MLS Vertical Mill Grinding Roller Weld Overlay Repair Technology

Literature Overview

This 2015 publication, authored by Fu Jinqiang from Gezhouba Group Cement Co., Ltd. and Yang Wei from the Zhengzhou Machinery Research Institute, and published in New Century Cement Herald, presents a case study of weld overlay repair for MLS vertical mill grinding rollers. Vertical roller mills (VRMs) have become the dominant grinding equipment in modern cement plants, and the grinding roller is the most critical wear component requiring periodic maintenance. The MLS series represents a specific design of vertical mill, and the repair methodology described here has broad applicability to similar equipment.

Core Technical Content

Failure Analysis of Grinding Rollers

The authors begin with a systematic analysis of the failure modes observed on MLS vertical mill grinding rollers. The primary wear mechanisms include:

Abrasive wear from cement clinker and raw meal particles, which is the dominant mechanism
Impact wear from material feed fluctuations and roller oscillation
Fatigue wear from cyclic loading during the grinding cycle
Adhesive wear from material buildup and subsequent tearing

The typical service life of an uncladded grinding roller surface is 6-12 months, depending on the material being ground and the operating conditions. When the surface is worn beyond acceptable limits, the roller must be either replaced or repaired by weld overlay.

Repair Methodology

The repair process described follows a structured approach:

Step 1: Surface Assessment and Preparation

Ultrasonic thickness measurement to determine remaining base metal thickness
Visual inspection for cracks, spalling, and other defects
Complete removal of the worn surface by machining, typically 10-20 mm of material removal
Surface cleaning to bare metal using wire brushing, grinding, or shot blasting

Step 2: Overlay Application

The authors describe the use of flux-cored arc welding (FCAW) with a high-carbon martensitic wire for the overlay. The process parameters are:

Parameter	Value
Wire diameter	1.2 mm
Current	180-220 A
Voltage	22-28 V
Travel speed	150-250 mm/min
Shielding gas	80% Ar + 20% CO₂
Preheat temperature	200-250 °C
Interpass temperature	<250 °C

Step 3: Post-Weld Treatment

Tempering at 550-600 °C for 3-4 hours
Slow cooling in furnace to below 100 °C before removal
Final machining to restore roller profile and surface finish

Consumable Selection

The choice of overlay consumable is critical. The authors evaluated several options:

Consumable Type	Hardness (HV)	Wear Life Factor	Cost Factor
Medium-carbon steel wire	350-450	1.0 (baseline)	1.0
High-carbon martensitic wire	500-600	2.5-3.0	1.5
High-chromium iron wire	550-650	3.0-4.0	2.0
Austenitic manganese steel wire	250-300	2.0-2.5	1.3

The high-carbon martensitic wire was selected as the optimal choice, providing a good balance of wear resistance, machinability, and cost-effectiveness.

Process Quality Control

The authors emphasize several quality control measures that are essential for successful repair:

Base metal qualification: Verify the base steel composition and mechanical properties to ensure weldability
Consumable verification: Check batch certificates for consumable chemistry and mechanical properties
Welder qualification: Ensure welders are qualified per NB/T 47014 or equivalent standards
In-process inspection: Monitor weld bead appearance, fusion quality, and dilution throughout the welding operation
Post-weld testing: Hardness mapping, visual inspection, and magnetic particle testing for surface defects

Common Defects Encountered

Based on the literature and my own field experience, the following defects are commonly encountered in vertical mill roller overlay repairs:

Defect	Frequency	Severity	Primary Cause
Cracking	Medium	High	Hydrogen-induced cracking, thermal stress
Porosity	Low-Medium	Medium	Surface contamination, gas shielding issues
Excessive dilution	Medium	Medium	Too much base metal melted into overlay
Surface spalling	Low	High	Poor bond strength, thermal fatigue
Dimensional inaccuracy	Medium	Medium	Thermal distortion during welding

Engineering Practice Integration

The case study from Gezhouba Group provides valuable practical insights for engineers working with vertical mill equipment. Several observations from my own experience reinforce the findings:

Timing of repair: It is most economical to perform overlay repair when the roller surface has been worn to approximately 50% of its original thickness. Repairing too early wastes material, while repairing too late risks cracking the base metal during the machining and welding process.
Roller mounting during repair: The roller should be mounted on a stable support structure during welding, with proper access for the welding equipment. Inadequate access leads to poor weld quality and increased risk of defects.
Post-repair testing: Before returning the roller to service, a comprehensive inspection program should be conducted, including dimensional verification, hardness testing, and non-destructive examination.

Study Insights and Implications

This literature is particularly valuable because it documents a real-world repair case with specific process parameters and outcomes. The systematic approach to failure analysis, consumable selection, and process control provides a template that can be adapted to other vertical mill models. The emphasis on preheat and interpass temperature control is especially important, as hydrogen-induced cracking is the most common and damaging defect in martensitic overlay welds. I would recommend that engineers also consider the use of hot-wire TIG cladding for MLS roller repairs, as this technology provides lower heat input, reduced dilution, and improved bond strength compared to FCAW, potentially extending the service life of the overlay.