Changes in toughness associated with the operating temperature or material condition can lead to unexpected performance
Lowering the toughness of a material by changing the material composition/grade or unintentional heat treatment can lower the toughness of the material at the operating condition to promote brittle behaviour - causing unexpected failure
The effect of temperature on material performance can be significant, even when there is no apparent change in temperature, but simply a change in manufacturing process. The issue is that a material’s performance is frequently affected by the temperature at which it operates, or by the temperatures to which it has been exposed. Two cases detailed below illustrate this contention.
In the first case repeated failure of a component led to the owner having a component made from a higher carbon steel (typically 0.4% carbon steel) rather than mild steel, thus 'making it stronger', and 'therefore it must be better'. Unfortunately the typical ambient operating temperature was in the vicinity of 12ºC and the ductile to brittle transition temperature of the 'new improved' steel was in the region of 45ºC rather than the -30ºC of the original material. At 12ºC the behaviour was clearly lower shelf with a fracture toughness comparable to cast iron. The consequence was very early fatigue initiation and premature failure at much smaller fatigue crack lengths – even though the owner thought he was improving the matter!
The other refers to a case of an aluminium diving tank, used to provide bottled oxygen for conventional scuba diving. The tanks normally were re-painted to the local supplier’s colours and logo, but in one particular case there was a rush to supply a couple of tanks needed for a diving trip. In an effort to 'speed up' the process, the local supplier arranged to have a batch of four such tanks repainted and put them in an oven 'to dry the paint'. While this process allowed the paint to dry and the tanks could be delivered on time it was not realised that this process was also changing the heat treatment condition of the peak aged aluminium alloy from which the tank was made, significantly lowering the strength. Unfortunately this had disastrous consequences, when the client was refilling the tank prior to a planned diving trip. When the reduced alloy strength was combined with the effects of a very small surface defect, the structural integrity of the pressurised tank was exceeded leading to the catastrophic failure of the tank and the clients death. The supplier was indeed liable, for a process he thought was merely 'drying the paint'.
These cases illustrate the need to be aware of the implications of the processes employed, especially for critically heat treated components.
Published in Technical Tips by Origen Engineering Solutions on 1 October 2015