Bolt is an important fastener. It is widely used in machine tools, general machinery, automobiles, engineering machinery, electric power engineering, petrochemical industry, bridges, rail vehicles, mining machinery, construction and other fields. It is often seen in our daily life that the quality and reliability of its products play an important role in the working performance and structural safety of the host.
According to the mechanical properties of bolts, they are divided into high-strength bolts and ordinary bolts. High strength bolts mainly refer to bolts with mechanical property grade ≥ 8.8. These products are mainly made of high-quality materials such as low and medium carbon alloy (such as boron or manganese or chromium) steel or medium carbon steel. Except for individual varieties and specifications, they are all subject to quenching and tempering treatment. They have high mechanical properties and service properties and are often used in occasions with high requirements.
1. Technical requirements for bolt heat treatment
Heat treatment is a key process in the production of high-strength bolts, which determines the final service performance of bolts and can make high-strength fasteners meet the requirements of design. It has certain strength, good plasticity, toughness, low notch sensitivity and high bending strength. Avoid relaxation and other comprehensive mechanical properties and service performance. So as to ensure the quality and reliability of fastener products and improve the market competitiveness of products.
2. Common defects and influencing factors of bolt heat treatment
There are many defects in bolt heat treatment, such as insufficient surface hardness, soft spots, insufficient tensile strength, large deformation, quenching cracking, surface oxidation, etc. Quenching cracking is one of the most dangerous defects in heat treatment. Directly resulting in product scrapping. There are many factors that affect the defects of heat treatment. Such as steel hardenability, chemical composition of austenite, quenching heating temperature, medium cooling rate, original structure, workpiece shape, residual stress and its distribution, heating equipment and atmosphere, etc
2.1. Chemical composition and hardenability of steel
The carbon content of steel determines its hardenability. The lower the carbon content in austenite, the greater the thermal stress relative to the structural stress; On the contrary, the higher the carbon content, the greater the effect of structural stress. Alloying elements can significantly improve the hardenability of steel. If the hardenability of steel is good and the workpiece is easy to harden, the effect of structural stress and specific tolerance is relatively large, and quenching cracking is easy to occur when the medium cooling speed is too fast; If the hardenability of steel is poor, the workpiece is not easy to harden, and the medium cooling speed is too slow, it is easy to have soft spots or insufficient hardness.
2.2 quenching temperature
Raise the quenching temperature of carbon steel and low alloy steel. It will not only increase the thermal stress, but also increase the structural stress due to the increase of hardenability, and eventually increase the tendency of deformation cracking. Too high quenching temperature is easy to cause warpage. For long and thin bolts, lower quenching temperature shall be used as far as possible on the premise of ensuring performance indicators.
2.3 original structure and residual stress
The original structure before quenching will also have a great impact on the heat treatment performance. The original structure includes the inclusion grade, banded structure, original grain size, degree of component segregation, distribution direction of free carbonized material points, and different structures obtained by different preparatory heat treatments.
The magnitude and distribution of residual stress before quenching will also affect the performance of products after quenching, such as machining, forging, welding, correction and so on. If annealing is not carried out before quenching to eliminate it, the tendency of deformation cracking will increase. Bolts tend to cause large mechanical stress in the process of heading, shearing and threading. Low alloy bolts (such as 40Cr, 40CrMo, etc.) are generally hot punched, while large-sized carbon steel bolts (such as 35K, ML35, 35) require good processing properties. Such as annealing. Avoid the formation of large residual stress, which will eventually lead to deformation and cracking.
2.4 medium cooling rate
The faster the cooling speed, the greater the quenching internal stress and the greater the tendency of deformation and cracking. Among them, the thermal stress mainly depends on the cooling speed above MS, while the structural stress mainly depends on the cooling speed below Ms. The cooling capacity of the medium mainly depends on its physical characteristics. Such as viscosity, heat capacity, wettability, etc., but the process parameters such as temperature, stirring and concentration (water-based) also have a great impact on the cooling capacity. Improper selection will inevitably lead to heat treatment defects. For carbon steel fasteners, most of them use water-based quenching media. But it has more process parameters than quenching oil. There are high requirements for process and operators, and maintenance and detection shall be done during use. Common problems of water-based products [3]:
2.4.1 medium concentration is too low
If the concentration is too low, it is easy to cause the defects of deformation and cracking. It is necessary to strengthen the concentration detection and adjust it in time. At present. Refractometer is mainly used for concentration detection on the production site. Due to pollution, medium aging and other reasons, the refractive index will be reduced. If new liquid is used, the refractive index concentration will be overestimated. For example. The reading of the refractometer is 6, and the refractive coefficient of the new liquid is 2.5. It is considered that the concentration is 6x2.5 = 15%. If the actual concentration is only 6x1.5 = 9%, the concentration is overestimated by 6%. This is very dangerous.
2.4.2. The temperature of the medium is too low or too high. The liquid temperature of the water-soluble quenching medium has a great impact on the cooling characteristics of the core ash. The circulating air cooling system should be equipped during use to adjust the bath temperature during use. If the temperature is too high or too low, the workpiece may be deformed and cracked. Polyglycol (PAG) water-soluble quenchants have a unique inverse solubility, that is, the solubility in water decreases with the increase of temperature. After the medium temperature rises, evaporation increases, the steam film stage is greatly prolonged, and the maximum discouraged cooling rate decreases. Generally, the operating temperature must be less than 60 ℃. When the temperature is too high, PAG and water will be separated, and the cooling characteristics will change greatly, resulting in heat treatment failure.
2.4.3 medium aging
The quenching and cooling process is a rather harsh use environment, and oxidative degradation, thermal degradation, mechanical degradation, etc. will cause the molecular chain of PAG to break and shorten. The shorter molecular chain means the smaller viscosity, while the depression stage of convective gas (low-temperature cooling) is closely related to the fluidity (viscosity) of the medium. The viscosity decreases and the convective cooling speed increases.
Therefore, under the condition of ensuring the same concentration, the viscosity will continue to decrease with the extension of time, resulting in the rapid cooling rate at low temperature, which will increase the tendency of deformation and cracking.
2.4.4 medium pollution
For the water-soluble quenching medium, foreign pollution must be strictly avoided, especially when the easily emulsifiable oil is mixed into the tank liquid, the water agent will be emulsified, which will cause its depression and great changes in its characteristics. But its own pollution should also be paid enough attention,
First, during the long-term replenishment of tap water, the dissolved substances in the water gradually accumulate in the quenching medium. Resulting in the high concentration measured by the refractometer.
Second. Pollutants from internal and external metamorphic products that can prolong the vapor film stage of quenching gas depression.
Third. The unbalance of the workpiece to the added quenching agent (solute) is brought out during quenching.
2.4.5 improper mixing
The mixing of the medium shall enable the medium to flow evenly and smoothly and eliminate the stagnant area.
Increasing stirring can break the steam film stage faster and make the cooling uniform. Insufficient mixing will easily lead to defects such as insufficient hardness and soft spots; If the stirring speed is too high, it is easy to mix air and cause depression but unevenness, and it is also easy to cause defects such as deformation and uneven hardness. Most mesh belt furnace production lines use propeller mixing, which must be inserted to a certain depth below the liquid surface, not less than 300mm. In addition. It is strictly prohibited to use compressed air for stirring, which not only promotes the gas to participate in heat exchange, but also makes the spirit depressed but uneven, which aggravates the deformation and even leads to quenching cracking; It also accelerates the aging and pollution of the medium and shortens its service life.
3. Countermeasures for common defects of bolts
3.1 insufficient surface hardness, soft spots and tensile strength
(1) For water-based quenching media.
The concentration of PAG quenchant can be reduced or the concentration of salt quenchant can be increased to accelerate the cooling speed;
(2) Reduce the temperature of quenching medium;
(3) Increase circulation and stirring to improve quenching intensity;
(4) Raise quenching temperature and prolong holding time. Fully austenitizing the structure;
(5) Select steel with higher carbon content or steel with better hardenability.
If the surface decarburizes. The surface hardness may be insufficient.
It can be verified by increasing the grinding amount of the surface and comparing the hardness changes before and after. If the surface decarburizes, the carbon potential of the heating furnace needs to be increased.
3.2 large deformation, quenching cracking, etc
(1) For water-based quenchants, the concentration of PAG quenchant can be increased or the concentration of salt quenchant can be decreased to reduce stress;
(2) Increase the temperature of quenching liquid;
(3) Reduce circulation and mixing intensity;
(4) Reduce the quenching temperature or precooling quenching;
(5) Select the steel with lower hardenability for slender bolts.
Sufficient attention must be paid to the way and moving direction of the workpiece immersed in the quenching medium to ensure that the workpiece is heated and cooled as evenly as possible. And avoid deformation due to self weight during heating. If quenched bolts have tooth burst, angle burst and other quenching cracks. It may be caused by structural defects or excessive mechanical residual stress. Especially for grade 8.8 35 steel series. General Ф Wires below 20 do not need to be annealed, Ф Above 20, annealing process (surface hardness below 20hrc) shall be adopted to reduce wire hardness, reduce mechanical stress and prolong the service life of wire drawing die.
3.3 after quenching, the surface is slightly oxidized and blackened, but there is no oxide skin
When the mesh belt furnace is used for mass production of fasteners, especially for water-based media, the head or the whole will often appear blue and black oxidation. This is because a large amount of water vapor is generated during the process of blanking and infiltration of water, and the oxidation of the surface of the workpiece is intensified in the oxidation atmosphere of high temperature and humidity.
(1) Increase the water pressure of the water seal to avoid water vapor entering the furnace body as far as possible;
(2) Clean the filter screen;
(3) Lower the liquid level of the tank;
(4) Strengthen mixing. Increase the cooling rate.
(5) The water content of methanol exceeds the standard, and the qualified product shall be replaced
3.4 serious oxidation after quenching. Peeling of oxide skin
The workpiece is seriously oxidized after quenching, and even the oxide scale falls off. It can be basically determined that it is caused by the heating atmosphere.
(1) The carrier gas (methanol) is used up, resulting in no atmosphere protection in the furnace;
(2) If the furnace body leaks, check the air tightness quickly;
(3) The water content of methanol seriously exceeds the standard, and the qualified product shall be replaced.
About Jinrui, whether you are a fastener demander / dealer / supplier, if you want to know more, you can visit the official website of Jinrui: www.chinajinrui.net
Display of some products of Jinrui
Jiangsu Jinrui metal products Co., Ltd. is the production base of Shanghai Jinrui standard parts. The factory covers an area of more than 20000 square meters, has a registered capital of 50 million yuan, a total investment of more than 100 million yuan, an annual output of more than 20000 tons, and its products are exported to more than 40 countries overseas.
Since its establishment in 2001, Jinrui has developed into a brand-new layout with Jiangsu Hai'an as its production base and Shanghai Baoshan and Henan Nanyang as its sales centers after more than 20 years of unremitting efforts.
At present, Jinrui's main products include the following six categories:
1. The main products of bolts include: metric and American round head oval neck bolts (also known as fishtail bolts), inner and outer hexagon bolts, hexagon flange bolts, carriage bolts, hub bolts, etc
2. Stud products: full thread thread, double headed studs, etc
3. Nut products: Hexagon nuts, flange nuts, heavy nuts, square nuts, butterfly nuts, etc
4. Screw products: self tapping screws, self tapping self drilling screws, wood screws, various machine screws, etc
5. Washer products: flat washer, spring washer, square washer, lock washer, etc
6. Fastener related non-standard products
Jinrui's wire drawing plant also provides processing services such as fine drawing, rough drawing and spheroidizing of wire rod, and supplies finished wire rod of various domestic big brand steel mills.
Based on the business philosophy of "pioneering innovation and pursuing excellence", Jinrui has successively introduced multiple models of starting and forming machines, threading machines, nut machines, hot heading and rolling machines, lathes, large-scale hot heading machines and their supporting testing equipment, such as magnetic particle flaw detection machines, metallographic analyzers, salt spray testing machines, direct reading spectrometers and optical screening machines, tensile machines, Vickers hardness meters, Rockwell hardness meters, etc., and has independent R & D departments, It can produce various high-quality fasteners such as GB, DIN, ANSI, JIS and ISO international standards. The products are widely used in petroleum, chemical industry, boilers, wind power generation, expressway guardrails, elevator guide rails, machinery manufacturing, construction sites, auto parts and medical devices.
Jinrui has flourished since its establishment. By integrating the strengths of various companies, careful design and strict material selection, the products are more durable and highly praised by the majority of users. Jinrui won the ISO9001:2000 international quality system certification in 2001, the special equipment manufacturing license in 2010, and the TS16949 automobile quality management system certification in 2012. Moreover, the products are covered by the "global product liability insurance" by the people's Insurance Company of China, ensuring that every screw can be used with confidence, firmness and peace of mind!
No matter now or in the future, Jinrui will always grasp the core service concept of "quality first, science and technology to prosper the enterprise, user first, and considerate service", and sincerely provide you with high-quality services and high-quality products. Jinrui will also become your most loyal and long-term partner.
