Research On Microstructure And Properties Of Chromium Carbide Iron Matrix Surface Gradient Composite By In-situ Technology

It was recognized that material failures, such as fatigue fracture, wear andcorrosion, occur on the material surface in most conditions. Thus the preparation of anovel surface enhanced composite material will greatly improve the service life ofmachine parts. At present, the composites comprised with metal matrix and gradientmicrostructure surface become the research hotspots in this area. The surfacecomposites performed by in situ techniques possess clean interface betweenreinforcement and matrix, continuously changing in the ingredient and structure, as wellas high hardness, high toughness and excellent chemical stability. The novel compositeshave a wide range of application in industries.This thesis focuses on Fe-Cr-C ternary alloy system, combines casting process andin situ technology, prepared chromium carbide iron matrix surface gradient compositewith different characteristics from pure grating and grey cast iron as raw materials. Themicrostructure, phase composition, and micro-area composition of the composite werecharacterized by scanning electron microscope (SEM), X-ray diffraction (XRD),differential thermal analyzer (DSC). The mechanical properties and wear performanceof the composite were tested by micro hardness tester, abrasive wear testing machine,friction wear testing machine and other equipments. The main results expressed asbelow.(1) In the Fe-Cr-C ternary system, four different reaction layers were preparedunder the setting of in situ reaction temperature (1180℃). The four reaction layers were:dense ceramic layer (Fe,Cr)7C3(I),(Fe,Cr)7C3granular layer (II), strip-like (Fe, Cr)7C3plate layer (III) and non-graphite layer(IV). (2) The hardness test results show that the microhardness were gradient distributedfrom the layer I to the matrix. The microhardness reached the highest in layer I, as1484HV0.1, as7time higher as the microhardness in matrix (214HV0.1). The averagemicrohardness of layer II and layer III (carbide and matrix composite area) was800HV0.1, about4times of the matrix. And the micro hardness in layer IV (no graphite area)was324HV0.1, slightly higher than that of the matrix. This is because a small amount ofchromium element solved into the matrix in the layer IV (no graphite area), whichenhanced the matrix strength.(3) The two body abrasion with different load experimental results showed that,under the same load, the dense ceramic (Fe,Cr)7C3layer presented the highest relativewear resistance. The main wear mechanisms were micro cutting and micro cracks. Andthe direction of the micro cracks was vertical to the wear direction. Increasing the load,friction coefficient decreases and the decline rate was gradually slow, wear quantityincreases. At low load (5N) wear mechanism was priority to the abrasive wear. Underthe high load (15N), wear mechanism was mainly abrasive wear too. However, fatiguewear mainly occurred in the high load (20N) condition.