Manthax Under surface cracks seen in HAZ of T 22 tubes could be due to residual stress at portions not totally relieved - even after proper PWHT . For this analyze boiler water chemistry at high temp. i.e, possibility of SCC due to caustic (caustic source is from steam at high temperature). Even small ppm of caustic in crevices and in HAZ can develop Caustic SCC at that temp even with stress relieved joints and with inconel 600 weld over lay. Inconel 600 tube sheet weld overlay given is immune to high temp hydrogen problem - H2 partial pressure at that temp. It looks like there could be two possibilities for this repetitive failure seen. One : Possibility of high temperature chloride & Two- Caustic SCC from steam is another possibility. Caustic/ Chloride concentration from steam (i.e., boiler water chemistry with chloride / caustic) on the weld and tube sheet may, it looks like to be the contributory cause for this problem - as it looks from the description given by you for the repetitive failure after 2 years service. Inconel 600 tube sheet over lay weld is not immune to caustic / Chloride SCC at temp above 350-450 deg cent with concentration of caustic from high temperature steam. Small concentration of ppm levels of chloride / caustic can result in SCC in inconel 600 tube sheet as well as in T 22 tubes. Conventional seal weld of T 22 with Inconel 600 weld overlay may crack if concentration of caustic / chloride in portions of HAZ where residual hardness is high. This could be one reason for the weld HAZ cracks. Which mode of SCC is predominant is the question to be diagnozed by in-situ metallography of the tube sheet at affected areas. If it is predominantly transgranular in nature, then it could be due to choloride concentration. If it is predominantly intergranular in nature, then possibility of caustic SCC is likely. If both modes of cracks are present, ie., IGSCC and TGSCC then it could be due to both modes of cracking. PWHT procedure for Inconel 600 weld overlay with T 22 if properly done and held should have alleviated this problem. Yet the repetitive nature indicates boiler water chemistry and the control in boiler water chemistry with contaminants of chloride / caustic concentration could have contributed to the failures. Suggest : Analyse from these angles. Trust this helps you C.V.Srinivasan Nishi Engineers Pvt Ltd India July 24, 2008 E-mail: nishi@vsnl.com >We have a high pressure boiler with HTHA positive >environment on tube side. The tubes and tubes are T22/F22, >the tubesheet also has overlay of 6mm thk inconel 600. The >tubes are welded (reckon seal weld) with inconel overlay. >The tube sheet was PWHTed after overlay and also after >welding the tubes during its manufacturing. > >Within two years of operation, sever under surface cracks >seen in most of the tube welds (on the tubesheet) , closer >to HAZ. The affected welds were ground till weld fusion zone >and re-welded followed by PWHT. After three years down the >line, same phenonmenon is seen in most of the tubes, I have >the following questions: >1. Could this be related to incorrect PWHT >2. Therotically, inconell 600 eliminates need for any PWHT >on tubesheet, still could this be the result of incorrect >PWHT. >3. The cracks seen are more on the tubesheet than tubes >themselve, while tubesheet is ovelayed and and doesnt come >in contact with hydrogen. >3. If this is due to hardness on tubesheet caused by the >process overlay, there is no direct either with hydrogen in >the service, >4. At last, is this combination of F22 with inconel 600 is >correct, as F22 alone (if heat treated correctly) should be >able to work well with given H2 partial pressure and temp. > > > Kind regards, Manthax

Manthax Yes it could be high temperature related delayed hydrogen embrittlement which can occur in T 22 tubes /HAZ/welds. High temp hydrogen embrittlement is also one form of SCC. Considering syn gas is at temp in the range of 350-380 deg cent at the tube side and shell side with steam - yes this is likely to be delayed hydrogen embrtillement where PWHT had not fully relieved the stresses in T 22 tube and HAZ areas. Yes it is usual / common to have Inconel overlay weld on F22 tube sheets in such syn gas +BFW service in ammonia plants handling high partial pressure of syn gas at the exit of ammonia converter. Trust this helps you C.V.Srinivasan Nishi Engineers pvt ltd India July 25,2008 E-mail:nishi@vsnl.com >>Manthax >> >>Under surface cracks seen in HAZ of T 22 tubes could be due >>to residual stress at portions not totally relieved - even >>after proper PWHT . For this analyze boiler water chemistry >>at high temp. i.e, possibility of SCC due to caustic >>(caustic source is from steam at high temperature). Even >>small ppm of caustic in crevices and in HAZ can develop >>Caustic SCC at that temp even with stress relieved joints >>and with inconel 600 weld over lay. >> >>Inconel 600 tube sheet weld overlay given is immune to high >>temp hydrogen problem - H2 partial pressure at that temp. >> >>It looks like there could be two possibilities for this >>repetitive failure seen. >>One : Possibility of high temperature chloride & Two- >>Caustic SCC from steam is another possibility. >>Caustic/ Chloride concentration from steam (i.e., boiler >>water chemistry with chloride / caustic) on the weld and >>tube sheet may, it looks like to be the contributory cause >>for this problem - as it looks from the description given by >>you for the repetitive failure after 2 years service. >> >>Inconel 600 tube sheet over lay weld is not immune to >>caustic / Chloride SCC at temp above 350-450 deg cent with >>concentration of caustic from high temperature steam. Small >>concentration of ppm levels of chloride / caustic can result >>in SCC in inconel 600 tube sheet as well as in T 22 tubes. >> >>Conventional seal weld of T 22 with Inconel 600 weld overlay >>may crack if concentration of caustic / chloride in portions >>of HAZ where residual hardness is high. This could be one >>reason for the weld HAZ cracks. >> >>Which mode of SCC is predominant is the question to be >>diagnozed by in-situ metallography of the tube sheet at >>affected areas. >> >>If it is predominantly transgranular in nature, then it >>could be due to choloride concentration. >>If it is predominantly intergranular in nature, then >>possibility of caustic SCC is likely. >> >>If both modes of cracks are present, ie., IGSCC and TGSCC >>then it could be due to both modes of cracking. >> >>PWHT procedure for Inconel 600 weld overlay with T 22 if >>properly done and held should have alleviated this problem. >>Yet the repetitive nature indicates boiler water chemistry >>and the control in boiler water chemistry with contaminants >>of chloride / caustic concentration could have contributed >>to the failures. >>Suggest : Analyse from these angles. >> >>Trust this helps you >>C.V.Srinivasan Nishi Engineers Pvt Ltd India July 24, 2008 >>E-mail: nishi@vsnl.com >> >>>We have a high pressure boiler with HTHA positive >>>environment on tube side. The tubes and tubes are T22/F22, >>>the tubesheet also has overlay of 6mm thk inconel 600. The >>>tubes are welded (reckon seal weld) with inconel overlay. >>>The tube sheet was PWHTed after overlay and also after >>>welding the tubes during its manufacturing. >>> >>>Within two years of operation, sever under surface cracks >>>seen in most of the tube welds (on the tubesheet) , closer >>>to HAZ. The affected welds were ground till weld fusion zone >>>and re-welded followed by PWHT. After three years down the >>>line, same phenonmenon is seen in most of the tubes, I have >>>the following questions: >>>1. Could this be related to incorrect PWHT >>>2. Therotically, inconell 600 eliminates need for any PWHT >>>on tubesheet, still could this be the result of incorrect >>>PWHT. >>>3. The cracks seen are more on the tubesheet than tubes >>>themselve, while tubesheet is ovelayed and and doesnt come >>>in contact with hydrogen. >>>3. If this is due to hardness on tubesheet caused by the >>>process overlay, there is no direct either with hydrogen in >>>the service, >>>4. At last, is this combination of F22 with inconel 600 is >>>correct, as F22 alone (if heat treated correctly) should be >>>able to work well with given H2 partial pressure and temp. >>> >>> > > >Kind regards, Manthax > >Thanks Shrinivasan, > >Sorry I was not clear in describing the whole scenario, the >synthesis gas is on the tubeside and that is why the tube >sheet is overlayed with inconel 600. The boiler feed water >is on the shell side, the back of the tubesheet and also the >crevice formed beween the tube and tuubesheet is also >exposed to boiler feed water. We have not seen any corrosion >on the backside, the cracks are seen on are on the synthesis >gas side of the tube sheet than water side. The tubesheet is >very thick and also well expanded, I reckon boiler feed >water may not reach on the other end. > >Could this be hyrogen damage through the t 22 tubes? It is >worth doing metallography of affected areas as suggested by >you, to rule out SCC phenonmenon. > >Is it usual to have inconel over alloy steel in hyrdrogen >service, is it done to avoid the PWHT of alloy steel tubes >to alloy steel tubesheet?