WELDING REPAIR of Titanium Condensate (Tie Rod)

Rabu, 13 April 2011 17:20 | Oleh Administrator | | |

WELDING REPAIR of Titanium Condensate (Tie Rod)
Vibration During Welding
At
Power Plant (Steam Turbine)
PT. Semen Tonasa – Sulawesi Selatan

ABSTRACT

WELDABILITY OF MATERIALS :

TITANIUM AND TITANIUM ALLOYS

* High strength to weight ratio
* Corrosion resistance
* Mechanical properties at elevated temperatures.

KEYWORDS : Weld Metal Porosity, Embrittlement, Contamination Cracking, Vibration During Welding, Condensate, Tie Rod, Repair Welding, Conductivity.

Introductions

MATERIAL TYPES ALLOY GROUPINGS

• Titanium; Commercially Pure (98-99,5 % Ti) or strengthned by small additions of oxygen, nitrogen, carbon and iron. The alloys are readily fusion weldable.
• ALPHA ALLOYS : This are largely single phase alloys containing up to 7% alluminum and a small amount (<0.3%) of oxygen, nitrogen and carbon. The alloys are fusion welded in the annealed condition.
• ALPHA-BETA ALLOYS : This have a characteristic two phase, microstructure formed by the addition of up 6% alluminium and varying amount of beta forming constituens - vadium, chromium and molybdenum. The alloys are readily welded in the annealed condition.

FILLER ALLOYS

* Titanium and its alloys can be welded using a matching filler. Composition; Compositions are given in
* The American Welding Society Specification AWS A5.16-2004.

WELD IMPERFECTIONS

• TIG and PLASMA processes, with argon or argon-helium shielding gas, are used for welding thin section components, typically <10 mm.
• The most likely imperfections in fusion welds are :

- WELD METAL POROSITY
- EMBRITTLEMENT
- CONTAMINATION CRACKING (Hydrogen Cracking)

WELD METAL POROSITY

• Porosity arises when gas bubbles are trapped between dendrites during solidification (in Titanium, Hydrogen from moisture in the arc environment or contamination on the filler and parent metal surface, is the most likely cause of porosity)
• ACTION : Cleaned by Pickling (HF-HNO3), light grinding or Stainless Steel brushing in the join surface area.

EMBRITTLEMENT

• Can be cause by weld metal contamination by either gas absorption or by dissolving contaminants such as dust (iron particles) on the surface. At temperatures above 500 deg. C, Titanium has a very high affinity for oxygen, nitrogen and hydrogen.
• The weld pool, heat affected zone and cooling weld bead must be protected from oxidation by an inert gas / back up gas (Argon or Helium).
• The color can indicate whether the shielding maximum. SILVER or STRAW color shows satisfactory gas shielding was achieved. DARK BLUE may be acceptable. Light blue, grey and white usually unacceptable (oxygen contamination).
• For small components, an efficient gas shield can be achieved by welding in a totally enclosed chamber, fill with shielding gas. Oxygen level should be reduced to approximately 40 ppm before striking the arc on scrap titanium and <20 ppm before welding the actual component.
• In tube welding, a fully enclosed head is equally effective in shielding the weld area and is preferable to orbital welding equipment in which the gas nozzle must be rotated around the tube.
• When welding out in the open, the torch is fitted with a trailling shield to protect the hot weld bead while cooling.

CONTENT

CASE STUDY / WELDING REPAIR

Power Plant at BTG Semen Tonasa - Pangkep Sulawesi Selatan is a coal steam turbine.
The condensate made from SS 410 with titanium lining (On tube sheet).
Join on Tube Sheet and Tie Rod is crack and sea water come inside to the boiler.
Problem..! So… we did repair welding on join tube sheet and tie rod.

CONTAMINATION CRACKING

• If iron particles are present on the component surface, they dissolve in the weld metal reducing corrosion resistance and, at a sufficiently high iron content, causing embritlement. Iron particles are equally detrimental in the HAZ where local melting of the particles from pockets of titanium-iron eutectic. Micro cracking may occur but it is more likely that the iron-rich pockets will become preferential sites for corrosion.
• Particular attention should be paid to separating titanium from steel fabrications, preferably by designating a specially reserved clean area. Welders should guard against embedding steel particles in to the surface of the material by :

- Avoiding steel fabrication operations near titanium components.
- Covering components to avoid airborne dust particles settling onthe surface.
- Not using tools, including wire brushes, previously used for steel.
- Scratch brushing the join area immediately before welding.
- Not handling the cleaned component with dirty gloves.
- To avoid corrosion cracking and minimize the risk of embritlement through iron contamination, it is best practice to fabricate titanium in a specially reserved clean area.

FILLER TITANIUM

Titanium 6Al-4V AWS Class ER Ti-5

Features

• High strength to 600^oF
• Excellent general corrosion resistance
• High strength-to-weight ratio

Applications

• Gas turbine compressor dishes and blades
• Airframe structural components (aircraft)
• Chemical process equipment

Performance Profile

Ti 6Al-4V is the most widely used of all the alpha beta titanium alloys. It may be heat treated for high strength in welded construction at service temperatures through 600^oF. Harden ability is limited and section over about one inch may not develop full properties. For cryogenic applications the extra low interstitial grade ELI (UNS R 56401) is preferred.

Annealing bar and forging is done at 1275-1325 ^oF 2hr air cool. Solution treat 1675-1725 ^oF 1hr water quench. Precipitation treat 975-1025 ^oF 3hr air cool. For increased fracture toughness but low tensile strength precipitation treat 1150 ^oF 4hr air cool.

Welding is by GTAW, direct current electrode negative, ¾ “ gas cup and gas lens. Limit electrode extension to 3/8 “. Inert gas shielding is required on all areas hotter than 800 ^oF. That is in addition to remarry torch shielding, trailing shield is required to protect the cooling weld metal, and back up shielding is required. Internal spaces should be purged 10 times the enclosed volume. Shield all tack welds.

Shielding and purge gas must be argon or argon helium mixture, NEVER nitrogen. Filler metal AWS A5.16 ERTi-5 may be used. ERTi-5 ELI preferred for greater toughness.

1. Sheet : Regular anneal furnace cool Duplex anneal. Mill anneal + 1435 ^oF, 15 minutes air cool. Triplex anneal. Mill anneal + 1850 ^oF, 5 minutes air cool, + 1375, 15 minutes air cool.

Bar and forgings :

Duplex anneal 1650-1850, 1 hour air cool + 1000^oF -1100 ^oF, 8-24 hours air cool.

2. Bar duplex anneal : Mill anneal + 1000^o – 1200^oF, ½-6 hours air cool.

3. Anneal furnace cool at 300^oF per hour maximum to 1000 ^oF to 1050 ^oF.

4. Stress relief may be accomplished at 800oF-15 hours, 850oF-5 hours, at 900oF-1 hour and 950 ^oF - ½ hour.

5. For 100% stress relief, 1000 ^oF -50 hours or 1200 ^oF -5 hours for 50% relief, 1000 ^oF -5 hours or 1100 ^oF – ½ hour.

6. Furnace cool at 300oF maximum from anneal temperature for maximum improved by holding at annealing temperature 24 hours.

7. Slow cool to 1000 ^o -1050 ^oF maximum from upper annealing temperature.

8. Anneal sheet at temperature for 20 minutes. For bar hold at anneal temperature 2 hours.

.

Tie Rod Titanium 6 Al 4 V

Tie rod titanium condensate is support at tube sheet with loaded sea water = 250m³/minute.

Prepare before welding (Turbine off)

• 1. Clean up join tube sheet and tie rod, by light grinding (Blending) and Stainless Steel brush.
  2. Make V groove on crack at join tube sheet & tie rod.
  3. Welding Process :
     • Welding machine inverter ARCMART DC 16li.
     • Ampere 80A DC Negative.
     • Tungsten Thor 2% ø 1.6 mm with trailing torch.
     • Argon Gas, HP with out back up, flow 15 LPM
     • Filler AMS 4954/ER Ti-5 ø 1.6 mm
  4. Test : Penetrant Test (P.T)

PENETRANT TEST (P.T)

After welding and cooling down, color on HAZ is looked silver (shielding gas maximum) Penetrant Test : accepted (No crack or porosity)

FUNGTIONAL TEST

Turbine on, water conductivity at condensate and hot well 70. When broken! Water conductivity 900. (Sea water conductivity 64,000).

Recommended for boiler <10, during operation before crack on join tube sheet & tie rod=8, After first repair conductivity 70…. Not accepted. After 2x24 hours… conductivity up to 4,000 and than shut down the turbine…!!

SECOND PREPAIRE (Turbine … Start Up)

1. Re-cleaned up on tube sheet & tie rod.

2. Re-make V groove on tube sheet & tie rod.

3. Re-welding process:

a. Welding machine inverter ARCMART DC 161i.

b. Ampere 80A DC Negative.

c. Tungsten Thor 2% ø 1.6 mm with out trailing / with gas lens.

d. Argon Gas HP with out back up with flow 20 LPM. e. Filler AMS 4954 / ER Ti-5 ø 1.6 mm.

VACUUM TEST

84 Bar vacuum test with water soap… No broken (accepted)

PENETRANT TEST

On HAZ color… SILVER.

Penetrant test… No Crack or porosity.

FUNGTIONAL TEST

After 2x24 hours… Turbine running, conductivity=2.4…very good

Electric power connect to transmission.

Root pass… RPM 1000 ( Start up)

Filling …… RPM 2000 ( Start up)

Capping …. RPM 3000 (Running)

CONCLUSION :

Clean , clean and clean …is very important

Shield, shield and shield …is very important

Vibration During Welding is a way to the good welding of Titanium

REFERENCES

• Welding hand book Metals and Their Weld ability Section Four Sixth Edition American Welding Society 1972 chapter 73.
• More than 20 years experienced in welding.

SUKARDI