Heat Treatment of Titanium tubes(S/T Exchangers) 1

[farzam]

Hi All,

My question is related to heattreatment of non-ferrous U-bends,based on our client spec. all non-ferrous u-bends shall be annealed after bending, but for carbon steel only four top rows shall be annealed.we proposed our client that for Titanium tubes also let us provide annealing only for 4-top rows, and we guarantee this matter, but they have not agreed.
Let me know your idea, that its mandatory to perform annealing for all Titanium U-bends.
It will be appriciate.

Farzam

 

[Goahead]

It is certainly recommended to stress relieve bent tubing.
You might have a look at an article at

http://www.key-to-metals.com/Article97.htm

This treatment could be possibly combined with the one due after welding, provided suitable heating reaches all tubing sections.

http://www.welding-advisers.com/

 

[metengr]

I would suggest you review the applicable ASME SB material specification for titanium and titanium alloy heat exchanger tubes (SB-338). This specification applies directly to seamless or welded titanium or titanium alloy tubing for condenser and heat exchangers.

The tubing can be supplied in various forms depending on Grade. Grades other than 9, 18, and 28 must be furnished in the annealed condition. Grades 9, 18 and 28 can be furnished in the annealed, cold worked or stress relieved condition (600 deg f for 30 minutes).

 

[kenvlach]

The bent titanium tubing should definitely be stress-relieved. Titanium is used due to corrosive conditions. If not annealed, corrosion preferentially occurs on the outsides of the bends due to residual tensile stress. I have seen a perfect illustration where a non-annealed, serpentine-style HXer (fabbed by a low bidder) having 6 U-bends developed perforations on the outsides of all 6 bends.

Clean before heating (if possible, in argon or vacuum to avoid H, N or O pickup) and give a light pickling afterwards (especially if heated in air). I suggest a brief pickling per ASTM B600-91(2002) Paragraph 4.3.2 at about 120^oF for 1-2 minute. Use a nitric acid-to-hydrofluoric acid ratio of (at least) 10:1 to avoid hydrogen embrittlement; a solution of 33.2 % nitric acid and 1.6 % hydrofluoric acid is recommended – ASM Handbook Vol. 5, Surface Finishing (1994), p. 839. Wash with clean water, high-pressure spray rinse with clean water at 130^oF, rinse in hot DI water and allow to dry.
This gives a very clean, bright, passive suface.