Spring wire standard and application grade

    Springs are commonly used parts in the mechanical industry and in daily life. The main function of springs is to use the energy stored in their own deformation to mitigate the vibration and impact of machinery or components and control the movement of machinery or components.

1 , the use of spring wire characteristics and uses
    The spring is used within the elastic range and should be returned to its original position after unloading. It is desirable that the plastic deformation be as small as possible. Therefore, the steel wire should have high elastic limit, yield strength and tensile strength. The higher the yield ratio is, the closer the elastic limit is to the tensile strength, and the more it is possible to increase the strength utilization, the stronger the elasticity of the resulting spring.

Springs rely on elastic deformation to absorb impact energy, so the spring steel wire does not have to have a high degree of plasticity, but at least it must be able to withstand the plasticity of the spring forming, and enough toughness to withstand the impact energy.

Springs usually work under alternating stress for a long period of time, and therefore have high fatigue limits, as well as good resistance to creep and relaxation.

Springs used in specific environments also have special requirements for steel wires. For example, springs used in corrosive media must have good corrosion resistance. Springs used in precision instruments should have long-term stability and sensitivity, low temperature coefficients, high quality factors, little aftereffects, and constant elastic modulus. Springs operating at high temperatures require sufficient elastic limit and good creep resistance at high temperatures.

In addition, spring steel wire forming process and heat treatment process should also be considered. Cold drawn spring steel wire and oil quenched and tempered spring steel wire are all directly wound by the steel wire in the delivery state. After the spring is formed, it is directly used by stress relief treatment. The tensile strength of cold-drawn spring steel wire is slightly higher than that of oil quenched and tempered steel wire. Large-size cold-drawn steel wire is too elastic. It is difficult to wind the spring. Therefore, the cold-drawn spring wire should have a specification of less than 8.0 mm , and the oil quenched and tempered wire should have a specification of less than 13.0 mm . Actually, the spring of 13.0mm in diameter is selected from the light-pull state spring steel wire, cold-drawn and formed into a shape and then quenched and tempered. Most of the steel wires with a diameter of 15.0mm or more use springs for heating and winding.

The spring can be divided into a static spring and a dynamic spring according to the operating state. Static springs refer to springs with a limited number of vibrations during service, such as safety valve springs, spring washers, pan springs, constant load springs, mechanical springs, watch hairsprings, etc. Dynamic spring refers to springs with more than 1×106 vibrations during service period , such as engine valve springs, vehicle suspension springs, shockproof springs, coupling springs, and elevator buffer springs. The static strength and stability are mainly considered in the selection of static springs, and fatigue, relaxation, and resonance performance are mainly considered in the selection of dynamic springs.

The springs can be classified into light load, normal load and heavy load according to the load conditions. Light load refers to a spring that receives static stress, low stress, and small deformation, such as springs for safety devices and springs for absorbing vibrations. Design life 103 to 104 times.

  The general load refers to the ordinary spring with a design life of 105 to 106 times and a vibration frequency of 300 times /min . Within the allowable stress range, the life guarantee is 1×106 times, and the lower the load stress, the longer the service life.

Heavy load refers to springs that work for a long time and have frequent vibrations. Such as valve springs, air hammers, presses, and hydraulic controller springs, which have higher loads, are often used at less than about 10% of the allowable stress , and the service life is greater than 1 x 106 , usually 107 .

The principle of selection of springs is: First meet the functional requirements, followed by the strength requirements, and then only consider the economy.

Carbon spring steel is widely used in spring steel, with a large amount of steel. The steel contains 0.60% to 0.90% of carbon and 0.3% to 1.20% of manganese, no other alloying elements are added, and the use cost is relatively low. Carbon spring steel wire can get high tensile strength, sufficient toughness and good fatigue life after proper processing or heat treatment. However, the carbon steel wire has low hardenability, poor relaxation resistance and corrosion resistance, and has a large temperature coefficient of elastic modulus ( up to 300×10-6/ °C ) , which is suitable for manufacturing smaller cross-sections and lower operating temperatures ( 120 °C >) Spring.
    Alloy spring steel generally contains 0.45% to 0.70% carbon and a certain amount of alloy elements such as Si , Mn , Cr , V , W and B. The addition of alloying elements improves the anti-relaxation performance of the spring steel, improves the toughness of the steel, and significantly improves the hardenability and the operating temperature of the steel, and is suitable for the production of springs with larger cross-sections and higher temperatures. The commonly used grades of spring steel at home and abroad are shown in Table 1 .

Table 1 Comparison of commonly used spring steel grades at home and abroad

2 , spring steel wire standards, scope of application and process characteristics

China's spring steel wire standard is based on ISO and JIS standards, developed by steel. Current standards and recommended industry standards including 6 carbon spring steel, standard steel alloy spring 4, as detailed in Table 2 standard.

Table 2 Spring wire standard details

2.1 Carbon spring steel wire standard, scope of application and process characteristics

There are two types of carbon spring wire current and industry recommendations: One is cold deformation reinforced wire, also known as cold drawn spring wire. The cold drawn carbon spring steel wire is first treated with lead quenching to obtain the sorbite structure, then the surface is phosphatized, drawing to the finished product size at a greatly reduced surface area, the steel wire structure is fibrous, and has high tensile strength and elastic limit. Good bending and twisting properties. Cold drawn spring steel wire has high dimensional precision, smooth surface, no oxidation and decarburization defects, and relatively stable fatigue life. It is a widely used spring steel wire.

Another type of carbon spring wire is martensite-reinforced steel wire, also known as oil quenched and tempered steel wire. Carbon steel wire through quenching and tempering, get a good comprehensive mechanical properties, when the wire size is smaller ( φ ≤ 2.0mm ), the oil quenched and tempered steel wire strength indicators than sorbitized cold drawn wire after . When the steel wire size is larger ( φ≥6.0mm ) , the sorbitized steel wire can't use the large reduction rate to obtain the required strength index, and the oil quenched and tempered steel wire can get the colder than the cold drawn steel as long as it is completely hardened. The higher performance of the wire. With the same tensile strength, the martensite-reinforced steel wire has a higher elastic limit than the cold-deformed reinforcement wire. The microstructure of the cold drawn steel wire is fibrous, with obvious anisotropy. The metallurgy of the oil quenched and tempered steel wire is uniform tempered martensite and is almost isotropic. At the same time, the anti-relaxation performance of the oil quenched and tempered steel wire is better than that of the cold drawn steel wire. The service temperature ( 150 to 190 °C) is also higher than the cold drawn steel wire ( ≤120 °C). In recent years, large and medium-sized oil quenching and tempering steel wires have greatly replaced the trend of cold drawn steel wire. The application scope and process characteristics of all types of carbon spring steel wire are described in the following standards.

( 1 ) YB/T5220-93 Carbon Spring Wire for Non-mechanical Springs
    The standard applies to sofa springs, seat cushions, backrest extension springs, snap springs, clip springs and other non-mechanical spring carbon spring wire. According to the requirements of different tensile strengths, the steel wire is divided into nine groups of A1 , A2 , A3...A9 . Each group wire is supplied according to a strength range regardless of the size, and the tensile strength deviation is ≤200Mpa . The A1 , A2, and A3 groups are used to make lower stress springs. A3 , A4, and A5 are used to make general stress springs. A7 , A8, and A9 are used to make higher stress springs. The mattress springs generally use the A3 and A4 groups.

From the use of state analysis, the standard steel wire is basically a static spring. The finished steel wire is only used to evaluate the three properties of tensile strength, winding and single bending. See Table 3 for details .

Table 3 Carbon spring wire for YB/T5220-93 non-mechanical spring

Explanation:
1Φ ≤ 4.0mm steel wire wound on 2d mandrel for two rounds without crack or break.
2Φ> 4.0mm steel wire bending test, the sample along the R = 10mm arc bent 90o in different directions, the bend must not have cracks or broken.

Steel wires in the A1-A3 group are generally selected from 45 to 70 steels. Groups A3 to A6 use 65Mn or 70 steel. Steels in the group A7 to A9 are selected from 70 or T8MnA ( 82B ). Because the steel wire supplied according to this standard is basically used for making static springs, the requirement for fatigue life is relatively loose, and the steel wire can be directly drawn by controlled rolling and cooling wire rods. Preheat treatment of steel wire can also be replaced with normal bath instead of lead bath treatment. At the same time, it is allowed to use converter-static steel as raw material.

(2) GB/T4357-89 Carbon Spring Wire

This standard is a universal standard for cold-drawn carbon spring wire and is mainly used for making static springs that work under various stress conditions. Wires can be supplied in three grades depending on the stress state of the spring: Class B for low-stress springs, Class C for medium-stress springs, and Class D for high-stress springs. The finished steel wire is used to evaluate the four properties of tensile strength, twisting, winding and bending. The mechanical properties of common specifications are shown in Table 4 .

Table 4 GB/T4357-89 carbon spring wire

Explanation :
1 D ≤ 4.0 mm D grade steel wire and Φ ≤ 6.0 mm B grade, C grade steel wire is wound on the mandrel equal to the wire diameter for 2 rounds, and the surface of the wound sample surface may not be cracked or broken.
②Φ> 4.0mm 2 class D wire winding turns on the mandrel 2 times the wire diameter, the surface of the sample can not be wound from cracking or breaking.
3Φ> 6.00mm steel wire should be checked for bending, and the sample is bent 900 in different directions along an R=10mm arc, and no cracks or cracks can occur after bending.

Steel grades B and C are generally 70 ( 67A , 72A ) or 65Mn ( 67B ), and grade D is T9XtA and T8MnA ( 82B ).

    This standard steel wire is mainly used for making static mechanical springs. Its frequency of vibration during service period is higher than that of non-mechanical springs. It has certain requirements for fatigue life, and finished products also increase the assessment of torsional performance. Therefore, the steel wire rod should be smelted by electric furnace or electric furnace + furnace refining method, P ≤ 0.030% , S ≤ 0.020% and so on. If free ferrite is contained in the microstructure of the finished steel wire, the fatigue life of the spring will be reduced. However , free ferrite of 5% to 1.5% is generally stored in the controlled rolling coils. Generally, wire rods are not suitable for the production of finished steel wires. . According to this standard, the steel wire supplied in principle should be treated with a lead bath before the elimination of free ferrite structure. The microstructure of the finished steel wire should be a fiber-reacted sorbite structure.

(3) GB/T4358-1995 " Useful carbon spring wire"
    The steel wire supplied according to this standard is mainly used for making dynamic springs that work under various stress conditions. According to the working stress state of the spring, the steel wire is supplied in 3 groups: E group is suitable for medium-stress dynamic springs, group F is for high-stress dynamic springs, and group G is for dynamic springs with high fatigue life. The finished steel wire is used to assess the five performance indexes of tensile strength, twisting, winding, bending and decarburization. The mechanical properties of common specifications are shown in Table 5 .

Table 5 GB/T4358-1995 Carbon spring wire for important purposes

Explanation :
1Φ< 4.0mm steel wire is wound on a mandrel equal to the wire diameter, and φ≥ 4.0mm steel wire is wound 5 times on a mandrel with 2 wire diameter, and no crack or breakage is allowed.
2Φ> 1.0mm steel wire bending test, the sample along the R- arc bending in different directions 900 , the bending can not have cracks or break, Φ ≤ 4.0mm , R = 5mm ; Φ> 4.0mm , R = 10mm .
3G steel wire decarburization layer ≤ 1.0d% .

    Since the steel wire supplied in accordance with this standard is used to make dynamic springs working under medium and high stress conditions, in addition to maintaining a high elastic limit and a good toughness index, the finished steel wire must also take into account the fatigue limit and the fatigue life of the spring. For this reason, there is a higher demand for the purity of steel, the content of non-metallic inclusions and gases, the content of ferrite and the degree of surface decarburization. Steel wire rod must be used refining furnace + furnace smelting, have higher requirements on the chemical composition of the wire rod: P≤0.025%, S≤0.020%, Cr ≤0.10%, Ni≤0.15% (0.12%), Cu≤0.20% . Actual production to improve the fatigue life will tend to control the upper limit Mn, E 70 or a group selected 70Mn (72B), F or a group selected T8MnA T9RtA, G group selected 65Mn (Mn adjusted to 0.9-1.2%) or 67B. The purpose of reducing the content of P and S in steel , increasing the content of Mn and adopting the refining outside the furnace is to reduce the content of non-metallic inclusions in the steel, improve the inclusion morphology and reduce the gas content, and increase the fatigue limit and fatigue life. If free ferrite is contained in the microstructure of the wire, the fatigue life will be greatly reduced, and the steel wire supplied in accordance with this standard must be subjected to the quenching prior to the formation.

    Group G steel wire is used to make valve springs that work under severe vibration. The fatigue life requirements are extremely high. Therefore, the more flexible 65Mn wire rod is used, although the tensile strength is reduced, but the fatigue life is more ensured. Decarburization of steel wire surface and formation of ferrite structure seriously affect fatigue performance. The standard adds decarburization layer test to Group G steel wire and stipulates that the depth of total decarburized layer shall not be greater than 1.0%d , but for larger size steel wire ( Φ > 4.0mm ) Due to the reduction of surface area, the decarburized layer caused by the hot-rolled wire rod is difficult to eliminate. The standard supplements: “With the consent of the purchaser, it is possible to supply steel wire with a decarburized layer that does not exceed 1.5% d in diameter . ”

(4) GJB1497-92 "Special Purpose Carbon Spring Wire Specification"

In some special occasions, the spring is required to have elastic force beyond the conventional requirements, such as the firearm spring. For the convenience of carrying and using, the space occupied by the spring is small, the elastic force is large enough, and the tensile strength is high and the toughness is better. Steel wire is used to make springs , and "Special Purpose Carbon Spring Wires" adapts to this need.

The standard wire points A, B, propyl group 3 supplier. Group C is adapted to higher stress springs, Group B to high stress springs, and Group A to ultra high stress springs. The standard recommended steel wire is T9A , T10A and T8MnA . In fact, the tensile strength of steel wire increases with the increase of carbon content, and the torsional performance (toughness index) decreases with the increase of carbon content. In consideration of comprehensive considerations, Dalian Three Steel Company of Special Steel Group selects T9A steel wire with rare earth for production of A, B, and C Group wire. The finished steel wire was evaluated for six performance indexes of tensile strength, torsion, torsional fracture, winding, tensile strength uniformity, and dimensional uniformity . Table 6 shows the mechanical properties of common specifications .

It should be pointed out that the ultra-high tensile strength of steel wire is obtained by sacrificing part of plasticity and fatigue life. It is only suitable for the production of springs with simple shapes and low requirements for fatigue life. This standard should not be promoted.

Table 6 Specifications for GJB 1497-92 Special Purpose Carbon Spring Wires

Explanation:
1 The difference between the tensile strength at both ends of each plate wire shall not be greater than 100Mpa .
2 When the wire is twisted, there must be no visible cracks and delamination within the specified number of twists.
3 The wire is wounded and tested, and the wire must not be broken or broken after it is wrapped around the mandrel for 1 to 5 turns. The mandrel diameter is equal to the wire diameter.

(5) YB/T5103-93 oil quenched - tempered carbon spring wire

The oil quenched and tempered steel wire is first produced to the finished product size, and then oil quenched and tempered to deliver the tempered martensite structure. The homogeneity of the structure is an important indicator to determine the properties of the oil quenched and tempered steel wire. Because the carbon steel spring wire has limited hardenability, the core oil cannot be completely transformed into martensite after quenching and tempering of the wire oil with too large specification. Therefore, the standard specifies oil quenching. Tempered carbon spring wire supply diameter is less than 12.0mm .

Compared with the cold-drawn carbon spring steel wire, the oil quenched and tempered steel wire with a diameter of ≤ 2.0 mm has lower tensile strength and elastic limit than cold drawn steel wire. However, after large-size steel wire ( Φ≥6.0mm ) sorbite treatment, it is impossible to obtain high tensile strength by drawing at a large surface reduction rate, and the oil quenched and tempered steel wire can obtain colder than cold drawn steel wire as long as it is completely hardened. Higher tensile strength. Even under the same tensile strength conditions, the elastic limit of oil quenched and tempered steel wire is also high with cold drawn steel wire.
    The anisotropy of the cold drawn spring wire is significant, while the oil quenched and tempered steel wire is almost identical. The fatigue life and stress relaxation resistance of the oil quenched and tempered steel wire is much better than that of the cold drawn steel wire. The working temperature is higher ( 150 to 170 °C) and the creep resistance is better. In addition, the straightness of the oil quenched and tempered steel wire is good. After the coil is opened, the steel wire springs straight, almost no bending, and the forming performance is better when winding the spring. Therefore, industrially developed large-size oil quenched and tempered carbon spring steel wire almost replaced cold drawn carbon spring steel wire.

(6) YB/T5102-93 Valve Oil Quenching and Tempering Carbon Spring Wire
    The relationship between YB/T5102-93 and YB/T5103-93 is equivalent to the relationship between GB/T4358-1995 and GB/T4357-89 . The former is used to make dynamic springs, the latter being a common standard and is mainly used to make static springs. As the dynamic oil quenching and tempering springs have more stringent requirements on the homogeneity of the structure, the hardenability of carbon steel is limited, and YB/T5102 stipulates that the supply specifications are smaller ( 6.0 mm in diameter ).
    Valve oil quenching and tempering The application scope and quality control requirements of carbon steel wire are basically the same as GB/T4358-1995 G group, and will not be repeated here.

2.2 Alloy spring steel wire standard, scope of application and process characteristics

China's alloy spring steel wire is divided into three categories, alloy spring wire, oil quenched and tempered silicon-manganese spring steel wire, valve oil quenched and tempered chrome-silicon alloy spring steel wire, from the perspective of the use of developed alloy springs, the ratio of oil quenched and tempered steel wire Increasingly, Japanese Industrial Standards ( JIS ) stipulate that alloy spring steel wires are all supplied in oil quenched and tempered state.

(1)    GB/T5218-1999 Alloy Spring Wire

This standard combines the original GB5218-85 " Si- Mn spring wire", GB5219-85 "chrome-vanadium spring wire" and GB5221-85 "Chromium-silicon spring steel wire" into one standard.

The standard is applicable to the production of spring steel wire for high and medium stress springs. The steel wire coil spring must be quenched and tempered before it can be used.

The standard lists three steel grades, 60Si2MnA , 50CrVA, and 55CrSiA , and provides that other steel grades can be supplied according to the requirements of the buyer.

The steel wire is generally delivered in a cold-drawn state. The finished product has a diameter of ≤5.0mm and a tensile strength of σb≤1035Mpa . A diameter of >5.0mm has a steel inspection of HB≤302 , which corresponds to a light-pull state. In the production of large size ( Ф>8.0 ), the cold-reduced surface reduction rate of finished products shall not exceed 25% , and the reduction rate of small-size steel wire ( Ф≤5.0 ) shall not exceed 30% . In the annealed state, the delivery wire is too soft and can easily cause dead bends. Springs tend to cause bad spring shapes and uneven pitches. When the cold reduction reduction ratio is too large, the steel wire is too hard and the bounce spring has a large rebound, making it difficult to form a thick spring. Therefore, the directly wrapped spring wire is supplied in a light-pull state. For steel wire repurchasing, users who want to perform cold processing (such as flattening, rolling into profiled sections, etc.) should be recommended to use steel wires in the annealed state.

In order to ensure the performance of the wrapped spring, the finished steel wire is subjected to a winding test. A wire diameter of ≤5.0 mm shall be wound six times on a mandrel equal to 1 to 2 times the diameter of the wire and must not be broken or broken.

Decarburization and surface defects severely reduce spring fatigue life and must be strictly controlled. In particular, 60Si2MnA and 55CrSi have a high silicon content and are easily decarburized during annealing. Rod annealing and semi-finished recrystallization annealing are recommended to use low temperature, long annealing process. As mentioned before, for springs with high fatigue life requirements, the polished steel wire must be selected, but whether it is annealed or cold drawn is necessary. When the 50CrV soft steel wire is polished, the grinding debris can easily adhere to the surface of the steel wire to form irregular small white spots. Therefore, the cold-grinding polishing process should be adopted as much as possible to reduce the surface “ white spots ” .

Under the conditions of surface quality assurance, the inclusion content becomes an important factor influencing the fatigue life. Therefore, the standard stipulates that non-metallic inclusions and graphitic carbon can be added when required by the buyer.

(2) YB/T5104-93 "oil quenching - tempering Si - Mn alloy spring wire" and YB/T5105-93 "valve oil quenching - tempering chrome-silicon alloy spring wire".