A Case study on failure of coconut husk chipping blades

Abstract

The exportation of coconut-based products is one of the major foreign currency earnings in Sri Lanka. Among those products, grow bags made from coconut husk chips is performed a main role. The quality of the grow bags is highly influenced by the chip quality. Existing coconut husk chipping machines consist of two circular blades which are encountered unexpected production difficulties due to fracture in the blade material and low lifetime. However, despite this component failing frequently, industrialists have no idea about the reason for the failure or a method to avoid the failure. This study has focused on the identification of the existing blade material, the reason for the material failure, and proposed a treatment to avoid the failure. Accordingly, a detailed failure analysis was carried out using Arc Spark Spectrometer, Optical Microscope and Micro hardness tester. The study has found that Type 01 blade has manufactured by cold work D2 steel. The undesirable primary carbide network (coarse carbides) present in there reduces the toughness of the steel and leads to material cracking and distortions. Also, the retain austenite is present in the cold work steel at the room temperature due to different cooling rates and thermal contractions between surface and the core. It produces comprehensive residual stresses that ultimately lead to a hardness gradient along the cross section of the blade. It favors the crack generation and mechanical failures of the blade due to high anisotropy. This anisotropic mechanical response along the cross section is mainly due to material type and lack of control of thermal treatments. In order to avoid the failure residual internal stresses and hardness gradient were eliminated by the tempering at 350 oC.