INCORPORATING COLD CHAIN

TALKING TECHNICALLY

Grant Laidlaw is currently the owner of the Air Conditioning and Refrigeration Academy( ACRA) in Edenvale. He holds a Bachelor of Business administration and an associate degree in educational administration. He has a National Technical Diploma and completed an apprenticeship with Transnet. He has dual-trades status: refrigeration and electrical. He has been involved with SAIRAC for 17 years and has been a Johannesburg committee member for the past eight( chairman in 2011 and 2012. Currently he is the vice-chairman( Johannesburg council) as well as vice-president( national council).

What is weld distortion? Distortion in a weld results from the expansion and the contraction of the weld metal and adjacent base metal during the heating and cooling cycle of the welding process. Doing all the welding on one side of a part will cause greater distortion than if the welds were to be alternated from one side to the other. During this heating and cooling cycle, many factors affect shrinkage of the metal and lead to distortion— these factors include physical and mechanical properties that change as heat is applied. For example, as the temperature of the weld area increases, yield strength, elasticity and thermal conductivity of the steel plate decrease, while thermal expansion and specific heat increase( Figure 1). These changes, in turn, affect heat flow and uniformity of heat distribution.

Contraction of the weld area on cooling results in both transverse and longitudinal

Coefficient of Expansion( 10 6 In / In.)

9- 8- 7- 6- 5-

Yield Strength( 1000psi)

50 40 30 20

Modulus Coefficient of thermal expansion

Yield strength

10 5

0 200 400 600 800 1000 1200 1400

The principal features of the more common forms of distortion for butt and fillet welds.

shrinkage. Non-uniform contraction( by way of thickness) produces angular distortion in addition to longitudinal and transverse shrinkage.

For example, in a single V-butt weld, the first weld run produces longitudinal and transverse shrinkage and rotation. The second run causes the plates to rotate using the first weld deposit as a fulcrum. Hence, balanced welding in a double side V-butt joint can be used to produce uniform contraction and prevent angular distortion.

Similarly, in a single side-fillet weld, nonuniform contraction produces angular distortion of the upstanding leg. Doublesided fillet welds can therefore be used to control distortion in the upstanding fillet, but because the weld is only deposited on one side of the base plate, angular distortion will now be produced in the plate.

Longitudinal bowing in welded plates happens when the weld centre is not coincident with the neutral axis of the section so that longitudinal shrinkage in the welds bends the section into a curved shape. Clad plate tends to bow

30 25 20 15 10

Modulus, E( 10 6 psi)

Heated bar

Bar at room temperature before heating and after cooling

( a) in two directions due to longitudinal and transverse shrinkage of the cladding— this produces a dished shape. Dishing is also produced in stiffened plating. Plates usually dish inwards between the stiffeners, because of angular distortion at the stiffener attachment welds.

In plating, long range compressive stresses can cause elastic buckling in thin plates, resulting in dishing, bowing or rippling. Distortion due to elastic buckling is unstable: if you attempt to flatten a buckled plate, it will probably ' snap ' through and dish out in the opposite direction.

Twisting in a box section is caused by shear deformation at the corner joints. This is caused by unequal longitudinal thermal expansion of the abutting edges. Increasing the number of tack welds to prevent shear deformation often reduces the amount of twisting.

Allowances for weld shrinkage It is almost impossible to predict accurately the amount of shrinking. Nevertheless, a rule of thumb has been composed based on the size of the weld deposit. When welding steel, the following allowances should be made to cover shrinkage at the assembly stage.

Transverse shrinkage Fillet welds: 0.8mm per weld where the leg length does not exceed 3 / 4 plate thickness. Butt weld: 1.5mm – 3mm per weld for 60 ° V joint, depending on number of runs.

Bar restrained during heating

Longitudinal shrinkage Fillet welds: 0.8mm per 3m of weld. Butt welds: 3mm per 3m of weld. Increasing the leg length of fillet welds, in particular, increases shrinkage.

Factors affecting distortion If a metal is uniformly heated and cooled there is almost no distortion. However, because the material is locally heated and restrained by the surrounding cold metal, stresses are generated higher than the material yield stress, causing permanent distortion. The principal factors affecting the type and the degree of distortion are the following: 1. Parent material properties 2. Amount of restraint 3. Joint design 4. Part fit-up 5. Welding procedure.

Reasons for distortion To understand how and why distortion occurs during heating and cooling of a metal, consider the bar of steel shown in Figure 2. As the bar is uniformly heated, it expands in all directions, as shown in Figure 2( a). As the metal cools to room temperature it contracts uniformly to its original dimensions.

But if the steel bar is restrained( as in a vice) while it is heated, as shown in Figure 2( b), lateral expansion cannot take place. But, since volume expansion must occur during the heating, the bar expands in a vertical direction and becomes thicker. As the deformed bar returns to room temperature, it will still tend to contract uniformly in all directions, as in Figure 2( c). The bar is now shorter, but thicker. It has been permanently deformed, or distorted.( For simplification, the sketches show this distortion occurring in thickness only. But in actuality, length is similarly affected.)

In a welded joint, these same expansion and contraction forces act on the weld metal and on the base metal. As the weld metal solidifies and fuses with the base metal, it is in its maximum expanded from. On cooling, it attempts to contract to the volume it would normally occupy at the lower temperature, but it is restrained from doing so by the adjacent base metal. Because of this, stresses develop within the weld and the adjacent base metal. At this point, the weld stretches( or yields) and thins out, thus adjusting to the volume requirements of the lower temperature. But

( b)

Restrained bar after cooling

( c)