SS304/304L Tubes to CS Tubesheet Mock Weld -Cracks found
PARAI Reaction Channel
patni engineers
Patni Engineers. Mumbai. India
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Vanchinath S.A.
PARAI Reaction Channel
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Ahmed Osman
PARAI Reaction Channel
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Hrushikesh Sangamnerkar
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Marco Nunes
What the Macro Suggests
From the image:
- Fusion is occurring on the upper groove walls.
- Root area shows an unfused triangular gap / cold lap zone.
- Weld metal is flowing around the joint rather than penetrating fully to the root.
- This is typical when the arc cannot adequately reach the root face or root land.
Likely Causes
1. Amperage Too Low for Root Pass
82 A for first pass on a restrained joint with 50 mm thick tubesheet may be too low, especially if heat is rapidly absorbed by the heavy SA-516 tubesheet.
The tubesheet acts as a heat sink, pulling heat away from the root.
2. Groove Design / Root Land Too Heavy
If the groove has:
- narrow included angle
- excessive land/root face
- insufficient tube projection or mismatch
then penetration becomes difficult at lower amps.
3. Arc Position / Torch Angle
If arc is focused too much on filler or upper bevel edges rather than root corner, fusion at the bottom will be poor.
4. Travel Speed Too Fast
If moving too quickly on root pass, puddle bridges the opening before full fusion occurs.
Recommendations to Achieve Sound Root Penetration
A. Increase Root Pass Current Slightly
Try:
For 19.05 mm tube:
- Root pass: 90–100 A
- Hot pass: 100–110 A
- Cap: 105–115 A
For 15.85 mm tube:
- Root pass: 80–90 A
(Exact depends on wall thickness)
Use pulse GTAW if possible.
B. Modify Groove Design (Very Important)
Need better root accessibility:
Suggested:
- Increase included angle to 37.5° to 45°
- Reduce root face to 0.5 mm max
- Maintain small root gap if code allows
- Ensure tube stickout/projection consistent
Current geometry may be choking penetration.
C. Use Arc Focus Technique
For root pass:
- Keep tight arc length
- Direct arc into root corner
- Slight pause at sidewalls
- Minimal filler addition initially until keyhole/fusion established
D. Preheat Tubesheet Slightly
For SA-516 thick section:
- 80–120°C local preheat
This reduces heat sink effect and improves fusion without excessive amperage.
E. Back Purge / Internal Shielding
For SS304L tube root integrity, ensure argon purge inside tube.
Poor shielding can also contribute to cracking/oxidation.
To Avoid Cracks Simultaneously
Since you also had cracking concerns earlier:
Use:
- ER309LMo is correct choice
- Control interpass <150°C
- Avoid excessive restraint if possible
- Do not overweld
- Slightly higher heat with fewer passes is often better than cold multilayer welds
Cold welds can crack too.
My Best Practical Recommendation
Trial Procedure:
19.05 tube:
- Preheat tubesheet to 100°C
- Root pass 95 A DCEN
- 1.6 mm tungsten
- Tight arc
- Slow travel
- 3 passes total
If still incomplete:
- Slightly open groove angle.
Important Observation
You moved from cracking due to too much restraint/heat imbalance to lack of fusion due to overcorrection (too cold).
Now you need the balanced middle zone.
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Vishwas Keskar
Ashishranjan - Domech Fabricators Pvt.Ltd.
Dear PRC,
What you’re seeing is a fairly classic situation with tube-to-tubesheet mockups so before changing filler, it’s worth understanding the mechanism.
1) Are these hot cracks?
Yes—what you’re describing strongly points to solidification cracking (hot cracking).
Why:
- They appear at the root and sometimes surface
- Not detected by PT, but show up after macro etch → typical of tight, interdendritic cracks
- Occur randomly, not in every weld
- You’re using austenitic filler (ER309) on a dissimilar joint (likely carbon steel tubesheet + possibly SS tubes)
These are textbook indicators of hot cracking during solidification.
2) Likely causes in your case
Several contributing factors are probably overlapping:
a) Dilution from SA-516 Gr.70 (high carbon steel)
- Excess dilution into ER309 reduces ferrite in the weld metal
- Low ferrite (FN < ~3–5) → high hot cracking susceptibility
b) Fully austenitic weld metal
- If your weld metal solidifies as fully austenitic, it’s very crack-prone
c) Heat input / bead profile
-
Your parameters (~145 A, 13 V, 2 passes, 1.6 mm wire) can produce:
- Narrow beads
- High restraint at root
- Segregation → cracking
d) Joint restraint
- Tube-to-tubesheet welds are highly restrained, which promotes hot cracking
e) Possible contamination
- Sulfur, phosphorus, or even minor surface contamination can aggravate cracking
3) Can you switch to ER309LMo?
Yes—you can, and in many cases it helps, but it’s not a guaranteed fix by itself.
Why ER309LMo may help:
- Mo improves resistance to solidification cracking
- Slightly better tolerance to dilution
- Often results in a more crack-resistant weld metal structure
However:
- The key factor is ferrite content, not just Mo
- If dilution is still high, even ER309LMo can crack
4) More effective corrective actions (important)
Instead of only changing filler, address the root causes:
✔ Control ferrite number (very important)
- Target FN 5–10 minimum
- If possible, check using FN meter or calculate via WRC-92 diagram
✔ Reduce dilution from tubesheet
- Use lower heat input at root
- Use stringer beads, avoid wide weaving
- Slightly reduce amperage if penetration is excessive
✔ Consider buttering approach (best practice in critical jobs)
- Butter tubesheet hole with SS (309/309LMo)
- Then weld tube → reduces dilution dramatically
✔ Root pass control
-
Ensure:
- Proper fit-up (no excessive gap)
- Smooth root profile (avoid concavity)
- Avoid overheating small root area
✔ Interpass temperature
- Keep controlled (typically <150°C for this type)
✔ Cleanliness
-
Ensure:
- No oil, moisture, or oxide
- Proper pickling/cleaning if needed
5) Why PT didn’t detect cracks
This is normal:
-
Hot cracks are often:
- Very tight
- Subsurface or micro-cracks
- PT only detects open-to-surface discontinuities
Macro etching exposes them by revealing grain boundary separation.
6) Practical recommendation
Do this in order:
- Switch to ER309LMo ✔ (good step)
- Reduce heat input at root
- Ensure ferrite presence (FN ≥ 5)
-
If still failing →
👉 adopt buttering technique (most reliable fix)
7) Bottom line
- Yes, these are hot cracks (solidification cracks)
- Yes, ER309LMo is a better choice, but not a standalone solution
- The real fix is controlling dilution + ensuring ferrite in weld metal
Regards,
Ashish Ranjan
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