A178 grade D finned tubes

[MFQC]

Dear friends
Have you got any experience in production of finned tubes with HFRW(high frequency resistance welding)?
In our new client spec for production of finned tubes from A178 grade D with A1008cs fin, the hardness difference between weld and HAZ shall be lower than 100vickers.
With any parameters we change, the hardness difference results not lower than 200 vickers.
Tube od:31.8mm
Tube tk:3.4mm
Fin tk: 0.8mm
Fin height:16mm
Could you please kindly help me in this issue?
Thanks

 

[Dhurjati Sen]

If the fins are capable of carrying out what they are supposed to do, i.e. heat transfer, then why this fuss about hardness?

I think punching / wrapping of fins and subsequent galvanizations are not viable options in your case.

DHURJATI SEN
Kolkata, India

 

[EdStainless]

What is the starting hardness of each material?
Aren't these both fairly soft to start with?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[SJmetallurgist]

Once finning is done, I dont see any importance of hardness difference. The more relevant checks for such resistance welded finned tubes are:
a. Depth of fusion in% of tune thickness.
b. Pull out strength.
c. Hardness with a maximum limit (not the differential values)
d. Fusion.
e. Micro & macro.
One general misunderstanding is that all heat transfer takes place due to conduction through fins, while infact, only 4 to 5% of heat transfer is due to conduction.

 

[MFQC]

Dear friends
As attached you can see the photo which is extracted from our finning procedure. As you see in points 5 & 6 the Hardness difference between base and HAZ is more than 100 HV0.3.
All other parameters such as bonding, dimensions ,Pull out, depth of fusion ,... are met.
I personally think that this limitation is perhaps because of the vibrations in HRSG during performance.
with changing what welding parameter do you think that we can meet the procedure?
Best regards

 

[Dhurjati Sen]

It seems you are staring at a big rejection. Pity unable to help you out.

Still,
1. check all the documentation
2. carry out PMI & hardness of the A1008cs fins and co-relate with standard
3. carry out PMI & hardness of the A178 grade D tubes and co-relate with standard
4. check all the parameters of the resistance welding set-up
5. check the hardness testing machine, if possible measure hardness with another one
6. check the actual location of the high hardness, perhaps you would find any adherent scale

All the best!

DHURJATI SEN
Kolkata, India