Spring steel grades - properties and scope of application

Structural carbon or high-carbon steel includes spring steel. To give it highly targeted properties, it is doped in small quantities with 2-3 elements, for a total of up to 2.5%. But the use of these steel grades is not limited to the manufacture of springs. This group is called that because the name most strongly reflects their main feature - elasticity.

Characteristics of spring steels

Spring steels are characterized by increased yield strength (δB) and elasticity. This is the most important characteristic of metal - to withstand mechanical loads without changing its original shape. Those. a metal subjected to tension or, conversely, compression (elastic deformation), after removing the acting forces from it, must remain in its original shape (without residual deformation).

Types and scope of spring steel

Based on the presence of additional properties, spring steel is divided into alloy (stainless) and carbon. Alloy steel is based on carbon steel with a C content of 65-85% and is alloyed with 4 main elements, all or selectively, each of which brings its own characteristics:

1. chromium;
2. manganese;
3. silicon;
4. tungsten.

Chromium - at a concentration of more than 13%, works to ensure the corrosion resistance of the metal. With a chromium concentration of about 30%, the product can work in aggressive environments: acidic (except sulfuric acid), alkaline, aqueous. Corrosion spring steel is always alloyed with a second accompanying element - tungsten and/or manganese. Operating temperature up to 250 °C.

Tungsten is a refractory substance. When its powder gets into the melt, it forms numerous crystallization centers, crushing the grain, which leads to increased plasticity without loss of strength. This brings its advantages: the quality of such a structure remains very high during heating and intense abrasion of the surface. During heat treatment, this element retains its fine-grained structure and eliminates softening of steel during heating (during operation) and dislocation. During hardening, it increases hardenability, as a result of which the structure becomes homogeneous to a greater depth, which in turn increases the service life of the product.

Manganese and silicon usually participate in mutual doping, and the ratio always increases in favor of manganese, up to about 1.5 times. That is, if the silicon content is 1%, then manganese is added in an amount of 1.1-1.5%.

Refractory silicon is a non-carbide-forming element. When it enters the melt, it is one of the first to take part in crystallization, pushing carbon carbides to the grain boundaries, which accordingly leads to strengthening of the metal.

Manganese can be called a structure stabilizer. By simultaneously distorting the metal lattice and strengthening it, manganese eliminates the excessive strength of silicon.

In some steel grades (when the product is operating in high-temperature conditions, at temperatures above 300 ºC), nickel is added to the steel. It eliminates the formation of chromium carbides along grain boundaries, which lead to matrix destruction.

Vanadium can also be an alloying element, its function is similar to that of tungsten.

Spring brands specify the element copper; its content should not exceed 0.15%. Since copper, being a low-melting substance, concentrates at the grain boundaries, reducing strength.

Spring brands include: 50HG, 3K-7, 65G, 65GA, 50HGFA, 50HFA, 51HFA, 50HSA, 55S2, 55S2A, 55S2GF, 55HGR, 60G, 60S2, 60S2A, 605, 70, 70G, 75, 80, 85, 60С2ХА , 60S2HFA, 65S2VA, 68A, 68GA, 70G2, 70S2XA, 70S3A, 70HGFA, SH, SL, SM, DM, DN, KT-2.

Grades of such steel are used for the manufacture of not only springs and leaf springs, although this is their main purpose, which characterizes the main property. They are used wherever there is a need to provide the product with elasticity, ductility and strength at the same time. All parts made from these grades are subject to tension and compression. Many of them experience loads that periodically replace each other, and with a huge cyclic frequency. This:

• bearing housings that experience compression and tension at each point with high frequency;
• friction discs experiencing dynamic loads and compression;
• thrust washers, most of the time they experience compression loads, but a sharp change in tension can also be added to them;
• brake bands, for which one of the main tasks is elasticity under repeated stretching. With this dynamic of increased aging and wear, stronger steel (with less elasticity) is susceptible to rapid aging and sudden failure.

The same applies to gears, flanges, washers, collets, etc.

Properties of spring steel

The main characteristics of this type of steel are high resistance to elastic deformation and low residual elongation coefficient. This is due to the inadmissibility of increasing or decreasing the structural size of the spring.

Steel spring

Good structural and operational properties are achieved by drawing a pre-patented wire at low temperatures, while tightly tightening the material.

The patenting process is carried out in the interval between two hoods, the steel is heated above the temperature point of austenite formation and then cooled in a bath of molten lead, while the austenite transforms into thin-plate sorbitol and its mechanical strength increases.

To achieve the same physical and chemical properties over the entire cross-section of the material, spring steel must undergo a calcination process using a through method, this will ensure a homogeneous structure throughout the entire cross-section. This method is especially important for the manufacture of springs of large diameter, when uneven properties of the starting material can lead to destruction of the finished product.

Like any other material, spring steel is characterized by the presence of carbon in its composition. In this case, its content can vary between 0.50-0.80% by weight of the alloy. Additionally, the following alloying additives are used:

• silicon – up to 2.5%;
• manganese – up to 1.3%;
• tungsten – up to 1.3%;
• nickel – up to 1.7%.

Microstructure of spring steel

It is worth noting that chromium and manganese, when alloyed together, increase the resistance of steel to low plastic deformations. Nickel and tungsten form a thin and uniform structure of the carbide fraction, which prevents dislocation.

Spring steel is very critical to deformations of the outer layer of the material, since these stresses are concentrators of possible defects in the finished product.

Hardening of this type is carried out at temperatures of 850–880 °C, but after such heat treatment the steel exhibits weak elastic properties due to the formation of martensite; to increase this type of properties, it is tempered at temperatures of the order of 420–510 °C, which promotes the formation of troostite and increased elastic deformation alloy to a tensile strength of 1200-1900 MPa and a yield strength of 1100-1200 MPa. At the same time, carrying out hardening isothermally - at a constant temperature - has a positive effect on the plasticity and viscosity of the material.

Steels of this type have good anti-corrosion properties due to the presence of alloying additives such as chromium and molybdenum in the alloy. This has a positive effect on the service life and prevents the formation of cracks during operation.

It is also worth noting several main disadvantages of spring steel:

• poor weldability - this is due to the destruction of the outer layer of the material and local overheating of the part;
• difficulty of cutting - some difficulties arise when trying to cut this type of steel, this is directly related to the high resistance to deformation.

Marking

Spring-spring steels can be grouped by position:

• unalloyed with a carbon content of 65-85% - inexpensive general purpose steel;
• manganese-silicon - the cheapest with high physical and chemical properties;
• chrome-manganese - stainless steel, works in aggressive environments at t -250 +250 C;
• additionally alloyed and/or tungsten, vanadium, boron - they are steels with an increased service life due to their homogeneous structure, an excellent strength-to-ductility ratio due to fine grains and can withstand high mechanical loads. They are used on such objects as railway transport.

Marking of spring steels is carried out as follows. Let's look at the example of 60S2HFA:

• 60 - percentage of carbon in tenths (carbon is not indicated in a letter value);
• C2 - letter designation of silicon with index 2, indicates a 2-fold increase in the standard content (1-1.5%);
• X - presence of chromium up to 0.9-1%;
• F - tungsten content up to 1%;
• A - the added letter index A at the end of the marking indicates the minimum content of harmful impurities of phosphorus and sulfur, no more than 0.015%.

Types and applications

From the name it follows that the scope of use of these materials is limited to the production of springs, leaf springs, knives and other engineering products, which require an optimal combination of strength and elastic properties. However, there are other, more exotic applications for this type of metal.

Spring steels have long been used to make music wire, most commonly found in pianos. Musicians use spring steel in pianos because the piano string needs to be very tightly tensioned, and in this condition it must withstand the constant blows of the hammers of the keys.

Spring steel is also used to make guitar strings.

The armor and swords of actors who star in popular medieval films or TV series are also made from spring steel. They must be light enough to allow the actor to move freely while maintaining the authenticity of the image he is creating. After all, it is much easier to swing a sword made of spring steel than a weapon that is forged from iron or other heavy metals. The best part is that the armor and weapons that you see on the big screen are completely identical to those worn by the winners of knightly tournaments or the usual hand-to-hand combat of those days.

Production

Depending on further processing and the final type of part, steel is supplied in sheets, wire, hexagons, and squares. High performance qualities of the product are ensured by 2 components:

1. the structure of the metal, which is determined by the chemical composition and subsequent processing;
2. the presence of non-metallic inclusions in the structure, or rather the minimum quantity and size, which is eliminated at the stage of smelting and casting;
3. the shape of the part (spiral, arc) and its dimensions, which is determined by the calculation method.

When the spring is stretched, the inner and outer sides of the coils experience different degrees of stress: the outer ones are less susceptible to stretching, while the inner ones experience the greatest degree of deformation. The same applies to the ends of the spring: they serve as attachment points, which increases the load in these and adjacent places. Therefore, steel grades have been developed that are preferably used for compression or tension.

Thermo-mechanical treatment

Without exception, all spring steels are subjected to thermomechanical treatment. After it, strength and wear resistance can increase 2 times. The product is shaped in an annealed state, when the steel has the maximum possible softness, after which it is heated to 830-870 C and cooled in an oil or water environment (only for grade 60 CA). The resulting martensite is tempered at a temperature of 480 ºC.

All requirements and recommendations for this type are described in GOST 14959-79. Based on them, the company develops more detailed technological sheets that meet narrow parameters.

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