1, an overview
　With the progress and development of science and technology, metallurgical microscope, whether in the structure, or the optical system has a great improvement and improved spring product materials requires a good hardenability to ensure that the entire cross section of the core After quenching, tempering, about 85% martensite, diameter more than 15mm of spring products is particularly important to ensure good hardenability. To this end, metallographic examination in the spring industry developed rapidly. Metallographic examination by means of metallographic microscope to study the internal organization of the metal material, but also macroscopic examination by the naked eye or low times magnifying glass.
　Metallographic sample before the use of mechanical polishing was prepared, but with the rapid development of technology of the metallographic examination, the electrolytic polishing was prepared metallographic sample has highlighted the unparalleled advantage, and the countries have already widely used technology. Electropolishing is not only capable of making the surface smooth and bright, there are other benefits. Such as surface stress to reduce the necessary amendments to GB / T 13298-1991 "metal microstructure inspection method to remove surface carbon and oxides, to reduce friction, etc.; into the technology.
　It is understood that the newly revised GB / T 13298-201X "metal microstructure inspection method" standard will be applied to the electrolytic polishing of metallographic samples. The main technical content, including: the characteristics and applications of electrolytic polishing; commonly used electrolytic polishing liquid conditions of use, safety precautions, electropolishing Frequently Asked Questions and improved methods; use and requirements of the electrolytic polishing device; test methods and inspection rules.
　Each one corresponding to the spring performance, chemical composition, the relationship between microstructure and mechanical properties, in general: with decreasing grain size, material strength and hardness increase; with the inclusion content increase the toughness of the material tends to decrease; anisotropic mechanical properties of grain and grain preferred orientation. Therefore, the identification of the microstructure, grain size and inclusion of material assessment and commonly used means of metallographic examination has become the material process control, quality assurance and failure analysis tools. The following spring commonly used metallographic examination standard interpretation of the application are outlined as follows.
　2, the macroscopic examination of the steel
　Steel smelting or thermal processing, due to certain factors (eg, nonmetallic inclusions, gases, and process selection or improper operation, etc.) the impact, resulting in an internal or surface defects of steel, thereby seriously affecting the materials or products quality, and sometimes also lead to the material or product obsolescence. Loose in the steel, bubbles, shrinkage and residual, non-metallic inclusions things, segregation, white spots, cracks, and a variety of abnormal fracture defects, can be found by macroscopic examination. The macroscopic examination usually sulfur print test, the etching test, pyramid test, fracture test, production test, according to the requirements of the test to select the appropriate macro-inspection method.
　2.1 The etching test
　The etching experiment is to show the low magnification of steel materials testing methods, test methods according to national standards GB/T226-1991 steel macrostructure and defect etching Inspection Act, this method is simple, easy to operate, it is clear to display various defects in the iron and steel materials, such as: cracks, inclusions, loose, segregation and bubbles and so on.
　The etching experiment is the use of acid erosion of the various parts of the steel materials, which clearly shows that the macrostructure and defect of the spring material. Under low magnification the distribution of defects, can know the quality of steel metallurgy, to infer the cause of the defects, in the process to take practical measures to achieve the purpose of improving product quality.
　This standard specifies the testing of steel macrostructure and defect of heat, cold acid etching method and electrical etching method. This standard applies to steel macrostructure and defect inspection. Arbitration inspection, if the technical conditions without special provisions, the hot acid etching method prevail. The standard has been implemented for 20 years, low times, steel etching method and etching solution has a great update. At present, China is seeking to galvanic corrosion to the ISO standard, necessary right GB/T226 standard be revised to provide technical support for the amendments to the ISO 4969:1980.
　Steel low magnification range of defect evaluation and assessment rules may GB/T1979-2001 of structural steels and defect map, which is applicable to carbon steel, alloy structural steel, spring steel, steel (forging, rolling, blank) cross section of the interview kind of defect assessment. The rating plan has six sets were applied to the low magnification and defects of the different sizes steel. Common macro-defect in the steel ① general loose; ② center porosity; the ③ ingot type segregation; ④ segregation; ⑤ subcutaneous bubbles; (6) internal bubbles.
　Classifying various types of defects listed in the picture to the standards in Appendix subject to assessment when various types of defects seen in order to visually limited, in order to determine the categories of defects, allows the use of more than 10X magnifying glass. When its severity between the adjacent two, half a grade. Does not require the assessment level defects, only to determine the defect categories.
　2.2 Other
　Fracture test (GB/T1814-1979 "steel fracture Inspection Act"), sulfur print test (GB/T4236-1984 "steel sulfur print test method"), pyramid test (GB/T15711-1995 steel tower hairline acid leaching test method "), steel macro inspection technology. In recent years, due to the technological progress of China's steel industry, steel smelting or thermal processing control capabilities to improve the internal surface of the steel produce fewer defects, in order to reduce the inspection fee, in the spring industry, the majority of enterprises are no longer three categories of projects to check. This article is not repeat them here.
　3 evaluation of the microstructure of the steel
　Non-metallic inclusions in steel 3.1
　GB/T10561-2005 "steel nonmetallic inclusions in the matter content in the standard diagrams for microscopic test method provides for the assessment using a standard rating map, the compression ratio is greater than or equal to three times the rolling or forging steel, non-metallic microscopic evaluation method of inclusions, the selection of test methods and results expressed, and the test report. General steel, non-metallic inclusions in standard ISO rating level patterns were measured. Standard Appendix A lists the assessment level map of the ISO standard, these ratings picture is equivalent to 100 times the vertical polished surface area of 0.5mm2 square field of view. Standard map is divided into A, B, C, D and DS five categories based on the morphology of the inclusions. Standard is equivalent to ISO4967: 1998 (E) steel non-metallic inclusion content determination - Standard diagrams for microscopic inspection method ".
　Sampling should be representative of nonmetallic inclusions, if the sample is unreasonable, it may cause the non-metallic inclusions missed.
　In order to ensure the formation of the test surface, to avoid the edge of the sample fillets, available fixtures or mosaic approach to protect them. Inclusions in the sample after grinding wheel tie rough grinding, fine grinding (emery paper); specimen polishing, pay attention to prevent spalling of the inclusions, deformation and the polished surface is contaminated, the appropriate choice of the polishing agent and polishing process, strict Run the operating specifications. See under the microscope 100X is a no scratches, no dirt mirror.
　3.2 average metal grain size
　GB/T6394-2002 "metal average grain size under the average grain size of the metal structure and assessment methods. This method is mainly applied to single-phase grain structure, but by the specific provisions applicable to specific types of multiphase or more sets of samples the average grain size measurement.
　Standard grain size, grain size, the length of the transversal unimodal distribution to measure the average grain size of the specimen. These distributions approximate normal distribution. The measurement method only applies to the measurement of the average grain size, does not apply to sample three-dimensional grain size measurement, the measurement of three-dimensional grain.
　The grain size refers to the measurement of grain size, commonly used length, area or volume to represent different approaches to the assessment or measurement of grain size. Regardless of the grain size of the grain size number of levels and methods of measurement and units of measurement. Grain size measurement methods commonly used method, area method, the cut-off point method. The actual measurement magnification of the grading of grain size prescribed in GB/T6394-2002 standard.
　The real impact of the use of steel, Austenite grain size, rather than the nature of grain size. The grain size is a function of austenitizing temperature and time, the assessment is not in a any specified temperature, grain size, but in the actual implementation in the heat treatment conditions of austenitic grain size, this is the conventional "austenite grain size. Appropriate display will grain apparent, depending on the grain size distribution measurement method and state, American ASTM standard has a series of standard measurement methods. Existing GB/T6394-2002 "metal, the average grain size determination method" modeled on the ASTM E112 to develop; metallographic surface grain (ALA Grain Size determination of the upcoming issue of GB / T × × × × " ) test methods "and GB / T × × × ×" Characterization of double grain size standard test method (developed by reference to ASTM E930 and ASTM E1181, has also been the relevant authorities to develop or translated into national standards, and validation, to be promulgated and implemented), will form a complete grain size measurement standard series.
　3.3 free inspection of graphite carbon
　Spring steel, high carbon content, and some spring steel alloy silicon content is also higher, silicon is to promote the graphitization elements graphitization phenomenon may occur in multiple annealing process. Once the precipitation of graphitic carbon steel, just like non-metallic inclusions, and separates the metal matrix, serious damage to the material properties such as tensile strength, yield strength, quenched hardness on the fatigue life of the spring have adverse effects. And the source of fatigue cracks tend to generate free graphite graphitization is a defect in the graphitization of steel is more serious, the extent of its impact on performance is more intensified. Inspection in accordance with the graphitic carbon GB/T13302-1991 graphite carbon in the steel Micrographic method using standard sample should be cut on the delivery status of the steel cross-section of the detection surface of the vertical steel axis. Graphitic carbon flocculent and two types of bar sheet, Metallographic examination flocculent graphite morphology easy and polishing pits in the sample preparation process, confusion, should pay special attention. Such as the sample with light etching of the 4% nitric acid alcohol solution, free graphitic carbon is surrounded by a carbon-poor areas, often ferritic structure, so you can distinguish between the graphite sample pits.
　3.4 steel depth of decarburization
　National standards GB/T224-2008 decarburization of steel Determination of depth applied to the determination of depth of decarburization of raw materials and bolts finished. Determination of depth of decarburization can be divided into the optical method, the hardness and chemical analysis.
　3.4.1 Metallographic method
　Metallographic organizational change in the sample from the surface to the heart as the carbon content changes observed under the microscope.
　① specimen selection and preparation
　Interception of the specimen can not detect surface heat changes. Edge of the sample shall not rounding, curling, the specimen should be mounted or fixed to the blessing. Corrosion with nitric acid alcohol to show the organizational structure of the steel.
　Determination of 2 decarburized layer
　Determination of the total decarburized layer - to distinguish between the relative amount of change in carbon steel, high carbon steel is composed of ferrite and pearlite or cementite organizations. By means of the micrometer eyepiece or measured directly in the microscope on the ground glass screen is no longer differentiated from the surface to its organization and matrix organization a little distance. For each sample, a the decarburization District in the most uniform field of view, should be randomly conducted several measurements (at least 5 times), the average of these measurements as the total depth of decarburization.
　Determination of the decarburized layer --- the whole decarburized layer obtained after the sample surface decarburization of the ferritic structure, therefore, the measurement should be measured from the surface cementite or pearlite point. or measuring the depth to produce the organization of the ferrite is full of depth of decarburization.
　3.4.2 hardness method
　The hardness measured decarburized layer is divided into the microhardness and Rockwell hardness method.
　① microhardness method
　The microhardness of only 300g load Vickers hardness measurements in the edge of the cross-section of the material online, 2:00 Vickers hardness values should be equal to or hardness values less than 1:00 30 Vickers hardness units. less than 30 Vickers hardness can be calculated for the depth of decarburization.
　The ② Rockwell hardness method
　Determination of Rockwell hardness, measured on the surface of the specimen. Rockwell hardness method according to GB/T230.1-2009 Determination of Rockwell hardness value of HRC, only used to determine eligibility.
　3.4.3 The determination of carbon content in France
　① chemical analysis
　Chemical analysis method for the determination of layer stripping the carbon content of the metal shavings to determine the depth of decarburization. Layer by layer stripping depth of each layer is 0.1mm thick. Layer can also be determined using the method of spectral analysis of carbon content, until the same location and the heart of the matrix carbon content, this location to the surface of the vertical distance is the total depth of decarburization.
　② spectrometry
　Plane sample layer by layer grinding on each interval of 0.1mm on each layer of carbon spectrometry. Applies only to the right size flat specimens.
　4, spring metallographic examination
　Ordinary common spring material is carbon steel or low alloy spring steel, the carbon content of 0.55% -0.75% range, often by adding the alloying elements of Si, Mn, Cr, V, Cr and Mn, mainly to improve the hardenability, Si improve the elastic limit, V improve the hardenability and grain refinement. Commonly used materials 60,70,65 of Mn 60Si2Mn, 50CrVA, and so on.
　The spring is the use of its elastic deformation to absorb and release of external forces, finished with a high elastic limit, high yield ratio, high fatigue strength and sufficient ductility and toughness. General control of the quenching temperature in Ac3 + (30 ~ 50 ° C) tempering temperature was controlled at 400 ~ 500 ° C, the austenite grain growth in the actual production is usually caused by overheating due to quenching, although the normal tempering steel The intensity was not obvious, but the ductility and toughness are greatly reduced, which affects the normal use of the spring. Therefore, metallographic examination according to the actual situation properly grasp the martensite qualified boundaries. Martensite based on the experience of daily production, silico-manganese steel springs after quenching martensite fine martensite or smaller, the reference JB/T9211-2008 standard 1-4 martensite qualified. The austenite grain size should be less than 5, more than five grains of martensite for the coarse martensite or coarse martensite. Inspection quenched martensite, with 3% nitric acid alcohol solution erosion, were observed and assessed in the 500X.
　JB/T9211-2008 standard medium carbon steel and medium carbon alloy structural steel martensitic grades of carbon spring steel and medium carbon alloy spring steel, the overall quenching or quenching and 200 ° C tempered martensite test and martensite metallographic picture of comparative law. The same sample preparation and sample erosion and general metallographic sample. The standard specifies the martensite grade 1-8 level, the martensite photo magnifications are 500X, just in the rating process will enlarge the sample 500X martensite with standard ratings pictures for comparison can be know the level of martensite in the samples.
　Martensite level in Table 1.
　Table 1 Level Description of the martensitic microstructure
　The martensite Level Microstructure
　A hidden needle martensite, fine-needle martensite, ferrite is not greater than 5%
　Fine needle martensite lath martensite
　Fine needle martensite lath martensite
　4 lath martensite, fine-needle martensite
　5 lath martensite, acicular martensite
　6 lath martensite, acicular martensite
　7 lath martensite, core needle martensite
　8 lath martensite, core needle martensite
　Austenitizing temperature, the martensite morphology and sample sizes. One of the low austenitizing temperature, the quenched structure is hidden needle martensite, fine-needle martensite and not more than 5% ferrite (volume fraction). 8 is overheating organization is thick lath martensite + rough piece needle martensite. Normal quenching control in the 2-4 grade, their organization as small lath martensite and plate martensite.
　Care's body is tempered by the organization after the normal quenching and tempering, the emergence of non-martensite, undissolved ferrite or bainite should not exceed 10%.
　5 Conclusion
　With the rapid development of materials science, a variety of spring materials, extensive use of materials, structural strength and fatigue strength of the increasingly high demand for precision of the quantitative analysis of the metallographic examination raised new expectations. Metallographic analysis the detection of spring material organization, to ensure that product quality is an important means indispensable.
　Microstructure analysis, not only organized to identify the assessment, both qualitative and quantitative, semi-quantitative detection on the basis that the relevant national standards or industry standards. This interpretation is the common standards of timeliness, due to the standard, Please note that the effectiveness of standards in the specific application.
　References
　GB/T226-1991 steel macrostructure and defect etching test method [S]. Beijing: China Standard Press, 1991.
　2, GB/T1979-2001 structural steel macrostructure and defect of diagrams for the [S]. Beijing: China Standard Press, 2001.
　3, GB/T10561-2005 steel, non-metallic inclusions matter content in the standard diagrams for microscopic test method [S]. Beijing: Standards Press of China, 2005.
　4, the GB/T6394-2002 metal average grain size determination method [S], Beijing: China Standard Press, 2002.
　5, GB/T13302-1991 steel, graphite microscopy evaluation method [S]. Beijing: Standards Press of China, 1991.
　GB/T224-2008 steel decarburization depth Determination of [S]. Beijing: Standards Press of China, 2008.
　JB/T9211-2008 carbon and carbon alloy structural steel martensitic grades [S]. Beijing: Machinery Industry Press, 2008.