Study On The Regulation Of Microstructure And Properties Of High Carbon Alloy Steel By Friction Stir Processing

Cr3 and Cr5 steels are two typical high carbon alloy steels.They have high hardness,good toughness and other mechanical properties.They are often used as large rolls,castings and forgings,and cold work dies for cold rolls and support rolls.In this paper,friction stir processing and isothermal heat treatment at different temperatures and holding time are carried out for the two steels.Advanced material analysis and testing methods were used to study the evolution law of the structure of the processing zone under various treatment conditions,to determine the microhardness and tensile mechanical properties of the processing zone,and to discuss the strengthening and toughening mechanism of the two kinds of steel processing zones.The research on Cr3 steel shows that with the increase of heat input,the content of retained austenite in the processing area increases,and the content of retained austenite in the upper part of the processing area is more than that in the lower part,which is attributed to the chemical and mechanical stabilization of retained austenite.The carbides in the upper part of 800 rpm-30 mm/min processing zone are all dissolved,while the carbides in the lower part and the edge of the processing zone are still undissolved.The structure of the processing zone is composed of lath martensite,acicular martensite,massive austenite and thin film austenite.The hardness of the processing zone is significantly higher than that of the quenched and tempered base metal,but the structure is brittle.After isothermal treatment at low temperature for a long time in 800 rpm-30 mm/min processing area,it was found that the retained austenite completely transformed into ferrite and carbide after holding for 20 hours at 300 ?.The yield strength and tensile strength of martensite were improved significantly,and the plasticity and toughness were improved significantly.After tempering for a short time(2 h)at different temperatures,the hardness gradually decreases with the tempering temperature increasing.After tempering at 550 ?-2 h,the yield strength and tensile strength are 1540 MPa and 2076 MPa respectively,and the elongation at break is 7.1%.The research on Cr5 steel shows that the structure of 800 rpm-30 mm/min processing zone of Cr5 steel contains lath martensite,massive twin martensite and a small amount of thin film retained austenite.As the carbon content is lower than that of Cr3 steel,the chemical stabilization of austenite decreases,and no high carbon massive retained austenite is found in the processing zone.The hardness and strength of the processing zone are much higher than those of the quenched and tempered base metal,but the structure is brittle.After tempering at different temperatures,with the increase of tempering temperature,a large number of dispersed carbides precipitated from martensite during tempering at high temperatures,resulting in secondary hardening of martensite,which led to the decrease of average hardness in the processing area first and then increase.The yield strength and tensile strength after tempering at 500 ?-2 h were 1930 MPa and 2140 MPa respectively,and the elongation at break was 4%.