How many times can be weld a P91 ?

Date 03-28-2003 13:39

I'm not in a position to answer your specific question but I can tell you my personal experience that may help you in making up your mind.
Back in my days of erector engineer, I was in charge of the erection of a crude oil refining plant in Argentina. The oil to be treated was highly paraffinic, so much that it was solid like a candle at room temperature.
For this reason, it used to burn up from time to time into the crude oil heater tubes (the one that heats the oil before sending it to the topping tower), leaving a carbonaceous residue which plugged the tubes. Those tubes were made of A-335 Gr P5 steel (5% chrome and 0,5% moly).

Each time one or more tubes got plugged, they had to be removed. There's no way to clean, either mechanically or chemically, an 8 inches tube 40 feet long plugged with carbon. Of course, had the tubes been welded, the only way to remove them was to cut the weld between the tube and the respective header.

And now comes the important information. In the client's (the refinery owner) experience, the maximum number of oxyacetylene cuts and rewelds that could be done in the A-335 Gr P5 tubes and their headers were three (3). The fourth cut and reweld would have damaged in a permanent manner the metallographic structure of the metal.

For this reason, the client didn't want welded connections in the crude oil heater. They used only expanded ones. An expanded tube can be removed from its header countless times if the correct method to break the expansion is used. By the way, Mr. G. Austin (pipewelder 99) has explained this method very recently in his answer to the question: "Is oxyacetylene dead?", posted on the Shop Talk section of this site.

Once the problem is solved, it would give a marvelous Marmaduke Surfaceblow story on Power magazine. Send the details to the magazine and you will contribute to the fascinating series of Marmaduke Surfaceblow stories, which began in 1948 and are still running.

Giovanni S. Crisi
Sao Paulo - Brazil

By Niekie3 (***)

Date 03-30-2003 15:46

Hi Mathew

Due to the metallurgical changes that occur during welding, there is generally not a maximum number of times associated with the welding of the material. (Subsequent welds tend to "renew" the material structural changes of the previous weld.) Usually the problem is rather the number of times that the material sees a PWHT. This is so because the PWHT is carried out over a wider area than only the weld.

With each successive PWHT, the material will tend to weaken slightly. After a couple of PWHT's, the material strength in this area might possibly dip below the minimum required in the specification. The way to calculate the effect is to use the holloman-joffe parameter to calculate the total time at temperature, and to then correlate this to material weakening.

Something to keep in mind is that the real limiting material property in your application will be the creep strength (operating at 600°C) and not the tensile strength. This gives you some leeway because grain growth (the typical weakening mechanism during PWHT) does not reduce creep strength. In fact, it improves creep properties!

Keeping this in mind, I would not be too nervous about the multiple welds at the same position.

Hope this helps

Regards
Niekie Jooste

By GRoberts (***)

Date 03-31-2003 16:16

Niekie,

I agree that multiple PWHT cycles can be detrimental in some cases, but am wondering about your reasoning. I have not heard of grain growth at PWHT temperatures. What is the mechanism in this case? I only know about grain growth in the austnitic temperature range (for ferritic or martensitic steels). I think that the main concern with PWHT for long/multiple times is embrittlement, where the P & S (and other tramp elements) migrate to grain boundaries to reduce thier free energy (G) and embrittle the grain boundaries. I think this is where the X and J factors come for calculating embrittlement potential for base and weld metal.

By Niekie3 (***)

Date 03-31-2003 19:55

Hi GRoberts

Theoretically grain growth will occur at all temperatures. Obviously the lower the temperature, the lower the energy available to allow the necessary migration within the material to take place. At PWHT temperatures (around 730°C for P91) the energy is certainly high enough to drive grain growth.

The larger the grain, the weaker the material and the lower the impact strength.

When the S & P levels are below specification limits, they should have no major impact on the properties of the steel under most circumstances.

Austenitic grain growth in steels that display solid state phase transformations are not really that much of an issue, because during the transformation, the grain structure at ambient temperatures are refined. This is what "normalising" is all about.

Regards
Niekie Jooste

By GRoberts (***)

Date 03-31-2003 21:00

Niekie,

Here are some informative sites regarding type IV cracking. While embrittlement is a major concern regarding pressure testing and startup, type IV cracking is a major cause of failure during service. Martensitic high temperature steels are the most prone.

http://enpov.aeat.com/carnot/case_studies/pdf/Modern.pdf

http://www.jsms.jp/kaishi/48/paper48-2-6.htm

http://216.239.53.100/search?q=cache:2EOMzOih9iEC:www.msm.cam.ac.uk/phase-trans/2002/papers/faulkner1.doc+%22type+IV%22+cracking&hl=en&ie=UTF-8

As far as the contaminants segregating to grain boundaires, this only occurs over very long times at temperatures. The X and J factors can be seen at the site below. Other contaminants that are included are tin arsenic and antmony.

http://www.btwcan.com/html/ppf8.html (I mis-remembered earlier about S, which is not included)

I do not recall hearing before about grain growth at PWHT temperatures, only at austenizing temperatures. I am interested to learn more if you know of any good reading materials on this subject.

The grain growth that occurs right next to the weld (the Coarse grain HAZ) does occur in the austenite phase on transformable steels, but even though they transform back to martensite or ferrite, those grains are also bigger due to the larger prior austnite grains. The larger austenite grains also make the CGHAZ more hardenable, since the larger austenite grains do not have as many nucleation sites for ferrite (i.e. grain boundaries) and retards the transfomation, paving the way for the shear transormation to martensite if the temperature gets low enough. On steels without PHWT this can be especially detremental to CTOD test values. This is also the main reason Ti is added to HSLA steels. The TiN is stable at very high temperatures and pins the austinitic grain boundaries from growing and reducing the toughness of the HAZ.

Here is a good article describing the grain growth and its attempted prevention in 12 cr steels.

http://www.aws.org/wj/supplement/Meyer12-01.pdf

As an additional note, I have read that the reason Mo decreases embrittlement, is that it allows the grains to be more cohesive across the grain boundaries, thus reducing the drive of the tramp elements to go to the grain boundaries in order to establish a lower free energy (G)
"Design of Alloys for the Energy Industries" by Bhadeshia avaivlable at the Universiy of Cambridge website.

This is a interesting post. Keep the answers coming!!

By Niekie3 (***)

Date 04-03-2003 19:47

Hi GROBERTS

Thanks for the references to type IV cracking. It is not something that I was familiar with. I am still reading the papers, to try and get my mind around it. From what I see so far, there seems to be a lot of debating of the causes, but not really any concrete reasons for the cracking. I will be investigating it further.

Your reference to the "X & J" factors are in fact a reference to the temper embrittlement effect that I mentioned. It does not appear to be related to the type IV cracking.

Regarding grain growth at PWHT temperatures: It certainly does happen rather slowly, compared to grain growth at higher temperatures, but occurs none the less. Especially where extended times at high temperatures are seen, grain growth will be found. I do not know of any paper on the net regarding this, but you will get references to it in metallurgy text books such as "Materials Science for Engineering" by Van Vlack. They use examples relating to brass, but the principles remain the same for all materials.

I agree totally with your statement regarding the origin of large grains adjacent to the weld metal, but this argument is really only valid for a single pass weld. Most materials used at high temperature will be rather thick, leading to multi-pass welds. In multi-pass welds the subsequent weld passes tend to temper and normalise the HAZ of the previous pass. As such, HAZ toughness is often comperable to that of the parent metal.

The article on the grain growth of the 12 Cr steel that you reference, is looking at a 3Cr12 "stainless steel". This steel is something of an oddity in that it is actually a type of low alloy duplex stainless steel. It suffers from the same grain growth problems that ferritic stainless steels experience in thicker sections. I therefore do not believe that it can cast too much light on the P91 debate at hand. Interesting paper non-the-less.

I must say that this post has certainly got me thinking, and challenging my current thinking, on these issues once again.

Thanks for your informative links.

Regards
Niekie Jooste

By GRoberts (***)

Date 04-21-2003 22:49

Hi Niekie,

I was getting a little bit off subject and a lot of the HAZ stuff I said was more general and not directly related to P91. But you are right in that the X and J factors have to do with grain boundary embrittlement. I was not trying to link them to type IV cracking. Sorry if I was confusing there.

After looking again, the grain growth reference was probably not the best one, it was just the first one I found in the stack of info I have. While I agree that mulitpass weldments perform better than single pass weldments with regard to HAZ toughness, there are always going to be unrefined/tempered areas from the previous bead unless a good temper bead technique is used. I have recently been doing CTOD tests, and most of the research I have found indicate that usually the coarse grain HAZ is the least tough area. A good paper that references a lot of this can be found at this link.
http://www.us.cbmm.com.br/english/sources/techlib/info/weldabil/weldabi.htm

There is also a good HAZ/CTOD research paper in the December 1991 Welding Journal if you have it.

By chall

Date 03-31-2003 16:48

I am sounding in on this one to compliment you for the question and the other responders for their answers.

I have always wondered how many times a weld may be cut out and welded over. We have not encountered the situation you describe yet, but I expect it will pop up on the horizon. We have been in situations where we have repeatedly removed hand-hole caps from boiler headers, for replacement. Generally, the boiler has operated for a year or so before the hand-hole cap is replaced. Those materials are carbon steel. From what I've been able to determine, carbon steel is very difficult to embrittle. Although that sounds nice and causes everyone to nod and proceed forward, I would be interested in opinions on rewelding SA106B pipe material as well as the P91 material originally discussed here.

Thank you, Charles Hall

By Niekie3 (***)

Date 03-31-2003 20:24

Hi Charles

The problems with repeated welding of SA106 B will typically be less than with the P91, because the phase changes are typically less complicated and quite predictable. There will be a zone of grain growth that will have lower strength and impact values, but this is usually small compared to the over design (high safety factors) that is used in the design of this type of equipment. In addition, if the material is in the creep region, this grain growth will not be detrimental. All that happens is that at low temperatures the impact properties are reduced. This is typically only a problem during pressure tests at relatively low temperatures. If the component survives the pressure test, then it will generally survive the operation.

Regards
Niekie Jooste

By M-Squared (**)

Date 03-31-2003 17:49

Personally, I do not allow this material to be welded and PWHT more that three times. The reason for this is the increased risk of Type IV cracking in the HAZ of this material.

As most people are aware, Weldments can be considered as a structure with five materials; base material, weld material, and three HAZ’s. The HAZ is made-up of a Coarse grain zone, Fine Grain Zone, and Soft Zone. At the edge of the weld metal there exists a coarse grained region that is created by grain growth resulting from the high temperatures of welding. Moving away from the weld metal further there is a fine grain region resulting from recrystallisation which occured from an intermediate temperature that was insufficiently high for grain growth. Finally, next to the parent metal there exist a region that has been heated to temperatures below Ac1 and is therefore over-tempered but not re-transformed (This layer is relatively soft compaired to the coarse zone).

Type IV cracking is observed in the fine grained part of the HAZ (Type IV zone). It is commonly associated with high alloy ferritic/martensitic steels like P91 but there have been studies of C-Mn and low alloy steels as well.

Since PWHT is carried put below the transformation temperature, there is little change in the coarse and fine grain structures of these weldments. Now if we cut the pipe and reweld it in the same place, the process described above for the HAZ is once again carried out. Therefore, the coarse and fine grain structure becomes coarser/finer and the likelihood of Type IV cracking increases. Also, one has to remember that PWHT of these materials is performed to temper the Martensite that has formed in the weld metal. Please remember the weakest link in this material is not the weld metal but the Coarse/fine grain zone in the HAZ. To date, there have been no failures associated with welds in P91 material; however, there have been numerous failures in the HAZ attributed to Type IV cracking.

Hope this helps.

By Niekie3 (***)

Date 03-31-2003 20:16

Hi M-Squared

I assume that the "type IV" cracking is the so-called "temper brittleness" found in some alloy steels. If I am wrong, please help me right, because I am unfamilliar with the term.

Temper brittleness is induced in alloy steels when they are held in the temperature range 480 - 650°C for extended periods of time. It should not be induced at the typical PWHT temperatures associated with a P91 material. (approx 730°C)

Certainly, there is a section of the material that will see this temperature range during welding, but it is for such a short period of time, that it should have very little detrimental effect. Of greater concern is the time that the material sees this during the heating and especially the cooling cycle of the PWHT. Obviously, if a "band PWHT" is performed, then there will be a section (far away from the weld) that sees this temperature for an extended period of time during the PWHT cycle. This could lead to potential embrittlement problems.

I believe that all this will be a drop in the ocean compared to the service conditions of the material. (600°C) This temperature lies nicely in the temper brittleness range, and should the material have a tendancy to temper brittleness, it will happen in service. Luckily additions of Mo tend to inhibit the temper brittleness susceptibility of materials. P91 has around 1% of Mo. For this reason, it is suitable to be used at elevated temperatures for extended periods.

Regards
Niekie Jooste

By G.S.Crisi (****)

Date 04-04-2003 19:14

Gentlemen,
quite a few statements have been posted here, all of them containing valuable and sound technical and scientific information. I`ve learned a lot from them and I`ve printed them to use them as reference for my lessons at Mackenzie University.
However, except for M squared, so far nobody has answered Matthew`s question. He`s in a hurry and wants to know if the A-335 Gr P91 pipe can be cut and rewelded once more, which nobody told him.
I know perfectly how he feels. They need steam to run their turbogenerator and the pipe must be put into service as soon as possible. In this right moment, Matthew is being pressed against the wall by his bosses, who are little interested in metallurgical considerations: all they want is to have the steam pipe fixed up and get started with the turbo.
Could any of you tell Matthew whether or not the pipe can be cut and rewelded again? You don`t need to base your opinion on sophisticated
metallurgical theories, your feeling as welding experts will be sufficient.
Agree, Matthew?
Giovanni S. Crisi
P.S. How do I know that the piping is used to deliver steam to a turbogenerator? Well, that`s the only thing steam at 150 bar and 600 C is good for.

By Niekie3 (***)

Date 04-05-2003 20:31

Good point Prof Crisi

My opinion is that it will be OK to go ahead with the repair weld. Following the repair weld, perform a hardness test on the weld, HAZ and adjacent parent metal. If the values are in the required range, I would go ahead and use the piping with a reasonably happy heart.

While the line is off and cold, it would also be a good time to take some replicas to estimate the creep dammage to date. You could then also get a replica of the current HAZ and adjacent PM for reference purposes should this repair become necessary again.

Regards
Niekie Jooste

By GRoberts (***)

Date 04-07-2003 14:32

Have we been getting a little bit off subject? Hard to believe in this forum!!

Anyway, if I was the one saying whether or not a repair would be allowed, I would allow the repair. If it was specified to remove not only the previous weld, but the previous HAZ, then the problems could be mimimized with the overlapping HAZs. The other reason I would allow it, is with high temperature applications, the mode of failure is creep, which can be monitored. So you can always keep an eye on it. If the repair was at risk of brittle fracture in an inaccessible situation, it would be another matter all together

By M-Squared (**)

Date 04-07-2003 15:22

Since this would be the third time, I would allow it. However, Niekie makes a very good point. I would perform a hardness check on the weld and HAZ but I would pay particular attention to the HAZ. When we talk about creep strength or resistance, the limiting factor will be the HAZ and not the weld. In addition, creep resistance of the weld and/or the HAZ can be somewhat related to hardness. Therefore, the higher the hardness of the HAZ, the better its creep resistance.

I would use a Krautkramer Model MIC, or some equivalent instrument to test the weld. This instrument works well for testing the HAZ. Using an instrument like a Telebrineller will not yield accurate results (ball is to big for HAZ testing).

The next and often heated debate will be what is the allowable range of hardness for the weld and HAZ. If you ask 10 people, you are sure to get 10 different answers. Personally, I use a range of 250 –300 BHN which is base on some research that was conducted by my company. The reason we go to 300 is that we have found that you will get some in service tempering. Others will argue that 300 is too high and that this material will not temper in service at temperatures of 1000 – 1200F. We found out that after 100 hours at a temperature of 1000F reduced the hardness by about 20 points (HBN).

By G.S.Crisi (****)

Date 04-07-2003 21:17

Fine, Gentlemen, now Matthew has a point to start from.

A word of warning to GRoberts: it would be fine if the HAZ is removed. However, this is an existing piping system which is already in place. Removing the HAZ would mean to cut a good 1/2 inch from the piping length (1/4 inch from each welding end). In order to reach the 1/8 inch gap or so between the two bevels to remake the weld, you would need to pull the two pieces of pipe one against the other by that distance (1/2 inch). This would take the vertical piping out of plumbness if the weld is in the horizontal portion; or take the horizontal piping out of level if the weld is in the vertical portion. Not to mention that you may create undesirable stresses on the piping.

A word to MSquared. If the hardness figures you mention are based on research made by your company, I'll take them for granted. Nevertheless, they seem a little high for me. I was accostumed to see a BHN of 240 as a maximum for welds. Does ASME/ANSI B 31.1 (Power piping, as the one we are talking about) say something on the matter?

Suggestion: now that you Gentlemen have solved Matthew's problem, could you MSquared give us some further details on how your company got to that conclusion? I'll appreciate that.

Giovanni S. Crisi

By GRoberts (***)

Date 04-07-2003 21:23

Sorry to not explain that well, but I was more thinking of buttering the pipe or making the joint wider as required by the process and equipment used rather than bend or displace the pipe.

By M-Squared (**)

Date 04-08-2003 14:24

Giovanni,

I was anticipating this question. As I had mentioned in my previous post you can never get 2 or more people to agree on hardness of P91 weld metal.

I would be glad to share our research with you but first; can I ask how did you arrive at your 240 BHN? Typically when I ask this question the answer is, “I saw it in a spec” or “That’s what our customer wants”. Better yet, “That’s what the manufacture of the material told me it should be” Your hardness number is for the weld, what should the HAZ be to avoid creep failure or type IV cracking in the HAZ?

Neither ASME B31.1 nor ASME Section I puts a hardness value on the weld material, they only tell you time and temperature for PWHT (which in most cases is not adequate enough).

The reason we put a maximum hardness value on this type of weld metal (B9) is to ensure adequate toughness during fabrication, transportation, installation, and during hydrostatic testing. Once the weldment is in service toughness, and therefore hardness, is no longer a concern. I say this because these materials typically operate between 1000 F – 1200F. Obviously, at these temperatures, brittle failure will not occur. Generally, the higher the hardness of the weld, the less tough it will be and the lower the hardness the more tough it will be.

Toughness is a measure of how much energy the weld will absorb (Charpy V-Notch testing). In industry today, the accepted value for energy absorbed is 15 ft-lbs. If a specimen breaks at or above this value, it is considered to be ductile and suitable for service. If it breaks below 15 ft-lbs, it is considered brittle. Our research showed that welds with a hardness value of 315 BHN broke at an average value of 28 ft-lbs, which is significantly higher than the accepted criterion of 15 ft-lbs. Therefore our conclusion was to allow a maximum hardness of 300 BHN for the weld metal. In addition, we would not allow a harness value of less that 190 BHN. The reason for this is in accordance with ASME and ASTM the minimum tensile strength of this material is 90,000 psi, which corresponds to a hardness value of approximately 190 BHN . A weld should be a least as strong as the base metal to ensure it has adequate load bearing capacity.
But truth be told, I would only allow this hardness value after careful assessment of the component and its service. A hardness value of 210 BHN is more realistic especially if the weld has not been volumetrically test (i.e. RT UT) such as the case with tube to header welds in HRSG’s superheater sections where the welds are PT tested. In those cases, the weld should be about 20% stronger than the base material.

In my last post, I used a minimum value of 250 BHN. The reason for this is, for this service condition for this weldmwent, we are concerned with creep failure. As I also mentioned in my last post, creep resistance of the weld/HAZ can be somewhat related to hardness. The higher the hardness, the better the creep resistance of the weld/HAZ. Since this will be the third time that this has been welded, and PWHT I would like to see the hardness value kept relatively high to ensure adequate creep resistance.

One question. Currently T23 material is being used for boiler tubes. It is currently a code case in ASME. The attractiveness of this material is that it does not in most cases require preheat or PWHT. In the as welded condition with no preheat, weld metal hardness has been shown to be between 268 and 313 HV (253 BHN - 298 BHN). Why is it that this is acceptable for T23 but P91 hardness should be substantially lower? They are used for the same operating conditions. Any thoughts or comments. When this material is accepted by ASME if will be under P5A grouping

By chall

Date 04-10-2003 14:41

M-squared,

ASME Section B31.3, Table 331.1.1 has a hardeness limit of 241 (BHR) for P5B materials. I know it isn't the appropriate standard, but it is a good starting point.

For what it's worth, we did some work using specifications from DFD a couple of years ago. (I only mention them specifically because they are backed up with excellent engineering.) They imposed a limit of 275 BHR for P5B materials (ASME Section I and B31.1 construction).

Charles

By G.S.Crisi (****)

Date 04-23-2003 18:27

M-Squared,
one half of the answer to your question (where did I see the 240 BH Number) was given by Charles Hall right above this posting.
The other half is this: I've seen it on the Handbook of Stress Relieving, published by Cooperheat Ltd.
Giovanni S. Crisi

By DPWeber (*)

Date 04-12-2003 15:06

Prof. Crisi, regarding your warning about removing the heat affected zone and subsequent fit up problems. As the pipe is being repaired in the cold position I would think that the piping system would already be out of plumb and level. Given the amount of "cut short" in a high energy piping system to allow for thermal expansion do you believe an additional 1/2" would be that significant an impact? Not being argumentative, just looking to learn from your opinion. I'm sure there might be some hanger impact, especially if the hangers were already nearly bottomed or topped out. Recently I had to replace a 39" piece of main steam pipe in a 50 year old plant. There was significant distortion of the Stop Valve, that required a 44" piece to be reinstalled to achieve a reasonably well aligned fitup. This did have a significant hanger setting impact, but I believe relieved some stress in the system caused by the movement of the stop valve. So differences between "as built" and "as is" may influence the repair process.