A study of the wear process related to twin-screw extruders
Extruders are used in a wide range of process industries and high reliability is essential if cost effective manufacturing is to be maintained. A critical part of twin-screw extruders is the barrel that must withstand many different wear and corrosion environments depending on the end user. For many applications the extruder barrel is a critical component and it is essential that it performs in a predictable manner, providing the necessary design life-time. This project has addressed these aims by considering the wear/corrosion behaviour of current and potential extruder barrel materials from which a life prediction model has been developed. A wide range of engineering materials has been evaluated in the laboratory for abrasive wear resistance using a dry sand abrasive wear test according to ASTM G 65-93. An appraisal of the tests and the applicability of the results to the in-service conditions of an extruder has lead to further testing for abrasion and abrasion-corrosion resistance of four materials, namely Mild Steel, 440C, N18 and N18+5%TiC+5%TiN. Plastic deformation was the main feature of the damaged surfaces in the form of ploughing which has been modelled in terms of a low-cycle fatigue process. The relative hardness between material and abrasive was found to be an important parameter in controlling the rate of material removal. It has also been shown that the synergistic effect of abrasion-corrosion results in an accelerated material removal rate. The information from these tests has been used to develop a model of the wear of extruder barrels by abrasive particles. It is shown that there is a correlation between the particle size, wear debris size and wear groove size distributions. From a knowledge of the particle flux, the particle size distribution and the loading conditions, metal recession is predicted based on a low-cycle fatigue process. The wear rates for a wide range of Fe- and Ni-based materials are predicted to better than a factor of two. When corrosion is also present, the mechanism of metal recession depends on whether passive surface films are formed. For the Fe-based materials which exhibit direct dissolution of material, the wear/corrosion rate can be estimated by combining the metal loss rate under pure wear and pure corrosion conditions only. For the Ni-base alloys, thin passive films form in all the aqueous environments studied and corrosion rates are extremely low. However, during abrasive wear the passive films are removed and the overall metal recession rate is a combination of metal loss due to abrasive wear of the substrate and the continual formation and removal of surface passive films.