Of Alloying Elements On Properties And Microstructure Of Powder Metallurgy Low Alloy Steels

In the article, based on the former studies about the effect of Ni addition to Fe, Fe-2Ni was chosen to be the base. The effect of the amount of Cr and Mo addition on the mechanical properties of Fe-2Ni alloys by sintering and by heat treatment was respectively studied to expect to attain an alloy with excellent properties.Fe-2Ni-xCr (x=0.5,1,3) alloy prepared by MIM were investigated. The results show that with Cr increasing, the density of alloys reduce, the amount of pores increases and the pores are larger; the intensity and the hardness of sintered Fe-Ni-Cr alloy increase, the elongation and the impact energy of sintered Fe-Ni-Cr alloy reduce. The fractures of the alloys change from dimple to cleavage fracture with river pattern. The microstructures of sintered alloys are pearlite and little ferrite. There are the enrichment areas of Ni in Fe-2Ni-3Cr alloy. The microstructures of alloy with Cr addition after heat treatment change to temper troostite and there occur meshy secondary cementite in Fe-2Ni-3Cr alloy.During the process of sintering, the sintering temperature and time directly influence the diffusion of alloys and the change of pores. In the article, Fe(CS)-2Ni-lCr+0.3C alloy is respectively sintered in 1120℃ 1250℃ 1336℃ and the results show that with the sintering temperature increasing, the density of alloys increases, the intensity, the hardness and ductility of sintered Fe-Ni-Cr alloy gradually increase. Diffusion function f (t, T) is used to reflect the input of the energy during the process of sintering and predict the properties of the alloy. The prediction results of intensity and hardness of Fe (CS)-2Ni-lCr+0.3C alloy by f (t, T) are respectively G=0.5015f(t,T)+329.91and hardness=0.0545f(t,T)+53.71Fe-2Ni-xMo+yC ( When y=0.3, x=0.5,1,2,5 ; when x=1,y=0.3,0.5,0.8) alloy prepared by P/M were investigated. The results show that with Mo increasing, the intensity and the hardness of sintered alloy increase, the elongation of sintered alloy reduce. The changes of the intensity, the hardness and the elongation of alloy after heat treatment have the analogic results. With Mo added, the microstructures of alloyschange from ferrite to bainite and martensite . The microstructures of Fe-2Ni-lMo+0.8C are all bainite and martensite. There occur the enrichment areas of Mo in Fe-2Ni-5Mo+0.3C alloy. The results of X-ray test of four alloys show that the alloys are almost composed of Fe solid solution. In addition, there occurs Fe3Mo3C in Fe-2Ni-5Mo+0.3C alloy which helps to increase the intensity and hardness.Based on anterior respective studies about the effect of adding Cr and Mo to Fe-2Ni, the alloy that Cr and Mo were together added to Fe-2Ni in the appropriate amount was investigated to expect an alloy with good properties and compared to Fe-2Ni-0.5Cr-0.5Mo alloy. The results show that the density of Fe-2Ni-1Cr-2Mo alloy is much lower than that of Fe-2Ni-0.5Cr-0.5Mo alloy. The amount of pores of Fe-2Ni-0.5Cr-0.5Mo alloy is a few and the pores are small. The intensity of sintered Fe-2Ni-lCr-2Mo alloy is 867MPa, 200MPa higher than that of Fe-2Ni-0.5Cr-0.5Mo alloy, while their elongation is close. The properties of Fe-2Ni-1Cr-2Mo alloy after low temperature tempering is equivalence to those of Fe-2Ni-0.5Cr-0.5Mo alloy after middle temperature tempering expect impact energy.