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Thermal cycling of grate-link material for iron ore pelletising process
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.ORCID iD: 0000-0003-2190-0158
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.ORCID iD: 0000-0003-3661-9262
Number of Authors: 32017 (English)In: Ironmaking & steelmaking, ISSN 0301-9233, E-ISSN 1743-2812, Vol. 44, no 4, p. 269-280Article in journal (Refereed) Published
Abstract [en]

A test-rig for thermal cycling has been developed to investigate the mechanisms behind some of the damages done to a grate-link in a pelletising indurator for iron ore pellets. The results from the test-rig were compared with the degradation of grate-links that had been in service for 8 months in the indurator. The damages on the grate-links induced both in the test-rig and at service were defined as internal oxidation which facilitated propagation of intergranular stress corrosion cracking (IGSCC). Internal oxidation was initiated by spallation or removal of the oxide layers. The test-rig was able to successfully perform a lab-scale simulation of the degradation experienced in the indurator. Thermal cycling was applied to plate-formed specimens by constraining their thermal elongation to form bending and thereby stimulate the mechanisms of spallation and sensibilisation to IGSCC by internal oxidation.

Place, publisher, year, edition, pages
Taylor & Francis, 2017. Vol. 44, no 4, p. 269-280
National Category
Other Materials Engineering
Research subject
Engineering Materials
Identifiers
URN: urn:nbn:se:ltu:diva-8244DOI: 10.1080/03019233.2016.1210404ISI: 000396722800005Scopus ID: 2-s2.0-84980010123Local ID: 6b903639-c677-4a13-93f4-39130abbb133OAI: oai:DiVA.org:ltu-8244DiVA, id: diva2:981135
Note

Validerad; 2017; Nivå 2; 2017-03-22 (rokbeg)

Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2018-11-19Bibliographically approved
In thesis
1. Degradation Mechanisms of Heat Resistant Steel at Elevated Temperatures: In an Iron Ore Pelletizing Industry
Open this publication in new window or tab >>Degradation Mechanisms of Heat Resistant Steel at Elevated Temperatures: In an Iron Ore Pelletizing Industry
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis focuses on the different degradation mechanisms of the stainless steel in a travelling grate in a Grate-Kiln iron ore pellet indurator. The travelling grate is a conveyor belt that transports green-body pellets to a rotary kiln while the pellets are being dried and pre-heated to a temperature of 900-1100 °C by recycled hot air. After unloading of the pellets to the rotary-kiln for further sintering, the travelling grate is cooled in room temperature while returning to the loading zone of the wet pellets.

The steel was tested during thermal cycling in a test-rig, in order to simulate the influence of thermo mechanical fatigue and oxide spallation. The influence of erosion-deposition was investigated in a modified horizontal industrial combustion kiln at 800 °C, with slag and coal from production used as erosive media and combustion fuel, respectively. The influence of minor alloying additions of Mn, Si and Ti on the microstructure was explored by eight different casted alloy compositions. Isothermal heat treatments were performed at 800 °C during 200 hours on steel immersed in deposits recovered from a travelling grate in production.

The three main degradation mechanisms found in this work are thermal spallation, erosion-deposition and deposit induced accelerated corrosion (DIAC). Thermal spallation of the oxide layer is caused by the thermal expansion difference between the oxide and the metal during heating and cooling. It has been found that Ti improves the spallation resistance while Si reduces it. Spallation of deposits is another cause believed to increase the degradation. Erosion-deposition appears due to simultaneous erosion and deposition of particles on the travelling grate that causes erosion or deposition depending on the amount of alkali metals in the environment. The velocity of the particles also influences erosion and deposition in the way that higher velocities increase erosion. DIAC is proposed to form on the travelling grate due to the concentration of chloride- and sulphate containing alkali metals in the deposits.

 Other than these major degrading mechanisms, minor degradation mechanisms such as internal oxidation, sigma formation, carburization and sensitization towards inter-granular attack have been found inside the steel during heating. Thermo mechanical fatigue (TMF) causes intergranular cracks in the material of the travelling grate. Casting issues such as micro-segregation have also been addressed in this thesis.

A few different ways to improve degradation resistance have been proposed, such as homogenization heat treatments, optimization of process parameters and inhibitor solutions. 

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2017
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Keywords
stainless steel, Grate-Kiln, microstructure, corrosion, as-casted, thermal cycling, erosion, deposition
National Category
Metallurgy and Metallic Materials Mineral and Mine Engineering Corrosion Engineering
Research subject
Engineering Materials
Identifiers
urn:nbn:se:ltu:diva-62162 (URN)978-91-7583-826-7 (ISBN)978-91-7583-827-4 (ISBN)
Public defence
2017-04-28, E632, Luleå, 09:00 (English)
Opponent
Supervisors
Available from: 2017-03-01 Created: 2017-02-24 Last updated: 2017-11-24Bibliographically approved

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Nilsson, ErikTegman, RagnarAntti, Marta-Lena

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