Caustic Embrittlement in Locomotive Engines 1

[Metcorr]

In early 50s, failure in carbon steel steam locomotive had occured due to 'caustic embrittlement' at the riveted structure. This may be due to accumulation of caustic at the riveted locations and concentrating therby to reach a critical limit where caustic embrittlement can happen. The problem was resolved by changing the rivetted structure to welded one. Should the stress due to welding would not be more than the rivetted one? Any comment or advice please.

 

[unclesyd]

The stress could be higher, though the stress around a rivet hole are quite high, the improvement come in that there are no crevices for the caustic to concentrate to the point of causing caustic embrittlement. Another point is that you can relieve some stresses on a welded connection and if you stress relieve a riveted connection you can loose some integrity. If you have a very small leak path the concentration will happen in a relatively short time.

 

[metengr]

I agree completely with unclesyd. Elimination of the crevice formed at overlapping plates reduces susceptibility to increased caustic concentration.

 

[Metcorr]

Thanks Metengr. But, should we not expect that rivetting would imposed compressive stress rather than tensile stresses and will actullay helpful in combating stress corrosion cracking?

 

[unclesyd]

Think about the hole that the rivet is in and how the rivet works. The hole is subject to hoop stresses, tensile, due to the expansion of the rivet in the hole. If you you would examine holes where there has been some caustic embrittlement problems you would see small radial cracks emanating form a lot of the rivet holes. Depending on how much effort went into the rivet set these cracks can be very short, microscopic to through ligament failure.