Preliminary evaluation of fatigue in carburized, conventionally and intensively quenched steels

Abstract

Steels treated through carburizing thermochemical treatment and quenching and tempering thermal treatment are broadly used in components that need to have hardness and superficial mechanical resistance together with good toughness in the core of the component. Additionally, it is possible to produce surface compressive residual stresses that normally improve fatigue resistance. Relatively unknown, the intensive quenching thermal treatment is a method that presents some advantages, one of them being the possibility of avoiding cracking by distortion due to extreme cooling. Other advantages are the increase in mechanical resistance, the use of shorter carburizing cycles, improvement of fatigue performance, among others. Once the cooling rate is high, low carbon steels can be used instead of low alloy steel such as AISI 8620. This way, it is possible to use less costly steel and to obtain the advantages of intensive quenching.
The present work aims to compare samples carburized during 6 hours at a temperature of 920oC and carbon potential of 0,9% both for AISI 1020 and AISI 8620 samples, through mechanical and metallurgical analyses, being the principal the production of Wohler curves together with fractographic analysis in low magnifying glass and scanning electron microscope. Results pointed out that the AISI 1020 steel presented grain size which is three times bigger than AISI 8620 steel grain size. The effect of optimizing intensive quenching when applied to AISI 1020 steel is practically covered by the fact that AISI 8620 steel presents a more refi ned structure, with smaller grain size comparatively and therefore better mechanical properties. This way, intensive quenching treatment can provide superior performance to non-alloy steels relatively to alloy steels only if grain size is equal or inferior.

Key words: carburizing, intensive quenching, fatigue.