Strengthening Technology of Cast Tungsten Carbide Iron-Based Composite Material Weld Overlay Layer on Mill Shoes

Literature Overview

This study by Liu Chengjie, Qiu Yaling, Song Zhenhua, Zhuang Jia, and Liu Qingyou (2007) from Southwest Petroleum University and Chengdu General Machinery Factory investigates the strengthening of cast tungsten carbide iron-based composite material weld overlay layers used on mill shoes (also known as shoe shoes or bottom shoes) in oil well drilling operations. Mill shoes are critical downhole tools that protect the drill pipe from wear and damage during drilling operations, particularly in high-wear formations.

Core Technical Points

Mill shoes are subjected to extreme wear conditions during drilling operations, including:

• Sliding contact with the wellbore wall at high velocity
• Abrasive contact with rock cuttings and drilling mud solids
• Impact loading from vibration and stick-slip phenomena
• Elevated temperatures from frictional heating and downhole environment

The traditional approach to enhancing mill shoe wear resistance involves using solid tungsten carbide (WC) components, but these are expensive and prone to catastrophic failure if chipped. The composite material approach combines a tough iron-based matrix with dispersed tungsten carbide particles, providing a balance between toughness and wear resistance.

The weld overlay strengthening technology described in this study involves depositing a tungsten carbide-containing iron-based composite material onto the surface of cast iron or steel mill shoe bodies. The overlay layer typically contains 30-60% tungsten carbide by weight, with the remainder being an iron-nickel-cobalt matrix that provides toughness and bonding capability.

Microstructure and Phase Composition

The microstructure of the tungsten carbide iron-based composite overlay consists of:

The key to the performance of this composite overlay is the uniform distribution of WC particles within the iron matrix. Poor distribution leads to localized soft spots that wear preferentially, creating uneven surfaces that accelerate further wear. The study demonstrates that the particle size distribution of the feed powder significantly affects the overlay quality.

Process Parameters for Overlay Welding

The overlay welding process parameters are critical for achieving a sound composite material deposit:

A critical process consideration is the melting behavior of WC particles. WC has a very high melting point (2870°C), which is above the melting point of most iron-based alloys. Therefore, WC particles are not fully melted during the welding process but are instead dissolved partially or completely at the interface with the molten pool. This partial dissolution creates a strong metallurgical bond between the WC particles and the iron matrix.

Wear Resistance Mechanisms

The wear resistance of the tungsten carbide composite overlay is attributed to multiple mechanisms:

1. Hardness contribution: The high hardness of WC particles (2000-2500 HV) provides resistance to abrasive wear through indentation resistance.
2. Matrix toughness: The iron-nickel-cobalt matrix provides sufficient toughness to prevent catastrophic failure and to accommodate plastic deformation without cracking.
3. Carbide network reinforcement: The secondary carbide phases (M7C3, M23C6) form a reinforcing network that impedes crack propagation.
4. Self-lubrication effect: At high sliding speeds, the iron matrix can form a thin tribolayer that reduces friction and wear rate.

The study demonstrates that the optimal WC content for mill shoe applications is in the range of 40-50% by weight. Below this range, the wear resistance is insufficient. Above this range, the overlay becomes brittle and prone to chipping and spalling.

Engineering Applications and Performance

Mill shoes with tungsten carbide composite overlays have been successfully applied in various drilling conditions:

The field performance data shows that mill shoes with tungsten carbide composite overlays can extend service life by 3 to 8 times compared to conventional steel mill shoes. This represents significant cost savings in drilling operations, as mill shoe replacement is a major downtime factor.

Quality Control and Inspection

The quality of the composite overlay is verified through the following methods:

The metallographic examination is particularly important for verifying the WC particle distribution. A non-uniform distribution can lead to premature failure, even if the average composition is correct. The study recommends examining multiple cross-sections through the overlay thickness to ensure consistent quality.

Study Insights and Reflections

The tungsten carbide iron-based composite overlay technology represents a practical solution to the wear problem in oil well drilling operations. The key engineering insight is that the optimal performance is achieved not by maximizing the WC content, but by finding the right balance between WC content, particle size, and distribution. This balance ensures sufficient wear resistance while maintaining the toughness needed to prevent catastrophic failure.

The study also highlights an important consideration for field applications: the overlay thickness must be sufficient to withstand the expected wear depth during the service life of the mill shoe. Typical overlay thicknesses range from 3 mm to 8 mm, depending on the expected service life and wear rate. Multiple passes may be required to achieve the desired thickness, with interpass temperature control being critical to maintain overlay quality.

From a cost-effectiveness perspective, the tungsten carbide composite overlay approach is significantly more economical than using solid tungsten carbide components, while providing comparable wear resistance for most drilling applications. The technology has been successfully commercialized and is now widely used in the oil and gas industry for mill shoe and other wear-critical components.