MACHINABILITY IMPROVEMENT THROUGH HEAT TREATMENT IN 8620 LOW-CARBON ALLOYED STEEL

Abstract

The effect of different heat treatments is evaluated on SAE 8620 low-carbon alloyed steel by means of drilling tests. Improving machinability through prior heat treatment in steels used for nitro-carburizing surface treatments is very important in the manufacturing of large series of parts, due to its impact in production costs. This is the case for the commonly used SAE 8620 grade, in its carburized and quenched and tempered state, for the production of gears, shafts and other transmission box components for the automobile industry. The machinability of the steel, determined by simple drilling tests (which are typical in industry labs), is a function of microstructure, which is determined by the state in which the steel is received and/or heat treatments prior to carburizing. This work shows that by employing some inter-critic annealing treatments, followed by sub-critic isothermal ones, the machinability of 8620 steel can be improved by ∼16% over the typical as-received cold drawn state.

Keywords:

• chip morphology
• drilling test
• heat treatment
• machinability
• metallography
• 8620 steel

ACKNOWLEDGMENTS

We acknowledge the fund “Apostar por el Futuro de México” granted by IPADE and Universidad Panamericana which made this work possible. We also thank Mr. García and Mrs. Quesada (Universidad de Oviedo, Spain), for their valuable contribution to the experimental work and revision of figures and text.

Notes

(CD: cold drawn, F: ferritic structure, IA: isothermal annealing, N: normalized, A: annealed, CC: controlled cooling, SR: stress relieved, S: spherodized, QT: quenched and tempered, G: totally globulized, and NA: normalized and air-cooled).