Microstructure and Properties of SMAW Overlay Metal Using CHR172 Electrode

Literature Overview

This paper by Zhong Yu, Qu Jinshan, Chen Wenjing, Pan Quanxi, and Luo Chaoyu from Xihua University and Liangshan Agricultural School was published in Welding Technology (Vol. 36, No. 3, 2007, pp. 10-12). The study investigates the microstructure and microhardness of overlay metal deposited using the CHR172 hardfacing electrode on 45 steel (C45) substrate via Shielded Metal Arc Welding (SMAW). The research systematically examines the effects of welding heat input and alloy composition on the resulting overlay microstructure and hardness distribution.

Core Technical Content

The CHR172 electrode is a chromium-based hardfacing consumable designed for producing overlays with excellent wear resistance through the formation of hard chromium carbide particles. The study reveals that the overlay microstructure and microhardness are governed by the interplay of several factors:

Factor	Effect on Microstructure	Effect on Hardness
Welding heat input	Controls cooling rate and grain size	Higher heat input generally reduces hardness through coarsening
Alloy composition	Determines carbide type and distribution	Higher Cr content increases carbide volume fraction
Hard particle type	Cr₇C₃ vs. Cr₃C₂ carbide formation	Cr₇C₃ is harder but more brittle than Cr₃C₂
Particle-matrix bonding	Quality of interface between carbides and matrix	Poor bonding leads to particle pull-out and reduced effective hardness
Particle distribution	Uniform vs. clustered distribution	Uniform distribution provides more consistent wear resistance

Microstructural Analysis

The study identifies several critical microstructural features in the CHR172 overlay:

Carbide morphology: The primary wear-resistant phase is chromium carbide, which can exist as Cr₇C₃ (hexagonal, very hard but brittle) or Cr₃C₂ (cubic, slightly less hard but more ductile). The relative proportion depends on the cooling rate and chromium content.
Matrix structure: The matrix surrounding the carbide particles typically consists of martensite or austenite, depending on the specific alloy composition and cooling conditions. The matrix provides the toughness necessary to prevent carbide-induced cracking.
Heat-Affected Zone (HAZ): The HAZ in the 45 steel substrate experiences rapid heating and cooling, leading to a narrow band of tempered martensite or bainite. The dilution at the overlay-substrate interface affects the hardness gradient.

Heat Input Effects

The study demonstrates that welding heat input has a profound effect on overlay properties:

Heat Input Range	Microstructural Consequence	Hardness Consequence
Low (rapid cooling)	Fine carbides, high hardness martensite matrix	Higher microhardness, increased brittleness
Moderate	Balanced carbide size and distribution	Optimal hardness-toughness combination
High (slow cooling)	Coarse carbides, potential carbide coalescence	Reduced hardness, potential carbide network formation

For practical welding operations, this translates into the recommendation that heat input should be controlled within a moderate range to achieve the best balance of hardness and toughness. Excessive heat input not only reduces hardness but can also promote the formation of continuous carbide networks, which severely degrade the fracture toughness of the overlay.

Engineering Practice Applications

In pipe and fitting manufacturing, CHR172-type overlay welding is commonly applied to:

Wear plates on material handling equipment
Cutter edges on earthmoving equipment
Valve seats and plugs in abrasive service
Pipe spools in slurry transport applications
Fitting surfaces subject to particle erosion

The following table summarizes typical process parameters for CHR172 overlay welding:

Parameter	Typical Value	Engineering Rationale
Electrode diameter	φ3.2 mm or φ4.0 mm	Balance between deposit thickness and control
Current range	120-200 A (φ3.2), 180-280 A (φ4.0)	Adequate penetration without excessive dilution
Interpass temperature	≤ 150°C	Prevent excessive heat input accumulation
Pass thickness	3-5 mm	Ensure adequate dilution control
Number of passes	2-3 for thick overlays	First pass for bonding, subsequent for hardness

Key Reflections

The study reinforces a fundamental principle in hardfacing technology: the hardness of the overlay is not solely determined by the electrode composition but is also strongly influenced by the welding process parameters, particularly heat input. This has direct implications for welding procedure qualification — two operators using the same electrode on the same base material can produce overlays with significantly different hardness values if their heat input levels differ.

For quality control purposes, this means that hardness testing of overlay welds should be conducted at multiple locations, and the welding parameters should be verified during production. The study also highlights the importance of carbide-matrix bonding quality, which is difficult to assess through conventional hardness testing alone and requires metallographic examination.

Concluding Remarks

This study provides a comprehensive understanding of how welding process parameters and alloy composition interact to determine the microstructure and properties of chromium-based hardfacing overlays. The key insight for practicing engineers is that optimal overlay performance requires balancing hardness (through appropriate carbide formation) with toughness (through matrix control), and that this balance is achieved not only through consumable selection but also through careful control of welding heat input. The CHR172 electrode represents a versatile hardfacing solution for moderate-to-severe abrasive wear applications, provided that the welding procedure is properly developed and maintained.