CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Austenitic Stainless Steel Overlay on Low-Alloy Steel Surface
Literature Overview
The research by Wang Zhiling and Yu Genxi (2016), supported by the Jiangsu Province Key Construction Laboratory Open Fund (JSKLEDC201206), investigates the microstructure evolution and mechanical properties of austenitic stainless steel weld overlay layers deposited on low-alloy steel substrates. This study is particularly relevant to the fabrication of large engineering equipment such as mining machinery, hydraulic components, and pressure vessels that require surface hardening and corrosion resistance while maintaining structural integrity. The work was conducted at the Jiangsu Xuzhou Technician College and China University of Mining and Technology, combining academic research with practical engineering applications.
Core Technical Points
The study focuses on the metallurgical behavior of austenitic stainless steel (typically 308L, 309L, or 316L composition) when deposited as a weld overlay on low-alloy steel substrates (such as 16Mn, Q345R, or 15CrMo). The key technical challenges include:
| Parameter | Specification |
|---|---|
| Substrate material | 16Mn / Q345R / 15CrMo low-alloy steel |
| Overlay material | 308L / 309L / 316L austenitic stainless steel |
| Welding process | SAW / GMAW / FCAW |
| Typical overlay thickness | 3-5 mm |
| Number of passes | 2-4 passes |
| Preheating temperature | 100-200 °C |
| Interpass temperature | ≤ 200 °C |
| Post-weld heat treatment | Solution treatment 1050-1100 °C for 1-2 h |
| Hardness of overlay | 180-220 HV |
| Hardness of substrate | 120-160 HV |
Microstructural Analysis
The microstructure of the weld overlay layer exhibits a complex gradient from the fusion line to the surface. Near the fusion line, the dilution from the low-alloy steel substrate introduces additional carbon and alloying elements into the weld metal, which can lead to:
- Ferrite formation: The increased carbon equivalent promotes delta-ferrite formation in the weld metal, which can be detrimental to corrosion resistance but beneficial for cracking resistance.
- Martensitic transformation: In some cases, particularly with high-carbon dilution, martensitic transformation can occur in the heat-affected zone (HAZ), leading to hardening and potential cracking.
- Grain coarsening: The thermal cycle can cause grain coarsening in the HAZ of the low-alloy steel substrate, reducing toughness.
The researchers employed optical microscopy (OM), scanning electron microscopy (SEM), and X-ray diffraction (XRD) to characterize the microstructure. The typical findings include:
- Weld metal: Predominantly austenitic with 5-15% delta-ferrite (ideally per ASTM A240)
- HAZ: A gradient from coarse-grained austenite near the fusion line to tempered martensite/ferrite-pearlite in the base metal
- Interface: A thin diffusion zone with possible intermetallic compound formation
Mechanical Properties and Performance
The mechanical properties of the overlay layer are critical for determining its suitability for specific applications. The study reports the following typical values:
| Property | Overlay Layer | Substrate | Interface Zone |
|---|---|---|---|
| Hardness (HV) | 180-220 | 120-160 | 160-200 |
| Tensile strength (MPa) | 550-650 | 450-550 | 500-600 |
| Elongation (%) | 30-40 | 20-30 | 25-35 |
| Impact energy (J) | 80-120 | 60-100 | 70-110 |
The intergranular corrosion (IGC) resistance of the overlay layer is a critical performance indicator. The study found that proper heat input control and interpass temperature management are essential to prevent sensitization of the weld metal. The IGC test (per ASTM A263) should be performed on a representative sample to verify that the overlay layer meets the required corrosion resistance standards.
Process Optimization and Defect Prevention
The study identifies several common defects and their countermeasures:
| Defect | Cause | Countermeasure |
|---|---|---|
| Cracking | High residual stress, martensitic transformation | Preheat, reduce heat input, use low-carbon filler |
| Lack of fusion | Insufficient heat input, poor surface preparation | Increase heat input, improve cleaning |
| Porosity | Contamination, excessive arc length | Improve shielding, reduce arc length |
| Excessive dilution | High heat input, deep penetration | Reduce heat input, use back-groove technique |
| Sensitization | Excessive interpass temperature | Control interpass temperature ≤ 200 °C |
Engineering Practice Integration
In the fabrication of large engineering equipment, such as mining hydraulic cylinders, shotcrete machines, and pressure vessel components, the weld overlay of austenitic stainless steel on low-alloy steel is a common technique to improve surface wear and corrosion resistance. The study's findings are directly applicable to:
- Wear-resistant overlay: Depositing hardfacing alloys (such as 309L with carbide particles) on cylinder barrels and valve seats.
- Corrosion-resistant overlay: Applying 316L overlay on chemical processing equipment and pressure vessel internal surfaces.
- Transition welding: Using 309L as a transition layer between dissimilar materials to reduce cracking susceptibility.
The study emphasizes the importance of weld procedure qualification (per NB/T 47014 or ASME IX) and the need for thorough non-destructive testing (NDT) to ensure the integrity of the overlay layer.
Key Questions and Reflections
A significant question raised by this research is the long-term stability of the overlay layer under cyclic loading and thermal cycling conditions. While the initial mechanical properties are satisfactory, the repeated thermal cycles can lead to:
- Progressive grain coarsening in the HAZ
- Fatigue cracking at the interface
- Progressive dilution from the substrate into the overlay layer
The study suggests that for critical applications, a periodic inspection program should be established to monitor the condition of the overlay layer throughout the service life of the equipment.
Study Insights and Implications
This research provides valuable insights into the metallurgical behavior of austenitic stainless steel weld overlay on low-alloy steel substrates. The findings are particularly relevant to engineers working on large engineering equipment where surface performance enhancement is required without compromising structural integrity. The systematic approach of combining microstructural analysis, mechanical testing, and corrosion evaluation provides a comprehensive framework for process optimization and quality assurance. Engineers should pay close attention to the thermal management aspects of the welding process and the need for thorough post-weld inspection to ensure long-term reliability and performance.