I hope it's not too late for my posting this answer.
Gas turbine blades can not be welded.
Firstly, they're made of very sophisticated material to withstand the high temperatures they're submitted to into the combustion chambers, and those materials are quite difficult to weld.
Secondly, they were submitted to sophisticated thermal treatments after fabrication to improve their properties, and those treatments make welding impossible.
Thirdly, they were machined into complicated shapes within rigorous tolerances of may be one thousanth of an inch or less. How are you going to restore the original shape after welding?
The only way to repair a worn out or broken turbine blade is to replace it with a new one. Actually, in many cases you'll have to replace the whole row of blades.
You're a student, and your professor should have told you that.
Anyway, what I say is not the Sacred Bible and I may be wrong. I'm ready to change my mind if someone tells me, based on actual experience, that he (or she) welded a turbine blade and it lasted for at least three months.
Giovanni S. Crisi
Sao Paulo - Brazil
Prof Crisi.
Turbine blades are routinely welded and weld repaired and can last as long as 20,000 hours (although the process is far from routine).
Even single crystal blades are weld repaired now a days. Controls are pretty tight, heat input and fixturing is key and finish machine/grinding operations are terribly expensive, but still cheaper than buying new blades.
All the major Airlines and engine manufactures have processes for rebuilding turbine blades. In fact the welding journal did a nice article in the early 1990s about a U.S. military facility that used a nifty robotic line to process blades with 6 axis micro plasma torches, and an adaptive feedback white light part positioning system. They were taking worn blades, welding, machining and FPI at the rate of about 300-600 blades per shift if I recall correctly, maybe somebody else remembers that article. I remember this keenly because my employer at the time built a similar line that was considered a failure due to unmatched equipment among other problems, we ended up hand welding the blades and using the automated line to so the finish grinding and FPI.
I diddn't respond to this post at first because I have little experience with Inco 738. However, it may have some repair practices in common with other super alloys so I'll toss a couple of links that may be slightly helpful.
These guys sell some of the best equipment for blade repair and have some cool photos http://www.liburdi.com/web_pages/tcs_rb211.shtml
http://www.msm.cam.ac.uk/phase-trans/2002/papers/APNickelWeldv2.pdf
http://www.netl.doe.gov/coal/turbines/projects/cost-reduction/tvbcFEAA064.html
https://www3.imperial.ac.uk/portal/page?_pageid=91,334220&_dad=portallive&_schema=PORTALLIVE
Also here is another fourm post regarding inco 738
http://aws.org/cgi-bin/mwf/topic_show.pl?id=5638#26034
Here is a newer Welding Journal article that includes some data on the Temper Bead approach. http://aws.org/wj/sept03/feature.html
Thank you, Lawrence. One learns something new every day. As I said, my words are not the Sacred Bible, and this time I was wrong.
You gave me an excellent idea. Here at Mackenzie School of Engineering in Sao Paulo, in order to get their degree, students must prepare what we call a TGI, i.e., a monograph on a certain subject they choose. The TGI is prepared under the supervision of a professor.
I'm going to propose my pupils to prepare his (her) TGI on "Repair of turbine blades by welding", and let's see if one of them accepts the idea. The basic bibligraphy would be that indicated by you on your posting. I'll keep you aware of what happens.
Giovanni S. Crisi
Prof. Crisi,
Thank you for keeping me in the loop. I very much look forward to seeing the work.
CFMI, General Electric, Rolls Royce and Pratt Whitney all have "in house" repair schedules (not unlike WPS) for each stage of blade they deem repairable (some are not). The schedules are often quite complete with data down to pulse amperages, durations and wire feed synchronization as well as drawings detailing repairable and non repairable areas of the blade and damage limitations. They may be willing to share some of the data compiled over the years.
Another interesting area is the expanded scope of surface coatings being applied to blades.
dear prof. crisi
dear lawrence
thanks alot for your attention.
I found something new about welding superalloys like IN 738.
if we overaged the material prior to welding to increase the dimension of primery gama prime particles and reduce the percent of fine secondary gama primes between larger ones, after welding and during PWHT the alloy will have enough ductility and wont crack becouse of residual stressws.
but here is a problem! during welding large gama prime and carbide particles are constitutionally liquated and will couse liquation cracking in HAZ.if we use solution heat tretment as preweld heat treatment, it reduse the amount of liquation cracking but PWHT cracking will increase largely.
Hi! I am a Spanish student and I have read some information about this for one of my university works.
I am not agree with Mr. Crisi because the replacing of damaged blades or nozzle vanes supposses greater costs than its repair by TIG or Brazing.
Nowadays, high pressure blades of gas turbines are made of nickel superalloys because they have good properties (creep and strength) at elevated temperatures.
The main problem is that they are difficult to repair by TIG and Brazing. The reason is their high susceptibility to HAZ cracking.
I think that this blades are directionally solidified but when you repair them you modified the initial microstructure and you can lost good creep properties in the repaired zone (now with equiaxic grains).
About HAZ cracking I supposse that is difficult to control the microstructure. The original microstructure was obtained with a high process control and when you repair it is difficult to avoid liquation cracking due to formation of euthectic compounds. I do not know the solution, but I have read somewhere the possibility of repairing by Nd:YAG laser welding to minimize heat input.
I hope that this words are useful, but as Mr. Crisi says, this is not the Sacred Bible and I may be wrong too.
Kind Regards.
Daniel
Gas Turbine blades are weldable. I know of a place which manufacture these turbines. The major problem is the working temperature for these turbines is 1900 degrees F. for the life-time of the part. Because of the amount of work it takes to fabricate these turbines......it is cheaper to build new and replace the old rather than trying to fix it. Post weld heat is important more so that pre-heat. Remeber too, shut down of a turbine is so expensive that no one can has time to spend repairing a part which has already seen service as it is too worn compared to the cost of replacement and the likelyhood that it will fail anyway.
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