Mechanical and Physical Properties of Differently Alloyed Sintered Steels as a Function of the Sintering Temperature

Milad Hojati,Christian Gierl-Mayer,Herbert Danninger

doi:10.3390/met12010013

Abstract

In this paper, the effect of processes occurring during the sintering of four powder metallurgy steel grades on the resulting properties were investigated. This included three grades prepared from plain iron powder with admixed graphite, one grade alloyed also with elemental copper and another with Fe-Mn-Si masteralloy. One further grade was prepared from Cr-Mo pre-alloyed powder with admixed graphite. The effect of the sintering processes was examined in the temperature range of 700–1300 °C in an inert atmosphere (Ar). In order to study oxygen removal, DTA/TG runs linked with mass spectrometry (MS) as well as C/O elemental analysis were performed. Charpy impact tests and fractography studies were performed to study the effect of the temperature on the formation and growth of sintering contacts. Characterization also included metallography, dimensional change, sintered density, and hardness measurements to describe the dissolution of carbon and alloying elements during the process. Physical properties that were measured were electrical conductivity and coercive force. The results showed that, in all steels, the reduction of oxides that occur during the heating stage plays a key role in the formation and growth of the sintering contacts as well as in the completion of alloying processes. In the chromium alloy steel, the presence of the stable chromium oxides delays these processes up to higher temperatures, while in the other steels that are based on plain iron powder, these processes take place earlier in the heating stage, at lower temperatures. Compared to the standard Fe-C and Fe-Cu-C grades, the Cr-Mo steel requires more sophisticated sintering to ensure oxygen removal, but on the other hand it offers the best properties. The masteralloy variant, finally, can be regarded as a highly attractive compromise between manufacturing requirements, alloy element content, and product properties.

Highlights

The powder metallurgy (PM) press-and-sinter route is a well-known manufacturing process for the large-scale production of precision parts in a cost-effective manner
Sintering of the samples was performed in a SiC rod heated electrical laboratory furnace equipped with a gas-tight Kanthal APM superalloy muffle at temperatures varying in the range of 700–1300 ◦ C for 1 hr under plain Ar (99.999 quality)
Compactibility is always a main property for pressed and sintered ferrous PM parts because almost all the mechanical properties of the sintered part depend on the as-sintered density, and since the parts hardly densify during sintering—in order to retain geometrical precision, this means the green density [10]

Summary

Introduction

The powder metallurgy (PM) press-and-sinter route is a well-known manufacturing process for the large-scale production of precision parts in a cost-effective manner. One of the main advantages of the powder metallurgy process compared with ingot metallurgy is the larger flexibility in the material usage and the choice of the alloying route This potential can usually be exploited when using conventional alloying elements of powder metallurgy such as carbon, copper, nickel, and/or molybdenum, e.g., by adding the elemental powders to the mix or by diffusion bonding them to the base powder. This method cannot be effective or practical when adding powders with higher sensitivity to oxygen. Some of these elements, such as chromium, silicon, and manganese, which are usually cheaper than

Methods

Results

Conclusion