Interrupted boriding of medium-carbon steels
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I. INTRODUCTION
THE surfaces of engineering components are subjected to higher stresses and greater fatigue, abrasion, and corrosive damages than the interior. Therefore, more than 90 pct of the service failures of engineering components initiate at, or near, the surface. Surface modification techniques are employed to improve the resistance to failure by producing a hard and wear-resistant case around a soft and tough core. Two major classes of treatments available for enhancing the surface properties are thermal and thermochemical. Thermal treatments, such as flame and induction hardening, modify the microstructure without modifying the surface chemistry, whereas in thermochemical methods, the surface chemistry is altered. Carburizing and nitriding are well known thermochemical methods.[1] Boriding or boronizing is a recent process, which is analogous to carburizing and nitriding. Boriding can develop surface hardness in the range of 1500 to 2000 HV, as compared to a hardness in the range of 600 to 1100 HV for nitriding, 700 to 850 HV for carburizing, and 950 to 1100 HV for chromium plating. Borided layers provide a wear resistance comparable to that of sintered carbides. The wear resistance of cold-working tools is increased by about 10 times and that of hot-working tools and dies by about 3 times as a result of boriding.[2] If the process has been performed properly and the right material and layer thickness have been chosen, boriding can extend the service life of engineering components beyond that imparted by traditional methods like carburizing or nitriding.[3,4] Various processes adopted for boriding include pack boriding, molten salt boriding, electrolytic boriding, gas boriding, vacuum boriding, etc. In pack boriding, the sample is cleaned and kept surrounded by solid mixtures consisting P. GOPALAKRISHNAN, Assistant Professor, Department of Metallurgical Engineering, and S.S. RAMAKRISHNAN, Dean, School of Metallurgy and Materials, and Professor, Department of Metallurgical Engineering, are with PSG College of Technology, Coimbatore 641 004, India. P. SHANKAR, Scientist, is with the Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India. M. PALANIAPPA, formerly ME Student, Department of Metallurgical Engineering, PSG College of Technology, is Doctoral Scholar, Indian Institute of Technology, Madras 600036, India. Manuscript submitted September 25, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
of boron carbide and borax mixtures.[5,6] Alternatively, boron carbide or amorphous boron has been used as the boron source, along with diluents such as silicon carbide or alumina or graphite and activators like KBF4 or NaF in the boriding mixtures.[4,6,8–10] Ferroboron can be considered as a boron source instead of boron carbide. However, it is reported that the commercial grades of ferroboron contain impurities like Si and Al; therefore, the use of this material leads to a degenerate layer. However, special-quality ferroboron can be used to get a good-quality boride c
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