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Degradation Mechanisms of Heat Resistant Steel at Elevated Temperatures: In an Iron Ore Pelletizing Industry
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. (Engineering Materials)ORCID iD: 0000-0003-2190-0158
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
Keyword [en]
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: urn:nbn:se:ltu:diva-62162ISBN: 978-91-7583-826-7 (print)ISBN: 978-91-7583-827-4 (electronic)OAI: oai:DiVA.org:ltu-62162DiVA: diva2:1076895
Public defence
2017-04-28, E632, Luleå, 09:00 (English)
Opponent
Supervisors
Available from: 2017-03-01 Created: 2017-02-24 Last updated: 2017-07-10Bibliographically approved
List of papers
1. Case study of grate-chain degradation in a grate-kiln process
Open this publication in new window or tab >>Case study of grate-chain degradation in a grate-kiln process
2013 (English)In: 7th EEIGM International Conference on Advanced Materials Research: 21–22 March 2013, LTU, Luleå, Sweden, IOP Publishing Ltd , 2013, 12012Conference paper (Refereed)
Abstract [en]

Austenitic stainless steels are often used in high temperature applications due to their inherent resistance to corrosion. The grate-chain in some Grate-Kiln processes for sintering of iron pellets is made of these austenitic steels to withstand the severe environment. It has been shown however that the grate-chain is affected by several degrading mechanisms in the harsh environment of the sintering process. A grate-chain that had been used for 13 months in production was investigated in order to find the mechanisms of degradation. Results show that slag products are accumulated on the grate-chain during time and interact with the steel mainly due to the content of alkali metals. The resistance towards degradation seems to decrease with time which is suggested to be caused by the depletion of chromium.

Place, publisher, year, edition, pages
IOP Publishing Ltd, 2013
Series
I O P Conference Series: Materials Science and Engineering, ISSN 1757-8981 ; 1
National Category
Other Materials Engineering
Research subject
Engineering Materials
Identifiers
urn:nbn:se:ltu:diva-34930 (URN)10.1088/1757-899X/48/1/012012 (DOI)84893631580 (Scopus ID)93c6163e-7375-45d7-9cbe-a1d2102ce860 (Local ID)93c6163e-7375-45d7-9cbe-a1d2102ce860 (Archive number)93c6163e-7375-45d7-9cbe-a1d2102ce860 (OAI)
Conference
EEIGM International Conference on Advanced materials research : 21/03/2013 - 22/03/2013
Note
Validerad; 2013; 20130926 (andbra)Available from: 2016-09-30 Created: 2016-09-30 Last updated: 2017-07-10Bibliographically approved
2. Thermal cycling of grate-link material for iron ore pelletising process
Open this publication in new window or tab >>Thermal cycling of grate-link material for iron ore pelletising process
2017 (English)In: Ironmaking & steelmaking, ISSN 0301-9233, E-ISSN 1743-2812, Vol. 44, no 4, 269-280 p.Article 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.

National Category
Other Materials Engineering
Research subject
Engineering Materials
Identifiers
urn:nbn:se:ltu:diva-8244 (URN)10.1080/03019233.2016.1210404 (DOI)000396722800005 ()6b903639-c677-4a13-93f4-39130abbb133 (Local ID)6b903639-c677-4a13-93f4-39130abbb133 (Archive number)6b903639-c677-4a13-93f4-39130abbb133 (OAI)
Note

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

Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2017-07-10Bibliographically approved
3. Effect of Si, Mn and Ti on High Temperature Resistance of a Travelling Grate in a Grate-Kiln Indurator
Open this publication in new window or tab >>Effect of Si, Mn and Ti on High Temperature Resistance of a Travelling Grate in a Grate-Kiln Indurator
(English)In: Ironmaking & steelmaking, ISSN 0301-9233, E-ISSN 1743-2812Article in journal (Refereed) Submitted
National Category
Metallurgy and Metallic Materials Mineral and Mine Engineering
Identifiers
urn:nbn:se:ltu:diva-62159 (URN)
Available from: 2017-02-24 Created: 2017-02-24 Last updated: 2017-07-10
4. Simultaneous deposition and erosion of iron ore particles in a 300 kW combustion kiln
Open this publication in new window or tab >>Simultaneous deposition and erosion of iron ore particles in a 300 kW combustion kiln
(English)In: Open Energy and Fuels Journal, ISSN 1874-2483Article in journal (Refereed) Submitted
National Category
Metallurgy and Metallic Materials Energy Systems
Identifiers
urn:nbn:se:ltu:diva-62157 (URN)
Available from: 2017-02-24 Created: 2017-02-24 Last updated: 2017-07-10
5. Deposit-induced accelerated corrosion of as-casted ACI HHII stainless steel in a Grate-Kiln indurator
Open this publication in new window or tab >>Deposit-induced accelerated corrosion of as-casted ACI HHII stainless steel in a Grate-Kiln indurator
(English)Manuscript (preprint) (Other academic)
National Category
Metallurgy and Metallic Materials
Research subject
Engineering Materials
Identifiers
urn:nbn:se:ltu:diva-62156 (URN)
Funder
Hasselblad Foundation, 22875
Available from: 2017-02-24 Created: 2017-02-24 Last updated: 2017-07-10

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