Electrode Strength Lower than Base Metal

Date 04-04-2005 01:46

I am not well versed in the ASME specs but I do know in Cromoly tubing for race chassis the filler metal is always of lower tensile strength than the base material, the reason is for elongation purposes. It was explained to me as, if the filler were stronger the car would be as if welded with glass---- shatters in a collision.

This propably isn't what your referring to but I hope this helps.

By - Date 04-04-2005 12:09

Maybe I'm missing something here, but if two metals have the same chemistry, how can they have different tensile strengths? Give us an example of what you're refering to, please.

CM

By chall

Date 04-04-2005 13:14

If you will attempt to qualify a WPS with a groove weld, you are required to pass the tensile test. From Section IX:

QW-153.1 Tensile Strength. Minimum values for procedure qualification are provided under the column heading "Minimum Specified Tensile, ksi" of QW/QB-422. In order to pass the tension test, the specimen shall have a tensile strength that is not less than:
(a) the minimum specified tensile strength of the base metal; or
(b) the minimum specified tensile strength of the weaker of the two, if base metals of different minimum tensile strengths are used; or
(c) the minimum specified tensile strength of the weld metal when the applicable Section provides for the use of weld metal having lower room temperature strength than the base metal;
(d) if the specimen breaks in the base metal outside of the weld or fusion line, the test shall be accepted as meeting the requirements, provided the strength is not more than 5% below the minimum specified tensile strength of the base metal.

Charles Hall

By - Date 04-04-2005 14:45

Charles,
With all due respect, I think the forum is referring specifically to the filler metal. I know of no where in Sec. IX that addresses his concerns. Your response referring to QW-153.1 is certainly true, but that does not address specifically whether the Code allows filler metal with lower tensile strength to be used. QW-153 addresses variables for base metals.

CM

By chall

Date 04-04-2005 15:25

An interesting point. 153.1(c) addresses this in my opinion:

QW-153.1 Tensile Strength. Minimum values for procedure qualification are provided under the column heading "Minimum Specified Tensile, ksi" of QW/QB-422. In order to pass the tension test, the specimen shall have a tensile strength that is not less than:
(c) the minimum specified tensile strength of the weld metal when the applicable Section provides for the use of weld metal having lower room temperature strength than the base metal

Occasionally we are faced with the task of welding superheater alignment clips to tubes. Our clients want a weld that attaches the clip sufficiently. However, in the event of high stress (caused by thermal expansion or movement due to other causes), they want the weld to fail. They want this as opposed to having the clip/weld tear away from the base metal leaving a thin spot in the tube.

It's getting a bit off track, but when we are faced with these situations, we qualify the WPS using a fillet weld joint design. The acceptance criteria doesn't consider tensile strength.

Charles.

You are right though, about the original question. It does seem to be a metallurgical question...

By - Date 04-04-2005 16:18

Charles,
Your points are well taken, but the forum does not mention if this a procedural question or not. But 2 important points should be noted here. No. 1 is that a filler metal and a base metal having the SAME chemistry will have the same tensile strength. That is what determines tensile strength, chemistry. No.2 is that the applicable filler metal specification will give the minimum required tensile requirements for any filler metal. Respectfully, QW153.1 and QW-153.1 (c) are referring tensile testing of a "tensile test specimen". This will require a welded specimen to be performed. The forum question specifically asks about a filler metal before weld deposition. This is totally different than what QW-153.1 is referring to. My question to the forum is how can a filler metal, having the same chemistry as the base metal, have a different tensile strength?

CM

By Lawrence (*****)

Date 04-04-2005 17:15

For the sake of discussion, though I'm not a section IX expert I do have a couple of thoughts.

I think the key to the original question is to define exactly what he means by *matching* filler metal.

One may refer to a handbook such as the ASM Metals Handbook Vol. 6 and find various suggested *matching* filler metals for the joining of a given alloy. Are they an exact chemical *Match* ? No, or almost never. Exact chemical matching such as shearing sheetmetal strips to use as filler metal to weld sheetmetal is common for only a very few alloys.

Those cross reference charts exist for the reason of choosing *matching* ie.(approved?) fillers with a selection of different properties. Many automobiles are joined (when fillers are used) with fillers that have greater ductility (less tensile) than the parent material so that under cyclic loads or heavy impact the welds don't shatter first.

Turbine engine manufactures provide standard practice manuals with wire cross reference charts that may provide several alloys to *match* a given parent material. I've seen specs call out 347, inco 718 and Hastelloy W as a *matching* filler for 321 stainless depending on service conditions.

Furthermore, parent material may be in an annealed state and the filler might have greater tensile values simply born from the drawing process. Both materials may have an exact chemical match but differing strength properties.

6061 Aluminum sheet and 6061 Aluminum filler wire would appear to be an exact match eh? They are not. The filler wire has additions to deal with hot cracking.

Heck, an as welded heat affected zone may have different strength properties than parent material or the filler giving us three possible strengths for a single chemical composition :)

Peterbil must give us more data.

By - Date 04-05-2005 02:30

The author specifically says "matching chemical" composition. To me, that is pretty obvious what he is saying. "Matching" filler metals designated to weld a certain base metal is always overalloyed to compensate for alloy loss in the weld arc. To be honest, selecting a filler metal should depend on the chemical composition of the base metal. The tensile strength should not be the deciding factor. I.E., if you are welding a base metal of 316L, then the filler should be a 316L filler metal unless otherwise directed to do so. Just using a 316L filler metal as an example, it does not normally ever match the chemical composition of the base metal. It is overalloyed to allow for the alloy loss. Even a LITTLE alloy difference will make a BIG difference once the weld process starts.

CM

By GRoberts (***)

Date 04-05-2005 03:54

I think I have to agree with you on this one Charles. ASME IX does not address what filler metals are or are not allowed to be used with specific base matals, it only requires certain properties of a welded joint, as you mentioned. (QW153, which addresses the properties required from welding procedure qualification) Since the original question was whether or not ASME IX addresses the issue, I think you called it right.

One example of the situation described in the original question I have had to deal with, is welding on a 5 Cr, 1/2 Mo alloy (8018-B6 or 80T5-B6) with a 90 minimum tensile strength. Normally the matching filler metal will not meet 90ksi after stress relief because most alloys in the 5Cr-1/2Mo family are 80Ksi or less, so that is what they are designed for. So you have to pick your filler metal manufacturer and get an electrode with the right chemistry. (In this case higher carbon) By design, most of the time the filler metal is lower in hardenability than the base metal. This is because of the drastic quenching effect that the surrounding base metal has on the weld as compared to the cooling rate experienced by the base metal during heat treatement. So the weld can reach the same strength without having as much alloy. Most of the time this is beneficial from the crack-resistance standpoint, but it can be detemental when higher strengths are required.

If you are talking stainless steels/nickels, etc, (as I think Mr. Meadows is- althought the original question did not indicate what alloys were involved), usually filler metal will over-match the base metal to aid in corrosion resistance. Mr. Meadows points out that before depositing the weld, the electrode overmatches even more to compensate for oxidation in the arc, but mostly we are concerned about the properties of the weld deposit, not the filler metal before welding. (Unless you are manufacturing the filler metal.) Except for solid wire where the chemical analysis is allowed to be determined from the wire, and therefore will be slightly different from the deposited weld metal.

I think the bottom line is that you cannot seperate the filler metal from the procedure qualification, which is how ASME addresses the minimum tensile strength required from the joint, which in turn dictates the minimum tensile strength of the weld metal.

By peterbil (*)

Date 04-06-2005 04:53

Hi Guys,
Wooh, i like this forum thanks for commenting now let me give you a detail regarding my question. I welded an API casing grade L80 vs AISI 4130 I've checked the chemical composition of this base metals specially L80 which I found out to be a chrome moly steel obviously 4130 is a chrome moly steel. When I said matching I mean that the composition is within or most elements in the base metal is found in the electrode / filler rod I've used ER80S-B2 for my root pass and E8018-B2 for hot and capping. The application of this is on geothermal. kindly checked this out for further detail.
http://www.cwa-acs.org/journal/Fall2003/Wilson.html

By - Date 04-06-2005 13:26

peterbil,
With respect, your original question to the forum differs drastically from what you are now presenting to us. First of all, 4130 is classified as an HTLA, or Heat Treatable Low Alloy AISI steel, not a Chrome-Moly. A Chrome-Moly steel contains 0.5 to 9% chromium and 0.5 or 1% Moly. 4130 is NOT considered a Chrome-Moly steel since it does not meet the minimum requirement for moly. 4130 has only 0.15-0.25% moly. To be considered a Chrome-Moly, the Moly will be either 0.5% or 1.0%. This is not to be taken personal or disrespectful, but you left out MANY vital details when you posted your original question.

CM

By Lawrence (*****)

Date 04-06-2005 18:54

Learn something every day.

I have been refering to 4130 as Chrome moly for years.

Even Lincoln Electric, and the Welding Journal have something to learn.

http://www.lincolnelectric.com/knowledge/articles/content/chrome-moly.asp

http://www.aws.org/wj/apr03/AWfeature.html

By jwright650 (*****)

Date 04-06-2005 19:13

"Learn something every day.
I have been refering to 4130 as Chrome moly for years."

Me too, Lawrence. I just didn't know any better and that's what I had always heard 4130 referred to as.
John Wright

By - Date 04-06-2005 23:35

Lawrence and John,
The assumption is quite understandable, but there is really more difference than normally taken. For one, the 4130 has a carbon content of 0.28-0.33%. The carbon content of the Chrome-Moly's are 0.20% max. except for the "modified" Chrome-Moly known as P91, which has a carbon limit of 0.08-0.12%. This affects the heat treatment temperatures for these grades. I agree, 4130 is called a "chrome-moly" many times and it is a low alloy material just like the real chrome-moly's, but there is a difference. Please refer to the ASTM Specification for Chromium-Molybdenum Steel Products Forms. I guess one of the big differences relating to the 4130 is that during welding the HTLA steels have high carbon limits that can promote the formation of martinsite, which makes the material more susceptible to hydrogen embrittlement. I apologize if I appeared to try to be getting too "technical". For a quick reference ou can refer to the AWS Welding Handbook, Vol. 4, Materials and Applications--Part 2, Eighth Edition. Page 48 and 49 reference the HTLA (Heat Treatable Low-Alloyed Steels. Page56 and 57 refer to the Chromium-Molybdenum Steels. Thank you...

Chuck (not Mr. Meadows) <smile>

By jon20013 (*****)

Date 04-06-2005 19:44

Mr. Meadows,

I've been watching and reading your posts with great interest. Very well thought out and professional responses, you are definately an asset to this forum.

Now then, if I may pose a personal question? My company has been tasked with doing some hardfacing overlays (Stellite 6) on 17-4PH materials. The required buildup of stellite 6 is fairly thick (approximately 0.250" required thickness).

In reading the manufacturers recommendations of keeping the 17-4PH down below 248F it simply cannot be done. We have found quite a bit of success placing the first layer at below 248F and then placing the parts in an industrial oven to preheat them to around 400F before depositing remaining layers. Once HFOL is achieved we then place back in oven and slow cool for about 8 hours.

I would welcome you comments on this application. (By the way, since I work in the nuclear industry getting our customer to modify HFOL material from Stellite 6 to Stellite 21 is not really an option....).

By - Date 04-07-2005 03:59

Hi Jon,
Your comments are certainly appreciated. Thank you for your kind words. Now, lets look at your "situation".
I'm a little surprised that you are being required to overlay a 17-4PH material in the first place. It is rather uncommon for this process. First of all, the 17-4PH is, as you already know,a precipitation-hardening stainless steel. The 17-4 differs from the other PH materials in that it is a martinsitic, moderate strength steel which requires no formal preheat upon welding. I'm at odds with the recommendation of "keeping the 17-4PH down below 248F". I don't understand the reasoning. The only reason I can possibly think of is that when the 17-4 cools down from 300 to 200F, the austenite transforms to martinsite, although a few stringers of ferrite can be in this martinsitic matrix. I'm not sure I have enough data to fully present an eduated opinion on this process, especially with the temperatures you are mentioning. If you are meeting all required metallurgical tests, I would assume you would want to proceed with the technique that is giving favorable results. Believe me, I've dealt directly with the nuclear regulatory commission and customers that work within the nuclear guidelines so I would not even begin to recommend changing form one HFOL to any other. Based upon all the data you have provided, I cannot see any problem with what you are currently doing. I just don't understand where the 248F is coming from...Again, thanks for the kind words.

Chuck

By jon20013 (*****)

Date 04-07-2005 11:50

Thanks Chuck. I've copied from the producers literature and pasted just below: (Sandmeyer is the producer by the way)

"Welding
Alloy 17-4 PH can be welded by the following welding processes: SMAW,
GTAW, PAW and GMAW. SAW should not be used without preliminary testing (to check freedom of cracks and toughness of the weld metal).
Due to a ferrite delta primary type of solidification, the hot cracking risk of the weld metal or the HAZ is reduced.
Generally, no preheating must be done and interpass temperature must be limited to 120°C (248°F). The better toughness is obtained in the weld after a complete heat treatment (solution annealing + precipitation hardening)."

Me again: As I mentioned, when applying Stellite 6, the 248°F interpass does not seem to work... perhaps the producer was simply speaking in "general" terms? I know the application of Stellite 6 to 17-4PH is very unusual and without our industrial oven I would say nearly impossible, at least for the thickness of deposit our company was required to deposit. Long story short, elevated preheats seem to work well. Incidentally, when trying this application without the additional benefit of our oven we experienced delayed or cold cracking (underbead cracking) and these indications didn't appear until days after completion (final HFOL was subjected to PT before machining).

By - Date 04-07-2005 13:23

I read your literature from Sandmeyer, and it all seems reasonable except for the part about SAW not being recemmended, but that is another subject. I'm not real sure that I fully agree, but it is not my place to contradict a manufacturers data, but please let me make a point. The 17-4 PH is a martensitic type of the PH grades. Upon cooling, only a few stringer of ferrite form, but in the martensitic matrix. Is that enough ferrite to prevent hot cracking? I'm not sure, but it doesn't seem to me that the ferrite is sufficient for that to take place. Not to say that you will automatically experience hot cracking, though. Actually, the primary structure solidification of the 17-4 is predominately austenite and will transfer to martensite upon cooling, with a few stringers of ferrite in that martensitic matrix. Most of the martensitic grades of the PH's are virtually ferrite free.
Now, the "semiaustenitic" grades of the PH's (PH 15-7MO, PH 14-8MO, and a few others) will contain 5 to 20% of delta ferrite in the solution annealed condition. This is definitely enoough ferrite to counteract hot cracking.
The rest of the manufacturers comments, I agree with.
I can see bringing the 17-4 up to 400F to allow the transformation from the austenitic structure to it's natural martensitic structure. I disagree that with a 17-4 "no preheating must be done". The martensitic grades of PH (17-4) are not subject to cracking. I think the manufacturer should state that no preheating is "required", not say "must not be done". Your use of your ovens are probably making your job more appropriate. Good job..

Chuck

By peterbil (*)

Date 04-07-2005 01:14

Hi Cmeadows

I appreciate your comment and thanks for correcting me with the issue about 4130 as HTLA and not chrome moly I've checked it myself from AWS Handbook and I agree with you. The details I presented here is a case that I am really concern about but it is still in connection with my original question. If you read the article in website included in my post you will see that E8018-B2 was used as electrode to weld L80 vs 4130 for a reason concerning control of hardness equal to or less than 22HRC in sour environment NACE MR0175. An L80 has 0.8-0.9% Cr, 0.29-0.30% Mo tensile 95 ksi min. based on my mil certificate 4130 Q&T 1.064% Cr, 0.226% Mo tensile 101.21 ksi and E8018-B2 has only 80 ksi 1.5%max Cr, 0.65%max Mo. (sorry the chemistry of electrode and filler rod is actually higher than base metal) ER80S-B2 has the same tensile value and the same chemistry obviously the electrode/filler rod strength is lower than my base metal going back to the original question what do you think about this case?

By - Date 04-07-2005 03:32

I commend you on doing a very detailed research on this subject. You certainly have documented evidence to back up your statements, and apparently mock up test data to satisfy NACE and the service involved. Again, the main concern, in my opinion, is the difference in carbon content which dictates to some degree the heat treatment, which is directly relative to the hardness requirements. Based upon the requirements you are having to adhere to, and based upon the article you submitted, I cannot argue with documented test data. We do not have to agree with certain things, but when in doubt, doing research like you did is the answer. I think your concerns, respectfully speaking, are more from a personal concern than from a metallurgical standpoint. With that said, and based upon the data submitted, I think you should accept the proven data and proceed with confidence.

Chuck