Welding 420HC stainless (SMAW)?

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Does anyone have suggestions for this project? It's an old broken knife tang made from a high carbon variant of 420 stainless (air hardening stainless). Local suppliers only have 308 and 312 rod available. Will that work,or do I need a specific 420 rod?Also, please explain to this newbie what happens if I try using mild steel electrodes on this piece? The tang doesn't need to be hard after welding-it just needs to be tough.
Reply:Squirrel,As you probably know, since 420 is air hardening, welding on 420 can result in a very hard and brittle heat affected zone (HAZ) in the base metal next to the weld, regardless of the filler composition.  Preheat and slow cooling may minimize this, but you may need to post weld temper the weld region to regain some toughness.  Although maybe if the weld is ductile, it could absorb stresses and prevent brittle failure in the HAZ.Since you don't need the weld to have the same properties of the 420, I think I would use 312, since this weld should remain austenitic and will be a tough and ductile weld.  Again not too sure about the need to temper HAZ.The 308 would probably result in a hardenable weld chemistry mix, maybe somewhat similar in hardenability to the 420, just guessing.And the mild steel electrode, guessing the weld would be hardenable, and maybe not corrosion resistant depending on the ratio of 420 and carbon steel in the weld mix.  Need roughly 11% chrome to make a steel stainless.Some links to info:http://www.brazing.com/products/Weld...tion_chart.asphttp://www.matweb.com/search/Specifi...assnum=NUWW014
Reply:Based on the charts,I'm going to try 312 with a propane preheat (it's what I have) and a very slow air cool down.
Reply:How about some 110ksi brazing alloy?  That 312 is only about 70ksi so the braze is stronger and you may be able to do it without changing the heat treatment.
Reply:If you get the steel over about 300-450 deg F, you'll likely change the heat treatment of the original steel (depending on original alloy and it's original heat treatment and tempering ).Yup, 420 HC stainless steel is air hardenable.  If the temp of the 420HC gets into the range of 800F-1025F, corrosion resistance and toughness of the steel will decrease (per TimkenLatrobe Steel).The brazing.com tech page indicates that a 309 filler should be used to join 420 to 420, the HC variant is not specifically listed.How close to the 'blade' part is the tang broken and how critical is this repair?  Are you just willing to try anything and sort off get the knife back together or are you trying to get it back to pretty much original 420HC steel specs?If the latter and you can't keep any welding/brazing heat out of the blade area (via chill bars or such), then I'd suggest you need to do a full anneal of the blade and tang pieces, then weld with 309 filler, then anneal again, then heat treat, and then temper.  The annealing time-temp profile needed is heat at a rate not exceeding 400F/hour to 1525F-1625F and hold at temp for 1 hour/inch of thickness (2 hour minimum), then cool slowly with the furnace at a rate not exceeding 40F per hour until temp is down to 1000F, continue cooling to ambient in the furnace or in still air.  Heat treatment will be heat at rate not exceeding 400F/hour to 1400-1500F, hold until temp is equalized, then rapidly heat to 1800F-1875F and soak for minimum of 30 minutes, quench in still air until 125F-150F.  Temper immediately after quenching, double tempering at 300F for a minimum of 1 hour but no less than 4 hours and then air cool to ambient should result in a hardness of Rc53-54, triple tempering is recommended for maximum toughness.  Higher tempering temperatures from 500F up to about 900F will result in toughness dropping by about half from about 18 ft-lbs to about 11 ft-lbs, unnotched.  Triple temper at 490F for maximum toughness and a hardness of about 52 Rc. Of course, all the elevated temperature stuff should be in an inert atmosphere, either locally or within the furnace.Using a mild steel electrode on stainless will turn the welded area into a witch's brew of steel of varying and varied alloy composition.  Can you weld it that way?  Sure, but the result may be rather unpredictable down the road with regard to the metal properties.
Reply:It's about 1-2" from the base of the blade. The tang itself is only about 1/4" thick and maybe 1/2" wide,so I won't be spending too much time welding it,and don't expect anything more than a very localized buildup of heat. I'm honestly not concerned with annealing the tang. It seems to me that the tang was too hard and brittle as it came from the factory,almost like it received the same heat treat as the blade,with no subsequent annealing to lessen it's brittleness. I may try to keep the blade cool during the process,since annealing the blade is obviously a bad idea. Any ideas for keeping it cool?  If I anneal part of the tang-oh well. It will just be easier to drill for the new bolsters,and it should make the tang more resistant to cracking.I don't have access to a forge right now,so heat treating is basically out of the question anyway....That brazing alloy is interesting stuff. I'd never seen that before (I'm just a hobby welder). It seems like it's basically fancier alloyed versions of nickel rod like you would use for cast iron.