Analysis and modeling of the failure behavior of carbonitrided parts

Abstract

This work aims at a better understanding and modeling of the failure of carbonitrided pinions made out of 20MnB5 steel. Carbonitriding is a thermochemical treatment inducing high surface hardness while preserving significant core ductility. This results in graded microstructure and properties which makes the prediction of failure particularly complex: brittle external layer, ductile core material. A test bench was specifically designed to load one tooth of the studied pinions with a lateral force until complete failure. In situ observations were performed and the load-displacement curve recorded, showing a variety of behaviors as a function of the teeth engagement depth. The presence of the carbonitrided layer induces only a slight force and toughness increase. The main failure mechanism comes from shear failure of the ductile core material which will therefore be studied in this paper. Experimental tests with various stress states were conducted to measure plastic properties as well as to calibrate fracture criteria for the core steel. Von Mises plasticity and a simple strain hardening curve fit very well all these experiments. As fracture criteria from the literature were unable to predict failure correctly for all the mechanical tests, an adapted criterion has therefore been proposed as an outcome of this extensive mechanical testing campaign. Fracture simulations in LS Dyna were performed using the element erosion technique, the limitations of which are discussed. Comparison with the experimental tooth fracture allows evaluation of the proposed failure criteria, and enables to highlight and discuss the present limits of the simulation. Keywords: carbonitriding; ductile - brittle failure; graded material; failure criteria; stress triaxiality ratio; Lode parameter