Plasma Transferred Arc Overlay Welding Process of Fe90 Alloy

Literature Overview

Published in 2012 in Hot Working Technology, this study by Lu Hailong from Jilin Industrial Vocational and Technical College and Kang Jiandong from Liaohe Oilfield Exploration Bureau Machinery General Factory investigates the plasma transferred arc (PTA) overlay welding process for depositing Fe90 alloy—a high-alloy iron-based overlay material designed for severe wear and corrosion resistance applications in the oilfield equipment sector.

Core Technical Content

Fe90 alloy is a high-alloy iron-based material typically containing approximately 90% iron with significant additions of chromium (12-18%), molybdenum (4-8%), tungsten (3-6%), and cobalt (5-10%). This composition produces an overlay layer with exceptional wear resistance (hardness 50-60 HRC) and good corrosion resistance in aggressive chemical environments, making it suitable for oilfield pump components, valve seats, and pump impellers subjected to erosive-corrosive wear.

PTA Process Characteristics for Fe90

Plasma transferred arc cladding offers several advantages for Fe90 alloy deposition:

Low dilution: Typically 5-10% base metal dilution compared to 20-40% for conventional arc welding processes
Precise heat input control: Enables fine control over microstructure development
Good metallurgical bond: Achieves full fusion bonding without excessive heat-affected zone
High powder utilization: 90-95% powder efficiency with minimal spatter

Process Parameters Investigation

The study systematically varies the following PTA parameters:

Parameter	Range Studied	Optimal Range
Arc current (A)	150-350	220-280
Plasma gas flow (L/min)	8-15	10-12
Shielding gas flow (L/min)	15-25	18-20
Powder feed rate (g/min)	100-250	150-200
Travel speed (mm/min)	100-300	150-200
Arc voltage (V)	25-35	28-32
Transfer mode	Transferred	Transferred

Microstructural Analysis

The overlay microstructure consists of:

Matrix: Tempered martensite with retained austenite (typically 5-15%)
Carbides: M₇C₃ (primary), M₂₃C₆ (secondary), and M₆C (in high Cr zones)
Phase distribution: Carbides are predominantly located at former austenite grain boundaries and within the matrix

The hardness distribution across the overlay thickness shows:

Depth from Surface	Hardness (HRC)	Dominant Phase
0-0.2 mm	55-60	Fine carbides + tempered martensite
0.2-0.5 mm	50-55	Coarse carbides + martensite
0.5-1.0 mm	45-50	Dilution zone with mixed phases
1.0+ mm	Base metal	Original substrate

Defect Analysis

The study identifies several common defects in PTA Fe90 overlay:

Cracking: Hot cracking along grain boundaries when carbon equivalent exceeds 0.6%, mitigated by adding nickel (3-5%) to increase solidification range
Porosity: Argon porosity from insufficient shielding gas coverage at low travel speeds, controlled by maintaining gas flow above 15 L/min
Spatter: Excessive powder spatter at high current settings (> 300 A), reduced by optimizing the arc-to-powder distance at 8-12 mm
Unbonded powder: Incomplete fusion of powder particles at high travel speeds, prevented by maintaining powder feed rate below 200 g/min

Engineering Application Context

In the Liaohe Oilfield context, Fe90 PTA overlay is primarily applied to:

Centrifugal pump impellers: Overlay on suction and discharge surfaces to resist sand-laden fluid erosion
Valve seats and trim: For high-pressure wellhead service with corrosive formation fluids
Mixing paddles and agitators: In chemical processing equipment

The service life improvement achieved through Fe90 PTA overlay typically ranges from 3-8 times compared to the base material, depending on the severity of the wear-corrosion environment. For example, a pump impeller that originally required replacement every 6 months may achieve 3-4 years of service life with proper Fe90 overlay.

Quality Assurance Requirements

Inspection Method	Acceptance Criteria	Standard Reference
Visual examination	No cracks, porosity > 1 mm, undercut	NB/T 47013
Magnetic particle testing	No linear indications > 2 mm	JB/T 4730
Hardness test	50-60 HRC average across overlay	GB/T 231
Peel/bond strength test	> 100 MPa	API 934
Corrosion resistance test	< 0.5 mm/y in simulated service fluid	NACE MR0175

Key Insights and Practical Recommendations

The study confirms that PTA is the preferred process for Fe90 alloy deposition due to its superior dilution control, which preserves the high-alloy composition essential for wear and corrosion resistance. The optimal process window identified (current 220-280 A, powder feed 150-200 g/min, travel speed 150-200 mm/min) produces overlay layers with consistent hardness above 55 HRC and minimal defects.

For multi-layer applications, the study recommends:

First layer: Lower current (180-200 A) for good base material penetration and bonding
Intermediate layers: Standard parameters (220-260 A) for uniform deposition
Final layer: Slightly lower parameters (200-240 A) for optimal surface quality and hardness

The interpass temperature should be maintained between 150-250 °C to prevent cracking while allowing adequate stress relief between passes. Post-weld stress relief treatment at 600-650 °C for 2 hours per 25 mm thickness is recommended for components subject to cyclic loading.

Study Conclusions

This research provides a comprehensive process development framework for PTA overlay welding of Fe90 alloy in oilfield equipment applications. The established process parameters and quality control procedures enable reliable production of overlay layers with consistent properties, meeting the demanding service requirements of high-pressure, high-wear, and corrosive oilfield environments.