Effect of Powder Spray Configuration on CO2 Shielded Arc Overlay Microstructure and Wear Resistance

Literature Overview

This paper by Ma Chunli and colleagues from the Engineering Research Center for Metallic Wear-Resistant Materials and Surface Technology at Jiamusi University investigates the influence of two distinct powder delivery configurations—internal gun (in-gun) versus external gun (out-gun) powder feeding—on the microstructure, hardness, and wear resistance of CO2 gas shielded arc overlay welds. The study employs an optimized Cr-Ti-Mn-B alloy powder system, examining how the powder injection geometry affects alloy utilization, microstructural uniformity, and tribological performance.

Core Technical Points

Powder Delivery Mechanisms

The fundamental distinction between in-gun and out-gun powder feeding lies in the trajectory and mixing dynamics of the alloy powder relative to the arc plasma. In the in-gun configuration, powder is introduced directly through the torch nozzle, resulting in intimate contact with the arc zone. This promotes more complete melting, higher alloy transfer efficiency, and superior dilution control. The out-gun method introduces powder from an external angle, which can lead to incomplete melting, uneven distribution, and higher powder loss.

Microstructural Analysis

The authors employed metallographic microscopy and X-ray diffraction (XRD) to characterize the overlay layer. Key findings include:

• In-gun powder feeding produced a more homogeneous microstructure with uniform carbide distribution
• The Cr-Ti-Mn-B system forms hard carbides (Cr7C3, Cr23C6, TiC, TiB2) that are critical for wear resistance
• Out-gun feeding resulted in localized segregation of alloying elements, leading to microstructural inhomogeneity

Hardness and Wear Performance

The surface microhardness being slightly lower than the mid-layer value (730 vs. 811 HV) is an interesting observation. This suggests that the mid-layer experienced more complete solidification with finer grain structure, while the surface layer may have undergone slight oxidation or had slightly different cooling rates.

Engineering Practice Integration

From a practical standpoint, this research has significant implications for the overlay welding of wear parts in mining, cement, and power generation industries. The in-gun powder feeding method is particularly advantageous when:

1. High alloy transfer efficiency is required to minimize material costs
2. Consistent microstructural properties across the overlay thickness are critical for predictable service life
3. The overlay is subjected to severe abrasive wear conditions where carbide distribution uniformity directly impacts performance

However, the out-gun method retains advantages in certain applications where the geometry of the workpiece makes internal gun access difficult, or where a slightly lower but still adequate hardness level is acceptable at reduced equipment complexity.

Key Reflections

The finding that the mid-layer hardness (811 HV) exceeds the surface hardness (730 HV) warrants further investigation. In practice, the surface layer bears the brunt of abrasive contact, so one would ideally expect the highest hardness at the surface. This gradient may be related to the cooling rate differential between the surface (exposed to atmosphere, faster cooling) and the interior (insulated by subsequent passes, slower cooling). For engineering applications, understanding this gradient is essential for predicting surface wear initiation and fatigue crack nucleation.

The Cr-Ti-Mn-B system represents a well-established wear-resistant composition, but the study's contribution lies in demonstrating that powder delivery geometry is a critical process parameter that can significantly alter the final performance. This reinforces the principle that overlay welding quality is not solely determined by consumable chemistry but also by the process parameters governing how that chemistry is delivered to the weld pool.

Study Insights

This research underscores a frequently overlooked aspect of overlay welding process optimization: the powder delivery configuration. Engineers often focus on wire/powder composition and arc parameters while neglecting the geometric aspects of powder introduction. The demonstrated improvement in microstructural uniformity and alloy utilization from in-gun feeding suggests that process development programs should systematically evaluate powder delivery geometry as a primary variable. For manufacturers of overlay welding equipment, this study provides justification for investing in precision in-gun powder feed systems as a value-added feature.