Dimensional changes on heat treating of sintered steel
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The dimensional changes that occur in the steel during heat treating is one of the most difficult problems facing the heat treater. While the influence of the alloy chemical composition, of the heat treatment cycle and of the shape and dimensions of the pieces on the deformations caused by heat treatments on the wrought steels are now a days well known, the presence of the porosity in the sintered steels arouses some inconvenients which influence directly the heat treatment effects. The present work concerns the study of the application of hardening surface treatments like nitriding, nitrocarburizing, carburizing and carbonitriding to sintered steel having different chemical composition and density. Beyond the hardening effects, the dimensional changes caused by the different processes have been well studied, pinpointing the influence of the alloys chemical composition as well as of the process. In particular the high temperature processes, like carburizing and carbonitriding especially, can cause very high dimensional change. Taking out from the industrial production PM automotive components (syncrohubs), manufactured employing Fe-P or Distaloy AB or Astaloy Mo powders, the effects of nitriding and nitrocarburizing processes have been studied. Molten salths bath, gaseous treatments as well as plasma processes have been adopted, some times performing steam oxidation before gas nitriding. Measuring the outer diameters increase of the pieces, it was clear that the execution of steam oxidation before nitriding treatments is advantageous because it contributes to a better dimensional control, even if the adopted nitriding cycles can be applyed to pieces, having chemical composition like Distaloy AB only, without any previous steam oxidation treatments. The Astaloy Mo pieces can be advantageously treated in plasma. In particular the adopted cycle allowed very little dimensional changes, while when subjected to gas nitriding processes without previous steam oxidation these alloys can undergo quite high dimensional changes. The molten salths bath, TF1 process, employed for the treatment of the P alloyed pieces allow satisfactory results, with good reliability, but they introduce ecological problems as well as possible future corrosion phenomena for the treated pieces. Being the surface hardening of stainless steels a problem of topical interest, a series of AISI 316L stainless steel sintered bushings have been plasma nitrided adopting different cycles, in order to search the proper treating parameters. Taking in account for the quite low density of these pieces, the dimensional changes caused by the treatment appear well controlled and relatively low. Moreover the axial density gradient of the pieces clearly influences the changes, allowing the greatest diameter increase in the center position of the bushings, where the density is the lowest.