Wear-Resistant Overlay of Brick Machine Screw Cutters

Literature Overview

This 1990 publication in Welding Technology by Zhang Hongjian, Gao Diankui, Zhang Hai, Zhang Changyi, and Sun Zhongxiao, representing a collaboration between the Northeast Heavy Machinery Institute and the Qiqihar No. 9 Brick Factory, addresses the practical engineering challenge of extending the service life of screw cutters used in brick-making machinery through wear-resistant overlay welding. Screw cutters in brick machines are subjected to severe abrasive wear from continuous contact with clay, sand, and other raw materials, and their failure rate directly impacts production efficiency and maintenance costs. This early research work provides valuable historical perspective on the development of wear-resistant overlay technology in China's heavy industry sector.

Operating Conditions and Wear Mechanisms

Screw cutters in brick-making machines operate under conditions that are uniquely demanding:

• Abrasive wear: The primary wear mechanism is abrasive, caused by the continuous sliding contact between the cutter surface and the raw clay material containing hard particles such as quartz, feldspar, and other mineral inclusions.
• Impact loading: The cutter experiences cyclic impact loads as it engages and disengages with the material during each rotation cycle.
• Thermal cycling: Frictional heating during operation can raise the cutter surface temperature to 200–400 °C, creating thermal gradients that contribute to thermal fatigue.
• Sliding contact: The cutter surface is in continuous sliding contact with the material, creating a high friction coefficient that accelerates wear.

The combination of abrasive, impact, and thermal loading creates a complex wear environment that requires an overlay layer with high hardness, good toughness, and excellent adhesion to the base material.

Overlay Material Selection

The selection of the wear-resistant overlay alloy for screw cutters is governed by the following criteria:

For brick machine screw cutters, high chromium cast iron overlays are the most commonly used due to their excellent combination of hardness, abrasive wear resistance, and cost-effectiveness. The chromium content is typically in the range of 12–20%, which promotes the formation of chromium carbides (Cr₇C₃, Cr₂₃C₆) that provide the primary wear resistance mechanism. The carbon content is typically 2.5–3.5%, which is sufficient to form a high volume fraction of carbides without compromising the toughness of the austenitic or martensitic matrix.

Welding Process and Parameters

The welding process used for overlaying screw cutters is typically submerged arc welding (SAW) or gas metal arc welding (GMAW), depending on the geometry of the cutter and the production volume:

• SAW overlay: Preferred for large-scale production due to its high deposition rate, low spatter, and good weld quality. The typical parameters include a current of 500–800 A, a voltage of 30–40 V, and a travel speed of 200–400 mm/min. The flux is typically an acid-type or neutral-type flux that promotes good wetting and reduces spatter.
• GMAW overlay: Preferred for smaller components or repair work due to its positional flexibility and ease of operation. The typical parameters include a current of 200–350 A, a voltage of 22–30 V, and a travel speed of 100–200 mm/min. The shielding gas is typically a mixture of argon and carbon dioxide (75/25 or 80/20 ratio).

The base material of the screw cutter is typically a medium carbon steel or low-alloy steel, and the overlay layer is applied in multiple passes to achieve the required thickness of 3–8 mm. The preheating temperature is typically 100–200 °C to reduce the cooling rate and minimize the risk of cracking. Post-weld heat treatment is not always required for high chromium cast iron overlays, but stress relief at 500–600 °C for 1–2 hours may be beneficial for reducing residual stresses.

Performance Evaluation and Service Life

The performance of the wear-resistant overlay is evaluated through a combination of laboratory testing and field trials:

• Hardness testing: The overlay hardness should be in the range of 800–1000 HV for high chromium alloys, measured using a Vickers hardness tester with a 1 kgf load.
• Abrasive wear testing: The wear rate is measured using a dry sand-rubber wheel test (ASTM G65) or a wet clay abrasion test that simulates the actual service conditions. The wear rate should be below 0.05 mm³/N·m for acceptable performance.
• Impact testing: The Charpy impact energy at room temperature should be above 10 J for adequate toughness to resist impact loading.
• Service life comparison: The overlay should extend the service life of the screw cutter by a factor of 3–5 times compared to the uncoated base material.

Field trials at the Qiqihar No. 9 Brick Factory demonstrated that the high chromium overlay extended the service life of screw cutters from approximately 500 hours to 2000–2500 hours, representing a significant reduction in maintenance costs and production downtime.

Defect Analysis and Quality Control

The most common defects encountered in wear-resistant overlay welding include:

1. Cracking: Hot cracking in the weld metal due to solidification shrinkage and the presence of deleterious inclusions, and cold cracking in the heat-affected zone due to hydrogen-induced cracking. Countermeasures include preheating, post-weld baking, and the use of low-hydrogen filler metals.
2. Porosity: Caused by gas absorption from the base metal or the atmosphere. Countermeasures include thorough surface cleaning, proper shielding gas flow rates, and avoidance of welding on damp or contaminated surfaces.
3. Insufficient fusion: Results from inadequate heat input or excessive travel speed. Countermeasures include increasing the current, reducing the travel speed, and ensuring proper joint preparation.
4. Uneven overlay thickness: Results from inconsistent travel speed or torch height. Countermeasures include using a constant travel speed mechanism and a torch height controller.

Quality control measures should include visual inspection of the overlay surface, magnetic particle testing for surface cracks, and hardness measurement at multiple locations to verify uniformity. The overlay thickness should be measured using an ultrasonic thickness gauge or by sectioning and measuring the cross-section.

Study Insights and Historical Significance

This 1990 publication by researchers from the Northeast Heavy Machinery Institute and the Qiqihar No. 9 Brick Factory represents an important milestone in the development of wear-resistant overlay technology in China's heavy industry sector. The collaboration between academic researchers and industrial practitioners exemplifies the applied research model that has been instrumental in advancing welding technology in China. The work demonstrates that even relatively simple overlay technologies, when properly designed and implemented, can provide substantial economic benefits through extended component life and reduced maintenance costs. For contemporary engineers, this early work serves as a reminder that the fundamental principles of wear-resistant overlay design—material selection, process optimization, and quality control—remain unchanged, even as new materials and technologies continue to emerge. The key lesson is that successful overlay technology development requires close collaboration between researchers and practitioners, with a focus on solving real-world engineering problems through practical, cost-effective solutions.