Failure Analysis of Drive Spindle Sleeve of Finishing Mill of Hot Strip Mill

Abstract

This paper analyses the failure of a bottom spindle head sleeve in a finishing mill stand. The sleeve, manufactured by Mill OEM and installed in 2022 (after overhauling), exhibited fatigue cracking after approximately 2.9 years of service. Metallurgical analysis revealed lower hardness and microstructural changes in the interphase weld zone, stemming from multiple previous refurbishments involving hard facing. Repetitive welding during these overhauls induced thermal cycles, leading to softening of the parent material and ultimately, fatigue failure. Finite Element Analysis (FEA) showed that stress levels at the failure location remained within the acceptable range at rated load but exceeded the safe limit at peak loads (150% of rated current). Recommendations include improved supplier oversight, enhanced quality assurance procedures for overhauling, and a more robust maintenance protocol to prevent recurrence.

Introduction

This study investigates the failure of a bottom spindle head sleeve used in the finishing mill of a hot strip mill (HSM), which plays a critical role in transmitting torque from the spindle to the work roll. The failure caused a breakdown and significant production downtime.

The sleeve was found cracked at multiple locations, with crack initiation observed near the wear pad region, which is directly involved in load transmission. The component had undergone five refurbishing cycles, and cracks were identified as fatigue-related, propagating from the cladded wear pad area toward the base material.

Material analysis showed that the base material (18CrNiMo7-6) and surface hardness were generally within acceptable limits. However, detailed metallurgical examination revealed a significant hardness drop in the interphase (HAZ) zone (~175 BHN) compared to the parent material (260–290 BHN). Microstructural changes were also observed, indicating transformation due to repeated thermal cycles and welding during overhauling.

The investigation concluded that the primary cause of failure was repetitive welding during refurbishing without adequate post-weld heat treatment, which led to microstructural degradation, reduced hardness in the heat-affected zone, and ultimately fatigue crack initiation and propagation. Inadequate quality control during overhauling, including missing inspection and heat treatment records, further contributed to the failure.

Overall, the study highlights that improper refurbishment practices and lack of controlled post-weld heat treatment significantly reduced the sleeve’s mechanical integrity, leading to premature failure. Preventive measures include strict overhaul limits, proper heat treatment procedures, and improved monitoring of material properties after repair.

Conclusion

1) Communicate overhauling defects and low hardness issues to the supplier. 2) Develop a comprehensive Overhauling Quality Assurance Plan (QAP) for spindle sleeves, including: • DPT and UT inspections after removing the hard face layer. • In-situ microstructure and hardness checks of the parent sleeve body. • Rejection criteria based on abnormalities. • Hardness criteria for interphase/parent material. • Complete test certificates, including post-weld heat treatment data. • Specifications for welding electrode type, size, and number of layers.

References

[1] Spindle assembly drawing (SMS/0447750-SPINDLE, R0), Jan’1998 [2] Article:- Failure Analysis of Work Rolls of a Thin Hot Strip Mill, January 2015, Case Studies in Engineering Failure Analysis, DOI: 10.1016/j.csefa.2015.01.001, Authors: Author: Piyas Palit Hrishikesh R. Jugade, Arvind kumar Jha Souvik Das Goutam Mukhopadhyay

Copyright

Copyright © 2026 Ankush Sharma, Himangshu S Mandal, Vinod Kumar, Anuj Kumar, Ajay Kumar Singh, Yoginder Singh. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.