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.

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

Heat treatment and strength test of welded joints

Because 65Mn steel has a tendency to overheat, the welding heat affected zone has a great influence on the mechanical properties of the joint. The 65Mn steel wire with a diameter of 0.7 mm is very hard and brittle after butt-welding by argon arc welding. When the welding point is slightly bent, it will be brittle fracture at the fusion line or weld, and the fracture will show obvious brittle fracture morphology. The resulting joint consists of a weld and a heat-affected zone, and the microhardness of each area from the center of the weld to the base metal is tested along the axis of the joint. The measurement results show that from the base material to the heat-affected zone and the middle of the weld, the microhardness increases sharply, and the hardness of the middle of the weld reaches HV 1 060, which indicates that the heat-affected zone and the middle of the weld have formed hard and brittle structures. For this kind of joint with hard and brittle structure, in order to improve its toughness and plasticity, reduce its hardness, and obtain an appropriate match of hardness, strength, plasticity and toughness, the welded joint must be properly tempered. After heat treatment, the brittleness of the heat-affected zone should be eliminated, and the base material should be able to maintain a certain strength and elasticity. The tempering is carried out in a box-type resistance furnace, and the tempering process is shown in Table 1. Carefully polish the welded joints of the steel wires after tempering to make their diameters approximately equal to those of the base material, and then conduct a tensile test on the WE-50 tensile testing machine. Take three samples of each tempering treatment and take the average value of their tensile forces.

It can be seen from the test that after heat treatment above 330℃, the elasticity of the base material basically disappears, and the fracture occurs at the base material, but not in the solder joint and its heat-affected zone. This shows that the brittleness of the heat-affected zone is completely Disappeared, but the strength of the base material was greatly shaved off (after tests, the tensile strength of the base material used was 1 663 MPa). When the temperature is kept at 260°C for 10 min, although the elasticity of the material remains basically unchanged, the brittleness of the heat affected zone cannot be eliminated. The effect is best when the heating temperature is 280℃ and the heat preservation time is 10 min. The tensile strength of the heat-affected zone is only about 20% lower than that of the base material, while the elasticity of the base material disappears less. The 280°C tempered welding head was tested for the microhardness of each zone on the longitudinal section along the axial direction. It was found that the highest hardness value at the weld was reduced to about HV 500, which was about 1 times lower than the untreated hardness.

The welded ring steel wire should not only meet certain strength and elasticity requirements, but also have a certain fatigue strength.

in conclusion

(1) Non-melting polar argon gas shielded welding can be used to weld φ0.7 mm 65Mn steel wire to obtain a good-looking welded joint, and no filler metal can be added during welding. The welding current intensity is more suitable for 10 A. If the current is too high, it will cause spatter and collapse. If the current is too small, it is difficult to start the arc and the arc is unstable.

(2) The welded joint is very hard and brittle, so proper heat treatment must be carried out. The use of a tempering process with a heating temperature of 280°C and a heat preservation time of 10 min for the welded joint can make the joint’s tensile strength up to 1 370 MPa and higher fatigue strength.

Improvement of heat treatment process for 65Mn steel spring support

Improve the heat treatment process for spring support, increase process holes, use new quenching hangers, and adopt surface sandblasting treatment to solve the problems of uneven hardness, shape changes and unclean surfaces after heat treatment.

The 65Mn steel spring support is the spring used for the left and right side support linings of the light passenger car body of our company, and the hardness after quenching is 45-50HRC. In response to the uneven hardness and shape distortion caused by heat treatment, and the surface is not clean, rust, salt, and not easy to spot welding, we improved the heat treatment process, used a new quenching hanger and the surface was sandblasted. Practice has proved that its effect is remarkable.

Quenching Process of 65Mn Steel Spring Support

When the 65Mn steel spring support is quenched in a medium-temperature salt bath furnace, the quenching temperature is 810~830℃. After heat treatment, it is found that the quenching hardness is uneven and the difference in height is very different. The hanging utensils used in the salt furnace are made of barbed wire, with pockets, 20 pieces each time. Because there are more point-to-surface contacts between parts. It is easy to cause uneven hardness during quenching, affecting the quality of the workpiece, and low or inelasticity. For this reason, the quenching hanger and the workpiece were improved respectively. At 5mm from the edge of the spring support, add a process hole of 1 mm, and tie each piece with iron wire, leaving a certain gap between the pieces. Each string is 20 pieces, each furnace is equipped with 4 strings, and the furnace capacity is 80 pieces. In this way, the injustice makes the quenching hardness of the workpiece uniform, and the furnace load is increased by 4 times, which improves the work efficiency.

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

Corrosion of materials and protection methods

Corrosion of materials

1. Chemical corrosion 65Mn is oxidized at high temperatures and is affected by carbon dioxide, sulfur dioxide, oxygen, hydrogen and other gases in a dry environment at room temperature, and causes chemical effects in non-electrolyte liquids (such as gasoline and lubricating oil). This is the chemistry of 65Mn corrosion.

2. Electrochemical corrosion 65Mn often comes into contact with other materials during work, or dust, dirt, etc. are often scattered on the work piece. The two different state parts or the parts where the two substances are in contact will cause a potential difference. If you contact the electrolyte or Absorbing sulfur dioxide, carbon dioxide, and moisture, etc., will cause micro-batteries to form an electric current and dissolve the metal that is the negative electrode. In the industrial atmosphere, all kinds of dust (including acid, alkali, salt) and dirt are often dropped, and the materials will corrode quickly.

Anti-corrosion measures for materials

1. Anti-corrosion To prevent 65Mn from rusting, it should be coated with preservatives or chemically treated to form a protective film before leaving the factory, or plated or packaged, and then transported to the warehouse. (The oiled material when entering and exiting, if found to be stained Or if the oil bottom is rusted or volatile and dry, it should be decontaminated and descaled, and repainted with oil.) During acceptance, the box should be folded according to the regulations, and the packaging should be restored immediately after acceptance. When storing, put a desiccant in the warehouse to reduce moisture and humidity.

2. Spraying anti-corrosion 65Mn is generally used in the open air. Spraying anti-corrosion coating on the surface of the material can isolate the material from air, rain and other corrosive media, and eliminate the chance of electrochemical corrosion of the material during storage. Especially the 73418 anti-rust oil, it can form a continuous film on the surface of the steel, after natural drying, firmly adhere to the surface of the steel, can prevent the steel from rusting for a year, and has low cost and easy use.

Welding of 65Mn steel wire:

The welding experiment of 65Mn steel wire with φ0.7 mm was carried out by the method of argon tungsten arc welding. Studies have shown that: when the welding current is 10 A, a perfect cylindrical welded joint can be obtained, but the joint is very brittle and hard. The use of a heating temperature of 280°C and a post-heating process of 10 minutes of heat preservation can greatly reduce the brittleness and hardness of the joint. The tensile strength of the welded joint after treatment is up to 1 370 MPa, and it has excellent fatigue strength. Therefore, if the welded ring-shaped steel wire is plated with diamond abrasive, it may become an efficient cutting tool.

Composition and performance of 65Mn steel wire

The steel wire used in this study is a 65Mn cold drawn steel wire with a diameter of 0.7 mm. The original structure is sorbite and a small amount of ferrite, which are distributed in fibrous form. The carbon content of 65Mn is 0.62% to 0.70%, and the content of Si and Mn are 0.17% to 0.37% and 0.9% to 1.2%, respectively. Mn moves the S and E points in the iron-carbon phase diagram to the lower left and lowers the A3 and A1 lines. Therefore, manganese steel has a tendency to overheat. 65Mn steel is a high-carbon steel, and the combined effect of Mn and Si makes its carbon equivalent more than 0.8%. This makes 65Mn steel have a great tendency to harden, and the weldability is extremely poor.

Argon arc welding butt welding process of 65Mn steel wire

In order to reduce the consumption of the electrode, the DC positive connection is selected for the butt welding test of the wire, that is, the DC power supply is selected, the wire is connected to the positive electrode of the power source, and the tungsten electrode is connected to the negative electrode of the power source.

The tungsten electrode containing 1% or 2% thorium oxide has high electron emission efficiency, good current carrying capacity, good anti-pollution performance, easy arc ignition and relatively stable arc. In order to facilitate the operation, a finer thorium tungsten electrode with a diameter of 2 mm was selected, and the tip of the electrode was sharpened.

Since the lower arc voltage characteristics of argon are particularly beneficial for manual arc welding of thin plates and wires, argon is selected as the shielding gas.

A DC manual argon arc welding machine was used for the test. Before welding, both ends of the steel wire were carefully polished flat. In order to prevent pores in the solder joints, the end oil was cleaned with acetone. Place the flat wire on both ends on a flat and clean alignment plate, align the ends without leaving a gap at the joint, and press the two sides of the joint with a weight. Connect the wire to the positive electrode of the welding machine and the tungsten electrode to the negative electrode, and adjust the current to 20 A, 15 A, 10 A, and 8 A for welding. When welding, ignite the ignition arc next to the joint and make it stable. Move the arc to the joint to melt the joint metal and quickly extinguish the arc. At the same time, apply a slight upsetting force to complete the welding process after cooling. Use filler wire.

The test found that when the welding current is 20 A, the arc burns violently, the metal spatter at the joint is serious, and the welding joint collapses seriously. When the current is adjusted to 15 A, the arc burns more smoothly and the molten pool splashes less, but the weld still collapses. But when the current drops to 10 A, the arc is easy to start, the arc burns stably, and there is no collapse in the weld. Figure 2 shows the shape of the welded joint taken with a digital camera under a Leica MZ6 stereo microscope when the welding current is 10 A. It can be seen that the cylindricity of the joint is good, and it can meet the requirements of a wire saw after grinding. When the current is adjusted below 8 A, it is difficult to start the arc and the arc is unstable, making it difficult to complete the welding process.

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

Investigation and analysis of material 65Mn
65Mn steel has the advantages of high hardness, good hardenability, low decarburization tendency, low price, good machinability, etc., but it is sensitive to overheating, is prone to quenching cracks, and has temper brittleness. 65Mn steel is widely used. Mainly produced into steel wire, steel belt, used to manufacture various flat and round springs, leaf springs and spring sheets with less cross-section. 65Mn steel is widely used in the automobile industry, electronics industry, trains and other transportation vehicles. It can manufacture circular saw blades for high-speed cutting of various types of steel, steel pipes and steel bars.

Brand, composition, organization, heat treatment, performance, use introduction

Material designation: 65Mn American ASTM: 1566, SEA: 1566 (1066) Former Soviet Union ГОСТ: 65Г

(The steel composition is C=0.64%, Mn=0.92%, Si=0.18%, S=0.005%, P=0.017%. The degree of crystallinity is 4 to 8. The austenitizing temperature is 830℃)

65Mn steel is generally used after quenching and tempering. When tempered below 450℃, it is tempered martensite, and when tempered above 450℃ it is tempered sorbite.

Heat treatment process:

Surface treatment process: using surface sandblasting treatment. 65mn steel circular saw blade is carbonitrided in advance to increase the carbon and nitrogen content, and then conventional heat treatment is performed to improve the tempering stability of the tooth, thereby increasing the hardness and wear resistance of the tooth, and improving the circular saw blade Life.

Heat treatment process parameters

Annealing Normalizing Temper Tempering Quenching Tempering Stress Relief Tempering (cold drawn spring steel wire)

810 810 680~700 810 360~570 250~360

Cooling test Furnace cooling Air cooling Air cooling Oil/water cooling Air cooling Air cooling

Density ρ=7.81 g/cm3

Welding performance: poor

Use of the material: The steel can be cold rolled into steel plates, steel strips and steel wires to make springs. 65Mn can also be made into tools such as fitter’s chisel and scribing needle. 65Mn steel can be used to make small springs with a general section size of about 8-15mm, such as various small flat and round springs, bottom springs, spring springs, and also suitable for making spring rings, valve springs, clutch springs, brake springs, etc. . 65Mnj is one of the materials used to make 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.

Influence of Mn:

It is generally believed that manganese is a beneficial element in steel. In 65Mn, most of manganese is dissolved in ferrite to form a replacement solid solution and strengthen ferrite. A part of manganese is also dissolved in Fe3C to form alloy cementite. Manganese can also increase the relative amount of pearlite and make It becomes thinner, thereby increasing the strength of the steel. Manganese can be combined with S to form MnS to reduce the harmful effects of S.

Heating defects and control

Heating defects and control
1. Overheating

We know that overheating during the heat treatment process will most easily lead to the coarsening of austenite grains, which will reduce the mechanical properties of the parts.

1. General overheating: The heating temperature is too high or the holding time at high temperature is too long, which causes the austenite grains to coarsen and is called overheating. Coarse austenite grains will reduce the strength and toughness of the steel, increase the brittle transition temperature, and increase the tendency of deformation and cracking during quenching. The cause of overheating is the out-of-control of the furnace temperature meter or mixing (often caused by not knowing the process). The superheated structure can be annealed, normalized or tempered at high temperature for several times, and then re-austenitized under normal conditions to refine the grains.

2. Fracture inheritance: steel with overheated structure, after reheating and quenching, although the austenite grains can be refined, sometimes there are still coarse granular fractures. There are many theoretical controversies about fracture heredity. It is generally believed that impurities such as MnS have been dissolved into austenite and enriched in the crystal interface due to excessive heating temperature, and these inclusions will precipitate along the crystal interface when cooling. It is easy to fracture along the coarse austenite grain boundary when subjected to impact.

3. Inheritance of coarse structure: When steel parts with coarse martensite, bainite and Widmanite structure are re-austenized, they are heated to the conventional quenching temperature at a slow speed, or even lower, the austenite crystal The grains are still coarse, and this phenomenon is called tissue heredity. To eliminate the heredity of coarse tissues, intermediate annealing or multiple high temperature tempering treatments can be used.

2. Overburning

Excessive heating temperature will not only cause coarse austenite grains, but also local oxidation or melting of the grain boundaries, leading to weakening of the grain boundaries, which is called overburning. The performance of steel deteriorates severely after overburning, and cracks are formed during quenching. The burned tissue cannot be recovered and can only be scrapped. Therefore, avoid over-burning during work.

3. Decarburization and oxidation

When steel is heated, the surface carbon reacts with oxygen, hydrogen, carbon dioxide and water vapor in the medium (or atmosphere) to reduce the surface carbon concentration, which is called decarburization. The surface hardness, fatigue strength and resistance of decarburized steel after quenching The abrasiveness is reduced, and the residual tensile stress on the surface is easy to form surface mesh cracks.

During heating, the iron and alloys and elements on the surface of the steel react with oxygen, carbon dioxide, and water vapor in the medium (or atmosphere) to form an oxide film called oxidation. The dimensional accuracy and surface brightness of the workpieces deteriorate after oxidation at high temperatures (generally above 570 degrees), and the steel parts with poor hardenability of oxide film are prone to quenching soft spots.

Measures to prevent oxidation and reduce decarburization include: coating the workpiece surface, sealing and heating with stainless steel foil packaging, heating with a salt bath furnace, heating with a protective atmosphere (such as purified inert gas, controlling the carbon potential in the furnace), flame burning furnace (Make the furnace gas reductive)

Fourth, hydrogen embrittlement

When high-strength steel is heated in a hydrogen-rich atmosphere, the plasticity and toughness decrease is called hydrogen embrittlement. Hydrogen embrittlement can also be eliminated for workpieces with hydrogen embrittlement (such as tempering, aging, etc.). The use of vacuum, low hydrogen atmosphere or inert atmosphere heating can avoid hydrogen embrittlement.

Of course, in actual work, some people use this phenomenon to serve others (such as alloy crushing treatment, etc.)

Process characteristics of multi-strand spring

Due to the limitation of product structure, multi-strand springs generally have the characteristics of high strength and good performance. The material is required to guarantee the ultimate performance in terms of strength and toughness. Therefore, the material is usually carbon spring steel wire (GB4357) or carbon spring steel wire (GB4360) with elastic modulus E=206×103N/mm2. The specific specifications of the steel wire shall be determined according to the structural characteristics and load requirements of the product.
According to the number of steel wire strands, multi-strand springs can be divided into three strands and four strands, of which three strands are common.

There are two ways to wind multi-strand springs: one is to wind the steel rope and the spring at the same time; the other is to twist the steel rope and the spring step by step, that is, twist the steel rope first, and then wind the spring. The steel rope can be twisted and wound on the lathe separately, and it can also be wound on the machine tool with automatic steel rope mechanism.

The typical process can refer to the following:

Loading→(twisting the steel rope)→winding→cutting→grinding head→visual inspection→surface treatment (depending on product requirements and whether welding head)→welding both ends (if necessary)→repairing welding head (if necessary)→parallel end Ring (parallel)→calibration→heat treatment→squeeze→calibration→end-dressing ring (if necessary)→inspection→surface treatment (there is no such process if surface treatment has been performed)→oil immersion

The calculation of typical process parameters can be carried out according to the following formula:

1. Calculation formula of unfolded steel wire length:

L1=(πDn1m)/(cosαcosβ)

D———Spring diameter (mm)

n1———Total number of turns

α———Helix angle (°)

m——–Number of spring strands

β——-Wire rope twisting angle (°)

2. Stiffness calculation formula:

P’=(Gd4mi)/(8D3n)

G——–shear modulus (N/mm2), generally 79×103~85×103

d——–Wire diameter (mm)

i———-Twisting coefficient, when m=3, β is between 15°~25°, take 1.05~1.2; when m=4, β is between 20°~30°, take 1.1 ~1.3.

n———-effective number of turns

During processing, the following points should be noted:

1. Springs without support rings and springs with too thin steel wire diameters should not be welded to the spring heads, but the end cables should not be significantly loose, and should be deburred. For multi-strand springs that need to be welded to the head, the length of the welded part should be less than 3 times the cable diameter (the longest is no more than 10 mm). The heating length should be less than one circle, and it should be polished smoothly after welding, and the welding part should be locally annealed at low temperature during gas welding.

2. The support ring can choose two methods: cold and hot, according to product requirements. It is not allowed to heat the spring to sparking or whitening by using heat and the method, and the temperature of silico-manganese steel should not be higher than 850℃. The support ring and the effective ring should be in effective contact, and the gap should not exceed 10% of the nominal gap between the rings.

3. The characteristics of the multi-strand spring can be determined by the adjustment lead, and the cable distance can be adjusted as necessary during winding. The twisting distance can be 3-14 times the steel wire diameter, but generally 8-13 times is better. The spring force is also closely related to the free height, the end ring, the outer diameter and the performance of the steel wire, and can be changed by adjusting one or several of them.

4. The pressing time of important springs is 24 hours, and that of ordinary springs is 6 hours or continuous compression 3~5 times, each time holding for 3~5 seconds. The gap between the spring and the mandrel during compression is preferably 10% of the diameter of the mandrel. If the gap is too small, it is difficult to operate, and if the gap is too large, the spring is likely to bend and deform. If one of the springs breaks during pressing, the rest should be processed again.

5. For multi-strand springs with a large H0/D2 value, attention should be paid to their deformation during heat treatment, considering whether to wear the mandrel and the placement method, and appropriate heat treatment equipment should be selected. Under the repairable condition, tempering and hot pressing can be performed many times to achieve the purpose.

6. The spring surface treatment is generally phosphating treatment, but other treatments can also be carried out. Where zinc and cadmium are to be coated, the hydrogen removal treatment should be carried out after electroplating, and 3% (not less than 3 pieces) shall be drawn after the removal of hydrogen. The spring should be cleaned of surface dirt, salt marks, and oxide scale. The method can be sand blowing or gasoline cleaning, but acid cleaning is not allowed.

Manufacturing process of cold formed spring

When using materials that do not need to be quenched and tempered to make a spring after forming, the process is
Spiral compression spring: coiling, stress relief annealing, grinding on both ends, (shot blasting), (aligning), (stress relief annealing), standing or pressure treatment, inspection, surface anticorrosion treatment, packaging.

Spiral tension spring: coiling, stress relief annealing, hook and loop production, (tail trimming), stress relief annealing, standing treatment, inspection, surface anticorrosion treatment, packaging.

Spiral torsion spring: coiling, stress relief annealing, torsion arm making, tail trimming, stress relief annealing, standing treatment, inspection, surface anticorrosion treatment, packaging.

The manufacturing processes of the spiral tension and torsion springs described above are all the hook loops or torsion arms at both ends after being wound on the ordinary coil spring. In recent years, many domestic and foreign manufacturers have produced and used computer forming machines or special forming machines, spring body and tail shape

The shape can be completed on the forming machine at one time, eliminating the need for processing shackles or twisting arms.

When using materials that need to be quenched and tempered after forming, the main difference from the above process is that they need to be quenched and tempered after forming. Sometimes the spring end processing needs to be normalized.

The process with brackets is a non-fixed process, and whether it is performed depends on the performance requirements of the spring.

New technology for surface treatment of mould spring

The current mold is the main process equipment for industrial production. The development of modern industrial products and the improvement of technical level depend largely on the development level of the mold industry. Since the Second World War, the increase in automation has led to the rapid development of foreign molds. Although my country is a big country in mold production, it has never been a strong country in mold manufacturing. For springs that match the mold, this detail is also the key to achieving quality. Affect the performance of the entire mold.
I am engaged in surface treatment engineering, and have conducted market research on the surface treatment of mold springs, that is, rectangular springs. I found the following questions and raised them for discussion.

The surface of the spring is identified by various paint layers. For example, light and small loads are represented by yellow paint; light loads are represented by blue paint; medium loads are represented by red paint; extremely heavy loads are represented by brown paint;

Performance analysis: A good paint layer is used to match the spring surface treatment, which can be described as two birds with one stone. Colors are used to identify various technical parameters, which is convenient for industrial production and operation, and the anti-corrosion performance of the spring surface can be better guaranteed.

At present, the main manufacturers that successfully use the above methods are many foreign manufacturers, such as Japan’s Dongfa Company.

In China, only a small number of manufacturers have effectively visited. At present, it is mainly due to some fixed technical requirements, such as the use of imported materials to reach foreign levels, but there is an obvious gap in the surface treatment effect of the entire spring.

Manual painting is generally used in China, and the appearance will appear sagging, leaking, poor combination, and poor rust resistance. The limitations of its own process caused a large number of products to be stripped and reworked.

Electrophoretic paint is used abroad. The workpiece is used as a cathode in a water-soluble paint tank, and a certain voltage and time are applied to form a paint layer with uniform thickness on all surfaces of the spring, which can then be cleaned and dried. Its appearance is clean and bright, no particles, no bottoming phenomenon, good binding force, it is not easy to fall off, and the hardness is 3-4H. If the polyurethane cathodic electrophoretic paint from UK LVH company is used, its flexibility is quite good. The operating environment has no strong solvent smell, less air pollution, and simple waste water treatment and discharge process. Just adjust the resin of the PH sedimentation electrophoretic paint, and then discharge the clear water.

We design the process flow for a domestic manufacturer: forming spring → shot blasting → upper hanger → alkaline degreasing → double water washing → (slightly acid 1% neutralization) phosphating → double water washing → double deionized water washing → cathode system polyurethane Color electrophoretic paint→recycling→double water washing→assisting washing→baking and curing→under hanging tools.

result:

The peened particles will be caught in the gap between the two ends of the spring.

Shot blasting causes a rough surface, and the paint layer should be thickened accordingly to have a bright and flat appearance, with a thickness of 10-20 microns, which in turn causes the shrinkable thickness of the spring to become smaller.

Phosphating can cause hydrogen embrittlement and reduce the service life of the spring.

Improvement process: forming spring → lye polish degreasing → centrifugal water drop → hanging → alkaline degreasing → double water washing → (micro acid neutralization 1%) phosphating → double water washing → double deionized water washing → cathode system polyurethane Color electrophoretic paint→recycling→double water washing→assisting washing→baking and curing→under hanging tools.

analysis:

The finishing (finishing) process is to put the spring directly into the inclined centrifugal, spiral vibrating, vortex and other series of finishing machines, and add an appropriate amount of abrasive, abrasive and water for finishing. The surface after finishing is smoother than that of shot blasting.

Moreover, traditional processes such as degreasing, cleaning, pickling, phosphating, dehydrogenation, etc. are reduced, the production process is simplified, and there is less chance of corrosion of the spring during the production process, so pickling is not required. The thickness of the paint on the surface can be controlled at about 10 microns.

The author used Hawking Fine Chemicals (Shanghai) Co., Ltd.’s Geelylon cathodic electrophoretic paint for the experiment. It has better performance in all aspects and the coating rate: when the coating is 10 microns, 45-50 square meters per kilogram of paint can be coated. Salt spray test: 500-1000H.

However, subtracting the shot peening process may reduce the strength of the spring itself. It is best to choose a steel shot with a suitable diameter according to different spring specifications, such as the size of the gap, to avoid the steel shot being caught in the spring gap.

Unqualified paint removal method:

At present, some factories use oil to burn the paint layer, and then use shot blasting to clean the surface, but the operation process causes considerable pollution, and fire is prone to occur during manual operation.

Use concentrated sulfuric acid to carbonize the paint layer, but strong acid itself can cause hydrogen embrittlement, so spring is not suitable.

The use of KRT paint remover, the swelling method, peel off the paint layer, does not damage the spring substrate, simple operation, is a relatively advanced method, but the use of commercial paint remover may lead to increased costs, but if the scrap rate is reduced, the overall The cost will not rise.