{"id":17938,"date":"2023-01-10T16:36:57","date_gmt":"2023-01-10T15:36:57","guid":{"rendered":"https:\/\/mollificiovalli.it\/the-material-structure-how-it-affects-the-quality-of-springs\/"},"modified":"2024-02-05T09:08:30","modified_gmt":"2024-02-05T08:08:30","slug":"the-material-structure-how-it-affects-the-quality-of-springs","status":"publish","type":"post","link":"https:\/\/mollificiovalli.it\/en\/the-material-structure-how-it-affects-the-quality-of-springs\/","title":{"rendered":"THE MATERIAL STRUCTURE: HOW IT AFFECTS THE QUALITY OF SPRINGS"},"content":{"rendered":"

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

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Also read:<\/strong><\/em>SPRING MANUFACTURING : WHICH MATERIALS ARE USED<\/a><\/p>\n<\/blockquote>\n

Briefly and broadly, let’s remember the steel<\/a> manufacturing process<\/strong>.<\/p>\n

In Italy, steel is almost exclusively manufactured in electric furnaces<\/strong>, 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.<\/p>\n

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

These first steel mill products are cooled and then transformed.<\/p>\n

The billet<\/em> is heated to temperatures close to 1,200\u00b0C, to make the solid plastic and prone to deformation, which occurs through drawing.<\/p>\n

During the drawing process, the wire<\/em> 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<\/em>, which is then supplied in coils and constitutes the steel mill’s finished product<\/strong>.<\/p>\n

Material Structure<\/h1>\n

Within this family, we find defects related to the following aspects:<\/p>\n

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  • Solidification processes<\/strong>: defects that occur during the transition from the molten cast (at a temperature greater than 1,500\u00b0C) to the first semi-finished product (bloom or billet) with intermediate consistency between solid and liquid (temperature 900-1,000\u00b0C).<\/li>\n
  • Drawing processes<\/strong> 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.<\/li>\n<\/ul>\n

    These are defects for which the steel mill is responsible<\/strong>.<\/p>\n

    The following defects are found in this area:<\/p>\n

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    • Non-metallic inclusions;<\/li>\n
    • Blowholes and micro-cavities;<\/li>\n
    • Microstructural defects;<\/li>\n<\/ul>\n

      Non-metallic inclusions;<\/h2>\n

      Non-metallic inclusions interrupt<\/strong> 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.<\/p>\n

      Inclusions can be of exogenous<\/strong> or endogenous<\/strong> origin.<\/p>\n

      Exogenous<\/strong> 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.<\/p>\n

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

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

      The danger of these “non-metallic islands<\/em>” 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<\/strong>. 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<\/em>” steels. Clean<\/em> or superclean<\/em> versions of pre-tempered SiCr<\/em>, SiV<\/em>, and SiCrNi<\/em> steels are available on the market, in which a maximum size of non-metallic inclusions is guaranteed<\/strong> in the area below the wire surface up to 1 mm.<\/p>\n

      Below is an example of a specification for clean<\/em> 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<\/sup> up to 1 mm below the wire surface. For a superclean<\/em> steel, the guarantee is extended to inclusions larger than 10 microns.<\/p>\n

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

      Blowholes and micro-cavities<\/h2>\n

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

      Microstructural defect<\/h2>\n

      To explain microstructural defects, we must first explain what the microstructure of steel<\/strong> is. Steels have a polycrystalline<\/em> 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<\/strong>. This structure is created during the casting process (nucleation<\/em>). 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<\/em>) grain is an indicator of the quality of the steel’s microstructure. The American regulatory agency (ASTM<\/em>) has created a standard that classifies grain size<\/strong> on a scale from 1 (coarse<\/em>) to 8 (fine<\/em>). For wire rods<\/em> 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.<\/p>\n

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