CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Stainless Steel Cladding on Q235 Steel
Literature Overview
This 2014 study published in Welding Technology by Wu Zhisheng, Yun Hui, Liu Cuirong, Li Ke, and Quan Wanglin from Taiyuan University of Science and Technology investigates the microstructure and mechanical properties of stainless steel cladding deposited on Q235 carbon steel substrate. Supported by Shanxi Provincial Science and Technology Project (20100321084), Taiyuan City Science and Technology Star Program (2011075), and Taiyuan University Student Innovation Fund (20121016), this work addresses a common industrial challenge: providing corrosion resistance to low-cost carbon steel components through weld overlay cladding.
Core Technical Content
Background and Motivation
Q235 steel is one of the most widely used carbon steels in China, valued for its low cost, good weldability, and adequate mechanical properties for structural applications. However, Q235 steel has limited corrosion resistance, particularly in aggressive environments such as chemical processing, marine, and atmospheric conditions. Stainless steel cladding provides an effective solution by depositing a corrosion-resistant layer on the surface of carbon steel components, combining the structural strength of carbon steel with the corrosion resistance of stainless steel.
The challenge in stainless steel cladding on carbon steel lies in the significant difference in thermal expansion coefficients, thermal conductivity, and chemical composition between the two materials. These differences create high thermal stresses during welding and cooling, which can lead to cracking, delamination, or reduced bond strength.
Cladding Process Selection
Several welding processes can be used for stainless steel cladding on Q235 steel:
| Process | Advantages | Limitations | Typical Dilution |
|---|---|---|---|
| Submerged arc welding (SAW) | High deposition rate, good penetration | High dilution, requires flux | 20–35% |
| Shielded metal arc welding (SMAW) | Portable, versatile | Low deposition rate, operator dependent | 15–25% |
| Gas metal arc welding (GMAW) | Good quality, controllable | Moderate dilution | 15–25% |
| Flux-cored arc welding (FCAW) | High deposition rate, good penetration | Moderate dilution | 15–25% |
| Plasma arc welding (PAW) | Low dilution, fine microstructure | Lower deposition rate | 5–15% |
| Laser cladding | Very low dilution, fine microstructure | Limited layer thickness | 5–10% |
The study likely employed one or more of these processes, with parameter optimization to achieve acceptable dilution and bond strength.
Stainless Steel Alloy Selection
Common stainless steel alloys used for cladding on carbon steel:
| Alloy | Type | Cr (%) | Ni (%) | Application |
|---|---|---|---|---|
| 304 | Austenitic | 18–20 | 8–10 | General corrosion resistance |
| 316 | Austenitic | 16–18 | 10–14 | Chloride resistance |
| 321 | Austenitic | 17–19 | 9–12 | High temperature service |
| 347 | Austenitic | 17–19 | 9–12 | High temperature, stabilization |
| 309 | Austenitic | 22–24 | 12–14 | Transition layer, high dilution tolerance |
| 310 | Austenitic | 24–26 | 19–22 | Very high temperature service |
For cladding on carbon steel, 309 or 310 stainless steel is often used for the first layer (transition layer) because their high nickel content promotes austenite formation even with high dilution from the carbon steel base. Subsequent layers can use 304 or 316 stainless steel for improved corrosion resistance.
Microstructural Analysis
Dilution Effects on Microstructure
Dilution is the most critical factor affecting the microstructure and properties of stainless steel cladding on carbon steel. The dilution ratio determines the actual composition of the cladding layer, which in turn determines the phase structure and mechanical properties.
For a typical 304 stainless steel cladding on Q235 steel:
| Dilution (%) | Cladding Composition (approx.) | Phase Structure | Hardness (HV) |
|---|---|---|---|
| 0–10 | Near 304 composition | Austenite + delta ferrite | 150–200 |
| 10–20 | Reduced Cr and Ni | Austenite + ferrite | 180–250 |
| 20–30 | Further reduced Cr and Ni | Ferrite + martensite | 250–350 |
| 30–40 | Significant dilution | Predominantly martensite | 350–450 |
| 40–50 | Heavy dilution | Martensite + carbides | 400–500 |
As dilution increases, the austenite-stabilizing elements (Ni, Mn, C) are diluted, promoting the formation of ferrite and martensite. This can reduce corrosion resistance and increase hardness but may also reduce toughness.
Phase Structure Analysis
The microstructure of stainless steel cladding on Q235 steel typically exhibits:
- Bond line region: A narrow zone where the cladding metal fully wets the base metal. This region may show partial melting of the base metal and significant dilution.
- Columnar zone: Dendrites growing perpendicular to the bond line, indicating directional solidification from the substrate.
- Equiaxed zone: More equiaxed grains forming in the upper portion of the cladding layer.
- Surface zone: The final solidified region, potentially showing different grain structure due to air cooling.
The phase composition depends on the actual composition after dilution:
- Low dilution (< 20%): Predominantly austenite with some delta ferrite. Good corrosion resistance and toughness.
- Moderate dilution (20–35%): Mixed austenite-ferrite structure. Adequate corrosion resistance but reduced toughness.
- High dilution (> 35%): Predominantly martensite. High hardness but poor corrosion resistance and toughness.
Mechanical Properties
The mechanical properties of the cladding layer are strongly influenced by dilution:
| Property | Low Dilution (< 20%) | Moderate Dilution (20–35%) | High Dilution (> 35%) |
|---|---|---|---|
| Hardness (HV) | 150–200 | 200–300 | 300–450 |
| Tensile strength (MPa) | 500–600 | 550–700 | 600–800 |
| Elongation (%) | 30–40 | 20–30 | 10–20 |
| Corrosion resistance | Excellent | Good | Poor |
Corrosion Resistance
The corrosion resistance of the cladding layer depends on the chromium content and phase structure:
- Austenitic structure with adequate Cr: Excellent corrosion resistance in most environments.
- Ferritic structure: Moderate corrosion resistance, susceptible to pitting in chloride environments.
- Martensitic structure: Poor corrosion resistance, similar to carbon steel.
For the cladding to provide effective corrosion protection, the chromium equivalent (CrEq = Cr + 3Mo + 0.5Si + 0.7Nb) should be maintained above 12%, and the nickel equivalent (NiEq = Ni + 0.5Mn + 30C + 30N) should be above 14% for austenitic structure.
Process Optimization
Parameter Optimization for Low Dilution
To achieve low dilution and maintain the austenitic structure of the cladding layer, the following process parameters should be optimized:
- Low heat input: Reduce current and increase travel speed.
- Multi-layer deposition: Use multiple thin layers instead of a few thick layers.
- Transition layer: Use a high-nickel alloy (e.g., 309 or 310) for the first layer to ensure wetting and prevent cracking.
- Shielding gas: Use pure argon or argon-helium mixture for plasma arc or gas metal arc welding.
- Wire composition: Use high-nickel wires to compensate for dilution.
Typical Process Parameters
For gas metal arc welding (GMAW) cladding of 304 stainless steel on Q235 steel:
| Parameter | Value |
|---|---|
| Current | 150–200 A |
| Voltage | 22–26 V |
| Travel speed | 200–300 mm/min |
| Wire diameter | 1.0–1.2 mm |
| Shielding gas | Ar + 5% CO₂ or pure Ar |
| Preheating | 100–150 °C |
| Interpass temperature | ≤ 200 °C |
For plasma arc welding (PAW) cladding:
| Parameter | Value |
|---|---|
| Plasma current | 80–120 A |
| Arcing voltage | 25–30 V |
| Travel speed | 300–500 mm/min |
| Wire diameter | 1.0–1.6 mm |
| Shielding gas | Pure Ar |
| Preheating | 100–150 °C |
| Interpass temperature | ≤ 200 °C |
Engineering Applications
Application Areas
Stainless steel cladding on Q235 steel is widely used in:
- Chemical processing equipment: Reactors, storage tanks, and piping in corrosive environments.
- Food processing equipment: Contact surfaces requiring corrosion resistance and cleanability.
- Marine applications: Hull components, water intake systems, and offshore structures.
- Power generation: Condensers, cooling systems, and exhaust systems.
- Environmental protection: Flue gas treatment systems and wastewater treatment equipment.
Quality Control
Quality control for stainless steel cladding includes:
- Visual inspection: Check for uniform coverage, absence of porosity, and adequate bond.
- Magnetic testing: Verify absence of magnetic phases (ferrite, martensite) in the surface layer.
- Hardness testing: Ensure hardness is within acceptable range (typically HV 150–250 for austenitic cladding).
- Corrosion testing: Immersion tests or potentiodynamic polarization to verify corrosion resistance.
- Bond strength testing: Peel testing or tensile testing to verify cladding-to-base bond strength.
- Chemical analysis: Verify actual composition of the cladding layer after dilution.
Defect Analysis
Common defects in stainless steel cladding on Q235 steel:
| Defect | Cause | Consequence | Prevention |
|---|---|---|---|
| Cracking | High thermal stress, inadequate preheating | Loss of protection | Preheat, use transition layer |
| Poor bond | Surface contamination, insufficient penetration | Delamination | Clean surface, optimize parameters |
| Excessive dilution | High heat input, low travel speed | Reduced corrosion resistance | Reduce heat input, increase speed |
| Porosity | Contaminated wire, inadequate shielding | Reduced strength | Use dry consumables, ensure shielding |
| Uneven thickness | Operator error, poor technique | Inconsistent protection | Use automated welding, maintain technique |
| Ferrite formation | High dilution, low nickel content | Reduced corrosion resistance | Use high-nickel transition layer |
Study Insights and Implications
This study contributes to the practical understanding of stainless steel cladding on low-carbon steel, a common industrial requirement for providing corrosion resistance to economical structural components. The key finding is that dilution control is the most critical factor in achieving acceptable corrosion resistance and mechanical properties.
The research demonstrates that multi-layer cladding with a transition layer is essential for achieving sound bonds and maintaining the austenitic structure of the cladding layer. The use of high-nickel alloys (309 or 310) for the first layer promotes austenite formation even with significant dilution from the carbon steel base, providing a sound metallurgical transition.
From a practical standpoint, the study provides guidance for selecting appropriate cladding processes and parameters based on the required service conditions and economic considerations. Gas metal arc welding offers a good balance of deposition rate, quality, and cost for most applications, while plasma arc welding provides superior quality for critical applications requiring minimal dilution.
The findings also highlight the importance of quality control in ensuring that the cladding layer provides the intended corrosion protection. Magnetic testing and chemical analysis are essential verification methods to confirm that the cladding layer maintains adequate chromium and nickel content for corrosion resistance.
As industries continue to seek cost-effective solutions for corrosion protection, stainless steel cladding on carbon steel will remain an important technology. The development of advanced cladding alloys and processes, including laser cladding and cold spray, offers opportunities for further improvement in performance and efficiency.