Engineers from one of TWI's Industrial Members delivered the remains of a welded manifold valve to TWI's defect characterisation and interpretation experts. They needed to know the reason the component cracked.
The valve was a substantial forging made from F22 alloy steel. A central hole had been machined around which a stub had been welded. Cracking occurred at the toe of this weld.
TWI took a methodical approach to solving the problem:
- visually examined and photographed it in intensive detail
- inspected it using liquid penetrant, magnetic particle inspection and ultrasonic testing
- cut it open to allow one of the dye penetrant indications to be broken open, then examined the fracture surfaces using light microscopy and scanning electron microscopy
- re-assembled the remaining parts of the fracture surfaces and took a metallographic section for examination using light microscopy. Features of particular interest were photographed at high magnification and hardness measurements were made in the parent material and heat affected zone
- analysed the chemical composition of the parent material using optical emission spectrometry
The results of these examinations focused attention on the discovery of a flaw about a half millimetre in depth. 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, i.e. during postweld heat treatment (PWHT), but it was not consistent with a forging defect. Original features of the flaw had been destroyed during PWHT and subsequent corrosion but close examination allowed TWI to rule out reheat cracking as a possible cracking mechanism.
Detailed discussion took place between TWI and its client based on the findings and TWI was able to advise that the defect formed due to hydrogen cracking, primarily as a result of inadequate control of preheating.
Five recommendations were made related to the importance of pre-heating control, post weld heat treatment and consumable drying to allow the client to overcome the problem.
To learn more about TWI's defect characterisation services, please contact us.