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October 22 - October 26, 2018

How do you prepare flame-cut edges for application of thermal spray metallic coatings? The problem is that these edges are often harder than the abrasive, so a very low profile is produced, resulting in lower-than-specified adhesion of the thermal spray to the steel.

Selected Answers

From Jon Cavallo of Sponge-Jet Inc. on October 25, 2018:
I agree with Web Chandler that 1) the flame cut edge is harder than essentially all of the standard blast abrasives and power tools and, as such, 2) a minimum of 1/16 in. of the flame-cut edge must be removed to facilitate specified surface preparation. During my career, I have observed several significant coating failures caused by the flame cut edge phenomenon and attendant inadequate surface profile.

From Brad Wilder of Intech Contracting on October 25, 2018:
The AASHTO/NSBA Steel Bridge Fabrication Guide Specification lists three possible solutions: grinding, machining, or heating. As the others have said, grinding is usually the most practical option in a field setting on an existing structure. Recent revisions to the AASHTO/NSBA guide highlight another important point: the cutting process can also leave striations or marks that can interfere with the use of replica tape. Inspectors should be aware of this and take care to confirm that edge profile measurements are actually measuring profile and not fabrication marks. For new structures that are to be thermal sprayed (or galvanized), it is worth considering non-thermal cutting methods to eliminate the issue prior to the coating process.

From Bryant Chandler of Greenman-Pedersen, Inc. on October 23, 2018:
Carbon steel above about 0.3% carbon content will develop a hard surface (martensite) along the flame cut edge. As the carbon content increases, the hardness (Rc) will increase during normal oxy-acetylene cutting processes. Abrasive wheel grinding along the flame cut edge to a depth of 1/32" to 1/16" should remove the hardened edge sufficient to obtain a good abrasive blasted profile. Grinding the edge is faster than trying to blast a profile by eroding away the hardened surface.

From Grant Blohm of Metalizing Pro Inc on October 22, 2018:
Due to the heat generated in flame-cutting process, the steel along the cut edge is physically changed as a result. This outer layer near the cut is known as the heat affected zone (HAZ), which is heated to a temperature in excess of 2600F and then rapidly cooled afterwards. The heat radiates into the rest of the plate. This narrow zone along the outer edge is then hardened by the process. To remove the hardened surface of the HAZ, it is necessary to physically grind the steel to a depth that will effectively anneal the HAZ, thereby allowing for conventional grit blasting to profile the steel. The depth of the HAZ will vary with the carbon level of the grade steel and the thickness of the plate. Thicker pieces or higher strength steels require more stock removal. While grinding is slow and labor intensive, it is the most common method to correct this condition. In a few instances where the HAZ depth is minimal, abrasive blasting with aluminum oxide has been able to profile the flame cut edges to an acceptable degree. Generally speaking, grinding is still the most effective method in most cases.

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Tagged categories: Coating Application; Paint application; Thermal spray

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