Heat resistant metal advice please

[Duner Wi]

A Danish made ' Morso ' model 1410 , Squirrel wood heater .
They are used a lot in small cabins and house boats .
Except for a few alpine areas our Winters are not very cold , it never snows , water never freezes , lowest temperature is perhaps between 2 and 4 degrees Celsius overnight.
That's why I'm so annoyed , it not as if we burn cubic metres of wood at a time , rather a steady trickle of good well seasoned hardwood gum .

Now I see what you are dealing with. Higher temperatures that my old agriculture simple wood stove. Morso 1410 baffle plate #44141100 subjected to higher temperatures do to superior design. I would if you can source it stainless 304 or preferably 309 or 310 as it can withstand higher temperatures. If you like to expirement check out ceramic coatings. Look at ND Coatings.com au. 1100 C should have room to spare. No idea as to cost. Some people with high output Turbo petrol engines like to keep the heat in the exhaust. Use your good judgement.

 

[Mark-Ia]

Maybe contact the manufacturer, perhaps they know there is a problem with a new alternative for replacement.

 

[Charles in Aus.]

Maybe contact the manufacturer, perhaps they know there is a problem with a new alternative for replacement.

Unfortunately Morso follow a classic denial argument and suggest that the user is at fault . The most common reason given is that the fuel used is the wrong type usually pine or softwood. I find this ironic considering that most wood burnt in Europe and Scandinavia is softwood.

 

[Charles in Aus.]

I have had a suggestion that a steel variety called ' Hardox' 450 could be suitable . Primarily used for wear surfaces on earth moving machinery . Supposedly welds easily , though a little difficult to bend .
Any thoughts on this if familiar with the material ?

 

[showcrop]

How old is your Morso? I had a Belgian coal stove that I had to make repairs to, but the parts were available. The underlying problem was that the joints had looseened up in thirty-five years and I couldn't damp it down enough so that constantly ran too hot which was hard on the cast iron.

 

[DanielW]

Hardox 450 is great for many uses - we use it a fair bit where I currently work (and we used it a lot more at my old job) and I've used it a fair bit around the farm. Although SSAB doesn't 'guarantee' the yield for Hardox, it's always going to be much higher your typical A36/44W. 450 is weldable without pre-heat (if less than 3/4"). And much better wear resistance (450 refers to the Brinell). But as far as heat resistance goes, it may not be a huge improvement at high temps. The same company (SSAB) makes a material called 'Strenx', which has very similar properties. Hardox is intended to have high/guaranteed hardness and wear properties and the higher yield is (for lack of a better term), a 'side effect'. Conversely, Strenx is intended to have high/guaranteed yield, and the higher hardness is a 'side-effect'. But both have similar eutectic points and only guarantee mechanical properties up to 250C, which you'll me much hotter than inside a stove.

It'll probably be better than regular mild A36/44W, but I'm doubtful that it will be a revelational improvement. SSAB does make a high temp Hardox (I think it's 350 Brinell, but I could be misremembering). But from what I remember it's hard to come by. Whereas Hardox 400, 450, and 500 are readily available (and many other companies make similar AR steels). I've forgot most of what I ever knew about metallurgy, but I'm thinking that Hardox (or equivalent) may be tougher and not yield as easily, but also that it would become more martensitic and brittle after being exposed to higher temps. It looks like that baffle plate would be formed if you had it made: That's something to be cautious above with Hardox. 450 is formable, but takes larger die openings and much higher brake force than regular plate. Some shops might not want to do it. And if you're forming with two small a bottom die, you can get stress cracking through the bend. We'd always try and avoid forming 450 thicker than 3/16" past about 60 degrees where possible.

I've done a fair bit of fabrication & re-working inside fireboxes for syrup evaporators and our old outdoor furnaces. While I'm not expert, in my experience cast is best for anything inside a firebox (at least of your common materials/alloys). But if you can't get cast, going with thicker material will give you better bang for your buck than using a different alloy. That's assuming you're not going to use iconel or similar - which would probably more than the stove's worth to get one made from. But again: I'm not an expert. Just my experience.

Not sure what you have for fabrication, forming, & steel supply around you. If it was my stove and with everything I have at my disposal (I'm lucky because where I work does side-jobs and has full plasma/laser-table and brake capabilities): I'd draft it in CAD, get it made formed out of slightly thicker plate. Maybe go with Hardox or a similar AR/high-yield plate if it's available (and if your shop can form it). But I wouldn't get too hung up on needing the Hardox. Most significantly, have the shop that makes them make three or four when they burn/form them. With the economy of scale, the cost to get three or four made will probably be only a little more than the cost to get one made. If you can go a little thicker and even if they only last two or three years, you'll still have enough to last over a decade.

 

[Stan Coryell]

I have had a suggestion that a steel variety called ' Hardox' 450 could be suitable . Primarily used for wear surfaces on earth moving machinery . Supposedly welds easily , though a little difficult to bend .
Any thoughts on this if familiar with the material ?

Hardox is Saab's brand name for AR. It comes in different grades.

 

[Charles in Aus.]

How old is your Morso? I had a Belgian coal stove that I had to make repairs to, but the parts were available. The underlying problem was that the joints had looseened up in thirty-five years and I couldn't damp it down enough so that constantly ran too hot which was hard on the cast iron.

Only two seasons , this is the reason I'm so annoyed and also why I won't buy a genuine replacement . $150 per season plus the inconvenience of having to swap it over ( not a straight forward job either) is too much .

 

[Fritz Maurer]

3 or 4 mm of cast iron....roughly 1/4 thick... is not a real heat-proof baffle... depending on how unevenly it may be be heated, no surprise that it cracked.

If you have room for 10mm, then GO 10mm. Get yourself some ordinary 3/8 plate and weld up whatever shape you need. No reason to go to the trouble of these space-age materials that will cost as much to buy, cut, and fabricate as much as the original replacement piece would cost.

 

[countryman]

Me being a retired Power boiler Engineer, fabricator, designer, certified welder, mechanical repairs yes.

Fab. it out of require shop stainless steel, here's why wood burns from 400 - 500 degrees the (carbon) in steel, cast steel, cast iron, burns out and shrinks, cracks and everything else. Stainless has no carbon it holds up pretty good but threw time it can fatigue.

 

[Dave G9N]

I think that Fritz Maurer and Countryman have it right. I suspected that the thickness might have been the issue, and based on countryman's experience, the casting is too thin. They want to keep it light, but conflicting requirements lead to compromises. If it is too heavy no one will buy one and if too light no one will buy a second one.

There is a post from another forum below. Someone used a 1/2 in. (6.2 mm) carbon steel plate and measured a 0.020 to 0.025" (~0.5 mm) thickness loss after 15 years in a wood stove. I would expect the the more rapid heating of a much thinner sheet would cause more loss.

Countryman's comment about decarburization is interesting, since cast iron has about 4% C while mild steel has only 0.2% C, but the temperature given for decarburization might be centigrade, not Fahrenheit, and the amount of oxygen is important. A fast burn with the damper wide open would cause more oxidation than a damped down burn. That is not saying he's wrong, it's just complicated. On the subject of burning, I like the manufacturer's excuse about burning pine. Hardwoods burn hotter than pine. If the pine is making soot and creosote, the oxygen content and temperature of the combustion gasses would be low. I guess they felt that telling you not to use hardwood would have been too obviously a load of bull manure.

Regardless of which steel you buy, it seems that thickness is your friend. Any carbon steel will work, but not quite as well as stainless. Any of the 18-8 stainless grades (301-305) or 409, which is used for car exhaust systems should be fine here. Performance will be about a wash between any of them, so price and availability are the main drivers. 409, if you can find it, is a little better. It appears that 1/8" of cast iron is too light, and 1/2" carbon steel plate is overkill. My guess, and it is only a guess, would be a minimum of 1/4" cast iron, 1/8" (10-11 gauge) carbon steel and just over 1/16" (14-16 gauge) stainless steel.

For the record, Fritz thinks in metric, and 3 mm is 1/8 inch. 6 mm is closer to 1/4. So yes, too thin, but 1/8 inch for those of us who think imperial. Not easy to think in both systems at once. I disagreed with Fritz based on the assumption that he is translating between metric and imperial and thinking that 1/4" wasn't working. I could be wrong.

Hardox, as stated before is a brand name AR steel. Almost exactly the same stuff as some armor plate, minus the expensive ballistic testing requirement. Hardness and wear resistance are closely related. AR steel contains maybe twice as much manganese as mild steel. Mn is also good for wear, but doesn't do anything for oxidation resistance. Pretty much the only thing to look for in a low alloy steel is the thickness.

https://app.aws.org/forum/topic_show.pl?tid=11829 is a thread asking the same question. The good answers were:

By MDG Custom Weld

Date 05-14-2007 16:48
We have repaired wood stoves in the past, and here are some things to keep in mind:
1. Clean and prep the steel as much as possible to remove any soot/ carbon build up. This is really important if you're welding in any type of stainless steel.
2. Remove the stove from the home or where ever it's at if at all possible. A spark in the flue can cause a fire if it lands on some built up creosol
3. Most inside stoves are made from carbon steel plate.
4. If you're welding in some ferritic SS, we always use 409 plate with 409 metal core wire.
5. USE CAUTION!!

and

By fbrieden

Date 05-14-2007 18:52
Replaced mine with 1/2" A 516 Grade 70 about 15 years ago and a UT test on it at several locations came in at .475-.480 (after power brushing). The baffle is lasting longer than the refractory bricks that I have to replace again.

 

[4wdtom]

The baffle plate on our Danish made wood heater has cracked and warped . A new one is hundreds of dollars .
Other owners have said that even new ' upgraded ' replacements only last a season before showing signs of failure .
Rather than succumb to a treadmill of cost and failure I'd like to make a heftier replacement myself.
It's a simple enough shape , easily fabricated and certainly not worth the cost asked .
Big question, what grade of steel , stainless or cast should I use ?
Originals are cast iron at about 3 or 4 mm , there is space to allow up to 10mm in thickness .
Thanks ; it's high Summer here but as the saying goes , Winter is coming!

Can you splice it together where it is cracked with some 1/4" or 3/8" steel bars? Bolts not tightened too much might allow it to expand and contract a bit as it is heated and cooled and now it is in smaller pieces, or the crack has relieved a stress area, maybe it won't crack any more.

 

[Charles in Aus.]

Update on baffle plate .

Our Autum has chilled quickly leaving little time to experiment with alternatives .
I succumbed and went to a supplier directly and bought a manufactured replacement. While there I was told that there had been a few upgrades and changes to the plate and fire bricks in the latest productions . It seems we were sold one of the last of the older iterations .
New baffles are a different shape and of thicker cast , fire bricks are also differently shaped and of dense ceramic rather than Vermiculite based . I made a pattern from the replacement for the future , at least now I can make one at leisure .
Thanks to all for the advice I know much more now than I did before.