Wear-Resistant Alloy Overlay Welding on Large Hot Rolling Bloom Rolls

Literature Overview

This paper by Zhang Zhongtao from Angang Northern Machinery Factory, published in Angang Technology in 1992, presents the development and industrial application of a dispersion-strengthened steel wire electrode designated H25Cr3Mo2MnVA for overlay welding on large hot rolling bloom rolls. The work represents a significant milestone in China's domestic efforts to develop wear-resistant overlay welding consumables by adapting and improving upon international technologies. The research was driven by the urgent need to extend the service life of bloom rolls, which are subjected to extreme thermal cycling, mechanical abrasion, and chemical erosion during the hot rolling of steel billets.

Core Technical Points

The H25Cr3Mo2MnVA wire electrode is a dispersion-strengthened type overlay welding consumable. Its designation indicates a composition rich in chromium (approximately 25%), with molybdenum (3%), manganese (2%), vanadium, and aluminum as key alloying elements. The hardening mechanism relies on the formation of fine, uniformly dispersed carbides and nitrides within the weld deposit, which resist coarsening even under the severe thermal conditions of hot rolling.

Microstructural Characteristics and Hardening Mechanism

The overlay weld metal achieves its exceptional wear resistance through multiple reinforcing mechanisms operating simultaneously. The chromium content promotes the formation of M7C3 and M23C6 type carbides, while vanadium and molybdenum contribute to the precipitation of extremely hard MC and M2C type carbides. These dispersion particles, typically in the sub-micron range, provide outstanding resistance to abrasive wear and thermal fatigue. The aluminum addition further refines the grain structure and enhances the stability of the carbide network during thermal cycling.

Parameter	Specification
Electrode designation	H25Cr3Mo2MnVA
Hardness (as-welded)	55-62 HRC
Hardness (after thermal cycling simulation)	50-58 HRC
Typical overlay thickness	6-12 mm
Base material	45 steel or low-alloy steel
Rolling temperature range	1100-1250°C

Welding Process Parameters

The overlay welding process requires careful control of heat input to maintain the fine dispersion structure while ensuring adequate metallurgical bonding with the roll base material. Preheating is essential to prevent cracking in the base metal, particularly for larger roll diameters where residual stresses are significant. The welding sequence follows a multi-pass approach: a transition layer is applied first to mitigate dilution effects, followed by the main overlay passes.

The recommended process parameters include:

Process Variable	Typical Range
Preheat temperature	250-350°C
Interpass temperature	200-300°C
Arc voltage	28-35 V
Welding current	280-380 A
Travel speed	250-400 mm/min
Number of overlay passes	3-5
Post-weld cooling	Controlled air cooling or furnace cooling

Engineering Practice and Economic Analysis

The industrial application on large hot rolling bloom rolls demonstrated a substantial improvement in service life compared to conventional overlay materials. The authors report that the H25Cr3Mo2MnVA overlay increased roll life by a factor of 2 to 3 times relative to previously used materials. This translates into significant economic benefits including reduced roll change frequency, lower production downtime, and decreased consumable costs per ton of rolled product.

From a metallurgical perspective, the key challenge in bloom roll overlay welding is maintaining hardness retention after repeated exposure to temperatures exceeding 1000°C. The dispersion-strengthened approach addresses this by relying on thermodynamically stable carbides that do not dissolve or coarsen rapidly at elevated temperatures. This distinguishes it from martensitic hardfacing materials, which suffer from softening due to tempering of the matrix under thermal cycling.

Key Defects and Countermeasures

During the application of this technology, several characteristic defects were identified and addressed through process optimization:

Defect Type	Root Cause	Countermeasure
Cracking at weld-base interface	Excessive cooling rate, high carbon equivalent of base metal	Increase preheat temperature, use low-hydrogen electrode
Porosity in overlay deposit	Inadequate flux coverage, contaminated base surface	Thorough surface preparation, controlled welding atmosphere
Undercut at pass boundaries	Excessive travel speed, improper torch angle	Optimize travel speed, maintain consistent torch angle
Hardness non-uniformity	Incomplete melting of previous pass, dilution variation	Ensure 70-80% overlap between passes, monitor dilution
Thermal fatigue spalling	Excessive overlay thickness, poor interlayer adhesion	Limit total overlay thickness, apply stress-relief annealing

Study Insights and Implications

This paper exemplifies the systematic approach required for developing specialized welding consumables for demanding industrial applications. The success of the H25Cr3Mo2MnVA electrode demonstrates that dispersion strengthening is a viable and reliable approach for hot rolling applications where thermal stability of hardness is paramount. The work also highlights the importance of integrating materials science with process engineering—no matter how excellent the consumable composition, improper welding parameters will destroy the microstructural benefits.

For contemporary practice, this research remains relevant as the fundamental metallurgical principles of dispersion-strengthened overlay welds continue to guide modern consumable development. Modern techniques such as plasma arc surfacing and laser cladding can now achieve even finer dispersion structures, but the compositional philosophy established in this work remains a cornerstone of wear-resistant overlay welding technology.