Discussion on materials for high strength bolts of carbon steel and alloy steel

Fasteners are a common base and are closely linked to the steel industry. The materials used to make fasteners are mainly steel, stainless steel, titanium alloys and non-ferrous metals. Bolts are used most in fasteners. Except for a small number of large gauges, which are manufactured by warm rolling (warm extrusion) and cutting, most of them are manufactured by cold heading (cold extrusion) forming process. The mechanical properties of the bolt depend to a large extent on the quality of the material from which the fastener is made.

1 Carbon steel and alloy steel bolt material selection The basic screw and stud 0 (referred to as the new standard) was officially implemented on October 1, 2011. The new standard is the mechanical properties of fasteners made of ISO8981:2009 carbon steel and alloy steels. Part 1 : Bolt, screw and stud coarse and fine thread (English version) with specified performance grades, and GB/T 3098.12000 Mechanical properties of fasteners Bolts, screws and studs (commonly known as the old standard) have been partially modified in terms of mechanical properties, materials and heat treatment, mechanical and physical properties.

1.1 Modification of mechanical performance grade The new standard has eliminated 3.6 grades and grades in the performance grade, and added 121 grades. It also added markings, signs, basic types and applicable test methods for reducing the performance level of load-bearing bolts. Adding 3 new standards before the number to modify the mechanical performance grade of fasteners is shown in Table 1. 1.2 Materials and heat treatment modification 6th, 4.8, 5.6, 5.8 and 6.8 grade low-strength fastener materials, the new standard uses "carbon steel Or the addition of elemental carbon steel "replaces the old standard "carbon steel", and the maximum boron mass fraction is replaced by 0.003% of the old standard; the minimum carbon mass fraction of 6.8 replaces the old standard by 0.15%. .

For the direct cold forging of grades 4.6 and 5.6, the new standard table 1 fastener mechanical performance level modification table clearly stipulates that ''in order to ensure the required plasticity and toughness, it may need to be used for cold heading Wire or cold heading fastener products are subjected to heat treatment "requirements.

The new standard for quenching and tempering of low-carbon alloy steels (such as boron or manganese or chromium) as mentioned in the old standard was modified to “harden and temper carbon steel (such as boron or manganese or chromium) with added elements”. In the new standard, 8.8 and 8 fasteners have added “alloy steel quenching and tempering” materials, and grade 12.9 has added “additional element carbon steel (such as boron or manganese or chromium or molybdenum) to quench and temper "Materials. The maximum mass fraction of sulfur and phosphorus of the 8. 8~12.9 high-strength fasteners that need heat treatment is reduced from 0.035% to 0.025% of the old standard, and the carbon steel of 9.8 grade added elements (such as boron or manganese or chromium) is quenched and The tempered material has a maximum carbon mass fraction of 0.40% instead of the old standard of 0.35%, and the 12.9 minimum carbon mass fraction is replaced by 0.30% of the old standard of 0.28%. Both the new and old standards have a carbon mass fraction below zero. 25% boron-added carbon steel, the minimum mass fraction of manganese is specified: 0.6% for grade 8.8 and 0.7% for grade 9. The new and old standards stipulate that the new standard eliminates 10.9 grade in the marking system, so it is in the material. Correspondingly, the material class of 1M grade low carbon alloy steel (such as boron, manganese or chromium) quenched and tempered (minimum tempering temperature 340C) was eliminated. The minimum tempering temperature of 12.9 is replaced by the old standard 380C by 425 C. The new standard also stipulates that “the white phosphide aggregation layer can be detected on the surface of the 12.9/12.9 surface without metallographic examination, and the phosphide is removed. The aggregate layer should be carried out before heat treatment."

The new standard also states that “it should be cautious when considering the performance level of 12.9/12. The fastener manufacturer's capabilities, service conditions and tightening methods should be carefully considered. In addition to surface treatment, the use environment may cause stress corrosion of fasteners. Cracking." The new standard adds "see the requirements of GB/T5267.3-2008 for the requirements of fastener materials for hot dip galvanizing. The new standard stipulates that when the dispute arises, the finished product analysis".

1.3 Modification of mechanical and physical properties HRE, "Rockwell hardness HRC," surface hardness HV0. 3", "minimum height of undecarburized layer of thread", "maximum depth of full decarburization layer (4) "," after tempering The hardness ', 'by bad torque (Mb), the new standard adds to the low-strength fasteners of 4. 8, 5.8 and 6.8 without heat treatment. The provisions of the physical fasteners are non-proportional 8, 5.8 and 6.8. Graded physical fasteners are specified for non-proportional elongation of stress values ​​of 0.0048 d. In the investigation, the existing values ​​are for calculation only, not experimental values.

Lower yield strength (yield point) (RA) of grades 4.8, 5.8 and 6.8; the new standard clearly states that “when the current yield strength ReL cannot be measured, a non-proportional elongation of 0.2% stress RP0.2 is allowed instead”. The new standard specifies the impact toughness (impact energy absorption) project at 5.6, 8.8, 9.8, 10.9 and 12.9/12.9. The new standard "except for the 12.9/12.9 class is under investigation and research", the other four grades The values ​​are all 27. The impact toughness values ​​are relatively large and the old standards are relatively large. The new standard uses “impact toughness” instead of the old standard “impact absorption work”, “the value is measured at -20C test temperature” and only "Applicable to d16mm, that is, impact toughness (KV2) is a low temperature impact test, and impact absorption work refers to the test at normal temperature. The required limit values ​​also vary greatly.

Both the new and the old standards stipulate that when measuring the surface hardness, "the surface hardness should not be 30 Vickers hardness values ​​higher than the core hardness." The surface hardness of 10.9 should not be greater than 390HV0.3. The new standard also increased to 12.9. The surface hardness requirement of /12.9 is "not allowed to increase the surface hardness, and the surface hardness should not exceed 435HV0.3."

Grades 9 and 12. stipulate the requirement that “the core of the thread section obtains a martensite structure with a volume fraction of 90% after quenching and before tempering”. The new standard extends the range to 8.8, ie 8.8, 9. Hardness arbitration. Test, the old standard is specified in the section "from the end of the thread diameter, 1/2 radius", and the new standard gauge 2 carbon steel and alloy steel high-strength bolt material selection 2.1 high-strength bolt material selection principle selection fastening The principle of materials is mainly considered from the following aspects. (1) According to GB/T3098.1-2010/ISO898-1:2009 standard; (2) mechanical properties of fastener materials, especially tensile strength requirements; (3) according to product type and size, according to specific Molding process and equipment, fastener manufacturing process requirements for material processing performance; (4) According to the service environment of the product and the specific requirements of the customer, the requirements of the working conditions on the corrosion resistance of the material, the working temperature to the material Requirements for heat resistance (high temperature strength, oxidation resistance) and low temperature cold brittleness (low temperature impact toughness); (5) according to the resource status of steel, as well as price, weight, procurement and other requirements. Based on the above comprehensive considerations, determine the grades, varieties, specifications and material standards for the selection of fastener materials.

2.2 Problems with high-strength bolt materials 2.2.1 Sulphur and phosphorus mass fractions are 0.035%H; GB/T699 high-quality carbon structural steel H and GB/T 3077 alloy structural steel 0 in "high-quality steel" regulations S The mass fraction of P and P is 0. 035%, which is incompatible with the mechanical properties of the new standard fasteners. The S and P mass fractions of the 8. 8~12.9 grades in the new standard are not more than 0.025%. The S, P mass fractions specified in the “Quality Steel” in the existing GB/T 3077 can no longer meet the requirements of the new standard, but GB The ''high-quality steel' and 'extra-quality steel' in /T 3077 meet the requirements of the new standard, and the "extra-quality steel" in GB/T 699 meets the requirements of the new standard.

Some of the imported high-strength fasteners adopt the corresponding foreign material standards, which leads to a large number of materials localization work. GB/T6478, GB/T699 and GB/T3077 have not adapted to the needs of the situation and should be revised as soon as possible. 6. 2.2. 2 Decarburization layer decarburization layer is an important factor affecting the quality of fasteners and the use of materials. 6478 for decarburization layer inspection regulations "The total decarburization layer depth (ferrite + transition layer) on each side shall not be greater than the nominal diameter of 1.0. % (high-quality steel) or 1.5% (high-quality steel), and supplied by the latter if not explicitly stated by the purchaser. GB/T 699 for decarburization inspection of steel with a carbon mass fraction greater than 0.30% It is stipulated that “the total decarburization depth (ferrite + transition layer) on each side shall be no more than 1.0% (high-grade steel) or 1.5% (high-quality steel) of the nominal diameter.” The carbon mass fraction in GB/T3077 is greater than 0.30. The decarburization layer inspection of % steel is defined as "the total decarburization depth (ferrite + transition layer) on each side is not more than 1.5% of the nominal diameter". GB/T3098.1 stipulates that the depth of the full decarburized layer of the thread shall not be greater than 0.015mm, 8.8 and 9.8 thread undecarburized layer height is 1/2 tooth height, 10.9 level is 1/3 tooth height, 12.9 level is 1/4

2.2.3 Non-metallic inclusions and acid-impregnated low-magnification non-metallic inclusions are the main cause of cold head cracking, and the influence of Class C (silicate) and D (spherical oxide) inclusions is greater. The larger the particle, the more easily it cracks. The inclusions within 2 mm from the surface should not exceed 0.15 pm. For high-strength fastener materials, the silicate inclusions are required to be no more than 0.5, and the spherical oxide inclusions are not greater than Grade 1. Main defects of material control: ingot type segregation of cold heading steel, surface decarburization during material and annealing, cold cracking, quenching cracking and deformation in quenching and tempering.

3077 Inspection of non-metallic inclusions clearly stipulates that non-metallic inclusions can be inspected according to the requirements of the purchaser, after consultation between the supplier and the buyer, and indicating the qualification level in the contract. See Table 2. Table 2 Acid Leaching Low-magnification Organization Technical Requirements Standard No. Alloy steels of alloying elements such as nickel and manganese, and steels for corrosion resistance are carbon steel and stainless steel which is mainly used for strength and hardenability, and a trace amount of boron is added to greatly improve hardenability. Boron steel.

Boron steel can produce 9.8 grade, 10.9 grade high strength bolts. Heat treatment method: low-temperature martensite is obtained by high-temperature quenching and rapid cooling, and after intermediate temperature tempering, the service conditions of the bolt are basically satisfied.

China's self-developed steel grade 20MnTiB is mainly used for steel structural joints. It is mainly used for 10.9 grade axle bolts, nuts and differential bolts. The name is changed to ML20MnTiB and listed in GB6478, but the bolt quality and new standard There is a fixed gap. First, the lower limit of carbon mass fraction of 20MnTiB and ML20MnTiB is less than 0.20%, which can not meet the requirements of the new standard. Secondly, when the hardness of 20MnTiB steel reaches 32 ~ 39HRC, it may not reach the minimum tempering temperature of the new standard. The requirements of 425C; Finally, the new standard stipulates that the maximum mass fraction of B is 0.003%, and the maximum mass fraction of B of 20MnTiB and ML20MnTiB steel is 0.0035%, and the upper limit exceeds its standard. The carbon mass fraction of 20MnTiB and ML20MnTiB should be above 0.22% in order to maximize the hardenability to increase the effective hardenability. When meeting the new Table 3 grades 矣8 and 10.9 bolts, it also adopts the Taiwan Steel Standard Series. Made of 10B21, 10B23 and 15B22 steel. The lower limit of carbon mass fraction of 10B21 and 15B22 can not meet the requirements of 10.9 grades on the carbon mass fraction of 0.20%~0.55% in the new standard; the lower limit of manganese content of 10B21 and 10B23 can not meet the new standard "for 8.8, 9.8, 10.9 grade 030%。 The minimum manganese mass fraction should be 0. 030%, the minimum mass of manganese should be 0. 030%, the minimum mass of manganese should be 0. 030%, the smelting of the low-carbon alloy steel (such as boron, manganese or chromium) is less than 0.25% (melting analysis), the minimum manganese mass fraction should be 0. 030% Other chemical composition requirements are shown in Table 4. 8.8 and 10.9 grade bolts made of CH35ACR and CH40ACR steel produced by Taiwan Steel Standard, also known as medium carbon alloy steel, which is a carbon steel with micro Cr content. Although the hardenability is worse than that of medium carbon steel 35 and 45 steel, it is not as good as 40 steel. Therefore, it is recommended to manufacture 8.8 bolts, but it is not suitable to manufacture 10.9 bolts.

Table 4 Chemical composition of 10B21, 10B23 and 15B22 steels Table 5 Grade 矣3 Conclusion The probability of accidents caused by the failure and damage of fasteners is very high, therefore, the materials used to manufacture fasteners must be strictly selected. In the future, fasteners are still mainly made of steel materials. With the advancement of technology in the fastener industry, more and more steels meet the requirements of modern fastener products and processes, which provides important for the production of high-strength bolts. Guarantee.

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