Carbide Extraction Methods

Carbide Extraction Methods

Chemical extraction methods

Berzellus reagent
Composition: 160 g CuCl2, 140 g KCl, 10 g tartaric acid, 925 mL distiled water and 75 mL HCl.
The sample was placed on a vibrating table to enhance dissolution. Solution filtration was then used to filter carbides from the solution. The filter paper was washed (100 ml of 0.25 N HCL and 50 ml H20), then dried at 120°C before final weighing. Carbide size and structure were determined by standard scanning electron microscopy and x-ray diffraction techniques.
Reference: H. Goldenstein, J. A. Cifuentes, Overall kinetics and morphology of the products of austenite decomposition in a Fe-0.46 Pct C-5.2 Pct Cr alloy transformed isothermally above the bay, Metallurgical and Materials Transactions A, June 2006, Volume 37, Issue 6, pp. 1747-1755.

Procedure for microalloyed steels
A bulk extraction technique was used to identify the kind of precipitates formed in each alloy. Two kinds of etchants were used for the bulk extraction technique: 10% HCl in methanol solution was used for 3Mo alloy, and 1 g of tartaric acid plus 100 ml of 10% HCl in methanol solution was used for 3W alloy. Use a heated solution (70 C) of HCl to dissolve the matrix. Residues after extraction were collected using centrifugal sedimentation.
Reference: Junfang Lu, Douglas Ivey and Hani Henein, Quantification of Nano-Sized Precipitates in Microalloyed Steels by Matrix Dissolution, 2006 International Pipeline Conference, Volume 3: Materials and Joining; Pipeline Automation and Measurement; Risk and Reliability, Parts A and B, Calgary, Alberta, Canada, September 25–29, 2006, pp. 1767-1784.

A new method
An innovative idea was inspired by making carbon extraction replica during which needs initial light etch to make the carbides stand out from the polished matrix surface. Based on this phenomenon, a sample was deeply etched by 6% nital and carbides could be clearly observed under SEM. However, when the sample was scanned by XRD, the trace only had a strong Fe peak.
A certain amount of experimentation was carried out to find the optimum conditions. It was found that by deeply etching in Villela's reagent for 24 hours gave a layer of carbide residues which remained on metal surface. The metal block was then picked up from Villela's reagent without touching the surface and immersed into alcohol for several minutes. After being dried under 50°C, all the carbide residues were deposited on sample surface and ready for XRD examination.
Reference: Cheng Li, EFFECT OF HEAT TREATMENT ON THE MICROSTRUCTURE OF A 2CrMoNiWV ROTOR STEEL, Sheffield Hallam University, Materials Research Institute, UK, PhD Thesis, 1996, pp. 40-42, 154-165.

Chemical extraction is a relatively simple method to separate the particles from the matrix, but because of the strong acid solution concentrations, small particles can also be lost via dissolution.

Electrolytic extraction methods

Microalloyed steels
Some important strengthening precipitates in microalloyed steels are typically in the 5 to 20 nm size range. Electrochemical dissolution procedures in this range seems to be the best choice, since more diluted solutions are used that are less likely to dissolve fine particles. This procedure involves anodic dissolution of the steel matrix in an aqueous or nonaqueous electrolyte, followed by the collection of undissolved particles in special filters. Potentiostatic methods have been commonly applied to aid dissolution. The main variables involved in these processes are the composition of the electrolyte used, and subsequent treatment of the residues after particle separation from the steel matrix. Most of the electrolytes used for steels that have been published in the literature are listed in Table 1, along with information of the experimental conditions employed and steels investigated. Residues collected in the filters can be directly analyzed, depending on the analytical technique selected.
The experimental steel used in this study was a Nb modified SAE 8620 grade. The laboratory-melted steel was cast as an ingot, reheated to 1,230°C, forged, and air-cooled to room temperature. The billets where then rolled after reheating at 1,260°C for approximately one hour. The thermomechanical procedure has been described in detail elsewhere. Specimens 70 mm x 18 mm x 1.5 mm of this material were sealed in evacuated and argon backfilled quartz tubes, solution treated at 1,300°C for 30 min, followed by iced-water quenching. Precipitation heat treatments were then carried out at 700°C for 2 h and 72 h. Additionally, some of the specimens subjected to precipitation heat treatments at 700°C for two hours, were heated to 1,050°C, held for one hour at this temperature and quenched in water to dissolve the cementite particles. Rectangular samples 15 mm x 10 mm x 1.5 mm were then cut from the heat treated specimens, to perform the electrochemical dissolution tests to isolate the particles from the steel matrix, and quantify the amount of Nb that was in solid solution and or in the form of a precipitate. Preliminary electrochemical dissolution trials were conducted to establish the parameters for particle extraction of carbides. Thus, potentiodynamic and potentiostatic tests using various electrolytes were carried out to guide in the selection of the test conditions. The methodology used for these experiments is described in detail below.
Potentiodynamic tests: Polarization curves: In order to define the appropriate conditions for electrochemical dissolution of the steel matrix, potentiodynamic tests were performed in three of the most commonly used electrolytes: a) 10 vol.% HCl + deionized water, b) 5% KBr + 6% citric acid + deionized water; and c) 10% acetyl acetone + 1% tetramethyl ammonium chloride + methanol. These solutions will be referred to in the text as “A”, “B” and “C”, respectively. To carry out these experiments, a copper wire was soldered to the back of 15 x 10 x 1.5 mm3 specimens. The specimens were then mounted in bakelite, ground through 600 grit, ultrasonically cleaned in methanol and dried. Polarization curves were obtained for each electrolyte, using a standard corrosion cell, two graphite counter electrodes and a standard calomel electrode (SCE).
Potentiostatic tests: To select the preferred electrolytic solution for electrochemical dissolution of the steel matrix, potentiostatic tests were carried out using the three experimental solutions employed in the potentiodynamic tests. These experiments were run for different times to determine the matrix dissolution rates, using specimens treated at 700°C for 2 h. To assess the rate of specimen dissolution for each electrolyte, 15 mm x 10 mm x 1.5 mm specimens were accurately weighed to ± 0.0001 g, then electrochemically dissolved at constant potential, and the weight lost monitored with time.
Reference: A.L. Rivas, E. Vidal, D.K. Matlock and J.G. Speer, Electrochemical extraction of microalloy carbides in Nb-steel, REVISTA DE METALURGIA, 44 (5), SEPTIEMBRE-OCTUBRE, 2008, pp. 448-450.

Microalloyed steels
Grade 100 microalloyed steel, with a thickness of 8 mm, was used in this study. To define the appropriate conditions for electrolytic dissolution of the steel matrix, potentiodynamic curves for the Grade 100 steel and several binary carbides and nitrides were obtained using a Gamry Potentiostat/Galvanostat system (Gamry PC4/750, Gamry Instruments, Warminster, PA). The electrolyte employed was 10 pct acetylacetone (AA) solution, which consists of 10 pct acetylacetone, 1 pct tetramethylammonium chloride and methanol. During each potentiodynamic scan, the steel sample, pure carbide, or pure nitride acted as the anode, with platinum sheet as the cathode. Using the same Gamry system, a static potential (300 mV vs saturated calomel electrode [SCE]) was applied during electrolytic dissolution to dissolve the matrix and to leave behind the precipitates. For chemical dissolution, an HCl acid solution (1:1 mixture by volume of HCl acid, with a specific gravity 1.19, and distilled water) was used at 338 K to 343 K (65 C to 70 C). Henceforth, dissolution using 10 pct AA will be referred to as electrolytic dissolution, and dissolution using the HCl solution will be referred to as chemical dissolution. After the steel matrix was dissolved, a Sorvall RC-6 super speed centrifuge (Mandel, Guelph, ON, Canada) made by Mandel, was used to separate solid particles from the solution by rotating the material rapidly at up to 40,000 RCF (relative centrifugal force) at 277 K (4 C). The centrifuging process was repeated several times to clean the precipitates. Inductively coupled plasma spectroscopy (ICP), with a Perkin Elmer Elan 6000 ICP-MS (Perkin Elmer, Waltham, MA), was used to analyze the concentration of the elements in the supernatant extracted by chemical dissolution, as a means of performing a mass balance. Calibration was done using four calibration levels (four-point calibration curve) for each element. The solution to be analyzed was diluted to keep the analyte concentration within the linear working range.
Reference: JUNFANG LU, J. BARRY WISKEL, OLADIPO OMOTOSO, HANI HENEIN, and DOUGLAS G. IVEY, Matrix Dissolution Techniques Applied to Extract and Quantify Precipitates from a Microalloyed Steel, METALLURGICAL AND MATERIALS TRANSACTIONS A, VOLUME 42A, JULY 2011, p. 1769.

W-alloyed 10 wt pct Cr Steel
Microstructure examination and microchemical analysis on precipitates in the initial specimens were carried out using a D/Max-2500/PC X-ray diffractometer (Rigaku, Japan) and a JEOL 2010 transmission electron microscope (Japan) equipped with an energy dispersive X-ray spectrometer. A small amount of precipitate residues for XRD measurement were electrolytically extracted in a solution containing 5% hydrochloric acid, 20% potassium chloride and 75% distilled water under the current density of 0.02– 0.05 A/cm2 and at the temperature of -10 C below. The extraction replicas for TEM observation were prepared as follows. The specimens were mechanically polished, etched in a solution containing 1 g picric acid, 5 mL hydrochloric acid and 95 mL ethanol, and then coated with a thin carbon film to embed the precipitates, which were finally released in acetone.
Reference: Xiaoqiang Hu, Namin Xiao, Xinghong Luo and Dianzhong Li, Transformation Behavior of Precipitates in a W-alloyed 10 wt pct Cr Steel for Ultra-supercritical Power Plants, J. Mater. Sci. Technol., 2010, 26(9), p. 818.

2.25Cr-1Mo-0.25V Steel
The hot–rolled steel plate with a thickness of 20 mm used in the investigation came from a 150 kg vacuum-induction melt. Then, the heat treatment was as follows: 1 hour at 940 C, a two-step water quenching, followed by 2, 5 and 10 hours tempering respectively at 710 C, air cooling. The precipitate was electrolytically extracted using the 5% KCl-1% Citric acid distilled water solution. The residue was separated using a membrane filter with 0.05 mm pores.
Reference: Zhang Yongtao, et al., Evolution Behavior of Carbides in 2.25Cr-1Mo-0.25V Steel, Materials Transactions, Vol. 50, No. 11 (2009) p. 2507.

Determination of the amount of undissolved carbides in tool steels
The extraction of undissolved carbides after austenitizing was carried out electrochemically. In an electrolyte of 5 % muriatic acid and 95 % methyl alcohol the matrix of the samples is dissoluted with an intensity of current of 5 mA/cm². The dissolution potential of the matrix is about 500 mV and much lower than that of the carbides with about 900 mV. Therefore it is possible to dissolve the matrix completely without attacking the carbides. The electrolytic dissolution of thematrixwas done galvanostatically, because under these conditions a quantitative extraction of carbides can be expected. The extracted amount of carbides can include small parts of the carbon dissolved in the matrix in colloidal solution. Thereby the measured amount of extracted carbides is a little too high. The fault in weight content is however less than 2% of the mass of the extracted carbides. Therefore the fault was not taken into account at the weight of undissolved carbides. Whereas the total amount of carbides wasmeasured on all tested samples, the nature of the extracted carbides was only tested in the annealed condition and after austenitizing at 1150 C or 1200 C. The carbide identification was made by X-ray structural analysis and by use of the ASTM card index.
Reference: S. Wilmes and G. Kientopf, CARBIDE DISSOLUTION RATE AND CARBIDE CONTENTS IN USUAL HIGH ALLOYED TOOL STEELS AT AUSTENITIZING TEMPERATURES BETWEEN 900 C AND 1250 C, 6TH INTERNATIONAL TOOLING CONFERENCE (TOOL), 10-13 September, Karlstad (Sweden), 2002, p. 535.

2.25Cr-1Mo steel
It was necessary to separate the minor carbide phases from the matrix to investigate the phase changes in detail. An electrochemical extraction set up was developed. It consists of Pt-Ir electrodes with 10%HCl -90% CH3OH. Metal coupons weighing 1-2 g were dissolved in this set up. The carbides are passive to the electrochemical reaction and the metal matrix is totally dissolved. The carbide phases are centrifuged out and dried. Phase analysis by XRD revealed various carbide phases M3C, M7C3, M2C, M23C6 and M6C. Two crystallographic forms of M2C were found to be present i.e. cubic and hexagonal. Carbide precipitates were characterised by TEM and various shape and sizes were presented.
Reference: Not known.

NCWV/D3 tool steels
The electrolytic extraction of carbides was carried out in a 5%-HCl acquaeous solution of specific gravity of 1.19 g/cm3, with the current density of 10 mA/cm2. The extraction time was 18 to 20 hours. The carbides extraction method in details has been presented in. Percentages of Cr, W, and V in matrix of the quenched steel samples were determined by performing chemical analysis of electrolytes obtained after the carbides extraction.
Reference: Tadeusz Nykiel, Tadeusz Hryniewicz, EFFECT OF AUSTENITIZING PARAMETERS ON DISSOLUTION OF CARBIDES IN GAMMA SOLUTION AND CHEMICAL COMPOSITION OF MATRIX IN THE HARDENED STEEL OF 2% C AND 12% Cr TYPE WITH ADDITIVES OF W, Mo, V, ADVANCES IN MATERIALS SCIENCE, Vol. 4, No. 2 (4), December 2003, pp. 50-51.

Carbide extraction from the Weldox steel

Pearlitic 12X1M0 heat resistant steel
The samples (60 mm × 20 mm × 20 mm, working area 2.2 cm2 ÷ 2.5 cm2) of 12X1M0. steel were soaked into 20 °C electrolyte solution and current measurements were carried out according argentums chloride reference electrode (EVL – 1M3.1), when the sample potential from the stationary value to 1.2 V varied at 1·10–3 V/s or 5·10–3 V/s rate. Performing anodic polarization of heat resistant steel, Fe ions, which forms insoluble bivalent and then trivalent ferrous hydroxides (Fe(OH)2, Fe(OH)3), enter the electrolyte solution. In order to escape pollution of electrolyte solution oxalic and citric acids, which forms soluble ferrous citrates and oxalates with Fe(OH)3. Anodic polarization curves were recorded in 0.05 % and 0.5 % hydrochloric acid solution and in hydrochloric solution with 0.01 % and 0.05 % oxalic or citric acid concentrations.
Power supply 1621A BK Precision was used for electrochemical etching of the steel samples (measurements 20 mm × 10 mm × 10 mm). Electrochemical etching lasted for 1 hour. Samples taken from the etching solution were washed by warm water weak flush and dried in a hot air stream. Under laboratory conditions samples of 12X1M. steel were aged in electric furnace SNOL 30/1300, in which temperature is automatically regulated from 50 °C to 1300 °C with an accuracy ±4 °C. The samples were isothermally aged at 600, 650 and 700 °C for 24, 48, 192, 288, 384, 576 and 864 h.
Reference: Arunas BALTUŠNIKAS, Kinetics of Carbide Formation During Ageing of Pearlitic 12X1M0 Steel, MATERIALS SCIENCE (MEDŽIAGOTYRA). Vol. 13, No. 4, 2007, p. 287.

Methods of extracting fine precipitates

Carbide extraction from the stabilised austenitic cast steel