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Inclusions and/or Particles Engineering for Grain Refining Purposes in Ferritic Fe-20mass%Cr alloys
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy. (Tillämpad Processmetallurgi, Applied Process Metallurgy)
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Compared to more common used austenitic stainless steels, ferritic stainless steels contain very low amounts of the expensive alloying element Ni. In addition, they have good corrosion properties, but are sometimes suffering from poor weldability and bad mechanical properties. This is mainly due to the presence of large grains after casting and large grain growth during heat treatment or welding. Processes for reducing the grain size (grain refining) of metal alloys are widely known and proven before to be suitable for many alloys. A successful grain refining process can increase the strength of an alloy without decreasing the ductility. This can be achieved by different methods, such as rolling or cooling. In this work, the focus has been on studying the aspect from a metallurgist point of view, to analyse the possibilities to create small particles in the liquid stage to enhance the solidification. The focus has been on oxide and nitride formation for nucleation of smaller grains during solidification.

This study was made by forming particles, develop the analysis methods and thereafter to study the effect of particles on the structure of a ferritic stainless steel. The particles were formed by additions of Ti, Ce and Zr in to a liquid Fe-20mass% Cr alloy containing different amounts of oxygen and nitrogen. The electrolytic extraction technique was used to achieve a good understanding of the reaction processes and the particles size, number, composition and morphology. The grain sizes and the particles were then studied in as-cast samples as well as in specimens heat treated for 60 minutes at 1200, 1300 and 1400°C in a Confocal Scanning Laser Microscope (CSLM). Also, based on Scanning Electron Microscope (SEM) determinations and Thermo-Calc calculations, the precipitated particles were divided as primary and secondary particles. Thereafter, the grain refining potential was studied for each of these types.

In this work, particles engineering by using small particles have been proven suitable for the Fe-20mass% Cr ferritic stainless steel alloys. Although the work has been based on small-scale samples and experiments, the results show good tendencies with respect to grain refining as well as a clear relationship between the particles in the steels and the microstructures. It was found that Ti-Ce additions in Fe-20mass% Cr alloys resulted in complex oxides including Ti, Ce and Cr. These oxides were observed to have high agglomeration tendencies and therefore to form larger particles or clusters. The grain refining potential on the solidification structure was insignificant, despite a relatively large amount of particles. However, Ti-Zr additions in Fe- 20mass% Cr alloys on the other hand resulted in primary precipitated particles such as ZrO2 and ZrO2+ZrN. Furthermore, ZrN nucleated ferrite during solidification, which lead to a clearly observed grain refining effect. This effect was also increased with an increased number of particles. The amount of particles (primary and secondary) was also found to increase with an increased amount of nitrogen. Secondary particles (mostly TiN) were precipitated near grain boundaries, which lead to a location based pinning effect of the grain growth during heat treatment at 1200 °C. This pinning effect was increased by an increased nitrogen content and thereby an increased number of particles. Finally, the pinning effect was clearly reduced during heat treatment at 1400 °C.

Place, publisher, year, edition, pages
Stockholm: KTH , 2010. , x, 55 p.
Keyword [en]
Inclusions, Particles, Grain size, High Cr, Ferritic, Grain refining, Solidification, Pinning effect, Stainless steel, Oxides, Nitrides
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-13012ISBN: 978-91-7415-599-0 (print)OAI: oai:DiVA.org:kth-13012DiVA: diva2:320220
Public defence
2010-05-20, F3, Lindstedtsvägen 26, KTH, Stockholm, 13:00 (English)
Opponent
Supervisors
Note
QC20100524Available from: 2010-05-24 Created: 2010-05-24 Last updated: 2011-09-19Bibliographically approved
List of papers
1. Characteristics of Ti-Ce Complex Deoxidation Products in a Fe-20mass%Cr Alloy
Open this publication in new window or tab >>Characteristics of Ti-Ce Complex Deoxidation Products in a Fe-20mass%Cr Alloy
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2009 (English)In: STEEL RES INT, ISSN 1611-3683, Vol. 80, no 6, 450-456 p.Article in journal (Refereed) Published
Abstract [en]

The particle characteristics such as size distribution, composition and morphology have been studiedin an Fe-20mass%Cr alloy as a function of holding time at 1600°C. The alloy was deoxidised with Tiand Ce, followed by holding at 1600°C and cooling to 1400°C and quenching. The inclusion particleswere investigated on a surface of film filter with an open pore size of 0.05 or 5 μm after electrolyticextraction of the metal samples. Different electric charge and electrolytes (2%TEA and 10%AA) werecompared for extraction of the Fe-20mass%Cr alloy. 300 Coulombs with 10%AA was found mostsuitable for the electrolytic extraction of particles to determine the particle composition and sizedistribution. Most of the particles were found to be complex oxides containing Ti, Ce and Cr.Furthermore, the composition of the particles was found to change from a high Ce-content to a highCr-content with longer holding time at 1600°C.

Keyword
Fe-Cr alloy, Ti and Ce deoxidation, electrolytic extraction, particle size distribution, composition, FERRITIC STAINLESS-STEEL, INCLUSIONS, NIOBIUM, GROWTH, AL, ZR
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-13046 (URN)10.2374/SRI08SP155 (DOI)000267546800012 ()2-s2.0-67749091108 (Scopus ID)
Note
QC 20100524Available from: 2010-05-24 Created: 2010-05-24 Last updated: 2011-03-14Bibliographically approved
2. Effects of Nitride-Oxide Particles on Solidified Structure inFe-20mass%Cr Alloy Deoxidised with Ti/M (M = Zr or Ce)
Open this publication in new window or tab >>Effects of Nitride-Oxide Particles on Solidified Structure inFe-20mass%Cr Alloy Deoxidised with Ti/M (M = Zr or Ce)
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2010 (English)Report (Other academic)
Abstract [en]

The ratio of equiaxed grains on a cross section and average grain sizein Fe-20mass% Cr alloy deoxidised with Ti/M (M = Zr or Ce) were studiedas function of nitrogen content and particle characteristics such as sizedistribution, morphology and composition. Fe-20mass%Cr alloys weremelted at 1600ºC, deoxidised, cooled to 1400ºC followed by waterquenching. The particles were investigated on a surface of film filter afterelectrolytic extraction of metal samples with 10%AA electrolyte. It wasfound that the number of particles increases with increasing the N content.The as-cast solidification structure and grain size were clearly affected bythe number of oxide and oxide-nitride particles in Ti/Zr experiments. Theeffect in the Ti-Ce case was almost undetectable.

Place, publisher, year, edition, pages
Stockholm: KTH, 2010. 40 p.
Series
KTH/MSE, 10:13
Keyword
Fe-Cr alloy; Ti, Zr and Ce deoxidation, grain size distribution
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-13327 (URN)
Note
QC20100611Available from: 2010-06-11 Created: 2010-06-11 Last updated: 2010-06-11Bibliographically approved
3. Effect of Primary and Secondary Particles on Grain Sizein a Fe-20mass% Cr Alloy Deoxidised with Ti and Zr
Open this publication in new window or tab >>Effect of Primary and Secondary Particles on Grain Sizein a Fe-20mass% Cr Alloy Deoxidised with Ti and Zr
2010 (English)Report (Other academic)
Abstract [en]

Because of the high and volatile Ni price in recent times, it becomesmore important to develop ferritic stainless steels with low level of Ni. Here,it is known that these steel grades usually contain oversized grains, which asa consequence leads to poor mechanical properties. One way to deal withthis problem is to control the microstructure and the size of grains in ferriticstainless steels. This would also make ferritic stainless steels morecompetitive in comparison to the more commonly used austenitic stainlesssteels. This study focuses on the grain refining effect of particles present in aferritic stainless steel. The particles were created by additions of Ti and Zr into a liquid Fe-20mass% Cr alloy, before the start of solidification. Aconstant O content (150 ppm) together with varying N contents (65, 248 and490 ppm) in the metal samples were used to vary the number, compositionand location of the precipitated particles. The grain sizes and particles werestudied in as-cast samples as well as for specimens heat treated for 60minutes at 1200 and 1400°C. It was found that the formation of particles isenhanced by an increased N content in the alloy. Based on SEMdeterminations, the precipitated particles were divided as primary (mainlyZr-oxides and Zr-nitrides) and secondary (mainly Ti-nitrides) particles andthe effect was studied for each of the types. An increased content of primaryparticles as “nucleators” for precipitation of α-ferrite during solidification ofthe melt lead to an increased formation of equiaxed small-size grains. Inaddition, an increased N content in the metal samples resulted in anincreased number of secondary particles, which are located near the grainboundaries. Therefore, the pinning effect of these particles on grain growthincreased at a holding temperature of 1200oC. However, most of the nitrideswere found to dissolve during heating and holding at a 1400 °C temperature.Thus, as a consequence, the pinning effect of these particles on grain growthdecreased rapidly with the holding time.

Place, publisher, year, edition, pages
Stockholm: KTH, 2010. 45 p.
Series
KTH/MSE, 10:14
Keyword
Fe-Cr alloy; Ti and Zr deoxidation; grain size distribution; grain growth, pinning effect, particle size distribution, N content
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-13324 (URN)
Note
QC20100611Available from: 2010-06-11 Created: 2010-06-11 Last updated: 2010-06-11Bibliographically approved
4. An experimental study on the influence of particles on grain boundary migration
Open this publication in new window or tab >>An experimental study on the influence of particles on grain boundary migration
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2010 (English)In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 45, no 8, 2233-2238 p.Article in journal (Refereed) Published
Abstract [en]

The pinning effect of particles on grain boundary migration was studied in a Fe-20 mass % Cr alloy deoxidised with Ti and Zr. The different nitrogen contents (65, 248 and 490 ppm) were used to vary the number of precipitated inclusions. The specimens from equiaxed zones of metal samples with different particle densities were examined by in situ observations during a 60-min holding time at 1200 and 1400 degrees C in a Confocal Scanning Laser Microscope. The change of particles pinning effect on the grain growth was described by an average grain size, (D) over bar (A), and the ratio between the perimeter and area of grains, P-GB/A(G). It was found that the pinning effect of particles (mostly complex Ti-Zr oxynitrides) on grain growth decreased with a decreased nitrogen content in the metal. Furthermore, the effect of particles decreased with an increased temperature of treatment, due to the reduction of the number of particles on the grain boundaries.

Keyword
STAINLESS-STEEL, GROWTH, ALLOY, SURFACE, MICROSTRUCTURE, TRANSFORMATION, OXIDE, SIZE, TI
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-13047 (URN)10.1007/s10853-009-3908-7 (DOI)000274656400039 ()2-s2.0-77949873531 (Scopus ID)
Note
2009 6th International Conference High Temperature Capillarity, Greece, Athens 6-9 May, 2009. QC 20100621Available from: 2010-06-11 Created: 2010-05-24 Last updated: 2017-12-12Bibliographically approved
5. Dissolution of Nitrides and Precipitation of an AusteniticPhase on the Surface of Fe-20%Cr alloys
Open this publication in new window or tab >>Dissolution of Nitrides and Precipitation of an AusteniticPhase on the Surface of Fe-20%Cr alloys
Show others...
2010 (English)Report (Other academic)
Abstract [en]

New phases have been observed on surfaces of metal specimens duringcooling after heat treatment at 1200, 1300 and 1400 °C of an Fe-20mass% Cr alloywith different nitrogen contents (65, 248 and 490 ppm) and deoxidised by Ti andZr. These phases were assumed to be related to a phase transformation. Theanalysis of nitrogen content in matrix metal and new phase crystals was carried outbased on point analysis of nitrogen using SEM. According to obtained results itwas concluded that the nitrogen had been dissolved from the nitrides during 60minutes of heat treatment at high temperature and diffused in to the matrix. Theseareas of enriched nitrogen content were then transformed to a new phase duringcooling. This new phase was assumed to be austenite because the nitrogen is a wellknown element for promotion of austenite formation. The possibility of austeniteformation in Fe-20mass% Cr alloys with different nitrogen content was consideredand confirmed thermodynamically by using the Thermo-Calc software.

Place, publisher, year, edition, pages
Stockholm: KTH, 2010. 30 p.
Series
KTH/MSE, 1015
Keyword
ferritic stainless steel, high Cr alloy; Ti and Zr deoxidation; N content, austenite
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-13335 (URN)
Note
QC20100611Available from: 2010-06-11 Created: 2010-06-11 Last updated: 2010-06-11Bibliographically approved

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