In the past TWI has received enquiries from Members who have suffered problems with welding, despite following advice on preheat. In one particular case the recommendation had been to preheat small medium carbon steel transmission components for tractors to 200°C before welding. Macro examination of one of the batch of components showed root cracking in the heat affected zone (HAZ) and subsequent ultrasonic examination indicated root cracks in many of the components. It turned out that the procedure had been to preheat a batch of 20 components to 200°C in a furnace and then load them into a basket which was transported to the welding station some distance away. After welding, each component was loaded into another basket, and when all the components had been welded, which took a total time of 1½ hours, they were transported to the machine shop.
That procedure would have caused a wide variety of thermal cycles in the HAZs of different components, and the last few components would have cooled to well below 100°C before welding - hence the cracking problem.
Solution - the components were welded successfully by a modified procedure in which they were preheated in a furnace close to the welding station and taken out one at a time for welding.
One of the more extreme examples of bad practice, reported some years ago, was a component that was preheated on Tuesday, allowed to cool, and welded on Wednesday. This was not as silly as it sounds because the purpose of preheat is not always adequately explained. Standards and Codes of Practice give some guidance on preheating, e.g. BS 499 defines preheating temperature as 'the temperature immediately prior to the commencement of welding resulting from the heating of the parent metal in the region of the weld' (NB: Some parts of BS 499 have been revised or superseded since this case study was written, please check the British Standards website for more information).
The AWS Structural Code states that 'the base metal shall be preheated in such a manner that the parts on which the metal is being deposited are above the specified minimum temperature for a distance equal to the thickness of thepart being welded, but not less than 75mm in all directions from the point of welding'.
Most instructions or Codes of Practice do not say that the minimum preheat temperature should be maintained throughout the welding operation. This is to reduce the cooling rate of the HAZ after welding and so avoid cracking. Applying this simple principle could prevent a lot of cracking problems.
An interesting example from some years back was a railway line that fractured when a train passed over it. The rail end had been hardfaced to build up a worn area and the conventional preheat of 100° had been specified for this operation which took place one night in February. Investigation of the failure showed HAZ hardnesses of 500HV and higher. Enquiries at the local meteorological station indicated that on the night in question the temperature was4°C. To investigate the effect of welding at 4°C ambient temperature, a short length of rail was cooled to 4°C in a mixture of dry ice and acetone and then preheated locally to 100° by a gas flame. When one minute had elapsed after preheating, the end of the rail was hardfaced. Examination showed HAZ hardnesses similar to those of the rail that failed in service, indicating the importance of maintaining the preheat temperature throughout the welding operation.
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