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Development of Technology to Control Mg Content in Molten Nickel Alloy of NW2201
2009 (English)In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 49, no 6, 837-842 p.Article in journal (Refereed) Published
Abstract [en]

Technology to precisely control Mg content has been developed to improve hot-workability of NW2201 Ni alloy. Relatively high standard deviation of Mg contents, 0.007, out of average Mg content of 0.02 mass% was problem among heats. This brought poor hot-workability leading to sliver-like surface defects on the strip when Mg content was relatively high or low. Laboratory-scale experiments were performed to understand the behavior of Mg in terms of equilibrium and mass-transfer of the slag/metal reaction of 3(MgO)+2Al=(Al2O3)+3Mg. Experiments were firstly carried out to understand Mg content in molten Ni equilibrated with CaO-Al2O3-MgO-CaF, system slags. It was found that Mg content in liquid Ni depended on Al content and C/A (CaO/Al2O3 ratio in mass%) in the slag. The measured Mg contents mostly agreed with thermodynamic calculation. Further, kinetic analysis was also undertaken experimentally by approaching from both higher and lower Mg contents to the equilibrium Mg content. As a result, mass-transfer of Mg in molten Ni is considered as the rate-determining step for the above reaction. By applying the countermeasures, introduced from the view points of both equilibrium and kinetics, for practice, the standard deviation of Mg content was improved from 0.007 to 0.003 among heats. Consequently, the defects have been improved successfully.

Place, publisher, year, edition, pages
2009. Vol. 49, no 6, 837-842 p.
Keyword [en]
NW2201, nickel, hot-workability, Mg, Al, slag, equilibrium, kinetics, mass-transfer
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-27766DOI: 10.2355/isijinternational.49.837ISI: 000267402900012OAI: oai:DiVA.org:kth-27766DiVA: diva2:380863
Note
QC 20101222Available from: 2010-12-22 Created: 2010-12-22 Last updated: 2017-12-11Bibliographically approved
In thesis
1. Control of Alloy Composition and Evaluation of Macro Inclusions during Alloy Making
Open this publication in new window or tab >>Control of Alloy Composition and Evaluation of Macro Inclusions during Alloy Making
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In order to obtain a good performance and predict the properties of alloys, it is necessary to control the contents of alloying elements and to evaluate a largest inclusion in the product. Thus, improved techniques for both control of alloy elements and evaluation of the large inclusion in products will enable us to provide better qualities of the final products.

In the case of one Ni alloy, (NW2201, >99 mass%Ni), the precise control technique of Mg content is important to obtain a good hot-workability. Hereby, the slag/metal reaction experiments in a laboratory have been carried out at 1873 K, so that the equilibrium Mg content and kinetic behavior can be understood. More addition of Al in the melt as well as higher CaO/Al2O3 value of slag resulted in higher amount of Mg content in Ni. For the same conditions of Al content and slag composition, the mass transfer coefficient of Mg in molten Ni was determined as 0.0175 cm/s. By applying several countermeasures regarding the equilibrium and kinetic process to the plant trials, the value of the standard deviation for the Mg content in an alloy was decreased till 0.003 from 0.007 mass%.

The size measurements of largest inclusions in the various alloys (an Fe-10mass%Ni alloy, 17CrMo4 of low-C steel and 304 stainless steel) were carried out by using statistics of extreme values (SEV). In order to improve the prediction accuracy of this method, three dimensional (3D) observations were applied after electrolytic extraction. In addition, the relationship of extreme value distribution (EVD) in the different stages of the production processes was studied. This was done to predict the largest inclusion in the products at an early stage of the process.

A comparison of EVDs for single Al2O3 inclusion particles obtained by 2D and 3D observations has clarified that 3D observations result in more accurate EVD because of the absence of pores. Also, it was found that EVD of clusters were larger than that of single particles. In addition, when applying SEV to sulfide inclusions with various morphologies, especially for elongated sulfides, the real maximum sizes of them were able to be measured by 3D observations. Geometrical considerations of these particles clarified the possibility of an appearance of the real maximum inclusion sizes on a cross section to be low. The EVDs of deoxidation products in 304 stainless steel showed good agreement between the molten steel and slab samples of the same heat. Furthermore, the EVD of fractured inclusion lengths in the rolled steel were estimated from the initial sizes of undeformed inclusions which were equivalent with fragmented inclusions. On the other hand, from the viewpoint of inclusion width, EVD obtained from perpendicular cross section of strips was found to be useful to predict the largest inclusion in the final product with less time consumption compared to a slab sample.

In summary, it can be concluded that the improvement of the techniques by this study has enabled to precisely control of alloy compositions as well as to evaluate the largest inclusion size in them more accurately and at an earlier stage of the production process.

Place, publisher, year, edition, pages
Stockholm: KTH, 2010. 53 p.
Keyword
slag/metal reaction, equilibrium, kinetics, largest inclusion, statistics of extreme values, electrolytic extraction, cross section, inclusion morphology
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-27773 (URN)978-91-7415-846-5 (ISBN)
Public defence
2011-01-14, B1、, Brinellvägen 23, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20101222Available from: 2010-12-22 Created: 2010-12-22 Last updated: 2012-03-28Bibliographically approved

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