Filler Metal Management 2
A radiator core retrieved from service was examined for a suspected premature corrosion related failure.
Upon closer metallographic examination, no evidence of corrosion was found at the failed area.
Header: AA4343/ AA3005
Tube: AA4343/ AA3003
It was concluded that the cause of the failure was in fact a mechanical failure occuring in the thinned wall area.
The following sequence of events proposes a rational explanation for the eroded tube area:
In service radiators are subject to internal pressure fluctuations and expansion and contraction due to heating and cooling. Mechanical failure was imminent and occured in the weakest part of the tube, the thinned down tube wall area adjacent to the tube to header joint.
Erosion of the base metal is undesirable since it reduces the wall thickness of the brazed component.
In addition Si penetration in the grain boundaries is known to increase the susceptibility to intergranular corrosion. Therefore proper filler metal management practices should be observed to prevent undesirable effects. One such factor easily controlled by the brazer is maximum peak brazing temperature.
It looks your company has good experimental knowledge and experience. it’s very interesting in solving the problem. so I have some questions on erosion problem of PF condenser.
Q1: How do we pick out the problematic condenser with erosion in productio line?. it’s not easy to sort it out due to micro phenomenon. what if testing pressure is getting raised?
Q2: Is there any specific method and know-how to improve radical erosion?
Q3: maybe if we have such problem, can you help us improve?
A1: There is no method to pick up from a continuous production a condensor with potential erosion. The level of erosion can be establish by optical microscope investigation. The practice is such that for a given process on a time basis (for example it could be ones week) a random part is collected from production and metallographic cross sections are prepared. An internal procedure dependently on the part type should determine the spot for examination, permissible level of erosion and frequency of the check. For a stable process, (no change in brazing temperature and time) it is safe to assume that the level of erosion is quite similar at each part.
A2: Erosion is caused by too high brazing temperature, too long time at temperature, temperature gradients and in case of extrude tubes too big the so called die lines. Those factors have additive character and therefore all should be check for corrective actions.
A3: I would need to know more details about your process and the parts in question to comment something more.
Thank you for your kind answer. And I have one questions on this.
Q: I believe we shall don’t permit the erosion during brazing process because it could be quite critical. but If we cannot avoid making the problem, what do you think about the allowable range of erosion? 5% or less?
Your comments are very helpful for me. thank you.
Producers of heat exchangers have established specifications for maximum levels of erosion/ core alloy dissolution – with allowable ranges for each type and design. A level of “5% or less” can be considered as quite demanding. In any case, the exact requirements depend on specific quality considerations.
thank you for your feedback.
Sorry, but I have one more question.
When there is some failures on brazing during leaking test, can we conduct re-brazing in the furance? I heard corrosion problem could be in field after certain period.
Yes, Many companies do the re-brazing on leaking parts. However the risk of erosion and intergranular diffusion of silicon which influences the corrosion resistance is in this case quite high. Therefore it should be considered obligatory to perform at least thorough metallographic examination on a re-brazed part to check possible degree of erosion and Si diffusion. If no bigger changes are observed when compared to standard single time braze part re-brazing is acceptable. When having doubts to the results of metallographic investigations further testing like SWAAT and cycle pressure should be performed, however the cost and time required for these latter tests can put the sense of re-brazing in question.
One additional matter might be worthwhile mentioning – which is that rebrazing is not suggested/ not suitable for Al heat exchangers with MPE tubes coated with Zn or Zn flux – i.e. when Zn diffusion layer is used as corrosion protection concept. During rebrazing, the Zn is diffusing further into the tube – so that the concentration gradient can degrade.