1. A youn g engineer working in a manufacturing shop producing spot welded low c
ID: 2073398 • Letter: 1
Question
1. A youn g engineer working in a manufacturing shop producing spot welded low carbon steel components is informed that all parts that had been produced in the last two hours had cracked immediately after welding. Prior to this, parts were being produced successfully. The engineer is also informed that the welding parameters have not been changed in recent weeks. a) What may be the reason for this occurrence? (5 Marks) (8 Marks) (7 Marks) (5 Marks) b) What tests would you carry out to confirm or otherwise your suspicion? c) What results /evidence would confirm your suspicion? d) How would you solve the problem? You are free to make ANY assumption, for example that all the testing equipment required is readily available! Your method should be one that ensures that the problem is solved within the least possible time and with the least costs.Explanation / Answer
a) The cracks that occured in the above case must be Hot Cracking because the parts cracked immediatley after welding. It is basically shrinkage cracks during solidification of weld metal. It occurs when there is insufficient supply of liquid weld metal to fill spaces between solydifying weld metal.
Hot cracking also occurs due to other reasons :
Elements that form a second phase are identified as increasing the risk of hot cracking.The weld metal composition also affects hot cracking.
b)The weld product can be tested by NDT methods.These methods are:
When these tests are done if the weld is proper , it would not indicate cracks in radiograph.It will show clear passage of sound through weld.These testing methods can give the reason for the formation of cracks in the weld.
Also the amount of liquid that was present can be inspected while testing.The equipment should also be inspected for damages ,arc voltage etc.
c) During the tests if the results show low amount of liquid present during welding, It could be the reason for the cracking and hence the weld maybe hot cracked.if there is no clear passage of sound during radiograph through the weld it could indicate cracking also.Different tests give different results.Based on these results the reason for craking can be understood and hence this reason can be avoided in the future.
d)The heat input during welding should be reduced hence we get more seggregation time.Also the voltage must be lowered as necessary during welding.There must be emough liquid to reach all the regions on the weld.Backfil technique can be used to provide greater thickness to crater.Areas between tack welds and weld pass should be filled thickly prior to welding.The parameters will not be changed.When all these problems are solved the welding can start again and hopefully there will not be any crack formation again. Also thebase metal should be cleaned and compostion should be checked. The batch of metals may have bad composition or mixtures.
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