Let’s discuss factors related to material manufacturing processes that can lead to spring failure. During wire production, defects can arise that affect the wire’s quality, specifically those related the surface condition of wire.

Also read:SPRING MANUFACTURING : WHICH MATERIALS ARE USED

Briefly and broadly, let’s remember the steel manufacturing process.

In Italy, steel is almost exclusively manufactured in electric furnaces, starting from scrap that is melted through an electric arc. Other chemical elements that contribute to defining the characteristics of the steel are then added.

The liquid bath, through a series of structures-devices, is transformed into a cast that solidifies, producing prismatic solids called blooms and billets, weighing around 2 tons.

These first steel mill products are cooled and then transformed.

The billet is heated to temperatures close to 1,200°C, to make the solid plastic and prone to deformation, which occurs through drawing.

During the drawing process, the wire undergoes a series of successive reductions, usually up to 40, by varying the advancement speed. The initial speed is typically around 5 m/min, increasing to 100 m/s at intermediate stages and finally reaching 150 m/min. This process produces a round wire, which is then supplied in coils and constitutes the steel mill’s finished product.

Material Structure

Within this family, we find defects related to the following aspects:

• Solidification processes: defects that occur during the transition from the molten cast (at a temperature greater than 1,500°C) to the first semi-finished product (bloom or billet) with intermediate consistency between solid and liquid (temperature 900-1,000°C).
• Drawing processes that involve heat treatments, reduction steps, wire lubrication, and cooling rates. All of this occurs during the transition from billet to wire rod. These are defects for which the steel mill is responsible.

These are defects for which the steel mill is responsible.

The following defects are found in this area:

• Non-metallic inclusions;
• Blowholes and micro-cavities;
• Microstructural defects;

Non-metallic inclusions;

Non-metallic inclusions interrupt the structural homogeneity of the material. Within the metallic matrix, there are elements embedded in the solid that do not belong to the metal’s crystal lattice.

Inclusions can be of exogenous or endogenous origin.

Exogenous inclusions are due to fragments of the refractory coating of the blast furnace that detach and fall into the bath or due to slag or foreign materials present in the scrap used to produce steel and pollute the charge. Their effect on the material structure is identical to that of endogenous inclusions.

Endogenous inclusions are generated by chemical reactions inside the casting bath with oxygen, sulfur, silicon, and nitrogen, forming oxides (globular OG or elongated OA), sulfides (SS), silicates (OS), and nitrides. They are classified into morphological classes. The purity of the wire is defined by a standard that provides for 9 classes (0 is the best and 8 is the worst). The class is assigned based on the number, distribution, and morphology of the inclusions.

These compounds are non-metallic, have a solid consistency, and can have deleterious effects even in very small sizes (15-20 microns). In fact, these zones are devoid of elasticity because the compounds are very hard, and in case of mechanical stress, they fracture.

The danger of these “non-metallic islands” is greater the more severe the application to which the springs made with this material are subjected and the closer they are to the surface. In highly stressed springs, whose fatigue life must be practically infinite, the presence of non-metallic inclusions must be prevented as much as possible. In this sense, casting processes have been developed that allow for “clean” steels. Clean or superclean versions of pre-tempered SiCr, SiV, and SiCrNi steels are available on the market, in which a maximum size of non-metallic inclusions is guaranteed in the area below the wire surface up to 1 mm.

Below is an example of a specification for clean steels for springs, which certifies the absence of non-metallic inclusions with dimensions greater than 15 microns in an inspection area of 1,000 mm² up to 1 mm below the wire surface. For a superclean steel, the guarantee is extended to inclusions larger than 10 microns.

As mentioned earlier, inclusions are more dangerous when they are near the surface. For this reason, in high-performance steels (such as pre-tempered valve spring steels), the surface layer of the rod is removed. This process is known as “peeling” or “skin passing” and results in a peeled rod, also known as a “peeled wire rod“. In general, for other uses, the starting billet is ground to eliminate the surface layers, which have irregularities and are more likely to contain non-metallic inclusions

Blowholes and micro-cavities

Blowholes are generated during the cooling process of the cast, where gases can remain trapped in the solid structure being formed. This type of defect is identified by performing radiographic and ultrasound tests on the billets. It is clear that blowholes are an area that interrupts the structural homogeneity of the steel and constitutes the starting point for fractures.

Microstructural defect

To explain microstructural defects, we must first explain what the microstructure of steel is. Steels have a polycrystalline structure, which is highlighted by inspecting a sample with a scanning electron microscope at specific magnifications. This is called the microstructure of steel. Under the microscope, the microstructure in section appears as a cluster of polygonal structures (the grains), juxtaposed like the tiles of a mosaic. This structure is created during the casting process (nucleation). The characteristics of the microstructure depend on the chemical composition of the cast and the control of the thermal gradient of the casting. The better the cooling control (slow and uniform), the smaller the grain size. A small (fine) grain is an indicator of the quality of the steel’s microstructure. The American regulatory agency (ASTM) has created a standard that classifies grain size on a scale from 1 (coarse) to 8 (fine). For wire rods of steel for springs, it is preferable to have a grade greater than 4, and for heavy-duty uses, a grade greater than 6 or 7.

Granular microstructure

Grain dimensions according to non ASTM scale

The microstructure of steel is also determined by the process used to produce wire rods from billets. The billets are heated to 1,200°C, and during the coiling process to form a coil, the temperature gradually decreases from 1,200°C to 500°C. This phase causes the material to undergo recrystallization, which affects the grain size.

Grain refinement can be achieved through heat treatments that involve specific and constant cooling gradients. These treatments are based on heating cycles that must occur at temperatures above AC3 (austenitization temperature 870-880°C) and are isothermal annealing with controlled cooling. As mentioned earlier, cooling is the key factor in the formation of the granular structure.

Microstructural Defects

The most common microstructural defects are as follows.

Segregation

Carbon is not distributed uniformly; it concentrates at the core and impoverishes the surface.

Inter-granular defects

The spaces between one grain and another are filled by compounds that have lower solidification temperatures, including impurities and Fe-C compounds with a rigid structure such as cementite.

In these structures, ruptures can occur that exploit the cracks that occur between the grains due to the rigid compounds between one grain and another.

Localized formation of martensite

Martensite is one of the possible structures of the Fe-C alloy, particularly hard and, therefore, brittle. It is the structure that typically forms in rapid cooling processes, following heating above AC3. In the quenching process, it is the intermediate structure, which is then modified with tempering to achieve the necessary degree of toughness for the elastic use of tempered components. Untempered martensite is a rigid structure from which cracks propagate.

Grain irregularity

Grains of different sizes can coexist. This lack of uniformity compromises the mechanical characteristics of the wire.

Decarburization

As the word suggests, a phenomenon that leads to a reduction in the presence of carbon in a specific area of the wire. This situation occurs at the surface level, with deleterious effects on the fatigue resistance of the spring. The decarburized zone appears white and less elastic under the microscope and is prone to crack formation. Decarburization occurs during heat treatment or as a result of processes that lead to localized heating of the raw material. For example, the grinding of billets. In the case of tempered wires, localized decarburization can occur due to an inefficient wire cooling process.

EN 10270-1 and -2 prescribe for steels containing carbon a maximum depth with superficial decarburization defects equal to 1.5% of the wire thickness.

Hydrogen embrittlement

Hydrogen penetrates between the grains of the polycrystalline structure and deteriorates the mechanical properties of the steel. This defect can occur during pickling of the wire rod, before the drawing reductions. Cleaning the wire rod from surface oxidation occurs with a passage in acid baths, normally sulfuric acid, which contains hydrogen.

If the wire rod is not adequately cleaned, hydrogen penetrates the material’s structure and embrittles it.

These defects are identified with non-destructive testing (ultrasound) and/or by performing microscope analyses on specimens taken at the beginning and end of the wire rod coil.

Angelo Passarotti

He graduated in electronic engineering from the Milan Polytechnic in 1992. Since 2000, he has been working at Mollificio Valli as technical sales manager.

Over the years, he has acquired extensive experience in the calculation and technical aspects of spring production.

He has always been passionate about mathematics and statistics, and has had the opportunity to apply his knowledge in statistical control techniques, metrological aspects and in general in the practical field of problem solving and continuous improvement.