- fr
- en
Pierre Michaud
Abstract
Influence of alloying elements on relationship between carbides precipitation and usual mechanical properties of 5% chromium tool steels
Pierre Michaud - 8 June 2006
Tempered martensitic steels containing 5% chromium, mainly used for forging and high-pressure die casting tools show a limited lifetime due to the severe thermo-mechanical working conditions. The resistance to stress at high temperature of these steels is directly related to the stability of alloyed carbides which are formed above 450°C during tempering. Carbides forming elements (W, Mo, V, Nb) as well as elements influencing the precipitation kinetics (Co, Ni) were added to a low-silicon AISI H11 steel previously studied. The characterization of carbides formed during the heat treatment was carried out using techniques such as : X-ray diffraction and electronic microscopy. To evaluate the size distribution, the volume fraction and chemical composition of the secondary precipitates of nanometric size, small angle neutron scattering (SANS) experiments were performed. A significant modification of the volume fraction and chemistry of nanometric precipitation are observed only for Mo, V and Ni additions. Moreover, results of mechanical properties showed that the volume fraction of small precipitates (VC, Fe3Mo3C) directly influences the mechanical resistance at high temperature but results in a detrimental effect on Charpy impact energy. Moreover, above a volume fraction threshold, a saturation of the yield strength is observed. Two different mechanisms dealing with interaction modes between dislocations generated during the quench and precipitates are presented to explain the saturation. Finally, different ways of improvement of steel grades are proposed.
Keywords :
Tempered martensitic steels, Hot work tool steels, Secondary
precipitation, Precipitation kinetics modelling, Isothermal fatigue.
Plan du site