CLADDING TECHNOLOGY SHANXI CO., LTDAlloying and TIG Arc Remelting Strengthening of Oil Pump Internal Surface
Literature Overview
This paper, published in Welding Technology (焊接技术) in 2000, presents a surface hardening and strengthening methodology for the internal surface of oil pumps using alloying combined with TIG arc remelting. The authors, An Xizhong, Liu Zhengjun, Lin Kegang, and Hao Xuefeng, represent Shenyang University of Technology (Welding Teaching and Research Section) and Shenyang Boiler and Pressure Vessel Inspection Research Institute. The work addresses the tribological degradation and corrosion wear problems encountered in oil pump internals operating under high-temperature, high-pressure, and chemically aggressive conditions.
Core Technical Content
Oil pumps used in petroleum refining and chemical processing are frequently subjected to severe service conditions involving abrasive slurry, corrosive media, and cyclic thermal loading. The internal surfaces of pump bodies, impellers, and wear rings are prone to erosion-corrosion, cavitation damage, and adhesive wear. Conventional surface treatments such as electroplating, chemical vapor deposition, and thermal spray coatings often suffer from poor bond strength and limited thickness, leading to delamination and premature failure.
The proposed approach involves two sequential steps: first, alloying the base steel surface with a high-alloy powder or flux to deposit a layer of hard, wear-resistant material; second, remelting this alloyed layer using TIG arc to homogenize the composition, eliminate porosity, and achieve a metallurgical bond with the substrate. The resulting remelted layer exhibits a gradient microstructure with a high hardness near the surface and a smooth transition to the base metal, providing both wear resistance and fatigue resistance.
Key Technical Points and Process Parameters
| Parameter | Typical Value / Range | Purpose |
|---|---|---|
| Base material | 20 steel, 45 steel, 16Mn | Pump body and internal components |
| Alloying powder | Cr-Mo-V-B system or Ni-Cr-Mo system | Provides hardness and corrosion resistance |
| Alloying thickness | 0.5–1.5 mm | Ensures sufficient remelted layer depth |
| TIG remelting current | 120–200 A | Controls penetration and dilution |
| Travel speed | 5–15 mm/min | Balances heat input and layer geometry |
| Shielding gas | Argon or He-Ar mixture | Prevents oxidation of remelted surface |
| Preheating temperature | 150–250 °C | Reduces residual stress and HIC risk |
| Remelted layer hardness | 45–60 HRC | Achieves wear resistance |
| Dilution rate | 30–50% | Controls final composition of remelted layer |
The alloying step is typically performed using a powder spray method or by placing the alloy powder in a groove machined on the surface. The powder is then fused into the substrate by a localized heat source (oxy-acetylene flame, induction heating, or electric resistance heating). The subsequent TIG remelting pass dissolves the alloyed layer completely and re-solidifies it under controlled cooling conditions, producing a homogeneous, dense, and crack-free surface layer.
Microstructural Analysis and Performance
Metallographic examination of the remelted layer typically reveals a fine-grained martensitic or austenitic-ferritic structure, depending on the alloy composition. The Cr-Mo-V-B alloying system produces a hard martensitic structure with carbide precipitation, achieving hardness values of 50–60 HRC. The Ni-Cr-Mo system produces an austenitic structure with superior corrosion resistance and moderate hardness (45–50 HRC).
The gradient transition zone between the remelted layer and the base metal is critical for preventing delamination under cyclic loading. A well-designed process ensures a smooth compositional gradient over a depth of 0.3–0.8 mm, which accommodates the differential thermal expansion between the hard surface layer and the ductile substrate.
Engineering Practice and Quality Control
In practical application, the alloying and TIG remelting process must be qualified per NB/T 47014 (Procedure Qualification for Welding of Pressure Vessel Materials) or equivalent standards. The qualification procedure includes:
- Visual inspection (VT) of the remelted surface for cracks, porosity, and unevenness.
- Magnetic particle testing (MT) or liquid penetrant testing (PT) for surface and near-surface defects.
- Hardness testing across the cross-section to verify the gradient profile.
- Bend testing or fatigue testing to confirm bond strength and cyclic durability.
For oil pump components operating under sour service conditions (containing H₂S), the remelted layer must also pass hydrogen-induced cracking (HIC) and sulfide stress corrosion cracking (SSC) testing per NACE MR0175/ISO 15156. The low-carbon, low-sulfur consumables used in the alloying step are essential to minimize the risk of HIC in the heat-affected zone.
Study Insights and Reflections
This research demonstrates a cost-effective and reliable approach to surface strengthening of pump internals that avoids the limitations of thin coatings. The combination of alloying and TIG remelting provides a thick, metallurgically bonded layer with excellent wear and corrosion resistance, suitable for severe service environments. The technique is particularly advantageous for repair and retrofit applications, where large pump bodies cannot be easily replaced or recoated.
The key engineering insight is that the dilution rate and cooling rate must be carefully controlled to achieve the desired balance between hardness, toughness, and corrosion resistance. Excessive dilution leads to softening of the surface layer, while insufficient dilution results in high hardness but poor toughness and increased susceptibility to cracking. The preheating and post-weld heat treatment (PWHT) steps are equally important for relieving residual stresses and stabilizing the microstructure.