In-Situ Overlay Welding Repair of Excavator Slewing Ring Inner Gear

Literature Overview

The paper by Wang Gongke (2000), published in Road Construction Machinery and Construction Mechanization (Vol. 17, No. 3, pp. 18-19), documents an in-situ overlay welding repair of an excavator slewing ring inner gear. This case study is particularly instructive because it addresses the practical challenges of field repair on large, heavy-duty components where disassembly is impractical or economically prohibitive.

Component Description and Failure Mode

The excavator slewing ring is a critical structural component that transfers loads between the upper and lower structures while permitting 360° rotation. The inner gear engages with a pinion to drive rotation. Key characteristics:

Parameter	Typical Specification
Outer diameter	1500-3000 mm
Ring thickness	80-150 mm
Material	Medium carbon steel (40Cr, 42CrMo, or equivalent)
Surface hardness (gear teeth)	28-32 HRC
Wear allowance	2-5 mm per side
Service life (before repair)	Variable, often 2-5 years

The repair scenario involves gear tooth wear or damage requiring material restoration. In-situ repair avoids the enormous logistics of shipping the ring to a workshop or replacing the entire assembly.

Repair Process Design

FMEA Analysis of In-Situ Repair Risks

Failure Mode	Severity	Occurrence	Detection	RPN	Countermeasure
Distortion of ring geometry	9	6	8	432	Fixturing, staged welding, low heat input
Cracking in HAZ	8	5	7	280	Preheat, low-carbon consumables, PWHT
Poor fusion at repair boundary	7	4	6	168	Proper surface prep, adequate penetration
Hardness mismatch	6	5	5	150	Matched consumable selection
Residual stress leading to fatigue	8	4	9	288	Stress-relief welding sequence, post-weld treatment

Welding Procedure

Surface preparation: Grinding away damaged material to create a uniform repair area with smooth transitions (blended edges with radius ≥3 mm)
Preheat: 200-300°C applied locally using oxy-fuel torch or induction heating
Welding consumables: Low-hydrogen electrodes (E5015, E5016) or flux-cored wire for deposit layers; hardfacing alloy (e.g., Ni-based or Cr-C based) for final surface
Weld sequence: Alternating opposite-side welding to minimize distortion; building up material in multiple thin layers (≤3 mm per pass)
Interpass temperature control: ≤300°C to prevent grain coarsening and excessive softening
Post-weld treatment: Localized stress relief heating at 550-600°C followed by controlled cooling

Key Technical Considerations

Geometric Accuracy

The slewing ring gear must maintain precise geometry after repair:

Runout tolerance: ≤0.1 mm TIR for the gear face
Tooth profile accuracy: must meet original specification
Surface finish: Ra ≤ 3.2 μm after machining

Post-weld machining (grinding or milling) is typically required to restore dimensional accuracy, which necessitates that the weld deposit include sufficient oversize (typically 3-5 mm above final dimension).

Material Compatibility

The base metal is typically a quenched-and-tempered medium carbon alloy steel. The repair weld metal must:

Match or slightly exceed the base metal strength
Have compatible thermal expansion coefficients
Resist cracking during cooling (low carbon equivalent, CE ≤ 0.45)
Provide adequate toughness at service temperature

Engineering Lessons

This case illustrates several important principles for field welding repairs:

In-situ repair is feasible for large components when proper procedures are followed
Distortion control is the primary challenge for ring-shaped components—symmetric welding sequences are essential
The repair must be designed to restore not only material volume but also mechanical properties and dimensional accuracy
Post-repair inspection should include dimensional checks, hardness verification, and NDT (MT for surface cracks, UT for subsurface defects)

Summary

The in-situ overlay repair of the excavator slewing ring demonstrates that field welding, when properly planned and executed, can restore critical components to serviceable condition. The key to success lies in careful procedure design, distortion management through welding sequence optimization, and comprehensive post-repair verification. This approach saves significant cost and downtime compared to component replacement, making it a valuable technique in maintenance engineering.