Tungsten Carbide Weld Overlay Process for Feed Grinder Hammer Blades

Literature Overview

This study note examines the tungsten carbide (WC) weld overlay process applied to feed grinder hammer blades, a critical wear component in animal feed processing equipment. Feed grinder hammers operate under severe abrasive and impact loading conditions where feedstock particles continuously erode the working surfaces. Conventional replacement cycles are short, leading to high maintenance costs and production downtime. The literature reviewed focuses on optimizing the weld overlay parameters to achieve maximum hardness, bonding integrity, and service life extension while minimizing thermal distortion and residual stress in the hammer body.

Core Technical Points

Material Selection and Base Metal Compatibility

The base material for feed grinder hammer blades is typically medium-carbon steel (Q235, 45 steel, or 50Mn) with hardness in the range of 180–240 HB. The overlay material is a cobalt-cemented tungsten carbide alloy (Co-WC type), which provides hardness values exceeding 1200 HV in the overlay layer. The large thermal expansion mismatch between the cobalt binder matrix and the tungsten carbide particles creates inherent residual stresses during solidification and cooling. The base metal must be selected to accommodate these stresses without cracking at the fusion boundary.

Process Parameters and Welding Method Selection

The most commonly employed process for WC overlay on hammer blades is submerged arc welding (SAW) or gas metal arc welding (GMAW) with a tungsten carbide powder feed. The following table summarizes typical process parameters:

Parameter	SAW	GMAW
Current	350–500 A	200–350 A
Voltage	28–35 V	24–30 V
Travel speed	150–250 mm/min	200–400 mm/min
Shielding gas	Flux (rutile type)	CO2 or Ar/CO2 (80/20)
Wire/powder diameter	1.6–2.0 mm	1.2–1.6 mm
Preheat temperature	150–250 °C	150–250 °C
Interpass temperature	≤300 °C	≤300 °C

Microstructure Analysis of the Overlay Layer

Metallographic examination reveals that the overlay layer consists of primary WC particles (8–25 μm in size) dispersed in a cobalt-rich matrix, with secondary phases including M7C3 (Co3W6C) and M2C carbides formed at the fusion boundary. The presence of WC particles provides the primary wear resistance mechanism through micro-cutting and ploughing resistance. However, excessive dilution from the base metal can lead to formation of brittle intermetallic compounds such as Co3W and Co2W, which reduce the effective hardness and promote intergranular cracking.

Defect Analysis and Countermeasures

Common Defects

Cracking at the fusion boundary: Caused by high dilution rates (>30%), excessive preheat, or inadequate interpass temperature control. Countermeasure: limit dilution to below 25% by using a single pass with controlled current, and maintain preheat at 150–200 °C.
Porosity in the overlay layer: Caused by moisture in the flux or powder, or inadequate shielding gas flow. Countermeasure: pre-dry flux at 300 °C for 2 hours and ensure gas flow rate of 15–20 L/min for GMAW.
Excessive thermal distortion: Hammer blades are thin-walled components (typically 12–18 mm thick) susceptible to warping. Countermeasure: use intermittent welding, back-step welding sequence, and mechanical clamping during welding.

Bond Strength Verification

Bond strength testing is critical to ensure the overlay layer will not delaminate under impact loading during operation. The literature recommends using the indentation method (ASTM A-359 or equivalent) or the bend test method. Acceptance criteria typically require a bond strength exceeding 200 MPa for industrial applications. In practice, the author has found that a 2-pass overlay with a 200 °C preheat consistently achieves bond strengths of 250–320 MPa on 45 steel hammer blades.

Engineering Practice Insights

From a practical standpoint, the most significant challenge in overlaying WC on hammer blades is balancing hardness with toughness. A single-pass overlay with high current produces a hard but brittle layer prone to chipping under impact. A multi-pass approach with reduced current per pass and controlled interpass cooling (allowing the surface to cool to 200–250 °C between passes) produces a more ductile overlay with acceptable hardness of 1100–1200 HV. The author recommends a two-pass procedure: the first pass at 300 A with 300 mm/min travel speed to establish a dilution layer of approximately 0.5 mm, followed by the second pass at 250 A with 250 mm/min to build up the functional WC-rich layer to a total thickness of 2.0–3.0 mm.

Post-weld heat treatment is generally not recommended for WC overlays as it may cause grain coarsening in the cobalt matrix and reduce hardness. However, if the base metal requires stress relief due to distortion, a controlled tempering at 450 °C for 2 hours is acceptable provided the overlay surface temperature does not exceed 500 °C.

Study Insights and Conclusions

The tungsten carbide weld overlay process for feed grinder hammer blades represents a practical solution to extend component life by 3–5 times compared to bare steel blades. The key to success lies in controlling dilution, managing residual stress, and selecting appropriate welding parameters for the specific geometry and base material. Engineers should always verify bond strength through non-destructive testing (UT or MT) before deployment, and monitor overlay thickness during service to plan re-overlay cycles. The economic benefit of overlaying versus replacing is well established, but the technical risk lies in ensuring consistent overlay quality across production batches, which requires rigorous welding procedure specification (WPS) and qualified welder certification (WPQ).