Outline the inspection of common springs

3. Evaluation of steel microstructure
  3.1 Non-metallic inclusions in steel
GB/T10561-2005 “Standard Grading Chart Microscopic Inspection Method for the Determination of Non-metallic Inclusions in Steels” stipulates the use of standard grading charts to assess non-metallic inclusions in rolled or forged steel with a compression ratio greater than or equal to 3 times Microscopic evaluation methods of objects, selection of inspection methods, expression of results and test reports, etc. In general, non-metallic inclusions in steel are determined using the ISO rating chart in the standard. Appendix A of the standard lists the ISO standard rating maps. These rating images are equivalent to a square field of view with an area of ​​0.5 mm2 on the longitudinal polished surface at 100 times. According to the shape of inclusions, the standard map is divided into five categories: A, B, C, D and DS. The standard equivalent adopts ISO4967: 1998 (E) “Determination of Non-metallic Inclusions in Steel-Standard Grading Chart Microscopic Inspection Method”.
   The sampling of non-metallic inclusions should be representative. If the sampling is unreasonable, it may cause missed detection of non-metallic inclusions.
   In order to ensure the flatness of the inspection surface and avoid rounded corners on the edge of the sample, it can be protected by clamps or inlays. Samples of inclusions should be smoothed, coarsely ground, and finely ground (metallographic sandpaper) with a grinding wheel; when polishing the samples, pay attention to prevent the inclusions from peeling off, deforming and contaminating the polished surface. You can choose a suitable polishing agent and polishing process. Implement operating specifications. What you see under the microscope at 100X is a mirror without scratches and dirt.
  3.2 Average grain size of metal
   GB/T6394-2002 “Metal Average Grain Size Measurement Method” specifies the average grain size expression and evaluation method of metal structure. This method is mainly suitable for single-phase grain structure, but it can also be applied to the measurement of the average size of specific types of grains in multi-phase or multi-component samples after specific regulations.
  The standard uses the unimodal distribution of grain area, grain diameter, and cross-sectional length to measure the average grain size of the sample. These distributions are approximately normal. The measurement method is only suitable for the measurement of the average grain size, not for the measurement of the three-dimensional grain size of the sample, that is, the measurement of the three-dimensional grain.
“Grain size” refers to the measurement of crystal grain size, usually length, area or volume are used to express different methods of evaluation or measurement of crystal grain size. The grain size expressed by the number of grain size levels has nothing to do with the measurement method and unit of measurement. The measurement methods of grain size are commonly used comparison method, area method and intercept point method. The actual measurement of the grain size rating under the magnification is in accordance with the GB/T6394-2002 standard.
   What really affects the use of steel is the actual grain size of steel austenite, not the “essential grain size”. The grain size is a function of the austenitizing temperature and time. The evaluation is not the grain size at a certain arbitrary specified temperature, but the grain size under the actual austenitizing condition in the heat treatment. This is conventional “Austenite grain size”. The crystal grains are revealed through an appropriate display method, and the corresponding measurement methods are used depending on the distribution and state of the crystal grains. The American ASTM standard has a series of measurement method standards. The current GB/T6394-2002 “Metal average grain size measurement method” is formulated with reference to ASTM E112 in the United States; and the upcoming GB/T×××× “Determination of the largest grain size on the metallographic test surface (ALA grain size) )” and GB/T ×××× “Standard Test Method for Characterizing Double Grain Size” (developed with reference to ASTM E930 and ASTM E1181, and has also been formulated or transformed into a national standard by relevant domestic units, and has been approved. It remains to be Release and implementation), will form a complete series of grain size measurement standards.
  3.3 Inspection of free graphite carbon
   Spring steel has a high carbon content, and some alloy spring steels also have a high silicon content. Silicon is an element that promotes graphitization, and graphitization may occur during multiple annealing processes. Once graphite carbon is precipitated in steel, it will split the metal matrix like non-metallic inclusions and seriously damage the properties of the material. For example, the tensile strength, yield strength, and quenching hardness will adversely affect the fatigue life of the spring. Moreover, the source of fatigue cracks often occurs at the free graphite, so graphitization is a kind of defect. The more serious the graphitization of steel, the more severe its effect on performance. The inspection of graphitic carbon is carried out in accordance with the standard of GB/T13302-1991 “Method for Microscopic Evaluation of Graphitic Carbon in Steel”. The sample should be cut from the steel in the delivery state, and the test surface is a cross section perpendicular to the steel axis. Graphitic carbon has two types: flocculent and strip-shaped. In metallographic inspection, flocculent graphitic carbon is easily confused with the polishing pits in the sample preparation process, and special attention should be paid to it. If the sample is lightly etched with a 4% nitric acid alcohol solution, the free graphitic carbon is surrounded by a carbon-poor area, which is often a ferrite structure, which can distinguish graphitic carbon from sample preparation pits.
  3.4 Decarburization depth of steel
   The national standard GB/T224-2008 “Measurement of Decarburization Layer Depth of Steel” is suitable for measuring the decarburization depth of raw materials and bolt products. The depth measurement of the decarburized layer can be divided into metallographic method, hardness method and chemical analysis method.
   3.4.1 Metallographic method
  The metallographic method is to observe the structure change of the sample from the surface to the core as the carbon content changes under an optical microscope.
  ① Selection and preparation of samples
   When cutting the sample, the detection surface cannot be changed by heating. The edge of the specimen shall not be rounded or crimped, and the specimen shall be inlaid or fixed in the holder. Corrosion with nitric acid alcohol to show the structure of steel.
  ② Measurement of decarburized layer
  Determination of the total decarburized layer—In medium-carbon steel and high-carbon steel, it is distinguished by the relative changes in the composition of ferrite and pearlite or cementite. Measure the distance from the surface to the point where there is no difference between the tissue and the matrix tissue by means of a micrometer eyepiece or directly on the ground glass screen of the microscope. For each sample, within one field of view in the deepest uniform decarburization zone, several random measurements (at least 5 times) should be performed, and the average of these measurements is taken as the total decarburization depth.
Determination of the fully decarburized layer-The fully decarburized layer refers to the fully ferrite structure obtained after the surface of the sample is decarburized. Therefore, the measurement should be measured from the surface to the point where cementite or pearlite appears. Or measure the depth of the full ferrite structure as the depth of the full decarburization layer.
   3.4.2 Hardness method
The    hardness method is divided into microhardness method and Rockwell hardness method.
  ① Microhardness method
The micro hardness method only uses a 300g load micro Vickers hardness tester. The measurement should be on the edge line of the cross section of the material. The Vickers hardness value of the second point should be equal to or greater than the hardness value of the first point minus 30 Vickers hardness. Unit. If it is less than 30 Vickers hardness units, it can be calculated as the depth of the decarburized layer.
  ② Rockwell hardness method
   When measured with a Rockwell hardness tester, it is measured directly on the surface of the sample. The Rockwell hardness method is used to determine the Rockwell hardness value HRC according to GB/T230.1-2009, which is only used to determine whether the product is qualified.
   3.4.3 Determination of carbon content method
  ① Chemical analysis method
   Use chemical analysis to measure the carbon content of the metal scraps stripped layer by layer to determine the depth of the decarburized layer. The depth of each layer is 0.1mm thick. It is also possible to measure the layer-by-layer carbon content by means of spectral analysis, until the position with the same carbon content as the core matrix. The vertical distance from this position to the surface is the total depth of the decarburized layer.
  ② Spectral analysis method
  The plane sample is ground layer by layer, with an interval of 0.1mm between each layer, and the carbon spectrum is measured on each layer. Only suitable for flat specimens with appropriate dimensions.