International Research Journal of Engineering and Technology (IRJET)
e-ISSN: 2395-0056
Volume: 10 Issue: 08 | Aug 2023
p-ISSN: 2395-0072
www.irjet.net
Failure Analysis of Billet Caster Tundish Tilter Bolts J N Mohapatra1, Rama Rao2, Maruti Prasad3 and D. Satish Kumar4 1Seniror Manager, R&D and SS, JSW Steel Ltd., Toranagallu, Bellary 583275 Karnataka, India 2Junior Manager, R&D and SS, JSW Steel Ltd., Toranagallu, Bellary 583275 Karnataka, India
Deputy General Manager, SMS-II, JSW Steel Ltd., Toranagallu, Bellary 583275 Karnataka, India 4 General Manager, R&D and SS, JSW Steel Ltd., Toranagallu, Bellary 583275 Karnataka, India ---------------------------------------------------------------------***--------------------------------------------------------------------Abstract - A detailed failure analysis was carried out on prematurely failed 60mm diameter bolts of billet caster tundish tilter. 3
The analysis involves visual examination, chemical composition, microstructure, inclusion, bulk hardness, charpy impact toughness, fractography of the failed and new bolt. The result show that the bolts were of 34CrNiMo6 grade with tempered martensitic microstructure, having MnS and alumina inclusions. All the bolts has failed at thread grooves with the river mark on the failed surface indicating fast fracture of the bolts. The bolts were found to be heat treated at a wrong tempering temperature leading to poor impact toughness and hence while tilting; the bots were failed prematurely due to impact load from the jerk. A mix mode of failure with numerus cracks are seen on the fracture surface indicating poor toughness of the bolts conformed by charpy test. It is recommended to correct the heat treatment as per the specification to prevent such premature failures. Key Words: Tundish tilter bolts, wrong tempering temperature, high hardness, poor impact toughness, premature failure
1. INTRODUCTION There are many reasons for which bolts and fastener fail such as overloading failure due to excess loading, fatigue failure resulting from presence of large inclusions, dents, corrosion pits, high roughness, high cyclic load, improper alignment, loose connection, corrosion fatigue, embrittlement such as temper martensite embrittlement, temper embrittlement, hydrogen embrittlement, stress corrosion cracking etc. [1-8]. 34CrNiMo6 steel is a medium-carbon low-alloy high-strength quenched and tempered steel widely used in various industrial fields. The steels are having excellent fatigue and wear properties in addition to high strength, high toughness and good hardenability [9]. It is also reported that the fasteners being stronger are not necessarily be better as hydrogen can have in the high strength fasteners to give the adverse effect [10]. The specification for 34CrNiMo6 steel is forging or hot rolling in the temperature range of 1100 to 850 oC, normalising in the temperature range of 850 to 880oC followed by air cooling, soft annealing in the temperature of 650 to 680oC followed by furnace cooling, hardening in the temperature range of 830-860oC followed by oil quench, tempering in the temperature range of 540 to 660oC followed by air cooling. With the quench and tempered condition for the thickness range of 40 to 100mm diameter, the properties should be minimum of 800MPa yield strength, 1000 to 1200MPa ultimate tensile strength, 11% elongation with 50% reduction in area and notch impact energy of minimum 45Joule [11, 12]. In the present investigation, 60mm diameter 34CrNiMo6 bolts of tundish tilter stand were analyzed to find out the root cause of failure.
2. HISTORY OF FAILURE The billet caster at SMS-II of JSW Vijayanagar Works, have approximate weight of empty tundish 35ton (Skull 7-10ton), with skull weight of tundish 45ton and in emergency condition tundish weight 70ton. The tundish tilter bolts of 60mm diameter were getting repeated failure in approximate 6 month of service. The bolts experience temperature in the range of 100-150oC in service. In the present case, all the four bolts were failed while tilting leading to fall of material. It is suspected that due to unbalancing of the tilter the accident occurred. A detailed failure analysis has been carried out to investigate the root cause of the bolt failure. Digital image for the visual examination, dimensional measurement, SPECTRO make OES for composition, optical microscopy using an Olympus make opto-digital microscope, Hitachi make scanning electron microscope (SEM), Bulk hardness (Brinell hardness) measurement through an Welspun make Universal hardness tester and Zwick/Roell make impact tester were used for the failure investigation to sort out the root cause of failure of the bolts.
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