Inclusion extraction methods

Inclusion extraction methods

Good characterisation of the shape requires extraction of the inclusions from the steel matrix. Methods are numerous and vary from electrolytic to chemical techniques, like dissolution of the steel in acids or halogenation. A still experimental technique is electron beam melting where inclusions are concentrated at the melted steel surface. However, it is recognised that the method gives rise to agglomeration and (partial) melting of inclusions. Therefore this technique is inappropriate to investigate the shape of inclusions. Whatever the extraction technique, care should be taken not to affect the inclusions. Except the carbon extraction method, which is commonly applied to prepare TEM specimens of precipitates, all extraction techniques lead to the loss of the original spatial distribution of inclusions. Further the image analysis of inclusions fails due to the contrast differences (shadows) on the particles in combination with low magnifications (large surface areas). An operator, who is per definition subjective, must therefore make a description of shapes. On the other hand no technique is more sophisticated than the human eye in observing shapes and surface characteristics. In comparison with Otot measurements, extraction of inclusions, Specimen preparation and detailed EM investigation is a very time-consuming and expensive methodology. Undoubtedly extraction of inclusions has advantages above looking at polished steel surfaces. Not only the shape can be observed, but also by dissolving the steel matrix, the analysis is more representative, because a large amount of inclusions is obtained. Additionally, the micro-analyses are not hindered by the steel matrix. Moreover, other techniques of inclusion assessment become feasible, such as powder X-ray diffraction (XRD) and automatic counting methods for size distributions.

It is an accepted fact that chemical extraction methods of non-metallic inclusions followed by analysis present several advantages over chemical analysis performed in situ.(by microprobe or EDS in SEM). The major advantages are the following:

1. Analytical results are far more representative because a much larger sample is taken for extraction than for in situ methods,
2. Phases can be specifically identified after extraction,
3. The total oxygen content and the total amount of most phases in steel can be estimated by chemical analys is of the residue.

There are, however, several disadvantages as well.

1. Some phases cannot be quantitatively extracted. This fact is due to either inclusion size ( <5 µm in diameter) or to the chemical nature of inclusions and reagents (too agressive).
2. Phases containing common elements and amorphous or/and isomorphic structures may interfere with certain analysis.
3. Since inclusion size in ESR-materials is relatively small (< 10 µm in diameter) analysis by X-rays is rather difficult.

Several chemical methods of inclusion extraction were performed. Among them bromine in methanol, iodine in methanol, bromine-ester-methanol and iodine-methyl acetatemethanol. From the above methods only the last two were suitable to this purpose. The iodine-methanol-methyl acetate however, was found to be the most convenient because of its accuracy and reproducibility.

Chemical extracting oxides from the steel matrix
Samples were taken after the last feeding of aluminum during the RH process with an automatic sampler, to extract oxides. The sampling point was located 20 cm below the slag surface near the center of the ladle. The samples were cut in hexahedron specimens of 1 g and the sizes of each edges were 1 cm, 0.5 and 0.3 cm. These specimens were put in a 100 mL solution of 90% methanol and 10% bromine. Within 24 h the Fe was dissolved and the remaining inclusions were observed by SEM. The components of the oxides were analyzed by EPMA.

Chemical extracting inclusions from the polished steel sample
After SEM investigation of the polished cross sections, the inclusions were extracted by dissolving the metal matrix in HCl (1:1). The non-dissolved inclusions collected on a membrane were investigated with SEM for size and morphology assessment.

Dekker's chemical method of extracting inclusions from the iron matrix
Extraction of inclusions from the steel was decided to be most appropriate to study the steel cleanliness. In particular it allows to describe in detail the morphologies of the inclusions. A steel sample of about 3 grams is ground with silicon carbide grit paper to remove surface oxides and is dissolved into a 100 ml aqueous solution of hydrochloric acid (1:1) under heating (80-100 °C). After a couple of hours all iron has been dissolved and 150 ml of hot de-ionised water is added. The hot iron solution is filtered over a Nuclepore polycarbonate membrane with pores of 0.2 µm (PC MB, 25 mm diameter, Nuclepore Track Etch Membrane, Whatman). The residue is washed alternately with hot (close to boiling) hydrochloric acid (1:25) and hot de-ionised water, 5 times each. The washing should ensure the removal of all iron salts. The procedure is based on a standard test method to determine the acid-insoluble content of copper and iron powders (ASTM: E194-90). The Nuclepore polycarbonate membrane is used because of its resistance to acid and its flat surface that facilitates SEM.
Source: Rob Dekkers, Ph.D. Thesis, Katholieke Universiteit Leuven, Leuven, Belgium (2002), PhD Thesis.

Electrolytic extraction method I
Electrolyte: (wt.%): 1%-1.5% of tetramethyl ammonium chloride, 6%-10% of triethanolamine, 6%-10% of glycerol, 0.5%-1% of diphenyl guanidine, and the balance of water. During electrolyzing, the current density is adjusted to 40-100 mA/cm; the electrolyte temperature is controlled to -5-+5 deg C; argon is introduced into an electrolyzing tank, and the flow quantity is 0.1-0.3 liter/minute; then, a centrifugal separator is used for performing centrifugal separation on the electrolyte. The invention has the advantages that the pH value of the electrolyte can be maintained between 7 and 8 for a long period of time to avoid damaging inclusions due to excessive acidity or alkalinity. The invention not only can completely extract the inclusions of stable Al2O3 and the like, but also can completely extract MnS inclusions which are difficult to completely decompose; therefore, the invention is suitable for extracting inclusions from different steel, and can achieve good effect.
Source: Electrolyte and method for electrolyzing and extracting non-metallic inclusions in steel by using - China Patent 200810228948.

Electrolytic extraction method II
In the electrolytic extraction (EE) method, inclusions after extraction from the metal sample are filtrated on a film filter. In the EE method, a metal sample is dissolved in an electrolyte using electric current. The solution goes through the filter with the help of an aspirator after that the dissolution of the metal in the electrolyte has been done. However, oxides and sulfides, which are not soluble in the electrolyte, remain in the solution and these are collected on the film filter after filtration has been done. The filter is removed for study of residue. The surface of each specimen was cleaned by fine grinding and washing by acetone and petroleum benzene in an ultrasonic bath, before electrolytic extraction. The specimens were dissolved using an electrolytic extraction method. The following settings were used: voltage: 150 mV, electric current: 40-60 mA and electric charge: 300-500 coulombs. For extraction of inclusions from metal samples of steel samples, a 10% AA (10% acetylacetone – 1% tetramethylammounium chloride – methanol) solution was used as an electrolyte. The total weight of the dissolved metal during the electrolytic extraction was in the range of 0.08-0.10 g. After electrolytic extraction, the obtained solution containing inclusions was filtrated using a polycarbonate membrane filter (PC) with an open-pore size of 0.4 ìm. The extracted non-metallic inclusions on the surface of the film filter were analyzed by an SEM equipped with EDS. Both micro area and qualitative element mapping analyses were performed.
Reference: H. Doostmohammadi, A 3-D Method for Determination of Non-Metallic Inclusions in Steel, Department of Materials Engineering and Metallurgy, Faculty of Engineering, Shahid Bahonar University, Kerman, Iran, 2015.