Impossible ARC-AIR gouging

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Hello everibody, I am very pissed of because we don´t get the right way to make a crackless arc-air gouging.I explain:The thing is that I have a gigantic (6 tons) valve body of LCC (casted and annealed high carbon steel), which inittialy had a cracking (not very big). I try to  take it out gouging with arc-air as ussually. But when I finished gouging I saw other new mini crackings near the first crack was. I mean, the arc-air gouging  makes other new crackings each time I try to repair the valve......................We have given a new heat treatment to the valve body to soften the annealing but nothing changes....Does anibody know what can be my negligence?? Or has anibody a REAL GOOD tutorial of arc-air gouging???Thanks!
Reply:can you explain what caused this to crack in the first place? was it in service? What % carbon or carbon equivalent? Any pictures?
Reply:Originally Posted by B.A.Baracus ...I have a gigantic (6 tons) valve body of LCC (casted and annealed high carbon steel), which inittialy had a cracking (not very big). I try to  take it out gouging with arc-air as ussually. But when I finished gouging I saw other new mini crackings near the first crack...
Reply:The time required for a slow cooling of such a large casting is very long; if not enough time was given, there may be some locked-in stresses. And, as Denrep said, the sudden heating and cooling caused by the gouging is sure to cause some surface micro cracks. You might try machining/grinding the surface down a bit and test with penetrating dyes or whatever is available to see when you have gotten below the cracking.I would expect you would need to put on a very ductile layer before adding the hardfacing layer..What kind of material does this valve 'regulate'?BTW, I am neither a metallurgist nor a professional welder; read my comments with that in mind. Also, because this is very interesting to me, I would appreciate some photos of the valve body and your work on it, if you have the time.
Reply:G'Day B.AAre you sure it's not an existing crack ? How deep is the excavation ?Sure cast steels can crack in the sections you are working with, the answer is preheat.Material: Most carbon steel valve bodies would not exceed 0.3% carbon IMO.If you can advise material grade - (ASTM or UNS), thickness, and service temperature, I might be able to help with a repair solution.CheersLast edited by chrispc66; 09-10-2008 at 08:34 AM.Reason: typo
Reply:sounds like there is a lot of pent up stress on that valve if its cracking that much.  I've used carbon arc on 120,000 psi High yield steel and had no problems with cracking.  Oh, check your air supply to make sure there isnt a ton of moisture in it.  If there is, bleed it out or (if you have no way of bleeding it) get a dryer/moisture seperator for your air line.  that ones a long shot but if water is hitting the steel it could be contributing, especially if the cracks are tiny.where is the crack running?  perpendicular to the gouge?  Parallel?"Real power is an arcair hand torch with a piece of 3/4 inch carbon and 1500 amps.  You feel like Zeus throwing a lightning bolt."
Reply:The heat from the arc air is the same or more concentrated than the heat from welding.  I would treat the piece as if you were welding it to gouge it.  Preheat it first, use a grinder or better yet a carbide cutter.David.Real world weldin.  When I grow up I want to be a tig weldor.
Reply:Whowh!I´m stoned with all that information.I will try to reply one by one!!!
Reply:Originally Posted by qaqccan you explain what caused this to crack in the first place? was it in service? What % carbon or carbon equivalent? Any pictures?
Reply:Originally Posted by denrepHowdy B.A. Baracus,I've carbon gouged on a lot of crack repair jobs.  With crack repair there is always a keen look-out for the crack's path. When looking closely for the ends of a crack, or additional cracks, I've often noticed tiny new fractures like you describe; and that's with common low and medium carbon steels.As you know, Air-arc is a concentrated brutal fast heating of metal, even the more mild steels can show micro cracking. I'm sure the tendency to crack during gouging would be much worse with the high carbon casting you've described.Whatever pre-heat process you expect to use when welding that 6 ton giant, is probably going to have to be used during the gouging too.If you don't want to heat twice, you could consider grinding the crack out with carbide cutters.Good Luck
Reply:Originally Posted by Oldiron2The time required for a slow cooling of such a large casting is very long; if not enough time was given, there may be some locked-in stresses. And, as Denrep said, the sudden heating and cooling caused by the gouging is sure to cause some surface micro cracks. You might try machining/grinding the surface down a bit and test with penetrating dyes or whatever is available to see when you have gotten below the cracking.I would expect you would need to put on a very ductile layer before adding the hardfacing layer..What kind of material does this valve 'regulate'?BTW, I am neither a metallurgist nor a professional welder; read my comments with that in mind. Also, because this is very interesting to me, I would appreciate some photos of the valve body and your work on it, if you have the time.
Reply:Originally Posted by chrispc66G'Day B.AAre you sure it's not an existing crack ? How deep is the excavation ?Sure cast steels can crack in the sections you are working with, the answer is preheat.Material: Most carbon steel valve bodies would not exceed 0.3% carbon IMO.If you can advise material grade - (ASTM or UNS), thickness, and service temperature, I might be able to help with a repair solution.Cheers
Reply:This sounds like a shrinkage crack caused during the manufacturing process (thick to thin). You might want to have this part x-rayed. Without knowing the extent of the cracking you don't know if your making things worse by repeated heat cycles. You could be not removing all cracks and the heat cycle can cause the existing cracks to grow. You will probably want to research a repair procedure that will account for preheat, post heat, welding technique, magnetic particle inspection of excavated areas, proper joint configuration for gouged areas, as mentioned earlier up the thread preheat prior to gouging, and final NDT after repair. Just my opinion
Reply:BAIMO = in my opinionASTM A352 LCC is a C,Mn steel with low temperature impact properties at -50°F and % C about 0.25%. At 10" thickness there will be precautions required.The crack is full thickness as stated so:1-Preheat to at least 100°C for gouging - Gouge out crack 50% depth2-Weld out 50% of excavation using FCAW ( E81T1-Ni 1 type and Ar / 20% CO2 shield) and 100°C preheat. Do not exceed 2.0 kJ/mm. ( A x V / Tsp (mm/min) x 0.06 ) and 250°C IPT.3-Gouge crack from other side until deposited WM is reached4 - Weld out full thickness (100°C preheat). Ensure the excavation is free from oxide etc before any welding.5- Allow to cool to ambient and UT examine to relevent appendix in ASME VIII.6- If Ok blend smooth then MT examine all surfaces to relevent appendix in ASME VIII.7 - Perform PWHT in furnace at 620°C for 1 hour per 1" of thickness at weld repair.(heat to 620°C no more than 100°C per hour)8 - Cool at no more than 100°C per hour to 400°C9 - At 400°C - remove from furnace and allow to cool in still air.10 - Perform NDE as previous to ensure freedom from reheat cracking.It is advised a welding procedure is qualified to either ASTM A488M or ASME IX before attempting this repair.Cheers
Reply:chrispc66,Do you typically UT cast material. I have always experienced that due to the large grain structure UT is difficult at best for this application.
Reply:qaqcDepends on the base material. Standard UT shear and compression probes are fine for cast carbon steels / low alloy steels. Austenitic / duplex types need angled compression probes at least.Radiography on thick, ( > 20mm), or variable thickness cast sections using gamma techniques is virtually impossible to get decent contrast / sensitivity to interpret radiographs sucessfully.Cheers
Reply:Originally Posted by chrispc66BAIMO = in my opinionASTM A352 LCC is a C,Mn steel with low temperature impact properties at -50°F and % C about 0.25%. At 10" thickness there will be precautions required.The crack is full thickness as stated so:1-Preheat to at least 100°C for gouging - Gouge out crack 50% depth2-Weld out 50% of excavation using FCAW ( E81T1-Ni 1 type and Ar / 20% CO2 shield) and 100°C preheat. Do not exceed 2.0 kJ/mm. ( A x V / Tsp (mm/min) x 0.06 ) and 250°C IPT.3-Gouge crack from other side until deposited WM is reached4 - Weld out full thickness (100°C preheat). Ensure the excavation is free from oxide etc before any welding.5- Allow to cool to ambient and UT examine to relevent appendix in ASME VIII.6- If Ok blend smooth then MT examine all surfaces to relevent appendix in ASME VIII.7 - Perform PWHT in furnace at 620°C for 1 hour per 1" of thickness at weld repair.(heat to 620°C no more than 100°C per hour)8 - Cool at no more than 100°C per hour to 400°C9 - At 400°C - remove from furnace and allow to cool in still air.10 - Perform NDE as previous to ensure freedom from reheat cracking.It is advised a welding procedure is qualified to either ASTM A488M or ASME IX before attempting this repair.Cheers