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snr12 (Mechanical) (OP)
27 Nov 12 14:20
I have a question regarding Titanium alloys. We have a tube component made of Ti 6Al-4V, and we nitride the tube at 1450F. After nitriding we are seeing a 0.003”-0.004” expansion in the diameter of the tube, which is not suitable for our application. Can anyone please suggest another Titanium alloy which would not produce such growth? We were thinking of pure alpha Titanium, and see if it grows the same. Please advise. Any suggestion is greatly appreciated.
cloa (Petroleum)
28 Nov 12 10:05
Distortion and Dimensional Changes. Distortion in nitriding may result from:


Relief of residual stresses from prior operations such as welding, hardening, machining, and so forth
Stress introduced during nitriding due to inadequate support in the furnace, or too rapid or nonuniform heating or cooling.
Stress is introduced by the increase in volume that occurs in the case. This change causes a stretching of the core, which results in tensile stresses that are balanced by compressive stresses in the case after the parts have cooled to room temperature. The magnitude of the permanent set in the core and case is affected by yield strength of the material, thickness of the case, and by the amount and nature of the nitrides formed.

Have you allowed for the above effects?

Did you try a stress relief for the Ti-6Al-4V- some list a temperature of 1000F?
snr12 (Mechanical) (OP)
28 Nov 12 11:30
Thank you for the information. Yes, we have tried stress-relieving after machining at 1000F for 2 hours and air-cool. But still the tube grows during nitriding.
Helpful Member!  EdStainless (Materials)
28 Nov 12 16:10
I think that the growth is a direct result of the Nitrogen being forced into the structure. There is no way around it. Every time to nitride or carborize a material there will be a volumetric growth. You either need to compensate for it to start with or plan on finish grinding afterward.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

Helpful Member!(3)  cloa (Petroleum)
29 Nov 12 3:35
Given the nitrided layer is about 3-6 micrometres- how can it produce a dimensional change of 76.2 micrometres. The density difference is something like +25%. Of course the nitrogen atoms would diffuse more than the case depth but it wouldn't be lot more as the nitride formation would impede the atomic flow.
Helpful Member!  TVP (Materials)
29 Nov 12 10:44
Nitriding of Ti produces a TiN surface layer (typically 1-3 micrometers) and a diffusion zone with N dissolved into the Ti lattice (typically 20-40 micrometers). Nitriding usually causes significant surface roughening as well, which will affect dimensional measurement in the range of 10's of micrometers. I don't have any immediate data available, but it is certainly possible to have that order of magnitude dimension change after nitriding. The beta phase (BCC) is kinda fluffy when compared to the hexagonal alpha phase, so it will allow more nitrogen diffusion and hence more distortion. It may be worth looking at an alpha alloy if you cannot adjust the initial tube size, eliminate residual stress effects, perform final machining after nitriding, etc.
Helpful Member!  snr12 (Mechanical) (OP)
29 Nov 12 16:14
Thank you all for the responses. We have been running some experiments. Heated the tubes at 1450F for 2 hours, air-cooled, measured and the part grows 0.003". Repeated the process and the part further grew 0.002". Finally after 4 heat-treatments the part growth reduced to 0.001". Overall the part expanded 0.007-0.008". This was all done prior to nitriding to see at what point the tube actually stops growing. We are planning of using a pure alpha Titanium (Ti 5Al-2.5Sn)and repeat the study. But it is so hard to find this alloy at any supplier!
snr12 (Mechanical) (OP)
30 Nov 12 10:10
Tried them, they do not carry the alloy.
Helpful Member!(2)  GregRubin (Marine/Ocean)
2 Dec 12 16:23
Snr12,

The grade 6 you are looking for is a pretty unusual grade. While there are some uses for it, grade 2,5,and 9 are far more common. The last time we did any grade 6 manufacturing the mills required us to buy 500kilos at a time before they would produce the alloy billets for us. If you would be using this amount then it wouldn't be that hard to source.

The other option is to design to the growth of the tubing. Just have a Grade5 tubeing manufactured that is .007" smaller than your finished product, and let the nitriding growth allow it to expand to the finished tolerances you would need. For this type of specialized production a typical run of 50kilos would be pretty standard.

My other question, is why were you speccing grade5 when grade2 might work? We typically recommend people stick with 2 unless they need the strength of 5 since the manufacturing cost is lower, and it is easier to work with in the field. Obviously where needed you should use 5, but if 2 will work it is typically a less expensive option.
snr12 (Mechanical) (OP)
3 Dec 12 14:19
Thank you, Yes we are thinking of under-sizing the parts and allow for growth. Since we are in the medical device field, we used the Grade 5 ELI material and also for its better strength properties.
GregRubin (Marine/Ocean)
3 Dec 12 19:52
Snr12,

Are these anywhere close to standard size tubes? You have mentioned four different alloys, and they may all have slightly different growth patterns. (CP2, Grade 5, Grade 6, and Grade 23.

My thought is if you are considering this many different alloys, you may want to do some testing on each of them individually to determine the growth pattern of them. It is absolutely possible that they may all exhibit slightly different growth tolerances. It wouldn't cost much to track down some cut offs in a few different alloys then process them all, long before worrying about exacally which alloy to use.

I am also a little curious about using grade 6 (5al2.5sn) for medical uses. Not that it couldn't be done, but I am not familure with anyone using a tin containing alloy in the medical field, at least not for implants. We don't do a lot of medical work, so I wouldn't state it can't/wouldn't be used, but it is pretty atypical.

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