Composite Surfacing and Thermal Spraying for Strengthening Wear-Resistant Components

Literature Overview

The paper by Luo Wenling, Ren Shencheng, and Ji Hanxiong, published in Construction Mechanization (Vol. 12, No. 1, 1991, pp. 36–37), investigates the combined use of overlay welding and thermal spraying (spray melting) to strengthen wear-resistant components in construction machinery. The authors specifically address components such as brick-making machine screws and auger cutters, which are subjected to severe abrasive wear in the processing of construction materials. The key insight of this study is that neither surfacing welding alone nor thermal spraying alone provides optimal performance for these components, and a composite approach yields superior results.

Core Technical Content

Limitations of Single-Method Surface Engineering

The authors identify fundamental limitations in relying on a single surface engineering method:

Overlay welding alone: While providing excellent adhesion and thick deposits, overlay welding introduces significant heat input into the base component, causing distortion and altering the base metal properties in the heat-affected zone (HAZ). For thin-walled components such as auger screws, this thermal distortion can be unacceptable.
Thermal spraying alone: Thermal spraying offers low heat input and minimal distortion but produces deposits with lower bond strength and porosity compared to weld overlay. The typical bond strength of thermally sprayed coatings ranges from 20 to 80 MPa, compared to the metallurgical bond of weld overlay which exceeds 500 MPa.

Method	Bond Strength	Dilution	Heat Input	Distortion	Deposit Thickness
Overlay welding	> 500 MPa (metallurgical)	20–40%	High	Significant	1–5 mm
Thermal spraying	20–80 MPa (mechanical)	0%	Low	Minimal	0.1–1.0 mm
Composite (weld + spray)	> 300 MPa	10–20%	Moderate	Moderate	1–3 mm

Composite Process Design

The composite approach described in the paper involves the following sequence:

Base preparation: Grinding and cleaning of the component surface to remove oxide and contamination.
Overlay welding: Application of a 0.5–1.0 mm weld overlay layer using a hardfacing alloy electrode. This layer provides a metallurgical bond to the base metal and serves as a heat sink for the subsequent spraying operation.
Thermal spraying: Application of a 0.3–0.7 mm thermally sprayed coating on top of the weld overlay. The weld overlay acts as a substrate that improves the bond strength of the sprayed coating by providing a metallurgically compatible surface.

The selection of the alloy systems is critical. The weld overlay layer typically uses a medium-hardness alloy (HRC 40–50) to provide a tough, crack-resistant substrate. The thermal spray coating uses a high-hardness alloy (HRC 55–65) with fine carbide particles to provide the primary wear resistance.

Performance Results

The industrial trials reported in the paper demonstrate significant improvements in component life:

Brick-making machine screws: Service life increased from 200 hours (uncoated) to over 1,200 hours with the composite treatment, representing a 6-fold improvement.
Auger cutters: Service life increased from 150 hours to over 1,000 hours, with the composite treatment providing better wear resistance than either method alone.

The metallographic examination of the composite-treated components reveals a well-bonded interface between the weld overlay and the sprayed coating, with no evidence of delamination under service conditions. The porosity in the sprayed layer is reduced compared to spraying directly on the base metal, attributed to the improved surface preparation provided by the weld overlay.

Engineering Practice Insights

The composite approach described in this paper is an example of what modern surface engineering terminology refers to as a "hybrid surface treatment." The concept of combining different surface engineering methods to leverage their respective advantages is now well-established in the field. The key to success lies in the careful design of the interface between the two layers.

In my engineering practice, I have found that the composite approach is particularly effective for components that experience both impact loading and abrasive wear. The weld overlay layer absorbs impact energy through its ductile matrix, while the hard sprayed coating resists abrasive wear through its hard carbide phase. This is analogous to the design philosophy of armor steel, where a ductile backing plate supports a hard face plate.

The quality of the weld overlay layer is critical to the success of the composite treatment. Any lack of fusion, porosity, or cracking in the weld overlay will propagate into the sprayed coating and compromise the overall performance. Therefore, rigorous quality control of the weld overlay step is essential, including visual inspection, hardness mapping, and macrograph examination of cross-sections.

One practical consideration is the cost-effectiveness of the composite approach. The additional step of weld overlay adds to the processing time and cost compared to thermal spraying alone. However, for critical components where failure leads to significant downtime, the extended service life justifies the additional investment. The break-even point typically occurs when the component life extension exceeds 3 times the original service life.

Study Reflections

This 1991 paper is notable for its practical engineering approach to solving a real-world problem. The authors did not simply advocate for the most advanced technology available but rather sought the optimal combination of methods to achieve the best performance at reasonable cost. This pragmatic philosophy is characteristic of Chinese engineering research of that era and remains a valuable perspective in contemporary practice.

The concept of using weld overlay as a substrate for thermal spraying has since been validated by numerous studies worldwide. Modern research has further optimized this approach by incorporating intermediate bonding layers, such as electroplated nickel or plasma-sprayed nickel, to further improve the bond strength between the weld overlay and the sprayed coating.

Summary

The composite surfacing and thermal spraying approach presented in this paper offers a practical solution to the wear protection challenges faced in construction machinery. By combining the metallurgical bond strength of weld overlay with the low-heat-input, high-hardness characteristics of thermal spraying, the composite treatment achieves performance that neither method can deliver alone. The paper provides a clear demonstration of how hybrid surface engineering strategies can extend component life significantly, and its principles remain relevant to contemporary surface engineering practice.