CLADDING TECHNOLOGY SHANXI CO., LTDAlloying and TIG Arc Remelting Strengthening of Heavy Oil Pump Internal Surfaces
Literature Overview
This 2000 publication from Shenyang University of Technology and the Shenyang Boiler and Pressure Vessel Inspection Institute addresses a practical engineering problem in the petroleum industry: the wear and corrosion resistance enhancement of heavy oil pump internal surfaces through surface alloying combined with TIG arc remelting. The work by An Xizhong, Liu Zhengjun, and colleagues represents an applied approach to surface engineering that combines two techniques — powder alloying and arc remelting — to create a functionally graded surface layer with superior tribological and corrosion resistance properties.
Technical Background and Problem Statement
Heavy oil pumps operate in extremely harsh environments characterized by:
- High viscosity hydrocarbon media with suspended abrasive particles (sand, catalyst fines, scale)
- Temperatures ranging from ambient to 350°C depending on the extraction process
- Corrosive components including H₂S, CO₂, organic acids, and chlorides
- Cyclic loading from reciprocating or rotary pumping action
- Particulate-laden fluid flow causing erosion-corrosion synergy
Conventional pump internals made from carbon steel or 304 stainless steel typically exhibit service life of 6–18 months before requiring replacement. Surface engineering treatments can extend this life by 3–10 times, providing substantial economic benefits.
Process Design: Alloying and Remelting
The two-step process involves:
Step 1: Surface Alloying
Powder alloying introduces a high-alloy composition onto the base surface. The typical alloying powders used include:
| Powder Component | Typical Composition | Purpose |
|---|---|---|
| Ni-Cr-BSiC | 35% Ni, 20% Cr, 10% B, 15% SiC, balance Fe | Hard, wear-resistant |
| Cr-C-Mo | 25% Cr, 3% C, 5% Mo, balance Fe | Corrosion-resistant, hard |
| Ni-Fe-Cr | 40% Ni, 30% Fe, 15% Cr, 5% SiC | Tough, corrosion-resistant |
| Co-Cr-W | 55% Co, 15% Cr, 10% W, balance Fe | High-temperature wear |
The powder is applied via flame spraying, cold spray, or simple packing and melting methods to create a 2–5 mm pre-alloyed layer on the base surface.
Step 2: TIG Arc Remelting
The remelting step uses a TIG arc to completely melt the pre-alloyed layer and a controlled depth of base material (typically 0.5–1.5 mm), creating a metallurgically bonded, functionally graded layer.
| Parameter | Range | Effect |
|---|---|---|
| Arc current | 80–180 A | Controls remelting depth |
| Travel speed | 100–300 mm/min | Balances dilution and penetration |
| Arc oscillation | 0–5 mm amplitude | Improves bead uniformity |
| Shielding gas | Ar or Ar/He (80/20) | Penetration enhancement |
| Wire feed (optional) | ERNiCrMo-3 | Adjusts final composition |
Microstructural Engineering
The functionally graded layer produced by this process exhibits a controlled gradient from the base material through the remelted zone:
- Surface layer (0–1 mm): Hard carbide and boride phases (Cr₇C₃, CrB, SiC) in a Ni-Cr matrix. Hardness 800–1200 HV.
- Transition zone (1–2 mm): Dissolved carbides in austenite/ferrite matrix. Hardness 400–600 HV.
- Base material (below 2 mm): Unaffected base steel. Hardness 150–250 HV.
This gradient provides:
- Surface hardness for wear resistance
- Toughness in the transition zone for crack arrest
- Compatibility with the base material for stress distribution
Performance Results
| Performance Metric | Untreated Surface | Alloyed + Remelted | Improvement |
|---|---|---|---|
| Dry sliding wear rate | 45–80 mg/100m | 3–8 mg/100m | 85–95% reduction |
| Corrosion rate in 5% HCl | 2.5–4.0 mm/y | 0.1–0.3 mm/y | 90–97% reduction |
| Service life | 6–18 months | 3–5 years | 3–10× extension |
| Surface hardness | 180–250 HV | 800–1200 HV | 4–6× increase |
Engineering Practice Considerations
The application of this technology to pump internals requires careful consideration of several practical factors:
- Geometry accessibility: Internal pump surfaces (volute, impeller, cylinder) require specialized torch fixtures or robotic systems for consistent remelting quality.
- Heat-affected zone management: Excessive heat input can cause distortion of thin pump walls (typically 6–12 mm), requiring controlled multi-pass remelting with interpass cooling.
- Inspection: Post-remelting surfaces require hardness mapping, microstructural examination, and non-destructive testing (MT or PT) to detect lack of fusion or cracks.
- Repairability: The functionally graded layer can be ground off and reapplied, providing a repair strategy for worn components.
Key Reflections
This work demonstrates the power of combining surface alloying with arc remelting as a practical, cost-effective alternative to full component replacement or expensive thermal spray coatings. The approach is particularly well-suited to the Chinese petroleum industry where heavy oil extraction dominates and component life extension provides significant economic returns. The technology bridges the gap between conventional welding and advanced surface engineering, using standard TIG equipment with modified consumables. The key insight is that the remelting step is not merely a bonding operation but a microstructural engineering tool that controls dilution, phase formation, and property gradients. This philosophy of "welding as surface engineering" has broad applicability to other wear-critical components including valve seats, turbine blades, and mining equipment.