Novel approaches for achieving full density powder metallurgy steels
Doctoral thesis, 2019
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
hot isostatic pressing.
cold isostatic pressing
liquid phase sintering
double pressing-double sintering
powder metallurgy steels
water atomised powder
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,; Vol. 74(2019)p. 244-250
XPS Analysis of Oxide Transformation During Sintering of Chromium Alloyed PM Steels
Powder Metallurgy Progress,; Vol. 14(2014)p. 85-92
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
Full Densification in PM Steels Through Liquid Phase Sintering and HIP Approach
Euro PM2018 Proceedings,; (2020)
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
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.
Innovative powder based manufacturing of gear wheels with high performance (HIPGEAR)
VINNOVA, 2014-10-01 -- 2017-03-31.
Nanotechnology Enhanced Sintered Steel Processing
Swedish Foundation for Strategic Research (SSF), 2016-01-01 -- 2020-12-31.
Full Density PM-steel through New Processing Routes
VINNOVA, 2018-07-01 -- 2021-06-30.
VINNOVA, 2017-08-01 -- 2018-03-31.
Novel sintering strategy for the competitive manufacturing of the high-performance PM components
VINNOVA, 2013-10-01 -- 2016-12-31.
Areas of Advance
Manufacturing, Surface and Joining Technology
Metallurgy and Metallic Materials
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4562
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