1. Medium carbon steel ML35, 35 or SRWCH35K, the bolts larger than 16mm8.8 can not be hardened, and it is easy to solve after changing to 45 steel.
Usually 8.8 bolts are made of ML35, 35 or SRWCH35K. When the section size is different, the mechanical properties of the 8.8 grade bolts are not consistent due to the different quenching degree. The higher the mass fraction of bolt steel carbon, the higher the martensitic hardness. After quenching the bolt with the entire section, the volume fraction requires about 90% of the martensite structure, indicating that the critical diameter of the specimen is completely hardened, and the higher the degree of hardening, the greater the yield ratio after quenching and tempering. An increase in the yield ratio means that the required mechanical properties can be achieved; otherwise, it is not hardened, the worse the degree of hardenability, and the worse the mechanical properties, which will affect the final quality.
Carbon has a strong influence on hardenability. The contribution to hardenability is equivalent to manganese at C ≤ 0.35%. When the carbon content is 0.37%, the hardenability of steel just meets the requirements of technical conditions. When C ≤ 0.40%, for every 0.10% increase, the HRC hardness value can be increased by 6HRC. The carbon content in the steel increased from 0.32% to 0.40%, and the HRC from the quenching section J7.5mm increased by about 8HRC to 56HRC, and the hardness value increased with the increase of carbon content.
We often use quenching hardness to control the quality of heat treatment. However, for 45 steel, due to the low hardenability, it is only suitable for high-strength bolts with a size less than 20mm. For many years, many companies have used 45 steel for bolts with a cross-section diameter of 22 to 27 mm, and the quenching and tempering treatment neglects the most important quality characteristics of hardenability. Due to the cross-sectional size effect, the larger the bolt size, the lower the hardness after quenching, the shallower the hardened layer, the larger the acicular and lath martensite on the surface of the metallurgical structure, and the ferrite plus pearlite structure in the core. At this time, the comprehensive mechanical properties of the high-strength bolts are also lower. When the size is larger than 30 mm, the surface may not even obtain martensite structure.
2. What are the requirements for the application of high-strength bolts on transmission towers?
High-strength steel will be widely used on transmission lines. The selection of high-strength bolts matched with high-strength steel is one of the concerns and discussions of suppliers. This problem mainly includes two aspects: the wall pressure of the rod and the anti-shear of the high-strength bolt.
The high-strength bolts used on the transmission tower are different from the high-strength bolts used in the civil structure and the bridge structure. They are made of high-strength and thick bolts, and their strengths meet the corresponding grades (8.8 and above), but the processing standard is still large hexagonal heads. Bolts (GB/T5780), the calculation of shear and tension is still calculated using ordinary bolts. Generally, when calculating and selecting bolt specifications, in principle, under the premise of meeting the requirements of force and bolt spacing, use small-diameter bolts as much as possible; under the premise of meeting the strength requirements, use low-level bolts as much as possible.
For components using Q345, Q390 and above high-strength steel, 6.8-class bolts are suitable for small-size angle steels; for large-size angle steels, 8.8-class bolts are more economical. It is not recommended to use 10.9 bolts. It is recommended to use equal strength steel plates for the gusset plates. Q420 high-strength steel is used in places with high force, and most of the joint bolts are in double shear state.
Since the 10.9 bolts are prone to hydrogen embrittlement fracture during pickling and rust removal, and lack of mature operating experience, it is recommended not to use 10.9 bolts. The price difference between the 4.8 bolt and the 6.8 bolt is not much different, and a 6.8 bolt can solve the practical problem of many small auxiliary materials, but it can be solved with two 4.8 bolts. Therefore, in the iron tower, bolts M16 and M20 use 6.8 grade hot dip zinc bolts, and M24 bolts should use 8.8 grade hot dip zinc bolts to reduce the gusset plate and reduce the tower weight.
3. Can boron steel be used to produce high-strength bolts? What should you pay attention to when manufacturing 10.9 high-strength bolts?
Boron-containing steel is a low-alloy steel based on manganese and boron instead of chrome or nickel steel. Boron is added as an alloying element to the steel. As long as it is 0.001% to 0.002%, the hardenability can be remarkably improved, the hardenable size of the steel can be increased, and the uniformity of the structure and properties in the steel section after quenching can be improved.
Cold heading steel has always used medium carbon steel or low alloy steel. These steels have poor deformation properties (requires spheroidizing annealing pretreatment and intermediate softening annealing), and have poor mechanical properties and severe decarburization. The basic significance of using boron steel is to reduce the carbon content, improve the cold head deformation ability, and add boron to increase the hardenability lost by carbon reduction.
Boron-containing steel replaces a large amount of other alloying elements with a small amount of boron. Therefore, compared with other alloy steels having comparable hardenability levels, the total alloy content is low, the high-temperature deformation resistance is small, and plastic deformation is easy.
It should be noted that: 1 thermal processing has the greatest influence on the variation of boron in various forms of solid steel. When the content of boron in steel exceeds 0.003%, a low melting point Fe-CB ternary eutectic appears at the grain boundary. , the hot brittleness is produced and the hot workability is deteriorated; 2 the main function of boron is to improve the hardenability, so it is necessary to ensure that the content of solid solution boron is in a suitable range, more than 0.001%, so as not to impair the hardenability, and boron Whether it can play its due role depends to a large extent on the choice and operation of the heat treatment system; 3 the quenching temperature should be selected according to the boron content of the steel, the chemical composition and the state of the boron before quenching, etc. It should not be fixed, and the tempering temperature in the heat treatment system will decrease, which will increase the delayed fracture tendency of the boron-containing steel bolts. Many research and practical experience have also proved that boron-containing steel has the advantage of adding normalizing before quenching, which can eliminate or reduce the boron phase and improve the performance of the boron-containing steel after quenching and tempering.
10B21, 10B28, 10B33, 20MnTiB and ML20MnTiB are used most in bolts. The final mechanical properties of bolts are determined by the material quality of the fasteners. The lower limit of carbon content of 10B21, 20MnTiB and ML20MnTiB is less than 0.20%, which cannot be satisfied. Standard requirements for materials; when the bolt strength is ≥1040MPa and the hardness is 33~39HRC, it is achieved by reducing the tempering temperature. When the tempering temperature test is used at 425°C, the hardness and strength are reduced, which cannot meet the standard. Performance requirements for minimum tempering temperatures. Production practice can be seen that the mechanical properties of the bolt depend not only on the heat treatment process and the metallurgical structure, but more importantly, it should have a good chemical composition.
4. Is there any way to wash the residual oil on the surface of the fastener?
There is no way to clean the cleaning method, mainly for the cooling oil residual oil attached to the surface of the fastener during cold heading or cutting, and the cleaning of the residual oil on the surface of the fastener after quenching in the heat treatment oil.
1 alkaline water cleaning method
The simplest alkaline water cleaning is a direct heating or indirect heating cleaning tank, which is 3% to 10% N2CO3 or NaOH aqueous solution. For self-tapping screws for carburizing or carbonitriding, the intermediate cleaning before tempering is usually carried out with a sodium carbonate solution heated to 50-70 ° C 5% to 10%. Connected mesh belt furnaces fall into this category. Fastener cleaning generally uses a dual method of spraying and immersing. If the effect is better with hot water, the high-pressure airflow is used for drying. The main disadvantage of the alkaline water cleaning method is that the cleaning effect is not very good, especially the blind holes and the groove portion of the fastener are prone to stains and residues. The wastewater generated by the alkaline water cleaning must be neutralized and the waste oil recovered to meet the relevant environmental requirements before being discharged.
2 metal cleaning agent cleaning method
In order to overcome the shortcomings of the alkaline water cleaning method and improve the cleaning effect, a special metal cleaning agent can be used at present, the cleaning agent is diluted into a 1% to 3% solution in 40-70 ° C water, and the fastener is soaked in the solution 15 ~ 20 min, then rinsed with hot water, and finally dehydrated with a dehydrator. Can be used in the washing machine. The disadvantage of the metal cleaning agent cleaning method is that it is expensive and has an unpleasant odor. It has a certain corrosive effect on the fastener, and the wastewater pollutes the environment. Therefore, the wastewater must be treated and the waste oil recovered.
3 organic solvent cleaning method
It is cleaned with an organic solvent such as trichloroethane or trichloroethylene, and the cleaning effect is good. When the quenched fastener is cleaned by immersion or spraying under normal pressure, the volatilization and splashing of the organic solvent pollute the environment. To this end, a complete set of closed vacuum decompression solvent vacuum cleaning method is used, the vacuum pump is used to discharge the air, and the fastener is contacted with high-density solvent vapor in the absence of air to further improve the cleaning effect, and the solvent and oil are recovered by vacuum distillation. . The solvent content of the recovered oil is reduced from 20% to less than 4%, which is lower than 50 ppm of the environmental sanitation regulations of trichloroethylene.
4 combustion degreasing method
Cold-pressed or machined fasteners need to be cleaned of the surface oil before entering the heat treatment process. To this end, the fastener can be heated in a degreasing furnace to 350-400 ° C to vaporize or burn the oil to achieve the purpose of de-oiling the fastener or the effect of surface phosphorus removal. The combustion degreasing method is only suitable for cleaning before heat treatment, and can also clean light, low viscosity and low boiling oil such as cooling oil and anti-rust liquid. After the high-viscosity, high-boiling heavy quenching oil is burned, a large amount of residue and carbon black adhere to the surface of the fastener, failing to clean the object.
For fasteners requiring high surface cleaning, such as aviation bolts, automobile engine connecting rod bolts, etc., advanced cleaning techniques such as vacuum or ultrasonic are currently used to replace the commonly used chlorinated hydrocarbons that have a destructive effect on the atmospheric ozone layer. The method can achieve efficient and environmentally friendly cleaning.
5. What is the cause of the heat treatment crack of 45# steel nut?
45# steel H10 grade M30 nut, after quenching and tempering, it is found that the hexagonal cross section has visible arc and radial cracks in the circumferential direction, and the bearing surface also has a circular crack visible to the naked eye. This is a common heat treatment crack, the surface of the fracture is mostly dull, the part has been blackened, a small part is shiny and has a tear-like strip (caused when the fracture is broken), and no other defects are seen.
The metallographic examination showed no decarburization layer on both sides of the crack, and most of the fracture surface of the open crack was oxidized and blackened, and a small portion of the fracture after the break had metallic luster, indicating that the surface oxidation blackening was caused by oxidation at high temperature tempering. The crack is cracked along the crystal, the tail is sharper, and the straightness is straight and strong. There are two possible causes for decarburization on both sides of the crack. One is that there is no crack before quenching, and cracking occurs during quenching and cooling. No oxidative decarburization is observed on both sides of the crack. Second, the crack is serious due to cracking of the nut. The sample is an extended crack when the crack propagates after heat treatment. The author believes that the crack is the quenching crack, the thermal stress and the superposition of the microstructure stress cause the quenching stress to increase significantly and cause the nut to crack.
It is recommended that the original quenching process be 830 ° C, there is local overheated structure in the microstructure, now changed to 800 ~ 810 ° C, the use of aqueous solution quenching cooling to fast quenching oil. The cooling ability of the oil quenching medium is lower than that of water, especially in the range of 300 to 200 ° C, and the cooling rate is much lower than that of water. Therefore, the quenching with the rapid quenching oil can alleviate the quenching purpose, and the deformation of the internal thread of the nut can be reduced.
6. What effect does the tempering temperature of the 42CrMo steel bolt quenching and tempering on the metallographic structure?
For quenching and quenching of 42CrMo steel 10.9 bolts, it should be fully austenitized to obtain about 90% martensite structure. When the tempering temperature is 540 °C, its structure and performance are the best, which is tempered at low and medium temperatures. At the same time, the strength of the martensite and ferrite two phases is large, which is easy to cause stress concentration and segregation, resulting in uneven organization and poor performance. When the 42CrMo steel bolt is tempered at 540 °C, the difference in the strength of the two phases is reduced, which is beneficial to improve the stress distribution and the homogenization of the structure.
The 42CrMo steel bolts are tempered after quenching, and the influence of the tempering temperature on the metallographic structure has undergone a process from coarse to small. Different tempering processes after quenching are mainly the elimination of quenching structure stress, the stabilization of the structure and the precipitation process of matrix microstructure. When tempered below 400 °C, the structure is tempered martensite, undissolved ferrite and a small amount of retained austenite. The carbides precipitated during medium and low temperature tempering are fine.
As the tempering temperature increases, carbon atoms continue to precipitate and form carbides. The precipitated carbides grow and form tempered torsite. At 540 °C, the carbides continue to grow and granulate, martensite. It turns into a ferrite structure and forms tempered sorbite and undissolved ferrite. At this time, the strength and hardness of the steel decrease, and the plasticity and impact toughness are greatly improved. Production practice shows that the tempering temperature of 42CrMo steel bolt quenching and tempering must be above 540 °C, otherwise the low temperature impact energy -45 °C ≥ 27J may be unqualified.
7. How to strengthen the austenitic stainless steel?
Such as commonly used 06Cr19Ni10 (304) or 12Cr18Ni9 (commonly known as 18-8 steel) austenitic stainless steel. The phase change does not occur during heating, and the mechanical properties cannot be strengthened by the heat treatment method, and the strength is low, and the plasticity and toughness are high. For austenitic stainless steels, it is generally required to improve corrosion resistance and plasticity, and fasteners that eliminate cold work hardening should be solution treated; for fasteners with complex shapes and unsuitable solution treatment, stress relief annealing can be performed; Stainless steel containing titanium or niobium can be subjected to stabilization annealing and sensitization treatment in order to obtain stable corrosion resistance.
Solution treatment is suitable for austenitic stainless steel of any composition and grade. The recommended heating temperature range is generally 950 °C ~ 1050 °C, and the method of rapid cooling after a certain time is maintained. Stabilization annealing heating temperature selection is very important, the selection principle should be higher than (FeCr) 23C6 dissolution temperature, generally between 750 ~ 860 ° C. The stress-relieving treatment mainly removes the stress generated during the machining process or removes the residual stress after the processing, and can be heated to 1010 to 1120 ° C, and then slowly cooled after heating and holding. Usually, the sensitization treatment system is heated to 650 ° C, and air-cooled for 5 to 6 hours. For some special occasions, in order to more rigorously evaluate the material's resistance to intergranular corrosion, different sensitization systems are adopted depending on the temperature of the fasteners to be used in the future and the carbon content of the materials, and whether or not molybdenum is contained.
Austenitic stainless steel cannot be strengthened by heat treatment, but it can be strengthened by cold working deformation (cold work hardening, deformation strengthening), strength is increased, and plastic toughness is lowered. Austenitic stainless steel bolts, nuts and products have large processing stress after deformation and strengthening by cold heading and cold extrusion. The presence of such stress increases the sensitivity of stress corrosion when used in stress corrosion environments. , affecting the stability of the size. To reduce stress, a stress relief treatment can be employed. Generally, it is heated to 260-400 ° C, and it is air-cooled or slow-cooled after 2 to 6 hours of heat preservation. The stress relief treatment not only reduces the stress of the fastener, but also improves the hardness, strength and fatigue limit without a large change in elongation after the break.
Austenitic stainless steel can not be adjusted by mechanical heat treatment, but it can be improved by cold work hardening. It is applied to the manufacture of stainless steel bolts. The effect of cold work hardening is related to the carbon content and deformation of steel, with the increase of cold working compression ratio. The effect of cold work hardening is enhanced.
8. How to reduce the consumption of heat treatment protective atmosphere to reduce costs?
When steel fasteners are heated in various gaseous media, due to the different effects of various gases on the steel, oxidation and decarburization occur on the surface of the steel, and the chemical composition and quality of the steel fasteners change. Commonly used controlled atmospheres contain hydrogen, carbon monoxide, methane, carbon dioxide, nitrogen, moisture, and traces of oxygen. To this end, the fastener should be heated to a protective atmosphere. At present, the mesh belt furnace adopts methanol and propane or toluene drip-type cracking as the main force of the protective atmosphere. Commonly used organic liquids such as methanol + propane, methanol + toluene, nitrogen methanol, etc. crack the controllable gas. Nitrogen-controlled gas is used in the furnace for heating fasteners. It can be decarburized and can be quenched and heated in low carbon steel and medium carbon steel. This gas has a very low Co content, so the price is much cheaper. The production practice proves that the use of nitrogen and methanol atmosphere can reduce the cost by about 20% to 30%, and the economic benefits are remarkable.
Methanol is decomposed above 800 °C by CH3OH→Co+2H2, and its composition is 1/3Co and 2/3H2, and a small amount of CO2, H2O and CH4. There is a tendency to form free carbon depending on the reaction temperature, and it is a gas having a weakly carburizing or non-carburizing gas and having a protective gas.
Nitrogen is an inert gas, non-toxic, non-polluting to the environment, and has no risk of combustion and explosion. It is called a green heat treatment atmosphere. The main components of the nitrogen-methanol atmosphere are N2, H2, Co, and trace amounts of H2O, CO2, and CH4. Adjusting the mixing ratio of nitrogen and methanol, Co in the furnace atmosphere can vary from 0 to 33%, and the concentration of H2 in the atmosphere is always twice the concentration of Co, and the balance is N2. Since methanol is a weak carburizing agent, when the carbon potential in the furnace reaches a certain level (generally specified as 0.35% C to 0.40% C), in order to rapidly increase the carbon potential in the furnace to meet the production needs, it is necessary to add a certain amount to the furnace. The amount of enriched gas is introduced into the furnace by introducing propane gas or instilling methanol in a total amount of 2% to 5% of the shielding gas, thereby rapidly increasing the carbon potential and achieving the process setting value. The proportion of gas after decomposition in the furnace is basically 20% CO, 40% H2 and 40% N2. It is worth noting that the flow rate of N2 is 10-15 L/min.
Propane is a flammable and explosive material. It will explode and burn when it encounters a strong collision. Toluene is a strong carburizing organic liquid cracking gas. Toluene is a reagent for easy drug production. It reduces the amount of propane and toluene, can effectively reduce unsafe factors, and can reduce the harmfulness of harmful gases to employees.
9. Heat treatment of beryllium bronze elastic washers and spring products?
Beryllium bronze is a typical ageing precipitation hardening alloy widely used in elastic gaskets and spring products in instrumentation, aerospace industry, nuclear power industry and electronic and electrical industry products. The main elemental components of beryllium bronze are copper, antimony and nickel. At present, there are mainly three types of beryllium bronze materials produced in China: QBe2.5, QBe2.0, and QBe1.9. For the convenience of testing, the hardness method is usually adopted, and the technical requirement is 320-380HV. The bismuth bronze plate material has various states, and the microstructure and hardness of the yttrium bronze after heat treatment are different. There are four states: C state (soft state, ie quenching state), CY4 state (1/4 hard), CY2 state (semi-hard), CY (state hard state, ie cold rolling state after quenching).
The heat treatment process of beryllium bronze is mainly solid solution + aging. The hardness and strength can be improved by solution and aging treatment, but have no effect on excellent electrical or thermal conductivity and corrosion resistance.
After solution treatment, the material not only has good plasticity, but can be processed and formed, and the most important thing is to prepare for subsequent aging strengthening. Test and production practice shows that the solid solution temperature of beryllium bronze is 760 ~ 800 ° C, which can avoid the disadvantages caused by too high or too low temperature. In order to improve the surface quality, heating is often carried out in a protective atmosphere. There are many kinds of protective media. Compared with ammonia decomposition gas, alcohol cracking gas has the advantages of high purity, stable atmosphere and less water.
The aging treatment is generally carried out in a vacuum furnace for 2 hours and then cooled with the furnace. The cooling rate of the beryllium bronze solution treatment is generally as fast as possible, especially in the range of 370 to 705 ° C, and more rapid cooling. It is generally stipulated that the solid solution heated material should be placed in water for stirring within 3 seconds, otherwise the solid solution effect will not be achieved, which will affect the quality of the beryllium bronze. The cooling medium is generally 10 to 18 ° C clean tap water, convenient, practical and inexpensive.
Beryllium bronze has excellent overall properties after solution and aging, such as high strength, hardness and fatigue properties. For fasteners with high elasticity requirements, normal ageing is required. By treating the state of different grades of beryllium bronze at different temperatures, it is found that the aging time of beryllium bronze in the C state (soft state) and the CY4 state (1/4 hard) cannot exceed 350 ° C, and the Vickers hardness is 380 HV and 401 HV, respectively; The aging time of yttrium bronze in CY2 state (semi-hard) and CY state (hard state) should not exceed 320 °C, Vickers hardness is 405HV and 399HV respectively; after this temperature, the hardness of the material begins to decrease, and the grain boundary reaction increases. That is, it has been out of date. To this end, the technical requirements should be adjusted to 360-410HV.
The aging temperature is generally about 320 ° C, and obvious lines are precipitated in the crystal, and local fine granular precipitates appear on the grain boundaries. When the temperature is aged at 320-340 °C, not only can the process time be shortened, but also the comprehensive mechanical properties that meet the requirements can be obtained. It is feasible for the elastic pad and the spring member to have a certain strength and high elasticity.
The aging treatment of beryllium bronze should be carried out in a protective atmosphere furnace, a vacuum furnace or a well gas circulating electric furnace. The vacuum aging is 325 ° C × 2.5 ~ 3 h, the tensile strength is 1245 ~ 1255 MPa, and the elongation after fracture is 8.5% - 9.0%. 385 ~ 395HV; and the appearance quality can also meet the requirements, no pollution to the environment.
10. How to optimize the quenching and toughening process of 45# steel high strength bolt?
In production, 45# steel is used to manufacture large-size 8.8 high-strength bolts. The heat treatment and quenching process is 830~840°C quenching + medium and high temperature tempering. Due to the large diameter, it is generally cooled by an aqueous solution, but water quenching tends to cause cracks and large deformation in the steel, especially at the head and hexagonal edges. Therefore, improving the heat treatment process of 45# steel has aroused great interest from peers.
For many years, everyone used to solve the quenching cracking problem of 45# steel by sub-temperature quenching. When quenching by sub-temperature, due to the presence of undissolved ferrite in the structure, it is embedded in the martensite after quenching and becomes residual ferrite. According to the latest GB/T3098.1-2010 standard, the martensite should be about after quenching. 90%, it is judged as a non-conforming product.
According to the latest data, the critical point temperatures of Ac1 and Ac3 of 45# steel are 724 °C and 802 °C, respectively, and 800 °C is selected just near the critical temperature. From the metallographic examination, it can be seen that residual ferrite seems to be absent. a critical state.
From the metallographic examination, it can be seen that as the quenching temperature is raised from 770 ° C to 840 ° C, the amount of residual ferrite is not obvious, and 830-840 ° C is conventionally quenched, and there is no residual ferrite in the structure. At 800 ° C quenching at a lower temperature, the lath and sheet martensite coexist, tend to be finer and more uniform, and the quenching stress is smaller, effectively reducing the risk of quenching cracking.
From the mechanical properties test, it can be known that the sub-temperature quenching at 770-790 °C embeds residual ferrite in the martensite matrix, causing a splitting effect on martensite. When tempering at medium and high temperatures, it will affect the tempered torsite. The continuity of the tempered sorbite has a certain degree of weakening effect on the strength. At the same temperature, the strength obtained after 800 ° C or conventional quenching and tempering is similar. The effect of tempering on strength is not only related to the tempering temperature, but also depends on the degree of solid solution strengthening of martensite and the relative relationship with carbide.
For 800 °C quenching, martensite in 800 ° C and conventional quenched microstructures is not only from the morphology of the tissue, but also because the ferrite in the microstructure has little effect on the solid solubility of martensite. The solubility should be similar, so after the sub-temperature quenching + tempering near Ac3 (800 ° C), the strength is not much different from the strength after conventional quenching and tempering. To this end, the 45# steel high-strength bolt quenching and toughening process is recommended to be 800 ° C quenching water.
Extrusion is a metal forming process , in which billet is passed through a die , to get required cross section of component .
Non cutting process
Production of Fixed cross section
Types of extrusion process
Direct extrusion process
Indirect extrusion process
Impact extrusion process
Hydrostatic extrusion process
Tube extrusion process
Extrusion Die Abutment,Nickel Based SuperAlloy Rene 88,Rene 88 Extrusion Die Abutment,Nickel Based SuperAlloy
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