Published on 28 January 2013
We simply want to know why it cracked?.asserted the puzzled engineers of one TWI's Industrial Members recently. They had just delivered the remains of a welded manifold valve to TWI's failure investigation team near Cambridge.
The casualty was a 275mm by 275mm by 275mm forging made from F22 steel. A central hole had been machined around which a stub had been welded. Cracking occurred at the toe of the weld between the manifold valve and the stub. The weld metal originally securing it, had been machined off the valve by the time of delivery.
TWI took an eight point approach to solving the cracking conundrum...photograph it in intensive detail.inspect it using liquid penetrant, magnetic particle inspection and ultrasonic testing.then cut it open to allow one of the dye penetrant indications to be broken open.
With the fracture surfaces now exposed examination using light microscopy and scanning electron microscopy was undertaken. The remaining parts of the fracture surfaces were re-assembled and a metallographic section prepared. It was examined using light microscopy.
Features of particular interest were photographed in extremely high magnification. Vickers harness measurements were made in the parent material and micro-hardness measurements taken in the parent material and heat affected zone.
Finally the chemical composition of the parent material was determined using optical emission spectrometry.
Armed with the results of these eight fields of examination, attention was focussed on the discovery of a significant flaw a mere half millimetre in depth. Once again, using high magnification photography examination of the metallographic section through the flaw revealed products of oxidation and corrosion on the flaw surface.
This indicated that the flaw was present when the component was at elevated temperature but it was not consistent with a forging defect. A form of heat affected zone cracking, namely reheat cracking, which is usually revealed as intergranular, was considered to be the cause of cracking but dismissed. This flaw was straight in orientation and probably, but not definitely, transgranular.
Detailed discussion took place between TWI and its troubled Member based on the findings of a fuller report of the investigators' findings. TWI was able to advise that the defect was probably formed during welding and postulated that hydrogen cracking arose primarily as a result of inadequate control of preheating.
Five recommendations were made to TWI's Member company. Each related to the importance of either pre-heating control, post weld heat treatment or consumable drying and is confidential to the client.
To learn more about TWI's failure investigation work, whether it be of a structural, material or environmental nature, simply call the experts on +44 (0) 1223 899000.
For information about TWI's capabilities please contact us.