News

Spring Steels

• satyendra
• May 2, 2013
• 2 Comments
• Fatigue strength, helical spring, high carbon steel, leaf spring, Relaxation, spring steel, stainless steel, Tempered and hardened steel,

Spring Steels

When the first spring was produced is not known; however, it very likely was a bow. The bow was used to store energy which could be released at a precise moment, upon command by the bowman. The spring is defined as ‘an elastic body with a primary function of deflecting or distorting under applied load and returning to its original shape when the load is removed’. In other words, a spring is component which is capable of storing energy, either temporarily or permanently. Today, springs are fundamental mechanical components found in many mechanical systems. Steel is an important engineering material for the manufacture of springs used in the mechanical systems.

Steel springs are made in many types, shapes, and sizes, ranging from delicate hairsprings for instrument meters to massive buffer springs for railroad equipment. The most common types of springs are small steel springs which are cold wound from wire, relatively large hot-wound springs, and flat and leaf springs. Wire springs are of four types namely (i) compression springs (including die springs), (ii) extension springs, (iii) torsion springs, and (iv) wire forms.

The two more common types of springs which are used are helical coil springs and leaf springs. Coil springs are very common springs used in the automotive industry and they are produced from a length of round steel wire which is formed into loops that allow for movement. Coil springs can be further classified as compression springs and extension springs. A compression spring is used where the energy of movement acts to compress the spring. These springs are used in applications such as shock absorber assemblies to absorb the movement arising from bumps and vehicle weight transfer. Conversely, an extension spring is used in applications where the energy of movement acts to extend the spring. Coil springs are typically produced by one of two processes namely (i) wire with the desired mechanical properties is wound into a finished spring, and (ii) soft-wire of the desired composition and hardenability is wound into a spring (sometimes by a hot-forming process), then quenched, tempered and finished into a part.

A leaf spring, also occasionally referred to as a semi-elliptical spring or cart spring is a long, flat, flexible piece of spring steel (or a composite material) which deflects by bending when forces act on it. Leaf springs are used primarily in suspensions to support the load of wheeled vehicles. Leaf springs can be used singly or in several layers, often with each successive layer being shorter than the one below it, which are bracketed together into one assembly. These days, leaf springs are predominantly used for heavy vehicles such as trucks because a leaf spring is stressed along its length and transmits a load over the width of the chassis. This action is to be contrasted to a coil spring which transfers a load to a single point. Leaf springs can perform a locating and to some extent damping functions as well as springing functions. Fig 1 shows the helical and leaf springs along with the tensile properties of some of the spring steels.

Fig 1 Helical and leaf springs along with the tensile properties of the spring steels

Compression springs are open wound with varying space between the coils and are provided with plain, plain and ground, squared, or squared and ground ends. The spring can be cylindrical, conical, barrel, or hour glass shaped. Extension springs are normally close wound, normally with specified initial tension, and, because they are used to resist pulling forces, are provided with hook or loop ends to fit the specific application. Ends can be integral parts of the spring or specially inserted forms. Torsion springs are normally designed to work over an arbor and to resist a force which causes the spring to wind. Wire forms are made in a wide variety of shapes and sizes. Flat springs are normally made by stamping and forming of strip material into shapes such as spring washers. However, there are other types, including motor springs (clock type), constant-force springs, and volute springs, which are wound from strip or flat wire.

Technically spring is an elastic component which is able to store an applied force effect. A very high degree of quality, reliability and service life is expected in springs since it is vital for the functioning of the mechanical system. While certain materials have come to be regarded as spring materials, they are not specially designed alloys. Spring materials are high strength alloys which frequently show the greatest strength in the alloy system. For example, in steels, medium and high-carbon steels are regarded as spring materials. Beryllium copper is frequently specified when a copper base alloy is required. For titanium, cold-worked and aged Ti-13V-11Cr-3Al is used.

The energy storage capacity of a spring is proportional to the square of the maximum operating stress level divided by the modulus. An ideal spring material has high strength, a high elastic limit and a low modulus. Because springs are resilient structures designed to undergo large deflections, spring materials are to have an extensive elastic range. Other factors such as fatigue strength, cost, availability, formability, corrosion resistance, magnetic permeability and electrical conductivity can also be important and are to be considered in the light of cost / benefit. Hence, careful selection is to be made to achieve the best compromise.

The spring’s capacity to take on static and dynamic loads over an extended period of time depends on the steel that goes into its making. Steel material for the production of the springs is normally called spring steel. Spring steels feature the unique characteristic of being able to withstand considerable twisting or bending forces without any distortion. Products made from these steels can be bent, compressed, extended, or twisted continuously, and they return to their original shape without suffering any deformation. This characteristic is defined as high yield strength and is the result of the specific composition and hardening of the steels. Chemical composition, mechanical properties, surface quality, availability, and cost are the principal factors to be considered in selecting steel for springs.

Spring steels are medium and high carbon steels, low alloy steels, or stainless steels, produced to very high yield strengths. Spring steels are also used when there are special requirements on rigidity or abrasion resistance. Spring steels are also to meet the different requirements from the technical point of view. These requirements are (i) high elastic limit which is the tension that can be applied on the steel material without a plastic deformation, (ii) high ultimate strain which is the value of the extension until rupture in relation to the original length, (iii) high contraction at fracture which is the change of the original cross section in comparison to the cross section at rupture, (iv) good creep rupture strength which is a kind of tensile strength taking in account temperature and time, (v) good endurance limit which is the reaction of the material on constantly changing maximum stresses till the plastic deformation begins, and (vi) low surface decarburization and a clean, free from fracture surface makes the outer shell of the material soft hence it is to be avoided.

The above special requirements of spring steels are being met by adding different alloying elements in the steels. These are silicon, manganese, chromium, vanadium, molybdenum and nickel (in case of stainless steels). Most of the spring steels are hardened and tempered to around 45 HRC. Most of the springs are made with medium and high carbon steels, alloy steels and stainless steels as given below.

Medium and high carbon spring steels – These spring steels are the most commonly used materials since they are less expensive, These materials can be easily worked and are readily available. These steels are not suitable for springs operating at high or low temperatures or for shock or impact loading.

Alloy spring steels – These spring steels are used for conditions of high stress and shock or impact loadings. These steels can withstand a wider temperature variation than high carbon spring steels and are used in either the annealed or pre tempered conditions. Silicon is the key element in most of the alloy spring steels. A typical example of alloy spring steel contains 1.5 % – 1.8 % silicon, 0.7 % – 1 % manganese, and 0.52 % – 0.6 % carbon.

Stainless spring steels – The use of stainless spring steels has increased in recent times. There are compositions available which can be used for temperatures upto 300 deg C. All these steels are corrosion resistant but only the stainless steel of 18-8 composition is to be used at sub-zero temperatures.

The selection of materials used by spring designers is frequently met with decisions about specification requirements, cost, availability, reliability and performance. The vast majority of spring requirements are normally met by the common high carbon spring wire materials such as music wire, hard drawn, or oil tempered carbon steels etc. These raw materials can meet many of the requirements of strength, reliability, cost, and availability. The starting materials for the production of most of the springs are given below.

Music wire – It is the most widely used of all spring materials for small springs since this wire is the toughest. It has the highest tensile strength and can withstand higher stresses under repeated loading conditions than any other spring material.  It is available in diameters from 0.12 mm to 3 mm.  It has a usable temperature range from 0 deg C to 120 deg C. Music wire contracts under heat and can be plated.

Oil tempered wire – This is a general purpose spring material used for springs where the cost of piano wire is prohibitive and for sizes outside the range of the music wire.  This material is not suitable for shock or impact loading.  This material is available in diameters from 3 mm to 12 mm.   The temperature range for this material is 0 deg C to 180 deg C. This wire does not generally change dimensions under heat and can be plated. It is also available in square and rectangular sections.

Hard drawn wire – This is the cheapest general purpose spring steel and normally used where life, accuracy, and deflection are not very important.  This material is available in sizes 0.8 mm to 12 mm.  It has an operating range 0 deg C to 120 deg C.

Chrome-vanadium steel – This is the most popular alloy spring steel for improved stress, fatigue and long endurance life conditions as compared to high carbon steel materials.  This material is also suitable for impact and shock loading conditions.  This steel wire is available in annealed and tempered steel in sizes from 0.8 mm to 12 mm.  It can be used for temperatures upto 220 deg C. It does not normally change dimensions under heat. It can be plated.

Chrome-silicon steel – This is an excellent spring material for highly stressed springs requiring long life and/or shock loading resistance.  It is available in diameters 0.8 mm to 12 mm and can be used from temperatures upto 250 deg C.   This material also does not normally change dimensions under heat. It can be plated.

Martensitic stainless steel – This is corrosion, resisting steel which is unsuitable for sub zero conditions.

Austenitic stainless steel – It is a good corrosion, acid and heat resisting steel for springs with good strength. It is useful in moderate temperature conditions.  It has low stress relaxation.

Spring steels are normally produced as bars with round or flat cross section, as well as wire, sheets or strips. Developments in material, design procedures and manufacturing processes permit springs to be made with longer fatigue life, reduced complexity, and higher production rate. Most springs are linear which means the resisting force is linearly proportional to its displacement. Linear springs follow Hooke’s Law, F = k. Dx, where F is the resisting force, k is the spring constant and Dx is the displacement.

Both carbon and alloy steels are used extensively. Steels for cold-wound springs differ from other constructional steels in four ways namely (i) they are cold worked more extensively, (ii) they are higher in carbon content, (iii) they can be furnished in the pre-tempered condition, and (iv) they have higher surface quality. The first three items increase the strength of the steel, and the last improves fatigue properties. For flat, cold-formed springs made from steel strip or flat wire, narrower ranges of carbon and manganese are specified than for cold-wound springs made from round or square wire.

Steels of the same chemical composition can perform differently because of different mechanical and metallurgical characteristics. These properties are developed by the steel producer through cold work and heat treatment or by the spring manufacturer through heat treatment. Selection of round wire for cold-wound springs is based on minimum tensile strength for each wire size and grade and on minimum reduction in area (40 % for all sizes). Rockwell hardness and tensile strength for any grade of spring steel strip depend on thickness. The same properties in different thicknesses can be achieved by specifying different carbon contents. The optimum hardness of spring steel gradually increases with decreasing thickness.

The modulus of elasticity in tension and shear is vital to spring design. For most steels and age-hardenable alloys, the modulus varies as a function of chemical composition, cold working and degree of aging. Normally variations are small and can be compensated for by adjustment of reference parameters of the spring design, (e.g. number of active coils, and coil diameter). For most materials, moduli are temperature-dependent and vary inversely with temperature by approximately 2 % per 55 deg C. Since non-ambient temperature testing is costly, design criteria for the springs are normally specified at room temperature after making appropriate compensation for the application temperature. For true isotropic materials, the elastic moduli (M) in tension (E) and shear (G) are related through Poisson’s ratio by the expression M = E/2G -1, so that, for common spring materials, any one of the parameters can be approximated using the other two.

Because of cyclic loading which accompanies the use of springs, there is an ongoing interest in improving fatigue strength of spring materials. Spring efficiency is related to its ability to store energy per unit weight. Typically, steel strengths of higher than 1,375 MPa are desired. There has been a continuing effort to develop spring steels to meet ever-increasing demands for improved mechanical properties with lower weight suspension materials to facilitate the larger effort of developing automotive vehicles with lower weights and lower cost.  For development of high-strength spring steels with improved sag strength and fatigue strength and improved quench embrittlement properties in addition to other thermo-physical and mechanical properties has been of part of this vitally important effort.

Fatigue strength is another important mechanical property of steel springs. However, this property is affected by many factors. Surface quality has a major influence on fatigue strength and is frequently not clearly delineated on national specifications. It is important to use only those materials with the best surface integrity for fatigue applications, particularly those in the high cycle region. In steels, for which processing costs are a large fraction of product cost, surface quality can vary over an appreciable range. Depth of surface imperfections, such as seams, pits and die marks, can be upto 3.5 % of diameter for commercial spring wire grades. Different intermediate qualities can be achieved. Highest levels are represented by music and valve spring quality (VSQ) grades which are virtually free of surface imperfections. Decarburization, which can also adversely affect fatigue performance, follows a similar pattern. Surface quality of spring materials is a function of the care exercised in their production and processes employed. Materials produced with a high level of surface integrity are more costly than commercial grades.

For most applications, the question of ‘magnetic or not’ is adequately answered with the use of a permanent magnet. For some applications, even very low levels of magnetic behaviour can be detrimental. Then, it is desirable to know the magnetic permeability of the steel materials and reach agreement between parties on a maximum allowable value. Since permeability can be altered by cold work, some variation can be expected. In general, low permeability steel materials are more expensive so spring designers are to specify low levels only when absolutely necessary. Frequently, nitrogen strengthened manganese stainless steels are good choices because they have good strength at moderate cost.

Frequently, operating environment is the single most important consideration for proper spring material selection. For successful application, material is to be compatible with the environment and withstand the effects of temperature and corrosion without an excessive loss in spring performance. Corrosion and elevated temperatures decrease spring reliability. The effect of temperature on spring steel materials is normally predictable. For the compatibility of spring materials and spring coating systems with corrosive environments, the spring designers normally rely upon previous experience.

Primary concern for the high temperature applications of springs is stress relaxation. Stress relaxation is the loss of load or available deflection which occurs when a spring is held or cycled under load.  Temperature also affects modulus, tensile and fatigue strength. For a given spring, variables which affect stress relaxation are stress, time and temperature, with increases in any parameter tending to increase the amount of relaxation. Stress and temperature are related exponentially to relaxation. Curves of relaxation versus these parameters are concave upward as is shown in Fig 2.Other controllable factors affecting relaxation include the following.

Alloy type – More highly alloyed materials are normally more resistant at a given temperature or can be used at higher temperatures.

Residual stress – Residual stresses remaining from forming operations are detrimental to relaxation resistance. Hence, use of the highest practical stress-relief temperatures is beneficial. Shot peening is also detrimental to stress relaxation resistance.

Heat setting – Different procedures can be used to expose springs to stress and heat for varying times to prepare for subsequent exposures. Depending on the method used, the effect is to remove a normally large first-stage relaxation and/or to establish a residual stress system which lessens relaxation influences. In some cases, the latter approach can be so effective that in application, compression springs can ‘grow’ or show negative relaxation. Increase in free length does not normally exceed 1 % to 2 %.

Grain size – Coarse-grain size promotes relaxation resistance. This phenomenon is used only in very high temperature applications.

Fig 2 Relaxation versus initial stress and temperature

The effect of a corrosive environment on spring performance is difficult to predict with certainty. General corrosion, galvanic corrosion, stress corrosion, and corrosion fatigue reduce life and load-carrying ability of springs. The two most common methods employed to combat effects of corrosion are to specify materials which are inert to the environment and to use protective coatings. Use of inert materials affords the most reliable protection against deleterious effects of all types of corrosion; however, this is often costly and sometimes impractical. Protective coatings are often the most cost-effective method to prolong spring life in corrosive environments. In special situations, shot peening can be used to prevent stress corrosion and cathodic protection systems can be used to prevent general corrosion.

Coatings can be classified as galvanically sacrificial or simple barrier coatings. Sacrificial coatings for high carbon steel substrates include zinc, cadmium (and alloys thereof) and, to a lesser degree, aluminum. Due to its toxicity, cadmium coating is only specified when absolutely necessary. Since sacrificial coatings are chemically less noble than steel, the substrate is protected in two ways. First, the coating acts as a barrier between substrate and environment. Second, galvanic action between coating and substrate cathodically protects the substrate. This characteristic allows sacrificial coatings to continue their protective role even after the coating is scratched, nicked or cracked. The amount of damage a sacrificial coating can sustain and still protect the substrate is a function of the size of the damaged area and the efficiency of the electrolyte involved. Use of conversion coatings, such as chromates, lengthens the time of protection by protecting sacrificial coatings.

Metallic coatings are normally applied by electro-plating. Since most high hardness steels are inherently very susceptible to hydrogen embrittlement, plating is to be carried out with great care to minimize embrittlement and subsequent delayed fracture. A baking operation after plating is also essential. Springs are almost always in contact with other metal parts. In a corrosive environment, it is important that the spring material is nobler than components in contact with it.

Characteristics of spring steel grade

The three types of wire used in the greatest number of applications of cold-formed springs are (i) hard-drawn spring wire, (ii) oil-tempered wire, and (iii) music wire.

Hard-drawn spring wire – Among the grades of steel wire used for cold-formed springs, hard-drawn spring wire is the least costly. Its surface quality is comparatively low with regard to such imperfections as hairline seams. This wire is used in applications involving low stresses or static conditions.

Oil-tempered wire – It is a general-purpose wire, although it is more susceptible to the embrittling effects of plating than hard-drawn spring wire. Its spring properties are achieved by heat treatment. Oil-tempered wire is slightly more expensive than hard-drawn wire. However, it is significantly superior in surface smoothness, but not necessarily in seam depth. Most cold-wound automotive springs are made of oil-tempered wire, although a small percentage is made of music wire and hard-drawn spring wire.

Music wire – It is the carbon steel wire used for small springs. It is the least subject to hydrogen embrittlement by electroplating and is comparable to valve-spring wire in surface quality.

Chromium-silicon and chromium-vanadium steel spring wire and strip – These are suitable for moderately elevated temperature service. The chromium-silicon steel spring wire, which has better relaxation resistance than the chromium-vanadium alloy, can be used at temperatures as high as 230 deg C. The cold-drawn spring wires of the chromium-vanadium and chromium-silicon alloy steels are heat treated before fabrication, while cold-rolled chromium-vanadium and chromium-silicon strip steels (and generally carbon strip steel as well) are heat treated after rolling and spring fabrication. The chromium-vanadium and chromium-silicon steel spring wires can be in either the annealed or oil-tempered condition before spring fabrication. Annealing can be performed before and after drawing, while oil tempering is performed after cold drawing.

High-tensile hard-drawn wire – It fills the gap where high strength is needed but where the quality of music wire is not required.

Valve-spring quality wires – All valve-spring wires have the highest surface quality attainable in commercial production, and most producers require that the wire conform to aircraft quality. Most VSQ wire producers remove the surface of the wire rod before drawing to final size. This practice improves the surface quality and eliminates decarburization.

Carbon steel spring wire – It is the least costly of the VSQ wires. The requirements for carbon VSQ wire is normally in oil tempered condition.

Chromium-vanadium steel wire – This steel wire is of valve-spring quality and is superior to the same quality of carbon steel wire for service at 120 deg C and above. A modified chromium-vanadium steel of valve-spring quality is also specified. This modified chromium-vanadium wire has a smaller range of preferred diameters and a lower minimum and maximum tensile strength for a given wire diameter.

Chromium-silicon steel VSQ wire – This steel wire can be used at temperatures as high as 230 deg C.

Annealed spring wire – Carbon steel wire of valve-spring quality, as well as chromium-vanadium and chromium-silicon steel wire of both spring and valve-spring quality can be used in the annealed condition. This permits severe forming of springs with a low spring index (ratio of mean coil diameter to wire diameter) and also permit sharper bends in end hooks. Although a sharp bend is never desired in any spring, it is sometimes unavoidable.\

Springs made from annealed wire can be quenched and tempered to spring hardness after they have been formed. However, without careful control of processing, such springs have greater variations in dimensions and hardness. This method of making springs is normally used only for springs with special requirements, such as severe forming, or for small quantities, because springs made by this method can have less uniform properties than those of springs made from pre-tempered wire and are higher in cost. The amount of cost increase depends largely on design and required tolerances, but the cost of heat treating (which frequently involves fixturing expense) and handling can increase total cost by more than 100 %.

Stainless steel spring wire – Cold-drawn type 302 stainless spring steel wire is high in heat resistance and has good corrosion resistance. The surface quality of type 302 stainless spring steel wire occasionally varies, seriously affecting fatigue resistance. Type 316 stainless is superior in corrosion resistance to type 302, particularly against pitting in salt water, but is more costly and is not considered a standard spring wire. Type 302 is readily available and has excellent spring properties in the full-hard or spring-temper condition. It is more expensive than any of the carbon steel wires for designs requiring a diameter larger than around 0.3 mm but is less expensive than music wire for sizes under around 0.3 mm.

In many applications, type 302 stainless can be substituted for music wire with only slight design changes to compensate for the decrease in modulus of rigidity. For example, a design for a helical compression spring was based on the use of 0.25 mm diameter music wire. The springs were cadmium plated to resist corrosion, but they tangled badly in the plating operation because of their proportions. A redesign substituted type 302 stainless steel wire of the same diameter for the music wire. Fewer coils were required because of the lower modulus of rigidity, and the springs did not require plating for corrosion resistance. The basic cost of this small-diameter stainless wire was, at the time, 20 % less than the cost of the music wire. Elimination of plating and reduction of handling resulted in a total savings of 25 %.