Novel approaches for achieving full density powder metallurgy steels
Doctoral thesis, 2019

Powder metallurgy (PM) is one of the most resource-efficient methods for manufacturing structural components with complex shapes. The utilisation of the metal powder to shape the components allows to minimise material waste and increase energy efficiency. However, with increased usage of PM parts in high-performance applications, there is a demand for components that can withstand extreme loading conditions with properties being equivalent or better than those of their wrought counterparts. The PM steel components fabricated through press and sinter route, even with all their advantages, have limitations due to the presence of residual porosity. Hence, it is desirable to reach full density to meet the highest performance demands. This study covers different powder consolidation approaches for water atomised steel powder with the aim of reaching near full density. This is achieved through the following processes: cold isostatic pressing (CIP) followed by sintering, liquid phase sintering (LPS), double pressing-double sintering (DPDS). These approaches were complimented by capsule free hot isostatic pressing (HIP) to reach full density.

Densification and subsequent enhancement of mechanical properties are to a certain extent directly connected to the successful removal of the surface oxide layer, covering the metal particles. This behaviour is especially critical in the case of powder pre-alloyed with oxygen-sensitive elements as chromium. The hydrogen in the sintering atmosphere reduces most of the surface iron oxide layer and any oxide residues are transformed into more stable oxides rich in Cr and Mn. Vacuum sintering provides oxide reduction through the formation of better local microclimate in the pores. When the powder is encapsulated and processed using HIP, the initial surface oxide is transformed into stable oxide particles that decorate the particle boundaries. Based on these results a model of oxide transformation during powder consolidation is proposed with regards to the alloy composition, powder properties and processing conditions.

In order to realise full density, CIP is utilised for consolidating iron powder and Cr-Mo pre-alloyed water atomised powder to reach a relative density of around 95% in sintered state to attain surface pore closure. This allows for subsequent HIP without capsule to reach full density. In case of Mo pre-alloyed powder, the LPS approach utilising Ni-Mn-B master alloy was established for enhanced sintering and densification. The best mechanical properties were then obtained with 0.12 wt.% of boron that allowed reaching as-sintered relative density of up to 96%. In addition, pore free surface was obtained after sintering that enabled capsule-free HIP to reach full density. Through the DPDS process, a pore free surface could also be achieved, which enabled reaching full-density through the subsequent HIP. Even though fine powder showed better densification, the density gradient in the compact persisted from the first pressing is there as the low-density region i.e., neutral zone, in the middle of the compact even after second pressing and HIP. Hence, optimisation during the first pressing is necessary to avoid this phenomenon.

All the above approaches represent different methods of achieving full density and selection of the appropriate method depends on the required geometry, alloy composition and hence resulting properties, number of components, cost, etc. Based on the analysis of the different methods it can be concluded that the combination of the tailored alloy concepts and consolidation techniques allows manufacturing of complex-shaped full-density PM components for high-performance applications.

Cr- and Mo-alloyed PM steels

vacuum sintering

high density

full density

hot isostatic pressing.

cold isostatic pressing

liquid phase sintering

master alloy

double pressing-double sintering

powder metallurgy steels

sintering

water atomised powder

Virtual Development Laboratory (VDL-room), M-huset, Chalmers Tvärgata 4C, Gothenburg
Opponent: Professor Mónica Campos, Universidad Carlos III de Madrid, Madrid, Spain

Author

Maheswaran Vattur Sundaram

Chalmers, Industrial and Materials Science, Materials and manufacture

Effect of density and processing conditions on oxide transformations and mechanical properties in Cr-Mo-alloyed PM steels

Vacuum sintering of chromium alloyed powder metallurgy steels

Metal Powder Report,; (2019)

Journal article

XPS Analysis of Oxide Transformation During Sintering of Chromium Alloyed PM Steels

Powder Metallurgy Progress,; Vol. 14(2014)p. 85-92

Journal article

Capsule-free hot isostatic pressing of sintered steel to full density using water atomized Fe and Cr-alloyed powder consolidated by cold isostatic pressing

Enhanced Densification of PM Steels by Liquid Phase Sintering with Boron-Containing Master Alloy

Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science,; Vol. 49(2018)p. 255-263

Journal article

Full Densification in PM Steels Through Liquid Phase Sintering and HIP Approach

Euro PM2018 Proceedings,; (2018)

Paper in proceedings

Experimental and finite element simulation study of capsule-free hot isostatic pressing of sintered gears

International Journal of Advanced Manufacturing Technology,; Vol. 99(2018)p. 1725-1733

Journal article

From a historical perspective, utilising metal powder for making goods, monuments and jewellery were adopted as early as 3000 BCE by Egyptians; Inca’s used gold powder for making ornaments and the Iron pillar in Delhi, India, was made using reduced iron ore by hammering and it exists even today as a standing monument. However, powder metallurgy as an industrial process has grown rapidly only within the last century. Earlier development was focused on tungsten filaments as it was not possible to process tungsten through other methods due to the high melting point. Manufacturing steel parts or components from metal powders has grown in the beginning of the 20th century. Powder metallurgy, as a metal forming technique, utilises raw material in the form of powder particles, which are shaped into desired form using die-tools by pressing at high pressures. Once shaped, they are heated in a furnace to form metallic bonds, to provide the necessary strength to the component which is called sintering. Further, utilising metal powder for manufacturing conserves raw material, as it is consumed only for the desired shapes. Moreover, the energy required to make a steel component using press and sinter route is much lower than the other manufacturing processes such as casting, forging, and machining. This makes it very much attractive for using this process route for the mass production of components.

However, the main drawback of this approach is the inability to reach full density, which limits the application of these materials. It is established that the properties of materials are a direct function of density; hence, increasing the density increases the properties and thus its potential applications. Therefore, the main focus of this study is to find the possible ways to reach full density in powder metallurgy steels, such that the process can be directly implemented for manufacturing. To do so, different processes utilising pressure, temperature, and combination of both were evaluated utilising low-alloyed steel powder. The challenges associated with powder processing at different stages were also addressed. From the results, it was demonstrated that full densification can be achieved through the proposed approaches based on the requirements. Hence, with this immense potential, these approaches provide opportunities for continuous progress of powder metallurgy in the future.

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Driving Forces

Sustainable development

Areas of Advance

Production

Materials Science

Subject Categories

Manufacturing, Surface and Joining Technology

Metallurgy and Metallic Materials

ISBN

978-91-7597-881-9

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4562

Publisher

Chalmers University of Technology

Virtual Development Laboratory (VDL-room), M-huset, Chalmers Tvärgata 4C, Gothenburg

Opponent: Professor Mónica Campos, Universidad Carlos III de Madrid, Madrid, Spain

More information

Latest update

3/7/2019 8