Zhuojin Pipe Fitting Co., LtdInconel 625 Alloy Overlay on 2.25Cr1Mo Substrate Microstructure and Property Analysis
Literature Overview
Published in Hot Working Technology (2010, Vol. 39, No. 17), this study from Liaoning Petrochemical University and Fushun Petrochemical Company investigates the microstructure and properties of Inconel 625 alloy overlay layers deposited on 2.25Cr1Mo steel substrates using the gas tungsten arc welding (GTAW) process. The research addresses a critical engineering need in the petrochemical industry: the protection of high-temperature pressure vessels and piping made of 2.25Cr1Mo steel against corrosion in aggressive environments. Inconel 625 (UNS N06625) is a nickel-chromium-molybdenum alloy with excellent resistance to both oxidizing and reducing environments, making it an ideal overlay material for components exposed to chloride-containing media at elevated temperatures.
Core Technical Points
Base Material and Overlay Material Properties
| Property | 2.25Cr1Mo Steel | Inconel 625 |
|---|---|---|
| C (wt%) | 0.17–0.25 | ≤0.10 |
| Cr (wt%) | 0.90–1.20 | 20.0–23.0 |
| Mo (wt%) | 0.40–0.60 | 8.0–9.0 |
| Ni (wt%) | ≤0.50 | 52.0–58.0 |
| Nb (wt%) | 0.5–1.0 (typical) | 3.15–4.00 |
| Yield strength (MPa) | 415–485 | 550–690 |
| Hardness (HV) | 180–220 | 250–350 |
| Thermal expansion (μm/m·K) | 12.0–12.5 | 13.0–13.5 |
The thermal expansion mismatch between the base metal and overlay material is relatively small (approximately 0.5–1.0 μm/m·K), which is favorable for minimizing residual stress and reducing the risk of cracking during cooling.
Microstructural Analysis
The study reports that the Inconel 625 overlay layer exhibits a dendritic austenitic microstructure, which is characteristic of rapid solidification conditions typical of GTAW welding. The dendrite spacing is influenced by the cooling rate, which is determined by the welding parameters and the thermal conductivity of the base material.
Key microstructural features observed:
- Dendritic austenite (γ): The primary phase in the overlay, providing good ductility and toughness.
- δ-ferrite: A minor phase that may form at the interdendritic regions, contributing to solidification cracking resistance.
- Carbides: NbC or TiC precipitates may form at the dendrite boundaries, contributing to precipitation hardening.
- Laves phase: In some cases, a Ni₃Nb-based Laves phase may form, which can adversely affect ductility if excessive.
The fusion zone between the overlay and the 2.25Cr1Mo base metal shows good metallurgical bonding without separation, cracking, or porosity. This is attributed to the compatible thermal expansion coefficients and the relatively low dilution ratio achievable with GTAW.
Corrosion Performance
The study conducted corrosion testing in a 10% FeCl₃ solution at 50 °C, a standard test for evaluating the resistance of nickel-based alloys to chloride-induced stress corrosion cracking. The results demonstrate that the Inconel 625 overlay provides excellent corrosion protection, significantly outperforming the bare 2.25Cr1Mo base metal.
| Test Condition | 2.25Cr1Mo Base | Inconel 625 Overlay |
|---|---|---|
| 10% FeCl₃, 50 °C, 24 h | Severe pitting and intergranular corrosion | No visible corrosion |
| Weight loss (mg/cm²) | High | Negligible |
| Corrosion potential (mV vs. SCE) | -200 to -300 | +100 to +200 |
Engineering Practice Integration
In the petrochemical industry, 2.25Cr1Mo steel is widely used for high-temperature pressure vessels, reactors, and piping systems operating at temperatures up to 550 °C. However, this alloy is susceptible to corrosion in environments containing chlorides, sulfides, or other aggressive species. The application of an Inconel 625 overlay provides an effective corrosion barrier while maintaining the structural integrity of the base material.
Typical applications include:
- Reactor internals: Tubing and supports exposed to hot chloride-containing process fluids
- Heat exchanger tubes: Corrosion-resistant cladding on the process side
- Piping systems: Overlay on critical sections where corrosion is anticipated
- Valve seats and trim: Wear and corrosion-resistant surfaces in aggressive service
Welding Process Parameters
The GTAW process used in this study typically employs the following parameters:
- Electrode: 1.6 mm or 2.4 mm tungsten (ceramic or lanthanated)
- Current: 120–180 A (DC)
- Arc voltage: 10–15 V
- Travel speed: 3–6 cm/min
- Shielding gas: Argon at 10–15 L/min
- Preheat: 150–200 °C (to minimize cracking in the 2.25Cr1Mo base)
- Interpass temperature: ≤250 °C
The use of a pure nickel or Inconel 625 filler wire (e.g., ERNiCrMo-3) ensures minimal dilution and maintains the corrosion-resistant composition of the overlay.
Key Questions and Reflections
One important consideration not fully addressed in the study is the long-term performance of the overlay under cyclic thermal loading. The thermal expansion mismatch, while small, can lead to fatigue cracking at the overlay-base interface over extended service life. Engineers should consider supplementing the reported data with thermal cycling tests to validate the overlay's durability under realistic operating conditions.
Another reflection concerns the cost-effectiveness of Inconel 625 overlay compared to alternative materials such as Alloy 625, Alloy 718, or duplex stainless steels. While Inconel 625 offers superior corrosion resistance, its high cost may not be justified for all applications. Engineers should perform a life-cycle cost analysis to determine the most economical solution for each specific service condition.
Study Insights and Implications
This study provides valuable data on the microstructure and corrosion performance of Inconel 625 overlay on 2.25Cr1Mo steel, confirming the effectiveness of this material combination for petrochemical applications. The findings support the use of GTAW for depositing Inconel 625 overlays with acceptable quality, provided that the welding parameters are carefully controlled and the base metal is properly preheated. For engineers designing corrosion protection solutions for high-temperature petrochemical equipment, this study reinforces the importance of material selection compatibility and the need for comprehensive testing to validate overlay performance under actual service conditions. The Inconel 625 overlay on 2.25Cr1Mo steel represents a proven technology for extending the service life of critical pressure equipment in aggressive environments.