| As the most productive and widely used material in the powder metallurgical industry, ferrous powder metallurgical(PM) materials have been applied to fields of automotive, mechanics, electronic industry ,and so on, because of its high material using ratio, low cost, excellent integrative properties. Though its application fields are also limited, due to the existence of pores and sintering structure asymmetry. Getting high density and nice structure is the way to obtain high performances. To increase the density depends on improvement in raw material powders, pressing and sintering technique. However, it increases the cost in the meantime. Nice structure can be achieved by alloy-strengthening and heat treatments after sintering. It is the easy way to put into practice and costs little in the industry, so we used chromium as strengthening alloy element in the form of Cr-Fe powders. Otherwise, we adopted high-frequency induction quenching to material surface after sintering. In this work, the ferrous powder metallurgical materials were made by adding 0.5%(wt) graphite powders, 0.8%(wt) zinc stearate and Cr-Fe powders to Fe-2.0Ni-0.85Mo-1.75Cu system pre-alloyed powders. We changed the content of Cr from 0.2% to 1.5% in order to learn the effect of Cr on the ferrous powder metallurgical materials. To make sure that Cr can diffuse completely to strengthen the materials when sintering, raw Cr-Fe powders were changed into micro-fine powders by mechanical milling. The PM steel samples were prepared through single pressing and single sintering, with the pressures of 425,500,560,610MPa. The samples were commonly sintered at 1150℃for 1h, then high-frequency induction quenched. The effect of element Cr on the sintered density, on the microstructure characteristics and on the mechanical properties of Fe-2.0Ni-0.85Mo-1.75Cu system steel were investigated by means of optical microscope(OM), scanning electron microscope(SEM), transmission electron microscope(TEM), microhardness measurements and X-ray analysis. The tribological characteristics of the PM samples were studied by using a MM200 tribotester.In this study, the size of the Cr-Fe powders after mechanical milling is mostly in the range of 100 to 300nm, and the powders are nearly changed into the shape of sphericity. When Cr content is in the range of 0.2% to1.5%, the sintered density increased with pressure. It goes to 7.04～7.08g/cm3 under the pressure of 610MPa, the pore ratio was less than 10%, so we used the samples under this pressure to test their mechanical properties.The microstructure of the samples after sintering is composed of ferrite and pearlite, and the microstructure become more symmetrical and finer with Cr content increasing. Apparent hardness value of the sintered samples increases with Cr content and pressure, and reaches 102HRB with 1.5% Cr addition under 610MPa. Meanwhile, microhardness value also increases with Cr content. After high-frequency induction quenching, we get a mixed structure composed of dislocation martensite and twinned martensite in the material surface, and associated with a few remained austenite. There is a big change in the microhardness value in the surface layer, with an increase by HV0.2300～400. Moreover, surface quenched samples show a kind of layered structure. From the surface to the centre, surface quenched materials can be divided into three layers. The first layer is complete hardened layer and has the highest microhardness value, the second layer is a transition layer, and the third layer is the matrix. The microhardness value and depth of the hardened layer all increase with Cr content, the depth of the hardened layer can almost reached 20～40μm.The tensile strength of the sintered materials increases from 480MPa without any Cr to 700MPa with 1.5%Cr., increasing by almost 46%. At the same time, the ductility of the materials has been improved, the elongation of them is among 5%～8%. The fracture surface shows a mixed characteristic of dimple, quasi-cleavage and cleavage river pattern. The cracks initiate from inclusions, interior pores and Cr-rich hard areas. The tensile fracture mechanism of sintered Fe-2.0Ni-0.85Mo-1.75Cu materials is a mixed mechanism of tough fracture and brittle fracture.Besides the tests above, we also carried out the wear test. The type of the wear test was dry sliding wear, with a sliding distance of 3000m and a pressure load of 150N. The results show that the wear resistance of sintered steel is mostly determined by microstructure and Cr content. The wear resistance of high-frequency induction quenched samples is better than sintered state samples because of their higher hardness. And it increases with the content of Cr element, the quenched samples with 1.5% Cr show the best wear resistance.On our experiment condition, it is concluded by both the wear tests of the sintered state samples and the quenched state samples that: it is a multi-mechanism that abrasive wear, oxidative wear, adhesive wear and delamination wear coexist during the process of wearing. The wear mechanism of the sintered samples, whose microhardness is lower, is dominated by adhesive wear. Otherwise, that of the high-frequency induction quenched samples which have higher microhardness is dominated by both oxidative wear and delamination wear.According to the experiments above, we can draw a conclusion that the properties of Fe-2.0Ni-0.85Mo-1.75Cu system powder metallurgical materials have been apparently improved with microfine Cr-Fe powders addition.
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