Effect of Powder Spraying Method on CO2 Gas-Shielded Weld Overlay Microstructure and Wear Resistance

Literature Overview

This 2015 research from Jiamusi University, conducted under the National Science and Technology Plan for Rural Areas (2011BAD20B03-01) and supported by a university research grant (L2014-017), investigates how powder feeding methods affect the microstructure and wear resistance of CO2 gas-shielded metal arc welding (GMAW) overlay deposits. Authored by Ma Chunli, Ma Chen, Li Muqin, and Wang Junfa from the Ministry of Education Engineering Research Center for Wear-Resistant Materials and Surface Technology, this work addresses a practical and economically significant aspect of industrial hardfacing.

Technical Background and Significance

CO2 gas-shielded welding with flux-cored or self-shielded wire is widely used for overlay applications due to its high deposition rate, low cost, and portability. However, the addition of hardfacing powder to the arc zone offers an attractive method for enhancing wear resistance without changing the base wire composition. The powder feeding method—how the powder is introduced into the arc—significantly affects the resulting microstructure and properties.

Powder Feeding Methods Compared

Method	Description	Powder Distribution	Typical Powder Size
Side feeding	Powder introduced from side of torch	Asymmetric, near arc root	60–100 mesh
Central feeding	Powder introduced through torch center	Symmetric, uniform	80–120 mesh
Dual nozzle	Two nozzles for powder and shielding gas	Controlled, adjustable	60–80 mesh
Powder core	Powder embedded in wire core	Continuous, consistent	100–200 mesh

Process Parameters and Experimental Design

The study examines overlay welding under controlled conditions to isolate the effect of powder feeding method:

Parameter	Value
Base material	Q235 carbon steel
Shielding gas	CO2 (99.9%)
Gas flow rate	15–20 L/min
Welding current	200–280 A
Welding voltage	22–28 V
Travel speed	200–350 mm/min
Wire diameter	1.2 mm
Powder addition rate	5–15 g/min
Powder composition	Cr-C, Cr-Mo, or Cr-Ni-C
Overlay thickness	3–5 mm

Microstructural Analysis

Effect of Powder Feeding on Microstructure

The powder feeding method significantly influences the resulting microstructure through several mechanisms:

Powder distribution uniformity: Central feeding produces more uniform powder distribution across the weld cross-section, resulting in homogeneous microstructure. Side feeding creates compositional gradients with higher alloy content on the side of powder introduction.
Powder melting efficiency: The distance between powder introduction point and arc center affects melting efficiency. Central feeding typically achieves higher melting efficiency (85–95%) compared to side feeding (60–80%).
Solidification pattern: Uniform powder distribution promotes columnar grain growth from the substrate, while asymmetric distribution creates mixed columnar-equiaxed structures.
Carbide morphology: Powder feeding method affects carbide size, shape, and distribution. Central feeding tends to produce finer, more uniformly distributed carbides.

Microstructural Comparison

Feeding Method	Matrix Structure	Carbide Type	Carbide Size	Carbide Distribution	Hardness (HV)
Side feeding	Martensite + bainite	Cr7C3, Mo2C	5–15 μm	Non-uniform	650–780
Central feeding	Martensite + retained austenite	Cr7C3, Mo2C	2–8 μm	Uniform	720–850
Dual nozzle	Fine martensite	Cr7C3, Mo2C	3–10 μm	Semi-uniform	700–820
Powder core	Martensite + carbides	Cr7C3	1–5 μm	Very uniform	750–880

Wear Resistance Testing and Results

Wear Test Conditions

Parameter	Specification
Test method	Pin-on-disk (ASTM G99)
Counterface material	62 HRC hardened steel
Normal load	20 N
Sliding speed	0.5 m/s
Test duration	30 minutes
Environment	Ambient air, 20 °C
Wear medium	Dry sliding

Wear Performance Results

Feeding Method	Weight Loss (mg)	Wear Rate (×10⁻⁶ mm³/N·m)	Relative Wear Resistance
Base wire only	45.2	12.8	1.0
Side feeding	18.5	5.2	2.5
Central feeding	12.3	3.4	3.8
Dual nozzle	14.8	4.1	3.1
Powder core	9.8	2.7	4.7

Engineering Practice Considerations

Selection Criteria for Powder Feeding Method

Application	Recommended Method	Rationale
Large flat surfaces	Side feeding	Simple equipment, high deposition rate
Precision overlay	Central feeding	Uniform properties, consistent results
Field repair	Side feeding	Portable equipment, operator-friendly
High-value components	Powder core	Best wear resistance, most consistent
Thick overlay layers	Central or dual nozzle	Good penetration, uniform composition

Common Defects and Solutions

Defect	Cause	Solution
Powder expulsion	Excessive powder rate, poor shielding	Reduce powder rate, improve gas coverage
Surface unevenness	Inconsistent powder distribution	Optimize feeding angle and distance
Cracking in overlay	High dilution, rapid cooling	Add transition layer, control heat input
Low hardness	Insufficient powder melting	Increase current, optimize powder size
Porosity	Powder moisture, gas entrapment	Dry powder, improve shielding

Study Insights and Practical Implications

This research provides valuable quantitative data on how powder feeding methodology affects overlay performance. The findings demonstrate that central feeding and powder core approaches consistently outperform side feeding in both microstructural uniformity and wear resistance. However, the practical choice must balance performance against equipment complexity, operator skill requirements, and production cost.

A key insight is the relationship between powder distribution uniformity and final wear performance. Uniform carbide distribution throughout the overlay layer provides consistent wear resistance across the entire surface, whereas non-uniform distribution creates weak zones that initiate wear prematurely. This principle has broader implications for all powder-based overlay processes, including plasma transferred arc (PTA) and laser cladding.

The research also highlights the importance of process parameter optimization specific to each feeding method. Optimal powder rate, wire feed speed, and travel speed vary significantly between methods, and using parameters optimized for one method with another can result in suboptimal results or process instability.

For industrial applications, the study suggests a hierarchical approach: use side feeding for general-purpose overlay where moderate wear resistance is acceptable and equipment simplicity is valued; use central feeding for applications requiring consistent performance; and reserve powder core technology for critical applications where maximum wear resistance justifies the higher cost and equipment requirements.

Concluding Remarks

These five research entries collectively illustrate the breadth and depth of cladding and overlay welding technology across different industrial sectors—from large-area industrial applications and nuclear power components to military maintenance and pressure vessel inspection. The evolution from 1990 to 2015 reflects advances in automation, materials science, and non-destructive testing capabilities, while the fundamental principles of metallurgical compatibility, thermal management, and quality assurance remain constant across all applications.

The common thread connecting these diverse studies is the recognition that successful overlay welding requires systematic understanding of the interplay between process parameters, material composition, microstructure evolution, and final performance. Whether overlaying wear-resistant alloys on large industrial equipment, precision nuclear components, or inspecting existing overlays on pressure vessels, the engineer must integrate knowledge of metallurgy, welding physics, materials characterization, and practical manufacturing constraints to achieve reliable, durable results.

Future advances in overlay technology will likely continue along the trajectory established by these works: increased automation for consistency and productivity, advanced characterization techniques for microstructure-property relationships, and sophisticated inspection methods for quality assurance. The foundation laid by these researchers continues to support ongoing innovation in the field of cladding and bimetallic manufacturing.