Answer MEP Question 2

Q. Describe TWO methods of tracing a superficial crack in a marine machinery component; Explain the procedure for arresting the propagation of a crack in a machinery component. 

1. Liquid penetration test:- It is a low-cost method of inspection for the detection of surface cracks in non-porous materials. It can detect casting & forging defects, cracks and leakages in new products, and fatigue cracks in components that are in service. This method does not require special skills. 

The red Dye penetration method is normally used on board. 

a. The surface is cleaned by using cleaning fluid in an aerosol can. 

b. The dye penetrant from the can is sprayed on the top of the surface. 

c. The penetrant seeps into the surface crack. 

d. After adequate penetration time, the excess penetrant is removed using the cleaning fluid. 

e. The developer is applied, to draw penetrant out of the flaw. 

f. This gives the red visible indication of flow, which can be seen clearly in the normal white light.

2. Magnetic particle test:- 

They are conducted on ferromagnetic materials and can detect surface and subsurface defects. These are used on ferrous components to detect the presence of any subsurface cracks. The material to be checked is magnetized and sprayed with a special powder. This collects on the location of the crack and highlights the area.  

a. Finely divided ferromagnetic particles, applied over the surface. 

b. The magnetic field is applied to the material under test.

c. The discontinuities in the surface or sub-surface will cause a leakage field, the particles will be gathered and held by the leakage field.

d. The collection of particles forms an outline of the discontinuity and indicates its location, size, shape, and extent.

Advantages:

Large surface areas of complex parts can be inspected rapidly.

Can detect surface and sub-surface flaws.

Surface preparation is less critical than it is in the penetrant inspection.

Magnetic particle indications are produced directly on the surface of the part and form an image of the discontinuity.

Equipment costs are relatively low.

Disadvantages:

Only the ferromagnetic material can be inspected.

Proper alignment of magnetic field and defect is critical.

large currents are needed for very large parts.

Requires a relatively smooth surface.

Paint or other non-magnetic coverings adversely affect sensitivity.

Demagnetization and post-cleaning are necessary.

The most appropriate method for crack detection in crankshaft:

The ultrasonic sound test is preferred over magnetic particle testing because of the following reasons:

A high current is required for magnetic particle testing and it's very hard to get the magnetic dust removed.

US test is easier and can be done in place, even if the shaft portion is not accessible it is easy to do at any surface of the crank present and the depth of the crack.

It is easy to compare the last result with the present one to see if the crack has propagated further.

With the latest technology, the testing tool size is reduced to the size of a handheld and the readings are obtained wirelessly on the receiver.

Explain how the Propagation of a crack in a machinery component can be arrested.

- Cracks are usually of 2 types, 

1. Cracks due to an accident or incident like impact, freezing or overheating

2. Cracks during normal operating conditions, which indicates a design issue or usage beyond the design capabilities.

Pinning:

- Pinning is the most commonly used technique for repairing cracks in cast iron heads as it’s fast, reliable and cheap. It can also be used to repair aluminium castings as well.

- Pinning involves drilling holes in both ends of the crack to keep it from spreading, then drilling holes at various intervals along the length of the crack, installing overlapping pins to fill the crack, then peening over the pins with an air hammer to seal and blend the surface. Either tapered pins or straight pins may be used.

- If a crack is along an outside edge or corner that requires support to hold the sides of the crack together, or if the crack is in an area that would open up or pull apart when the casting is under load or gets hot, ordinary pins can’t be used. Therefore, use “locks” to hold the two sides of the crack together, and use special pins that have a “spiral hook” or "reverse pitch" thread pattern.

These pins can actually hold a crack together rather than just fill it.

- Welding is also considered on some occasions as crack propagation arresting method. The basic idea is to melt the surrounding metal and fill the crack with molten metal and a filler rod.

Furnace Welding Cast Iron

- To the furnace weld a cast iron head, the head is first preheated to 700°C (cherry red) in an oven.

- When the head preheat, the temperature has stabilized (it takes about an hour), a “neutral flame” oxyacetylene torch with slightly more acetylene than oxygen is used to melt the cast iron (which melts at 1315° to 1400° C). The crack can then be filled using a 1/4” cast iron filler rod and Borax flux.

Flame Spray Welding.

- For repairing cracks in cast iron heads and blocks is flame spray (powder) welding. The process is similar to brazing but uses nickel powder and a special torch to produce high strength repairs,

- The main advantages of this technique are that it is easier and faster than furnace welding. The process goes very quickly and can fill a three-inch hole in less than a minute.

TIG Welding Aluminum

- Cracks in aluminium heads are often repaired by TIG welding (though pinning also works with small accessible cracks). The first step is welding aluminium is preparing the crack. The head must be clean, grease-free and dry before grinding out the crack.

The crack must be ground all the way out. After grinding, the surface of the metal should be cleaned with a stainless-steel wire brush.