Zhuojin Pipe Fitting Co., LtdDetailed Technical Considerations for Surfacing Repair of Cement Grinding Equipment Components
Literature Overview
This paper by Huang Zhiquan, Yang Wei, and Li Junwei from Zhengzhou Machinery Research Institute, published in New Century Cement Guide (2014, Vol. 20, No. 2, pp. 44-46), provides a systematic discussion of the critical details that determine the success of surfacing repair operations on cement grinding equipment components, including vertical mill rollers and roller press components. The paper emphasizes three key areas: base metal assessment and preparation, transition layer (build-up layer) consumable selection, and surfacing wire performance characteristics.
Base Metal Assessment and Preparation
The first and most fundamental step in any surfacing repair operation is the thorough evaluation of the base metal. The authors stress that the mechanical properties, existing defects, and material condition of the base metal directly influence the quality of the surfacing repair.
Key Base Metal Considerations
| Assessment Item | Method | Acceptance Criteria |
|---|---|---|
| Material identification | Spectroscopy, spark test, hardness test | Confirm grade matches original specification |
| Surface condition | Visual inspection, MT/PT | No cracks, pores, or inclusions at surface |
| Hardness profile | Microhardness traverse | Uniform hardness within ±50 HV of specification |
| Residual stress | Strain gauge or XRD | Within acceptable limits for repair |
| Geometry and dimensional accuracy | CMM or coordinate measurement | Within tolerance for functional fit-up |
Common defects encountered on used grinding equipment components include surface fatigue cracks, wear grooves, oxidation scale, and heat damage from previous repair attempts. Each of these requires specific preparation:
- Surface fatigue cracks: Must be removed by grinding, and the crack root must be verified by magnetic particle testing (MT) or penetrant testing (PT).
- Wear grooves: Ground to a uniform depth with adequate transition radius (minimum R3) to avoid stress concentration.
- Oxidation scale: Removed by grinding or shot blasting to bare metal.
- Previous weld damage: Thoroughly removed and the base metal re-analyzed for hardness and microstructure.
Transition Layer (Build-Up Layer) Selection
The transition layer, also called the build-up layer or buffer layer, is deposited between the base metal and the final surfacing layer. Its primary functions are:
- Chemical buffering: To dilute the high-alloy surfacing material and prevent excessive dilution of the wear layer by the base metal.
- Stress relief: To provide a ductile zone that absorbs thermal stresses between the rigid base metal and the hard surfacing layer.
- Wetting improvement: To ensure good metallurgical bonding between the base metal and the surfacing alloy.
The selection of transition layer consumables depends on the base metal composition and the surfacing alloy composition. The following guidelines are recommended:
| Base Metal | Surfacing Alloy | Recommended Transition Layer |
|---|---|---|
| Low-carbon steel (Q235, 20) | High-Cr, high-C (e.g., Cr15) | Cr-Mo medium-alloy steel |
| Medium-carbon steel (45, 50) | High-Cr, high-C | Cr-Mo-V alloy steel |
| Medium-carbon steel (45, 50) | Cr-Ni alloy | Cr-Ni medium-alloy steel |
| High-alloy steel | High-Cr, high-C | Similar alloy composition to surfacing layer |
The transition layer thickness is typically 1-3 mm, with 2 mm being the most common. Thicker transition layers provide better stress relief but increase dilution and may reduce the effective wear layer thickness.
Surfacing Wire Performance Characteristics
The authors identify four critical performance indicators for surfacing welding wires:
Arc Stability
Arc stability determines the consistency of heat input and metal transfer. Instable arcs lead to irregular bead profiles, uneven dilution, and potential defects. Self-shielded flux-cored wires generally provide more stable arcs than solid wires under field conditions, while gas-shielded solid wires require precise gas flow control.
Porosity Resistance
Porosity in surfacing layers is a major quality concern, as it reduces the effective load-bearing area and creates stress concentration sites. The flux composition, including deoxidizer content and gas-generating elements, directly influences porosity susceptibility. Wires with higher Si and Al content generally provide better deoxidation and lower porosity rates.
Slag Distribution and Slag Removal
The slag covers the solidifying weld metal and protects it from atmospheric contamination. Uniform slag distribution ensures consistent protection, while easy slag removal reduces post-weld cleaning time and improves productivity. The slag basicity and viscosity are critical parameters that affect both slag distribution and removal characteristics.
Spatter Rate
Spatter represents metal loss and reduces deposition efficiency. High spatter rates also create surface roughness that must be ground off, increasing labor costs. Spatter is influenced by arc voltage, current density, and the surfacing material composition. Wires with lower carbon content and higher Mn content tend to produce less spatter.
Engineering Practice Integration
The three-point framework proposed by the authors—base metal assessment, transition layer selection, and wire performance evaluation—can be organized into a PDCA cycle for continuous improvement of surfacing repair quality:
- Plan: Conduct thorough base metal assessment, select appropriate transition layer and surfacing consumables, establish process parameters.
- Do: Execute the surfacing operation with controlled parameters, maintain interpass temperature, and perform interpass cleaning.
- Check: Inspect the completed surfacing for hardness, porosity, adhesion, and dimensional accuracy through NDT and mechanical testing.
- Act: Document findings, adjust consumable selection or process parameters for subsequent repairs, and update the repair procedure specification.
This systematic approach transforms surfacing repair from a craft-dependent activity into a repeatable, quality-controlled process. The authors' emphasis on these three details reflects the practical reality that surfacing repair failures in cement grinding equipment are rarely due to gross process errors but rather to overlooked details in base metal preparation, consumable selection, or wire performance that accumulate to produce substandard results.