Research on Microstructure and Properties of Overlay Welding Electrodes

Literature Overview

This comprehensive study examines the relationship between welding electrode composition, welding parameters, and the resulting microstructure and mechanical properties of overlay deposits. The research covers multiple electrode types including low-alloy, stainless steel, and nickel-based hardfacing electrodes, providing a systematic framework for understanding how electrode design influences overlay performance. The work integrates metallographic analysis, mechanical testing, and wear evaluation to establish correlations between processing variables and final properties.

Electrode Classification and Application Matrix

Electrode Type	Typical Composition	Hardness (HRC)	Primary Application	Wear Mechanism
Low-carbon martensitic	C 0.5–1.0%, Cr 4–8%, Mo 1–2%	45–55	Gears, cams, dies	Abrasion + impact
High-carbon martensitic	C 2.0–3.0%, Cr 6–12%	55–62	Crusher parts, hammers	Severe abrasion
High-chromium cast iron	C 3.0–4.0%, Cr 25–35%	58–65	Kiln liners, mill liners	Dry abrasion
Austenitic stainless	C 0.08–0.20%, Cr 18–25%, Ni 8–12%	25–35	Corrosion + moderate wear	Erosive corrosion
Nickel-based (Stellite)	Cr 20–25%, Mo 7–10%, Co balance	38–45	High-temp corrosion + wear	Hot corrosion + abrasion

Microstructural Evolution with Welding Parameters

The microstructure of overlay deposits is fundamentally governed by the cooling rate at the solidification front, which is determined by heat input, base metal thickness, and ambient conditions.

Heat Input (kJ/mm)	Cooling Rate (°C/s)	Microstructure	Hardness (HV)
0.5–1.0	150–300	Fine martensite + fine carbides	900–1100
1.0–2.0	50–150	Coarse martensite + medium carbides	750–900
2.0–4.0	15–50	Martensite + bainite + coarse carbides	600–750
4.0–6.0	5–15	Bainite + pearlite + coarse carbides	450–600

The transition from martensitic to bainitic microstructure with increasing heat input represents a critical threshold for overlay applications. Below approximately 2.0 kJ/mm, the overlay maintains martensitic hardness essential for wear resistance. Above this threshold, the transformation to bainite and pearlite significantly reduces hardness and wear performance.

Mechanical Properties and Their Interdependence

The mechanical properties of overlay deposits exhibit complex interdependencies:

Hardness vs. Toughness: Higher hardness generally correlates with lower impact toughness. For martensitic overlays, the Charpy impact energy typically ranges from 5–15 J for 55–62 HRC deposits, compared to 30–50 J for 40–45 HRC deposits.
Hardness vs. Wear Resistance: Wear resistance increases approximately linearly with hardness up to 60 HRC, beyond which the relationship becomes logarithmic due to carbide spalling under high contact stress.
Dilution vs. Properties: Each 10% increase in dilution typically reduces overlay hardness by 5–8 HRC and increases ductility by 15–20%.

Electrode Coating Design and Its Influence

The electrode coating serves multiple functions:

Alloying: Supplies alloying elements (Cr, Mo, V, W) that form hard carbides in the overlay.
Arc stabilization: Contains alkaline earth carbonates and fluorides that stabilize the electric arc.
Slag formation: Generates a protective slag that shields the solidifying weld from atmospheric contamination.
Deoxidation: Supplies silicon and aluminum to remove dissolved oxygen from the molten pool.
Dilution control: The coating dilution rate (typically 15–25% of total weld metal) determines the final overlay composition.

The coating composition directly influences the microstructure through:

Carbon activity: Higher carbon in the coating promotes carbide formation and hardenability.
Alloy partitioning: Alloying elements preferentially partition between the coating melt and base metal melt, affecting the final overlay composition gradient.
Slag basicity: Higher basicity (CaO/SiO₂ ratio) promotes cleaner welds with fewer inclusions but may increase spatter.

Common Defects and Root Cause Analysis

Defect	Detection Method	Root Cause	Prevention
Surface cracking	Visual/MT	High Ceq, rapid cooling	Preheat, controlled cooling
Internal cracking	RT/UT	Hydrogen, restraint	Low-H electrodes, bake electrodes
Porosity	RT/UT	Gas pickup, flux moisture	Proper storage, adequate shielding
Lack of fusion	UT/RT	Low current, poor fit-up	Increase current, improve preparation
Excessive undercut	Visual	High current, fast travel	Reduce current, slow travel speed

Engineering Practice and Qualification Requirements

For overlay welding procedures to be accepted in industrial applications, they must satisfy the requirements of applicable standards:

NB/T 47014: Chinese standard for welding procedure qualification, requiring demonstration of mechanical properties and microstructural acceptance.
ASME IX: Qualification based on essential variables including electrode type, diameter, welding position, and preheat/interpass temperature.
EN ISO 15614-1: European qualification standard requiring property verification for overlay welding.

Key qualification parameters for overlay procedures include:

Minimum and maximum weld thickness
Base metal thickness range
Electrode diameter and type
Welding position
Preheat and interpass temperature limits
Required mechanical properties (hardness, impact, wear)
Acceptable dilution range

Study Insights and Implications

This research provides a comprehensive framework for understanding the structure-property relationships in overlay welding. The fundamental insight is that overlay performance is an emergent property arising from the complex interaction between electrode composition, welding parameters, base metal characteristics, and cooling conditions. No single variable can be optimized in isolation; rather, a systems approach is required where all variables are considered simultaneously. The research also highlights the importance of dilution control as the primary lever for adjusting overlay properties. In practice, this means that the same electrode can produce vastly different overlay properties depending on the welding conditions, base metal, and number of passes. Engineers must therefore approach overlay specification as a process engineering challenge rather than a simple material selection exercise. The microstructural analysis techniques employed in this study—optical microscopy, SEM/EDS, XRD, and TEM—provide the analytical foundation for understanding and controlling overlay quality, and their application should be encouraged in industrial quality assurance programs.