Magnetic Fe16N2

New possibilities in material design

Industry currently makes extensive use of the nitrocarburising process for surface hardening of stainless steel. The process gives stainless steel excellent properties, making it far more durable against wear and fatigue. Nitrocarburising is a special surface treatment whereby nitrogen and carbon are added in a gas atmosphere at temperatures of 400 to 600°C. The positive effects of the treatment are well-known, but to date there has been insufficient theoretical understanding of the thermodynamics and the processes the materials undergo during the treatment. Professor Marcel Somers from DTU Mechanical Engineering has been involved in several parallel and complementary research projects and arrived at a coherent understanding of what happens in stainless steel when nitrogen and carbon are dissolved, following comprehensive interdisciplinary work. This work is paving the road for targeted surface engineering of stainless steels and design of new materials with even better performance.

A broad and far-reaching cooperation
The ThInSol project, financed by the Danish Research Council for Independent Research, has been a broad academic collaboration between various departments at DTU, as well as internationally with Pennsylvania State University. Two PhD projects have investigated the synthesis and experimental characterization (Bastian Brink), and modelling of the thermodynamics from first principles (Morten Bjørn Bakkedal).

At DTU Chemistry Associate Professor Kenny Ståhl assisted with characterizing the crystal structures in the materials with X-ray diffraction; Professor Catrine Frandsen at DTU Physics assisted with characterizing the materials' magnetic properties with Mössbauer spectroscopy. Associate Professor Mikkel Fougt Hansen at DTU Nanotech assisted in measuring magnetic properties as a function of temperature.

Heating, magnetism and deformations
Stainless steel behaves in many different ways when it is heated, and when nitrogen is added in the treatment process. “I find it both strange and exciting,” says Marcel Somers, “that stainless steelcan be both magnetic and non-magnetic. For example, when you pick up a knife from the cutlery drawer at home, it is magnetic, but a fork or spoon is not. This is because the knife has a different crystal structure. When the spoon or fork is heavily deformed it becomes magnetic” During nitrocarburising of stainless steel, nitrogen is gradually added to the material. In the course of this process it will become magnetic without changing its crystal structure, but then loses its magnetic properties when even more nitrogen is added. “It is very relevant,” says Marcel Somers, “not for the sake of publishing new knowledge, but because we have demonstrated it so systematically. This knowledge is extremely important when you try to make a proper model for how the material behaves during heating or cooling, because the magnetic properties are closely coupled to the expansion of the material.”

A design example from ThInSol:
Designing a new synthesis route for nano-porous materials

An iron foil can be converted into an iron sponge by a series of steps where nitrogen is dissolved (a process called nitriding) from an ammonia atmosphere into the iron, followed by total removal of nitrogen with hydrogen gas (called denitriding). In the nitriding step, iron nitrides which are not thermodynamically stable are formed. As a consequence, nitrogen gas filled bubbles develop within the iron nitride. Repetition of the nitriding/denitriding steps at successively lower temperature leads to finer bubbles at shorter distance from each other.  Finally, an extremely porous iron foil remains where the wall thickness in between successive cavities is of nanometer size. Such a nano-porous foil can be converted quickly into iron nitrides, iron carbides or iron oxides. The series of micrographs shows fracture surfaces of porous iron foils after nitriding/denitriding steps at the temperatures indicated, and consequently the development of the structure in the material. The foil indicated with 380
ºC was subsequently converted into the magnetic Fe16N2 phase at a low temperature. The top row shows the entire thickness of the foils; the bottom row shows a higher magnification. The new porous material has properties that can be an advantage for application as a precursor for manufacturing permanent magnets, catalyst carriers or scaffolds for deposition of other materials.

A new material for hard magnets
Iron nitride Fe16N2 powder is considered a promising material for permanent magnets, and is considered a candidate for the next generation of permanent magnets after the rare-earth-based magnets.

 Magnetic Fe16N2 This Fe16N2 phase was synthesized directly from nano-porous iron foil, crushed and mounted in transparent resin. The magnetic properties of a small amount of powder magnetized by another magnet are sufficient to attract a thin steel plate.

In the EU PressPerfect project with several European academic and industrial partners, which has been running in parallel with ThInSol, PhD student Federico Bottoli, has worked on the project,“Influence of steel composition and plastic deformation on the surface properties induced by low-temperature thermochemical processing”, under the supervision of Grethe Winther, Thomas L. Christiansen and Marcel Somers. The project team discovered a method to make the low temperature surface hardening treatment independent of the deformations, an approach which one of the industry partners in PressPerfect may attempt to implement in their production of electric shavers.

Finally, DTU has financed the PhD project on the modelling of composition and stress profiles in nitrided stainless steel. In this project Freja Jespersen worked under the supervision of Marcel Somers, Jesper Hattel (MPP) and Viggo Tvergaard (FAM) and succeeded as the first in the world to realistically simulate the simultaneous evolution of composition and stress profiles in nitrided stainless steel from physical data, without any fitting.


Kontakt

Marcel A. J. Somers
Sektionsleder, Professor, Dr.ir.
DTU Mekanik
45 25 22 50
http://www.mek.dtu.dk/Forskning/Temaartikler/ThInSol
21 NOVEMBER 2017