Cladding Repair Process for Ball Mill Journal Bearings

Literature Overview

This 1994 technical paper by Wang Zhengjian from the Yongchuan Phosphate Fertilizer and Cement Plant describes the practical application of weld overlay technology for repairing the journal bearings (trunnion bearings) of ball mills used in phosphate and cement processing. Ball mill trunnion bearings are critical rotating components that support the entire mill shell weight and transmit the driving torque. Damage to these bearings—typically in the form of brinelling, spalling, or excessive wear of the bearing raceway or housing—can cause catastrophic mill shutdowns, making rapid and reliable repair essential for production continuity.

Technical Background and Failure Analysis

Ball mill journal bearings operate under extreme conditions: high radial loads (often exceeding 100 tonnes), continuous rotation, and exposure to abrasive cement or phosphate dust. The typical failure modes include:

Failure Mode	Root Cause	Typical Location
Brinelling	Excessive static loading or vibration	Raceway surface
Spalling	Fatigue cracking beneath surface	Inner ring raceway
Housing wear	Misalignment or lubrication failure	Bearing seat bore
Corrosion pitting	Moisture ingress in humid environments	Bearing housing

The repair approach described involves removing the damaged bearing, assessing the extent of housing damage, and applying a weld overlay to restore dimensional accuracy and surface integrity of the bearing seat before reinstalling a new bearing.

Cladding Repair Process Details

Surface Preparation

The bearing housing bore must be thoroughly prepared before overlay welding. This involves:

Complete removal of the damaged bearing using hydraulic pullers or thermal expansion methods
Inspection of the bore surface for cracks, pitting, or excessive out-of-round condition
Grinding or machining of the damaged area to remove all affected material, creating a clean, flat weld preparation surface
Application of a nickel-based or iron-based surfacing layer to improve weldability and reduce dilution

Welding Process Selection

For ball mill bearing repair, the preferred welding processes are submerged arc welding (SAW) for building up large areas and gas metal arc welding (GMAW) or flux-cored arc welding (FCAW) for finishing and contouring. The selection depends on the extent of damage and the required dimensional accuracy.

Process	Application	Advantages	Limitations
SAW	Building up large worn areas	High deposition rate, low dilution	Requires accessible geometry
GMAW	Finishing passes, contouring	Good control, versatile	Higher dilution
FCAW	Intermediate builds	Good penetration, weather-resistant	Consumable cost
GTAW	Precision repair, thin sections	Excellent control, low heat input	Low deposition rate

Consumable Selection

The overlay consumables must be selected based on the bearing housing material (typically medium carbon steel such as Q345 or equivalent) and the operating conditions. Common choices include:

Nickel-based surfacing: Ni-Cr or Ni-Cr-Mo alloys for excellent weldability and reduced cracking tendency on carbon steel substrates
Iron-based surfacing: Fe-Cr-Mo or Fe-Ni-Cr alloys for high hardness and wear resistance
Stainless steel surfacing: 309L or 310L for corrosion resistance in humid environments

The critical requirement is achieving a deposit with hardness of at least HRC 35–45 to resist bearing housing wear while maintaining sufficient toughness to avoid cracking during thermal cycling.

Process Parameters and Quality Control

Welding Parameters

Typical welding parameters for ball mill bearing repair include:

Parameter	SAW	GMAW	FCAW
Current	350–500 A	200–300 A	250–400 A
Voltage	30–38 V	24–30 V	30–38 V
Travel speed	150–250 mm/min	200–400 mm/min	150–300 mm/min
Wire diameter	3.2 mm	1.2–1.6 mm	1.6–2.4 mm
Interpass temperature	≤ 250°C	≤ 300°C	≤ 300°C

Post-Weld Treatment

After overlay welding, the bearing seat must be machined to the required bore diameter and surface finish (typically Ra 0.8–1.6 μm). The machining process also serves to relieve residual stresses introduced during welding. A final dimensional check and hardness survey of the overlay deposit are performed before bearing installation.

Engineering Practice Insights

The practical success of this repair approach depends on several factors that are often overlooked in theoretical discussions:

Geometric accessibility: Ball mill bearing housings are often in confined spaces, requiring careful planning of electrode/wire access angles
Thermal distortion control: The massive bearing housing can develop significant thermal distortion during welding, which affects the final bore geometry
Bearing seat roundness: The overlay must be machined to achieve a true cylindrical bore with runout below 0.05 mm/m for proper bearing alignment
Residual stress management: Post-weld stress relief or controlled machining is essential to prevent delayed cracking

From a reliability standpoint, properly executed bearing seat repair can restore the component to near-original service life, extending the overall mill availability by months or even years. The economic benefit is substantial given that ball mill bearing replacement can require days of downtime and significant material costs.

Summary and Reflections

This paper exemplifies the practical application of overlay welding technology in heavy industrial maintenance. The ball mill journal bearing repair case demonstrates how weld overlay serves as an effective alternative to component replacement, reducing downtime, material costs, and environmental impact. The key technical insight is that successful repair requires careful integration of welding process selection, consumable choice, thermal management, and post-weld machining—all working together to achieve both dimensional accuracy and metallurgical integrity. For engineers in cement and mineral processing industries, this approach represents a proven maintenance strategy that should be included in planned maintenance programs rather than treated as an emergency measure.