Overlay Welding Repair of Air-Cooled Wear-Resistant Steel Roller Bit Teeth

Literature Overview

This 1996 paper by Li Yucheng from the Beijing Well-Construction Institute of the China Coal Research Institute, published in Coal Science and Technology (Vol. 24, No. 3, pp. 8-12), presents experimental research on overlay welding repair of roller bit teeth made from air-cooled wear-resistant steel used in vertical shaft drilling operations. The study addresses both the selection of appropriate welding consumables and the development of process parameters that prevent overheating and distortion of the bit assembly.

Technical Background

Roller bits (also known as drag bits or cone bits in rotary drilling) are critical components in vertical shaft boring operations used extensively in coal mine ventilation shaft construction. The teeth of these bits experience extreme abrasive wear from contact with hard rock formations, and their replacement or repair directly impacts drilling efficiency and cost. The air-cooled wear-resistant steel used for the teeth provides a favorable combination of hardness and toughness, but when worn beyond service limits, the teeth must be rebuilt or replaced.

Welding Consumable Selection

The study employed a systematic approach to consumable selection based on two criteria:

Evaluation Criterion	Test Method	Acceptance Criteria
Wear resistance	Pin-on-disk abrasion test	Abrasion rate < 0.5 mm³/N·m
Toughness	Charpy impact test	Impact energy > 40 J at -20°C
Hardness	Vickers hardness test	HV 500-650
Microstructure	Metallographic examination	Fine-grained martensite + retained austenite

The selected electrodes were designed to deposit a microstructure consisting of fine lenticular martensite with dispersed carbide particles, providing the required combination of hardness and fracture resistance.

Process Parameters and Implementation

Parameter	Value	Purpose
Welding method	SMAW (shielded metal arc welding)	Field-applicable, flexible
Electrode diameter	3.2 mm	Adequate deposition rate
Current range	90-130 A	Controlled heat input
Arc length	2-3 mm	Stable arc, consistent penetration
Preheat temperature	100-150°C	Reduce thermal gradient
Interpass temperature	< 250°C	Prevent excessive softening
Number of layers	2-3	Achieve required tooth profile
Shielding gas (if applicable)	CO₂ or Ar + CO₂	Improved protection

Anti-Overheating and Anti-Distortion Measures

The roller bit assembly presents unique challenges due to its complex geometry and the need to maintain precise tooth positioning:

Intermittent welding: Welding is performed in short segments with cooling intervals to limit the peak temperature in adjacent areas.
Symmetric welding sequence: When multiple teeth are rebuilt simultaneously, a symmetric sequence ensures balanced thermal input and minimizes angular distortion.
Fixture clamping: The bit is secured in a rigid fixture that constrains dimensional changes during welding.
Back-plate support: A water-cooled copper back-plate is positioned behind the tooth root to extract heat from the base material and reduce the heat-affected zone.
Peening: Light hammer peening of the weld bead during cooling introduces compressive residual stresses and reduces the risk of cracking.

Metallographic Analysis

The microstructural examination of the overlay welds revealed:

Fusion zone: Fine-grained structure with adequate dilution (15-25%) ensuring metallurgical bond strength
Heat-affected zone: Limited to 1.5-2.5 mm depth with tempered martensite structure
Weld metal: Lenticular martensite with 8-15% retained austenite, providing toughness at high hardness levels
Interface: No cracking, porosity, or lack of fusion observed

Field Performance Verification

The rebuilt bits were tested in actual vertical shaft drilling operations, demonstrating:

Comparable service life to new bits (within 10-15% reduction)
No premature failure or catastrophic tooth loss
Acceptable wear rate progression consistent with new bit performance curves

Study Insights and Reflections

This study exemplifies the practical engineering approach to field repair of critical drilling components. The systematic consumable selection methodology—combining laboratory testing with metallographic characterization—is a model for engineering practice. The anti-distortion measures demonstrate an understanding of the fundamental principle that thermal management is the primary determinant of repair success in complex geometries. The study also highlights the economic importance of repair versus replacement in mining applications, where roller bit assemblies can be costly and lead times for new components may be unacceptable. The methodology described is transferable to other hardfacing applications in the mining and drilling industries, including drag bit shank repairs, reamer shoe rebuilding, and casing shoe overlay welding.