The cracking issues are non existent in a 409 furnace and is simply a story used by other manufacturers to scare customers into buying their product.
First of all, let me say that I'm not a manufacturer of OWB's nor do I sell them. BUT, I am a welder/fabricator of over 30 years with extensive experience in stainless steels, so I know a little about the subject of stainless steels when used in OWB's. I don't think the issues associated with stainless OWB's is '
simply a story used by other manufacturers to scare customers into buying their product', but more a case of someone not fully knowing what they are dealing with in terms of materials and manufacturing processes. It is essential that anyone considering buying an OWB, stainless or otherwise, perform an extensive amount of research in both the history of the company and its' service record as well as how they are putting their units together. A factory tour during production operations should not be discouraged by the manufacturer, nor should answering questions about their processes. When I was looking at different manufacturers I had one tell me they don't allow factory tours. That just sent up a huge red flag for me and eliminated them from my list of prospective suppliers.
Here's a little information that may help you in your search and comparison of OWB's:
At first glance, 409 stainless would seem to be the choice for an application such as an OWB, but many precautions have to be followed in its' manufacture in order to achieve success. ANY stainless fabrication will be prone to cracking and failure at the weld joints if proper welding procedures are not followed. There are a number of welding problems with the ferritic stainless steels, which is what 409 is. Although 409 is not regarded as hardenable, small amounts of martensite can form during welding, resulting in a loss of ductility. In addition, if the steel is heated to a sufficiently high temperature, (such as what might happen with a very slow welding process or repeated welding in one area), very rapid grain growth can occur, also resulting in a loss of ductility and toughness. Although the ferritic steels contain only small amounts of carbon, rapid cooling can 'sensitize' the steel making it susceptible to inter-crystalline corrosion. When this is associated with a weld it is often known as weld decay and is what makes a weld brittle and prone to cracking or outright failure in a heating and cooling application. When I refer to 'heating and cooling', I'm talking about a few hundred degrees of difference, such as what may be seen when a fire in a fire box is sitting at 'idle' and then when the fire is at full flame.
Matching welding consumables, (wire or rod), are available for most grades so that corrosion resistance and mechanical properties can be matched to those of the parent metal. To reduce the risk of hydrogen induced cracking, low hydrogen welding processes are essential and preheat temperatures of 390 to 572°F are recommended. A weld that has been completely transformed to untempered martensite by allowing the joint to cool rapidly to room temperature can be extremely brittle and great care is needed in handling to prevent brittle failure. In addition, such joints are sensitive to stress corrosion cracking even in a normal fabrication shop environment if not handled properly. An alternative is to weld with austenitic stainless steel fillers, type 309 for example, but the weld may then not match the tensile strength of the ferritic steel and this must be recognized in the design of the weld. Nickel based alloys may also be used; alloy 625 for instance, which has a 0.2% proof strength of around 450MPa; and will give a better match on the coefficient of thermal expansion.
Besides the grade of stainless being used in an OWB, the welding process in which it is fabricated would be my biggest concern. I know of one manufacturer that used 409 stainless with disastrous results in each unit they built, while another manufacturer using the same grade of stainless has had more favorable results. I actually ended up buying my OWB from the company that had the disastrous results with the stainless because they recognized their lacking of knowledge in stainless and stopped using it rather than keep experimenting with it, were honest with me about it and allowed me into their shop to witness the whole manufacturing process. They even provided me answers to questions I asked that might not have been in their best interests, but again, they were honest.
I would do a LOT of research in how a boiler is put together before making a purchase you might later regret!
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Topic: Portage and Maine vs Heatmaster (Read 12195 times)