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Jim2 Jim2 is online now
7.0:1 > 11.3:1
Join Date: Oct 2003
Location: Calgary
Posts: 1,876
I weld 321 regularly (3hrs today alone) and quite honestly I don't think I can tell it apart from 304, though when I weld 304 the joints are usually more crude because the applications don't demand such precise fitment so I can't say for certain that someone else has not identified a characteristic which shows up when welding. Spark characteristics when touching materials to a grinding wheel is a method often used to narrow down what a material is, but a quick test with 321 and 304 does not display any obvious difference. - see pictures below.

The dominant attribute of 321 over 304 as it pertains to aviation exhaust systems is it's resistance to oxidation (corrosion) at sustained high temperatures (1000+).

I agree it's nice to know what you are getting when buying a product so as to not pay for bullsh__ marketing, but personally I would not go out of my way spending significant added dollars for 321.

321 can and will crack just like 304. I've seen plenty of cracks in 321 aviation exhaust systems ranging from high hour parts, to a pipe sitting on my bench from a brand new aircraft with less than 50 hours.

Good to see thread which will capture all the details of the different brands Noah.

Even though this quick test did not tell me the difference between 321 and 304 some of you might find this interesting. Examine the sparks from these different materials, look at spark length and tail end.

Spark volume can be related to pressure against the wheel, though some materials produce a higher volume of finer sparks such as carbide of cobalt cutting tools.

Known 304

Known 321 (has the same characteristics as 304 spark)

Known 316 (generally same characteristic of spark as 321 and 304)

4130 (chromolly) low carbon steel: you can see some of the sparks burst at the tail end because the steel is oxidizing from contact with oxygen at high temperature. By comparison, the stainless materials never ignite into a secondary spark.

Here is a Cessna 172 tailpipe made from 321 material, from a high hour application. The wall thickness appears to be about .050 but the oxidation which resulted from sustained exhaust gas heat along with a continuous blast of atmosphere air against the tubing eventually eroded through the tubing wall. I took a pair of pliers and ripped the right one like it was a pop can. If the tailpipe were made from 304 the eroded hole would have occured sooner in it's life. Edit: this tail pipe has seen something in excess of 10,000 hrs flying time, with atmosphere air blasting it at roughly 125 mph.

Edit: excuse me for rambling, I'm bored. Not enough new threads in the turbo forum.

Last edited by Jim2; 06-02-2007 at 08:20 AM..
Old 06-01-2007, 10:53 PM
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