Fracture Toughness: A Quality Index for Railway Solid Wheels

Abstract

About 10 years ago, the European Standard EN13262 was introduced, and, for the first time in railways, the toughness characteristic of the rim has to be proofed for tread-braked solid wheels. For wheels of steel grade ER7 the average fracture toughness obtained from six test pieces shall be greater than or equal to 80 MPa m1/2 and no single value shall be less than 70 MPa m1/2. The present paper focuses on the efforts in research and, of course, application of fracture mechanics to railway solid wheels. The obtained results demonstrate that the capability to measure fracture toughness of materials and a proper collection of the related metallurgical parameters help to develop the manufacturing process of wheels. The main challenge is to improve toughness in parallel with yield strength by maintaining a desired microstructure of the material. Some important correlations between structure and fracture toughness are specified and document how the toughness can be tuned. The properties on the production of solid wheels, monitored over a long period, have confirmed the ability to guarantee both high and uniform values of mechanical characteristics and toughness. This is an important result concerning product quality and safety of railway wheels. Fracture toughness and yield strength represent an appropriate quality index for any material and steel grade. A production index (PI), which is the product of yield strength and fracture toughness, is introduced to trace the quality of a material in production.