Sintered Alloy Flux Overlay Welding of Phi-850 Steel Rolling Mill Rolls

Literature Overview

This 1993 paper by Zhang Yazhi, Zhang Wen, and Zhang Bingxiang from Ansteel Technical School, published in Ansteel Technology, documents the overlay welding of sintered alloy flux onto 80CrMnMo forged steel rolling mill rolls with a diameter of 850 mm. The work describes a complete process including pre-weld preparation, welding parameters, post-weld heat treatment, and economic evaluation. The overlay welding of four rolls saved a total of 398,700 RMB, demonstrating significant economic value.

Process Parameters and Technical Specifications

The paper provides detailed welding parameters that are valuable for engineering reference:

The use of sintered alloy flux is significant because flux-cored or submerged arc welding with sintered flux allows for high deposition rates and deep penetration, which are advantageous for large cylindrical components like rolling mill rolls.

Pre-Weld Preparation and Inspection

The paper emphasizes a rigorous pre-weld preparation sequence:

1. Non-destructive testing of the roll to identify internal defects such as inclusions, shrinkage cavities, and cracks.
2. Machining of the roll surface to remove 15-20 mm of material, which eliminates surface defects, scale, and any decarburized layer from previous service.
3. Re-inspection after machining to confirm the integrity of the prepared surface.
4. Preheating of the roll to prevent thermal stress and volumetric stress during welding.

The machining depth of 15-20 mm is substantial and indicates that the rolls had experienced significant surface degradation during service. This preparation step is critical because overlay welding quality is directly dependent on the quality of the base material surface.

Welding Process Control

The welding parameters described in the paper reflect careful consideration of the thermal characteristics of 80CrMnMo steel. This alloy contains 0.8% carbon, 1.0% chromium, 0.3% manganese, and 0.3% molybdenum, giving it high hardenability and a tendency toward martensitic transformation during welding. The high carbon and alloy content create significant risk of cold cracking in the heat-affected zone.

The interpass temperature of 260-280 °C serves multiple purposes: it reduces the cooling rate of the weld metal, promotes the formation of softer microstructures, and limits the hardness of the HAZ. The requirement that the roll temperature never fall below 300 °C during welding is a strict constraint that ensures continuous thermal management throughout the welding operation.

The welding speed of 25-30 m/h for a roll of 850 mm diameter implies that each circumferential pass takes approximately 10-12 minutes. For multiple passes required to build up the overlay layer, the total welding time per roll is substantial, requiring careful scheduling and thermal management.

Post-Weld Heat Treatment

The post-weld tempering at 550 ± 10 °C for 8 hours is a critical step that serves to:

• Relieve welding residual stresses that could cause delayed cracking or distortion.
• Temper any martensitic phases formed in the HAZ and weld metal to reduce hardness and improve toughness.
• Prevent cracking in the overlay layer or base material due to stress relaxation.

The tempering temperature of 550 °C is selected to be below the Ac1 transformation temperature of 80CrMnMo steel (approximately 780 °C) to avoid phase transformation, while being high enough to achieve significant stress relief and microstructural softening. The 8-hour holding time ensures uniform temperature distribution throughout the massive roll.

Economic Evaluation

The reported savings of 398,700 RMB for four rolls translates to approximately 99,675 RMB per roll. Given that a new 850 mm forged steel roll costs significantly more than this amount, the overlay welding approach provides substantial economic benefit. The paper's economic analysis demonstrates that overlay welding is not merely a technical exercise but a practical solution for extending the service life of expensive rolling mill components.

Key Reflections

This paper is a classic example of systematic engineering practice in the welding of large heavy components. The emphasis on preheat, interpass temperature control, and post-weld tempering reflects deep understanding of welding metallurgy for high-carbon, high-alloy steels. The process parameters provided are directly applicable to similar applications, and the economic justification makes a strong case for overlay welding as a cost-effective alternative to roll replacement. The methodology described here can be adapted for other large cylindrical components requiring surface hardening or repair.