Internal Friction Study On The Mechanism Of Bake Hardening And Cottrell Atmosphere

This paper focus on Bake Hardened ability for low carbon steels deeply,explore the formation of Cottrell atmosphere at initial aging stage,precipitation of carbide after baking a long time and Bake Hardened ability under annealing state.BH value,stress-strain curves and internal friction have been measured to reveal bake hardened mechanism from atomic level.The main results are as follows:(1)When the pre-deformation is 2% and the bake temperature is 170?,with the increase of baking time the BH value and the height of SKK peak is linear.Their relationship can be expressed as BH=17119*hSKK-33;the main hardening mechanism of low carbon steel is Cottrell atmosphere strengthening,the main reason of the increase of yield strength is the increasing of dislocation density which is pinned by Cottrell atmosphere.The concentration of carbon atoms in dislocation is constant.(2)Under different aging processes,as the pre-deformation increased from 2% to 5%,the BH value decreased and the height of SKK peak increased;when the baking temperature is 170 ?,as the pre-deformation increased from 5% to 16%,BH value decreased and the yield point elongation increased.(3)With the increase of heat treatment time,the average grain size increases and the bake hardening value decreases gradually.The heat treatment increases the BH value of the material.With the increase of heat treatment time,the concentration of carbon atoms and the height of SKK peak increase.The BH value is affected by C atoms and Cottrell atmosphere.After bake treatment the height of Snoek peak becomes lower and the SKK becomes higher.(4)Compared with the original sample,the thermal treatment sample has high BH value;with the holding time prolonged,the carbon content decreased and SKK peak increased gradually;during the aging process,and the carbon atoms diffuse into grain boundaries and dislocations and format the fine carbide;the p2 peak,pitting and decrease of sample strength were respond to carbon stripping process;the process leads to the soften of sample and the decrease of BH value.