Cladding of New Rolling Mill Frame Surfaces

Literature Overview

This 2000 study by Zhang Liangfeng from Xiangtan Institute of Electromechanical Engineering investigates the application of weld overlay cladding to the surfaces of new rolling mill frames. Rolling mill frames are massive structural components that must withstand enormous compressive and bending loads during the rolling process. The working surfaces of the frame, particularly the bolted joints, pin holes, and contact surfaces with other mill components, are subject to severe wear, fretting, and corrosion. Applying a wear-resistant overlay cladding to these surfaces during fabrication, rather than as a repair after damage, represents a proactive approach to extending component life and reducing maintenance costs.

Core Technical Content

Service Conditions and Wear Mechanisms

Rolling mill frames operate under extreme mechanical and thermal conditions. The wear mechanisms affecting frame surfaces include:

Wear Mechanism	Location	Severity	Description
Abrasive wear	Guide surfaces, bolt seats	High	Contact with moving mill components
Fretting wear	Bolted joints	Moderate	Micro-motion under cyclic loading
Corrosion wear	Exposed surfaces	Moderate	Oxidation, chemical attack from coolants
Fatigue wear	Pin holes, bearing seats	High	Cyclic contact stress
Galling	Sliding contact surfaces	Moderate	Adhesive wear under high pressure

The base material of rolling mill frames is typically low-carbon structural steel (Q235, Q345) or medium-carbon steel (45 steel), with hardness in the range of HV 120–250. These materials provide adequate structural strength but insufficient surface hardness to resist the wear mechanisms described above.

Cladding Material Selection

The selection of overlay material depends on the specific wear mechanism and service conditions. The following materials were evaluated in the study:

Overlay Material	Hardness (HV)	Composition	Application
Cr-C composite hardfacing	800–1100	20-30% Cr, 3-5% C, balance Fe	Abrasive wear surfaces
Fe-Cr-Ni-C hardfacing	500–700	15-20% Cr, 8-12% Ni, 3-4% C	High-temperature wear
Ni-based hardfacing	400–600	Ni-Fe, Ni-Cr-Fe	Corrosion + moderate wear
Stellite 6	350–450	Co-Cr-W	High-temperature, severe wear
Austenitic stainless steel	200–280	304, 316	Corrosion resistance

Welding Process and Parameters

For new frame cladding, the following welding processes were investigated:

Submerged arc welding (SAW): Suitable for large, flat surfaces. High deposition rate (3–5 kg/h), low spatter, and good weld quality. Requires flux and wire feed equipment.
Flame spraying (oxy-fuel): Suitable for large areas and complex geometries. Moderate deposition rate (0.5–1.0 kg/h), lower equipment cost.
Electroslag welding (ESW): Suitable for thick overlay layers on large flat surfaces. Very high deposition rate (5–10 kg/h), uniform microstructure.
Plasma transferred arc (PTA) cladding: Suitable for high-quality overlay with precise dilution control. Moderate deposition rate (1–2 kg/h).

Process	Deposition Rate	Overlay Thickness	Surface Quality	Equipment Cost
SAW	3–5 kg/h	2–10 mm	Good	Moderate
Flame spray	0.5–1.0 kg/h	0.5–3 mm	Moderate	Low
ESW	5–10 kg/h	5–20 mm	Good	High
PTA	1–2 kg/h	1–5 mm	Excellent	High

Recommended Process Parameters for SAW Cladding

Parameter	Value	Rationale
Welding current	400–600 A	Adequate for 2–3 mm bead width
Arc voltage	28–35 V	Controls bead profile and penetration
Travel speed	200–400 mm/min	Balances deposition rate and quality
Flux coverage	Minimum 10 mm	Prevents oxidation and spatter
Preheat	100–150 °C	Reduces residual stress
Interpass temp	≤ 200 °C	Prevents grain coarsening
Wire diameter	1.6–2.4 mm	Compatible with current range

Process Development and Quality Control

Multi-Pass Cladding Strategy

For new rolling mill frame cladding, a multi-pass strategy is recommended to achieve the required overlay thickness and properties:

Bonding pass: A thin first pass (1.0–1.5 mm) using a filler metal compatible with both the base steel and the final overlay material. This pass ensures good metallurgical bonding and acts as a transition layer to reduce thermal stress.
Build-up passes: Subsequent passes (1.5–2.0 mm each) using the final overlay material. The number of passes depends on the required total thickness (typically 3–8 mm for mill frame surfaces).
Final pass: A final pass optimized for surface quality and hardness uniformity. This pass may use slightly different parameters to produce a smooth, dense surface suitable for subsequent machining or grinding.

Quality Control Requirements

Inspection Method	Purpose	Acceptance Criteria
Visual inspection (VT)	Surface defects	No cracks, undercut > 1 mm, porosity > 5% area
Magnetic particle testing (MT)	Surface/subsurface cracks	No indications
Ultrasonic testing (UT)	Bond integrity, internal defects	No lack of fusion, no volumetric defects > 1 mm
Hardness testing	Overlay properties	Within specified range (e.g., HV 800–1100 for Cr-C)
Macrographic examination	Microstructure, dilution	Uniform carbide distribution, no segregation
Wear testing	Service performance	Wear rate < 0.05 mm³/N·m

Integration with Engineering Practice

Design Considerations for Frame Cladding

When specifying cladding for new rolling mill frames, the following design considerations must be addressed:

Thermal stress management: The massive frame geometry creates significant restraint, leading to high residual stresses in the overlay welds. Stress-relief annealing (550–650 °C for 2–4 hours) should be performed after cladding to reduce residual stresses to acceptable levels.
Dimensional tolerance: The cladding must not compromise the dimensional accuracy of the frame. Machining allowances of 2–3 mm should be provided on cladded surfaces.
Stress concentration: Sharp transitions between cladded and uncladded areas create stress concentrations. Fillet transitions or gradual tapering should be used at cladding boundaries.
Welding sequence: A balanced welding sequence should be used to minimize distortion. Symmetrical welding from the center outward is recommended for large frames.

Economic Analysis

The economic justification for new frame cladding depends on the comparison between the initial cladding cost and the savings from reduced maintenance and extended component life. A typical economic analysis includes:

Cost Item	Estimate (per m² of cladding)
Material cost (filler + flux)	$50–150
Welding labor	$100–200
Equipment and consumables	$20–50
Inspection and testing	$30–80
Post-weld treatment	$50–100
Total	$250–580

Compared to the cost of frame replacement or major overhaul ($5,000–20,000 per frame), the cladding investment is typically recovered within the first 1–2 years of operation, depending on the severity of the service conditions.

Study Insights and Implications

The 2000 study from Xiangtan Institute of Electromechanical Engineering represents a significant contribution to the field of preventive cladding for heavy industrial equipment. Key insights include:

Applying wear-resistant cladding during fabrication, rather than as a repair, eliminates the need for disassembly, surface preparation, and reassembly that would be required for post-damage repair. This approach is particularly advantageous for large, complex structures like rolling mill frames where access is limited.
The selection of cladding material must be matched to the specific wear mechanism. A single material cannot address all wear types simultaneously, and multi-zone cladding with different materials may be required for different frame surfaces.
Residual stress management is critical for massive structural components. Without proper stress relief, the overlay welds may crack during service under cyclic loading.
The economic case for preventive cladding is compelling when the component life extension exceeds 2–3 times the original design life, and the downtime cost for repair is significant.

For modern rolling mill design, the integration of cladding specifications into the original engineering drawings and fabrication procedures is becoming increasingly common. This approach aligns with the philosophy of total productive maintenance (TPM) and life-cycle cost optimization. The study provides a valuable technical foundation for engineers developing cladding specifications for new heavy industrial equipment.