Flame cutting structural steel

I dont use a cutting torch very often but when I have used it to cut steel plate I have had good success with it. However when I am cutting angle iron or I beams or other structural steel I cant seem to keep the cut going. I dont know why but every so often I hit a spot that the torch wont cut. I have to zigzag a little with the torch to get the cut going again and that leaves a ugly cut. Is this because the steel in angle iron is cheaper than plate or am I just doing something wrong. I sometimes think that slag is building up in front of the torch but I am not sure if this is the cause or is this just the norm with structural steel?

Charles

Structural steel is often a mishmash of recycled steel. It is not unheard of to find unmelted ball bearings in them so maybe you hit a little piece of stainless mixed in.

incorrect preheat. Usually the problem is too low preheat but your description of slag building up leads me to think maybe you have too much preheat causing the steal to burn prematurely. Look at the edges of the cut section, does the torch side edge appear soft and melted? This would indicate too much preheat usually caused by an excess of acetylene.

The cross section of milled shapes varies over the width so the preheat that was adequate near the edge of the shape may not be enough near the middle. This problem can be compounded compounded by a heavy oxide coat which can deflect heat, too rapid a travel speed, improper torch orientation, or improper torch to work distance.

Can't say that I've ever noticed a difference between plate and structural steel so I'm confident some changes in your technique can solve the problem. Preheat the whole of the section not just the flange. Pay particular attention to the fillet where the web and flange meet as the steel is thickest here. Make sure the cutting tip is oriented such that one of the preheat nozzles is leading the cut and not offset. This seem like a small thing but was showed to me by an old hand and I notice the difference. Reduce your travel speed slightly. The best way to gauge your speed is to watch the stream of metal coming off the backside and slow down if it starts to deflect. You will have to travel faster near the edges of the section and slower near the middle. If these fail to cure your problem change to the next tip size up to increase preheat.

starbolin

Starbolin, Thanks for the quick response. I am sure one or several of your suggestions will probably be the answer to my problem. Flame cutting is not something I do enough of and Ill bet I am useing to big of a torch. I need to find my little cheat book and get the right size. I hadnt thought of too much heat causing a problem, I usually just grab the torch and go with whatever tip is already in it. Last time I used the torch I was cutting some 7/8 thick steel, now I am cutting 1/4 in angle. I should have thought about that one on my own.

Charles

Structural steel is often a mishmash of recycled steel. It is not unheard of to find unmelted ball bearings in them so maybe you hit a little piece of stainless mixed in.

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I think that's an old wives tale. I have worked with (welded, machined and fab.) a lot of structural and hot rolled steel and have never run across this problem. Because it is "structural" it has to meet an established level of quality.

I have never found structural steel any different to cut than plate or even low alloy steels and I have cut a lot of steel.

What you are probably experiencing is loss of cut by travelling too fast (unsteady travel speed gives the same effect) or from scale on the surface of the material cutting down the preheat.

Once the cut is lost the oxygen blast cools the spot just past the end of the cut. To get a clean restart stop the oxygen immediatly wait a few seconds for the metal to reach correct temperature them move back to the edge of the lost cut and restart the oxygen. If I am doing demolition cutting I just restart the cut on the side where the cut was lost to avoid wasting the few seconds required to preheat the quenched area at the end of the cut.

Yes to all of those suggestions, more to due with my improper techniuqe than anything else. Just as with all skills the less you use the more you forget. It has been a long time sence I last needed to cut something with the torch. I have been using the plasma cutter a lot lately and it does spoil you rotten. Thanks for all the advice and the more I have been using the torch last week the better I got with it.

Charles