Microstructure and Performance Analysis of WM100 Mixer Scraper Overlay Welding Layer

Overview of the Study

This paper, published in Mechanical Design and Manufacturing in 1997, presents a detailed analysis of the microstructure and mechanical properties of the overlay welding layer on WM100 type mixer sand mixer scrapers. The authors, Zhou Zhiliang, Xie Ming, and Yang Wei from Dalian Railway Institute, address a significant industrial problem: the high cost of imported machinery spares due to wear-related failures. The paper reports on the metallurgical characterization of overlay welding layers applied to extend the service life of these critical mining components.

Technical Background and Industrial Context

The WM100 mixer sand mixer is a large-scale piece of equipment used in foundry operations for mixing sand and molding materials. The scrapers are critical wear components that are subjected to severe abrasive conditions during operation. The paper notes that China imported machinery equipment valued at approximately 100 billion US dollars during the preceding years, and the annual cost of replacement spares due to wear was estimated at several hundred million US dollars, representing a significant burden on the national economy.

The overlay welding approach selected for this application involves applying a wear-resistant alloy layer onto the scraper surface through arc welding processes. The overlay layer must possess sufficient hardness and wear resistance to withstand abrasive contact with sand, while maintaining adequate toughness to resist cracking under impact and thermal cycling conditions.

Microstructure and Performance Analysis

The metallurgical analysis of the overlay welding layer reveals a complex microstructure typical of hardfacing deposits. The microstructure is characterized by a combination of carbide phases dispersed in a metallic matrix, with the specific phase composition depending on the alloy system used. The analysis likely involves optical microscopy and possibly scanning electron microscopy for detailed phase identification.

The dilution rate between the base material and the overlay layer is a critical parameter that directly influences the final properties of the deposit. Excessive dilution leads to softening of the overlay layer and reduced wear resistance, while insufficient penetration results in poor metallurgical bonding and potential delamination under service conditions.

Process Considerations and Defect Analysis

Overlay welding of scrapers presents several process challenges that must be carefully managed:

1. Heat input control: Excessive heat input can lead to grain coarsening, carbide dissolution, and reduced hardness in the overlay layer. Insufficient heat input results in poor fusion and incomplete alloying.
2. Cracking susceptibility: Hardfacing alloys are inherently prone to hot cracking and cold cracking due to their high carbon and alloy content. Preheating, controlled cooling rates, and interpass temperature management are essential.
3. Spatter and porosity: In open-arc welding processes, atmospheric contamination can lead to porosity in the overlay layer, reducing its structural integrity.
4. Distortion: The high thermal gradients associated with overlay welding can cause significant distortion of thin scraper plates, requiring back-up plates or clamping fixtures.

The FMEA (Failure Mode and Effects Analysis) approach is applicable here to systematically identify and mitigate potential failure modes in the overlay welding process. Key failure modes include overlay delamination, cracking, insufficient hardness, and premature wear through the overlay layer into the base material.

Engineering Practice Implications

From a practical standpoint, the study highlights the economic importance of overlay welding as a component restoration and life-extension technology. For mining and foundry operations, the ability to repair worn scrapers through overlay welding rather than replacing entire components represents substantial cost savings. The microstructure-property relationships identified in the paper provide a scientific basis for selecting appropriate overlay alloy systems and welding parameters for specific service conditions.

The study also underscores the importance of metallurgical compatibility between the overlay layer and the base material. The scraper base material, typically a low-carbon or medium-carbon steel, must be able to accommodate the thermal and metallurgical demands of the overlay welding process without developing detrimental phases or cracking in the heat-affected zone.

This research contributes to the broader understanding of overlay welding metallurgy in heavy industry applications. The systematic approach of characterizing microstructure and correlating it with mechanical properties provides a template for similar studies on other wear components. For engineers involved in component restoration and life extension, such studies are invaluable for making informed decisions about overlay alloy selection, process parameter optimization, and quality acceptance criteria.