Zhuojin Pipe Fitting Co., LtdApplication of Online Surfacing Repair Technology for Slag Vertical Mill Rollers and Grinding Discs
Literature Overview
Sun Zuowei's 2020 paper in Shandong Metallurgy (Vol. 42, No. 2, pp. 79-80) documents the successful application of online open-flame flux-cored wire surfacing repair on slag vertical mill rollers and grinding discs at Shiheng Special Steel Group. The work addresses two persistent operational problems: wear-resistant layer spallation and excessive maintenance downtime. The solution achieved microfine powder production rates of 90,000-120,000 tonnes per unit, demonstrating significant economic and operational benefits.
Weldability Analysis and Material Selection
Base Material Assessment
The slag vertical mill rollers typically consist of medium-carbon alloy steel substrates with surface hardness in the range of 30-38 HRC. The grinding discs exhibit similar metallurgical characteristics but with different geometric configurations that influence heat dissipation patterns. The weldability analysis must consider:
- Carbon equivalent (CE) of the base material to assess cracking susceptibility
- Residual stress state from prior service and thermal cycling
- Surface contamination levels from slag and abrasive materials
- Geometric constraints affecting torch access and interpass cooling
Consumable Selection Rationale
| Consumable Type | Composition Characteristics | Application Rationale |
|---|---|---|
| Flux-cored wire (open-flame) | High Cr, Mo, W alloy content | Produces hard carbide-rich microstructure |
| Base wire | Low hydrogen type | Minimizes cracking risk in HAZ |
| Flux | Basic type with deoxidizers | Provides adequate shielding in open conditions |
The selection of open-flame flux-cored wire surfacing (SFAW with flux-cored consumables) was driven by the need for high deposition rates, good penetration control, and the ability to work in various positions during online repair. This process offers superior productivity compared to solid wire alternatives while maintaining acceptable metallurgical properties in the surfacing layer.
Process Implementation Details
Welding Sequence Strategy
The online repair procedure follows a systematic sequence designed to minimize distortion and ensure uniform coverage:
- Surface preparation — mechanical grinding to remove existing worn layer and contaminated surface to a minimum depth of 3 mm
- Edge profiling — V-groove preparation at the boundary between existing surface and repair zone to ensure proper fusion
- Root pass — using slightly higher amperage to achieve full penetration at the base-metal interface
- Fill passes — 2-3 intermediate passes to build up to target thickness while controlling dilution
- Cap pass — final pass with optimized parameters for surface quality and hardness uniformity
Process Parameters
| Parameter | Value | Notes |
|---|---|---|
| Wire diameter | 1.6 mm | Balance of deposition rate and control |
| Open-circuit voltage | 28-32 V | Ensures stable arc in open conditions |
| Welding current | 220-280 A | Depends on pass number and position |
| Travel speed | 350-450 mm/min | Adjusted for position and thickness |
| Shielding flux coverage | Complete | Critical for oxide-free surface |
| Interpass temperature | ≤200°C | Monitored with infrared thermometer |
Performance Results and Analysis
The reported production improvement from 90,000 to 120,000 tonnes per unit demonstrates that proper surfacing repair can restore — and potentially exceed — original equipment performance. This outcome depends on several factors:
- Hardness profile: Target surface hardness of 55-62 HRC in the wear layer with a gradual transition to base material hardness
- Carbide distribution: Uniform dispersion of Cr₇C₃ and Cr₃C₂ carbides throughout the surfacing layer
- Layer adhesion: Minimum tensile bond strength of 350 MPa at the base-metal/surfacing interface
- Dimensional accuracy: Surface flatness within 0.5 mm per meter for grinding disc applications
Study Insights
The practical value of this paper lies in its demonstration that online surfacing repair — long considered a compromise solution — can achieve production results equivalent to new equipment when properly executed. The key success factors identified are:
- Systematic weldability analysis preceding consumable selection
- Use of flux-cored wire technology that provides high deposition rates suitable for large surface areas
- Attention to the metallurgical interface between base material and surfacing layer
- Integration of repair scheduling with production planning to minimize downtime
From a broader perspective, this case study supports the economic viability of repair-over-replacement strategies for large wear components, provided that rigorous technical protocols are established and maintained. The maintenance culture shift required — from reactive replacement to proactive repair with quality assurance — represents perhaps the most significant organizational challenge.