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1 What are the effects of non-metallic inclusions on high-strength bolts? Non-metallic inclusions in steel exist mainly in the form of oxides and sulfides. In accordance with GB/T 10561—2005 \"Standard grading chart for the determination of the content of non-metallic inclusions in steel – Microscopic examination method\", non-metallic inclusions are classified using the JK chart (Class A for sulfides, Class B for alumina-based substances, Class C for silicates, Class D for spherical oxides, and Class DS for single spherical particles). Inclusions in steel are usually detrimental to its properties, as they cause unevenness in the steel’s structure ； Inclusions often serve as sources of cracks; therefore, inclusions in steel are important factors affecting its fracture toughness, among which strip-shaped MnS has the most significant impact. Its presence disrupts the continuity of the metal; once it is removed, it forms pits or cracks. It easily becomes a source of cracks during cold heading, and it can cause stress concentration during heat treatment, leading to quenching cracks. Therefore, strict control over inclusions is required for high-strength fasteners; given that China’s steel standards GB/T 3077 and GB/T 6478 do not yet specify clear requirements regarding inclusions. Classes C (silicates) and D (spherical oxides) have the greatest impact on heat treatment; it is advisable that silicate inclusions be at level 1.5 or lower, and spherical oxide inclusions be at level 2 or lower ； The sum of oxide and sulfide inclusions should not be greater than grade 3. The shape and distribution of inclusions in steel can affect the overall performance of high-strength bolts. If heat treatment changes the shape and distribution of inclusions in steel, these inclusions will have different effects on the material before and after heat treatment. If heat treatment does not change the shape and distribution of inclusions but only causes changes in the matrix structure, it is necessary to consider the influence of these inclusions on the changes in the matrix structure during heat treatment. During heat treatment, the effect of inclusions on the microstructure is primarily to promote ferrite nucleation, thereby effectively dividing grains and refining the structure. Practice has shown that although the size, grade, and state of inclusions change after heat treatment, the poor microstructure of the raw material cannot be fundamentally improved and remains intact; as a result, this has a significant impact on the impact tests of high-strength bolts, often leading to failure in the low-temperature impact test. This issue is crucial. Re-quenching and tempering heat treatment also cannot meet the technical requirements. 2 For large-sized 10.9 grade high-strength bolts of steel structures above M82, what type of steel should be used? This involves an issue related to composition and hardenability. The commonly used steels such as 42CrMo, B7, and 40CrNiMoA can meet the requirements for hardness and strength using current manufacturing processes; however, it is difficult to meet the requirements for other properties such as elongation after fracture and impact toughness. 34CrNi3Mo steel is generally used; it is an alloy steel designed for quenching and tempering of large cross-sections. Its chemical composition consists of 0.30%～0.40% C, 0.17%～0.37% Si, 0.50%～0.80% Mn, 0.70%～1.10% Cr, 0.25%～0.40% Mo, and 2.75%～3.25% Ni. The oil quenching critical diameter can reach φ60–80 mm, offering advantages in hardenability that are unmatched by other steel grades. For 34CrNi3Mo steel, the critical temperature values are Ac1 = 725°C and Ac3 = 790°C. The quenching temperature is set between 860–880°C for cooling in water, and between 880–900°C for rapid quenching in oil ； For 10.9 grade bolts, the tempering temperature is generally 560–590°C, followed by water quenching. High-strength bolts specify that the core of the threaded section must be in a \"hardened\" state, with approximately 90% martensite structure obtained prior to tempering. To ensure hardenability, for diameters larger than φ50mm, the quenching medium has the greatest impact; if oil quenching fails to meet the mechanical property requirements, water quenching must be used. The principle is to employ aqueous solutions as much as possible, provided that cracking does not occur. This is because water quenching has many advantages, such as a deeper hardened layer, better mechanical properties, and lower production costs. For those of us in heat treatment, it’s impossible to do the job properly if we don’t prioritize the composition. Steel with a carbon content of ≥0.34%–0.38% can be used; the banding segregation should be kept at level 1.5 or below. The levels of non-metallic inclusions, such as sulfides, alumina, and spherical oxides, should also be at level 1.5 or less (including both coarse and fine types). If the impact toughness is insufficient, consider adding a tempering step at 580–600°C followed by rapid cooling. 3 What are the five harmful elements? What harm does it cause to cold heading steel? H, O, N, P, and S are known as the “five harmful elements,” which severely damage cold heading steel used for fasteners. For fastener manufacturers, it is essential to test the steel used in fastener production. The reduction of these five harmful elements leads to a decrease in the amount of sulfide, oxide, and nitride inclusions within the crystals and at their boundaries, which can act as sources of cracks in cold heading steel. This reduction weakens the effect of these boundaries, thereby increasing the fracture toughness and impact toughness of the material. Studies have shown that in cold heading steels containing 0.60%–0.90% Mn, S mainly forms MnS inclusions; these MnS inclusions act as crack sources within the steel matrix. The segregation of S and P elements at the grain boundaries weakens those boundaries. As a result, an increase in the contents of S and P reduces the plasticity of the steel, leading to a decrease in its reduction ratio. In steel, N mainly forms nitride inclusions; therefore, an increase in N significantly reduces the reduction of area for the steel, while the elongation after fracture remains largely unchanged. In steel, O mainly forms oxide inclusions; an increase in O leads to more oxide inclusions, resulting in a significant decrease in the steel’s reduction of area. H in steel causes defects such as white spots and granular segregation; as the H content decreases, these defects reduce, and the reduction in cross-sectional area increases significantly. For steel used in large-size high-strength bolts, reducing the five harmful elements helps to lower the content of non-metallic inclusions as well as the segregation of these elements at grain boundaries, thereby increasing the resistance to crack initiation and propagation – which is very effective in ensuring the proper performance of the bolts. 4 How is the grain size of steel evaluated? The grain size of steel is related to its heat treatment condition. Grain size is a function of the austenitization temperature and time; it does not refer to the grain size at some arbitrarily specified temperature, but rather to the grain size under the actual conditions of austenitization during heat treatment – this is what is commonly referred to as “austenite grain size”. Austenite grain size refers to the “actual grain size of austenite”” ； That is, the austenite grain size obtained through austenitization treatment when steel is used in practice. The “austenite grain size” mentioned in GB/T6394-2013 \"Methods for determining the average grain size of metals\" refers to the \"actual grain size of austenite\". The standard specifies the methods for expressing and evaluating the average grain size of metal microstructures ； This method is mainly applicable to single-phase grain structures, but with specific provisions, it can also be used to measure the average grain size of certain types of grains in multi-phase or multi-component specimens. Grain size refers to a measure of the size of grains; different methods are used to evaluate or measure grain size, typically expressed in terms of length, area, or volume. What truly affects the use of steel is the actual grain size of its austenite, rather than the \"inherent grain size\". The grains are made visible through appropriate display methods, and corresponding measurement methods are employed based on the grain distribution and condition; American ASTM standards provide a series of standard measurement methods for this purpose. The current GB/T6394-2002 \"Method for determining the average grain size of metals\" was formulated with reference to the American ASTM E112 standard ； Together with GB/T 24177-2009 \"Methods for the characterization and measurement of dual grain sizes\" (formulated with reference to ASTM E1181) and JB/T 4290-2012 \"Method for determining the maximum grain size grade on metallographic examination surfaces (ALA grain size)\", which are based on standards such as ASTM E930-99(2007), these standards form a complete set of methods for measuring grain size. This enables better alignment with international standards and creates a comprehensive series of standards for grain size measurement. The standard uses unimodal distributions of grain area, grain diameter, and intercept length to measure the average grain size of the specimen. These distributions are approximately normal. The measurement method is only applicable to the measurement of average grain size, and not to the measurement of the three-dimensional grain size of the specimen, that is, the measurement of volumetric grains. The grain size of steel is a very important testing parameter, and it is a mandatory inspection item for critical fasteners. The grain size of steel has a significant impact on the impact absorption energy value. 5 What are recast steel and resulfurized steel? High-quality carbon structural steels per the GB/T699, JIS G4051, JIS G3507–3509, SAE J403 standard series ； Steels for cold heading and cold extrusion, such as those in the GB/T6478 standard series, are the most widely used in fastener manufacturers. For such steel fasteners, they continue to play an important role, both in the past and today. In particular, cold heading steel is used to manufacture fasteners such as bolts, nuts, screws, rivets, and pins through cold heading processes, while cold extrusion steel is used to produce mechanical parts for vehicles, bicycles, and similar applications through cold extrusion; it is also known as rivet and screw steel. Cold heading and cold extrusion involve the plastic deformation of metals at room temperature, serving as a substitute for machining. They not only offer high efficiency and good quality, but also reduce material usage and lower costs by 10% to 30%. In contemporary China, further progress has been made in smelting technology. Due to the emphasis on resource conservation, there has been an increase in the variety of steels produced by reusing scrap steel in the smelting process. Over 80% of molten iron is used in converter steelmaking, and even in electric arc furnaces more than 60% of molten iron can be utilized. The pre-desulfurization of molten iron is applied in the production of cold-heading steel or high-purity steel; if a sulfur-reintroduction process is not carried out after ladle refining or vacuum degassing, the sulfur content in the steel ranges from 0.005% to 0.015%, 0.010% to 0.030%, 0.025% to 0.040%, and 0.030% to 0.045%, respectively. The higher the sulfur content, the better the machinability of the steel ； The higher the sulfur content, the more severe the forging flow lines in the steel, and the more pronounced the decline in its transverse properties. At the cold heading deformation area, if intense lateral flow occurs in the metal, it will cause the thread-like and rod-shaped sulfides to flatten, thereby enhancing the effect of these sulfides in splitting the matrix. This leads to the formation of tiny cracks in the bolt head or the hexagonal portion of the nut, as well as on the supporting surfaces, increasing the tendency for cracking during quenching or grinding. For bolt components of the same performance level, steel that has been reheated, steel with re-added sulfur, and cold-heading steel lack the low-temperature austenite recrystallization process at 820–880°C; as a result, their grain structure is coarser than that of cold-heading steel, which is another important reason for their poor toughness. Therefore, steel with recycled content or sulfur-recovered content should not be used for high-strength bolts and similar applications. Standard GB/T3098.1-2010 specifies the materials to be used for bolts of grades 8.8 to 10.9. For high-strength bolt steels that require heat treatment, the maximum allowable limits for sulfur and phosphorus are reduced from 0.035% to 0.025%. In the case of carbon steels with added elements (such as boron, manganese, or chromium), it is not permitted to add calcium or calcium alloys to the steel; all reasonable measures must be taken to prevent the introduction of elements that could affect hardenability, mechanical properties, and performance characteristics, originating from scrap steel or other alloy materials used in production. The 45# and 40Cr steels widely used in our country often contain excessive residual alloying elements, making it very difficult to control them. In particular, there are some impurity elements that are hard to remove during the smelting process; the contents of S and P are generally between 0.025% and 0.035%, or even higher. Other impurities include Cu, Pb, Sn, and Se. When the harmful elements P and S increase in the steel, defects such as central porosity, general porosity, and square segregation will occur at levels 2–3 when examined under low-power microscopy. Defects like white spots, shrinkage cavities, bubbles, and scale formation are also common. Due to their lower price, many fastener manufacturers also prefer to use this type of steel. During bolt quenching and tempering, cracking accidents often occur, mainly due to the presence of numerous impurity elements and non-metallic inclusions; no measures taken during heat treatment can prevent such cracking. For important products, it is recommended to use 45# and 40Cr steel sparingly or not at all. 6 Why is metallographic testing required after quenching and tempering of high-strength bolts? GB/T 3098.1—2010 \"Mechanical properties of fasteners – Bolts, screws and studs\" emphasizes that materials for products of grade 8.8 and above must possess sufficient hardenability, so as to ensure that the core of the bolt’s threaded section achieves approximately 90% martensite before tempering. In production, hardness is often used to measure the degree of tempering transformation after bolt quenching. To achieve good hardenability, the uniformity of the quenched microstructure is particularly important for high-strength bolts of grade 10.9 and above. Some companies, when performing heat treatment quenching, fail to achieve the highest hardness that steel martensite can attain; in other words, they do not achieve maximum hardenability (the ability of steel to harden during quenching). Instead, they adjust the tempering temperature by lowering it in order to ensure a minimum tensile strength value for the bolts, thereby masking poor quality of heat treatment. To this end, standard GB/T 3098.1 stipulates that it is essential for the bolt to achieve approximately 90% martensite structure prior to tempering. The fineness of martensite after quenching can be evaluated in accordance with JB/T9211-2008 \"Martensite Grades for Medium Carbon Steels and Medium Carbon Alloy Structural Steels\". Due to different austenitization temperatures, the morphology and size of martensite vary. Grade 1 corresponds to a relatively low austenitization temperature; the quenched structure consists of secreted needle martensite, fine needle martensite, and no more than 5% ferrite (by volume fraction) ； Grade 8 corresponds to overheated tissue, which consists of coarse lath martensite + coarse platelet martensite. During normal quenching, the hardness is controlled at level 3 to 5; the microstructure in this case consists of fine lath martensite plus plate-like martensite. Level 6 provides higher impact toughness, yield strength, and tensile strength, making it suitable for bolts of larger sizes that require high hardenability. The quality of quenched and tempered parts is generally evaluated in accordance with GB/T 13320-2007 \"Grading Diagrams and Evaluation Methods for the Microstructure of Steel Die Forgings.\" After polishing, the specimens are etched using an nitric acid-alcohol solution with a volume percentage of 2% to 5%. The metallographic structure is rated from 1 to 8, with grade 1 representing the best structure ； Grade 8 tissue is the worst. The third set of grading charts in this standard is applicable to quenched and tempered structural steel components, especially for the quenching and tempering inspection of high-strength bolts. When the evaluated quenched and tempered structure falls between two grades, the lower grade is used as the determination grade; for example, if it is greater than grade 3 but less than grade 4, it is classified as grade 4. The microstructural analysis of the quenched and tempered material is carried out under an optical microscope at 500x magnification. The acceptable grade can be agreed upon through negotiation between the supplier and the buyer; in the absence of such an agreement, grades 1 to 4 are considered acceptable. Production experience shows that for bolts operating in low-temperature environments and requiring low-temperature impact resistance, grades 1 to 3 serve as the acceptance criteria. If there is a dispute regarding the rating, the results of mechanical property tests can be used as a reference for making a decision. 7 How to use quenching media correctly? Today, water-soluble quenching media are becoming increasingly widely used by fastener manufacturers in China. In addition to medium and low carbon steels, products such as alloy steels, martensitic stainless steels, and heat-resistant die steels are also being processed using these media. The expertise in process control is improving as well. However, there are often inaccuracies in controlling the concentration of these water-soluble quenching media, which leads to various quality issues in heat treatment. At present, domestic fastener manufacturers primarily use rapid quenching oils and water-soluble quenching media for heat treatment. During use, these materials come into direct contact with high-temperature workpieces, which inevitably leads to changes in their physical and chemical properties; as a result, their cooling capabilities also change to some extent, and effective monitoring is necessary. First, improve the stability of the quenching medium to ensure that the properties and deformation of the quenched workpieces remain within controllable limits ； Secondly, it provides effective methods for monitoring cooling performance, and at the same time develops optimized quenching medium solutions tailored to specific operating conditions, in order to ensure the long-term stable use of such media. Based on the production practices of most fastener manufacturers, rapid quenching oil is primarily used for alloy structural steel grades, as well as thin-walled bolts, nuts, and custom-shaped components ； Water is mainly used for carbon structural steel and fasteners with simple shapes ； The quenching medium for PAG polymers lies between water and oil; under certain conditions, it can replace either water or oil, and it offers a wide range of cooling intensities. Polymer quenching media can eliminate the fumes produced by quenching oil. Reducing carbon emissions and promoting the use of polymer quenching media is a global trend. Polymer quenching media can fully meet the requirements for safe production, and it can be confirmed as an important process material for clean production in the heat treatment of fasteners in China. Currently, polymer quenching media are being used more and more widely due to their unique advantages in terms of environmental protection, technology, and cost. In recent years, to reduce the emissions of fumes from heat treatment, China has developed various new environmentally friendly and carbon-reducing quenching media. Currently, the use of polymer quenching media accounts for approximately 30% to 40% of all quenching media (oil + polymer quenching media). In production practice, to improve the quality of quenching and the reproducibility of tests on the effectiveness of polymer quenching media, it mainly depends on the reproducibility of stirring, which is essentially turbulence. The operating temperature, concentration, and stirring of the polymer quenching medium are key factors in ensuring the quality of quenching. In PAG polymer quenching media, bacteria use organic substances as raw material and catalyze their transformation, which accelerates the aging of such media, shortens their service life, and increases the cooling rate during the convection stage. This leads to an increased tendency for bolts and nuts to deform and crack during quenching, thereby making it difficult to ensure the quality of fastener quenching. At the site, only adjusting the concentration to slow down the cooling rate was attempted, but the effect was not significant. The use of the bio-stable polymer quenching medium AQ245, along with an environmentally friendly rapid cooling agent, can effectively address problems related to deformation and cracking. First, quenching fluids of different concentrations are prepared using a new solution, and the cooling rate of these fluids at 300°C at various concentrations is measured using a cooling characteristic tester. This allows for the creation of standard tables or charts relating different concentrations to their cooling rates; using a cooling characteristic tester is an ideal method for this purpose. Secondly, it is best to replace the quick quenching oil with fresh oil once a year ； Water-soluble quenching media are different from quenching oils; temperature, concentration, and stirring intensity have a significant impact on their cooling capacity. It is recommended to use propellers for vigorous stirring in order to ensure uniform cooling, thereby achieving uniformity in the microstructure and hardness. 8 Why does rapid quenching oil age over time? Currently, rapid quenching oil still plays a dominant role as a cooling medium in the heat treatment of fasteners. An increase in the operating temperature of quenching oil significantly accelerates the rate of its aging. For every 10°C increase in oil temperature, the rate of chemical reactions increases by about 2 to 4 times; in principle, the operating temperature for quenching oil should be between 60 and 80°C. The use of quenching oil for heating and repeated contact with hot workpieces cause the oil to oxidize and age, which represents the biggest threat to it in production. Prolonged quenching operations using bolts and nuts cause numerous complex chemical changes in the quenching oil, leading to significant alterations in its properties. This process also results in the gradual aging of the quenching oil; hydrocarbons in the oil undergo thermal decomposition to produce volatile components or gases, and certain compounds oxidize. The oxidation products, through polymerization, cause an increase in the oil’s viscosity – this is the mechanism behind the aging of quenching oil. At the same time, the oil in contact with the hot surface of the workpiece is heated to a high temperature, causing the oil chains to break and secondary reactions to occur, resulting in decomposition products. The oxidized hydrocarbons are in a highly unstable state; they can polymerize to form sludge, and this can even lead to serious consequences such as contamination of the workpiece and blockage of the pipelines. Clearly, the mechanism of hydrocarbon oxidation during the use of quenching oil should be given attention. Why does rapid quenching oil age over time? This is due to the continuous oxidation of the quenching oil during use, which is the main reason for the formation of sludge and stains on the surface of the workpieces. As the usage time increases, the oxidation of the oil becomes more severe, its viscosity rises further, and sludge formation increases as well. All these factors reduce the cooling capacity of the quenching oil, resulting in soft spots in the workpieces or even an inability to achieve proper hardening. Microstructural analysis shows an increase in granular ferrite structures in carbon steel ； A lower bainite structure is present in the alloy steel. At this point, the essence of the problem is that the quenching oil has undergone severe oxidation during use. As the quenching oil oxidizes over time, its carboxylic acid content – where a carboxyl group is a COOH group in organic compounds that contains carbon, oxygen, and hydrogen – gradually increases. Therefore, the acid value of the oil can be measured to determine the degree of its aging. After being used continuously for over 2 years, the acid value of quenching oil keeps rising; when the acid value reaches around 1.0–1.5 mgKOH/g, the rate of carboxylic acid formation increases significantly. After aging, the cooling curve of quenching oils shows a shortened vapor film stage, an increased maximum cooling rate; the entire cooling curve shifts upward to the right, the cooling rate at high temperatures increases, and the hardenability of the workpiece declines. An acid value of 1.5 mgKOH/g is a quantitative indicator of the aging of quenching oil, and it also serves as a basis for changing the oil; generally, the oil should be replaced after one year of use. Measures to prevent the aging of quenching oils include: using hydrogenated refined base oils and composite antioxidants, which can effectively improve the oil’s antioxidant properties. To enhance the anti-aging capabilities of quenching oils, the following precautions should be observed when using them: keep the oil in a circulating state, filter it regularly, prevent water from entering the oil, avoid partial oil changes, try to reduce the operating temperature of the quenching oil, and minimize contamination. 9 The problem of bacterial growth in polymer quenching media. Currently, polymer quenching media are being used more and more widely due to their unique advantages in terms of environmental protection, technology, and cost. However, unlike water and oil, polymer quenching media are rich in organic nutrients, which facilitates the growth of bacteria; as a result, this accelerates the aging of the polymer quenching media and shortens their service life. It increases the cooling rate during the convection stage, thereby making it difficult to maintain consistent quality in the quenching of fasteners. It may also clog the filtration system of the belt furnace, affecting its proper operation. In fact, many companies are forced to replace their current quenching agents ahead of schedule due to bacterial growth. When bacteria multiply, they release acidic substances that lower the pH value; a decrease in pH is a result of bacterial growth. By regularly testing the pH value of the polymer quenching medium, it is possible to identify trends or signs of bacterial growth, and take appropriate measures to prevent such growth. A lower pH value is one of the main reasons for bacterial growth ； Test results show that a pH value below 8.0 significantly promotes bacterial growth. When the pH value is below 8–8.5, it should be raised promptly to prevent bacterial growth and a decline in rust resistance. The pH value can be adjusted by adding the stock solution, or by using a pH adjuster provided by the supplier. Inorganic salts or bases such as NaOH and Na2CO3 should not be used, as they will contaminate the polymer quenching agent, affect the accurate measurement of concentration, and impact heat transfer during the quenching process; therefore, in-field use of inorganic bases or salts to raise the pH value is not advisable. The scale and carbon black generated during the quenching process continuously accumulate within the system; they not only clog the filters but also facilitate the growth of bacteria. Therefore, regular cleaning is necessary to keep the quenching medium as clean as possible. To this end, the growth of bacteria is related to factors such as the quality of the polymer quenching medium and the water used for preparation, the circulation of the quenching system, the pH value, impurities and oils, as well as bacteria in the air. Appropriate measures should be taken to address these factors in order to reduce bacterial growth and ensure the cooling efficiency of the polymer quenching medium. Why is the hardness uneven when 10 carbon steel is quenched with water cooling? When carbon steel is quenched in water, a white networked or granular structure appears in the microstructure as observed through metallographic examination; as a result, hardness testing reveals uneven hardness with one side being harder and the other softer. When the chemical composition of the steel is constant, during the quenching of bolts with large cross-sections, the cooling rate decreases from the surface to the core due to the effect of hardenability. The white network-like or granular structures that appear in this process are generally considered to be pro-eutectoid ferrite; their morphology transitions from discontinuous, fine semi-network patterns to coarser networks. Since there are usually no significant deviations in the quenching temperature, the likelihood of undissolved ferrite appearing in the quenched structure is low. The morphology of pro-eutectoid ferrite is related to the cooling rate. Therefore, when conducting a tissue analysis, one should first examine the cooling rate, and then consider the chemical composition as well as the heating temperature, in order to identify the true cause.