Wear-Resistant Alloy Hardfacing on Large Hot Rolling Billet Rolls

Literature Overview and Background

Large hot rolling billet rolls operate under extreme thermal and mechanical loading conditions, with surface temperatures reaching 800–1000 °C and sustained compressive and bending stresses. The surface wear of these rolls is primarily due to adhesive wear, abrasive wear from scale, and thermal fatigue. This literature addresses the application of wear-resistant alloy hardfacing to extend roll service life and reduce replacement frequency. The study covers alloy design, welding process selection, microstructural analysis, and field performance evaluation.

Core Technical Content

The hardfacing alloy system investigated is based on high-chromium white cast iron (Cr15–Cr25) with additions of vanadium, molybdenum, and titanium to refine carbide morphology and improve thermal stability. The microstructure consists of a martensitic matrix with evenly distributed MC and M7C3 type carbides, providing a balanced combination of hardness and toughness.

Parameter	Specification
Base roll material	45 steel or 50Mn2
Hardfacing alloy	High-Cr white iron (Cr 15–25%, V 2–4%, Mo 1–3%, Ti 0.5–1.5%)
Target hardness (HRC)	55–65
Hardfacing thickness	3–6 mm
Preheat temperature	250–300 °C
Interpass temperature	≤ 350 °C
Welding process	Submerged arc welding (SAW) or manual shielded metal arc welding (SMAW)

The welding procedure involves a multi-pass approach: the first pass provides a transition layer with lower hardness to minimize thermal stress concentration, while subsequent passes build up the high-hardness overlay. The SAW process is preferred for large rolls due to its high deposition rate and consistent bead geometry, with a typical current of 500–700 A, arc voltage of 30–36 V, and travel speed of 200–400 mm/min.

Microstructural Analysis and Performance

Metallographic examination reveals that the hardness gradient from base metal to overlay surface follows a predictable pattern. The transition zone (first 0.5–1.0 mm) shows a mixed microstructure of base ferrite-pearlite and overlay martensite-carbide, which serves as a critical buffer against thermal shock. The overlay surface exhibits a refined carbide network with carbide size controlled below 5 μm, which is essential for maintaining wear resistance at elevated temperatures.

Field testing demonstrated that rolls with the hardfacing overlay achieved a service life of 4500–6000 tons of rolled product, compared to 2500–3500 tons for uncladded rolls. This represents a 60–80% improvement in productivity. However, the literature also notes that thermal fatigue cracking can initiate at the overlay surface after prolonged service, particularly at the roll neck region where thermal gradients are most severe.

Defect Analysis and Countermeasures

The primary defects encountered during hardfacing of hot rolling rolls include:

Defect Type	Root Cause	Detection Method	Countermeasure
Surface cracking	Excessive thermal stress	Visual / MT	Increase preheat; add Ti for grain refinement
Delamination	Poor base metal cleaning	UT	Thorough grinding and degreasing
Hardness non-uniformity	Inconsistent travel speed	Hardness mapping	Automated welding; speed monitoring
Roll barrel distortion	Excessive heat input	Dimensional check	Interpass cooling; symmetric welding sequence

A key insight from this literature is the recommendation for a controlled cooling rate after welding — specifically, the roll should be cooled to below 200 °C before removal from the welding fixture. Rapid cooling can induce residual stresses that promote early failure during hot rolling service.

Study Insights and Implications

This literature demonstrates that alloy design for hot rolling roll hardfacing must balance hardness, thermal stability, and toughness. The addition of titanium to promote fine carbide precipitation is a particularly effective strategy. For engineering practice, the literature reinforces the principle that hardfacing of large cylindrical components requires careful management of heat input distribution to prevent distortion. The multi-pass approach with a transition layer is validated as essential for reliable service performance. Engineers should also consider the thermal cycling conditions during actual rolling operations when specifying overlay thickness and alloy composition.