended to reduce the cooling rate in the HAZ and minimize the risk of cracking.
- The welding procedure should be qualified under NB/T 47014 with attention to the essential variables of welding current, voltage, travel speed, and filler metal composition.
- Multi-pass cladding is preferred over single-pass to achieve adequate thickness while controlling dilution. The first pass should use lower current and higher travel speed to minimize base metal melting.
- Interpass temperature should be maintained below 250°C to avoid excessive grain growth in the cladding layer.
- Post-weld heat treatment is generally not required for austenitic stainless steel cladding but should be considered if the base metal thickness exceeds 20 mm.
Key Reflections
This study underscores the fundamental challenge of dissimilar metal cladding: achieving a metallurgical bond between materials with vastly different thermal expansion coefficients, melting points, and microstructural stability. The Q235 steel has a thermal expansion coefficient of approximately 12×10⁻⁶/K, while austenitic stainless steel has a coefficient of approximately 17×10⁻⁶/K. This difference creates thermal stresses during cooling and during subsequent thermal cycling in service, which must be managed through careful process design.
The hardness gradient analysis provides a practical tool for quality assessment in the field. Engineers can perform rapid hardness surveys across the cladding joint during inspection to identify potential weak zones without the need for destructive testing. This non-destructive approach to quality evaluation is particularly valuable for in-service inspection of cladded pressure vessels and storage tanks.
The study reinforces the principle that successful cladding of dissimilar metals requires a holistic approach that integrates metallurgical understanding, process optimization, and rigorous quality control. Engineers should not rely solely on the selection of a "universal" welding procedure but must tailor the process parameters to the specific combination of base metal, cladding material, and service conditions.