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x39
10-03-2005, 11:42 PM
I need to weld some flanges on a 4140 shaft. I intend to stick weld it using 7018 rod. I'm not sure as to how hot to preheat before welding. Any suggestions? Thanks.

3 Phase Lightbulb
10-04-2005, 02:20 AM
I think it all depends on how thick the shaft is and how much heat you'll be using when you weld to the shaft. I think stick will heat the entire shaft up anyway so you might not even need to pre-heat. I find with TIG, the heating is so localized that when it cools, it will crack if all of the base material hasn't been pre-heated. The heat from ARC welding isn't as localized as TIG so I bet you might not have to pre-heat but it all depends on how thick the material is.

When I preheat, or when I stress releave a joint, I heat with O/A until I start seeing a color change (before it gets red). It seems to work but I might not be doing it right either.

If you can, I would try pre-heating and then welding a sample first and see how it goes...

-Adrian

Forrest Addy
10-04-2005, 03:53 AM
4140 has significant chrome and 40 points of carbon. You're on the right track to pre-heat it. I'm sure your welding supply house has a procedure for your project which is a common in welding.

For full strength welds in 4140 I recall a reccommendation to use 11018D-2 with 400 degrees of preheat followed by a 1200 degree post heat and a slow cool.

A general purpose welding procedure for sticking on appurtinances where full strength is not required may very well reccommend 7018. You cannot bypass the preheat if you hope for a reliable weld. A tack or a false start can result in a crack at the fusion line that doesn't become fully coalesced in the subsequent weld. Subsurface cracks are seeds for metal fatigue.

x39
10-04-2005, 07:50 AM
Gentlemen, thank you both for the information, very helpful indeed. As in the situation Forrest alluded to, the weld will not be under a great deal of stress, it will only serve to prevent the shaft from turning due to a moderate radial force. I'm off to the shop, thanks again! http://bbs.homeshopmachinist.net//smile.gif

torker
10-04-2005, 08:17 AM
Without looking it up I'd have to say that Forrest is pretty darn close here.
Also, you should never be shy with the preheat even when stick welding especially with the hi-alloy steels.
Preheat will cut down on the possibilities of thermal shock induced cracking.
This is something we become very aware of up in the frozen north. Thermal cracking is real when the metal is cold.
I always hava a toolbox full of different temp sticks to try to control problems like this.
Years ago I'd watch the good weldors..the guys who never fail a coupon and they always used preheat.
Russ

3 Phase Lightbulb
10-04-2005, 10:58 AM
<font face="Verdana, Arial" size="2">Originally posted by Forrest Addy:
For full strength welds in 4140 I recall a reccommendation to use 11018D-2 with 400 degrees of preheat followed by a 1200 degree post heat and a slow cool.
</font>

Thanks for the info! Are there any tricks to obtaining 400 deg for the preheat, then 1200 deg slow cool for the stress releave? It sounds like a Propane torch could be used for both? Could I get two torches (one pre-set for 400, one preset for 1200)? I'm just wondering how you really can apply/verify how much heat you've put into something. I've been going by color change, but I don';t even know what temp that is, and I bet thats probably different each time.

-Adrian

jkoper
10-04-2005, 01:55 PM
Adrian,
You need to get some temp sticks, they are like a crayon that melts at a given temp. You should be able to get them at your welding supply.

Jim

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Jim Koper
J&R Machining

3 Phase Lightbulb
10-04-2005, 02:00 PM
<font face="Verdana, Arial" size="2">Originally posted by jkoper:
Adrian,
You need to get some temp sticks, they are like a crayon that melts at a given temp. You should be able to get them at your welding supply.

Jim

</font>


I always had problems with drawing outside the lines with those.. http://bbs.homeshopmachinist.net//smile.gif

Actually, I just thought about those Infrared temp guns.. It would be nice to just point one of those at the joint and get a good digital temp readout. Does anyone know if 400/1200 is within their calibrated range?

-Adrian

Michael Az
10-04-2005, 09:45 PM
My temp gun only goes to about 500 I think.
Michael