fire cradle

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I bought a fireplace insert several years ago and it came with a fire cradle.  The fire cradle has now burned through and the company that made the insert is no longer in business.  I can't find a fire cradle to fit but i have some mild steel half inch bar stock and welding up a new one would be simple, however, i need to know if mild steel would be sufficiently heat resistant or do i need to use cast iron?  would the expansion and contraction of the steel break the welds?
Reply:I made a log cradle using 1/2" rebar and welded it with flux core wire.    It's been 3 seasons and no welds have broken, nor has the rebar sagged.There are no small projects
Reply:By log cradle are you talking about a cradle to hold split or cut logs, or one thats actually holding firewood inside of a fireplace? I believe you are talking about the former and the first poster is referring to the latter. I am new to the welding scene but have a welding metallurgical class under my belt. Lets see what I can remember from that class.According to the Iron-Carbon Phase Diagram metals will go through a couple states as you weld or heat them.Now there are a few variables that I am unsure of. One of those is the temp that a firebox can reach. Most firebox paints are tested to about 1200*F. Lets then assume that the temp inside will not reach much more than that.As you probably know steel is an alloy of C (Carbon) and Fe (Iron). One of the ways steel is classified is by its percentage of Carbon. Low Carbon has .025-.30% C (Mild Steel,Cold Rolled),Med Carbon Steel has .30-.60% C(Vehicle Parts) and High Carbon has .60-1.76% (Tool Steels)Mild Steel in solid form from the refinery is in a Pearlitic State. Pearlite is Ferrite & Cementite. Pearlite is a very good state for metal to be in.It has plenty of strength on its own.According to the Phase Diagram above, Pearlite is the very bottom box, and ends at the first critical line. Thats 730*C which is about 1333*F. The firebox shouldn't get too close to this first critical line as temps will probably not reach higher than 1000*F. When you heat a metal higher than 1333*F you are now in an Austenitic state. The grains are much smaller than the previous Pearlitic state. Upon rapid cooling the Austenite cools and is now in a martensitic state. Martensite, untouched is very hard but very very brittle. That is why martensite must be tempered to have any strength.If you were to Pre-Heat,Weld,Post Heat you lessen the chance of forming martensite.Anyways to answer your question, I wouldnt see a problem with using mild steel as a fire rack as the temps. of the fire wouldn't be high enough to form martensite. However when welding the pieces together, you are running the temp. up much higher and the cooling rate may cause martensite depending on thickness of parent metal and also carbon percentage.Regarding the need to preheat:-No preheat required for <.20% C-No preheat on plates less than 1/2" thick with a C% of .20-.40%(This is where your mild steel would fall.)
Reply:I was referring to the cradle that fits inside my wood burner (Vogelzang Box Stove) and holds the logs (that are on fire) up off the bottom of the stove.   I found that I got more heat and cleaner combustion by allowing air to get under the wood that was burning.There are no small projects
Reply:Quattro.. Excellent post but... you just made me feel so stupid...
Reply:ImagineerExcellent explanation.  Well done.  I've seen those words (Austenitic, Martensite and Pearlite)  never really clear on the definitions. Thanks.
Reply:I got fed up of making new rack evrey 2 to 3 years, I made one with stainless 306 seven years ago still intact.SS is the way to go! MichelLast edited by Michel; 11-30-2005 at 07:14 PM.PM300Python gunV205TWeldcraft Crafter torch 300 ampLincoln water cooler Plasma Cutmaster 70Ranger 250 GXT
Reply:Well tap, if you'd like I can copy up some of my notes that I've taken in class if there is anything else that may interest you or that you are unclear of. Who knows if its good enough maybe it can be made a sticky at the top of the page. Let me know and I can see what I've got in my notes.
Reply:Originally Posted by imagineerI was referring to the cradle that fits inside my wood burner (Vogelzang Box Stove) and holds the logs (that are on fire) up off the bottom of the stove.   I found that I got more heat and cleaner combustion by allowing air to get under the wood that was burning.