X-ray fluorescence analysis technology can meet the special requirements of metallurgical analysis. A multichannel X-ray fluorescence spectrometer can analyze 20~30 elements in one minute, and the analysis accuracy can be completely comparable to that of wet chemical analysis, with a wide analysis range from several ppm to 100%. This can save a lot of manpower and improve work efficiency. It seldom uses acid and special chemical reagents, and will not pollute the environment.
However, X-ray spectrometry is not an absolute method, but depends on the comparison with standard samples for analysis. Take steel analysis as an example, there are standard samples at home and abroad, but if we don't pay attention to the surface effect, the analysis results of the best standard samples will be wrong. Generally, metal samples can be directly sampled and condensed from the furnace, or test pieces can be cut from large pieces of metal or raw materials to be analyzed in solid state, which has the characteristics of high speed, simple method and high analysis accuracy. The disadvantages are that internal standard cannot be added or diluted, chemical separation cannot be used for trace element analysis, and it is difficult to obtain suitable standard samples and synthesize them artificially.
⑵ Preparation of solid samples Generally, the surface of samples cut or directly cast is rough, and it usually needs further polishing. Grinding can be carried out on the grinder, and the surface with high finish can also be machined on the grinder. Commonly used abrasives are alumina (corundum) or silicon carbide (diamond sand) with various particle sizes. Generally, special treatments such as polishing or chemical etching are not needed. When measuring short spectral lines of molybdenum, nickel, chromium and other elements, the smoothness of sandpaper with particle size of about 80~ 120 can meet the requirements, but measuring long spectral lines requires higher surface smoothness of samples. It is especially important that the surfaces of analytical samples and standard samples must have the same smoothness.
When measuring samples, it is best to rotate by yourself to reduce the influence of surface effect, particle size and unevenness. If the sample has no autorotation device, the sample must be placed at the position where the wear marks on the surface of the sample are parallel to the plane formed by the incident and outgoing X-rays, so that the absorption is minimum, and if they are perpendicular to each other, the absorption is maximum.
In the process of grinding the sample, it is possible to grind off the inclusions in the sample, resulting in low analysis results of some elements or surface pollution. When analyzing low aluminum, if alumina is used as abrasive, the surface may be polluted. At this time, it is best to use silicon carbide abrasive. Conversely, if low silicon is analyzed, alumina abrasive should be used. For non-ferrous metals, such as aluminum alloy and copper alloy, they are much softer than steel samples, so they can't be polished with sandpaper. Only a lathe can ensure the surface smoothness of the samples.
This method of testing surface pollution measures the spectral line intensity ratio of polluted elements. For elements with atomic number below 60, the intensity ratio of La 1Ka can be measured, and for heavy elements with atomic number above 60, the intensity ratio of Ma/La 1 should be measured. This test can be compared with the contaminated samples and the same alloy samples that are known to be uncontaminated, and can even be used as an inspection method to eliminate pollution.
⑶ The X-ray fluorescence analysis of pig iron carbon exists in elemental state. The carbon in gray iron is spherical graphite and flaky graphite. The graphite pores falling off the surface during grinding will also pollute other analytical elements, leading to analysis errors. The cast samples are not uniform and are not suitable for X-ray fluorescence analysis. However, the grains of quenched samples are very fine and evenly distributed, and carbon forms cementite (Fe3C), which is a brittle and hard intermediate compound, and its surface can be machined by grinding.
(4) X-ray fluorescence analysis is sensitive enough for medium and low alloy steel. Generally, the measurement time of multichannel X-ray fluorescence spectrometer is only 20 seconds. It is best to use rhodium target X-ray tube to monitor the sample measurement for 60 seconds to improve the analysis accuracy. If necessary, the overlapping spectral lines should be deducted, and standard steel samples NBS116-165 and BAS should be used.
5. Analysis of Stainless Steel X-ray fluorescence analysis of stainless steel is difficult, because there is a serious enhancement and absorption effect between nickel, chromium and iron, and mathematical analysis must be adopted. After correction, the analytical results of chromium and nickel are satisfactory.
[6] Non-metallic material analysis Non-metallic material analysis includes raw material analysis such as slag and ore. Its analysis methods can be roughly divided into two categories. One is to crush the sample with a vibration mill, then press it into a disk with a diameter of 40 mm, and directly analyze it with an X-ray fluorescence spectrometer. This method is characterized by high speed, and the results can be obtained in about five minutes, which is suitable for rapid analysis, but it has "particle size effect" and "mineral effect", and the particle size of the sample must be strictly controlled. Especially for the analysis of light elements, it is particularly serious. Diluent, adhesive and heavy absorbent, such as boric acid, starch and vanadium sulfate, can be added appropriately to reduce the matrix effect and can be pressed into circular tablets. Another method is melting method, which can add flux such as lithium tetraborate to the sample and melt it into glass beads at high temperature. The melting time is generally 10~20 minutes, and the bubbles should be removed by shaking in the middle. For some reagents, oxidants such as sodium nitrate should be added. In order to prevent the sample from breaking, bromide can be added properly to make it easy to demould. If it is melted in a platinum-gold (5%) crucible, the sample can be used directly after cooling and demoulding. This method has high accuracy and can eliminate "granularity effect" and "mineral effect", but the analysis speed is slow and the sensitivity to some elements is poor.