Plasma Hardfacing Process Optimization of Fe90 Alloy on Low Carbon Steel

Literature Overview

This paper by Lu Hailong and Kang Jiandong (2012), published in "Hot Working Technology," examines the plasma arc hardfacing of Fe90 alloy onto low carbon steel substrates, with specific focus on the effect of welding current on the microstructure, hardness, and wear resistance of the hardfacing deposit. The study was conducted at Jilin Polytechnic Institute and Liaoyuan Oilfield Exploration Bureau Machinery General Factory, reflecting a strong industry-academia partnership oriented toward practical oilfield equipment repair and enhancement.

Core Technical Findings

Fe90 is an iron-based alloy widely used for hardfacing applications requiring high hardness and wear resistance in abrasive service. The authors systematically varied the hardfacing current and evaluated the resulting deposit quality:

Parameter	Optimal Condition (180 A)
Surface Hardness	67.7 HRC
Wear Resistance	Best among tested conditions
Surface vs. Side Performance	Surface superior to side
Primary Testing Methods	Rockwell hardness, wet sand-rubber wheel wear test, OM, SEM

The study demonstrates that the hardfacing current directly governs the thermal input, which in turn controls the solidification behavior, dilution rate, and final microstructure of the Fe90 deposit.

Process Analysis and Technical Discussion

Plasma Arc Hardfacing Process Parameters

Plasma arc hardfacing (PAHW) is a highly concentrated heat source process that offers several advantages over conventional arc welding methods for overlay applications:

High energy density enables rapid melting and solidification
Lower dilution compared to GTAW or SAW
Good process control and repeatability
Suitable for both new manufacturing and repair applications

The optimal current of 180 A represents a balance between adequate melting of the Fe90 alloy and controlled dilution from the low carbon steel substrate. At lower currents, incomplete melting of the alloy powder or wire may occur, resulting in a deposit with insufficient hardness and poor bonding. At higher currents, excessive dilution introduces soft ferrite and pearlite from the substrate into the deposit, reducing hardness and wear resistance.

Microstructure and Hardness Relationship

The Fe90 alloy typically solidifies with a martensitic matrix containing carbide precipitates (such as Cr₇C₃, Mo₂C, or Fe₃C, depending on the specific alloy composition). The high hardness of 67.7 HRC at the optimal condition indicates a predominantly martensitic microstructure with fine carbide distribution. The surface hardness exceeding the side hardness is a common observation in plasma hardfacing and is attributed to:

Faster cooling rates at the surface, promoting finer martensite
Potential for preferential segregation of alloying elements to the surface
Possible differences in dilution between the top and side regions

Wear Mechanism Analysis

The wet sand-rubber wheel wear test simulates sliding wear under abrasive conditions, which is representative of many oilfield and mining applications. The superior wear resistance at 180 A can be attributed to the combination of high matrix hardness and appropriate carbide morphology and distribution. Excessive current may cause carbide coarsening due to prolonged time at elevated temperatures, while insufficient current may result in incomplete alloy melting and segregation of unmelted particles.

Engineering Practice Integration

Application Context in Oilfield Equipment

Fe90 hardfacing deposits are commonly applied to:

Drill pipe tool joints
Valve components
Crusher jaws and plates
Pump shafts and impellers
Conveyor rollers

The study's findings directly inform the welding procedure specification (WPS) development for such components. A typical WPS for plasma hardfacing Fe90 might specify:

WPS Parameter	Recommended Value
Current	180 A
Voltage	25-30 V
Travel speed	200-400 mm/min
Shielding gas	Argon or Argon-Helium mixture
Preheating	Generally not required for low carbon steel
Interpass temperature	Below 150°C

Quality Control Considerations

For production hardfacing operations, the following quality control measures should be implemented:

Visual inspection for cracks, porosity, and incomplete fusion
Hardness testing at multiple depths to ensure uniform overlay
Wear testing on qualification specimens
Dilution analysis through microhardness traverse across the fusion boundary

Key Questions and Reflections

The study raises the question of whether the optimal current of 180 A is universal or specific to the particular equipment and consumable geometry used. Different plasma torch designs, nozzle diameters, and alloy wire/powder configurations would require individual optimization. Additionally, the study does not address the effect of layer thickness or multi-pass hardfacing, which are critical for thick overlay applications. The authors also do not discuss the effect of preheating on residual stress and cracking susceptibility, which is important for thicker substrate sections.

Study Insights and Implications

This research provides a practical, process-oriented contribution to the hardfacing community, demonstrating that relatively simple parameter optimization (current variation) can significantly impact overlay performance. The finding that surface performance exceeds side performance has direct implications for component design—orienting the service face to correspond to the surface of the hardfacing deposit maximizes wear life. For oilfield maintenance engineers, this study supports the adoption of plasma hardfacing as a reliable and efficient method for extending the service life of critical iron-based alloy components, provided that welding parameters are carefully controlled and validated through qualification testing.