Welding Dissimilar Metals: What filler do I use?

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Trying to figure out what weld filler I should use for the following application:303 stainless steel to Inconel 625.Newbie welder- first job on my own. This is a test I have to pass!I was told to use 625 filler rod, but it just doesn't sound right.
Reply:The first thing that comes to mind is the 303 is a "free machining" austenitic stainless steel that has a very high sulfur content to increase machinability by making the chips break off easier.  This creates a big problem for welding because sulfur causes hot cracking, generally in the form a centerline crack formed as the weld solidifies.  As I recall a filler with high ferrite content will help prevent the hot cracking, I think that may be 312.Other factors to consider are the material thicknesses, the joint design, does it require filler, how many passes, welding process, and application requirements like strength and corrosion.I think the primary issue may be the high potential for hot cracking, so I would research this in relation to your application.  For example, maybe you have a thick, multipass weld, and you find 312 is best for avoiding hot cracking, then you could butter the 303 side of the joint with a layer of 312 and then fill the joint with 308.
Reply:Originally Posted by jlc6367Trying to figure out what weld filler I should use for the following application:303 stainless steel to Inconel 625.Newbie welder- first job on my own. This is a test I have to pass!I was told to use 625 filler rod, but it just doesn't sound right.
Reply:I would contact the Special Metals Corporation which owns the  names and makes the alloys; call them and see what the experts know. The contact page can be found by looking here:Special Metals Corp. Contact
Reply:Looked up some references on welding 303 SS.http://cartech.ides.com/datasheet.as...=255&c=TechArtExcellent info on SS from Lincoln:http://content.lincolnelectric.com/p...ure/c64000.pdf
Reply:A very technical investigation of SS hot cracking, with some nice photos of cracks.http://files.aws.org/wj/supplement/03-2003-BROOKS-s.pdfAnother effect of sulfur that I had not mentioned, its affect on weld penetration due to surface tension and temperature gradient driven fluid flow, known as the Marangoni effect, or thermocapillarity.  Your Inconel 625 will most likely be very low sulfur content and thus have an outward fluid flow and produce a wide/shallow weld, where as the 303 is very high sulfur and will have an inward/downward fluid flow and produce a narrow deep weld.  But a very unusual thing happens when joining low sulfur steel to high sulfur steel, the flow and penetration goes to the low sulfur side of the joint.  See figures 1 -3 in this paper.http://www.micromagazine.com/archive/04/07/collins.htmlAnd one more thing, Inconel 625 contains Niobium (Columbium) which tends to make this allow sensitive to microfissuring, a form of hot cracking similar to the sulfur effect.Last edited by pulser; 06-04-2009 at 10:18 AM.
Reply:Think "10' Pole".That's what I "wouldn't" try to weld those two materials with.Sounds almost to me that someone is "testing" you to see if you know when to say no.Syncro 250 DX Dynasty 200 DXMM 251 w/30A SG XMT 304 w/714 Feeder & Optima PulserHH187Dialarc 250 AC/DCHypertherm PM 1250Smith, Harris, Victor O/ASmith and Thermco Gas MixersAccess to a full fab shop with CNC Plasma, Water Jet, etc.
Reply:Originally Posted by pulserThe first thing that comes to mind is the 303 is a "free machining" austenitic stainless steel that has a very high sulfur content to increase machinability by making the chips break off easier.  This creates a big problem for welding because sulfur causes hot cracking, generally in the form a centerline crack formed as the weld solidifies.  As I recall a filler with high ferrite content will help prevent the hot cracking, I think that may be 312.Other factors to consider are the material thicknesses, the joint design, does it require filler, how many passes, welding process, and application requirements like strength and corrosion.I think the primary issue may be the high potential for hot cracking, so I would research this in relation to your application.  For example, maybe you have a thick, multipass weld, and you find 312 is best for avoiding hot cracking, then you could butter the 303 side of the joint with a layer of 312 and then fill the joint with 308.
Reply:Note, while reading through Welding Metallurgy and Weldability of Stainless Steels by Lippold and Kotecki, I found the following information:"If solidification occcurs in the FA mode, resistance to cracking is very high, irrespective of impurity level. As shown on the Suutala diagram, if solidification occurs in the FA mode (above 1.48 Creq/Nieq), very high levels of S+P can be tolerated with no cracking. This has been verified by Lundin, et al and Brooks, et al, who have shown that high-sulfur, free machining steels (Type 303) can be welded without cracking if solidification is maintained in the FA mode."Now, I realize that many people here likely have no idea what that paragraph means, so let's dissect it. During welding, you are physically melting material and resolidifying it as a 'cast structure.' During solidification, there are several 'ways' in which a stainless steel weld metal can solidify. One of these ways is FA-type solidification. FA, meaning primary ferrite solidification with austenite present in the fully-solidified weld metal. This microstructure is especially resistant to cracking, even when sulfur and phosphorous are present. Creq and Nieq are Chromium and Nickel equivalents. I highly suggest you familarize yourself with these terms and the corresponding charts - the WRC-1992 diagram is highly recognized and used for welding different grades of stainless, ferritic and nickel-base metals together. In this paragraph, Lippold and Kotecki mention Suutala - a rare diagram that also lists its own Ni and Cr equivalents:Creq = Cr+1.37Mo+1.5Si+2Nb+3Ti, Nieq = Ni+0.31Mn+22C+14.2N+Cu When you find the ratio of Creq/Nieq for SS303, the ratio is ~2 - which should be resistant to cracking. However, when you find this ratio for 625, the ratio is ~0.7, which will not resistant cracking - especially in light of the phosphorous and sulfur in the SS303. Taking these factors into account, I would NOT suggest attempting to weld these two materials whatsoever. You will NOT be able to make this weld using 625 filler metal, if you can even do it.However, if you absolutely HAD to, I would suggest using GTAW process (for low dilution), favor the SS303-side and use a stainless filler metal with FA-type solidification behavior, such as a 308L. The more I think about it, a duplex stainless (2205) might work best. Keep dilution from the 625 side as low as possible. I am still not certain that either of these filler metals will work. The next time you post or anyone does, please provide more information - especially the process you intend to use and the application for the weld.