Why SS and CS material can not mix up in vessel fabrication time.......?[ponder] 1

[ramesh349]

Hi...

Good to see you all once again,

I have a query on vessel fabrication.

1. We are fabricating a stainless steel vessel in our shop. During the fabrication time our shop people separating all carbon steel plates near by the SS Vessel during the welding time. is it really effect the CS material...?

Is there any particular reason behind it.

2. If above is true, Why the SS vessel external attachments are directly welding with CS material.

3. Can we directly weld stainless steel and carbon steel. is there any corrosion reason behind it.

Please clarify the above 3 statements.


Thanks in advance,
Ramesh Kumar

 

[pressvessel]

The reason for separating SS and carbon steel components during handling & manufacture is not to protect the CS components, but to protect the SS components.

The grinding residue/dust from manufacture of CS components in the proximity of SS components would contaminate the surfaces of the SS components causing localized break down of the oxide layer. When I worked for a factory which manufactured both CS and SS vessels, there were dedicated areas cordoned off for SS vessel manufacturing. Strict rules were in place which prohibited even stepping-/walking on unprotected SS plates for the risk of boots/shoes contaminated with dust from the CS section or contaminated with oils/greases.

One of the first steps in manufacturing SS vessels in our factory was to "wrap" st/st plates with a protective film which was only removed prior to delivery of the completed vessel.

 

[PVDEngg]

SS is superior material as compared to CS. A superior grade material can be welded on to low grade material. Please refer ASME BPVC Section IX and you should get answers to all your queries.

 

[SnTMan]

ramesh349, to elaborate on pressvessel's post, the breaking of the oxide layer causes the SS to rust.

Ref your questions 2 & 3, it can be done but it is a poor practice. Best practice is to weld CS attachments to SS pads.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[EdStainless]

Many SS vessels will be passivated (acid cleaned) after fabrication to assure that you have uniform corrosion resistance.

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P.E. Metallurgy, Plymouth Tube

 

[r6155]

Depends of service and dimensiones, my best practice is external attachment same SS as pressure vessel, or SS pad + SS attachment.

Regards
r6155

 

[metengr]

1. We are fabricating a stainless steel vessel in our shop. During the fabrication time our shop people separating all carbon steel plates near by the SS Vessel during the welding time. is it really effect the CS material...?

Separation must be done to avoid contamination of the stainless steel with carbon steel. It is good practice to keep these materials separate.

2. If above is true, Why the SS vessel external attachments are directly welding with CS material.

They are not directly welded using carbon steel. Typically a 309 or Inconel filler metal is used.

3. Can we directly weld stainless steel and carbon steel. is there any corrosion reason behind it.

Only with a suitable filler metal made for dissimilar metal welding.

 

[EdStainless]

I should point out that dust, grinding debris and even simple contact with steel tools (lifting, positioning, and forming) will result in Fe being embedded in the surface of the SS. This will greatly reduce corrosion resistance and can lead to localized pitting failures.
This is why it not uncommon to see serious rust staining and pitting on SS decorative and architectural items, because fabricators and installers were not well versed in proper practice.

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P.E. Metallurgy, Plymouth Tube

 

[r6155]

In some cases equipments made of SS shall be fabricated in clean room (nuclear, pharmaceutical,... etc.).

Regards
r6155