| In this paper, a system research on the effects of Al element addition and heattreatment on the phase transformation and mechanical properties of Fe-1.5C-1.5Cr-xAlUltra High Carbon Steel (UHCS) was done based on theory analysis combined withexperimental tests. The control mechanism of Al element on the separation of theproeutectoid carbide was also discussed. Based on optimizing proper Al element addition,a comprehensive heat treatment technics without deformation to gain excellent combinedproperties with ultrafine grained microstructure in Fe-1.5C-1.5Cr-2.0Al UHCS wasproposed. The main research conclusions were presented as follows:(1) Thermodynamics phase graphs of Fe-1.5C-1.5Cr-xAl UHCS were calculated under"Thermo-Calc" thermodynamics calculating system. The addition of Al element wasfound to make the austenitic mono-phase region of UHCS become narrower andshift towards left. The temperature to reach liquid and solid binal phases decreases.With increasing the Al content, the onset temperature as well as phase transformationinterval for the eutectoid transformation increases. The eutectoid transformationtemperature interval of Fe-1.5C-1.5Cr-2.0Al UHCS is in about 40℃, and the starttemperature of eutectoid transformation is about 790℃.(2) Remarkable restrain effect of Al element on proeutectoid carbide (netlike carbide andcarbide in wildmanstatten tissue) was found in UHCS. When the Al content reaches1.0%, wildmanstatten tissue is suppressed. As Al content increases further close to2.0%, netlike carbide begins to be broken.(3) The mechanism for the Al element suppressing the formation of proeutectoid carbideis proposed: The increase of undissolved carbide after austenite treatment decreasesthe carbon content in austenite which results in a decrease in the amount ofproeutectoid carbide during cooling. The unresolved carbide suppresses the growthof austenite grains. Refinement in austenite grains reduces the number of carbidedistributed in grain boundary. Aggregation of Al in grain boundary greatlysuppresses the nucleation and growth of carbide in the boundary.(4) Space between pearlite flakes decreases and the temperature for pearlite tospheroidization totally decreases with increasing Al content. When Al contentreaches 2.0%, a totally glomeration tissue can be gained after 850℃×1h isothermalspheroidization treatment. However, when Al content exceeds 2.54%, graphitizationis found. The proper addition quantity for Al in Fe-1.5C-1.5Cr-xAl UHCS is 2.0%。 (5) Al element can increase thermal stability of UHCS against tempering and retardscementite formation fromε-carbide, which attribute to the low plastic after mediumtemperature tempering. Based on this research, we bring forward that it is verydifficult to increase the combined properties of Fe-1.5C-1.5Cr-2.0Al UHCS underquenching-tempering treatment.(6) Isothermal transformation points and Ms temperature of Fe-1.5C-1.5Cr-2.0Al UHCSwere measured. The shortest gestation time is about 30minutes at 300℃, while itsMs is 150℃.(7) A comprehensive isothermal quenching heat treatment technology withoutdeformation to gain excellent combined properties was proposed forFe-1.5C-1.5Cr-2.0Al UHCS. After isothermal quenching of 300℃×10h, a kind ofultrafine complex phase structure composed of low-bainite ferrite with item width of100nm,residual austenitic less than 100 nm in width and spherical cemenite withdiameter of about 250nm was gained. The yield strength reaches 1568MPa, fracturestrength 1744 MPa, the total elongation 6.9%, and microhardness HV532.
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