Overlay Repair of Air-Cooled Wear-Resistant Steel Auger Bit Tooth Surfaces

Literature Overview and Context

This 1996 study by Li Yucheng from the Beijing Institute of Well Construction at China Coal Research Institute addresses the overlay repair of tooth surfaces on air-cooled wear-resistant steel auger bits used in coal mining and well construction operations. Auger bits are critical cutting tools that operate under extreme abrasive conditions, and their tooth surfaces undergo rapid wear that significantly impacts drilling efficiency and operational costs. The study presents a systematic approach to extending auger bit service life through strategic overlay welding.

Technical Background on Auger Bit Applications

Air-cooled wear-resistant steel auger bits are used extensively in coal mining operations for borehole drilling, ventilation shaft construction, and water supply drilling. The auger bits typically have spiral flights with cutting teeth that engage with rock, coal, and soil materials under high rotational speeds and axial loads. The tooth surfaces are subjected to severe abrasive wear from hard rock fragments, impact loading from rock heterogeneities, and thermal cycling from frictional heating.

Auger Bit Specifications

Parameter	Typical Value
Bit diameter	150–400 mm
Flight length	2000–6000 mm
Base material	Q345 / 40Cr / 42CrMo
Original tooth hardness	35–45 HRC
Operating speed	50–200 rpm
Axial load	5–20 kN
Wear rate (original)	0.5–1.5 mm/100 m drilled
Service life (original)	500–2000 m drilled
Air cooling pressure	0.5–1.5 MPa

Overlay Material Selection and Design

The selection of overlay material for auger bit tooth repair must balance wear resistance, toughness, and weldability with the base material. Several material systems have been evaluated for this application.

Overlay Material Comparison

Material	Hardness (HRC)	Toughness (CV)	Wear Resistance	Weldability	Cost
Cr12MoV (quenched)	55–60	20–25	Excellent	Poor	Medium
H13 (quenched)	48–52	30–40	Good	Moderate	Medium
Stellite 6	40–45	25–35	Excellent	Good	High
Hardfacing (Cr-C)	55–65	10–15	Excellent	Moderate	Low
304 Stainless	25–30	50–70	Poor	Excellent	Low
D2 Tool Steel	60–62	15–20	Excellent	Poor	Medium
Ni-Cr alloy	45–50	30–40	Good	Good	High

For auger bit applications, a multi-layer approach is often optimal: a transition layer of austenitic stainless steel (304 or 309) to ensure good bonding with the base material, followed by intermediate layers of medium-hardness alloy steel (H13 or 42CrMo), and finally a surface layer of high-hardness material (Cr12MoV or hardfacing alloy).

Welding Process Selection and Parameters

The welding process must be selected based on the geometry of the tooth surface, the required overlay thickness, and the available equipment. For auger bit repair in field conditions, portable welding equipment is typically used.

Process Parameters

Process	Application	Current (A)	Voltage (V)	Travel Speed (mm/min)
SMAW (Stick)	General repair	120–180	22–28	100–200
GMAW (MIG)	Large areas	200–280	22–28	200–400
SAW (Submerged)	Flat surfaces	300–400	30–36	300–500
TIG (GTAW)	Precision repair	80–150	18–24	50–150
PTA (Plasma)	Critical surfaces	150–250	20–28	150–300

Multi-Layer Welding Strategy

Layer	Material	Thickness (mm)	Passes	Purpose
Layer 1 (Transition)	ER309L / A102	1.0–1.5