CLADDING TECHNOLOGY SHANXI CO., LTDSingle-Layer Cladding Technology for High-Temperature High-Pressure Separators
Literature Overview
This paper addresses the engineering challenge of applying a single-layer weld overlay to high-temperature and high-pressure separators used in oil and gas processing facilities. Conventional multi-layer cladding approaches, while providing excellent corrosion resistance, often result in excessive thermal distortion, high residual stresses, and significant production costs. The study evaluates the feasibility and performance of a single-layer overlay using a nickel-based alloy (Inconel 625) applied by submerged arc welding (SAW) to a carbon steel (SAW 350) separator shell, and compares the results with conventional multi-layer approaches. The separator operates at a design temperature of 450 °C and a design pressure of 25 MPa, which imposes stringent requirements on the overlay's mechanical integrity and corrosion resistance.
Design and Fabrication Approach
The single-layer overlay was applied using a continuous strip cladding technique with a 3 mm thick Inconel 625 strip and a consumable electrode of matching composition. The welding parameters were optimized to achieve a dilution ratio of 15–20%, which is higher than the typical 5–10% for multi-layer overlays but acceptable for the single-layer approach given the alloy composition of Inconel 625. The preheat temperature was set at 200 °C to minimize thermal gradients and reduce residual stresses. The interpass temperature was maintained below 250 °C to ensure adequate cooling rate for the formation of a fine microstructure.
| Parameter | Single-Layer | Conventional Multi-Layer (3 passes) |
|---|---|---|
| Overlay thickness | 3.0 mm | 3.0 mm |
| Number of passes | 1 | 3 |
| Dilution ratio | 15–20% | 5–10% |
| Preheat temperature | 200 °C | 150 °C |
| Interpass temperature | < 250 °C | < 200 °C |
| Thermal input per pass | 18–22 kJ/mm | 8–10 kJ/mm |
| Total thermal input | 18–22 kJ/mm | 24–30 kJ/mm |
| Residual stress (longitudinal) | 120–150 MPa | 180–220 MPa |
| Production time | 45 min/m | 120 min/m |
The single-layer approach reduces the total thermal input by approximately 30% compared to the multi-layer approach, which directly translates to lower residual stresses and reduced thermal distortion. The production time is reduced by more than 60%, providing significant cost savings for large-diameter separators.
Microstructure and Mechanical Properties
The microstructure of the single-layer Inconel 625 overlay consists of a columnar dendritic structure with L12-type Ni3(Fe,Cr) precipitates at the interdendritic regions. The higher dilution ratio compared to multi-layer overlays results in a slightly lower chromium content in the overlay (approximately 22 wt% versus 25 wt% for low-dilution multi-layer), but the corrosion resistance remains adequate for the operating environment. The hardness of the overlay is 280–310 HV, which is slightly lower than the multi-layer counterpart (320–350 HV) but still well within the acceptable range for high-temperature service.
| Property | Single-Layer | Multi-Layer | Specification Requirement |
|---|---|---|---|
| Hardness (HV) | 280–310 | 320–350 | ≥ 250 |
| Tensile strength (MPa) | 680–720 | 750–800 | ≥ 600 |
| Elongation (%) | 35–40 | 30–35 | ≥ 30 |
| Creep strength at 450 °C (MPa) | 280–300 | 310–330 | ≥ 250 |
| Corrosion rate (mm/y) | 0.02 | 0.015 | ≤ 0.1 |
The elongation of the single-layer overlay is actually higher than the multi-layer counterpart due to the more homogeneous microstructure and reduced residual stresses. This is advantageous for pressure vessel applications where ductility is critical for avoiding catastrophic failure.
Non-Destructive Testing Results
The single-layer overlay was inspected using magnetic particle testing (MT) and ultrasonic testing (UT) according to JB/T 4730. No surface cracks or lack of fusion defects were detected. The bond strength was verified by a tensile bond test according to ASTM B677, with results of 260–280 MPa, exceeding the minimum requirement of 200 MPa. A hydrostatic pressure test at 1.5 times the design pressure (37.5 MPa) was conducted without any leakage, confirming the integrity of the overlay-substrate bond.
Engineering Practice Implications
The single-layer cladding technology represents a significant advancement for high-temperature high-pressure separator fabrication. The key advantages are reduced thermal input, lower residual stresses, shorter production time, and lower cost. However, engineers must carefully evaluate the dilution ratio and ensure that the resulting alloy composition maintains adequate corrosion resistance for the specific operating environment. For applications involving chloride-containing media or highly aggressive chemical environments, the higher dilution ratio of single-layer cladding may compromise the corrosion resistance, and a multi-layer approach may still be preferred. The decision should be based on a thorough risk assessment that considers the operating conditions, material compatibility, and inspection requirements.
Summary
The study demonstrates that single-layer Inconel 625 cladding by SAW is a viable and economically attractive alternative to conventional multi-layer cladding for high-temperature high-pressure separators operating at 450 °C and 25 MPa. The reduced thermal input results in lower residual stresses and less distortion, while the mechanical properties and corrosion resistance meet or exceed specification requirements. Engineers should consider this technology for new separator designs and retrofits, provided that the dilution ratio and resulting alloy composition are verified through chemical analysis and the overlay is inspected according to applicable standards. This approach aligns with the industry trend toward lean manufacturing and reduced waste while maintaining the safety and reliability required for pressure equipment.