Pengujian Dan Analisis Metalurgi Terhadap Cacat Yang Terbentuk Pada Dinding Bagian Dalam Lubang Utama Sebuah Aileron Block Hasil Proses Permesinan [Metallurgical Assesment of A Broken Gearbox Intermediate Shaft of A Reciprocating Compressor]

Abstract

This paper presents the results obtained from the metallurgical assessment on a broken gearbox intermediate shaft of a reciprocating hydrogen make-up compressor. This gearbox intermediate shaft is splined at its end and made of a machinery steel of AISI 4340. This gearbox intermediate shaft was reported to have been failing frequently since the compressor was installed about thirty years ago. In the early operation during which the intermediate shaft was still supplied as original part, the shaft was reported to last for about three years, but later after the shaft was made by local manufacturer, its service life decreased significantly to less than one year or even only a few months. To perform metallurgical assessment, a number of specimens were prepared either from the broken shaft or from the unbroken shaft for laboratory examinations including macroscopic examination, chemical analysis, tensile test, metallographic examination, hardness test, and SEM (scanning electron microscopy) examination equipped with EDS (energy dispersive spectroscopy) analysis. Results of the metallurgical assessment obtained showed that the gearbox intermediate shaft had experienced predominantly to fatigue fracture caused by a high nominal stress due to the combination of shear stress, bending stress and torsion stress. The fatigue fracture was initiated from the tooth root of the shaft splines where high stress concentration present, and propagated into two directions, one in the anticlockwise transverse and radial direction approximately 45° to the shaft axis, and the other was to form a radial “whirlpool” crack pattern prior to the fast growing final fracture to form cup and cone like fractures. The high nominal stress experienced by the intermediate shaft during its operation may have been caused by the low strength of the material used for the intermediate shaft. The results of chemical analysis obtained showed that the material used for the intermediate shaft was very much close and met to the material specification of AISI 4340. However, from the results of mechanical tests obtained, the material used apparently did not meet to the material specification of AISI 4340 in the as-normalized condition. The low mechanical property of the intermediate shaft material in comparison with the standard material was very much influenced by its microstructures which contained a mixture of bainite or tempered martensite, pearlite and ferrite. The presence of the pearlite and especially ferrite in the microstructures could reduce the mechanical properties quite significantly and this may have been associated with some improper manufacturing and/or heat treating processes applied to the shaft. In addition, the acceleratioof fatigue fracture occurred on the intermediate shaft was likely not contributed by any corrosion.