Welding Nitride coated steel

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I have a splined shaft that has been Nitride coated.  I need to cut the shaft to shorten it and reweld it back together.  The weld will be 3/4-1" away from the spline area.If I cold cut the shaft and TIG weld it together being carefull of heat, will it affect the Nitride coating?  I have been told I cannot Nitride again after the fact.DrX
Reply:tough question..hmmm nitride is just a hardening coating..used on end mills often..mostly gold colored...i'm pretty sure its a alloy steel of some kind..before nitride coating..you just dont do that to mild steel... try to get about 1/8" of the coating "missing" from where the weld is going..but thats all for not..whats the diameter of the shaft?you need to..(this is gonna get involved)do this...you need to cut out what your going to loose..but you just cant go welding it back together and expect it to hold.. what you need to do is bore out each end..even if its only to 3/8" diameter or so..to both pieces..at least 1" deep..3/8 for 1 to 1 1/2" shaft..5/8" for 1 1/2 to 2 1/4" and so on..then you need to shrink fit a new shaft thats .002 bigger into each half and "marry" tham together before you weld..and make sure you have a good champher to weld into..yes thats how its done..something that has that coating on it is made for abuse...and it wont last without "marring" the pieces before welding... ...zap!Last edited by zapster; 11-22-2006 at 06:47 PM.I am not completely insane..Some parts are missing Professional Driver on a closed course....Do not attempt.Just because I'm a  dumbass don't mean that you can be too.So DON'T try any of this **** l do at home.
Reply:Zap, shrink fitting involves heating the bored out sections to open the bore up?If you don't have the time to do it right, then you definitely don't have the time to do it over.
Reply:You can probably get away with heating the bores and cooling the intermediate shaft.  I've done that to remove the pins out of bulldozer tracks.  When it's time to insert the pins we found it easier to heat the saddles and keep the pin in an ice chest with dry ice.  The shaft will shrink down just a bit and slide in without any issues.  Might work here without having to impart too much heat on the bores.
Reply:Originally Posted by ClanweldZap, shrink fitting involves heating the bored out sections to open the bore up?
Reply:The main part of the shaft is .77' diameter, while the splined ends are closer to 1" diameter.I have been told its carbon steel that had the ends heated and the splines forged, which you tell by looking at it.  I may be mistaken in calling it Nitride coated since the color is the same as steel, but it has a Rockwell hardness of 60, which exlains why the splines don't shear under load.I had plans to but weld the pieces together, then put a 2 piece collar over the weld (splines are larger diameter than shaft) and weld that on for added strength, plus it would provide an area to place a bearing to act as a carrier bearing for the longer shaft.I guess I may have to try one to see what happens.  Any ideas on how to test strength of splines without running the part and ruining other connecting components?Dr X
Reply:FYI, nitriding is a heat treatment 'speciality' done to certain alloys of steels to impart a very hard surface layer to the steel.  It is similar in concept to case-hardening or carburizing, except the hard surface layer is is usually shallower and harder in the nitriding case.  Another advantage over carburizing or case-hardening is usually better dimensional stability during (before/after) the treatment.  Typical nitriding depth is about 0.001 inch, case-hardening/carburizing may be 3 thou or so.  It is done in an ammonia-rich environment at high heat-treating temperatures, so it really isn't a do-it-yourself thing.  It is a change in the actual surface layer of the steel part and is not an applied coating.The 'gold' coating on some tooling is usually a titanium nitride vapor-deposited coating.  It is also very hard and wear-resistant, but it is an applied coating on top of the metal part.Further FYI, the splines were probably nitrided for the benefits of increased SURFACE hardness and wear-resistance, not to increase the strength of the splines.  Since the nitrided layer is so thin (about 1 thou), the strength of the steel is from the steel itself and not the nitrided layer.My guess is that the steel is an alloy steel and not just a plain carbon steel.  If the part was made with nitrided splines, I don't think the basic properties of carbon steel would be up to the task.  Probably the alloy is in the 4xxx family, but it could be something else.Some splines are made to allow some small axial mismatch (crowned splines), and some are straight splines.  The spline profile may be standard, or it may be a modified spline profile.  Are you up to modifying the part and holding the tolerances in the tenths-or-better range on the splined area?  Because that's often what is needed for precision splines.The splined area is a bigger diameter than the main shaft diameter for machining purposes (finish grinding of the splines) and for the appropriate strengths of the splined area and the shaft area (the spline is weaker than a same-diameter section of solid shaft).What specifically are you modifying and why?  I would be pretty wary of welding up a splined shaft precisely because of the precision needed in the splines and their alignment.  Cut the shaft?  Sure, part it off on a lathe.  Shorten the shaft on the lathe too.  Weld the shortened shaft with splines on each end?  Nope, not me.  The length of the original shaft and the particular spline profiles on that shaft -may- have been designed to be appropriate for each other.  Shorten the shaft and your splines may no longer be 'right' for the job.Or it might be that someone said "Hey, splines are neat and nitriding them is neat too, so let's jack the cost of this simple shaft up by a factor of 100 instead of just using a piece of plain shaft and some set-screws!"  But that's not too likely IMHO.
Reply:Moonrise, Nice description of the nitriding/spline machining etc.
Reply:I agree, nicely done and I learned a bunch.If you don't have the time to do it right, then you definitely don't have the time to do it over.
Reply:MoonRise,Thank you for the detailed explanation, it appears you know exactly what was done to the shaft that I could not put into words.The shaft is from a motorcycle.  The splines on one end go to the universal joint, which is where the cutting and welding will be done, the other go into the differential.  I have not checked the chemistry of the steel in the shaft, it could be a chrome steel, but will do so this week.  The splines are a good fit, not what I would call precision, but no problems mechanically with anything to date.I would cut the shaft cold, then TIG weld it together in a jig to keep everything running true.  once welded, I will machine down so weld is flush with shaft, then weld a split sleeve over for added strength.Dr X
Reply:You're welcome for the info.Now, let me say part of it again, in clear and uncertain terms.Don't do it.Let me say that part again, in case you missed it.Don't do it.As I said in the earlier post, splines are generally in the 'precision machining' category.  They look quite simple, but the tolerances in machining them are really usually very precise.  General tolerances for the tooth forms of the splines themselves are usually in the tenths of thousandths range, the tip OD tolerance of male splines could be to a 'loose' 5 thou or so but the teeth themselves are often machined to tolerances in the tenths of thousandths.The fact that the splines are for a motorcycle and that they are nitrided lead me to believe that they are indeed an 'engineered' item.  By that I mean that most likely an engineer designed and analyzed that shaft and those splines and specified a nitride treatment to the alloy steel for a reason (surface wear on the spline teeth).BTW, I doubt that your motorcycle has a differential.    The differential is used on vehicles with multiple tire/wheels assemblies on the same axle to allow rotational speed variations between the 'inside' tire and the 'outside' tire  on the same axle when the vehicle is going around a curve or making a turn.  A differential may also be used between multiple powered axles on a vehicle (like a 4x4) for the same reasons.  Unless your motorcycle is a 2-wheel drive version like a Rokon(?) or is a trike, it most likely does not have a differential.The big 'hub' thing at the rear wheel of a shaft drive bike that the driveshaft leads into is a right-angle gearbox, not a differential.   And if I may ask, just how do you propose to TIG weld the very center of a solid approx 1 inch diameter alloy steel shaft?     And then to continue welding the shaft completely?  And all while holding the splines at the very end of one piece of the shaft in place and oriented to tolerances of 0.005 inch or better?   Remachining the shaft OD and runout after welding is certainly do-able, but what are you going to do about those splines at the end of the shaft?  You're not going to true them up, if you could grind splines then you could just make up a brand new shorter shaft, grind the splines and then nitride your new splines on your new short shaft.   I would not even -try- to do what it sounds like you are proposing to do.   If you really-really-really insisted on trying to shorten your splined motorcycle driveshaft (btw, Why???? ), about the only way I would propose to do so would be as follows: - take driveshaft and part it off on a lathe; - shorten driveshaft on the lathe; - make appropriate precision collar, of the same alloy steel as the driveshaft,  on the lathe to join the cut-apart driveshaft pieces; - rejoin the driveshaft pieces with a medium-to-heavy shrinkfit to the joining-collar (freeze the driveshaft pieces and heat the collar); - TIG weld the collar to the driveshaft with a fillet weld (proper chamfer on the ID of the bore of the collar, don't notch or groove or whatever on the shaft itself just barely true up the OD if needed so you can get the proper shrinkfit into the collar), remember to preheat and post-cool properly for the specific steel alloy (btw, find out if it is even weldable before you start!); - hope your welding didn't distort the shaft runout at all, because if you distort the shaft then your spline fit (position, orientation, runout, etc) is screwed and you are SOL.  Hope your welding didn't screw the metallurgy of the driveshaft, or you are SOL.BTW, don't make the splice at the half-way point (that is the first vibration mode maximum excursion point).  Don't put it at the 1/4 point (that is the second vibration mode maximum excursion point).  Put it as close to an end as you can without having your machining or welding impact/change that end, or put it at the 1/3 point (that would be the third vibration mode nodal point).
Reply:Thank you for the information, it is a lot to think about.  I did come across a company that can take a shaft and machine the ends to create a splined collar.  This would eliminate the need for welding, but the cost would be the next factor.Back to the old drawing board.Dr Xtreme