MECHANICAL AND TRIBOLOGICAL PROPERTIES OF POWDER FORGED Fe-P ALLOYS

Abstract

Phosphorus (P) is an undesirable element in steels. During cold working, phosphorus induces cold shortness or brittleness due to which its content in modern steels is limited to < 0.04 wt. %. It has been seen that the embrittlement of phosphoric iron is due to segregation of phosphorus along the grain boundaries. The inter-granular failure occurs due to decohesion of the grain boundary. The resistance to brittle fracture is reduced by addition of phosphorus in the steel. On the other hand, phosphorus addition has been reported to improve magnetic permeability, lower coefficient of friction and improve wear resistance and corrosion resistance. Phosphorus provides solid solution strengthening to the ferrite matrix. The Iron-Phosphorus alloys are two-phase materials, which have the potential to be used as wear resistant materials. Their microstructure consists of hard phosphide phase in the ductile matrix of the ferrite. The presence of phosphide phase in these alloys also leads to an increase in hardness. Despite these advantages, it is rarely used because of the low ductility associated with phosphorus-containing alloys. Fe-P alloys may be prepared by melting and casting route. Here, the last liquid to solidify is rich in phosphorus and this can lead to embrittlement. Fe-P Powder metallurgy alloys developed by conventional pressing and sintering route of metal powders takes a long time to complete the powder metallurgy process. To expedite this process, liquid phase sintering (LPS) has been used. Most powder metallurgy (P/M) approaches for Fe-P alloys use liquid phase sintering by using the eutectic reaction where liquid decomposes into Fe (α) and Fe3P. In LPS the liquid at the particle boundaries is enriched in phosphorus; therefore, both cast as well as conventional powder metallurgy (P/M) routes, have limitations. In the present work, the Fe-P alloys have been developed by hot powder forging process to overcome the problem of segregation at the grain boundary and heavy volume shrinkage. In this process, phosphorus was admixed with water-atomized iron powder by a chemical route. Further, the powder was heated below the melting point (liquid phase) of the alloy (upto1050 oC) in the presence of hydrogen gas (H2 gas) prior to forging. The surface oxides were reduced during heating by the continuous flow of H2 gas. This facilitates proper bonding between the powder particles. Care was taken that no liquid phase formation took place during the processing. Powder forging ensures a higher density and may lead to improved properties. In the present work, the powder forged alloys with phosphorus contents ii ~1wt. % or less exhibited significant ductility. Possible reasons for this are discussed. The solid-state processing used may lead to the development of ductile phosphorus containing alloys with attractive properties. Iron-phosphorus alloys were developed by hot powder forging route to avoid the problem of segregation and shrinkage as well as to control the dimensional stability. The process produced forgings with negligible porosity. In the investigation, hot powder forging was carried out in such a manner that the formation of the liquid phase is avoided during forging. In the present study, six binary Fe-P alloys such as Fe-0wt.%P, Fe-0.35wt.%P, Fe-0.65wt.%P, Fe-1.3wt.%P, 2wt.% and 3wt.%P have been developed by hot powder forging technique. In P/M alloys prepared by conventional pressing and sintering route, small additions of carbon (<0.05wt.%C) to the Fe-P alloys may also reduce brittleness. A ternary alloy with carbon i.e., Fe-0.65wt.%P-0.20wt.%C was developed by the same route. The samples were cut off from the forged slabs after the removal of end caps and mild steel skin for the characterization. Mechanical properties were improved by the addition of P in the iron. Strength was observed to increase from 302MPa to 495MPa with the addition of phosphorus in the iron by scarifying the ductility. The Fe-0.65wt.%P showed the best combination of mechanical properties (UTS 365MPa and elongation 12%) among the six binary alloys. The significant amount of ductility accompanied with high density is one of the important achievements of the present investigation. Significant increase in strength and hardness of Fe-0.65wt.%P alloy occurred with the addition of 0.20wt. %C. The strength and hardness were improved from 365MPa to 552MPa and 111HV to 200HV respectively by addition of 0.20wt.%C in Fe-0.65wt.%P. The influence of P addition (0 to 3wt.%) on the dry sliding behavior of Fe-based alloys developed by hot powder forging has also been investigated in the present work. The effect of normal load on the wear behavior of Fe-P alloys is also studied. The wear mechanisms are oxidative, abrasive and delaminations depending on the amount of P addition. The Fe-3wt.%P alloy shows the minimum wear loss and lower coefficient of friction among all the binary alloys developed in the present investigation during dry sliding condition. Without P content alloy exhibited a higher wear rate which indicates that the ferrite matrix did not support wear in this alloy. Also, the tribological investigation was carried out on Fe-0.65wt.%P-0.20wt.%C alloy. The wear rate, Archard wear coefficient and the average coefficient of friction observed for Fe-0.65wt.%P-0.20wt.%C are less than for Fe-0.65wt.%P alloy. For alloy Fe-0.65wt.%P- iii 0.20wt.%C these values are 0.07 x 10-5mm3/m, 0.05 x 10-7 and 0.44 to 0.40 for three loads respectively and for Fe-0.65wt.%P the values are 0.46 x 10-5mm3/m, 1.4 x 10-7 and 0.54 to 0.48 for three loads respectively. This is attributed to the higher hardness of steel containing pearlite apart from phosphide. The results show that wear rate decreased by the addition of C in the Fe-P alloy which indicates that the pearlite resists the wear of material. The corresponding coefficient of friction was lower which indicated the real contact area decreased by the formation of oxides. Among all the developed alloys in the present work, the carbon containing alloy was found to undergo minimum wear loss and lower coefficient of friction during the dry sliding test. The mechanism of wear was observed to be mainly oxidative in the behavior