Weld durability in hot environment

[Stueeee]

Several years ago I needed to weld a 10mm diameter rod which lives in the firebox of my oil fired kitchen range. This component probably runs at or somewhere near dull red heat when the cooker is running. The rod material is steel, but is quite hard, can be sawn but doesn't feel like a mild steel.

At the time I needed to modify this component it already had a welded joint. I cut through the original weld and MIG welded it back together in the different position I needed. The problem is, about every 12-18 months this weld fails and needs to be redone, which is a pain as it always fails at the most inconvenient time. My MIG weld deposit has pretty much disappeared when this failure occurs.

My question is, what rod could I use that would have the kind of hot strength/endurance needed here. I have TIG, ARC and OA kit here as well as the MIG.

PS. Apologies as I know this isn't necessarily a TIG related question but couldn't find anywhere anywhere where it seemed to exactly fit.

Thanks, Stuart.

 

[TechnicAl]

Is it just the weld thats affected?

It would be nice to know what steel the rest is made from. As an initial thought Im thinking 310 stainless. It is heat resistant to about 1000 oC and is suitable for welding other steels without cracking.

But its not cheap and maybe its a sledgehammer to crack a nut

 

[Justme]

My MIG weld deposit has pretty much disappeared when this failure occurs.

So the metal is actually corroding away before it fails?

 

[FranV8]

Hmm, is it fired all the time?

Welds should be pretty good up to the temperature of the parent metal if they're made with similar compostion. I suspect it's corrosion from combustion products that's doing it.

The compositon of the MIG wire is probably just normal mild steel - how about moving the weld out of the heated section (i.e. if it is just a rod passing through, get another section of rod and cut it and the damaged section so the welded sections are outside of the flame?

 

[Shox Dr]

How about as the metal cools it will oxidise, as it is heated the oxidation is removed. And so the process continues

 

[TechnicAl]

How about as the metal cools it will oxidise, as it is heated the oxidation is removed. And so the process continues

Thats exactly what happens. The steel oxidises and spalls off just like rust. Its corrosion but by heat + the atmosphere rather than chemicals.

 

[nickk]

i suspect its the same conditions experienced on a vehicle exhaust,the temperature is just burning it away,even though its not reaching the melting point of steel,its relativeley close & over time it fails

 

[TechnicAl]

i suspect its the same conditions experienced on a vehicle exhaust,the temperature is just burning it away,even though its not reaching the melting point of steel,its relativeley close & over time it fails

An exhaust also has the chemicals from the engine but its similar. The fire will also have the products of combustion.

 

[FranV8]

I was trying to think of it in terms of the internals of an industrial boiler - but the bit I missed was the cycling on and off - industrial stuff tends to be lit and keep going, maybe switched off a couple of times a year (well, in the context I've worked with them on).

So, move the weld? Sounds like the rod itself is of a more resistant composition?

 

[nickk]

I was trying to think of it in terms of the internals of an industrial boiler - but the bit I missed was the cycling on and off - industrial stuff tends to be lit and keep going, maybe switched off a couple of times a year (well, in the context I've worked with them on).

So, move the weld? Sounds like the rod itself is of a more resistant composition?

maybe a nickel chrome mma rod is needed?

 

[hotrodder]

On the grounds that you're used to repairing it (so presumably nothing bad happens when it fails, it's just a PITA) i'm thinking try whatever stainless wire or electrode is to hand, one of the disimilar flavours if you've got a choice

 

[Stueeee]

I was trying to think of it in terms of the internals of an industrial boiler - but the bit I missed was the cycling on and off - industrial stuff tends to be lit and keep going, maybe switched off a couple of times a year (well, in the context I've worked with them on).

So, move the weld? Sounds like the rod itself is of a more resistant composition?

Thanks for the replies.

Yes the original rod is apparently completely unaffected by the heat cycling. Can't really move the weld, it's a sort of fork device on the end of the rod which needs to be there to operate a damper on top of the firebox. It was welded on originally but in a different orientation. Whatever filler it was originally welded with had been there for some time without any ill effect.

So what's the hot strength of stainless TIG filler wire like? I've got some 300 series TIG rods somewhere that I could try, or would a MMA hardfacing electrode (think there are some of these in the airing cupboard somewhere) be a better bet.

Stuart.

 

[hotrodder]

Have you got any 309? It's more common (although not as good as the 310 Al mentioned) and designed for disimilar welds so less likely to produce a brittle weld deposit than 308/316 when the parent alloy is unknown. There's an overview of stainless alloys with regards to high temp resistance here... http://www.azom.com/article.aspx?ArticleID=1175

 

[Windy Miller]

Stuart, I've got some stainless mig wire you could try if you like. If you think it's worth a trip to Rochester?