Posted in Manufacturing Engineering, Manufacturing process, Manufacturing technology, Mechanical by TheMech On August 29, 2013. No comments

Welding Defects-Solidification Hot Crack. Solidification cracking (Fig. 1) normally occur when: Weld.

Thermit welding steps:

It is referred to the cavities which are formed due to shrinkage of weld metal during its solidification. The shrinkage cavity usually occurs during welding of thick plates in a single pass using submerged arc welding or electro slag welding processes. Proper amount of filler material has to be supplied for compensation during shrinkage to avoid this king of defect. It is referred to the cavities which are formed due to shrinkage of weld metal during its solidification. The shrinkage cavity usually occurs during welding of thick plates in a single pass using submerged arc welding or electro slag welding processes. Proper amount of filler material has to be supplied for compensation during shrinkage to.

Various steps involved in the non pressure fusion thermit welding of metal parts is explained below. The mold is non-repetitive in nature and is used for repair welds.

1. Clean the joint:

An oxy-acetylene torch may be used to clean the metal surfaces by heating. During cleaning all dirt, grease, loose oxides, scale etc., must be removed.

2. Allow for contraction: Frozen fever full movie in hindi dubbed subtitles.

After cleaning, the part to be welded are to be lined up with a space of about 1.5 to 6 mm between the ends, depending upon the size of the parts to be joined.

This space makes you for

i. The contraction of the thermit steel in cooling

ii. The shrinkage of the base metal which has been heated during the welding operation

3. Construct the mold:

After the parts have been cleaned and spaced properly, the next stage is the making of the wax pattern from which the mould will be formed and which must in shape constitude a replica of the eventual weld.

The molding materials should be about 100 mm thick between the wax pattern and the molding box at all points.

The mold should be provided with the necessary number of pouring gates, heating gates and risers depending on the size of the weld.

4. Preheating the mold:

The mold prepared as above is then preheated in order to:

i. Melt away and remove the wax thereby leaving a mold cavity in the exact shape of the weld.

ii. Dry the mold thoroughly otherwise the superheated molten metal will form steam within the mold and cause porous weld.

5. Crucible and its charging:

Thermit mixture is charged in the container knows as crucible or reaction vessel. This vessel is of conical shape and is lined with magnesia tar lining.

The outside shell of this vessel is made up of steel sheet. Located at the bottom of the vessel is a magnesia stone and magnesia thimble through which the tapping pin is suspended.

The thimble is plugged by suspending the tapping pin through the thimble and placing a metal disc above the pin. This disc is then covered with refractory sand.

After drying the crucible, a small quantity of the thermit powder is first introduced, the object being to avoid damage to the refractory sand layer and to cushion off the plugging material in the bottom of the crucible from the impact of the full weight of the thermit charge.

6. Igniting the thermit mixture:

A low ignition point thermit in the form of a powder is placed on the top of the thermit in the crucible.

To initiate the reaction, the low ignition – temperature thermit is contacted with a hot rod. This ignition immediately starts the reaction in the main thermit charge.

7. Opening the mold:

The actual period for which the mold is left unopened depends upon the dimensions of the weld, being shorter (2 to 3 hours) for small sections and longer (about 4 hours) for heavy sections. The longer the mould can be left unopened, the better it is.

8. Finishing the weld:

After removing the mold, the risers and gates are cut away with a cutting torch. In case of shafts or parts requiring specific finished contour the same can be given by either machining and grinding.

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Even when you control the metal sheets with clamps, the welding operation creates a number of defects in the welded joints. These welding defects can reduce the static resistance of the joint and often have a major influence on its fatigue strength over time.

The shot peening treatment of the welded joint helps improve its fatigue strength and reduces the geometrical stress concentration. Peening can be applied to both shallow surface cracks and fillet welds that have not cracked to improve the fatigue resistance of the weld.

Weld Distortions

Before we dive into peening lets talk a bit about welding distortions. Welding distortion is caused by the expansion and contraction of the weld metal. This occurs when the base metals are heated during the welding process. There are multiple reasons why this happens.

When you perform all the welding on one side of a metal joint, it will cause much more distortions than if the welds are alternated from one side to the other.

During the heating and cooling cycle that is common in welding, many factors affect the expansion and shrinkage of the metal and cause distortions to appear. These factors include the physical and mechanical properties of the metal that changes as heat is applied.

As the temperature of the area under welding increases due to the arc application, the elasticity of the metal and conducting capacity of the steel plate decreases. These changes in metal properties affect heat flow and uniformity of heat distribution across the weld area.

Peening To Reduce Weld Distortions

Peening is an effective method used to balance the shrinkage of a weld puddle as it cools down. Generally, peening the weld bead thins it out by stretching it over the surface. This helps reduce the stresses induced by the contraction of the metal as it gets cooler.

Peening is considered a cold working process. The surface of the weld component is deliberately deformed, with the help of an air hammer.

During the peening process, the surface layer of the weld may appear to expand laterally. However, it is prevented from actually expanding by the elastic nature of the bulk material under the surface. This leads to the formation of compressive but beneficial internal stresses in the surface layer that gets a corresponding balance by internal tensile stresses elsewhere in the weld.

The process of preening creates a layer of compressed surface material that resists the development and propagation of cracks. It also improves the weld’s resistance to fatigue failure, stress corrosion, strength fatigue, and cavity erosion.

Types of Peening Techniques

There are four main types of peening techniques used in welding.

Shot Peening

This is the most common method f

Shrinkage Cavity Welding