Alternative to Nital Etch Process? 1

[THC75017]

I was wondering if there were any alternative nital etch process? We are not wanting to put acid on the part due to it being a primary part. We did have it on our plans to do. Although we want to find something else or remove it. If we remove it we have to find something to justify the removal. Or just find an alternative to Nital Etch. Does anyone know of something that might help in either removal or alternative?

Thank you for your help.

 

[unclesyd]

A little better description what your are try to do is required for an answer as Nital etch is normally used for metallographic samples for microscopic analysis.

The Nital etch is normally used on CS surfaces that have been polished.

 

[TVP]

THC75017,

Nital etching is also used for observing defects like grinding burn on bearings, gears, shafts, etc. Is this what you are referring to, or is it part of a metallographic examination for determination of microstructure (grain size, phase distribution, etc.)?

 

[THC75017]

The part we are dealing with is a Main Rotor Yoke for a helicopter. Now the area that is being called out for inspection is the spindal of the yoke where the bearings and the gribs go. Now the Mat'l that the yokes are from is 4340 Alloy Stl per AMS 6414 Normalized. From what our P.S. states Nital Etch is done to find grinding burns and other surface defects on case hardened and through hardened steel parts, and that abusive machining defects can also be detected. Basically, I've been looking for a alternative Process Spec for Nital Etch or data that I can show as to why we can remove it from our plans. And I'm just coming to a dead end on everthing. Thought I might could post here to see if anyone might know of an alternative or data I can look at that might be able to help. I guess another question if I might be able to ask is what is the newest and more recomended Process Spec for Nital Etch? We are currently using a Process Spec from 1977.

 

[THC75017]

Tensile is 150-170,000 PSI

 

[redpicker]

While there may be other processes that will reveal defects a nital etch would reveal in this application, I would consider the liability too great not to perform the nital etch as directed. If you use an alternative method and miss a defect the nital etch could have revealed, the consequences would be too great, in my opinion.

rp

 

[metengr]

I have seen some form of ECT used in an attempt to detect grinding burns on gear and pinion teeth (for wind turbine gear boxes). In my opinion, there is no acceptable alternative to the Nital etch for detection of grinding burns.

 

[TVP]

THC,

I agree with the others, eddy current and barkhausen noise are possible non-destructive methods, but they require lots of development, time, etc., and still may not detect potential defects. Also, your customer quite possibly requires nital etching rather than NDT. If you want to reference a contemporary standard, MIL-STD-867C was revised in 2008:

http://www.everyspec.com/MIL-STD/MIL-STD+(0800+-+0899)/MIL-STD-867C_20426/

 

[mfgenggear]

We do temper etch on a daily basis. on AISI 9310, 4340, 4330
& so on.

just make sure to post embrittlement bake below the tempering temperture & what the specification requires.

 

[THC75017]

The part does goes through magnaflux inspection also.
Thanks TVP for that spec. I'll take a look at it.
Would there be a issue if there are some dim. that are + or - .0005 talerance? Not sure if that would be a issue with how much the acid would eat away at the part?

 

[mfgenggear]

THC75017

only temper etch once, & make sure the the etch times not violated. normally hold OD diameters at the high limit.
there will be very little stock removal if done correctly.
.00005" max or less

Mfgenggear