Galling in large diameter steel threads 1

[NickA]

Hi, I don't know if anyone here will be able to help me but I have a question regarding similar materials galling in a threaded application.

First a little background,

My company produces heavy load hydraulic equipment. We normally design hydraulic cylinders with the end caps threaded into the tube (10-24" diameter threads 6 threads per inch). The material we normally use is ASTM A516 Gr. 70 (70KSI Tensile) for the end caps and ASTM A106 gr. C (70 KSI Tensile) for the tube.

Now for the challenge at hand,

We currently have a contract to deliver some equipment to a very cold environment (-40C ambient temperature). We have chosen materials that will have good fracture toughness because of the low temperature. (We have some pretty strict Charpy V-notch test requirements at -40C). The materials we chose to use are ASTM A514 (110 KSI Tensile)for the end caps and a low allow centrifugally cast steel for the tube (112.7 KSI Tensile). When we try to thread the endcap onto the tube the end cap threads tend to cut into the tube threads and then material starts to gall. Most of the damage occurs on the tube.

We realize that the metals are fairly similar in hardness and therefore not an ideal match. However the materials we normally use are even more similar and don't give us a problem!

If anyone has any comments or suggestions I would love to hear them. I am not a metallurgist just a lowly mechanical engineer, so if you have any knowledge that may help me or if you can point me in the direction of someone who can I would greatly appreciate it.

-Nick Atallah

 

[funnelguy]

Hi Nick,
I'm just guessing, but I don't think metallurgy is the source of your problems. The mating parts will not even assemble properly. Are you single point threading the ID of the caps? Perhaps there is a problem with the thread form? Are you galling on the root, flanks or major diameter of the tube? I've seem quite a few problems caused by interference between the roots and crests of the threads of mating parts. Just guessing.

 

[usaidit]

Thread form is my first guss as well, but one other thing did you use any dry lube? and have you sectioned a piece to look at the cross section? I would like to see the cross section photo for analysis this could tell the story.

usaidit

 

[CoryPad]

Dimensional fit is one cause of galling, but other major causes are surface roughness and material microwelding. I would recommend using some type of lubricant for the threads, which can be as simple as an oil/wax put on the threads, or as complex as a multilayer coating with integral lubricant. By the way, the similar hardness idea is a red herring, and you have your own data to prove it.


Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[yates]

NickA
Be wary of using oil or grease as a thread lubricant in -40°C conditions. MoS2 dry lube or just plain old graphite should keep you out of trouble if you ever need to unscrew in sub-zero conditions.
If lube does not help then you have a dimensional problem.

 

[NickA]

Thanks to everyone for the input. Upon further review, I am now thinking that there is a surface roughness problem on the male threads. Or could be that the tips are too sharp, even though they don't come to a point, they aren't rounded like they probably should be.

We use our single point technique on threads of even larger diameter with different materials so we didn't really consider changing that for this application. However, when we cut the threads in the male part, according to my machinist, they were very very hard and I can see a little bit of chatter on the flanks.

Unfortunately I don't know another way to cut threads of this diameter. (I was mistaken when I said 20" it is actually 21" diameter thread.)

 

[TVP]

Cutting threads on a high toughness low alloy steel will require very sharp tools and lots of lubricant. Look into optimizing the process for this material.

 

[EdDanzer]

The method you use to single point thread will have a major effect on assembly. Chatter is common in tubular parts, this will lead to high spots that have point loading when tightening. If you should be using carbide threading inserts that are designed for the pitch. It is common to not have enough clearance on the cutting tool and this causes heal dragging, leaving a small thing burr to gall.
In diameters over 12” , 4 TPI will work better.

 

[jetmaker]

Can't remember for sure if this as the application, but I've seen threads coated with silver to make installation/dsiassembly/corrosion issues become less significant. The silver acts as a lubricant, a load equalizer, and a inhibitor to galvanic corrosion.

Cheers.

 

[NickA]

Does anyone know a good method to reduce the sharpness of these threads.

The edges on the internal threads are very sharp. When we attempt to screw in the mating end caps the threads seem to cut into each other. We would like to round the ends of the threads near the lands.

My first thought was emery cloth or very fine sandpaper but I don't know how effective that will be.

Reverse-Current (Electro-cleaning) plating technique was suggested to me as a possible solution to remove small burrs and sharp edges but I don't know much of anything about it. Anyone done this before?

 

[CoryPad]

Emery cloth or sand paper should work - just remember to remove the debris afterwards as it can lead to thread fill and poor performance.


Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[funnelguy]

Have you looked into full profile threading inserts? They are available in a laydown style for your thread pitch. Call in the carbide reps and let them earn their keep.

 

[atad]

In my experience it could be that you are seeing the result of a certain amuount of ovality casused by distortion during the threading and possibly other machining processes.

For example, is there a lot of other machining taking place before the thread is cut thus causing the distortion to occur when the component is removed from the chuck?

Is it possible that the components could be rough machined first and then finally put back in the machie for threading only?

Is there any welded structures on the outside of the cylinder tube, i.e. feet or gimbals? If there are then you definitely need to rough machine first to allow any stresses to materialise.

 

[NickA]

We did think of ovality. We checked the bore and major diameters in several loactions to ensure that they were all the same. Also, we cut the threads at a different time than the honing of the bore. You are right though the temperature of the steel does get high when the tube is honed.

We are going to put the tube back in the machine to clean up the sharp edges of the threads. and chase the threads just incase a sharper tool will cut some of the rough material off. I will look into a full profile threading insert

Thanks

 

[coolbreeze]

The key to galling prevention is in surface preparation. Once the proper surface treatment is applied it raises the galling resistance threshold of the couple. The base metal composition is irrelevant once the proper coating or plating is applied.

Larger threads will have more tendency to gall, as there is more surface area, more relative motion, and typically heavier components. Alignment is critical due to side loading that can occur from high contact stresses. Rotional speed is also important, go slow to avoid heat caused by friction.

The surface treatment can be as simple as shot peening or aluminum oxide blasting. Some of the more preferred surface treatments on larger threads are: Zinc Phosphate coating, Teflon coat, and Copper plating.

Check for a good thread profile, (dimensions) apply a surface finish, and use a good lubricant, then when it works, vote me Tip Master of the week and we'll be cool.

 

[funnelguy]

NickA, are these tubes being threaded on an engine lathe? One end chucked and the other with a plug in it supported by a live center?

Those long tubes can really resonate. A roller backrest may give just enough damping to really help with this threading operation. The tube OD may not be true to the bore... I have had some luck by using a rubber tube as a resilient contact point for the roller. We're simply changing resonance. Some machinists will partially fill the ID of tubular parts with chips for the added damping.

The crests of the threads could be dehorned by a light turning/boring cut after threading. This would probably be preferable to filing or abrasives on threads this large. The full profile insert would be simplest, but the cutting forces might compound your chatter problems.

Apologies if these tips seem simplistic or obvious to you.

 

[unclesyd]

Looked up A514 (?) This material is T-1 steel. This material is your problem. This material was developed by USS as a high strength, high yield structural steel. This material will also work harden very easily and is abrasion resistant. Here is your tendency to cut the mating threads and to gall. I've seen it used only on pressure vessel supports. I have never seen it threaded as you have done. I"ve heard people cussing this steel with only ½" threaded holes. At onetime I had a lot of info on fabrication of T-1, will try to recover it.

 

[NickA]

THanks to everyone for the tips.

We are using a steady rest at the end of the tube to support it. And we have checked for concentricity. We always take a light pass over the threads of the male parts to reduce burrs but I'm not sure about the internal threads of the tube, I'll have to ask the machinist about that.

In regards to the A514, I am a little concerned about the work hardening that I think is going on. We use this material all the time but never for this diameter threads. The largest thread diameter we've done with A514 is 6.5". I would appreciate the fabrication info on the T-1. It may not have anything that is new to us but if it does, that would be great.

Thanks again. Hopefully when this is over we can all walk away with some new insight.

 

[funnelguy]

Nick, I was suggesting the steady rest about a third of the way back towards the chuck AND having the end of the tube supported by a live center in the tailstock. Supporting the tube in three places will make a BIG difference. Just in case we misunderstood each other...

 

[unclesyd]

NickA,

I found the information T-1, very little on machining other than to use carbide tools, nothing on outside threads. It was all about cutting, welding, and heat treating. I checked with a friend who does a lot of machine work for the pulp and paper industry and he also hasn’t threaded anything of your dimensions. Based on his experience, they use considerable T-1 for the abrasion resistance, he would not recommend large threads on this material. I ask about cutting threads on thick C/S plate edges in general. His answer was the same, not recommended. Looking at some of my notes if you cut the disc by flame how much material did you allow for removal prior to machining. I made some side notes stating we had cut too close to dimensions on some parts.
None of my books have a machine ability rating on this material. The closest I could come is AISI 1330, which is 55. Is the lathe that you are using to thread the parts ridged enough. If it a very ridged machine there are also some newer multipoint carbide tool that will cut a asymmetric thread form that could help.

It is definitely work hardening. Any additional work on the threads could aggravate your problem. Do you have the ability to check the external threads over wire.
This ability to work harden is the reason its’ used for grapplers, scrapes, bed liners, and chutes, etc.

I’m still trying to line out(in my mind) the interchangeability of the various grades, T-1, A415, A517. I hope that you received the PVQ material. The MTR should verify this. I have always been leery of dual certification due to the possibility of mixups along the way.

I also found a copy of USS plate handbook, they still have it on their website. The is very little information on machining any of the USS plate steel. The handbook has a great section on welding. I would contact the plate manufacturer and see if he can be of any help.

USS is planning to or has sold their plate division to ISG.

I’ve got feelers out, so hang in there.