CLADDING TECHNOLOGY SHANXI CO., LTDApplication of Online Cladding Repair Technology for Slag Vertical Mill Grinding Rollers and Discs
Literature Overview
Published in 2020 in the Shandong Metallurgy journal by Sun Zuowei from Shiheng Special Steel Group Co., Ltd., this paper reports on the application of online cladding repair technology for grinding rollers and discs in slag vertical mills. Slag grinding is a growing segment of the cement and metallurgical industries, driven by the need to utilize blast furnace slag as a supplementary cementitious material. The grinding environment in slag vertical mills is particularly aggressive due to the high abrasiveness of ground granulated blast furnace slag (GGBFS), and the wear on rollers and discs can be severe. This paper documents a successful application of online cladding repair and discusses the technical decisions that enabled it.
Operating Conditions and Wear Mechanisms
Slag vertical mills operate under conditions that are more demanding than typical cement raw meal or clinker grinding. The ground slag particles are angular and possess a high specific surface area, which increases the abrasive action on the grinding surfaces. Additionally, the presence of moisture and the tendency of slag to adhere to metal surfaces introduce an adhesive wear component. The combined abrasive-adhesive wear regime requires overlay materials that possess both high hardness and good toughness.
The typical overlay alloys used for slag vertical mill rollers and discs include:
| Alloy Type | Typical Composition | Hardness (HRC) | Application |
|---|---|---|---|
| High-carbon martensitic | C 2.5-3.5%, Cr 5-8%, Mo 1-2% | 55-62 | Roller surface, disc surface |
| Cr-Mo cast iron | C 3.0-4.0%, Cr 10-15%, Mo 1-3% | 55-65 | Disc surface, high-wear zones |
| Medium-carbon martensitic | C 1.5-2.5%, Cr 8-12%, Mo 2-4% | 45-55 | Transition layers, lower-wear zones |
| Austenitic | C 0.5-1.5%, Cr 8-12%, Ni 8-12% | 30-40 | Where toughness is prioritized |
The selection of the overlay alloy depends on the specific wear zone and the desired balance between hardness and toughness. For slag grinding applications, the author recommends a high-carbon martensitic overlay for the primary grinding surfaces, with a medium-carbon transition layer to manage the dilution gradient.
Online Repair Procedure
The online repair procedure described in the paper follows a systematic approach that can be broken down into the following stages:
- Assessment and planning: The wear pattern on the roller and disc is documented, and the remaining base metal thickness is measured. The repair plan specifies the number of weld passes, the alloy sequence, and the target final dimensions.
- Surface preparation: The worn overlay material is removed by grinding or milling to expose sound base metal. The surface is cleaned to bare metal, and any cracks or defects in the substrate are identified and addressed.
- Preheating: The substrate is preheated to 150-250 °C using induction heating or gas torches. The preheating temperature is monitored at multiple locations to ensure uniformity.
- Welding: The overlay is applied in multiple passes using FCAW or GMAW. A transition layer of medium-carbon alloy is applied first, followed by the final high-carbon martensitic overlay. The welding sequence is planned to minimize thermal distortion.
- Post-weld treatment: The overlay surface is ground to the specified profile and roughness. Stress relief is applied if required, typically by controlled heating to 550-650 °C.
- Inspection: MT, hardness testing, and dimensional inspection are performed before returning the component to service.
Technical Parameters and Process Windows
The paper provides specific process parameters that were used in the application, which are summarized below:
| Parameter | Value | Notes |
|---|---|---|
| Welding process | FCAW (flux-cored wire) | Wire diameter 1.2 mm |
| Shielding gas | CO2 (100%) or mixed gas | Selected based on alloy type |
| Current | 280-350 A | Adjusted per pass |
| Voltage | 28-32 V | Maintained for stable arc |
| Travel speed | 150-250 mm/min | Slower for first pass |
| Preheat temperature | 200 °C | Monitored by thermocouple |
| Interpass temperature | 200-300 °C | Maintained throughout |
| Number of passes | 4-6 layers | Including transition layer |
| Post-weld stress relief | 600 °C for 2 hours | Controlled cooling |
| Final hardness | 55-60 HRC | Measured at multiple points |
Performance Results
The paper reports that the repaired rollers and discs achieved a service life comparable to new components, with the overlay layer showing no signs of cracking, spalling, or excessive wear after several months of continuous operation. The hardness profile was uniform across the overlay thickness, with a gradual transition from the overlay hardness to the substrate hardness, indicating good metallurgical bonding without excessive dilution.
The economic analysis presented in the paper demonstrates that online repair is significantly more cost-effective than replacement. The cost of online repair was approximately 30-40% of the cost of new roller and disc replacement, with the additional benefit of reduced downtime since the components did not need to be removed from the mill.
Engineering Practice Insights
The case study presented in this paper is particularly instructive because it demonstrates that online repair of slag vertical mill components is not only technically feasible but also economically superior to replacement in many scenarios. The key success factors identified by the author are:
- Thorough surface preparation with verification of substrate integrity
- Use of a transition layer to manage dilution and ensure metallurgical compatibility
- Controlled thermal input with continuous temperature monitoring
- Systematic post-weld inspection including hardness mapping and MT
One observation that resonates with my own experience is the emphasis on the transition layer. In slag grinding applications, the high carbon content of the final overlay alloy creates a significant dilution challenge when applied directly to a low-carbon steel substrate. The transition layer acts as a buffer, gradually increasing the carbon and alloy content from the substrate to the final overlay, thereby reducing the risk of cracking and ensuring a sound bond.
The paper also highlights the importance of stress relief. The high residual stresses in the overlay layer, combined with the high carbon content, create a susceptibility to delayed cracking. The controlled stress relief treatment at 600 °C for 2 hours is a critical step that should never be omitted, even if it extends the repair time.
Summary and Implications
This paper provides a well-documented case study of online cladding repair for slag vertical mill components, demonstrating that the approach is technically sound and economically attractive. The detailed process parameters and performance results serve as a valuable reference for engineers planning similar repairs. The emphasis on transition layers, thermal control, and post-weld stress relief reflects a mature understanding of the metallurgical challenges involved. For engineers in the cement and metallurgical industries, this paper reinforces the message that online repair, when properly executed, is a reliable and cost-effective alternative to component replacement.