Spring steel grades:
Spring steel - application and specification
Spring steels are a separate group of construction alloy steels. They are characterized by a high elasticity limit, fatigue resistance, where the return of the element to the conventional form after compression is reduced, resistance to stress relaxation, high yield strength, damping and shock absorption, and in specific applications depending on the chemical composition of selected grades, also corrosion resistance (eg. X10CrNi18-8 and 1H18N9 steels).
They are used for the production of springs, spring washers, show excellent elastic properties, fracture toughness (in case of overload of the element is deformed, not cracked), resistance to plastic deformation of finished products (fatigue), relatively long time and the possibility of forming suitable shapes for springs and spring elements thanks to the high plasticity of the materials in this group.
In order to extend the life of the element even in fatigued conditions, the surface of the springs should be properly machined, smooth, without notches, scratches and unevenness. Heat treatment of spring steel consists in austenitization of the product in the temperature range of 800-870 ℃, hardening in water (about 30-40 ℃) or oil (about 50-70 ℃), and tempering at 300-520 ℃ depending on grade of steel. As a result of this heat treatment, the spring element will have a tempered martensite structure.
Chemical composition and types of spring steel
Structural alloys have a fairly wide range of carbon - C: 0.40-1.25%. The resilience of these steels and other characteristic properties of the materials corresponds to, among others, Silicon. As a result of ongoing research, development of construction technology and machinery, it should not be stereotyped that a sub-group of alloy spring steels is only used for springs. The fact is that many types of tool steels, high-alloyed corrosion-resistant steels and even bearing steels have been shown to be suitable for use and treated as spring steel in exceptional applications. Spring steels are supplied in an annealed condition, softened, heat-treated, and in the raw condition.
There are several types of basic spring steel subgroups:
- carbon spring steels, (f.ex. C55S, 65 / C67S, 75 / C75S, 85 / C85S, C90S, C100S, C125S)
- silicon and silicon-manganese steels (f.ex. 38Si7 / 40S2, 46Si7, 51Si7 / 50S2, 56Si7 / 55S2, 60S2A, 65Mn4 / 65G, 60SG)
- chrome steels with additives of vanadium, manganese, molybdenum, and nickel (f.ex. 51CrV4 / 50HF, 50HS, 50HG, 60SGH)
Carbon and alloy spring steels
The use of carbon steel is mainly for smaller and slightly less responsible spring elements. Its properties owes mainly to high carbon content of about 0.55-1.10%. Compared to the rest of the grades, carbon steels have a small range of alloying additives such as manganese and silicon. The products of these grades are applied to smaller springs because of low hardness, low strength and ductility - that is, low load plates and spring parts with small cross sections made of sheets, strips, bars and wires up to 10mm in diameter or thickness. The material should be used in environments with a maximum temperature of 150 oC. Spring carbon steels are characterized by high yield strength and good spring properties after thermal improvement.
Chromium steels with vanadium and manganese are a group of grades with even better parameters compared to silicon and manganese steels. Their hardness is much better, they are designed to work at much higher temperatures (up to 300 ℃) and can be used as particularly loaded springs, and parts of springs of much larger dimensions.
Chromium with vanadium (CrV) and chromium with molybdenum (CrMo) in spring steel as a combination of carbide elements causes less inclusions of nonmetallic materials and hence the material's resistance to fatigue is considerably higher than that of silicon steels.
The plasticity of the spring steel is also an important feature, and here we also place the chromium steels in the first place in terms of high deformation coefficient.
Particular SiCrV alloy grades, chrome-silicon containing vanadium and SiCrMo, chromium-silicon-molybdenum have the highest minimum value of Re. By adding to this group CrV, and CrMoV steels, all four have the highest operating temperature of up to 300 ℃. Carbon range in chromium steel is about 0.40%-0.65%.
Silicon, Manganese, or Si-Mn alloys are characterized by a lower carbon content compared to carbon-based steels. Additives such as silicon and manganese make these steels more durable, and have higher impact strength, ductility and high resistance to stress relaxation. Compared to carbon steels and silicon steels, during tempering are easily decalcified and are graphitized. Toughness is similar to carbon steel. Their plasticity is due to a higher content of silicon, and in some grades Manganese (e.g. 65Mn4 steel), it gives even more hardening capacity. These steels are also not suitable for use in high temperatures. The range of carbon in silicon steel is about 0.38-0.60%
Sheets, strips, and bars made of spring steel
The above described grades of alloy steel and carbon spring steel are defined in Polish standards PN-74/H-84032, PN-75/H-84019, PN-60/H-84030 (PN-H-84032:1965), GOST 2052-53, PN-EN 10083 and PN-EN 10089:2005 according to which the following are delivered:
- Hot rolled plates made of spring steel according to PN-EN 10029, and PN-H-92135, PN-H-92134
- Spring steel forged bars and forgings according to PN-79/H-94500, PN-84/H-94004, PN-EN 10250-3,
- Cold-rolled spring tapes according to PN-EN 10132-4:2004, PN-EN 10131, PN-75/H-92335, PN-74/H-92329
- Spring steel rolled bars according to PN-96/H-93003,PN-64/H-92604 PN-EN 10025-6, and PN-EN 10060,
- Spring steel rolled flat bars according to PN-H-93219, and PN-EN 10092-1,
- Spring steel drawn bars according to PN-85/H-93210, and Spring steel peeled bars according to PN-96/H-93005, PN-56/H-93006, PN-96/H-93009
- Spring steel mechanical wire according to PN-EN 10270, PN-91/H-84028 and PN-96/H-93005, PN-55/H-80057