Study Of Microstructure And Mechanical Properties Of Novel High-strength Bainitic Steel

We found a medium carbon low alloy bainitic steel with its mechanical propertiesmatching the critical properties of140-maraging steel and its heat treatment process, thatis, the35MnSiCrNiAlMo steel was cooled slowly from its martensite starting point (Ms)to the temperature30℃below the Ms point (Ms-30℃) and carbide-free bainitic ferriteand retained austenite duplex structure with the thickness of thin and the length of longwas obtained.The microstructure and mechanical properties of the medium carbon low alloy steelthat was subjected to a series of different austempered heat treatment processes werestudied by X-ray diffraction(XRD), transmission electron microscopy(TEM) and tensionand impact tests. Results show that the steel has achieved the highest combination ofstrength and toughness and matches those of the140-maraging steel, which is at leasttwenty times more expensive. The optimal comprehensive mechanical properties can beobtained when the steel is subjected to a continuous cooling bainite transformation fromMs to Ms-30℃at a cooling rate0.5℃·min-1, which the comprehensive mechanicalproperties are better than those of the austempered samples at any constant temperature.This heat treatment process can make the comprehensive mechanical properties ofpotential of the bainitic steel to develop perfectly. That is because a large number ofbainite microstructures with smaller thickness and longer length that are generated belowthe Ms point are obtained when the steel is subjected to this kind of heat treatment process.This type of microstructure, which is potentially used for the large components, can not beobtained by the conventional heat treatment process.