Pipe Problem: Cracks in Water Jacketing

One of the causes of cracks might be the change in metallurgy in the HAZ. Was adequate post-weld heat treament applied?

Thank you for your reply.

The pipe is 6mm thk no pwht applied and no need.

PT done and result shows no indication.

Sometimes we have to use preheat and PWHT for service even though the applicable code does not require it.

A36 carbon is pretty mild steel, but sometimes for longer weld life will be PWHTed when used in acid or H₂S service.

Its an water jacket external pipe.

so the water is in service.

may be water contaminated. but the crack/pinhole occured during commissioning time.

does sound like contamination.

Also, what is the start-up/shut-down processdure. Could there be thermal shock to the jacket?

it seems no, since the crack developed 1/2 days later.

crack size also app 1.5mm

Please did you check the hardness of heat affected zone? Also sometimes before to cut is needed a heat treatment as we say hydrogen removal treatment.

No we didn't follow your recommendations

You have not mentioned the thickness of the material and type of electrode used and where the crack have initiated. Is it from the junction of the weld and base metal towards HAZ (or) towards the weld.

As you pointed out it could have been contaminated (or) a combination of thickness factor, welding procedure and type of electrode used for repair work. Repair welding need not follow the same WPS, used for new plates and if required to be modified to accommodate the service conditions it underwent and other factors.

One way of overcoming the problem is to (1) pre heat the HAZ area on either side of the weld to a min of 150-200⁰ C ( of course it depends upon the thickness factor also) and maintain an inter pass temp. of 150⁰ C or so. (2) If you are using E 6013, type electrodes, please change it to E 7018 Type (use it after backing the electrodes at 300⁰ C for one hour to remove moisture content – if any in the flux coating) (3) After initial pass, please visually check for cracks and if you are satisfied complete the welding.

Please also make sure that you have removed all the existing cracks and have a DP check ( at room temp.) before proceeding with welding. The same procedure could be followed for jacket pipe welding also.

THK 6mm

ER70s2 and E7018

some modification done on the jacket pipe, ultimately the pipe will have 6 pieces to form a pipe.

some cracks is in the junction between HAZ and weld toe and some cracks at HAZ

all cracks are longitudinal i.e. parallel to the weld seam.

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repair work carried out,while water was in service.

i.e they put patch plate and welded ( which is not correct procedure)

and did MPI ( dry powder technique).

MPI shows no indication and 1 month over no further complaints .

I'd suspect hydrogen embrittlement. Did you use low hydrogen rod for the welding? The fact that you're welding around water leads to the possibility of hydrogen being introduced into the weld. Another possibility is increased stress in the weld and HAZ caused by differential contraction. The suggestion to check for hardness is a good one. That could indicate the introduction of hydrogen into the weld.

one possible reaons may be as per your comment.

to cross check we need destructive testing, which is not possible since the plant started up

how can i check hardness of welding

is there any thumb rule to check hardness

There are small hand held portable hardness testers available to do this. Since there doesn't seem to be a lot of information on your material, you would have to take measurements some distance from the problem area and then adjacent to the crack and compare the two. The device makes a small indentation which is subsequently measured under a portable microscope and compared to a standard. This is not an ideal way to do it but it is about all you can under the circumstances.

If you can't get one of these testers you could do a rough test with a file. That would be very subjective however. Depends on how different the two areas really are.

Another thing would be to acid etch the area adjacent to the weld and HAZ and the do the same a distance away from the problem area. You'd need a microscope here too to make a comparison.

Sorry I can't be more help. Perhaps a metalurgist contributer could comment and correct or expand on my contribution.

Good Luck!

I don't know what you mean by "proper WPS was used while welding" but the fact that this cracking is limited to the heat affected zone testifies that it was not done properly for the circumstances.

Postweld heat treatment to allow for diffusion of Hydrogen out of the work is great advice already given.

I suggest that you re examine what constitutes proper process parameters for cutting and rewelding including pre and post heat treatments as well as rod / filler metal selection, and deposition practices.

Was the cutting done mechanically or was it also done by arc cutting?

milo

You mention "cut". I suspect that the cutting portion of this work wasn't covered by a welding procedure. I have experience problems using Air-arc to cut out components if the surfaces of the cut were not ground back. In essence you will have two heat affected zones, though a post weld heat treatment would solve this problem. I have also experienced problems with saw type cutting and the use of petroleum based lubricants.

Try doing a PWHT (Post Weld Heat Treatment) prior to welding to cook out any contaminants that have infiltrated the metal.

How old is the WHB?

Are you using a preheat? rose-bud, electrical?

Has the boiler been overheated?

Does your welders really follow the WPS?

Is your weld area really - really clean?

Have you kept your welding rods in a rod oven - how long does the welder keep them out?

I believe that a pre weld bake-out, a properly prepared weld area, and a good preheat would eliminate your problem.

I believe that a pre weld bake-out, a properly prepared weld area, and a good preheat would eliminate your problem.

POST HEAT I'm sure is what you meant. Probaly a microsoft sticky keys phenomenon made it come out as pre heat a second time.

milo

OK - I meant pre-weld bake out following the same procedure as if you were doing a PWHT. 0-800 F at a moderate rate, 800 to 1150 F +/- 50 F, at 400 / inches thick, hold 1 hour per inch of thickness, cool at 400 / inches of thickness!

what is HAZ?

where i can get good knowledge of welding?

Heat Affected Zone. Its the material adjacent to the weld, and because ofthe heat, may harbor all sorts of issues depending on the materials hardenability, structrure and the welding process as has been so wonderfully pointed out by the many posters.

milo

HEAT AFFECTED ZONE:

Heat Affected Zone (HAZ) is the portion of the base metal lying next to the fusion line of weld, which had not melted but whose mechanical properties or micro-structures have been altered by the heat of the welding. The HAZ is subjected to a complex thermal cycle (sudden heating & cooling) in which all temperatures from melting range of the steel down to more mere warming are involved and therefore consists of series of graded structures ringing in the weld zone.

A typical HAZ structure for carbon steel can be divided into four distinct regions according to maximum temperature reached. In the 1^st region nearer to weld, the zone is heated high into to austenite temperature range and is extremely coarse grain structure. It will be high hardness zone and if the cooling rate is high enough, can readily transform to brittle martensite. This is the most potential dangerous area of HAZ and susceptible to cracking.

The second adjacent zone, heated slightly above the 1^st zone is of fully austenite range, fine grained normally and with moderate hardness.

The third zone will have partial austenite range and acquires mixed structure. A small amount of martensite may form in it if cooling rate is high enough. It may not be very hard but brittle is martensite is present.

The fourth region is not heated high and may not pose any problem and tempered at times.

The HAZ microstructure is determined by chemical composition and welding thermal cycle. Higher the carbon and the harder the zone are vulnerable to cracking. More alloy elements will also increase the hardenability and they both are expressed through Carbon Equivalent formulas.

We should always avoid martensite formation as it is very brittle. In addition to it, presence of Hydrogen generated through contamination, moisture present during welding ( use of Rutile coated electrodes or hydro carbons like oil or grease strains near the plate edges) and restraint induced stresses, can cause delayed cold cracking in the HAZ.

HAZ hardness ( could be checked with portable units now a days) is usually a good indication of the presence of martensite structure and thus potential cracking. But cracking rarely occurs if the hardness is 250 HV or less. Critical hardness of 350 HV and above will create problems if precautions like preheating the job, use of low hydrogen electrodes with good baking temperatures are not followed up.

Hence use of minimum pre heat, welding with low hydrogen type E 7018 electrodes will help to avoid cold cracking in most of the cases.

Sridhar

what is austenite and martensite heat range.

Iron is a allotropic metal because of its two phase existence due to temperature change. The change in crustal structure occur at a precise temperature i.e., it forms body - centered cubic (BCC) crystal below 913⁰C (1675⁰F). Iron changes to form Face Centered Cubic (FCC) crystal above 913⁰C. The BCC crystal is called alpha ferrite (-Fe) and FCC crystal is called austenite (-Fe). Iron reverts to BCC structure when the temperature is above 1400⁰C and till reaches 1540 ⁰C. This is know as delta ferrite (-Fe). This show that iron has two transformation range 910⁰ and 1400⁰ C at which phase changes occur during heating and cooling.

Austenite is a non magnetic form of iron and has the power to dissolve carbon and alloy elements. Iron accepts more alloys and they stay trapped at high temperature.

Martensite is the hardest of the transformation products of austenite and formed due to rapid cooling. It prevents austenite to form in softer constituents at high temperatures. Martensite is very hard and brittle and useless for most engineering applications except for hard facing. BY suitable Post heat treatment, Martensite is converted to modified microstructure called tempered martensite. Tempering helps carbon to form carbide precipitates and provide required toughness and hardness depending up on the chosen temperature and time cycles.

Larger parts (big mass) requires preheating to avoid faster cooling rates and Martensitic structure formation.. Similarly small parts have a faster reach to melting temperature. Gasses like Hydrogen developed due to moisture in the flux coating, humid air, contamination through organic compounds, rust, dirt on the joint surface gets trapped in the microstructures of metal as supersaturated solutes. They can cause under bead cracks known as delayed or cold cracks upon three conditions. (1). High stress state, (2), Martensitic microstructure, (3). Critical level of Hydrogen.

Similarly, Hot cracks are caused due to tearing of grain boundaries of welds that have not completely solidified. They develop immediately after welding as low melting materials like sulfur, phosphorus ( partially available due to contamination) with its high concentration intersect at the center of the weld. These partially melted, weak grain boundaries are stressed while the weld metal shrinks causing them to rupture thus forming cracks.

Sridhar

you can see latest experience i faced in my boiler where the water jet enters in shell and we have fire in tubes.

if you see in picture it cracks again in different place.hydraulic test pressure required is 27 bar. is it enough for testing. because its only the pressure test. what will happen after increase in temperature.

we cut that perticular portion and weld another plate with orgon welding is it ok ?

its a 22 mm thick plate what size of chamfer i have to maintain for filling.and is it necessary to weld on both side?

hi

you can see the leakage on the welding. i removed the copleate patch and put another. still it is leaking from same place after one week.

please guide me.

Hire a new welder - do NDE before and after welding. Before to make sure there is not a flaw in the parent metal.

material of this boiler plate is ASTM A216 WCB

Which welding process i should follow?

please guide me.

If this is the system I suspect it is, then hydrogen imbritlement should not be an issue because the steel is a Cr/Mo carbon steel alloy that is well on the safe side of the Nelson curve. We specified 1.25 Cr/0.5 Mo steel for this application.

what is hydrogen imbritlement?

what is caustic imbritlement?

what is nelson curve?

please explain,