Wear-Resistant Overlay Welding of Concrete Pump Truck Lower Shell in Construction Machinery

Application Background and Wear Mechanism Analysis

The lower shell (also referred to as the S-tube or delivery tube lower section) of a concrete transport pump truck is a critical wear component that guides the flow of concrete slurry from the pump cylinders to the discharge hose. This component is subjected to extremely severe abrasive wear due to the continuous passage of concrete, which contains aggregate particles (sand, gravel, and crushed stone) with hardness up to Mohs 7 (quartz). The wear mechanism is a combination of abrasive wear from hard aggregate particles, erosive wear from high-velocity concrete flow, and adhesive wear from the sticky cement paste. In typical service conditions, the lower shell can lose significant material thickness within a few thousand operating hours, leading to reduced flow efficiency, increased pumping pressure, and eventual structural failure.

The research published by Yang Zhaobin in 1996 at the Tai'an Hoisting Machinery Factory addressed the challenge of extending the service life of this component through wear-resistant overlay welding. The concrete pump truck industry in China was rapidly expanding in the 1990s, and the availability of wear-resistant overlay solutions was critical to reducing maintenance costs and improving equipment uptime.

Overlay Material Selection and Metallurgical Design

The selection of overlay material for the concrete pump truck lower shell is driven by the specific wear mechanism. Since the dominant wear mechanism is abrasive wear from hard aggregate particles, the overlay material must contain hard phases (carbides, oxides, or borides) that are harder than the wear particles, embedded in a ductile matrix that can accommodate plastic deformation without cracking.

For the concrete pump truck lower shell, the most commonly used overlay materials are high chromium cast iron (Cr15 or Cr20) or nickel-chromium-carbide composite alloys. The high chromium cast iron provides the highest wear resistance due to the abundant chromium carbide particles, but it has limited toughness and is prone to cracking if the thermal cycle is not carefully controlled. The nickel-chromium-carbide composite offers a better balance of wear resistance and toughness, making it more suitable for the cyclic loading conditions experienced by the pump truck shell.

The metallurgical design of the overlay must also consider the base material. The lower shell is typically fabricated from carbon steel (Q235 or Q345) or low-alloy steel plate. The weldability of the base material is generally good, but the large thickness of the shell plate (typically 12-20 mm) means that the thermal mass is significant, which can be leveraged to moderate the cooling rate and reduce the risk of cracking in the overlay deposit.

Welding Process and Technique

The overlay welding of the concrete pump truck lower shell is typically performed using submerged arc welding (SAW) or flux-cored arc welding (FCAW) for the bulk of the overlay, with gas tungsten arc welding (GTAW) or gas metal arc welding (GMAW) for the first pass (root pass) to ensure a clean bond with the base material.

The overlay is typically applied in multiple passes. The first pass (root pass) uses a compatible filler metal that ensures a clean, crack-free bond with the base material. Subsequent passes use the wear-resistant overlay material. The number of passes depends on the required overlay thickness, which is typically 3-8 mm for the lower shell. The overlay should be applied in a pattern that ensures uniform coverage and minimizes distortion, with each pass overlapping the previous pass by 50-70% to ensure complete fusion and uniform hardness.

Defect Analysis and Prevention

The most common defects in overlay welding of the concrete pump truck lower shell include cracking, porosity, lack of fusion, and excessive dilution. Each defect has distinct causes and countermeasures:

Cracking is the most serious defect because it can lead to premature failure of the overlay in service. The risk of cracking is particularly high when using high chromium cast iron overlay materials, which have a high carbon equivalent and are prone to forming brittle martensite during cooling. The countermeasures include preheating the base material to 150-200 °C, maintaining a low interpass temperature, using a hydrogen-free filler metal, and applying post-weld stress relief at 550-600 °C. In some cases, a transition layer of austenitic stainless steel (such as E309L) is applied between the base material and the wear-resistant overlay to reduce the risk of cracking.

Performance Verification and Service Life Assessment

The performance of the overlay weld is verified through hardness testing, wear testing, and in some cases, actual service trials. The hardness of the overlay deposit should be uniform across the surface, with no soft spots that could indicate excessive dilution or incomplete melting of the hard phases. The wear resistance is typically evaluated using a pin-on-disk or block-on-ring wear test, where the overlay surface is tested against a counterface of hardened steel or abrasive slurry.

In service, the overlay on the concrete pump truck lower shell can extend the service life by 3-10 times compared to the uncoated base material, depending on the overlay material, process quality, and operating conditions. The key to achieving maximum service life is not only the overlay material selection but also the process quality—ensuring a clean, crack-free, uniformly hard overlay with good bond strength to the base material.

Study Insights and Engineering Implications

The study of overlay welding for the concrete pump truck lower shell, though published in 1996, highlights several enduring engineering principles. First, the overlay material must be matched to the specific wear mechanism; a material that is hard but brittle will fail prematurely under cyclic loading, while a material that is tough but soft will wear quickly. Second, the process quality is as important as the material selection; a poorly executed overlay can fail even with the best material. Third, the economic benefit of overlay welding is substantial for components that experience severe wear but do not require complete replacement; the cost of overlay welding is typically 10-30% of the cost of a new component, with a service life extension of 3-10 times.

This work also reflects the practical engineering approach of Chinese construction machinery manufacturers in the 1990s, where the focus was on developing cost-effective maintenance solutions for rapidly expanding equipment fleets. The principles established in this work—material matching, process control, defect prevention, and performance verification—remain the foundation of modern overlay welding practice in construction machinery, mining equipment, and other heavy wear applications.