Effects Of Stress On Magnetic Properties And Microstructure Of Carbon Of Steel

3178 words | 6 page(s)

This paper focuses on various literatures that focus on the effects of stress on magnetic properties and microstructure of carbon steel. Based on the different literature it is evident that properties of ferromagnetic materials react to micro-structural and mechanical alterations. Additionally, differences in functional stress composition and geometry lead to alteration of the magnetization process of a material (Sablik, Kwun, Rollwitz & Cadena, 1992). The introduction part gives detail concerning the topic under study. Research background discusses the important aspect on the topic and different findings and conclusions on the topic under study. The last part is the state of art explaining what other have done, possible challenges and a recommendation for future research.

Carbon steel is obtained through a combination of iron and carbon. Carbon is the best material for iron to link with and it is used so as to create some hardening of the end product. Additionally with presence of carbon, steel properties such as density, hardness and malleability can be adjusted. The classification of iron is done on the basis of how much carbon is in it. High carbon is used to make fashioning cutting equipment due to its high level of hardness. On the other hard those classified to have lower to medium level of carbon are used to create metal sheeting that applied in construction as result of their hardness and malleability (Davis & ASM International, 1996, In Gray & Metallurgical Society of AIME, 1973). Therefore carbon steel being inexpensive it is often used in Heat Exchangers, Air Fin Coolers and Boilers. Water flowing through the pipes causes a lot of corrosion. Due to this, there is need for often inspection using verified and reliable NDE methods. The verified techniques include visual inspection, Remote Field Eddy Current, Magnetic Flux Leakage, Ultrasonic Internal Rotating Inspection System, Partial Saturation Eddy Current and Laser Optics. In this paper, Current, Magnetic Flux Leakage will be much focused on (Mandache, 2005).

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Research Background
Non-Destructive Testing
Non-destructive testing (NDT) has dramatically been acknowledged in the current industrial processes. The major aim of using this technique instead of other method is the fact that it cuts time consumption and it improves safety and productivity. The magnetic methods for NDT are magnetic flux leakage (MFL), electromagnetic acoustic transducer (EMAT) and lastly hystereis testing (International Conference on Advanced Nondestructive Testing & Lee, 2008).

The MFL is often used to detect cracks or defects on the surface of the ferromagnetic material such as steel. The effects of cracks are that they obstruct the flow of magnetic flux. To assess the leakage field hall effect sensors is used. The leakage field on the surface of the steel material enables description of the defect. On the other hand EMAT technique uses acoustic wave in the steel material. Reflection of the acoustic waves travels back into the EMAT transducer via magneto-elastic properties of the ferromagnetic material. Lastly hysteresis technique reacts to bulk properties. During this process, large volume of the material is subject to complete magnetization process (International Conference on Emerging Technologies in Non Destructive Testing, et.al, 2004, Sablik, Kwun, Rollwitz & Cadena, 1992). What is used as indicators of change on physical properties include coercivity and remanence.

Magneto-optical Kerr Effect
This is also known to be among the magneto-optic effect. It defines the alterations of light replicated from a magnetized source. It offers the best ways to achieve any magnetic alteration at various fields and temperatures.

The principle applied while applying this method is that when magnetic film produces linearly polarized light, the polarization changes into elliptic (Kerr ellipticity) while the principle axis is turned around (Kerr rotation). The degree of rotary motion and the level of ellipticity is proportional to an element of the magnetization of the film. The elements to be measured rely on optical configuration applied. Mostly used include perpendicular incidence and grazing incidence at 30 degrees angle and s-polarized light (Early, et.al 1987, Li, 2010, Salghetti-Drioli, 1999).

Effect of Stress on the Magnetic Properties and Microstructure of Carbon Steel
Carbon steel is obtained through a combination of iron and carbon. Steel qualifies to be a carbon steel in a situation where the proportions of particular elements are below specific percentages. For instance copper and silicon should be below 0.5% while manganese should be 1.65% and copper 0.4%. The major aim why manufacturers add carbon to steel is to make it harder and also quicken solidification process and more so cheaper alloying material (ASM International, 1990). If the content of carbon is changed, steel properties are also altered such that we have low, medium, high and ultrahigh carbon steel.

Low carbon steel has less carbon of about 0.05% to 0.3%. It is level of ductility is high and therefore cannot be machined and are usually used to make flat rolled sheets. They are also of low cost and cannot be changed be heat treatment. An addition of carbon hardens steel and decreases its weldability and ductility (In Gray & Metallurgical Society of AIME, 1973). Steel is considered to medium carbon steel if it has carbon content of about 0.3% to 0.6%. It is simpler to machine. Often silicon and manganese are added to get better quality. It is cheap. High carbon steel contain carbon content of about 0.6% to 1% are usually difficult to machine due to high content of carbon. It is susceptible to heat and therefore can be altered into various forms. Ultrahigh carbon steel on the other hand contains carbon of about 1.25% to 2%. It is very brittle and are extremely hard. It is very susceptible to heat and can be machined and has high resistance to wear (American Iron & Steel Institute, 1971).

It conventionally understood that magnetic properties of ferromagnetic materials react to micro-structural and mechanical alterations. Difference in functional stress, composition and geometry lead to alteration of the magnetization process either, by its own or through its combinations (Brookhaven National Laboratory, United States, Luhman & Snead, 1980). The measurement of this process is possible by use of hysteresis cycle. The monitoring process is done as the external situations vary to determine whether there is any kind of flaw. The major aim of this technological advancement is to enable monitoring services or quality management tool to material suppliers (Belfiore & Wiley Inter-Science, 2010). Magnetic testing meets the current needs for NDT of carbon steel elements.

To investigate this there are a several factors to consider. First is if the stress is in the elastic brackets of the material or it is above the limit and therefore the material is experiencing plastic deformation. Next is finding out if the material has positive or negative magnetostriction. Magnetostriction is defined as an alteration in dimension in ferromagnetic materials the process of being magnetized. Magnetostriction have a critical role in determination of the reaction of ferromagnetic material to stress. Increased magnetic induction is often witnessed in case where tensile stress is used. Stress generates effective magnetic field that reacts in combination with used magnetic field. Depending on the level of carbon in steel, the level of magnetostriction varies in relation to applied magnetic field and stress. To understand effect of stress on magnetization of a material, it is important to study the domain walls and structure (Brookhaven National Laboratory, United States & Welch, 1982).

There has been a lot of debate on matters regarding mechanical stress and its influence on magnetic properties of steel in general. From different literatures it is evident that permeability at specific values of magnetizing force is almost twice in a case where some stress is introduced. Based on this result is worth concluding that influence of the different factors such as chemical differences or alterations in metallographic composition on magnetic properties may be determined by the level of stress initiated during mechanical process or heat operations. Based on this explanation it is clear that in a process of using magnetic examination for the detection of errors the indication of tools used should be free from the impact of stress (Fischer, 1928, Sanford, 1924).

Despite the fact that effect of stress is at its maximum at reasonable values of magnetizing force, it is not a simple task to carry out a quantitative research on its magnitude due to lack of ways of placing the magnetic properties into an easy mathematic formula. Therefore to enable the observation, magnetic force should be expressed in terms of Kennelly’s law also known as reluctivity relationship. This force illustrates a linear connection of metallic reluctivity to magnetizing force (Dhaka, Berge, Kirschner, &Widdra, 2012). The reluctivity graph for carbon steel will tend to deviate from the straight line due to the availability of impurities. On an investigation on the stress effects on microstructure it is evident that the grains tend to be located around the fracture region. And on the other hand stress and strain curved about the pearlitic region of the microstructure (Durnten-Zurich, et.al 2013).

The functional principle of NDT which is also known as Non Linear Harmonic Analysis (NLHA) is created on the basis of magneto-mechanical analogy as illustrated below as fig 1. What is depicted defines the relationship between mechanical and magnetic instants of an atomic magnet. Taking into consideration the macro-scopic of different materials the outcome of the comparison is depicted in the relationship of macro-mechanical characteristics of the magnetic material such as yield strength, tensile strength and its anisotropy (Karama, 2011, Wolf, Vogel, Vogel & Vogel, 2010). On the other hand magnetic characteristics determined by the shape and magnetic hysteresis curve and the amounts defining it formation such as permeability, coercivity, remanence and saturation magnetization.

Fig 2 is an illustration of the working principle of NLHA. To carry out the measurement, there is application of sinusoidal input so that it can create magnetic field strength as illustrated. The indication is adjusted by the hysteresis graph and the level of output is determined by the receiving coil. This can be discussed in terms of electrical engineering such that hysteresis curve represent a medium of transfer or communication of the steel under investigation (Sablik, Kwun, Rollwitz & Cadena, 1992). The outlook of the hysteresis is determined by the level of harmonic received. It is important to note that NLHA method ought to be well illustrated by determining the relationship between the higher harmonic and the mechanical values achieved by the destructive investigations (Non-Destructive Testing Society of Great Britain & British Institute of Non-destructive Testing, 1959, Woodward, & Transport and Road Research Laboratory, 1989).

State of Art
What other have done
A lot of research has been carried out on this topic under study. From the study conducted by Jiles, it is concluded that the more the carbon content in plain carbon steels the higher the coercivity and hysteresis will be lost and the initial variation in permeability will be reduced. This influence is contributed by the high pinning of the domain walls. In this study the alteration of carbon content did not have much effect on the remanence (Jiles, 1999). On a study conducted to investigate the heat operation of AISIS 4130, it was found that the three microstructures generated, pearlite, bainite and martensite, pearlitic microstructure showed the least coercivities, hysteresis but greatest original variation in permeability. Martensite showed the greatest coercivities and hysteresis loss and less original differential permeability. Bainite on the other had showed in-between values as those of the three constraints (Jiles, 2001).

It is further evident that the size of the particles of carbon steel has little or no effect on the magnetic properties. It is a little interesting due to the fact that it would be anticipated that the greater the displacements linked to smaller particles would affect the magnetic properties (Kwun, Sablik, Burkhardt & Southwest Research Inst, 1993). Another study focused on difference of remanence and highest induction with change in temperatures. It was evident that remanence and highest induction was not static across the tempering period and matched to alterations on microstructure (Gorkunov&Batukhtina, 1989, 1987).

From the literatures reviewed it is evident that researchers experienced some difficulties in during their study. For instance one of the challenges associated to MFL is obtaining accurate assessments on the degree and diffusion of flaws despite not having elevated stress levels. The availability of high casing stress brings about high levels of ambiguity on the interpretation of MFL. Additionally, another challenge is that during the research process it is difficult to identify a tool of measurement that is independent of stress such that the end results are not distorted (International Conference on Structural Integrity and Failure & Ferguson, 2011). Despite the fact that effect of stress is at its maximum at reasonable values of magnetizing force, it is not a simple to carry out a quantitative research on its magnitude due to lack of ways of placing the magnetic properties into an easy mathematic formula.

In conclusion it is evident that there are distinct tendency in the hysteresis constraints as tensile stress in raised towards the direction of function of the magnetic field. The hysteresis decrease and coercivity reduces with stress in flexible range. The remanence and highest differential permeability as shown a rise with an increase in tensile stress until the Villari reversal point is achieved. The alteration of stress is linked to the contribution of stress to anisotropy ((Jiles, 1999). After mechanical yielding is achieved, the trends observed before are altered. The reason behind this is the generation of displacements from the plastic deformation changing the observed coercivity of the material ((Jiles, 2001). From the study it is evident that microstructure has a significant role in verifying the magnetic reaction of a ferromagnetic material.

Future Work
Too much research for pipelines application has been conducted on the effect of stress on MFL signal. Some of the case studies include uniaxial tension and firmness, biaxial loading and loops. Based on this, there is need to further research so as to understand more on the impact of stress on MFL assessments in casing and more so distinguish corrosion defects using the same assessments (Non-Destructive Testing Society of Great Britain & British Institute of Non-destructive Testing, 1959).

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