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Up Topic Welding Industry / Technical Discussions / Plasma cutting heat input
- - By CHGuilford (****) Date 01-20-2009 19:20
Is there any practical way to determine the heat input to the base metal from plasma-arc cutting? (in the field; not in the lab)

Obviously, we can use the AWS formula variables to calculate kJoules per inch for welding, but I have been asked about how to do that for plasma cutting. 
While the plasma-arc temperature is very high, the quantity of heat is fairly low and much of the heat is blown away with the ejecta.  And we know that  plasma-cut plate ends up at a cooler temp than an oxy-fuel cut doing the same thing.  So it seems like the overall heat input is probably relatively low, but I wonder if that can be determined.

I do have a request for info in with HyperTherm but while I'm waiting for them to get back to me, I'm curious if anyone has run into this already?
Parent - By jwright650 (*****) Date 01-20-2009 19:22
Typically the plasma cutter (in the right hands) travels a bit quicker too, so again less heat input.
Parent - By Metarinka (****) Date 01-20-2009 22:54
I wish I had some useful input but I'm just as curious to see if there is a formula or rule of thumb exists for this problem. similar to machining the majority of heat will go in the removed material leaving the base material relatively cool, secondly  travel speed is quite high which confrims the fact that plasma cutters have quiet a small HAZ.   So I would imagine the heat input is quite low. However just like a laser there still a haz which can lead to metallurgical issues
Parent - - By aevald (*****) Date 01-20-2009 23:24
Hello Chet, I believe all of this is still based on time at temperature and the variables that could be included here might make for a sizeable margin of error. I believe you might need to take cutting gases into consideration in this equation as well since there could be some considerable differences when comparing nitrogen, shop air, argon, and any of the other associated cutting or cooling gases. Even the selection of amp settings, nozzle diameter choices, and cutting gas pressures could have a great effect. Quite obviously, travel speeds are likely to be across the board depending upon the experience and proficiency of a particular operator and or his/her particular style or understanding of the process and what constitutes a "good" or "efficient" cut. I am making some of my statements on this topic based on a comparison to GTAW welding. You can make welds with the GTAW process based on a higher amperage level and a faster travel speed and achieve appropriate penetration/fusion or you can opt to run with a slower travel speed due to a possible lack of confidence or tentativeness and end up inputting a greater amount of heat along the weld path. If you choose the latter choice you will certainly have a greater HAZ area due to the longer time at temperature variable. I will be interested to see some of the responses to your inquiry.
     Also Chet, is there a specific reason for the need of this information? Is there a concern with hardening or other types of metallurgical issues? If so, couldn't you use a form of hardness testing leading up to the edge of the cut in order to determine the affected zone? Just curious. Best regards, Allan
Parent - - By CHGuilford (****) Date 01-21-2009 15:09
Allan (and others)
What sparked my question is that one of our divisions is working on a large "crusher" drum and a section has to be cut out with water jet and returned to the manufacturer.  This is about 14 feet diameter, 2.25 inches thick and I'm not sure of the base metal but welding has been with 90 series filler metal - and I don't know if PWHT is to performed but I highly doubt it.

While the section is away, we need to joint prep the adjoining sections insitu.  The question came up as to how to cut double bevels with a low heat input.  The section is being cut apart at the weld joints and the engineer is concerned because it was welded once, gets welded again, and he would like to "avoid additional heat input".

I guess the water jet apparatus will not be available to cut the bevels; oxy cutting will add too much heat; there are no cutters we could find similar to pipe bevelers; it would take 3 generations of people to grind the bevels...........

I suggested using a plasma cutter mounted to some Bug-O equipment, then grind the cut surfaces to clean them up.  So the question turns to: "How much heat input will that cause?" 
Submerged arc is being used to weld all the joints so I wondered how much the plasma heat input really matters but........whatever.

I don't have much more information than that - I'm not personally involved with the project but I did say I would ask around to see if I could find the answer.  And the information could be useful to any of the forum readers in the future.
Parent - By aevald (*****) Date 01-21-2009 15:50
Hello Chet, thanks for the additional information. I do believe you have the correct solution regarding the machine-mounted plasma system and then finishing with a grinder. You did mention bevelers, did you look into the Heck bevelers or similar machines? I believe you also hit on something with regard to whether this even matters, I would venture to say that the sub-arc process will provide plenty of HAZ due to the fairly large rate of deposition at a fairly high amperage and heat input. I also cannot see that this project won't require some pre-heat, post-heat, and interpass temperature monitoring to avoid cracking and other stress issues. Please keep us informed on this if you can, I would like to hear what you ended up doing. Best regards, Allan
Parent - - By Lawrence (*****) Date 01-21-2009 16:27
Chet,

Your solution seems like the best to me also.

But it doesn't really answer the question eh?

I wonder if you might go to your AWS section library.. I hear they have all the "Welding Journals" on Disc now a days... I would think that some egg head as written about HAZ and thick plasma cuts over the years and with a searchable database now available with the digitized Journals might provide a semi-quick answer with real data to back it up.
Parent - By CHGuilford (****) Date 01-21-2009 17:16
Good point Lawrence, thanks.  I think I'm about 200 miles from the library but I know who I can ask to look into fore me.
Up Topic Welding Industry / Technical Discussions / Plasma cutting heat input

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