What are the material and heat treatment requirements for Single Stand Multi-Groove Reducing Rolling Mill

       The rolls of Single Stand Multi-Groove Reducing Rolling Mill are subjected to alternating rolling loads and uneven wear across multiple passes. This article covers commonly used roll materials, performance indicators, heat treatment specifications, and critical heat treatment considerations for multi-pass rolls.

       Single-stand multi-pass sizing reducing mill serves as core equipment for tube forming. During operation, its rolls are continuously exposed to rolling pressure, alternating impact loads, high-temperature friction, and cyclic thermal stress. Compared with conventional single-pass rolls, the roll body of multi-pass rolls contains multiple grooved passes. Uneven load distribution among passes easily triggers local wear, spalling, cracking and chipping. Therefore, roll material selection and heat treatment process directly determine roll service life, tube dimensional accuracy and stable production line operation.

1. Operating Conditions Affecting Material Selection for Multi-Pass Rolls of Single-Stand Sizing Reducing Mill

       1. Multiple grooved passes are machined on one roll barrel. Stress concentration is prominent at local regions due to inconsistent load on each pass.

       2. Continuous frictional wear occurs during rolling. Wear rates differ greatly between pass bottoms and pass shoulders, leading to premature failure of individual passes.

       3. Alternating hot and cold conditions induce thermal fatigue under hot reducing conditions, which tends to generate network cracks on roll surfaces.

       4. Periodic biting impact requires roll matrix to possess higher toughness and spalling resistance than ordinary rolls.

       Given the above working conditions, multi-pass reducing rolls cannot rely solely on high hardness. A balanced combination of surface wear resistance, matrix strength, thermal fatigue resistance and impact toughness is mandatory.

2. Common Material Selection for Rolls

2.1 High-Chromium Cast Iron Rolls (Primary Choice for Hot Reducing)

       High-chromium cast iron is the most widely adopted material for rolls of single-stand multi-pass hot tube sizing reducing mills.

       Advantages: High hardness carbides deliver excellent wear resistance and thermal wear performance, suitable for high-temperature reducing processes.

       Limitations: Relatively low toughness; spalling on pass surfaces may occur under heavy impact loads.

       Application: Hot reduction, medium rolling load and mass continuous tube production.

2.2 Alloy Ductile Iron Rolls

       Advantages: Balanced comprehensive mechanical properties, better impact resistance and thermal cracking resistance than high-chromium cast iron, good machinability.

       Limitations: Slightly lower surface wear resistance compared with high-chromium cast iron.

       Application: Rolling scenarios with heavy impact and strict anti-cracking requirements for multi-pass reducing units.

2.3 Forged Steel Rolls (Cold Reducing / High Precision Applications)

       Common grades: CrMo, CrNiMo series alloy forged steel.

       Advantages: Dense matrix, high toughness, strong impact resistance and good hardenability with sufficient regrinding allowance.

       Limitations: Inferior wear resistance to cast iron grades and higher production cost.

       Application: Cold tube sizing reduction, high-precision thin-walled tube production and harsh working conditions with heavy impact.

2.4 High-Speed Steel Rolls (High-End Applications)

       High-speed steel rolls feature outstanding wear resistance and red hardness with superior thermal fatigue resistance. However, they come with high material cost and strict control requirements for heat treatment processes. They are deployed in high-speed, high-demand tube reducing production lines.

       Selection Principle: Evaluate maximum rolling load, rolling temperature and pass depth when selecting multi-pass rolls. Toughness indexes shall be properly upgraded at pass shoulders where stress concentrates to avoid cracking at pass corners.

3. Heat Treatment Technical Requirements for Corresponding Rolls

       The objective of roll heat treatment is to achieve a wear-resistant working layer with high hardness together with a tough matrix. The surface layer guarantees wear resistance of grooved passes while the matrix prevents roll fracture.

3.1 Heat Treatment Process for High-Chromium Cast Iron Rolls

       Process route: Casting → Stress Relief Annealing → Rough Machining → Quenching → Tempering

       Annealing: Eliminate casting residual stress and improve cutting machinability.

       Quenching: Air quenching or air blast quenching to raise surface hardness.

       Tempering: Low-temperature tempering to remove quenching stress, stabilize microstructure and reduce cracking risk.

       Hardness specification: Roll surface hardness HSD 80~92 while maintaining high toughness of roll core.

3.2 Heat Treatment for Alloy Ductile Iron Rolls

       Process route: Casting → Annealing → Rough Machining → Quenching + Tempering

       Key control: Stabilize pearlite fraction in matrix to improve thermal fatigue resistance.

       Hardness range: HSD 75~88. Compared with high-chromium cast iron, it provides greater toughness margin to accommodate alternating impact loads of multi-pass rolling.

3.3 Heat Treatment for Alloy Forged Steel Rolls

       Process route: Forging → Post-forging Annealing → Rough Machining → Quenching and Tempering → Surface Induction Hardening of Roll → Low-temperature Tempering

       Quenching and tempering: Ensure high strength and high toughness of roll matrix.

       Induction surface hardening: Only increase hardness of grooved working surface while keeping toughness at core to avoid overall embrittlement.

       Hardness: Working surface HRC 48~58.

3.4 Heat Treatment for High-Speed Steel Rolls

       Process: High-temperature quenching + multiple tempering to fully precipitate alloy carbides.

       Characteristics: Hardness decays slowly at high temperatures (good red hardness). Strict control of heat treatment deformation is required. Dimensional accuracy control of multi-pass grooves is challenging.

4. Special Heat Treatment Considerations for Single-Stand Multi-Pass Rolls

1. Stress Concentration Control for Multi-Pass Geometry

       Pass shoulders and pass corners are stress-concentrated zones. During heat treatment, heating and cooling rates must be controlled to prevent quenching cracks at pass corners. Full tempering after quenching reduces residual stress. This is the most critical distinction between multi-pass rolls and single-pass rolls.

2. Hardness Uniformity Control

       Multiple grooved passes are machined on a single roll. Hardness deviation of working surfaces across all passes after heat treatment must be kept within acceptable limits. Non-uniform hardness leads to inconsistent wear of different passes and fluctuation of tube outer diameter tolerance.

3. Heat Treatment Deformation Control

       Multi-pass rolls feature long barrel and multiple grooves. Heat treatment deformation directly affects concentricity of passes. Optimization of heat treatment fixture and cooling method is required. Excessive deformation increases subsequent regrinding workload.

4. Residual Stress Inspection

       Residual stress shall be inspected after heat treatment. High residual stress may initiate fatigue cracks starting from pass corners under superimposed alternating rolling loads and result in premature roll failure.

5. Correlation Between Roll Material, Heat Treatment and Failure Modes

       Rapid pass surface wear: Insufficient surface hardness or low carbide content of roll material.

       Cracks and crazing at pass corners: Excessive residual stress after heat treatment, insufficient toughness and thermal fatigue damage.

       Chipping and spalling of pass surface: Brittle hard surface layer with inadequate matrix toughness or improper quenching process.

       Significant wear discrepancy among passes: Uneven hardness after heat treatment combined with unreasonable rolling load distribution.

6. Conclusion

       Material and heat treatment schemes for rolls of single-stand multi-pass sizing reducing mill cannot be directly adopted from standards for single-pass rolls. The stress concentration and uneven load introduced by multi-pass grooved geometry require a balance between wear resistance and toughness in material selection. Heat treatment should focus on residual stress at grooved regions, hardness uniformity and heat treatment deformation. Proper matching of material and heat treatment processes extends roll service cycle, stabilizes tube outer diameter, ovality and other finished indexes, and reduces downtime and roll replacement costs.