NDT X Ray Testing | NDT Radiographic Testing | Radiography Test For Casting

Interpretation of Casting Radiographs

Castings are made of different metals and by various methods. Sand molded castings often have more surface irregularities than casting produced by metal mold and investment methods. Knowledge of various casting processes and casting defects helps effective radiographic evaluation. The major defects in castings and weldments are given below:

1. Gas porosity become visible as round or elongated soft shady spots, taking place individually or in clusters or distributed throughout the casting. This is caused by gas formation during solidification by evaporation of moisture or volatile material from the mold surface. Insufficient core baking, venting or entrapment of air in the cope surface of the casting before complete solidification could also be the cause. The term ‘gas porosity’ is used to refer to shady dark spots on the radiograph, whose diameters are usually within 0 to 1 mm.

01-gas porosity in castings

01-NDT Testing of Gas Porosity

2. Gas holes appear as dark circular images, isolated or in clusters. Gas holes are caused by gas trap in molten metal. Suppose the molten metal solidifies in the casting before all gases escape, it results in gas entrapment producing gas holes.

01-gas holes - flaws - NDT radiographs

3. Micro-porosity/Shrinkage porosity/Micro-shrinkage: These appear as an overall mottled appearance in aluminum alloy castings, and dark streaks or a spongy appearance in magnesium alloys. These are very fine cavities, usually around the grain boundaries. This defect occurs in casting when overall metal shrinkage is more than the normally expected shrinkage factor. The defect is due to improper feeding of the molten metal and occurs when the pouring temperature is higher than the ideal temperature.

4. Shrinkage appears as dendrite, filamentary or jagged darkened areas. These are caused due to contraction of metal while the casting solidifies. This defect usually occurs when there is change in section thickness of the casting and non-uniformity of temperature at different thicknesses.

01-NDT Testing of shrinkage

5. Cracks: These occur as hot tears or cold cracks (also called stress cracks). Hot tears appear as rugged dark lines of variable width and numerous branches with no definite line of continuity. Hot tears occur during or immediately after solidification. Cold state cracks shows generally as a continuous single narrow, sharp dark line all over the length. Such cracks take place whereas internal stresses are found out by a thermal gradient.

01-NDT Testing of cracks - hot tears - stress cracks

6. Dross appears as a dark, round or irregularly shaped images due to slag filling up the void entrapped in castings. These may look lighter if the density of inclusion is more than the density of the parent material. Inclusions may be due to slag, sand or oxides.

01-radiography test measurements - x ray tests of ndt

7. Cold shut appears as a dark line of variable length with a definite, smooth outline. Cold shuts are formed when two streams of molten metal flowing from different directions fail to unite. The creation of a cold shut is due to slow pouring, interrupted pouring or pouring the molten metal at too low temperature.

01-cold shuts - NDT radiographic test

8. Segregation appears as lighter or darker patches on the radiograph depending on the density of segregated constituents of the alloy. During the melting and casting processes, certain constituents of the alloy may separate from the alloy. This local concentration of the constituents results in a difference in densities on the radiograph, provided the density of the segregated portion is different from the density of the casting alloy. It is possible to have local segregation, in which shrinkage or a hot tear are filled with segregate. The terms used for such indications are shrinkage segregation and sealed hot-tear, respectively.

9. Misruns seem as a major darkened space of variable dimensions with a distinct, smooth outline. Misruns are produced by failure of the molten metal to completely fill a section of casting leaving the region void. This could occur as a result of lack of fluidity or pouring at too low a temperature.

10. Diffraction mottling appears as a spurious image on the radiograph and isn’t a casting defect. This mottling effect appears as star shaped representation in case of austenitic steel or in aluminum alloys. This effect of visualization are caused by the diffraction of radiation by the crystals of the metal. The mottling appearance vanishes if the radiograph is taken at a slightly changed angle of incidence.

11. Diffused chaplets are small bars with end plates used for maintaining the portion of mould core. These chaplets normally ruse with the casting. If not fused, these appear on the radiograph as darks, smooth lines conforming to the shape of the chaplet. This is caused by pouring the metal at too low a temperature to fuse the chaplet.

What Is Fuel Cell Technology | Fuel Cells Pros And Cons

Introduction to Fuel Cell

A fuel cell is an electrochemical mechanism that produces power without combustion by joining hydrogen and oxygen to generate water and heat. The voltage created by a single cell is typically rather small (< 1 volt), so many cells are connected in series to create a useful voltage. Hydrogen fuel cells are feasible to modernize the way the globe visions, generates, and utilizes energy. So far, hydrogen has been used to power, mobile phones to NASA’s space programs etc.

At room temperature, hydrogen is:

· The easiest and most rich constituent

· Lighter than air

· Colourless

· Odourless

· Harmless

· Additional energy per weight than any other energy mediums.

· More proficient than current fuels used in transportation

· The only by products of Hydrogen Fuel Cells are water vapour, electricity, and heat translating to Zero Carbon Emissions

Fuel cells are really attractive from an ecological stand point.

01-Hydrogen Fuel Cells - what is fuel cell technology

Hydrogen Fuel and air contains oxygen react when they come into make contact with a porous membrane (electrolyte) which split them. This reaction end results in a transmission of electrons and ions across the electrolyte from the anode to the cathode. If an external load is fastened to this arrangement, a complete circuit is created and a voltage is generated from the stream of electrical current.

Fuel Cell vs. Battery

Essential operating principles of both are very similar, but there are numerous intrinsic differences.

Hydrogen Fuel Cell Galvanic Cell (Battery)

· Open system

· Anode and cathode are gases make contact with a platinum catalyst.

· Reactants are externally supplied, no recharging required.

· Closed system

· Anode and cathode are metals.

· Reactants are internally conducted, require periodic recharging.

Fuel Cell Vs. Internal Combustion Engine

Similarities Differences

· Both use hydrogen-rich fuel.

· Both employ compressed air as the oxidant.

· Both require cooling.

Fuel cell:

· Output is electrical work.

· Fuel and oxidant respond electrochemically.

· Modest to nil pollution produced.

I.C. Engine:

· Output is mechanical work.

· Fuel and oxidant reacts by the process of combustion.

· Use of fossil fuels can generate considerable pollution.

Hydrogen Fuel Cell Efficiency

· 40% efficiency generated, converting methanol to hydrogen in reformer

· 80% of hydrogen energy content converted to electrical energy

· 80% efficiency for inverter/motor – Converts electrical to mechanical energy

· Overall efficiency of 24-32%

Auto Power Efficiency Comparison

Technology System
Efficiency
Fuel Cell 24-32%
Electric Battery 26%
Gasoline Engine 20%

A hydrogen fuel cell is similar to a battery in that it generates electricity from an electrochemical reaction. Both batteries and hydrogen fuel cells exchange chemical energy into electrical energy and also, it generates heat as a by-product of this process. However, a battery holds a closed circuit which stores energy within it and once this is run down the battery must be disposing of, or recharged by using an external supply of electricity to force the electrochemical reaction in the reverse direction.

01-working of PEM fuel cells - Fuel Cell Operation

Likewise a fuel cell uses an external delivery of chemical energy and it can run for an indefinite period of supply with a resource of hydrogen cylinder and a source of oxygen (usually air). The resource of hydrogen is in general referred to as the fuel and this presents the fuel cell its name, even though at this instant, no combustion happened. Oxidation of the hydrogen as an alternative takes place electrochemically in a very proficient way. During oxidation, hydrogen atoms react with oxygen atoms to form water; in this method electrons are freed and flow through an external circuit as an electric current.

01-fuel cell stack - portable fuel cell

Fuel cells can differ from miniature devices producing only a few watts of electricity right up to large power plants producing megawatts. All fuel cells are based just about a central design using two electrodes divided by a solid or liquid electrolyte that carries electrically charged particles between them. A catalyst is regularly used to speed up the reactions at the electrodes. Fuel cells are normally classified according to the nature of the electrolyte they use. Each type requires particular materials and fuels and is suitable for different applications.