Hot-dip galvanizing post-treatment technology

The post-treatment process includes passivation, pre-phosphating and oiling. Passivation treatment can improve the surface structure and gloss of the galvanized layer, enhance the corrosion resistance of the galvanized layer, prolong its service life, and enhance the bonding force between the coating and the base metal. The current passivation treatment mainly uses chromate passivation. Add some activators, such as fluoride, phosphoric acid or sulfuric acid, etc. to the passivation solution to get a thicker chromate film after passivation. When there is fluoride in the passivation solution, it can weaken the surface tension of the strip steel, accelerate the film-forming reaction, and can enhance the chemical polishing effect to make the passivation film fine and bright. For hot-dip galvanized steel sheets for construction, Japan Steel Pipe Company has developed a special chromate passivation coating with excellent corrosion resistance and black rust resistance.
Over the years, relevant researchers have conducted a lot of research on non-toxic or low-toxic inorganic corrosion inhibitors as passivating agents. Molybdate is one of them, its toxicity is lower than that of chromate, but the corrosion resistance after passivation is only equivalent to low Cr passivation. A small amount of molybdate and phosphate are added to the non-toxic water-soluble acrylic resin to obtain a passivation solution instead of toxic chromate for passivation treatment. The results show that the hot-dip galvanized layer is passivated with the non-toxic passivation solution, which can delay the appearance of white rust on the galvanized layer, and its corrosion resistance is close to that of chromate passivation water. Although various chromium-free passivation processes have been reported in the literature, there is no chromium-free passivation process that can completely replace the chromate passivation process. Some chromium-free passivation processes are equivalent to chromate passivation in some aspects, but its market prospects, application scope and environmental protection effects need to be further studied. However, the replacement of chromate passivation by chromium-free passivation is a general trend.

At present, there are few hot-dip galvanizing units with pre-phosphating capacity. The pre-phosphorized film can not only solve the problem of rust prevention during storage and transportation of hot-dip galvanized sheets that are relatively easy to rust before, but also play a role as a solid lubricating film in the stamping and forming process of automobile sheets. Reduce the surface friction coefficient of the hot-dip galvanized sheet, and effectively reduce the defects such as zinc layer shedding and powdering that are easy to produce after the hot-dip galvanized sheet is rubbed with the mold. Therefore, hot-dip galvanized pre-phosphorized sheets have been widely promoted in some steel plants in Europe, the United States, Japan, etc., and have become an effective technology to solve the cracking problem of difficult stamping parts. Therefore, it is foreseeable that the unit online pre-phosphating has great development potential.

The control measures of spring steel reed brittleness and heat treatment crack defects

1. Brittleness is one of the common defects of reeds, which can be divided into heat treatment brittleness and hydrogen embrittlement after galvanizing. Microscopically, hydrogen embrittlement fractures have intergranular, dimples, secondary cracks, etc., and hair lines and hydrogen micropores can also be observed on the fracture surface.
1) Heat treatment brittleness

Spring steel has a tendency to heat sensitivity and temper brittleness, and its tempering temperature is often just right at the junction of the first type of temper brittleness and the second type of temper brittleness. If it cannot be tempered, kept warm and cooled in time, it will cause the spring The flakes are brittle.

Solution: Control the tempering temperature, and timely temper, heat preservation and cooling.

2) Hydrogen embrittlement

After hot-dip galvanizing, it is easy to increase the brittleness of the reed. This is because before galvanizing, pickling must be carried out to remove the oxide scale, which will cause a part of hydrogen to penetrate into the coating and the base metal, weakening the bond between the metal crystal atoms on the grain boundary Force and generate internal stress, making the reed brittle, that is, “hydrogen embrittlement.”

Solution: Control the pickling time and temperature, standardize the galvanizing process, and after galvanizing, keep it at 180°C for several hours before proceeding with the hydrogen removal process, which can reduce the brittleness of the coating and structure of the reed.

2. Heat treatment cracks

Heat treatment generally has a relatively small probability of crack defects in the reed. If cracks occur, the reasons are: improper heat treatment process, improper adjustment of tooling, and operator error. Improper heat treatment process means that the heating temperature of the reed is too high and the holding time is too long, which causes the austenite grains in the reed structure to be coarsened. After quenching, the martensite needles are coarse, resulting in the increase of internal stress and brittleness of the reed. Form heat treatment cracks.

Solution: Use a mesh belt furnace to process the reeds, control the heating temperature to 790℃, control the holding time, direct oil quenching, and temper in time after cleaning, which can effectively solve the heat treatment crack defects of the reeds.

Sangao mining machinery gold ore dressing equipment new technology improves grade

The new technology of Sangao Mining Machinery’s gold beneficiation equipment meets the different requirements of customers. It is equipped with a variety of gold beneficiation processes. The successfully developed and successfully developed gold ore flotation leaching desulfurization process has been well applied in practice and effectively improved the beneficiation grade. And the recovery rate can reach 98%.
The content of gold in the ore is extremely low. In order to extract gold, the ore needs to be crushed and finely ground and pre-enriched or separated from the ore by beneficiation methods. Gravity separation and flotation are frequently used in gold beneficiation. Gravity separation occupies a very important position in the production of placer gold. Flotation is a beneficiation method widely used in rock gold mines. At present, about 80% of rock gold mines in my country Using this method to select gold, the level of beneficiation technology and equipment has been greatly improved.

The amalgamation gold extraction process is an ancient gold extraction process, which is simple and economical, and is suitable for the recovery of coarse-grained monomer gold. Many gold mines in my country still use this method. With the development of gold production and the progress of science and technology, the gold extraction process of amalgamation method has been continuously improved and perfected. Due to the increasingly stringent environmental protection requirements, some mines have cancelled the amalgamation operation and replaced it with the gold extraction process by gravity separation, flotation and cyanidation.

In gold production, the gold extraction process by amalgamation still plays an important role, and there are application examples at home and abroad. Many gold mines such as Zhangjiakou in Hebei, Erdaogou in Liaoning, Jiapigou in Jilin and Yinan in Shandong have applied this process. The Erdaogou gold mine in Liaoning was originally a single flotation process. According to the nature of the ore, it was changed to a combined amalgamation and flotation process. The total recovery rate was increased by 7.81% (the amalgamation recovery rate was 64.6%), and the tailing grade was increased from 0.74g/t Reduced to 0.32g/t, the annual benefit is 1.58 million yuan. The key to the amalgamation gold extraction process is how to take protective measures to eliminate mercury poisoning pollution.

The Sangao gold beneficiation production line is composed of jaw crusher, ball mill, classifier, flotation machine, leaching adsorption tank, thickener and dryer, etc., and can be combined with feeder, hoist and conveyor to form a complete beneficiation production line. The production line has the advantages of high efficiency, energy saving, high processing capacity and reasonable economy.

The production process of the beneficiation production line is as follows: the mined ore is initially crushed by a jaw crusher, and after being crushed to a reasonable fineness, it is evenly sent to the ball mill through a hoist and a miner, and the ore is crushed and ground by the ball mill. The ore fines ground by the ball mill enter the next process: classification. The spiral classifier cleans and classifies the ore mixture based on the principle that the solid particles have different specific gravity and different precipitation speeds in the liquid. When the cleaned and graded mineral mixture passes through the magnetic separator, due to the different specific magnetic susceptibility coefficients of various minerals, the magnetic substances in the mixture are separated through magnetic and mechanical forces. After the initial separation of the magnetic separator, the mineral particles are sent to the flotation machine, and different drugs are added according to the characteristics of different minerals to separate the desired minerals from other substances. After the required minerals are separated, because they contain a large amount of water, they must be initially concentrated by a thickener and then dried by a dryer to obtain dry minerals.

Heat treatment stress and its influence

Heat treatment residual force refers to the residual stress of the workpiece after heat treatment, which has an extremely important influence on the shape, size and performance of the workpiece. When it exceeds the yield strength of the material, it will cause deformation of the workpiece, and when it exceeds the strength limit of the material, it will crack the workpiece. This is its harmful side and should be reduced and eliminated. But under certain conditions, controlling the stress to make it reasonably distributed can improve the mechanical performance and service life of the parts, and turn harmful to beneficial. Analyzing the distribution and change law of stress in steel during heat treatment, and making it reasonable distribution has far-reaching practical significance for improving product quality. For example, the influence of the reasonable distribution of surface residual compressive stress on the service life of parts has attracted people’s attention.

(1) Heat treatment stress of steel

In the process of heating and cooling of the workpiece, due to the inconsistency of the cooling rate and time between the surface and the core, a temperature difference is formed, which will cause uneven volume expansion and contraction to produce stress, that is, thermal stress. Under the action of thermal stress, because the surface temperature is lower than the core part, and the contraction is greater than the core part, the core part is stretched. When the cooling is over, the final cooling volume contraction of the core part cannot proceed freely and the surface layer is compressed. Stretched. That is, under the action of thermal stress, the surface of the workpiece is compressed and the core is pulled. This phenomenon is affected by factors such as cooling rate, material composition and heat treatment process. When the cooling rate is faster, the carbon content and alloy composition are higher, and the uneven plastic deformation under the action of thermal stress during the cooling process is larger, and the residual stress formed in the end is larger. On the other hand, due to the change of the structure of the steel during the heat treatment process, that is, the transformation of austenite to martensite, the increase in specific volume will be accompanied by the expansion of the volume of the workpiece, and the phases of each part of the workpiece will change sequentially, resulting in inconsistent volume growth and structure. stress. The final result of the change of tissue stress is the tensile stress on the surface and the compressive stress on the core, which is just the opposite of the thermal stress. The size of the structure stress is related to the cooling rate, shape, and chemical composition of the material in the martensite transformation zone.

Practice has proved that as long as there is phase change in any workpiece during heat treatment, thermal stress and structural stress will occur. It’s just that the thermal stress has been generated before the structure transformation, and the structure stress is generated in the process of the structure transformation. During the entire cooling process, the result of the combined effect of the thermal stress and the structure stress is the actual stress in the workpiece. The result of the combined effect of these two stresses is very complex, and is affected by many factors, such as composition, shape, heat treatment process, etc. As far as its development process is concerned, there are only two types, namely thermal stress and tissue stress. When the direction of action is opposite, the two cancel out, and when the direction of action is the same, the two are superimposed. Regardless of whether they are mutually offset or superimposed on each other, the two stresses should have a dominant factor. When the thermal stress is dominant, the result is that the core of the workpiece is pulled and the surface is compressed. The result of the effect when the tissue stress is dominant is the tension of the compressed surface of the workpiece core.

(2) The effect of heat treatment stress on quenching cracks

The factors that can cause stress concentration (including metallurgical defects) in different parts of the quenched part have a promoting effect on the generation of quenching cracks, but only in the tensile stress field (especially under the maximum tensile stress). Come out, if there is no promoting effect in the compressive stress field.

Quenching cooling rate is an important factor that can affect the quality of quenching and determine the residual stress, and it is also a factor that can have an important and even decisive influence on quenching cracks. In order to achieve the purpose of quenching, it is usually necessary to accelerate the cooling rate of the parts in the high temperature section and make it exceed the critical quenching cooling rate of steel to obtain the martensite structure. As far as the residual stress is concerned, this can increase the thermal stress value that offsets the effect of the structural stress, so it can reduce the tensile stress on the surface of the workpiece to achieve the purpose of suppressing longitudinal cracks. The effect will increase as the high temperature cooling rate increases. Moreover, in the case of hardening, the larger the cross-sectional size of the workpiece, although the actual cooling rate is slower, the risk of cracking is greater. All this is due to the fact that the thermal stress of this type of steel slows down with the increase in size, the thermal stress decreases, and the structural stress increases with the increase in size, and finally the tensile stress mainly formed by the structural stress acts on the workpiece. Caused by the characteristics of the surface. And it is very different from the traditional concept that the slower the cooling, the smaller the stress. For this kind of steel parts, only longitudinal cracks can be formed in high hardenability steel parts that are quenched under normal conditions. The reliable principle to avoid quench cracking is to try to minimize the unequal time of martensite transformation inside and outside the section. Merely implementing slow cooling in the martensite transformation zone is not enough to prevent the formation of longitudinal cracks. Under normal circumstances, cracks can only be generated in non-hardenable parts. Although the overall rapid cooling is the necessary formation condition, the real reason for its formation is not the rapid cooling (including the martensite transformation zone) itself, but It is the local position of the quenched part (determined by the geometric structure), and the cooling rate in the high temperature critical temperature zone is significantly slowed down, so there is no hardening. The transverse fractures and longitudinal splits produced in large non-hardenable parts are caused by residual tensile stress with thermal stress as the main component acting on the center of the quenched part, and at the center of the quenched section of the quenched part, cracks are first formed and caused by Caused by expansion from the inside out. In order to avoid such cracks, the water-oil double liquid quenching process is often used. In this process, the purpose of implementing rapid cooling in the high temperature section is only to ensure that the outer layer of metal obtains the martensite structure. From the perspective of internal stress, rapid cooling is harmful and useless at this time. Secondly, the purpose of slow cooling in the later stage of cooling is not to reduce the expansion rate of martensite transformation and the value of organizational stress, but to minimize the temperature difference of the section and the shrinkage rate of the metal in the center of the section, so as to reduce the stress value and finally The purpose of suppressing cracking.

Several common heat treatment concepts

1. Normalizing: The heat treatment process of heating steel or steel parts to an appropriate temperature above the critical point Ac3 or Acm for a certain period of time and then cooling in the air to obtain a pearlite structure.

2. Annealing: heat the hypoeutectoid steel workpiece to 30-50 degrees above Ac3, after holding for a period of time, slowly cool with the furnace (or be buried in sand or lime) to below 500 degrees and cool in the air. Craft

3. Solution heat treatment: heat the alloy to a high-temperature single-phase zone to maintain a constant temperature, so that the excess phase is fully dissolved and rapidly cooled to obtain a supersaturated solid solution heat treatment process

4. Aging: After solution heat treatment or cold plastic deformation, the performance of alloy changes with time when it is placed at room temperature or kept slightly higher than room temperature.

5. Solid solution treatment: fully dissolve various phases in the alloy, strengthen solid solution and improve toughness and corrosion resistance, eliminate stress and soften, so as to continue processing and forming

6. Aging treatment: heat and keep warm at the temperature at which the strengthening phase precipitates, so that the strengthening phase precipitates out, hardened and improved strength

7. Quenching: After austenitizing the steel, it is cooled at an appropriate cooling rate to make the workpiece dissolve into the solid solution in all or a certain range in the cross section, and then heat treatment for the transformation of unstable structure such as martensite. Craft

8. Tempering: heat the quenched workpiece to an appropriate temperature below the critical point Ac1 for a certain period of time, and then use a method that meets the requirements to cool it to obtain the required structure and performance of the heat treatment process

9. Nitriding and carbonitriding of steel

(1). Nitriding of steel (gas nitriding)

Concept: Nitriding is a process of infiltrating nitrogen atoms into the surface layer of steel. Its purpose is to improve surface hardness and wear resistance, as well as fatigue strength and corrosion resistance.

It uses ammonia gas to decompose active nitrogen atoms when heated, and forms a nitride layer on the surface after being absorbed by the steel, while diffusing to the center.

Nitriding is usually carried out using specialized equipment or pit carburizing furnace. Suitable for various high-speed transmission precision gears, machine tool spindles (such as boring bars, grinder spindles), high-speed diesel engine crankshafts, valves, etc.

Process route of nitriding workpiece: forging-annealing-rough machining-quenching and tempering-finishing-stress removal-rough grinding-nitriding-fine grinding or grinding.

Because the nitrided layer is thin and brittle, it requires a high-strength core structure. Therefore, it is necessary to perform quenching and tempering heat treatment first to obtain tempered sorbite and improve the mechanical properties of the core and the quality of the nitrided layer.

After nitriding, the steel no longer needs to be quenched and has high surface hardness and wear resistance.

The nitriding treatment temperature is low and the deformation is small. Compared with carburizing and induction surface quenching, the deformation is much smaller

(2) Carbonitriding of steel: Carbonitriding is a process of simultaneously infiltrating carbon and nitrogen into the surface of the steel. Carbonitriding is customarily called cyanidation. At present, medium temperature gas carbonitriding and low temperature gas nitrocarburizing (ie, gas nitrocarburizing) are widely used. The main purpose of medium temperature gas carbonitriding is to improve the hardness, wear resistance and fatigue strength of steel. Low temperature gas carbonitriding is mainly nitriding, and its main purpose is to improve the wear resistance and seizure resistance of steel.

10. Quenching and tempering: It is generally used to combine the heat treatment of quenching and high temperature tempering as quenching and tempering. Quenching and tempering treatment is widely used in various important structural parts, especially those connecting rods, bolts, gears and shafts that work under alternating loads. After quenching and tempering, the tempered sorbite structure is obtained, and its mechanical properties are better than the normalized sorbite structure of the same hardness. Its hardness depends on the high-temperature tempering temperature and is related to the tempering stability of the steel and the size of the workpiece section, generally between HB200-350.

11. Brazing: a heat treatment process for bonding two workpieces together with brazing filler metal

Annealing-Quenching-Tempering

(1). Types of annealing

1. Complete annealing and isothermal annealing

Complete annealing is also called recrystallization annealing, generally referred to as annealing. This annealing is mainly used for casting, forging and hot-rolled sections of various carbon steels and alloy steels with hypoeutectoid compositions, and sometimes also used for welded structures. Generally used as the final heat treatment of some unimportant workpieces, or as the pre-heat treatment of some workpieces.

2. Spheroidizing annealing

Spheroidizing annealing is mainly used for hypereutectoid carbon steels and alloy tool steels (such as steel grades used in manufacturing cutting tools, measuring tools, and molds). Its main purpose is to reduce hardness, improve machinability, and prepare for subsequent quenching.

3. Stress relief annealing

Stress relief annealing is also called low temperature annealing (or high temperature tempering). This annealing is mainly used to eliminate residual stress in castings, forgings, welded parts, hot rolled parts, cold drawn parts, etc. If these stresses are not eliminated, the steel parts will be deformed or cracked after a certain period of time or in the subsequent cutting process.

(Two) quenching

In order to improve the hardness, the main form is heating, heat preservation, and rapid cooling. The most commonly used cooling media are brine, water and oil. Salt water quenched workpieces are easy to obtain high hardness and smooth surface, and it is not easy to produce soft spots that are not hardened, but it is easy to cause serious deformation of the workpiece and even cracks. The use of oil as the quenching medium is only suitable for the quenching of some alloy steels or small-sized carbon steel workpieces with relatively large stability of undercooled austenite.

(3) Tempering

1. Reduce brittleness and eliminate or reduce internal stress. There is a large internal stress and brittleness of steel parts after quenching. If not tempered in time, the steel parts will often deform or even crack.

2. Obtain the required mechanical properties of the workpiece. After quenching, the workpiece has high hardness and high brittleness. In order to meet the different performance requirements of various workpieces, the hardness can be adjusted by appropriate tempering to reduce the brittleness and obtain the required Toughness, plasticity.

3. Stable workpiece size

4. For some alloy steels that are difficult to soften by annealing, high temperature tempering is often used after quenching (or normalizing), so that the carbides in the steel are properly aggregated and the hardness is reduced to facilitate cutting.

Metal heat treatment process

The heat treatment process generally includes three processes of heating, heat preservation, and cooling, and sometimes there are only two processes of heating and cooling. These processes are connected and uninterrupted.

Heating is one of the important steps of heat treatment. There are many heating methods for metal heat treatment. The earliest was to use charcoal and coal as heat sources, and then to use liquid and gas fuels. The application of electricity makes heating easy to control without environmental pollution. These heat sources can be used for direct heating, or indirect heating by molten salt or metal, or even floating particles.

When the metal is heated, the workpiece is exposed to the air, and oxidation and decarburization often occur (that is, the carbon content of the surface of the steel parts is reduced), which has a very negative effect on the surface properties of the parts after heat treatment. Therefore, metals should usually be heated in a controlled atmosphere or protective atmosphere, molten salt and vacuum, and can also be heated by coating or packaging methods.

The heating temperature is one of the important process parameters of the heat treatment process. The selection and control of the heating temperature is the main issue to ensure the quality of the heat treatment. The heating temperature varies with the metal material being processed and the purpose of the heat treatment, but it is generally heated above the phase transition temperature to obtain the required structure. In addition, the transformation takes a certain time. Therefore, when the surface of the metal workpiece reaches the required heating temperature, it must be maintained at this temperature for a certain period of time to make the internal and external temperatures consistent and complete the microstructure transformation. This time is called the holding time. When using high-energy density heating and surface heat treatment, the heating speed is extremely fast, generally there is no holding time or the holding time is very short, and the holding time of chemical heat treatment is often longer.

Cooling is also an indispensable step in the heat treatment process. The cooling method varies from process to process, and the main thing is to control the cooling rate. Generally, annealing has the slowest cooling rate, normalizing cooling rate is faster, and quenching cooling rate is faster. However, there are different requirements due to different steel grades. For example, hollow hard steel can be quenched at the same cooling rate as normalizing.

Metal heat treatment processes can be roughly divided into overall heat treatment, surface heat treatment, partial heat treatment and chemical heat treatment. According to the different heating medium, heating temperature and cooling method, each category can be divided into several different heat treatment processes. The same metal uses different heat treatment processes to obtain different structures and thus have different properties. Steel is the most widely used metal in the industry, and the microstructure of steel is also the most complex, so there are many types of steel heat treatment processes.

Overall heat treatment is a metal heat treatment process that heats the entire workpiece and then cools it at an appropriate speed to change its overall mechanical properties. The overall heat treatment of steel has four basic processes: annealing, normalizing, quenching and tempering.

Annealing is to heat the workpiece to an appropriate temperature, use different holding time according to the material and workpiece size, and then slowly cool it, the purpose is to make the internal structure of the metal reach or close to the equilibrium state, obtain good process performance and use performance, or for further quenching Prepare for organization. Normalizing is to heat the workpiece to a suitable temperature and then cool it in the air. The effect of normalizing is similar to annealing, except that the resulting structure is finer. It is often used to improve the cutting performance of materials, and sometimes used for parts that are not demanding. As the final heat treatment.

Quenching is to quickly cool the workpiece in a quenching medium such as water, oil, other inorganic salts, and organic aqueous solutions after heating and holding the workpiece. After quenching, the steel parts become hard, but at the same time become brittle. In order to reduce the brittleness of steel parts, the quenched steel parts are kept at an appropriate temperature higher than room temperature and lower than 710°C for a long time, and then cooled. This process is called tempering. Annealing, normalizing, quenching, and tempering are the “four fires” in the overall heat treatment. Quenching and tempering are closely related and are often used in conjunction with each other.

The “Four Fires” have evolved different heat treatment processes with different heating temperatures and cooling methods. In order to obtain a certain strength and toughness, the process of combining quenching and high temperature tempering is called quenching and tempering. After some alloys are quenched to form a supersaturated solid solution, they are kept at room temperature or a slightly higher temperature for a longer period of time to improve the hardness, strength, or electrical magnetism of the alloy. Such a heat treatment process is called aging treatment. The method of effectively and tightly combining pressure processing deformation and heat treatment to obtain good strength and toughness of the workpiece is called thermomechanical heat treatment; heat treatment in a negative pressure atmosphere or vacuum is called vacuum heat treatment, which can not only make The workpiece is not oxidized or decarburized. It can keep the surface of the workpiece smooth and clean after treatment and improve the performance of the workpiece. It can also be injected with a penetrant for chemical heat treatment.

Surface heat treatment is a metal heat treatment process that only heats the surface of the workpiece to change the mechanical properties of the surface. In order to heat only the surface of the workpiece without introducing excessive heat into the interior of the workpiece, the heat source used must have a high energy density, that is, to give a larger heat energy to the workpiece per unit area, so that the surface or part of the workpiece can be short-term or instantaneous Reach high temperatures. The main methods of surface heat treatment include laser heat treatment, flame quenching and induction heating heat treatment. Commonly used heat sources include flames such as oxygen acetylene or oxypropane, induction current, laser and electron beam.

Chemical heat treatment is a metal heat treatment process that changes the chemical composition, organization and performance of the surface of the workpiece. The difference between chemical heat treatment and surface heat treatment is that the latter changes the chemical composition of the surface of the workpiece. Chemical heat treatment is to heat the workpiece in a medium (gas, liquid, solid) containing carbon, nitrogen or other alloying elements, and keep it for a long time, so that the surface of the workpiece can infiltrate elements such as carbon, nitrogen, boron and chromium. After infiltration of elements, other heat treatment processes such as quenching and tempering are sometimes required. The main methods of chemical heat treatment are carburizing, nitriding, metalizing, composite infiltration and so on.

Heat treatment is one of the important processes in the manufacturing process of mechanical parts and molds. Generally speaking, it can guarantee and improve various properties of the workpiece, such as wear resistance and corrosion resistance. It can also improve the structure and stress state of the blank to facilitate various cold and hot processing.

For example, white cast iron can obtain malleable cast iron after long-term annealing treatment, which can improve plasticity; gears adopt the correct heat treatment process, and the service life can be doubled or tens of times longer than gears without heat treatment; in addition, cheap carbon steel can be penetrated Certain alloying elements have the properties of some expensive alloy steels, which can replace certain heat-resistant steels and stainless steels; almost all tools and molds require heat treatment before they can be used.

Development Trend of Stainless Steel Grades

Stainless steel is one of the important inventions of the 20th century. After nearly a hundred years of research and development, a series of more than 300 grades of steel has been formed. In the special steel system, stainless steel has unique properties and a wide range of applications, which can not be replaced by other special steels. Stainless steel can almost cover any other special steels.
1 The evolution of austenitic steel

In developed countries, about 70% of the stainless steel consumed each year is austenitic stainless steel. Although my country’s consumption level is not high, the consumption of austenitic stainless steel has reached about 65% of the total consumption. Therefore, to see the development trend of stainless steel grades, we must first look at the trend of austenitic stainless steel.

Early researchers have discovered that carbon is the main cause of corrosion damage to the grain boundary of austenitic stainless steel. Limited to the level of metallurgical equipment at that time, it is difficult to control carbon below 0.03%. Finally, they came up with adding Ti and Nb to steel. It preferentially reacts with carbon to generate TiC and NbC, and the method of fixing the carbon prevents the precipitation of carbon at the grain boundary to generate Cr23C6, causing intergranular corrosion. Due to the high cost of Nb, until the mid-1970s, Ti-containing stabilized steel 1Cr18Ni9Ti still dominated stainless steel.

The 1Cr18Ni9Ti molten steel is viscous, and the surface quality of the continuous casting billet is difficult to pass. With die casting, the surface quality of the steel ingot is not good, so it must be peeled and ground, and the yield rate is very low. The finished steel contains TiN inclusions, has low purity, poor surface polishing performance, and many broken wires in the drawing. By the end of the 1960s, breakthroughs had been made in stainless steel smelting technology. AOD and VOD methods were widely used to make steel, reducing the carbon in stainless steel and no longer apologizing. Qunopiao? ⒘ ship pirates and weapons to swallow their own system? Tun a few Zheng? I stabilized steel is gradually replaced by low-carbon and ultra-low-carbon steel. In the 1970s, the United States, Japan and other countries have eliminated 1Cr18Ni9Ti from the standard. Although 0Cr19Ni11Ti (321) was retained, its output only accounted for 0.7 to 1.5% of the total. The transition from titanium-containing stabilized steel to low carbon was successfully completed. And the transition of ultra-low carbon steel.

The production and application of stainless steel in my country are relatively lagging. Although the national standard GB1220-84 “Stainless Steel Rods” was promulgated in 1984, 1Cr18Ni9Ti was listed as not recommended, but the dominant position of 1Cr18Ni9Ti has not changed. Until 1995, with the development of the national economy, especially the intervention of joint ventures, the domestic market gradually integrated with the international market. In just 5-6 years, my country’s austenitic stainless steel has completed the transition from titanium-containing stabilized steel to low-carbon And the transition of ultra-low carbon steel. At present, except for a few traditional industries that still use 1Cr18Ni9Ti, 304 (0Cr19Ni9) and 316 (0Cr17Ni12Mo) have become the leading brands of stainless steel.

2 Replace carbon with nitrogen and develop nitrogen-containing stainless steel

In austenitic stainless steel, nitrogen and carbon have many common characteristics, such as increasing the stability of austenite and effectively improving the cold working strength of steel. Increasing the carbon content will reduce the intergranular corrosion resistance of stainless steel. The affinity of nitrogen and chromium is smaller than that of carbon and chromium. Austenitic steel rarely sees Cr2N precipitation. Therefore, adding a proper amount of nitrogen can improve the strength and oxidation resistance of steel without reducing the intergranular corrosion resistance of stainless steel. Substituting nitrogen for carbon and developing nitrogen-containing stainless steel has become a hot topic.

Process and heat treatment of spring steel

The spring mainly works under dynamic load, that is, under the conditions of impact and vibration, or under the action of alternating stress, using elastic deformation to absorb impact energy and play a buffering role.
Because springs are often subjected to vibration and work under the action of shared stress for a long time, the main fatigue failure is, so the spring steel must have high elastic limit and high fatigue limit. In addition, it should have sufficient toughness and plasticity to prevent sudden brittle fracture under impact force.

In terms of process performance, spring steel should have good hardenability and low overheating and decarburization sensitivity. Reducing the surface roughness of the spring can increase the fatigue life.

In order to obtain the required performance, spring steel must have a high carbon content. The carbon content of carbon spring steel is between 0.6-0.9%. Due to the poor hardenability of carbon spring steel, it is only used to make springs with a cross-sectional dimension not exceeding 10-15mm. For springs with larger cross-sectional dimensions, alloy spring steel must be used. The carbon content of alloy spring steel is between 0.45-0.75%, and the added alloying elements are Mn, Si, W, V, Mo, etc. Their main function is to improve hardenability and tempering stability, strengthen ferrite and refine grains, and effectively improve the mechanical properties of spring steel. Among them, Cr, W, and Mo can also increase the high temperature strength of steel.

Springs formed under hot conditions (diameter or thickness generally above 10mm)

Springs formed in a cold state (diameter or thickness generally less than 10mm)

Heat treatment process of hot formed spring

Most of the springs formed by this method are hot-forming and heat treatment combined, while most of the coil springs are heat-treated after hot forming. The heat treatment method of this kind of spring steel is quenching + intermediate temperature tempering, and the structure after heat treatment is tempered troostite. This kind of organization has high elastic limit and yield limit, and has certain toughness.

Heat treatment process of cold formed spring

For springs made of cold-rolled steel plates, steel strips or cold-drawn steel wires, the materials have been strengthened due to cold plastic deformation and have reached the required performance of the spring. Therefore, after the spring is formed, it is only necessary to perform stress relief treatment within a temperature range of about 250°C for about 30 minutes to eliminate the internal stress of the cold-formed spring and to finalize the shape of the spring.

Heat treatment of heat-resistant spring steel

The valve springs of internal combustion engines work at higher temperatures, and some still have corrosive atmosphere, so special spring steel and appropriate heat treatment specifications must be selected.

Common defects and preventive measures during spring quenching

1. Decarbonization (reducing service life)

Preventive measures:

(1) Use salt bath furnace or controlled atmosphere heating furnace for heating.

(2) Adopt rapid heating process.

2. After quenching, the hardness is insufficient, the number of non-martensite is large, and ferrite appears in the core (residual deformation occurs, reducing service life)

Preventive measures:

(1) Choose materials with better hardenability.

(2) Improve the cooling capacity of quenching coolant.

(3) The temperature of the spring entering the coolant should be controlled above Ar3.

(4) Increase the quenching heating temperature appropriately.

3. Overheating (increased brittleness)

Preventive measures:

(1) Strictly control the forming and quenching heating temperature.

(2) Strengthen the metallographic inspection during quenching.

4. Cracking (increased brittleness, severely reduced service life)

Preventive measures:

(1) Control the quenching heating temperature.

(2) When it is cooled to 250-300℃ during quenching, take out air cooling.

(3) Tempering in time

Measures to improve spring quality:

(1) Thermomechanical heat treatment–combining steel deformation strengthening and heat treatment strengthening to further improve the strength and toughness of steel. Thermomechanical heat treatment is divided into high, medium and low temperature. High temperature thermomechanical heat treatment is quenched immediately after deformation occurs in a stable austenite state, and can also be combined with forging or hot rolling, that is, quenched immediately after hot forming. Thermomechanical treatment has been used in the production of automobile leaf springs. (60Si2Mn)

(2) Austempering of springs-Austempering can be used for springs with small diameters or sufficient permeability. It can not only reduce deformation, but also improve strength and toughness. It is best to perform tempering again after austempering. The elastic limit can be increased, and the tempering temperature is the same as the austempering temperature.

(3) Spring relaxation treatment-the spring works under the action of external force for a long time, and the result of stress relaxation will produce a small amount of permanent (plastic) deformation, especially the spring that works at high temperature, the stress relaxation phenomenon is more serious at high temperature. Reduce the accuracy of the spring, which is not allowed for general precision springs. Therefore, this kind of spring should be relaxed after quenching and tempering-pre-load the spring so that its deformation exceeds the deformation that may occur when the spring is working. Then heat it at 20℃ higher than the working temperature for 8-24h.

(4) Low-temperature carbonitriding-the process of combining tempering and low-temperature carbonitriding (soft nitriding) can significantly improve the fatigue life and corrosion resistance of the spring. This process is mostly used for coil springs.

(5) Shot peening-surface defects such as scratches, folds, oxidative decarburization, etc. tend to become stress concentration places and sources of fatigue fracture during spring operation. If a small steel shot is used to spray the surface of the spring at high speed, it will not only improve the surface quality of the spring, increase the surface strength, and put the surface in a state of compressive stress, thereby increasing the fatigue strength and service life of the spring.

65Mn spring steel material analysis, heat treatment, technology, use, market, price, technical support, etc.

Improve the pre-heat treatment process

Quenching process improvement

Due to the influence of the spring support distortion snares on subsequent welding, it must be kept in nitrate salt at 160℃ for 3-5 minutes, and then air-cooled graded quenching process. The stress and distortion after quenching are small, and the hardness is 58-60HRC.

Surface treatment

After oil quenching, the surface of the spring support appears salty, which is not easy to clean, and is easy to rust in long-term storage. It is not easy to weld during the next process of spot welding. To solve this problem, the workpiece is quenched and tempered before the next use of spot welding , Perform sand blast cleaning on the principal support, make the surface of the part clean, easy to spot welding, and meet the technical requirements of the workpiece.

The production and development status of this material at home and abroad

Through experiments, the 65mn steel circular saw blade is pre-carburized with the teeth to increase the carbon and nitrogen content, and then subjected to conventional heat treatment to improve the tempering stability of the teeth, thereby increasing the hardness and wear resistance of the teeth. Improve the service life of circular saw blades.

65mn steel circular saw blades are widely used in metallurgy, steel rolling, machinery, building materials and other industrial fields for high-speed cutting of various types of steel, steel pipes and steel bars. A carbonitriding method is carried out in advance through the teeth to achieve the purpose of improving the service life of the saw blade.

After the 65Mn saw blade has just been pre-infiltrated, a continuous hypereutectoid layer with a depth of about 0.7-1.0mm is formed along the tooth row, and the outermost layer is a white ribbon carbonitriding layer with a thickness of 5-10um. The inner side contains a large amount of granular carbonitride layer. After quenching and tempering, the metallographic structure of the tooth is uniform tempered troostite and a large number of dispersed granular carbonitrides, while the base structure is only tempered troostite .

65Mn steel is a medium carbon steel, and its tempering stability is poor. In order to meet the strength and toughness of the saw blade substrate, the hardness of the saw blade will be significantly reduced from HRC60 above to HRC44-50 after the implementation of medium temperature tempering (300-400℃). Before quenching, pre-carbonitriding treatment on the teeth of the saw blade can make the teeth enter the surface to obtain eutectoid carbon and a certain number of nitrogen atoms. After quenching, a large number of dispersed particles will be formed. Carbon and nitrogen compounds, and at the same time, a large amount of carbon and nitrogen elements dissolved in the matrix will significantly reduce the martensite transformation point of the steel, thereby obtaining more retained austenite. Such a structure is then tempered at medium temperature and exists in large amounts. The carbonitrides can hinder the decomposition of martensite and the accumulation of precipitated carbides. At the same time, due to the catalysis of carbon and nitrogen atoms, more retained austenite has the ability to further transform into secondary martensite. Therefore, the pre-infiltration treatment significantly improves the tempering stability of 65Mn steel, that is, at the same tempering temperature, the 65Mn steel circular saw blade with pre-infiltration treatment can maintain a higher tempering hardness than the non-pre-infiltration saw blade. Improve the wear resistance of the teeth.

In a domestic steel pipe factory, a batch cutting of cold drawn seamless steel pipes was carried out on the machine. The cut steel pipes were smooth and free of burrs and deformation. The statistical results proved that the average service life of the pre-permeated saw blades was significantly higher than that of the conventional heat treated saw blades. .

The 65Mn steel circular saw blade is pre-carbonitrided before quenching, which can significantly improve its tempering stability. The teeth of the saw blade after medium temperature tempering can obtain a large amount of granular carbonitride and uniform tempered troostite Its hardness is HRC5~8 higher than that of conventional heat-treated saw blades, and its wear resistance is doubled, while maintaining the strength and toughness of the matrix. The experimental production and application prove that the pre-infiltration treatment can effectively increase the service life of the 65Mn steel circular saw blade.

Market development prospects of this material

65Mn is one of the materials for making various leaf springs and wire springs. It is used in vehicles, trams, trains and other transportation vehicles. It is also widely used in the manufacture of meters, furniture, and children’s toys. This kind of steel has very good fatigue strength, excellent elasticity, and quite good plasticity and hardness. Because of the low price of this steel, the future of this material is bright, and it will be widely used now and in the future.

The surface treatment of 65Mn can be shot peening treatment, which is a work hardening treatment, thereby improving the strength and corrosion resistance. This work hardening can cause residual compressive stress, which can not only prevent stress corrosion cracking SCC , And it can also improve the fatigue strength. If the vortex jet is used, the same effect can be obtained, because the vortex impact around the vortex jet can strengthen the material, and the fatigue strength can also be improved. The vortex jet causes the residual compressive stress on the surface of the material to strengthen For metallic materials, this is better than shot peening. Therefore, both methods can improve the strength and corrosion resistance.

The surface quality of the spring has a great influence on the service life, and decarburization can cause stress concentration and reduce the fatigue strength of the spring. For decarburization, controlled atmosphere heat treatment or vacuum heat treatment can be used to prevent decarburization.