Metallographic inspection is one of the routine inspection methods for steel quality by observing the internal structure of metal mechanical parts by various metallographic means, and qualitative, semi-quantitative or quantitative inspection of grain size, shape, type, microstructure, relative quantity and distribution.
After a certain composition of steel, many properties of fasteners will be determined by structure, structure, macro and micro defects. In order to ensure the scientificity, objectivity and authority of the inspection, it is very important to standardize, standardize and internationalize the metallographic inspection method.
The manufacturing quality of fasteners directly affects the overall level and operational reliability of complete sets of equipment such as automobiles, high-speed railways, wind power and nuclear power. Fastener manufacturing quality is divided into intrinsic quality and extrinsic quality.
The intrinsic quality mainly refers to the mechanical properties, microstructure and defect distribution of the fastenerThe external quality mainly refers to the dimensional deviation, weight deviation and surface quality.
As one of the important methods for testing and analyzing intrinsic quality, metallographic examination can provide information about the relationship between material composition, thermal processing technology, microstructure and mechanical properties.
From the perspective of detection range, it includes macroscopic low-power test, high-power observation of optical microscope and electron microscope, as well as energy spectrum, Auger and other analysis. From the perspective of time span, it runs through the whole process of fastener manufacturing, covering the whole process of raw material composition, upsetting, welding and heat treatment.
Metallographic is a practical technology for the purpose of solving practical problems in production. Through inspection, we can not only find defects and rejects of fasteners (especially thermal processing), identify the performance and quality of fasteners, but also judge the degree of defects and the causes of them, so as to take corresponding corrective methods and preventive measures to improve the quality of the whole process of fastener manufacturing. The level of metallographic level of an enterprise represents the level of an enterprise's quality inspection ability. 01. Work effectiveness
At present, the standardization work in the field of metallographic in China is responsible for the metallographic inspection sub-technical committee of the National Steel Standardization Technical Committee (SAC TC183 SC14), and the standards include low-magnification organization inspection and high-power organization inspection.
By the end of 2021, there were 43 standards in the field of metallographic inspection, including 25 national standards and 18 industry standards, and a relatively complete metallographic inspection method standard system has been established.
1.1. The metallographic inspection standard realizes the qualitative inspection to the quantitative inspection.
In recent years, metallographic inspection has evolved from traditional manual methods to automated methods, and metallographic inspection standards have also been extended from traditional high-magnification and low-magnification qualitative tests to scanning electron microscopy spectroscopy (EDX), electron backscatter diffraction analysis (EBSD), X-ray diffraction analysis (XRD), transmission electron microscopy analysis (TEM) and other microbeam inspection methods, and from qualitative methods to semi-quantitative and quantitative methods, and a series of new standards for metallographic inspection methods have been formulated.
For example, in addition to the commonly used standard rating spectrum method (GB T 6394-2017), the standard GB T 36165-2018 "Determination of Average Grain Size of Metals - Electron Backscatter Diffraction (EBSD) Method" has been developed for the detection of average grain size by EBSD methodThe revised GB T 224-2019 "Decarburization Layer Depth Determination Method for Steel" adds glow spectroscopy analysis (GDOES) and electron probe analysis (EPMA) methods.
The formulation of the new metallographic inspection method has improved the automation level and quantitative analysis level of the metallographic inspection method, and met the requirements of faster and more accurate detection of the current fastener product quality inspection.
1.2. The international standardization work has been realized from active adoption to leading formulation.
China's metallographic inspection standards, from the 80s of the 20th century began to actively adopt international standards, to 2010 really began to substantively participate in the formulation and revision of international standards in this field.
At present, China has led the formulation and revision of three international standards:
1) ISO 16574:2015 "Method for Determination of Sostenite Content in High Carbon Steel Wire Rod", based on China's YB T 169 standard, is the first time to transform the Chinese standard into an international standard
2) ISO4969:2015 "Steel Macroscopic Corrosion Test Method", based on China's GB T 226 standard to ISO4969:1980 revision, incorporating the "electrolytic corrosion method", the method has the advantages of time-saving, acid-saving, easy operation, safety, and wide range of application
3) ISO 3887:2017 "Determination of the depth of steel decarburization layer", which is revised simultaneously with GB T 224-2008;The current standard GB T 224-2019 "Method for Determining the Depth of Decarburized Layer of Steel" greatly improves the accurate method of measuring the depth of decarburized layer.
The release and implementation of the above three international standards have won the recognition of experts in the world, promoted China's successful inspection methods to the international co-prosperity, and will provide strong technical support for the development of domestic and foreign fastener technology exchanges.
02. Work focus
Heat treatment quality control of fasteners, most of the main objects are processed semi-finished products or finished parts, heat treatment production is continuous batch into furnace production, once there is a heat treatment quality problem, the loss is very large.
If the quality of the heat-treated fastener is not accurately detected according to the required specifications in the quality inspection, the quality of the fastener is not effectively controlled, and it is easy to have a serious failure accident in the later use, which will cause greater losses.
Strengthening, standardizing and improving the inspection level of high-strength fasteners is one of the main means to control quality risks.
2.1. Quenching and tempering heat treatment.
Over the years, the metallographic structure analysis of quenched and tempered heat treatment has been observed under an optical microscope with 500X, and the use of GB T13320-2007 "Metallographic Structure Rating Diagram and Evaluation Method for Steel Die Forgings" has been adopted, and grade 1 and grade 4 have been used in a room temperature environment as the acceptance criteria.
Such as 10Level 9, 12Grade 9 high-strength fasteners, if in service at low temperatures, the bolts calibrated at -40 impact performance requirements grade 1 and grade 3 are qualified, if there is controversy in the rating, you can refer to the mechanical property test results for judgment.
Low-carbon or low-carbon alloy steel is more practical, generally refer to JB T72933-2014 "Internal Combustion Engine Bolts and Nuts Part 3 Metallographic Inspection of Connecting Rod Bolts", the first drawing (low carbon steel) inspection, according to this standard to determine the tempering organization 1 3 qualified.
In the new era, with the promotion and promotion of industrial foundation, intelligent manufacturing, green manufacturing transformation and application, as well as the detection of key basic parts such as mechanical equipment and nuclear power basic parts, mechanical parts, and high-strength fasteners after heat treatment, it is urgent to do so.
As one of the important methods to monitor the quality of heat treatment of fasteners, the metallographic inspection standard also puts forward higher requirements, and it is necessary to establish a unified, objective and quantifiable grade standard to facilitate reference, and meet the needs of different industries, different fields, and different mechanical parts quenching metallographic inspection and evaluation, so as to ensure that the inspection quality and heat treatment quality are effectively monitored.
The national standard GB T38720-2020 "Metallographic Structure Inspection for Quenching of Medium Carbon Steel and Medium Carbon Alloy Structural Steel" stipulates the material classification and quality requirements, process, inspection methods, quenching microstructure grade and microstructure evaluation of medium carbon structural steel and medium carbon alloy structural steel.
The new standard is applicable to the inspection and evaluation of the metallographic structure of medium carbon structural steel and medium carbon alloy structural steel fasteners after quenching and tempering. It is not suitable for the evaluation of decarburization, overburning, isothermal quenching and other tissues.
The standard takes martensite and ferrite as the two main control elements, quenching 1 6 grades, respectively taking the heating temperature as the consideration object, and adopts the difference in martensite needle length formed when overheating, heating temperature is high, heating temperature is normal, and heating temperature is low.
Quenching 7 10 grades, mainly considering the influence of cooling factors leads to the volume fraction and morphology of ferrite and the influence of other non-martensitic structures on the performance of grading, the appearance of reticulated non-martensitic structure and martensitic volume fraction less than 80% of the state will have a serious impact on the mechanical properties of materials or fasteners, which is not allowed.
For the control and grade classification of ferrite content and morphology, refer to the requirements of JB T 8491, GB T 8539, JB T 9173 and other relevant standards for core ferrite (fastener products are controlled at 10%).
2.2. Spheroidized annealing.
For most mechanical components, or for sub-eutectic steels for fastener products (bolts, screws, studs, rivets, etc.), soft annealing or spheroidizing annealing is required.
Spheroidization annealing is a process method to obtain a fine-grained (spherical) pearlite structure diffusely distributed on a ferrite matrix, the purpose of which is to improve the processing performance of cold heading, cold forging, cold punching or cutting, reduce the deformation and cracking tendency during quenching and tempering, and make the workpiece obtain fairly uniform final mechanical properties.
However, steel with a global, evenly distributed spheroidal body is not readily available through a simple process.
Generally, the spheroidization annealing process of steel has a long cycle, which not only requires the control of the heating rate, but also often requires the control of the cooling rate and other links.
The fluctuation of any link will affect the final spheroidization effect, and different spheroidization effects are very sensitive to the adaptability and process quality of different cold deformation processing of steel. Therefore, it is very necessary to achieve effective control of the inspection and rating of steel spheroids.
Due to the long cycle of spheroidization annealing process, high energy consumption and large environmental impact, the control requirements are very urgent for the improvement and optimization of spheroidization annealing process, which also requires a quantitative evaluation of the spheroidization annealing results, so as to analyze the weight of the influence of various factors on the spheroidization effect, and the inspection and measurement standards of the nodularization rate need to be unified.
The national standard GB T38770-2020 "Low and Medium Carbon Steel Spheroidization Structure Inspection and Rating" standard replaces the original JB T 5074 "Low and Medium Carbon Steel Spheroidization Rating" to avoid the judgment of nodularization rate being too strict, or causing an increase in the cost of wire rod heavy (new) spheroidization treatmentOn the contrary, the nodularization rate is judged too loosely, resulting in poor spheroidization, and cold cracking occurs during subsequent cold heading and cold stamping processing.
The new national standard divides spheroids into 6 levels according to their number and distribution, with the best level 6 and the worst level 1. At 500 times or 1000 times, the comparison and evaluation are carried out against the standard drawings in each standard.
03. Talent difficulties in metallographic inspection
Metallographic inspection is a very professional profession, the growth cycle of talents is long, some enterprises have eaten up the dividends of technical talents cultivated before, and most of them are eaten by foreign companies, resulting in the lack of high-level talents in enterprise metallographic inspection.
In the process of fastener manufacturing, quality problems often occur, and difficult to control, correctly find out the cause of defects, timely solve the technical problems of the production site, to ensure the normal production of the personnel, is a huge test for metallographic personnel.
At present, in order to meet the requirements of enterprises, although there are many training courses with qualification certificates, the training is only according to the map of the metallographic inspection personnel, and there is still a big gap from the real quality analysis and traceability ability.
For example, an undergraduate student majoring in computer science can start his own business during school, but a person who is engaged in metallographic can only be integrated as a whole after working for many years.
It's easy to train a qualified, and it's easy to lose it for a variety of reasons. Metallographic examination is a kind of reverse thinking, which observes the flow of plastic deformation, heat treatment phase transformation and defect formation of metal plastic deformation through the structure.
The concept of metallographic theory is abstract, logical and profound, and it has two characteristics:
The first is comprehensive, which includes upsetting metallographic, heat treatment metallographic and fractured metallographic, covering the basic knowledge of materials science, heat treatment principles and processes, failure analysis technology and experimental methods, etc., which require comprehensive use of technology, traceability and experience.
The second is practicability, with a strong production and application background, involving improving and improving product quality, technical research, risk assessment, quality arbitration, etc.
Quality is the life of the enterprise, due to the relevant systems and measures of the enterprise have not kept up, a large number of metallographic professional and technical personnel have been lost, talents are extremely scarce, and the prospects are worrying.
In view of the current situation of the fastener industry, it is difficult to get out of the predicament by relying on the spontaneous behavior of enterprises alone. It is hoped that with the support and support of the industry technology platform, the development of the enterprise will not only rely on the market, management, direction and strategic choice, but also rely on technology, quality and brand to compete.
04. Screw Jun's experience and summary
The fatigue life of fasteners has always been an issue that has been taken seriously. The data show that most of the failures of high-strength fasteners are caused by fatigue failure, and the fasteners have almost no symptoms when fatigue failure, so major accidents are easy to produce fatigue damage in the position of unhardened tissue.
Heat treatment can optimize the material properties of fasteners and improve their fatigue strength, and it is more important to improve the fatigue strength of materials through heat treatment for the increasingly high requirements of fasteners.
China is a big fastener manufacturing country, and the formulation of unified specifications for inspection methods and standards is the basis for China's fastener products to "go global".
The new standard will fully incorporate and reflect the advanced technological achievements of today's new products, new technologies and new processes, solve the problem of standard aging, ensure the timeliness of the standard, provide strong technical support for the manufacturing, use, installation and promotion of fasteners, and provide a basis for guiding and standardizing the production and acceptance of high-strength fasteners and metal special-shaped parts, which is conducive to improving the technical performance, safety and reliability and environmental protection performance of products.
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