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We performed welding on a tube sheet of our urea stripper having following material:
CLADDING (10 MM THICK) = ASME SB 265 GR-2
TUBES = ASME SB-338 GR-3
Cracks are observed after welding, my question is:
Is this possible that commercially pure titanium of above grades transforms into martensitic structure? if yes then under what conditions?
No... But could you give more info of your welding technique, most common cause for Ti cracking is contaminated material or poor gas coverage.
If I might make a slight clarification, you can get slight martensite in CP's with annealing in the beta region and a water quench. Probably depending upon Fe and O content. I'm guessin Grades 3 and 4 are most susceptible. But that don't sound like your problem.
Steve I think is dead on, as far as your welding issue is concerned, its sounds like contamination to me too.
More info would be helpful.
How bout hardnesses?
I agree with the Gas coverage. You MUST weld it in a 100% ARGON ATMOSPHERE.
Sounds like a process control issue.
Most Cladding (surface build up) operatons are pretty fast pased. Welders putting down as much metal as they can in a short time. This kind of pace won't work with Titanium. Laying one pass right next to another red hot pass is out of the question.
CP Ti welds like butter. But;
Your tubes will have to be cleaned (total oxide removal) both inside and out with an inert purge/coverage on any surface that will exceed about 800F. Overheating the inside of a tube or the back side of a sheet will draw bad things (Oxygen, Nitrogen or any contaminant on the surface opposite the weld) into the weld and they may be carried interstitially through the entire thickness of the weld if you get too hot.
It is surely not necessary to bag or chamber your project in 100% argon, but your heat affected zone will need inert coverage until it is below 800F (on both sides). Special trailing gas shields are designed to accomplish this on the weld side and special argon backups must be fabricated to cover the back side of plate and of course purge for pipe.
Surface prep and inert gas process control are much more detailed with Ti than with steels or inconels.
If your cladding worn areas take extra care if there is any chance of melt thru.
DEAR MR. LAWRANCE,
WONDERFULL ANALYSIS. WE DO QUALIFY AND UPGRADE THE SKILL OF OUR WELDERS PERIODICALLY. YOU CAN HELP ME IF YOU HAVE LATEST PROCEDURES FOR WELDER QUALIFICATION.
DEAR MR. LAWRANCE,
WE DO FREQUENTLY WELD SCREW CONVEYORS. FOR LONGER LENGTH, ie, MORE THAN 12 METERS, MACHINING OF SHAFTS AFTER WELDING OF SCREW FLIGHTS WITH CENTER PIPE BECOMES DIFFICULT. ANY ADVANCED METHODS DO AVAILABLE TO AVOID DISTORTION DURING WELDING, SO THAT MACHINING OF SCREW SHAFTS AFTER WELDING CAN BE ELIMINATED?
i can send you complete microscopic analysis of the failure if u give me your email address. Hardness from 200-500 Hv which is much much greater than that of recommended.
Contaminants such as oxygen and nitrogen (from the air or other places) are great strengtheners (and cause embrittlement with too much content). A high hardness reading is a good sign that there are too many intertitial elements in the Ti, such as oxygen, nitrogen or carbon.
is it possible that Ti can martensitic structure due to surface contamination
IMO, no. Surface contamination (generally due to inadequate trailing) will only cause oxides on the surface (TiO2). Though the hardness of these oxides can be crack initiation points (thank you Lawrence). But I think you have, as Lawrence stated, a process control problem wherein contaminants entered the puddle.
This opens you up for oxygen as well as nitrogen as Greg stated, and then the possibility of martensite, which with hardnesses of 500 Hv seems pretty sure to me.
I don't know if a surfce oxide would give you 500Hv unless its really thick. Thats really up there. What was the bead color? Perhaps Lawrence or Greg can chime in here. In fact, 200 is high for a CP.
The only problem I've ever encountered with cracking in CP titanium is autogenous welding of tubes to tube sheets,and the problem was contamination of machining oil during machining of the tube sheet holes.
What type of weld joint is being performed? Where do the cracks occur? Does the crack take place throughout the duration of the weld or in the start or stop of a weld?
Type of joint = tube to tube sheet welding
Area of cracks = Around whole weld (weldments)
cracks observed after welding
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