| With the gradual development of mechanical machining in high-speed,automation and precise, especially the rapid development of automobile industry,household appliances and precision instrument industry, free-cutting steel hasobtained considerable development, the lead series free-cutting steel is second only tosulfur series one in production, possessing good mechanical properties and heattreatability, it is often used in producing various of mechine parts and autocomponents. However, the element lead can cause environmental pollution, so theresearch of free-cutting steel without lead can have great realistic significance. Inorder to develop this new kind of environment friendly free-cutting steel based oncurrent studies on free-cutting steel at home and abroad, several environment friendlyBi-Mn-Fe alloys were designed, and their preparation technology, microstructure aswell as the properties and microstructure of free-cutting steels were study in the paper.Alloy fusion casting method and powder metallurgy technique were adopted toobtain Bi-Mn-Fe alloys with different composition and scanning electron microscopycoupled with energy dispersive X-ray spectroscopy were used to study theirsolidification microstructure, heat treatment microstructure, and the microstructure ofBi-free cutting steel into which the alloy had been added, the properties of the steelwere also studied using the electro-hydraulic servo universal testing machine andcommon lathe, and the results can provide references to use them in the environmentfriendly free-cutting steel production.Following research conclusions have been obtained:(1) Using the fusion casting method, the Bi-Mn-Fe alloy with high bismuthcontent, eg70wt.%-80wt.%, was prepared and its microstructure mainly consisted ofthree phases, i.e., BiMn, Mn(Fe) and Bi. When the liquid Bi-Mn-Fe alloy was cooledin permanent mould, the amount of BiMn and Mn(Fe) phases increased with theincrease of Mn content and the optimal melting temperature was1200℃. When thealloy was quenched in water, the water temperature had great effect to themicrostructure, the optimal water temperature was45℃. When the Bi-Mn-Fe liquidalloy was cooled in melting furnace or just cooled in air, the microstructure could beparticularly courser and larger.(2) After homogenization treatment, the microstructure of the Bi-Mn-Fe alloy which was prepared using the fusion casting method still consisted of three phases,i.e., BiMn, Mn(Fe) and Bi, the morphology of BiMn phase in Bi-Mn-Fe alloychanged from discrete distribution or isolation to continuous distribution, the amountof Mn(Fe) and Bi phase decreased. For permanent cast Bi-Mn-Fe alloy, the longer thehomogenization time is, the more the amount of BiMn phase, however, the amount ofBiMn phase changed no longer after16h homogenization at400℃. For liquidBi-Mn-Fe alloy quenched in water, the amount of BiMn phase did not change whenthe homogenization time was longer than4h at400℃. The amount of BiMn phase inBi-Mn-Fe alloy annealed at300℃was more than that at400℃and it changed nolonger when the homogenization time exceeded8h.(3) Using the fusion casting method, the Bi-Mn-Fe alloy with high bismuthcontent, eg lower than20wt.%, was prepared and there exist serious regionalsegregation, the microstructure on the center of the Bi-Mn-Fe alloy specimenconsisted Bi+BiMn+Mn (Fe) phase, and that on the surface of the alloy mainlyconsisted of Mn (Fe) phase and small bismuth particles which distribute uniformlywithin Mn (Fe) phase. It is difficult to prepare low bismuth content Bi-Mn-Fe alloyby fusion casting method which could conform to the quality requirements offree-cutting steel production.(4) Using the powder metallurgy method, the obtained Bi-Mn-Fe alloy with lowbismuth content, eg lower than20wt.%, was prepared and its microstructure consistedof Mn (Fe) phase and bismuth phase, BiMn compound was not found in the alloy.When Bi-Mn-Fe alloy was sintered at temperatures near the melting point of bismuth,the adequate sintering time should less than30min at250℃while the time shouldless than15min at300℃. Powder metallurgy method is an effective method for thepreparation of low content bismuth Bi-Mn-Fe alloy.(5) Bi free-cutting steels were fabricated using Bi-Mn-Fe alloy prepared withpowder metallurgy and casting method, and Bi exists in the form of fusiform, conespherical and near spherical shape in Bi free-cutting steel. With the increase of Bicontent in steel, the tensile strength of the steel decreases and the cutting performancecould be improved. When the adding amount of Bi in steel is the same, the Bi yield ofBi-Mn-Fe alloy prepared with casting method is higher comparing with powdermetallurgy method. |
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