Research on Cladding Materials and Processes for Hot Rolling Mills

Overview and Technical Background

Hot rolling mill rolls are subjected to extreme operating conditions including high temperatures (up to 1200°C), severe mechanical contact with hot steel strips, thermal cycling, and mechanical impact. Cladding technology is widely used to enhance the surface properties of hot rolling mill rolls, extending their service life and improving the quality of the rolled product. This study note examines the selection of cladding materials and processes for hot rolling mill roll applications.

Core Technical Points

Operating Conditions of Hot Rolling Mill Rolls

Parameter	Typical Range	Impact on Cladding
Strip temperature	800–1200°C	Requires high-temperature strength
Contact pressure	100–500 MPa	Requires high hardness and toughness
Thermal cycling rate	10–100 cycles/hour	Requires thermal fatigue resistance
Oxidation environment	Air, scale	Requires oxidation resistance
Cooling method	Water spray, air	Requires thermal shock resistance

Cladding Material Selection

The selection of cladding materials for hot rolling mill rolls depends on the specific application, including the type of steel being rolled, the rolling conditions, and the desired surface properties.

Material Type	Composition	Application	Key Properties
High-speed steel	W18Cr4V, W6Mo5Cr4V	Finishing rolls	High hardness, red hardness
Cemented carbide	WC-Co, WC-TiC-Co	Backup rolls	Extreme hardness, wear resistance
High-chromium cast iron	Cr13, Cr20	Intermediate rolls	Cost-effective, good wear resistance
Nickel-based alloys	Inconel 625, 718	Special applications	High-temperature strength
Hardfacing alloys	Fe-Cr-C, Co-Cr-W	Wear zones	Abrasive wear resistance
Stainless steel	310, 304, 316	Corrosive environments	Corrosion resistance

Cladding Process Selection

Process	Dilution Rate	Heat Input	Cost	Application
PTA cladding	5–15%	Low	High	High-performance coatings
SAW cladding	15–30%	Medium	Medium	Thick coatings
GMAW cladding	20–40%	Medium	Low	Economical coatings
Laser cladding	5–15%	Low	High	Precision coatings
Explosive cladding	0–5%	Very low	High	Bimetallic rolls
Electroslag welding	30–50%	High	Low	Thick bimetallic rolls

Process Parameters for Hot Rolling Roll Cladding

For a typical PTA cladding application on a hot rolling mill roll:

Parameter	Value	Notes
Arc current	250–350 A	Depends on roll diameter and coating thickness
Travel speed	10–20 cm/min	Circumferential travel
Powder feed rate	200–350 g/min	Matched to travel speed
Shielding gas	Ar 99.99%	Prevent oxidation
Preheat temperature	200–300°C	Reduce thermal stress
Interpass temperature	< 250°C	Control residual stress
Coating thickness	2–5 mm	Depends on service life requirement

Engineering Practice and Quality Control

Roll Cladding Quality Requirements

Requirement	Specification	Test Method
Coating thickness	2–5 mm ± 0.5 mm	UT or caliper measurement
Hardness	500–700 HV	Vickers hardness test
Dilution depth	< 1 mm	Metallographic examination
Bond strength	> 250 MPa	Shear or tensile test
Surface roughness	Ra < 6.3 μm	Surface profilometer
Cracking	None	Visual and MT inspection
Porosity	< 1%	Metallographic examination

Common Defects and Countermeasures

Defect	Cause	Countermeasure
Cracking	Thermal stress, high carbon	Preheat, reduce heat input, use low-carbon filler
Delamination	Poor bond, oxide inclusion	Clean base metal, optimize parameters
Hardness variation	Inconsistent parameters	Monitor and control process parameters
Surface roughness	Uneven deposition	Optimize travel speed and powder feed
Incomplete melting	Low heat input	Increase current, reduce travel speed

Inspection Protocol

Visual inspection: Check for surface defects, cracks, and porosity.
Magnetic particle testing (MT): Detect surface and near-surface cracks.
Ultrasonic testing (UT): Measure coating thickness and detect subsurface defects.
Hardness testing: Verify hardness profile across the coating.
Metallographic examination: Assess microstructure, dilution, and defects.
Bond strength test: Evaluate interface integrity.
Thermal cycling test: Verify thermal fatigue resistance.

Study Insights and Reflections

The cladding of hot rolling mill rolls is a critical technology that directly impacts the productivity, quality, and cost of steel rolling operations. The selection of cladding materials and processes must be carefully matched to the specific operating conditions of the mill, including the type of steel being rolled, the rolling temperature, and the desired surface finish.

A key insight from this study is that the performance of the cladding is not solely determined by the coating material but is also strongly influenced by the process parameters, the base roll material, and the service conditions. For example, a high-speed steel coating may perform excellently on a finishing roll but may be unsuitable for a backup roll due to the higher impact loads.

The PTA cladding method is generally preferred for hot rolling mill roll applications due to its low dilution rate, precise control over coating composition, and ability to produce dense, defect-free coatings. However, the relatively high cost of PTA equipment and consumables must be justified by the extended roll life and improved product quality.

From a standards perspective, hot rolling mill roll cladding should comply with relevant industry standards such as JIS H 7001, ASTM A 232, or EN 10279 for roll materials, and the welding procedures should be qualified in accordance with NB/T 47014 or ASME IX.

In conclusion, the cladding of hot rolling mill rolls requires a systematic approach that integrates material selection, process optimization, and quality control. The success of the cladding depends on a thorough understanding of the operating conditions, careful selection of materials and processes, and rigorous quality control to ensure consistent performance and extended service life.