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Steel Brazing- Brazing to Laser Cut Steel

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Steel Brazing- Brazing to Laser Cut Steel

 Why You Need To Gum Saw Plate 

(You can buy saw plate that has been sandblasted to remove the heat affected zone.  This can work but it takes a lot of sandblasting.)   

A laser cuts steel by burning a series of holes really fast through the steel.   The laser melts the steel and a gas is used to blow out the melted steel. 

This is the edge of a laser cut saw plate.  It is not a straight edge.  It is a rippled or scalloped edge.  



When it is cutting it looks like a series of dots.  What you also have is a series of overlapping heat affected zones.   In these heat affected zones the carbon has changed from carbon in steel to free carbon or graphite.  Because steel is typically only 2% or so carbon this is not a big affect.  However the hot laser also creates a heat effected zone of a heavy oxide, slag-like material.  This slag zone is hard to braze to and makes for a very weak braze.  



The dark spheres in the left drawing are carbon atoms backed inside a matrix of metal atoms.  A tennis ball in a chin link fence is a two dimensional version of the same thing.  The carbon atoms distort the lattice or matrix of the iron atoms and change its properties from plain iron to steel.  

This is accomplished with heat during steel making.  The heat of laser cutting is enough to remelt the steel and allow the carbon atoms to come lose from the matrix and combine to form graphite.  

At about 850 F (455 C) the carbon in steel segregates and becomes graphite.    This is a diffusion controlled process so higher temperatures will produce graphitization in shorter times.  The process accelerates at a rate of an order of magnitude (10 times) for each rise in temperature of 40 – 50 F.  



Here are two pictures of notches where the tips came out.   On the left you can see the lines from laser cutting at the top of notch.  At the right you can see the laser lines at both top and bottom with some braze alloy remaining in the middle.  

There is a practice of sandblasting steel plate to remove the heat affected zone.  This may be effective but it is hard to remove any significant amount of steel with sandblasting.   A light pass down the notch with a gumming wheel is a much surer way to remove enough metal.  You should grind back the plate 0.006” to 0.008”.  This is past the ridges from the laser cutting.  

Human hair ranges from really fine blonde at maybe 0.0007” to coarse black hair at 0.008” so the old guys who say that “you need to grind her back a hair were right”.    Incidentally punched plate has its own edge problems.