Ferrari Mailing List: Re: Why a 308GT4?.......sodium valves

[Peter Pless (ferrarilist]

Found out some interesting information....not applicable all areas


"Why do sodium cooled exhaust valves fail?



When valves are removed inspection revealed that some corrosion in the area
where the divergence from the stem to the head occurred on high mileage
cars. Low mileage cars showed no corrosion.



I have found that if valves are dropped onto a concrete floor from 1200MM
some valves snapped. Also when adjusting valve clearances if the cam bucket
hold down tool was removed quickly causing the valve to snap shut a valve
head may snap off.



Inspection of the fractured area revealed spots where fractures had been in
existence prior to the final drop shock that separated the head from the
valve.

Eccentricity existed between the outer diameter and the sodium filled core. 
This was very much predominant on one valve, but not detected by naked eye
on the remainder of the valves.



The quality of sodium in the core was highly irregular from one valve to 
another. This was determined by carefully removing the sodium with a 
suitable hand held drill bit.



Analysis of valve material by spectrometer revealed the following:-

                        Carbon                         0.70%

                        Manganese                   5.70%

                        Silicon                          1.00%

                        Sulphur             0.005%

                        Potassium                     0.02%

                        Nickel                          1.40%

                        Chromium                    20.00%

                        Molybdenum                0.58%

                        Copper                                    0.12%

                        Titanium & Nioberin     0.01%

                        Remainder                    Iron



This analysis matches very closely with German valve steel known as German 
Standard (DIN) 1.4881











The metallurgical report on these valves examined is as follows:-



1.        The alloy appears susceptible to interangular corrosion and 
pitting when used with leaded fuels. This corrosion causes stress 
concentration and structural weakness just below the head of the valve. 
(This was observed by dark patches at the fractu

2.        re face aforementioned).



2.    This metal when maintained at its critical temperature of between 
540-800 deg C for

prolonged  periods suffers from a reduction in toughness caused by the 
chromium precipitating out and the alloy becoming in the embrittling "Sigma"

phase.



3.    Eccentricity of the bore to OD of the stem would lead to some 
localised stresses, thus causing failure.



4.    Variation in sodium quantity on individual valves would not make the 
heat transfer optimum.



5.    As the valve appears to be investment cast, the final product could be

quite variable in analysis from batch to batch, hence exacerbating the 
problem.



In conclusion, the original sodium cooled exhaust valves are basically 
under-designed for our standards. Thicker stem and increased gas resistance 
flow would solve problem from the start.



Vehicles that have dropped valves are 246, 308, 365GT4, 365BB, 512BB 
400.etc. These valves have letters stamped above collets like EL BM ST FS. 
I would suggest  changing exhaust valves to Nimonic 80A or equivalent 
material of local manufacture".