PSI - Issue 13

Tatiana Oršulová et al. / Procedia Structural Integrity 13 (2018) 1689–1694 Tatiana Oršulová et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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There are many techniques that allow determination and evaluation of mechanical changes in materials. One of them is the nondestructive evaluation (NDE). The rapidly expanding role of the NDE methods in manufacturing, power, construction and biomedical industries has generated a large demand for practitioners, engineers, and scientists with knowledge of the subject. The NDE presents current practices, common methods and equipment, applications and the potential and limitations of current NDE methods, in addition to the fundamental physical principles underlying the NDE. Those methods can have a dramatic effect on the cost and reliability of products. The methods can be used to evaluate prototype designs during the product development, to provide feedback for process control during manufacturing and to inspect the final product prior to service. Additionally, the NDE methods permit products to be inspected throughout their serviceable life to determine when to repair or replace a particular part. In today’s economy, the concepts ‘‘repair or retire for cause’’ and ‘‘risk - informed inspection’’ are becoming very important (Shull, 2002). The NDE offers a margin of safety for this equipment and gives users the means with which to determine when equipment must be repaired or retired. The basic principle of NDE is quite simple. To determine the quality or integrity of an item nondestructively, one should simply find a physical phenomenon that will interact with and be influenced by a test specimen without altering the specimen’s function. This article focuses on one possible electromagnetic NDE application: evaluation of mechanical conditions of the austenitic steels through a measurement of their magnetic properties. The main goal is to measure and thus separate different levels of applied plastic deformation of concrete conductive biomaterials. Commercially available magnetic field sensor and device Magnet Physik are used for this purpose and obtained results are presented and discussed. The austenitic stainless steels are the ternary alloys of Fe-Cr-Ni. Their microstructures consist of very clean FCC (Face Centered Cubic) crystals in which all the alloying elements are held in a solid solution. Those steels are called austenitic because of their final structure. They are austenitic at the room temperature. The austenitic stainless steels are widely applied in chemical, petrochemical, biomedical and many other fields. The most widely used austenitic stainless steels are the following grades: AISI 304, 316L and 316Ti. Those materials belong among the types of the so-called high-alloy TRIP (Transformation Induced Plasticity) steels. These types of steels contain substantial number of alloying elements such as Cr and Ni, which improve pitting and corrosion resistance (Rodríguez- Martínez et al., 2011). The most common of selected steels is the AISI 304 - this steel contains essentially 18% of Cr and 8% of Ni. Content of C is limited to maximum of 0.08%. This material is paramagnetic and it has a cubic closed γ -phase. After the plastic deformation, the phase is transformed to a BCC (Body- Centered Cubic) α' - martensite phase. Thus, this material becomes partially ferromagnetic after the plastic deformation. Many reports refer that the magnetic effects of martensite content in AISI 304 is caused by the progressive cold rolling (Tourki et al., 2005; Tukur et al., 2014; Vertesy et al., 2005). The AISI 304 has the following selected properties: a high ductility, excellent drawing, forming, and spinning. A low carbon content means less carbide precipitation in the heat-affected zone during the welding and a lower susceptibility to intergranular corrosion. It also resists to most oxidizing acids and salt spray. Those properties make the AISI 304 widely used stainless steel. Both the AISI 316L and AISI 316Ti include higher percentage of alloying elements. Different chemical composition is reason why these steels change their primary deformation mechanism from twining (in 304 grades) to slipping: typical for 316 grades (Correa et al., 2017). Differences in the microstructure after the heat treatment were observed by an optical microscope. In microstructure of the AISI 304 in initial state, there were visible austenitic grains with different size. The annealing twins were also present and a relatively large amount of MnS based inclusions. A representative sample of the evaluated microstructures, the AISI 304 steel, additionally contains a significant proportion of α' -martensite (Fig.1a). Solution annealing (1050 ° C / 35 min) resulted in the structural homogenization of the materials and the effect of the prior technological treatment was eliminated. Rapid cooling in the water prevented phase and interstitial excretion inside the matrix and also along the grain boundaries. In the examined microstructures there was a significant reduction of plastic deformation, with simultaneous dissolving the bulk of sulphides, oxides and other compounds. The grains boundaries are affected by heat treatment. In the AISI 304 microstructure, significant annealing twins are visible (Fig.1b). 2. Experimental material