Fracture and fatigue crack propagation properties of hardened 52100 steel
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INTRODUCTION
S T E E L of 52100 composition is universally used for through-hardened bearings, and the philosophy and justification for its use have been reported in Reference 1. This steel is not only an excellent through-hardening bearing steel but is also very successfully applied for induction surface-hardening applications. 12,3~ For present day steelmaking methods, 52100 is typically of less than 15 p p m oxygen, and commonly less than 8 ppm, which can result in infinite bearing lives for correctly lubricated bearings at normal application loads. Bearing element fatigue strength is not only related to the basic steel cleanliness; the correct microstructure must be developed by heat treatment, and full surface integrity must be maintained upon grinding. Good toughness to tolerate the assembly, mounting, and applied structural stresses must also be present. Tolerance to shock loads is particularly important; the selected steel composition and heat treatment procedure are, therefore, a compromise between static load carrying capacity, fatigue strength, and toughness. Little systematic work has been reported on the influence of steel quality on fracture properties of hardened 52100 steel, and all of the reported work on the toughness has been for soft annealed bar. Spheroidize annealed and cold pilgered tubing is the most c o m m o n material form for bearings, and from previously reported work, tal it is known that the prior microstructure and, particularly, prior cold work have a significant influence on the heat treatment response of 52100 steel.
J.M. B E S W I C K , Department Head, is with the Department of Physical Metallurgy, SKF Engineering and Research Centre BV, 3430 DT Nieuwegein, The Netherlands. Manuscript submitted February 24, 1988. METALLURGICAL TRANSACTIONS A
In this paper, not only the influence of martensitic hardening and tempering conditions on the fracture properties of 52100 steel are reported but also the influence of isothermal treatments, carbon content within the 52100 range, steel cleanliness, residual copper, prior structure, and cold deformation during tube manufacture. The resuits are discussed, where appropriate, with respect to microstructural relationships for 52100 steel. II.
EXPERIMENTAL PROCEDURES
Ball bearing steels of various compositions and quality levels were used for the investigation. The normal "soft working" structure for 52100 steel is the soft annealed condition, and this was the starting structure for all the materials and heat treatments. The material code compositions and sizes are listed in Table I. The cold-worked tube material had been given a 50.9 pct area reduction in the cold rolling operation. The fracture toughness tests were performed on compact tension specimens of various sizes; however, the specimen size was always constant for each investigated parameter. The "standard" compact tension (CT) specimens had dimensions B = 12 and W = 50 mm. "Mini C T " specimens, B = 5 and W = 20 m m , were used for the tube material tests and thin CT specimens, B =
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