cracks using fluxcore??

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Does anybody know what would cause cracks using dual sheild fluxcore.Using outershield 71m, gas co2, mild steel plate.This has happened to me after air arc gouging out a bad weld. could it be that i am not cleaning the steel out good enough after carbon arcing it out. i,m running about 300 wire speed. and about 27 volts .
Reply:Cracks..... could you post some pics or describe the cracking issue, where the crack is. There are a lot of things that would cause a weld to crack period so without a pic it's a bit hard to help._________________Chris
Reply:Internal stresses, maybe.  If the joint is too rigid as it cools, it can tear the joint apart.  Try preheating.
Reply:I'm with Chris, I think we really need some more info.  It's kind of hard to say from what we know now. ~JacksonI'm a Lover, Fighter, Wild horse Rider, and a pretty good welding man......
Reply:Agree, need more info.Pic would really help.Where does the crack occur, in the centerline of the weld, or along the fusion zone at the toe of the weld?What is the base metal thickness and what is the joint, butt weld in a V-groove, or fillet weld in a T-joint?etc.?
Reply:yeah like they said
Reply:it was a crack straight down the center of the joint this is like 1 1/4" plate beveled . sorry for the late response and the weld has been gouged out and ground clean now so unfortunately no pictures. thanks for the response though.
Reply:This was taken from this website....http://nhml.com/resources_NHML_Defective-Welds.phpWeld cracks are serious defects that have little margin of safety. The two types of cracking most likely to occur are hot cracking (just after the weld has solidified) and cold cracking (which occurs near room temperature after the weld has cooled).Most weld cracks are caused by hot cracking, with the weld being pulled apart during cool-down. If the configuration of the part does not allow the weld area to contract as it cools, then hot cracking becomes likely. If the width-to-depth ratio in the weld cross section is too high, edge cooling can pull the bead apart causing centerline cracking.Hot cracking is common when high phosphorus, sulfur or lead content is present in the base metal. Chromium steels are particularly sensitive to microcracking, but this can be avoided with appropriate preheating procedures.Methods to control or eliminate hot cracking are usually detailed in work instructions or weld procedures. Variables such as the type of alloy or type of weld can affect the weld sequence and the part's susceptibility to cracking.In general, microcracking is less of a problem in metals that show good elongation in tensile tests. If you work with certified materials, be sure the elongation measurements of the lot meet or exceed the elongation specification for the type/grade of material.Unlike hot cracking, which occurs right away, cold cracking can occur in the weld metal, hours or even days after welding. A single visual part inspection made directly after a weld may not be adequate to detect cold cracking.The most damaging form of cold cracking is hydrogen embrittlement. This occurs when hydrogen is absorbed into the metal. Hydrogen embrittlement is usually associated with dust and dirt that has accumulated on the weld surface or from using a damp electrode.This embrittlement is extremely harmful in alloy steels and it often happens in carbon steels. Structures suffering from hydrogen embrittlement lose their toughness and often fail. If you suspect hydrogen embrittlement, especially if porosity is present in alloy steel, invest in a metallographic examination of a weld sample._________________Chris
Reply:Could of been when you first gouged out the metal you touched the electrode to the base metal a few times and introduced carbon deposits to your metal. This would of caused the area of the gouge to become hard, brittle and very crack sensitive. If you know you touched next time go over the area real good with a grinder, and it should be ok
Reply:The general rule is that all plate over 3/4 inch must be preheated.  In shops around where I work anyone not preheating one inch plate before welding will find that he will have serious employment problems.  I suspect you did not preheat to at least 200 F. ???  I would be preheating to 300 +