Breakage of Spring Steel During Manufacturing: A Metallurgical Investigation

Abstract

Spring steel is a name given to a wide range of steels used in the manufacture of springs, prominently in automotive and industrial suspension applications. High carbon of HC82BCr grade steel is drawn in the range of 4.0–3.0 mm as per customer’s requirement for production of spring mainly in automobile components. This spring steel grade is produced from continuous cast billet of size 150 mm × 150 mm and then hot rolled into the size of 5.5 mm diameter wire rod. Quality aspects of these spring are significant as any inherent abnormality in the material could lead to unwanted productivity losses or quality claims or sometimes serious safety issues. In this present study, spring samples broken during coiling stage were collected from plants for understanding their quality aspect and potential causes of breakage during production. Detailed microstructural investigation revealed that the root cause of breakage of the spring is related to the segregation of phosphorous and chromium in the billet which leads to the formation of martensite (brittle and less ductile) in wire rod during rolling process. This volume change due to formation of martensite increases the stress level at the interface of pearlite and martensite. During wire drawing, this increased stress level causes formation of internal crack in subsequent passes causing breakage during spring formation.