Characterization of Industrial Powder Metallurgy Produced 410L ODS Steel

Zeybek, Asim (2012). Characterization of Industrial Powder Metallurgy Produced 410L ODS Steel. PhD thesis The Open University.



The overall aim of this project is to characterize a powder metallurgy produced Oxide Dispersion Strengthened (ODS) 410L steel in terms of mechanical and microstructural analysis. An industrial/commercial existing production route was followed instead of a laboratory production. A professional powder metallurgy company did the production of the materials with high-tech manufacturing facility.

Materials were based on 12 Cr 410L steel. Four kinds of materials were manufactured based on 410L: "410L HIPed", which is 410L powder compacted via hot isostatic pressing (HIP) without mechanical alloying process; "410L MA", mechanically alloyed and then HIPed; "ODS 0.9 μm", mechanically alloyed 410L + 0.25 wt. % Y2O3 powder of 0.9 pm size and HIPed; and "ODS 50 nm" mechanically alloyed 410L + 0.25 wt. % Y2O3 powder of 50 nm size and HIPed.

Tensile tests and creep tests were performed for mechanical characterization. Yield strength and ultimate tensile strength of the samples at room temperature are comparable and around 680 MPa and 830 MPa respectively.

The best creep life belongs to non-ODS HIPed sample with a creep life of 679 hours at 625 °C under 75 MPa stress. Mechanical performance of the materials are found not promising when compared to other ODS alternatives from the literature, owing to compositional variations and non-metallic inclusions (up to 2 μm) in the materials. It was expected to have significant increase in the strength in ODS samples as it is seen in other ODS alternatives. However mechanical strengths of all the samples were comparable to each other.

The 410L powder was not clean with contaminations and many inclusions in it. Those inclusions are found as SiO2 and MnS with scanning electron microscope (SEM) and energy dispersive X-Ray (EDX) analysis. These inclusions have a negative effect on mechanical behaviour of the materials as they ease void nucleation and propagation because of very weak interface bonds to the matrix. Another effect of SiO2 particles are found as those particles play a role in formation of oxide clusters during mechanical alloying and high temperature compaction. It is concluded that either SiO2 particles hinder Y2O3 dissolutions via reacting in ball milling and/or serving as nucleation sites to Y2O3 during precipitation. In both cases Y2Si2O7 particles are observed due to the interaction between SiO2 and Y2O3.

Small Angle Neutron Scattering (SANS) revealed pure Y2O3 particles in the system as well and clarified the effect of heat treatment on the size of oxide clusters. It is found that yttria particles had not been dissolving during ball milling. It is discovered that the size of the initial yttria powder is as important as the amount. Heat treatment increases the size of the Y2O3 particles but decreases the size of complex Y2Si2O7 particles. This can be considered as there is a Y and/or Y2O3 move from complex particles to pure Y2O3 particles at high temperatures.

The main outcome of this project is: understanding possible manufacturing problems and contaminations due to industrial production route and causes on material performance; mechanical and material characterization of a novel ODS alloy from 410L matrix; effects of steel inclusions on ODS manufacturing like hindering yttria dissolution and/or chemical interaction with yttria and forming complex oxides; importance of size of initial yttria powders on ODS manufacturing.

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