Heat treatment of spring steel

Root forming method The manufacture of spring steel can be divided into two types: cold forming and hot forming (also called forming after strengthening and strengthening after forming).
1. Heat treatment of cold formed spring steel

For small springs, such as coil springs or spring steel strips with a wire diameter of less than 8 mm, they can be formed after heat treatment strengthening or cold deformation strengthening, that is, cold drawing and cold coil forming. The cold drawn steel wire has high strength, which is obtained by the work hardening of the cold drawn deformed steel. Cold drawn spring steel wires can be divided into three situations according to their different strengthening processes:

(1) Lead bath austempered cold drawn steel wire. That is to say, the wire rod is first cold drawn to a certain size, heated to Ac3+80~100 austenitized, quenched in a 450~550 lead bath to obtain a fine flake pearlite structure, and then cold drawn many times to the required diameter. By adjusting the carbon content of the steel grade and the cold-drawn type variable (the type variable can reach 85~90%), the spring steel wire with high strength and certain plasticity can be obtained. This lead-quenching wire drawing treatment is actually a kind of deformation heat treatment, that is, the pearlite deforms after it is deformed, and the steel wire strength can reach about 3000 MPa.

(2) Cold drawn steel wire. This kind of steel wire is mainly strengthened by cold drawing deformation, but it is different from lead-quenched cold-drawn steel wire. It adds an intermediate spheroidizing annealing of about 680 degrees in the middle of the cold-drawing process to improve the plasticity, so that the steel wire can continue to be cold drawn. The required final size has a lower strength than the lead quenched cold drawn steel wire.

(3) Quenched and tempered steel wire: This steel wire is cold drawn to the final size, then quenched and tempered at a medium temperature, and finally cold rolled into shape. The disadvantage of this kind of strengthening is that the process is more complicated, and the strength is lower than that of lead quenched cold drawn steel wire.

The steel wire strengthened by the above three methods must be subjected to a low temperature tempering process after cold coil forming, the tempering temperature is 250-300, and the tempering time is 1 hour. The purpose of low temperature tempering is to eliminate stress, stabilize size, and improve elastic limit. In practice, it has been found that the elastic limit of steel wire that has been strengthened is often not high after cold coil forming.

This is because cold coil forming will be easy to move and the number of errors will increase, and the initial plastic deformation resistance is reduced due to the Bauschinger effect. Therefore, it is necessary to perform a low-temperature tempering after cold coil forming, which has caused a multi-change process and increased the elastic limit.

2. Heat treatment of hot formed spring

Hot forming springs generally combine quenching and hot forming, that is, the heating temperature is slightly higher than the quenching temperature. After heating, the hot coil is formed, then quenched with waste heat, and finally tempered at 350 to 450 degrees at a medium temperature to obtain tempered yield. The body structure. This is a thermomechanical heat treatment process, which can effectively improve the elastic limit and fatigue life. Generally, large leaf springs on automobiles use this method. The neutral coil spring can also be formed under cold conditions, and then quenched and tempered.

In order to further exert the performance potential of spring steel, three points should be sacrificed during heat treatment of spring:

(1) Spring steel is mostly silico-manganese steel, silicon has the effect of promoting decarburization, and manganese has the effect of promoting grain growth. The fatigue strength of surface decarburization and grain growth is greatly reduced. Therefore, the heating temperature, heating time and heating medium should be selected and controlled. Such as rapid heating by salt furnace and heating under protective atmosphere. It should be tempered as soon as possible after quenching to prevent delayed fracture.

(2) The tempering temperature is generally 350 to 450 degrees. If the steel surface is in good condition (such as after grinding), the low temperature tempering should be used; otherwise, the upper temperature tempering can be used to improve the toughness of the steel and reduce the sensitivity to surface defects.

(3) Spring steel has a high silicon content, and the steel is prone to graphitization during the retreat process, which must be paid attention to. Generally, the graphite content is required to be tested when steel enters the factory.

Spring steel heat treatment method

Spring steel is divided into two types: hot forming and cold forming according to its processing and forming methods. Due to the different processing methods, the subsequent heat treatment methods are also different, as follows:
1 Heat treatment of hot formed spring

Large springs with a diameter or plate thickness greater than 10-15mm are mostly made of steel wire or steel plate drawn from hot-rolled wire rods.

Processing and heat treatment are as follows: first heat the spring steel wire to a temperature 50-80℃ higher than the normal quenching temperature, and then heat it to form, then quench + tempering at medium temperature to obtain tempered sorbite with excellent elastic limit and fatigue strength. Spring steel quenching and heating should use low-oxygen or non-oxidizing equipment such as salt bath furnaces and protective atmosphere furnaces to prevent oxidative decarburization.

After heat treatment, spring steel must be shot peened to strengthen the surface, generate residual compressive stress, and improve fatigue strength.

The process of hot-rolled spring steel is: flat steel shearing -> residual heat quenching after heating and bending forming + medium temperature tempering + shot peening -> packaging.

2 Heat treatment of cold formed spring

Spring parts with a diameter less than 8mm are usually formed by cold drawn steel wire and cold coil. The cold-drawn steel wire manufacturing process and subsequent heat treatment mainly fall into the following three categories:

1) Lead bath treatment cold drawn steel wire

First, the steel wire is drawn continuously for three times, the total deformation reaches 50%, and then it is heated to a temperature above Ac3 to austenitize, and then treated in a lead bath at 450-550℃ at an intermediate temperature to transform austenite into sorbite structure . The yield strength is 1600Mpa. After the cold coil is formed, it can be annealed at 200-300℃ to eliminate the stress.

2) Oil quenched and tempered steel wire

After the steel wire is drawn to the processed size, it is oil quenched and tempered. The strength of this kind of steel wire is not as good as that of the steel wire treated by the lead bath, but the performance is uniform and the cost is lower. After the cold roll is formed, it is subjected to stress relief.

3) Steel wire in annealed condition

The steel wire is drawn to the specified size and then annealed. After the softened steel wire is cold coiled, it needs to be quenched and tempered at medium temperature to obtain the required mechanical properties.

Surface treatment of spring

In recent years, the surface finishing (finishing) treatment of springs has also been recognized by spring designers. 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. Normally, it takes about 20~30min. The specific time depends on the shape of the spring and the amount of the device.

After finishing, take out the spring and rinse it with tap water, then immerse it in the SM series water film replacement anti-rust oil for several minutes and then take it out. At this time, a layer of 5μm anti-rust oil film is attached to the surface of the spring, which protects the spring from corrosion. This treatment method greatly supports the traditional complicated procedures, such as degreasing, cleaning, pickling, and dehydrogenation.

At present, the springs adopting the finishing process include: plunger springs for oil pump nozzles and pressure regulating springs for fuel injectors; support springs for refrigerator compressors. The comparison of performance (life) and corrosion resistance between the light-finished (finished) and conventional oxidized or plated springs does not wait for further tests and practices to draw more reliable conclusions.
The corrosion of the spring can be divided into chemical corrosion and electrochemical corrosion according to the type of reaction. They are the result of the change of the metal atoms on the surface of the spring or the gains and losses of electrons into ionic states.

If the metal on the surface of the spring only reacts chemically with the surrounding medium, the corrosion caused by the spring is called chemical corrosion. For example, the spring oxidizes in a particularly dry atmosphere to form an oxide film, and the spring chemically changes with the liquid or the impurities in the liquid in the non-electrolyte liquid, etc., which belong to chemical corrosion.

If the spring is in contact with the electrolyte solution, the corrosion caused by the action of the micro battery is called electrochemical corrosion. For example, the spring is in contact with acid or salt solutions. Such solutions are all electrolytes. Due to defects or impurities on the surface of the spring, electrodes with different potential differences are formed, so that the spring is constantly subject to electrolytic corrosion. Another example is that the spring is in a humid atmosphere.

The water vapor in the atmosphere condenses into a water film or water droplets on the surface of the spring, and the corrosive gases in the atmosphere (such as sulfur dioxide and hydrogen sulfide in industrial waste gas or salt spray in the ocean atmosphere) are dissolved in the water film or water Electrolytes are formed in the beads. In addition, the impurity or defects of the spring metal can also form electrodes with different potential differences, and the spring also produces electrolytic corrosion. These are all electrochemical corrosion.

The chemical corrosion of the spring is small and slow, while the electrochemical corrosion is the main and common. But generally speaking, chemical corrosion and electrochemical corrosion exist at the same time.

In the process of manufacturing, storage, and use, the spring often suffers from the corrosion of the surrounding medium. Since the spring works by elastic force, the elastic force will change and lose its function after the spring is corroded. Therefore, preventing the corrosion of the spring can ensure the stable operation of the spring and prolong its service life.

The anti-corrosion method of spring generally adopts a protective layer. According to the nature of the protective layer, it can be divided into: metal protective layer, chemical protective layer, non-metal protective layer and temporary protective layer, etc. The first three methods are introduced here.

Stainless steel springs and copper wire springs have certain anti-corrosion ability, so anti-corrosion treatment is generally not carried out.

1. The metal protective layer of the spring

There are many types of metal protective layers. In terms of springs, gold plating is generally used to obtain the metal protective layer. The electroplated protective layer can not only protect from corrosion, but also improve the appearance of the spring. Some electroplated metals can also improve the working performance of springs, such as increasing surface hardness, increasing wear resistance, improving thermal stability, and preventing radiation corrosion.

But if it is purely for the corrosion of the spring, the electro-galvanized layer and the electro-cadmium layer should generally be used.

Zinc is relatively stable in dry air, hardly changes, and is not easy to change color. A white film of zinc oxide or carbon zinc carbonate is formed in humid air. This dense film prevents further corrosion. Therefore, the galvanized layer is used as the anti-corrosion protection layer of the spring under normal atmospheric conditions. All springs that are in contact with solutions such as sulfuric acid, hydrochloric acid, caustic soda, and working in humid air such as sulfur trioxide, should not be coated with zinc.

Generally, passivation treatment is carried out after the galvanized layer, which can improve the protective performance of the coating and increase the appearance of the surface.

In oceanic or high-temperature atmospheres, springs in contact with seawater, and springs used in hot water at 70°C, cadmium is relatively stable and has strong corrosion resistance. Cadmium coating is brighter and more beautiful than zinc coating, softer, and has better plasticity than zinc. The coating has less hydrogen embrittlement and is most suitable for springs as a protective layer. But cadmium is scarce, expensive, and cadmium salt is highly toxic, which is very harmful to the environment. Therefore, it is restricted in use. Therefore, most of the springs used in aviation, navigation and electronics industries use cadmium plating as a protective layer.

Car spring modification

The coil spring is the most commonly used spring of the suspension system because it is easy to manufacture, high performance and efficiency, and low price. The definition of a spring in physics is to store energy. When we apply a fixed force to the spring, it will deform. When we remove the force, the spring will have a tendency to return to its original state, but the spring will shake when it rebounds. The amplitude tends to exceed its original length, and it will not slow down the free shock caused by the spring rebound until there is frictional resistance, which is usually the task of the shock absorber.

The general spring is the so-called “linear spring”, that is, when the spring is stressed, its compression deformation follows the “Hooke’s law” in physics: F=KX, where F is the applied force, K is the elastic force coefficient, and X Is the amount of deformation. For example, when a linear spring is subjected to a force of 40Kg, it will cause a compression of 1cm, and then every increase of 40Kg of applied force by 1cm will definitely increase the amount of compression. In fact, there are other pressures on the suspended springs. Even when the springs are fully extended, the springs are still under pressure to fix the springs on the car.

In the traditional spring and shock-absorbing cylinder suspension design, the spring acts to support the body and absorb the impact of uneven roads and other forces on the tires. The so-called other forces here include acceleration, deceleration, braking, turning, etc. The force caused by the spring. What’s more important is to keep the tires in constant contact with the road during the vibration elimination process to maintain the tracking of the car. Improving the contact between the tire and the road is our primary consideration for improving handling.

The main function of the spring is to maintain the comfort of the car and keep the tires in full contact with the ground. Using the wrong spring will have a negative impact on the driving quality and handling. Imagine if the spring is completely rigid, the suspension system will not work. When the car jumps up on uneven roads, the tires will completely leave the ground.

If this happens when accelerating, braking or turning, the car will lose track. If the spring is very soft, it is easy to “sit on the bottom”, which means that the suspension travel will be exhausted. If sitting on the bottom occurs when cornering, it can be considered that the spring coefficient becomes infinite (there is no compressed space), and the body will have an immediate weight transfer, resulting in loss of tracking.

If this car has a long suspension stroke, it may be possible to avoid the “sit bottom” situation, but the relative body will also become very tall, and a very tall body means a very high center of gravity of the body. The high or low has a decisive influence on the handling performance, so too soft shock absorbers will cause handling obstacles. If the road is absolutely flat, then we don’t need springs and suspension systems.

If the road is rough, a softer spring is needed to ensure that the tire is in contact with the road, and the travel of the spring must be increased. The choice of spring hardness is determined by the ruggedness of the road. The more rugged the spring, the softer the spring, but how soft it is is a key issue. Usually this requires the accumulation of experience and is also an important issue for various car manufacturers and teams.

Generally speaking, a soft spring can provide better comfort and maintain better tracking performance when traveling on a rougher road. However, when traveling on general roads, the suspension system will swing up and down, which will affect the handling. In a car equipped with good aerodynamic components, the soft spring will cause the height of the car to change when the speed increases, resulting in different handling characteristics at low and high speeds.

actual

The front suspension system of Xiaoqiu has two types of coil springs: 9 coils and 10 coils. The former is dedicated for 4-cylinders and the latter is dedicated for 6-cylinders. In the case of not increasing the front counterweight, generally there is no need to change the modification, just add a set of rubber pads. However, after replacing the front bumper and installing a winch, you must change the coil spring with more turns to ensure the controllability of the vehicle and the life of the related mechanical parts.

Wire processing technology

Metal cold processing is an important part of steel industry production. There are many types of metal cold processing, such as: steel wire, steel wire rope, steel strand and other metal products, etc. The production of special steel wires has a history of nearly 70 years in my country. It is a typical industry in the metal cold processing industry.
With the continuous development of the industry, the varieties of special steel wires (hereinafter referred to as steel wires) have been increasing, and the quality and quality have been continuously improved. The old workshop-style production model is gradually replaced by semi-automated and automated production. However, no matter how the equipment changes, in terms of steel wire production, the production method of steel wire is actually two lines: continuous line and periodic line.

1 Basis for steel wire production

Steel wire production is mainly based on the product standard requirements to determine the production method. Product standards are the link between suppliers and customers. According to product standards, the use of reasonable technology and scientific management are the basic guarantees to ensure that the products are qualified.

In the production of steel wire, the same steel grade (same chemical composition) adopts different standards, and the production methods are different. Take T9A as an example: when the GB/T4357-89 elastic spring steel wire standard is implemented, continuous line production must be used, and the finished steel wire produced can be used to manufacture various springs and weave wire ropes. When implementing the GB/T5952-86 elastic tool steel wire standard, it is necessary to adopt the cycle line production, and the finished steel wire produced can be used for manufacturing tools. Such as: knives, drills, needles, etc. Because the steel wires produced by continuous and periodic lines have different microstructures, different mechanical properties, and different process properties, they have different uses.

It is worth clarifying: whether it is continuous line production or periodic line production, heat treatment, surface treatment, processing deformation, three elements are indispensable.

2 Steel wire production factors

2.1 Steel wire heat treatment

Heat treatment is one of the main elements of steel wire production. Some people outside the industry thought: “Wire drawing is cold processing, simple operation, no heat treatment, as long as there is a corresponding die for extrusion and drawing.” This is a misunderstanding of steel wire production. The so-called cold working refers to the deformation and production process carried out at room temperature. Before the steel wire is processed and deformed, heat treatment must be used to improve the internal quality of the steel, achieve the microstructure required by the standard, and obtain good physical shrinkage and ductility in order to smoothly process and deform. This is the important point of heat treatment.

The heat treatment of steel wire production has four steps: they are raw material heat treatment, intermediate heat treatment, pre-finished heat treatment, and finished product heat treatment (excluding products delivered in cold drawn state).

There are 8 main types of heat treatment:

(1) Complete annealing: The steel wire is heated to complete austenitization and slowly cooled to obtain a structure close to equilibrium;

(2) Incomplete annealing: the steel wire is heated to a temperature between Ac1 and Ac3 to achieve incomplete austenitization, followed by slow cooling;

(3) Spheroidizing annealing: heating the steel wire (wire rod) to above Ac1 temperature, so that the carbide of the steel reaches the spheroidizing state;

(4) Recrystallization annealing: the cold-worked and deformed steel wire is heated to above the recrystallization temperature and kept for a proper time to recrystallize the deformed grains into uniform equiaxed grains and eliminate cold work hardening;

(5) Bright annealing: the steel wire is annealed in a protective atmosphere or vacuum to prevent oxidation and keep the surface of the steel wire bright;

(6) Normalizing: Heat the steel wire to 30~50℃ above Ac3, keep it warm for an appropriate time, and then cool it;

(7) Soxtenitizing treatment (Pedentoff treatment): After austenitizing medium-carbon or high-carbon steel wire, quickly move it to an appropriate temperature (about 500℃) below Ar1 and cool it in the middle temperature or air. , To obtain the sorbite structure;

(8) Aging treatment: After the steel wire undergoes solution treatment or cold shrinkage deformation, it is kept at room temperature or a certain temperature to achieve the purpose of precipitation hardening (1).

According to the specific conditions of steel wire production, such as: wire status-raw materials, semi-finished products, pre-finished products, finished products; product standard requirements; it is very important to determine the type of heat treatment. Of course, the heating method directly affects the heat treatment quality. So far, there are many heat treatment and heating methods for cold working in my country, each with its own advantages.

2.1.1 Heating method

There are five heating methods:

(1) Coal-fired heating;

(2) Coal burning-local gas heating;

(3) Natural gas heating;

(4) Pipeline gas heating;

(5) Electric heating.

The first heating method has a lower cost, but the furnace temperature is not easy to stabilize, and the surface of the steel wire is prone to decarburization; the second heating method has a slight advantage over the first method; the third and fourth heating methods are superior. The temperature is relatively stable; the fifth heating method is the most ideal. Although the cost is higher, it has no pollution, stable furnace temperature, uniform heating, and the internal quality of the product will be greatly improved. It is the most wise choice for the production of high-grade steel wire.

2.1.2 The main purpose of heat treatment

There are four main purposes for heating wire rod (raw material) or steel wire:

One is sorbitization (grain refinement) to improve tensile strength, increase toughness and ductility;

The second is spheroidizing annealing to obtain a reasonable microstructure and increase shrinkage and ductility;

The third is recrystallization annealing to recrystallize deformed grains into uniform equiaxed grains, eliminate cold work hardening, and facilitate cold work again;

The fourth is softening annealing, no microstructure change, only to eliminate processing stress, meet the mechanical requirements (mostly used for finished steel wire).

2.2 Surface treatment

Surface treatment is the second major element of steel wire production. There are four procedures for surface treatment: one is to remove the oxide scale (oxide film) produced by heat treatment; the other is to neutralize, the purpose is to prevent the residual acidic substances from corroding the steel matrix; the third is to coat, to increase the carrier to ensure lubrication, there is Conducive to cold working deformation and ensure the surface quality of the steel wire; the fourth is to remove harmful impurities on the surface (including decoating) to ensure that the steel wire is corrosion-free and the surface is smooth and clean.

2.3 Cold working deformation

Cold working deformation (hereinafter referred to as drawing) is the third and most intuitive element of steel wire production. This element is the main process of steel wire production.

Stainless steel grade group introduction

200 series—chromium-nickel-manganese austenitic stainless steel
300 series—chromium-nickel austenitic stainless steel

Model 301—Good ductility, used for molded products. It can also be hardened by mechanical processing. Good weldability. Abrasion resistance and fatigue strength are better than 304 stainless steel.

Model 302—The corrosion resistance is the same as that of 304, and the strength is better due to the relatively high carbon content.

Model 303—It is easier to cut than 304 by adding a small amount of sulfur and phosphorus.

Model 304—general model; that is, 18/8 stainless steel. The GB grade is 0Cr18Ni9.

Model 309—Compared with 304, it has better temperature resistance.

Model 316-after 304, the second most widely used steel grade, mainly used in food industry and surgical equipment, adding molybdenum to obtain a special corrosion-resistant structure. Because it has better resistance to chloride corrosion than 304, it is also used as “ship steel”. SS316 is usually used in nuclear fuel recovery devices. 18/10 grade stainless steel usually also meets this application level. [1]

Model 321—Except for the addition of titanium to reduce the risk of corrosion of the material welds, other properties are similar to 304.

400 series-ferritic and martensitic stainless steel

Model 408—Good heat resistance, weak corrosion resistance, 11% Cr, 8% Ni.

Model 409—the cheapest model (British and American), usually used as car exhaust pipe, is a ferritic stainless steel (chrome steel).

Model 410—Martensite (high-strength chromium steel), with good wear resistance and poor corrosion resistance.

Model 416—Add sulfur to improve the processing performance of the material.

Model 420—”tool grade” martensitic steel, similar to the earliest stainless steel such as Brinell high chromium steel. It is also used for surgical knives, which can be very bright.

Model 430—Ferritic stainless steel, for decoration, such as car accessories. Good formability, but poor temperature resistance and corrosion resistance.

Model 440—High-strength cutting tool steel with slightly higher carbon content. After proper heat treatment, higher yield strength can be obtained. The hardness can reach 58HRC, which is among the hardest stainless steels. The most common application example is “razor blades”. There are three commonly used models: 440A, 440B, 440C, and 440F (easy processing type).

500 series—heat-resistant chromium alloy steel.

600 series—Martensitic precipitation hardening stainless steel.

Model 630-the most commonly used precipitation hardening stainless steel model, usually also called 17-4; 17% Cr, 4% Ni.

Galvanized

Galvanized
7.1 Properties of zinc and zinc coating

(1) Color: light gray metal

(2) Proportion: 7.14

(3) Atomic weight: 65.37

(4) Atomic value: 2

(5) Standard potential: -0.7628v

(6) Melting point: 419℃

(7) Atomic number: 30

(8) Boiling point: 907℃

(9) Density: 7.13

(10) Characteristics: brittle. Relatively hard. It has better plasticity when heated to 100~150℃. It can be rolled and forged, but it becomes brittle at 250.

Zinc is relatively stable in dry air, and the white mold layer that easily forms carbonate or oxide on the surface in water and humid air is protective

, Zinc is an amphoteric metal, which corrodes in acid, sulphide and sulphide. The standard potential of zinc is relatively negative, so its coating is an anodic protection.

Low cost, easy to process, good effect, many steel parts are galvanized, but it is not easy to make frictional coating

7.2 Zinc Plating

(1) Acid zinc plating

(2) Alkaline non-cyanide zinc plating (alkaline plating)

(3) Cyanide zinc plating

7.3 Acid zinc plating

The simple shape of plating parts and the base paint are used more, and the advantages and disadvantages are as follows

advantage

(1) Available gloss, smooth, galvanized coating similar

(2) Can be directly plated on carbonized and nitrided steel and cast iron

(3) Higher current efficiency

(4) The waste liquid is easy to handle, just use high pH to precipitate zinc

(5) Good conductivity, saving electricity

(6) Low hydrogen embrittlement

(7) Glossy coating can be obtained at higher temperature

(8) The plating bath is stable, with low toxicity and low cost

Disadvantage

(1) The plating bath is highly corrosive, and the plating tank and auxiliary equipment need to be lined

(2) Welding and assembly plating parts are not suitable, there will be bleadout

(3) Poor ductility of thick coating

(4) Rough crystals

(5) Poor uniformity

(6) Filtering, cooling pipes and refrigeration equipment need to have

7.4 Alkaline Non-Cyanide Zinc Plating

advantage

1. Low toxicity

2. The waste liquid is easy to handle, only the zinc is precipitated

3. Low cost

4. Good uniformity

5. Available just plating tank

Disadvantage

1. The composition of the plating bath needs to be strictly controlled and analyzed every day

2. The pre-treatment requires high-quality springs

3. When the zinc content is small, the current efficiency is reduced

4. Sensitive to metal impurities and hard water impurities

5. The coating is cyanide galvanized and brittle

6. Additives are needed, otherwise the dark coating will appear

7. Special additives, non-general raw materials, need to be provided by the manufacturer

7.4.1 Alkaline-non-cyanide bath

Can be formulated in two ways

1. Anode zinc is soluble in caustic solution

2. Dissolve zinc oxide and caustic

The preparation procedure is as follows

1. Use a small amount of water to mix with zinc oxide in the plating tank to form a slurry, or it is best to use another matching bath to do it

2. Slowly stir caustic soda and water into one-third of the amount of water

3. Stir fully until the zinc oxide is completely dissolved

4. Add 6 lbs/100 gallons of zinc powder and stir for 30 to 60 minutes

5. Precipitate and filter or pour into the plating tank

6. Put in the anode zinc plate and use low current density electrolysis to remove impurities for one night

7. Analyze the content of zinc and caustic soda and adjust the ingredients to add additives

The formula is as follows

Zinc oxide 1.5 oz/gal

Caustic soda 10 oz/gal

Addition agents as indicated

Bath temperature 70 ~115 F

7.5 Gloss cyanide galvanized spring

Advantages

1. Long history of use and rich experience

2. The pre-processing requirements are not strict

3. Excellent coverage

4. The analysis and control of bath ingredients are relatively easy

5. Good coating ductility

6. The plating bath is less corrosive

Lack

1. The plating bath is highly toxic and must be treated with cyanide waste

2. Heat treatment and cast iron parts cannot be plated

3. Higher temperature operation is required, otherwise the gloss coating cannot be obtained, and the temperature must be above 105F

4. Poor conductivity of the plating bath

5. Under high current density, current efficiency drops sharply

Cadmium plating

8.1 Properties of cadmium and cadmium coating

(1) Color: off-white

(2) Hardness: 300~500 Mpa

(3) Atomic weight: 112.40

(4) Proportion: 8.65

(5) Melting point: 321C

(6) Atomic value: 2

(7) Standard potential: -0.4029V

(8) Atomic number: 48

(9) Resistance: 6.83m W –cm

(10) Strength: 70Mpa

(11) Boiling point: 767C

(12) Chemical properties: Similar to zinc, but not dissolved in lye, does not change color in room temperature air, and will oxidize in humid air

The protective film can prevent continued oxidation and corrosion. Cadmium oxide is insoluble in water. Cadmium is less soluble in acid than zinc, but has a stronger effect on nitric acid.

Cadmium produces toxic gases when it melts, and its soluble salts are also toxic. Cadmium is a cathodic protection coating on steel substrates, but at high temperatures

Or in the marine climate, it is a sacrificial anode protective coating.

8.2 Cadmium plating

In the marine environment and high temperature (above 70C) hot water, the cadmium coating has strong corrosion resistance and is relatively stable. Good weldability and lubricity

, Strong contact resistance, good resistance to alkaline solution, low hydrogen embrittlement, good gloss and strong adhesion. So aviation, aviation

Many parts in the sea and electronics industry are plated with cadmium. The cadmium coating is easy to polish and can be used as the bottom layer of the paint after phosphate.

Toxic, food utensils cannot be plated with cadmium.

8.3 Types of cadmium electroplating bath

It can be divided into the following four types:

(1) Alkaline cyanide plating bath. (3) Acidic boron fluoride salt plating bath.

(2) Alkaline sulfate plating bath. (4) Neutral sulfate plating bath.

The type of plating bath should be selected according to the shape of the plated part, the requirements of the plating layer, and the uniformity of the plating bath. General simple shape plating

An acid plating bath is used, while an alkaline cyanide plating bath is used for plating parts with complex shapes. Alkaline plating bath has fine crystals and good uniformity.

However, it is toxic, troublesome to dispose of the waste liquid, the electroplating bath is unstable, and the current efficiency of the anode and the anode is different.

Sodium carbonate and hydrocyanic acid. The acidic bath is less hydrogen embrittlement.

8.4 Alkaline cyanide cadmium plating bath formulation

Cadimum oxide 3 oz/gal

Sodium carbonate 4~8 oz/gal

Sodium cyanide (sodium cyanide) 13.5 oz/gal

Sodium hydroxide (sodium hydroxide) 1.9 oz/gal

Current density 5~90 ​​ASF

The bath temperature is 60~100. F

8.4.1 Preparation of alkaline cyanide cadmium plating bath

The steps are as follows:

(1) First check whether the quality of the medicine is in compliance.

(2) Strictly observe the safety rules for the use of highly toxic substances and good ventilation equipment.

(3) Dissolve the required amount of sodium cyanide in a small amount of hot water (50-60°C), and the temperature should not be too high to avoid decomposition.

(4) Dissolve the necessary caustic soda in water in another container.

(5) Pour the caustic soda solution into the sodium cyanide solution.

(6) Mix the required cadmium with water to form a paste, slowly add it to the above-mentioned mixture, stir it fully, and dissolve completely, the temperature can be raised

But not higher than 85°C.

(7) Dissolve other additives in an appropriate amount of water and add them in sequence.

(8) Add 1.5 to 2 pounds of zinc powder per 100 gal, stir fully for 30 minutes, let the solution stand for 4 hours, and filter

Into the plating tank.

(9) Sampling, analysis and calibration of each component.

(10) Air electrolysis with 0.25~0.5V current for 24~48 hours.

(11) Add gloss agent.

(12) The plating bath is normalized with 10ASF plating about two angels.

8.4.2 Additives for alkaline cyanide cadmium plating bath

Additives affect the structure and appearance of the coating in the electroplating bath. Inorganic additives such as nickel, cobalt, molybdenum and selenium compounds are mainly

Improve the appearance and physical properties of the coating, organic additives such as aldehyde, glucose, milk, sugar, gum, molasses

, Sulfonic acid, Turkish red oil, sulfonated castor oil, etc. promote fine-grained coatings, improve uniformity, and hide

Shield metal impurities. When the additive exceeds a certain amount, the coating will become brittle and generate bubbles.

8.4.3 Defects and causes of cyanide cadmium plating

(1) The coating is discolored and stained, the reasons are

1. The bottom metal has holes.

2. The pre-treatment is not clean.

3. Contact with dirty hands.

(2) The coating is burnt black, rough and dark, the reasons are

1. The current density is too large.

2. Too little free cyanide.

3. There are floating impurities.

(3) Poor coating adhesion and surface blistering due to

1. Insufficient cyanide.

2. The alkalinity is too high.

3. Poor pretreatment.

4. The plating part absorbs hydrogen when it is acidic.

(4) A large amount of cyanide gas is generated at the cathode, and the anode is in a crystalline state, and the coating is poor. The reasons are

1. Free cyanide is too high.

2. Insufficient metal salt.

(5) The anode dissolves unevenly and there is slag, the reasons are

1. Insufficient free cyanide.

2. Insufficient hydroxide.

 

8.5 Non-Cyanide Plating Baths

In recent years, due to pollution control and reduction of the formation of hydrogen during electroplating, many non-cyanide cadmium plating baths have been developed, which can be divided into neutral sulfur

Salt plating bath, acid borofluoric acid plating bath and acid sulfate plating bath.

Non-cyanide plating bath has high current efficiency and low hydrogen embrittlement, so high-strength steel should use non-cyanide plating bath. Of which borofluoric acid plating

The bath maintains high current efficiency even at high current density.

8.5.1 Neutral sulfate bath formulation

Ammonium chloride 1.5~3 oz/gal

Ammonium sulfate 10~15 oz/gal

Cadmium (content) 0.5~1.5 oz/gal

Current density 2~15 ASF

The bath temperature is 60~100. F

8.5.2 Formulation of acidic borofluoride cadmium plating bath

Ammonium fluoborate 8 oz/gal

Boric acid 3.6 oz/gal

Cadmium 12.6 oz/gal

Cadmium fluoborate 32.2 oz/gal

Current density 30~60 ASF

The bath temperature is 70~100. F

8.5.3 Formulation of acid sulfate cadmium plating bath

Cadmium oxide (cadmium oxide) 1~1.5 oz/gal

Sulfuric acid (salfuric acid) 4.5~5 oz/gal

Current density 10~60 ASF

The bath temperature is 60~90. F

8.6 Stripping and repair plating of cadmium plating

(1) Steel, copper and copper alloy plated parts: Dissolve and peel off the cadmium coating

1. Dilute hydrochloric acid. spring

2. 100g/l ammonium nitrate.

3. Waste chromic acid solution.

(2) Aluminum and its alloy plating parts: Dissolve and peel off the cadmium coating with 5-10% nitric acid.

(3) Plated parts with poor appearance: Some plated parts can be directly repaired without peeling off the coating. First perform electrolytic cleaning for 1 to 2 minutes,

Remove surface dirt and film, then activate, clean, and stay in the cadmium plating bath for 10 to 30 seconds, and then re-plating.

The time depends on the plating parts.

8.7 Process of cadmium plating

Inspection before plating→solvent degreasing→chemical or electrolytic cleaning→hot water washing→cold water washing

Electrolytic cleaning←cold water cleaning←on the shelf←neutralization←cold water cleaning←acid leaching rust removal

Hot water washing → cold water washing → pickling activation → cold water washing → cadmium plating → cold water washing → drying

Cold water washing ← passivation ← dry ← hot water washing ← cold water washing ← light out ← hydrogen removal

Warm water washing → drying → inspection

Flow Description:

Step 1: Check the surface condition and size of the plated parts.

Step 2: Clean the inside and outside of the parts.

Step 3: Make the surface of the parts wet with water, and clean the cathode first and then the anode after electrolysis. The anode time should be relatively long.

The spring leaf is anodized. Electrolyte composition 30~50g/l NaOH, 20~30g/l Na2CO3, 30~50g/l Na3PO4 and

10~15g/l Na2SiO3.

Step 6: The composition of the solution is 50~100g/l sulfuric acid and 100~150g/l hydrochloric acid.

Step 8: The solution is 50~70g/l sodium carbonate.

Step 11: Same as step 3.

Step 14: Use 30-50g/l sulfuric acid solution to remove the oxide film.

Step 19: Bake for 2~4 hours at 200~250℃.

Step 20: Soak in chromic acid solution for 0.1 to 0.5 minutes.

Step 24: The passivation solution consists of 80~150g/l sodium dichromate and 8~10g/l sulfuric acid. Special spring

8.8 Standards and specifications related to cadmium plating

(1) QQ-A-671 Federal Specification (cadmium anode standard specification)

(2) JIS H8611 Electroplated Coatings of Cadmium on Iron and Steel

(3) ASTM A165 Specification for Electrodeposited Coatings of Cadmium on Steel