CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of GH2132 Alloy Overlay Layer on Flat Plate
Literature Overview
GH2132 (equivalent to Inconel 625 / UNS N06625) is a nickel-chromium-molybdenum superalloy widely used in high-temperature, high-corrosion environments such as chemical processing, petrochemical cracking furnaces, and marine heat exchangers. The literature under review examines the microstructure evolution, mechanical properties, and bonding characteristics of GH2132 weld overlay layers deposited on carbon steel and low-alloy steel base plates. The study provides critical data on how deposition method, heat input, and interpass temperature influence the final quality of the overlay, which is essential for engineers designing cladding solutions for severe service conditions.
Core Technical Content
The overlay process investigated covers multiple techniques including submerged arc welding (SAW), gas metal arc welding (GMAW), and plasma transferred arc (PTA) cladding. The base material studied is typically Q345R or 16MnR carbon steel with a thickness range of 12–80 mm. The GH2132 overlay is deposited in multiple passes, with the first pass serving as a transition layer to minimize dilution from base metal elements.
Microstructural Characteristics
The as-deposited overlay layer exhibits a fully austenitic microstructure with a small fraction of delta ferrite (1–3 vol.%) depending on the heat input regime. The grain structure is columnar near the fusion boundary and equiaxed in the upper layers. The key alloying elements and their role in the overlay microstructure are summarized below:
| Element | Typical Content in GH2132 Overlay | Role in Microstructure |
|---|---|---|
| Ni | 57–62% | Stabilizes austenite, suppresses sigma phase |
| Cr | 20–23% | Provides pitting and intergranular corrosion resistance |
| Mo | 8.5–9.5% | Enhances resistance to reducing acid environments |
| Nb | 0.1–1.0% | Grain refinement, possible carbide precipitation |
| Fe | Balance | Contributes to dilution from base metal |
The presence of niobium carbides (NbC) and chromium carbides (Cr7C3) at grain boundaries is a critical observation. When dilution from the carbon steel base exceeds 25%, there is a significant risk of chromium carbide precipitation, which can deplete the matrix of chromium and create intergranular corrosion sensitization pathways.
Mechanical Properties
The overlay layer hardness typically ranges from 210 to 250 HV in the as-deposited condition. After solution treatment at 1050°C followed by water quenching, the hardness drops to approximately 185–210 HV with a more uniform microstructure. The tensile strength of the overlay layer exceeds 620 MPa, and elongation is typically 35–45%, reflecting the ductile nature of the fully austenitic matrix.
Bonding Interface Analysis
The fusion boundary between the GH2132 overlay and the carbon steel base is the most critical zone for service integrity. Metallographic examination reveals a dilution zone approximately 0.2–0.5 mm thick where the composition transitions from base steel to overlay alloy. In this zone, martensitic or bainitic microstructures may form due to the high carbon and manganese content from the base metal, creating a brittle interlayer susceptible to cracking under thermal cycling.
Process Parameter Analysis
The following table summarizes the recommended process windows for GH2132 overlay deposition on carbon steel:
| Parameter | SAW Overlay | GMAW Overlay | PTA Cladding |
|---|---|---|---|
| Current | 400–600 A | 150–250 A | 200–350 A |
| Voltage | 28–34 V | 22–28 V | 20–26 V |
| Travel speed | 150–250 mm/min | 300–500 mm/mm | 400–800 mm/min |
| Heat input | 1.5–3.5 kJ/mm | 0.8–1.5 kJ/mm | 0.5–1.2 kJ/mm |
| Interpass temperature | ≤250°C | ≤200°C | ≤150°C |
| Preheat temperature | 100–150°C | 50–100°C | Ambient–100°C |
The interpass temperature control is particularly critical. Exceeding 250°C between passes promotes grain coarsening and increases the risk of sensitization. The preheat temperature serves dual purposes: reducing thermal gradient to prevent cracking in the brittle dilution zone and ensuring adequate wetting of the base surface.
Engineering Practice Integration
In practice, GH2132 overlay is commonly applied to pressure vessel heads, pipe spools, and heat exchanger tubesheets where the corrosion environment demands nickel-based alloy resistance but the structural requirements necessitate a steel backing. A typical application is the cladding of hydrogenation reactor internals operating at 350–450°C in the presence of hydrogen sulfide and organic acids.
The dilution rate is the single most important quality indicator. For critical applications, the dilution should be controlled below 15% for the first pass and below 10% for subsequent passes. This is achieved through:
- Using a transition layer of matching composition between the base and the final overlay.
- Employing low-heat-input processes for the first pass to minimize penetration into the base metal.
- Applying post-weld solution treatment to homogenize the dilution zone.
Common Defects and Countermeasures
| Defect | Cause | Countermeasure |
|---|---|---|
| Intergranular cracking at fusion boundary | High dilution, rapid cooling | Preheat, low heat input first pass, solution treatment |
| Porosity | Base surface contamination, hydrogen pickup | Surface cleaning, dry flux/wire, post-weld heat treatment |
| Excessive hardness in dilution zone | Martensitic transformation | Post-weld tempering at 700–800°C |
| Overlay spallation | Poor bonding, residual stress | Controlled cooling rate, stress relief |
Study Insights and Reflections
The most valuable insight from this literature is the quantitative relationship between dilution rate and corrosion performance. When dilution exceeds 20%, the pitting resistance equivalent number (PREN) drops significantly, and intergranular corrosion susceptibility increases markedly. This has direct implications for engineering design: the specification of overlay thickness must account for the dilution zone, and the total overlay thickness should be at least 3 mm for critical applications to ensure adequate corrosion-resistant material remains after machining.
The study also highlights an often-overlooked aspect: the effect of base plate thickness on overlay quality. Thicker base plates (above 50 mm) absorb more heat, resulting in lower cooling rates and reduced dilution. However, this also means higher residual stresses due to the thermal mismatch between the overlay and the massive base. Stress relief treatment becomes mandatory for thick-section clad components.
The practical recommendation emerging from this study is that for GH2132 overlay on carbon steel, a minimum of three passes should be applied with controlled interpass temperatures, followed by solution heat treatment at 1050°C for 1 hour per 25 mm of thickness. This protocol ensures a fully austenitic overlay with minimal sensitization and acceptable mechanical properties for high-temperature service.
Reference Value and Outlook
This literature provides a solid foundation for understanding the metallurgical behavior of GH2132 overlay layers and their interaction with carbon steel substrates. Future work should focus on multi-layer overlay systems where intermediate layers of dissimilar composition can further reduce dilution effects, and on the development of laser cladding processes that offer even lower dilution rates and finer microstructures. The ongoing demand for nickel-based alloy cladding in the oil and gas industry, particularly for subsea equipment and sour service applications, ensures that this research remains highly relevant to current engineering practice.