Weld pratical questions

[Enhineyero]

To engineers who have are very knowledgeable in welding please help.

Have a few questions about steel welding - building structures (tried browsing the forum but cant find the answers).

1. Most standards tell you that minimum size of fillet weld is 3mm - why is this?
2. If you are to use the 3mm min. fillet weld into a thin hollow section (say 2mm thick) wont the weld distort the hollow section? how do you then weld a 6mm plate to a thin hollow section?
3. Can you bevel weld a 6mm plate into a thin (2mm) hollow section?
4. What is the minimum/maximum weld thickness that you can make on a single pass? I know its related to the welding rod size but not sure what is the commonly available welding rod.
5. Max. thickness of weld is usually dictated by the thickness of plates being welded together - is there a practical maximum when you have very thick plates?
6. When is it most appropriate to use partial penetration welds?

 

[JStephen]

1) One limitation if you put a small weld on a large piece of metal, it chills the weld faster than anticipated, not sure if that's the motivation for the 3mm or not. That'll also be related to the procedures anticipated under the code.
2), 3), No answer
4) You can weld 1.5" plate or thicker with a single pass with electroslag process.

https://www.youtube.com/watch?v=S6I80EN3gm8

5) Don't know, the beams at the SF Transit center were 4" thick, though. I think the ASME code has an 8" limitation. I suppose one practical limit is getting somebody to pay for it.
6) When strength and code allow?

 

[CANPRO]

1 and 2) what kind of codes are you referencing? 3mm welds aren't really appropriate for the typical plate thickness you'd see in the structural steel side of the building. Maybe look to cold formed codes or standards related to light gauge welding. I'm not involved in a lot of light gauge stuff so I'm not sure whats out there for references and standards. In my world a 3mm weld is a seal weld.

3) As in taper the 6mm plate down to 2mm and butt weld with the 2mm plate? I'm sure you can. Whether or not it is appropriate for the application and the weld can be qualified is another question.

4) As JStephen notes, it depends on the weld process. Your question leads me to believe you're asking about stick welding(SMAW). In this case a standard maximum single pass fillet would be 5/16" (8mm).

5) I've personally seen some 100mm plate with CJP welds, which was impressive. As far as welding codes are concerned (CSA W59) and the way the welds are qualified, once you get beyond a certain thickness it qualifies the procedure up to "unlimited" thickness. There may be application specific restrictions in other codes.

6) When it is required for strength and it is the most economical solution. A PJP would be restricted in certain applications, like a single side PJP in direct tension. Sometimes a PJP might be your only option due to access considerations. No easy answer here, its case by case.

 

[Enhineyero]

1. 3mm is mentioned as a minimum in AISC, CISC, AS 4100
2. There are several hollow section 1mm to 3mm thick, it is cold-formed but covered by the above codes (which are generally for hot rolled). Curious to know how to weld a plate into these thin sections.
3. What I meant is- can you connect the 6mm plate using bevel weld into the face of a 2mm hollow section?
4. what is the common thickness done on a single pass? or do welders just use 8mm whenever possible and do further passes if a thicker weld is required?
5. no further question
6. I see - PJP is more of a special case when full penetration is not easy to do.

 

[JStephen]

On 4), it will also depend on the materials, the position, the equipment, the code, the specific weld procedures used.

 

[dauwerda]

Enhineyero, you have references to AISC, CISC, and AS 4100, So I believe it is safe to say we are talking about welding structural steel. I am US based and only familiar with AWS D1.1 (welding code) and AISC so my answers will be with respect to those codes, but I assume many of these things will be similar.


1. Most standards tell you that minimum size of fillet weld is 3mm - why is this?
As JStephen stated the reason for this (at least in the AISC code) is to ensure a minimum amount of heat is put into the weld to prevent the weld metal from cooling too quickly. If it does cool to quickly, the weld will be too brittle.

2. If you are to use the 3mm min. fillet weld into a thin hollow section (say 2mm thick) wont the weld distort the hollow section? how do you then weld a 6mm plate to a thin hollow section?
Excess heat caused by large welds can be an issue and cause distortion. This should be considered when selecting material sizes and welds. Where AISC does specify a minimum weld size, it changes based on the thickness of the steel being welded and is based on the thickness of the thinner part joined.

3. Can you bevel weld a 6mm plate into a thin (2mm) hollow section?
I'm assuming this question is referencing a full penetration weld in a "T" configuration.
Can you? yes. Should you? probably not. It seems like the thin 2mm plate would be the limiting factor in the strength of the joint, I would think it would be more appropriate to size a weld based on the capacity of the the joint.

4. What is the minimum/maximum weld thickness that you can make on a single pass? I know its related to the welding rod size but not sure what is the commonly available welding rod.
This depends on what code you are following as well as welding process, weld type, welding position, and whether it is a prequalified weld or not. If you are using a prequalified weld from AWS D1.1 the requirements of the following table need to be met:

5. Max. thickness of weld is usually dictated by the thickness of plates being welded together - is there a practical maximum when you have very thick plates?
No

6. When is it most appropriate to use partial penetration welds?
As opposed to what? If a full penetration weld is not required a partial penetration is "always" going to be more economical. Often, a fillet weld will be more economical than a pjp however this will depend on both the size of the weld as well as the fabrication shop performing the weld (based on equipment available to them for prepping for a pjp weld).