Advanced material and processing for the high temperature carburising of microalloyed case hardening steels

Case hardening steels are a group of high quality structural steels, which are mainly used in gear systems and driving elements in the automotive, aerospace and general engineering industries. With regard to long process times for carburising at standard carburise furnace conditions, it is very effective to accelerate the carburising process by using higher carburising temperatures. However, it has to be considered that carburising temperatures up to 1100 degrees C require a precise austenite grain size control. Recent research activities have shown that an adequate austenite grain size control during case hardening at temperatures above 1000 degrees C can be realized by the application of microalloying elements (Nb, Ti) as well as an aligned ratio of N and Al. However, it is not predictable in which way different process steps during manufacturing - like additional annealing, different degrees of deformation, -temperatures and deformation rates - affect the appearance of the precipitates which are responsible for grain size control. The objective of this investigation is the determination of the effect of varying degrees of deformation and -temperatures on the austenite grain size stability of the microalloyed case hardening steel 16MnCr5. The appropriate process steps and parameters have been reproduced in laboratory scale to enable a specific austenite conditioning during high temperature carburising. The evaluation and documentation of the grain size as a function of heat treatment temperature was made using statistical methods and quantitative metallography. For analyze of the chemical composition as well as distribution and size of the precipitates TEM-investigations were carried out.