Overview
Engaged in fastener technology for many years, always believe that the mechanical properties of fasteners are related to the material of the fasteners, the parameter setting and molding method of the cold forming process, the heat treatment process control and the heat treatment process parameters. The surface treatment has little effect on the mechanical properties of the fasteners. The surface treatment only solves the appearance or corrosion resistance of the fasteners and prolongs the service life of the fasteners. However, the actual situation is not the case. The method of surface treatment is not suitable or the operation does not meet the requirements of the process parameters, which directly affects the mechanical properties of the fastener, and is not limited to the surface corrosion resistance or appearance.
1, the significance of metal corrosion protection
Metals are affected by environmental factors. Destructive damage caused by chemical or electrochemical reactions is called corrosion.
Almost all metal products, in a certain environment, will have several forms of corrosion, and the fasteners are metal products, naturally it is impossible to avoid metal corrosion problems. When the environment and time are different, the corrosion phenomenon is also significantly different. Especially in the coastal areas, the salt and humidity in the atmosphere are extremely high, and there are many polluting industries, such as: steel, petrochemical, thermal power generation, etc., and with the increase in cars this year, the exhaust gas makes the air full of corrosive Sulfides and particulates are the main causes of metal corrosion.
Metal corrosion can result in significant equipment maintenance costs and severe economic losses, such as increased design, manufacturing, maintenance and repair costs due to corrosion, and even catastrophic hazards. Fasteners are used in a wide range of applications and their quantity requirements are very large. The durability of their corrosion resistance determines the durability of their use and has a great impact on the safety and reliability of equipment. Therefore, it is necessary to study the effect of corrosion resistance on the mechanical properties of fasteners.
2. Mechanical properties of fasteners
Automotive fasteners account for only 1% of the total vehicle cost, but in terms of the number of parts of the vehicle, 70% of the total vehicle assembly workload is mostly installed fasteners. The reliability of a car is to a large extent related to the quality of the fastener itself and the quality of the assembly. Low cost, large dosage, and important role are the unique features of automotive fasteners.
Bolts and nuts are the most practical fastening combinations in the entire industrial field. They can withstand high working loads and can be disassembled for reuse. Bolted joints are created based on detachable couplings that can be joined by one or more sets of bolts. In this way, the connected parts can move or move like two pieces. The bolted joint must have the function of resisting external force, so that the connected parts will not move, otherwise the bolted joint will loosen, break or even fall off. This is why the bolts designed by the engineer must be able to withstand the various functions imposed on it while performing its functions. The assembly is done by bolting, the task of which is to fasten the assembled parts together so that external forces cannot separate them. Therefore, when designing the bolt connection, the force acting directly around the screw should be specially considered, that is to say, the mechanical properties of the bolt directly affect the coupling performance of the bolt.
3, metal corrosion mechanism
Surface anti-corrosion treatment refers to the application of a protective layer on the metal surface by various methods. The function of the surface is to isolate the metal from the corrosive environment to inhibit the generation of corrosion or reduce the contact between the corrosive medium and the metal surface to prevent or mitigate The purpose of corrosion. The protective layer should meet the following requirements: 1. Corrosion resistance, anti-wear, high hardness; 2. Tight structure, completeness and small gap; 3. Strong bonding with the base metal and good adhesion; 4. Uniform distribution and certain thickness. The protective layer is usually divided into two types: metal coating and non-metal coating. Metal coating means that the corrosion-resistant metal forms a protective layer on the easily corroded metal surface. This coating is also called coating. There are many methods and types of metal plating, the most common being electroplating, followed by molten metal immersion plating (hot dip galvanizing) and chemical surface treatment.
The non-metallic coating means that a protective layer is formed on the metal surface with an organic polymer material such as Teflon, and an inorganic material such as ceramic. The protective layer completely isolates the base metal from the environmental medium and prevents corrosion of the metal substrate due to contact with other media.
Electroplating means that in a solution containing metal ions to be plated, a plated material or article is used as a cathode, and plating can be obtained on the surface of the substrate by electrolysis. During the electrolysis process, a chemical reaction takes place at the interface between the electrode and the electrolyte, an electron is emitted from the anode to cause an oxidation reaction, and a cathode absorbs electrons to undergo a reduction reaction. Electroplating can cover a layer of material or article with a relatively uniform and good bonding force to change its surface characteristics and appearance to achieve the purpose of protecting or decorating the material. DC (low voltage, high current) is required for electroplating. The current density and energization time can be appropriately controlled to obtain the required plating thickness.
The fasteners are galvanized. Because zinc is cheaper than other metals for electroplating, and the corrosion resistance of zinc is far superior to steel in most cases, the galvanized layer can protect in discontinuous parts, small holes, and cracks. And the steel exposed at the edge. Therefore, general fasteners or fasteners that are susceptible to atmospheric corrosion are mostly galvanized. The fasteners do not have the same shape or smooth surface as the sheet or bar, so the thickness of the galvanized layer is difficult to be uniform in all parts, and the thickest part of the head, the crest of the thread and the flank, and the thinnest thread valley are often present.
4, the impact of surface treatment on mechanical properties
Processes for producing high-strength fasteners include: spheroidizing annealing of wire, pickling, phosphating and saponification, fastener forming, pre-cleaning, austenitizing, quenching, intermediate cleaning, tempering, surface treatment (eg plating) And re-tempering treatment for hydrogen removal. Although spheroidized in a nitrogen and natural gas protective atmosphere, hydrogen is still absorbed during the annealing process, and hydrogen is also absorbed during pickling and phosphating. If the fasteners are not clean, further hydrogen embrittlement can occur during subsequent heat treatment.
Before the subsequent heat treatment, the surface of the fastener must be cleaned so that there are no non-metallic contaminants. It is very important that if the lubricant and phosphorus remain on the surface of the fastener, they will diffuse to the substrate. Medium, and form triangular ferrite, which increases the absorption of hydrogen and causes brittle fracture. Based on this, pre-cleaning before heat treatment is very important.
The pre-cleaning process consists of degreasing, descaling, rinsing and drying processes. If the fastener does not eliminate the scale, it may cause brittle cracking of the subsequent heat treatment.
The quenching and tempering of the fasteners is usually carried out in a protective atmosphere in which the endothermic reaction is concentrated with propane, and the mixture of nitrogen and methanol is also used as a protective atmosphere in the furnace. Hydrogen in the protective atmosphere varies between 31% and 67% depending on the composition of the carrier gas. Hydrogen molecules penetrate the surface of the fastener and are dissolved at the austenite temperature. The solubility of hydrogen in the matrix is ​​6 times higher than that at high temperatures. Therefore, after quenching, the hydrogen content is found inside the fasteners. When the temperature exceeds 300 °C, the hydrogen will penetrate into the matrix. .
Surface treatment is usually electroplating, coating or phosphating. All surface treatments have a common premise that the black layer must be removed, or the surface of the fastener is metallic. In order to obtain the surface of the metal light, the part must be Pickling treatment.
Pickling is carried out in dilute hydrochloric acid. The fasteners are soaked in hot hydrochloric acid for 10 to 15 minutes. It can be considered that during the pickling treatment to the surface treatment (such as electroplating), most of the hydrogen is immersed. Fastener surface. In order to avoid or reduce the risk of fracture caused by delayed hydrogen, the fastener is subjected to continuous heat treatment after the surface treatment of the plating. This reannealing process is carried out at a temperature of about 200 ° C. The treatment time depends largely on the acid. The amount of hydrogen absorbed during the wash process and the permeability of the coating applied to the surface of the fastener. The sole purpose of the fastener reannealing process is to accelerate the bleed out of hydrogen.
The atomic hydrogen produced by the fastener degreasing, pre-plating pickling and electroplating process is easily adsorbed by the base metal and enters the interior of the metal. High-strength fasteners thus create a hydrogen embrittlement hazard. When a hydrogen atom enters a steel matrix, under stress, it causes a decrease in toughness and load-bearing force, and a fracture or sudden brittle failure occurs. Products with this phenomenon cannot be found to have reduced toughness by conventional tensile tests, which are delayed brittle failure or hydrogen stress fracture caused by hydrogen embrittlement. In order to reduce the hydrogen embrittlement sensitivity of the fastener after plating, any fastener with tensile strength ≥ 1050Mpade steel should be dehydrogenated within 4 hours after electroplating.
Oxidation treatment is to immerse steel in an alkaline solution containing oxidant, such as potassium nitrate or a solution of sodium nitrate and sodium hydroxide, and keep the boiling point of about 140 ° C, or immerse the steel in the melting of sodium nitrate grade potassium nitrate. In the salt, it is taken out and quenched in water, and a black oxide or iron oxide film is obtained on the surface thereof. The composition is a mixture of ferrous oxide and iron oxide, which is a simple and economical method for improving the corrosion resistance of steel. The thickness of the steel oxide film is extremely thin, and the higher the oxidant concentration of the solution during the oxidation treatment, the faster the rate of the oxide film is formed, and the film layer is dense and firm. The concentration of the alkali in the solution is appropriately increased, the thickness of the oxide film is increased, and the content of the alkali is too low, and the oxide film is thin and brittle. The temperature of the solution is appropriately increased to increase the oxidation-induced density. The higher the carbon content of the workpiece, the easier it is to oxidize and the shorter the oxidation time. The oxidation treatment time is mainly adjusted according to the carbon content of the steel and the oxidation requirements of the workpiece. The length of the adjustment time and the concentration of the oxidizing solution directly affect the performance of the product. High-strength bolts with tensile strength higher than 1000Mpade may cause alkali brittleness, resulting in spontaneous cracking under internal or external stress during black oxidation, ie high-strength fasteners with a residual stress of 10.9 or higher in lye Oxidation easily causes "alkali brittleness", resulting in the performance of fasteners not functioning properly.
5 Conclusion
1. When selecting high-strength fasteners, the surface should be specified in the technical documentation and indicate that the dehydrogenation treatment should be carried out in time after electroplating and before chromate passivation (within 4 hours). When the dehydrogenation treatment is incomplete or overlaps with the heat treatment temper brittleness, the function of the high-strength fastener is lost, and the consequences are very serious.
2. High-strength fasteners above 10.9 are prohibited from surface oxidation treatment in alkaline solution. Otherwise, “alkaline brittle†cracking will occur, resulting in normal performance of fasteners and fatal defects.
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