Influence of Free Ferrite Content on the Machinability of GCI using CBN Tool

Abstract

Over the years, industrial partners experienced issues related to inconsistent free ferrite in gray cast iron. In the course of a hot summer and a cold winter, machinability balance varies up to 350%. During warm months it is better and cold months worse. The inconsistent machinability increases the scrap rates by up to 400% and reduces tool life by up to 70%. The ultimate objective of this research is to collaborate with an automotive industrial partner to investigate the periodic machinability variability of gray cast iron engine sleeves, with a goal to reduce the cost and scrap rate of tool inserts. In this study, a significant amount of work was conducted concerning sample preparation for a metallography check. Samples from different months with varying amount of “free ferrite” were collected to study the seasonal effect on their machinability and high-speed machining under similar industrial conditions. Furthermore, a room temperature age strengthening of gray cast iron was conducted to demonstrate how the hardness increase from aging could improve the machinability of gray cast iron. A CBN insert is the second hardest cutting tool after the diamond insert, and is widely used in industry for machining cast iron. It has a high cutting speed is commonly used due to its high hardness and impressive wear resistance. It is known that gray cast iron can naturally age at room temperature or artificially age under a controlled temperature. Under different aging temperatures and times, gray cast iron exhibits a greater hardness after age strengthening, which affects the CBN tool life. The latter is usually limited by flank wear length, however the content of free ferrite in gray cast iron can generate ferrous built-up on the CBN cutting edge and significantly shorten the tool life of the CBN cutting tool.