Brazing 4130N steel fittings

[Ian Law]

My plans call for a number of 4130N steel washers to be brazed to the wing fittings to locally reinforce the areas around vital holes. A number of fabricated steel shackles require the adjoining "faces" to be brazed. These are obviously critical components.

The aircraft welder who will be welding and brazing my wing fittings next Tuesday has suggested TIG welding some of the fittings, rather than brazing them. I am reluctant to deviate from the drawings, but my Inspector has agreed with the welder, so that is likely to be the way the shackles and a couple of small brackets are done. The washers will still be brazed in place and have been jigged carefully to align the pilot holes and make it a bit easier to braze - or so I hope.

Having searched the web for comments on the merits of brazing 4130N, I find conflicting opinions. What a surprise. There are suggestions that this can lead to cracks developing in the 4130N.

I am now left wondering whether in fact all the fittings should be TIG welded, including the washers.

As far as I can tell, most of the examples of "my" aircraft have wing fittings with the washers and some other parts brazed. There are no reports of any problems.(A number of the wing fittings are, of course, welded, where appropriate and there is no question of brazing any parts that would be obviously unsuitable for this method.)

Has anyone any knowledge or experience of brazing 4130N?

Ian

[Bill McCarthy]

You have to watch out with brazing - you may end up with just a fillet of braze around the washer with lack of fill through the whole width. The only way of checking penetration is with ultrasonic NDE. You can get cracking too. If you decide on the TIG method (I would) it would be a good idea to wrap each completed weld in fire retardant material in order to cool the zone as slowly as possible as a method of stress relieving the HAZ.

[Ian Law]

Many thanks Bill. That sounds like wise counsel to me and I will suggest this to my welder.

Ian

[bertdeleporte]

Hi all,

I come late in discussion. The N in the material designation stands for "normalized" which means the material got a specific heat treatment to get relaxed of final format processing stress, and get grain normalized (100% austenitic).

By brazing the fittings, if properly done and using the appropriate welding surface preparation and flux,you may be able to stay within accceptable temps to avoid to get the material grain changed.

By welding (ie reaching melting temp), you are bound to harden the material in the heat affected zone, as mentioned by Bill. Once done, the only way to releive stress is to make a monitored post curing heat treatment.

I attach the TTT (Temperature Time curve for 4130

http://www.metalravne.com/selector/steels/VCMO125.html

and a quick "how to read the TTT curves", including a little bit of literature on austenite, martensite, pearlite, bainite (the letters A,M, P, B on the TTT curves meaning that).

http://bama.ua.edu/~ywei5/spring2009me3 ... iagram.pdf

You will find extensive documentation on the web or in any metal engeneering school or university library. The subject is centenary old, don't gamble with steel and heat, you are bound to fail. Use the methods as receips as in cooking, the subject is very deep, some spend their entire life in research on those subjects.

Bertrand

[Ian Law]

Many thanks Bertrand and I appreciate all your advice.

Although I have read items regarding the properties of 4130N many times in the past, it is not easy to remember the details when working with the metal is merely a hobby and it is useful to be reminded of the correct procedures.

In all the circumstances and following recommendations of my LAA Inspector and the CAA approved welder, I have had the various fittings TIG welded. The welder was aware of the problems with brazing, unless done under industrial standards of quality control. He recommended welding as the most appropriate method for my particular requirements, with these particular parts and stressed that he was using a modern type of welding rod that was far less prone to cracking. I can't remember the details of the type of rod - which he mentioned while he was welding; I was too busy racing about in his workshop placing parts in jigs, removing them and resetting them while trying to avoid picking up very hot parts by mistake! It was difficult to keep up with him and he doesn't suffer fools gladly ( which, of course, puts me at a distinct disadvantage!).

As a matter of great interest, where would most ordinary mortals have their welded steel fittings heat treated if this is a requirement? I know that Tony Bingelis recommends using a gas torch, but this method is somewhat discredited in some accounts ( not aircraft related ) I have read, on the web.
Is there any point in having this done after parts have been welded?

Thanks again for your comments.

Ian

[Ian Law]

PS

Forgot to mention I've saved copies of the "TTT" and "How to read the curves" for reference.

Thank you again Bertrand.

Ian

[Bill McCarthy]

Ian, the commercial stress relieving process is a very expensive and prolonged process. See "Cooperheat" if you just need info. So long as you have a good weld in appearance, good penetration, no inclusions etc, then all you may need to do is heat it up again with a gas torch (not concentrationg the heat in one spot). Since you were probably using thin walled tubing (?) anyway I would discuss this with your welder and he may advise to leave it alone. You could also carry out a dye penetrant NDE inspection of each weld yourself as a double check for cracks.

[Ian Law]

Thanks again Bill and I will heed your advice.

The parts were substantial 0.062" sheet fittings, with washers of the same material welded around their peripheries to the fittings, to act as reinforcements for the main wing attachment bolts and the attachment points for the external wing rigging fittings/shackles.
The welds appear to have been very expertly completed, as is usual by my welder and I would happily bet that his work would stand any scrutiny. To follow through the advice given to me here, I will apply dye penetrant checks.

All advice is much appreciated.

Ian

[hazelpilot]

My welding book says do NOT braze EN4130 as the copper in the filler will eventually cause a grain boundary crack. Note - EN4130 (advertised as chromoly) bicycles are welded, never brazed.
-Hazel

[macconnacher]

Note that EN4130 is a European Standard for Aerospace bolts and 4130 the steel commonly used in steel tube aircraft manufacture is an US standard steel

[Ian Law]

Thanks Hazel and Stuart and this confirms my findings.

My fittings have been welded, as mentioned.This appears to have been successful and has avoided another problem that might otherwise have arisen, had they been brazed, by ensuring the pilot holes in the adjoining parts remained in alignment & uncontaminated by brazing material and so allowed these to be more easily reamed to size, without losing positional accuracy.

Ian

[Jeremy Liber]

Ian

I've only just spotted your thread on brazing v welding so apologies for the late input.

I would be very reluctant to weld rather than braze for three reasons:
1. As pointed out by Bertrand you will (not might or could) change the material propeties of the steel by welding and to an unknown degree. The only certainty is that you will have lost the carefully chosen (by the designer and stressman) material properties that were used to justify the static and fatigue strength of the parts. You indicate that these are critical parts...!
2. By deviating from the design in critical areras you are invalidating the type-approval issued by CAA/LAA and potentially creating an un-airworthy (in the engineering sense) aeroplane.
3. I'm sure that your welder is an expert craftsman but I would be rather reluctant to take the advice of a welder on matters of aircraft stress analysis, especially on critical parts.

This might sound a little heavy but I'd hate to read about your first (or any) flight in the AAIB bulletin. What you are doing might be fine but you really do need to obtain LAA Engineering advice and approval for such a crucial devaition.

Good luck with the build.

Jeremy

[Ian Law]

Many thanks for your advice Jeremy. I feel damned if I do and damned if I don't, in this situation.

The only consolation is that there are many other equally vital components of a similar nature that are welded (as specified in the drawings), including the fuselage fittings ( similar gauge 4130N) that mate with the wing fittings and which would clearly not be strong enough, were they to have been brazed.

I will refer the whole business back to my Inspector and then on to LAA, if this is the right thing to do.

Thanks again.

Ian

[mikehallam]

One more tiny point Ian, & regardless of the cracking not cracking potential of either attachment method.

AFIK and my not having knowledge of the stress or load path(s) intended by designer: welding attaches the periphery of the washer to the structure, whereas brazing could/should provide a bond to attach the area of the whole washer.

Of course I've no idea of the duty imposed. It could be the 'washer' is in compression & merely spreads the load. If OTOH it takes a shear load from a through bolt, then the washer's possible half share of that load is distributed into the structure though a smaller area if welded round the edge. That assumes the hole is reamed so the bolt transfers the load more or less equally to the structure.

mike hallam.

[Ian Law]

Many thanks Mike.

I assumed at the outset that brazing had, perhaps, been called for in the drawings for the reason you suggest. I was also concerned that welding may not provide a complete “seal” around the bolt hole and so allow moisture to find its way in.

Where appropriate I shall be reaming the holes to size, as called for in the drawings. I expect the washer mainly spreads the load over the shaft of the bolt; the material adjacent to the washers is 0.062”, so the washer only adds to the integrity of the fittings to a limited, though obviously valuable extent, I assume.

Bill made the point that the braze may not entirely fill the gap anyway and this would surely result in an even weaker joint.

I have seen brazing carried out with a torch and the temperature required to do the job certainly looked hot enough to cause some pretty serious changes in the condition of the metals being joined. In support of brazing: some time ago, I spent many hours of hard labour trying to remove old brazing material to separate some damaged parts. There was no chance they would have parted company in service. The parts so brazed are still in one piece and I’m not volunteering to take them apart again.

As mentioned already, I have copied all the messages here and forwarded them on to my Inspector and asked for his guidance again.

Ian