Discussion

Figure 2.1

Figure 2.1 is the actual specimen of number 2, it is appeared that the defect for this surface area is root crack. The crack can be seen just next to the weld carter. It can be seen after 3 to 5 minutes after dye penetrant is applied.

A root crack is the crack formed by the short bead at the root (of edge preparation) beginning of the welding, low current at the beginning and due to improper filler material used for welding. Major reason for happening of these types of cracks is hydrogen embrittlement. These types of defects can be eliminated using high current at the starting and proper filler material. Root cracks start at the root and extent part way into the weld. They are the most common type of longitudinal crack because of the small size of the first weld bead. If this type of crack is not addressed then it will usually propagate into subsequent weld passes, which is how full cracks (a crack from the root to the surface) usually form.

ConclusionMagnetic Particle Inspection (MPI) is a Non-Destructive Testing (NDT) process for detecting surface and slightly subsurface discontinuities in ferromagnetic materials such as iron, nickel, cobalt, and some of their alloys. The process puts a magnetic field into the part. The piece can be magnetized by direct or indirect magnetization. Direct magnetization occurs when the electric current is passed through the test object and a magnetic field is formed in the material. Indirect magnetization occurs when no electric current is passed through the test object, but a magnetic field is applied from an outside source. The magnetic lines of force are perpendicular to the direction of the electric current which may be either alternating current (AC) or some form of direct current (DC) (rectified AC). There are pros and cons of this testing.

The advantages are, it can detect both surface and near-surface indications. Second, the surface preparation is not as critical compared to other Non-Destructive Testing methods. Most surface contaminants will not hinder detection of a discontinuity. Next, Magnetic Particle Inspection is relatively fast method of examination and the indications are visible directly on the surface. This type of testing also is a very low-cost compared too many other methods. Post-cleaning is generally not necessary for this test. It is a relatively safe technique; materials generally not combustible or hazardous and lastly, the Magnetic Particle Inspection is a portable Non-Destructive Testing method, especially when used with battery-powered yoke equipment.

The disadvantages of the Magnetic Particle are, it is restricted to ferromagnetic materials; usually iron and steel, and cannot be used on austenitic stainless steel. It is sometimes unclear whether the magnetic field is sufficiently strong to give good indications. This type of test cannot be used if a thick paint coating is present because some of the paints and particle suspension fluids can give a fume or fire problem, particularly in a confined space. Lastly, Magnetic Particle Testing is spurious, or having non-relevant indications, which is probable, and thus interpretation is a skilled task.

In conclusion, although Magnetic Particle Testing is not the best method for all applications, it is a useful and often be the best Non-Destructive Testing method for detecting surface and slightly subsurface discontinuities and also for quick, low-cost inspections.