Magnets are often used to adsorb stainless steel to verify its quality and authenticity. If they are not magnetic, they are considered good and genuine. If they are magnetic, they are considered fake. However, it is an extremely one-sided and unrealistic method of error identification.
There are many kinds of stainless steel, which can be divided into two categories according to its structure at room temperature.
1. Austenite type: 304, 321, 316, 310, etc.
2. Martensite or ferrite type: 430, 420, 410, etc.
Austenite type is nonmagnetic or weak magnetic, martensite or ferrite is magnetic. Most stainless steel used as decorative tube sheet is 304 austenitic material, generally speaking, it is non-magnetic or weak magnetic, but because of the fluctuation of chemical composition caused by smelting or different processing conditions, it may also appear magnetic, which can not be said to be fake or unqualified, because of the composition segregation or improper heat treatment during smelting, it will cause. A small amount of martensite or ferrite is found in Austenitic 304 stainless steel. Make 304 stainless steel with weak magnetism. In addition, 304 stainless steel after cold processing, the structure will also be transformed to martensite, the greater the cold processing deformation, the more martensite transformation, the greater the magnetism of steel. In the same batch of steel strips, 76 pipes are produced without obvious magnetic induction; in the production of 9.5 pipes, the magnetic induction is more obvious because of the larger cold bending deformation, while in the production of square rectangular pipes, the deformation is more intense and the magnetism is more obvious because the deformation is larger than that of round pipes, especially in the corner-bending part. In order to completely eliminate the magnetism of 304 steel caused by the above reasons, the stable austenite structure can be restored by solution treatment at high temperature, thus the magnetism can be eliminated.
The magnetism of 304 stainless steel caused by the above reasons is totally different from that of other stainless steels, such as 430 and carbon steel. That is to say, the magnetism of 304 steel always shows weak magnetism. If stainless steel has weak magnetism or no magnetism at all, it should be distinguished as 304 or 316 material. If the magnetism of 304 stainless steel is the same as that of carbon steel, It shows strong magnetism because it is not 304 material.
I. Conditioning Treatment
For medium carbon steel and low alloy steel products, in order to achieve the required strength for use, they are heated to the temperature above 50 above the GS line, then quenched in water and oil, and then tempered at the appropriate temperature. Quenching and high temperature tempering operations were carried out, and the "quenching and tempering" treatment was completed.
The two most important parameters of heat treatment process are temperature and time.
1. Temperature and time of quenching
For medium carbon steel and low alloy steel fasteners, if the temperature is heated below the GS line during quenching, that is, below 850 C for cooling, after cooling, the workpiece is hard and brittle, it should be considered that the temperature above the GS line should be heated at 50 C. Because IF quenching temperature is lower than GS line, the structure will become austenite and ferrite, in this case, timely quenching, soft ferrite will completely survive in its original state.
The closer the quenching temperature is to the GS line, the less soft ferrite is.
2. Cooling speed
Cooling rate is the key link in quenching process, that is to say, different cooling rate will make different changes in austenite. Rapid cooling can transform austenite into hard and brittle martensite. If cooled at a slower rate, ferrite or pearlite will be mixed in the martensite and become incomplete quenched structure.
For products above M12, when the cooling time is long, the cooling speed of the surface and the center of the workpiece is different, which will result in different hardness of the surface and the center. This is an unavoidable phenomenon in the heat treatment process of large diameter carbon steel workpiece. The effective way is to mix the cooling medium or the cooling medium in the cooling tank sufficiently.
3. Tempering process
When the quenched workpiece is reheated to 200 - 300 C, the carbides dissolved in martensite begin to precipitate. Almost all precipitates occur at about 400 C. The precipitated carbides gradually coagulate and grow into grains, i.e. troostite, which becomes low hardness and brittle-free structure. At 600 C, further coagulation enlarges into spherical structure, which becomes sorbite structure. The needle-like grain of martensite disappears completely and the toughness can be brought into play.
2. Carburizing and Quenching Treatment
Steel tapping screw requires tapping thread performance on its surface and toughness on its interior. It needs to be made of low carbon steel or low carbon alloy steel, carburized on its surface at a certain temperature, and then quenched. Current carburizing and quenching,
Tempering treatment is a gas carburizing process with high production efficiency and good working environment and its corresponding equipment.
The mechanical properties of steel tapping screw must be guaranteed by carburizing process. If the hardening layer is too shallow and the surface hardness and torsional strength are insufficient, the screw thread will be deformed or bare buckled when twisted. On the contrary, the hardened layer is too deep, brittle and easy to turn around or break.
Carburizing is also easy to achieve according to the standard.
3. Protective Atmosphere Heat Treatment
Protective atmosphere heat treatment process has been widely used in recent years. Its principle is that the non-oxidizing gas is fed into the furnace to form a non-oxidizing atmosphere in which a bright heating without decarburization and carburization is carried out.
In addition, there are many heat treatments in vacuum without using protective gas, and most of them are stainless steel products. Protective gases of carbon steel or low alloy steel products are decomposed into hydrogen (H2), carbon monoxide (CO), nitrogen (N2) and other gases by using liquefied propane, butane and any kind of gas or natural gas as raw materials, mixing gases with air in a certain proportion at the contact edge of the nitrogen catalyst heated to about 1050 degrees C, and decomposing them into hydrogen (H2), carbon monoxide (CO), nitrogen (N2). Composition, which also contains trace amounts of carbonate gas (CO2) and water.
The carbon concentration of endothermic gas denaturing furnace varies by changing the proportion of raw gas mixed with air. The relationship between carbon concentration and variation temperature, gas composition (dew point) is the same as quenching and tempering temperature. It only considers a general objective. In fact, only through gas analysis and material test analysis in furnace, can we grasp its characteristics.
Furnaces can be classified as:
(1) Distribution furnaces, gas furnaces and oil-fired furnaces by heat source
(2) According to its use, it can be divided into general treatment furnace, fixed carburizing furnace (sealed heat treatment furnace), liquid carburizing furnace (salt melting furnace), protective atmosphere furnace (gas carburizing furnace), vacuum furnace, high frequency heating device and flame heat treatment device.
(3) According to the structure of box-type furnace, cylindrical furnace, rotary furnace, continuous furnace, intermittent furnace (horizontal, well type), bottom-shaking furnace, rotary steamer furnace, bottom-shaking furnace, Pusher-type continuous heating furnace, trolley furnace.
In addition, temperature distribution, furnace gas control, safety rate, heating capacity, thermal efficiency, pollution prevention and other necessary conditions for the furnace are not related to the furnace form.
The following table shows the heat treatment process and mechanical properties of some alloy steels for bolts and nuts.