Investigation and analysis of the hottest material

Investigation and analysis of material 65Mn

65Mn steel has the advantages of high hardness, good hardenability, less decarburization tendency, low price and good machinability, but it has overheating sensitivity, easy to produce quenching cracks, and temper brittleness. 65Mn steel is widely used, mainly produced into steel wires, steel strips, flat and round springs with less cross-section, leaf springs and spring sheets. 65Mn steel is widely used in automobile industry, electronic industry, train and other transportation tools. It can produce circular saw blades for high-speed cutting of various types of steel, steel pipes and reinforcing bars

key words: 65Mn welding corrosion prevention and heat treatment

Chapter 1 Introduction to the brand, composition, structure, heat treatment, performance and use of this material

1.1 material brand: 65Mn American astm:1566, sea:156 launch the special action for the treatment of low-speed electric vehicles 6 (1066) the former Soviet Union ГОСТ： sixty-five Г

1.2 the chemical composition of the material is shown in table [1] the chemical composition of 65Mn

1.3 the structure of the material [2]

1.3.1 the phase transformation temperature is shown in table

1.3.2 the time thermometer structure transformation curve is shown in the figure. The isothermal transformation curve of 65Mn Steel

(the steel composition is c=0.64%, mn=0.92%, si=0.18%, s =0.005%, p=0.017%. The crystallinity is grade 4 ~ 8. The austenitizing temperature is 830 ℃)

1.3.3 alloy structure:

65mn steel is generally used after quenching and tempering. Tempering below 450 ℃ is tempered martensite, and tempering above 450 ℃ is tempered sorbite

1.4 heat treatment process:

1.4.1 surface treatment process: adopt surface sandblasting treatment. The 65Mn steel circular saw blade is pre carbonitriding the tooth to increase the carbon and nitrogen content, and then conventional heat treatment is carried out to improve the tempering stability of the tooth, so as to increase the hardness and wear resistance of the tooth and improve the service life of the circular saw blade

1.4.2 see table for heat treatment process parameters

name annealing normalizing temperature regulating tempering quenching tempering stress relief tempering (cold drawn spring steel wire)

heating temperature/℃ 810 810 680 ~ 700 810360 ~ 570 250 ~ 360

cooling square test furnace cold air cooling air cooling oil/water cooling air cooling air cooling

Table

1.5 material properties

1.5.1 mechanical properties [3]

1.5.2 density ρ= 7.81 g/cubic centimeter

1.5.3 process performance

weldability: poor

1.6 purpose 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 a bench worker's chisel, scratch needle, etc. 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 pad springs, spring springs, and also suitable for making spring rings, valve springs, clutch springs, brake springs, etc. 65mnj is one of the materials for making all kinds of plate springs and wire springs. Cars, trams, trains and other means of transportation use a lot; It is also widely used in the manufacture of instruments, furniture, and even children's toys

1。 7 Effect 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 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 thinner, so as to improve the strength of steel. Manganese can be synthesized with s into MNS to reduce the harmful effect of S

Chapter II corrosion of materials and protection methods

2.1 corrosion of materials

chemical corrosion - oxidation of 65Mn at high temperature and the role of carbon dioxide, sulfur dioxide, oxygen, hydrogen and other gases in normal temperature and dry environment. Thank you for browsing!, And cause chemical action in non electrolyte liquids (such as gasoline and lubricating oil), which is the chemical corrosion of 65Mn

II electrochemical corrosion 65Mn is often in contact with other materials or the workpiece is often scattered with dust, dirt, etc. in these two different state parts or the parts where the two substances are in contact will cause potential difference. If it contacts the electrolyte again or absorbs sulfur dioxide, carbon dioxide and moisture, it will cause a micro battery, form a current, and dissolve the metal that is the negative electrode. In the industrial atmosphere, all kinds of dust (including acid, alkali and salt) and dirt often fall into the air, and the materials will soon corrode

2.2 anti corrosion measures for materials [4]

I. anti rust to prevent 65Mn corrosion. When leaving the factory, it should be coated with preservatives, or chemically treated to form a protective film, or plated or packaged, and then transported into the warehouse. (when entering and leaving the oiled materials, if it is found that they have been stained, or the oil bottom is rusted or evaporated dry, they should be decontaminated, descaled and reapplied). During acceptance, the boxes should be folded according to the regulations, not more than one fold. After acceptance, the packaging should be restored immediately. During storage, desiccant shall be placed in the warehouse to reduce moisture and humidity

II. 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 corrosive media such as air and rain, and eliminate the opportunity of electrochemical corrosion of the material during storage. In particular, 73418 antirust oil can form a continuous film on the surface of steel. After natural drying, it can firmly adhere to the surface of steel, so that the steel will not rust for a year, and the cost is low and easy to use

Chapter III technical special report

3.1 welding of 65Mn steel wire: [5]

argon tungsten arc welding method is used to weld φ The welding test of 0.7 mm 65Mn steel wire was carried out. The research shows that when the welding current is 10 A, the cylindrical welded joint with perfect shape can be obtained, but the joint is very brittle and hard. The post heating process with heating temperature of 280 ℃ and holding for 10 minutes can greatly reduce the brittleness and hardness of the joint. The tensile strength of the welded joint after treatment is up to 1370 MPa and has excellent fatigue strength. Therefore, if diamond abrasive is coated on the welded ring steel wire, it may become an efficient cutting tool

1 composition and properties of 65Mn steel wire

the steel wire used in this study is 65Mn cold drawn steel wire with a diameter of 0.7 mm. Its original structure is sorbite and a small amount of ferrite, which is distributed in fibrous form. The carbon content of 65Mn is 0.62% - 0.70%, and the contents of Si and Mn are 0.17% - 0.37% and 0.9% - 1.2% respectively. Mn makes the s and e points in the iron carbon phase diagram move to the left and down, reducing the A3 and A1 lines. Therefore, manganese steel has the tendency of overheating. 65Mn steel belongs to high carbon steel. With the combined action of Mn and Si, its carbon equivalent reaches more than 0.8%. This makes 65Mn steel have great hardening tendency and poor weldability

2 argon arc welding butt welding process of 65Mn steel wire

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

tungsten electrode containing 1% or 2% thorium oxide has high electron emission efficiency, good current carrying capacity, good pollution resistance, easy arc striking and relatively stable arc. In order to facilitate operation, a thinner thorium tungsten electrode with a diameter of 2 mm is selected, and the front end of the electrode is sharpened

since the low arc voltage characteristic of argon is particularly beneficial to the manual arc welding of thin plates and wires, argon is selected as the shielding gas

DC manual argon arc welding machine is selected for the test. Before welding, both ends of the steel wire are carefully ground flat. In order to prevent pores in the welding points, clean the oil stains on the ends with acetone. Place the wire rod ground flat at both ends on a flat and clean alignment plate (Fig. 1), align both ends, leave no gap at the joint, and press both sides of the joint with a pressure iron. Connect the wire to the positive pole of the welding machine and the tungsten electrode to the negative pole, and adjust the current to 20 A, 15 A, 10 A and 8 a respectively for welding. During welding, ignite the ignition arc next to the joint and make it burn stably. Move the arc to the joint to melt the joint metal, and then quickly extinguish the arc. At the same time, slightly apply the upsetting force, and then complete the welding process after cooling. No filler wire is used in the welding process

the test found that when the welding current is 20 A, the arc burns violently, the metal splash at the joint is serious, and the solder joint collapse is serious. When the current is adjusted to 15 A, the arc combustion is relatively stable and the spatter of the molten pool is less, but the weld still collapses. However, when the current drops to 10 A, it is easy to start the arc, the arc combustion is stable, and there is no collapse at the weld. Figure 2 shows the shape of the welded joint taken with a digital camera under a Leica mz6 stereomicroscope 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 wire saw after grinding. When the current is adjusted below 8 A, it is difficult to start the arc and the arc is unstable, so it is difficult to complete the welding process

3 heat treatment and strength test of welded joints

because 65Mn steel has the tendency of overheating, the welding heat affected zone has a great impact on the mechanical properties of the joint. 65Mn steel wire with a diameter of 0.7 mm is very hard and brittle at the joint after butt welding by argon arc welding. If the welding spot is gently bent, it will be brittle at the fusion line or weld joint. It is required that two instruments of the same model or enough spare parts are broken, and the fracture surface shows an obvious brittle fracture morphology. The obtained joint is composed of weld and heat affected zone. The microhardness of each area from the weld center to the base metal is tested along the joint axis. The measurement results show that the microhardness increases sharply from the base metal to the heat affected zone and the middle of the weld, and the hardness in the middle of the weld reaches HV 1060, which indicates that hard and brittle structures are formed in the heat affected zone and the middle of the weld. For this kind of joint with hard and brittle structure, in order to improve its toughness and plasticity, reduce its hardness, and obtain the appropriate combination 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 at the same time, the base metal should maintain a certain strength and elasticity. Tempering is carried out in a box type resistance furnace. See Table 1 for the tempering process. The tempered steel wire welded joint shall be carefully polished to make its diameter roughly equal to the diameter of the base metal, and then the tensile test shall be carried out on the we-50 tensile testing machine. Take three samples of each tempering treatment, and take the average value of its tensile force

it can be seen from the test that after heat treatment above 330 ℃, the elasticity of the base metal basically disappears, and the fracture occurs at the base metal, not at the welding joint and its heat affected zone. This shows that although the brittleness of the heat affected zone completely disappears after heat treatment, the strength of the base metal is greatly reduced (after test, the tensile strength of the base metal used is 1663 MPa). When the temperature is kept at 260 ℃ for 10 minutes, although the elasticity of the material is basically unchanged, the brittleness of the heat affected zone cannot be eliminated. When the heating temperature is 280 ℃, the effect is the best when it is kept for 10 minutes. The tensile strength of the heat affected zone is only about 20% lower than that of the base metal, and the elasticity of the base metal disappears less. The microhardness of each area on the longitudinal section of the weld head tempered at 280 ℃ was tested along the axis direction, and it was found that the maximum hardness value at the weld was reduced to about HV 500, which was about 1 times lower than that without treatment

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

4 conclusion

(1) non consumable argon shielded welding is adopted φ 0.7 mm 65Mn steel wire can get a welded joint with good appearance, and no filler metal can be added during welding. The welding current intensity of 10 A is more appropriate, and excessive current will cause splash and edge 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 tempering process with heating temperature of 280 ℃ and insulation for 10 min can make the tensile strength of the welded joint reach 1370 MPa, and the fatigue strength is also high

3.2 improvement of heat treatment process of 65Mn steel spring support [6]

improvement of heat treatment process of spring support