Nitrogen In Type 321 Stainless Steel 4

[Maddog914]

What is the basis for the differences in minimum titanium content in Type 321 stainless? SAE-AMS specifications for aircraft tubing (e.g., AMS-5557) require minimum titanium = 5 x (carbon PLUS NITROGEN). Some comparable ASTM Specifications say the same thing (e.g., A-269) while others say required minimum titanium = 5 x carbon (e.g., A-213). Why the differences? What is the beneficial / detrimental influence of nitrogen?

 

[NickE]

Its my understanding that Nitrogen has much the same effect on steels as Carbon. Thou I'm not sure about the sensitization preventing effects of Ti when TiN is forming. That might be the reason the SAE-AMS tubing spec uses Carbon + Nitrogen. The nitrogen might combine with the Ti forming TiN reducing the amount of Ti available to combine with the Carbon preventing sensitization.

(mcguire will be along shortly I'm sure)



Nick
I love materials science!

 

[GRoberts]

I think NickE hit it on the head with his last sentence. The nitrogen takes up Ti, so it isn't available to tie up carbon.

 

[hnaziri]

You would want some Ti available to tie up carbon, otherwise chromium carbide is going to form if the carbon is not tied up. That's what you don't want to happen because formation of chromium carbide in 300 series stainless steels means that the material is sensotized. I think in recent years it has been found out that if nitrogen is also not tied up in type 321, it could leach out the chromium in the form of nitride very similar to the chromium carbide causing sensodization. therefore, the specs or standards that still call out for 5 X carbon as a minimum for the amount of Ti in type 321 may be older versions.

 

[mcguire]

Hi guys
321 was designed before they were able to make low carbon 304. It's kind of obsolete now. Some specs linger long after their usefulness, and I can't address why some are 5 x carbon. The most erudite specs call for Ti=5(C+N).
In this grade the titanium combines with the carbon before the carbon can combine with chromium and cause sensitization. Titanium combines preferentially with oxygen, nitrogen, sulfur and then carbon. It's the carbon one is trying to eliminate. ( Sulfur and oxygen are also bad, but this wasn't understood when the grade was designed.) Unfortunately, the nitrogen, which is quite beneficial, has to become TiN before the carbon can become TiC ( actually Ti2CS ).
Modern practice when making this grade of steel is to reduce C and N to as low as possible, around 0.015 each, and then use the minimum titanium to get the four times C+N.
Titanium nitride makes a slaggy residue that is hard to float off and makes surface defects. That is the only bad thing about nitrogen in 321.
Make sure you really want 321 and aren't just following obsolete practice. 304L is the more modern answer and it's cheaper.

 

[Maddog914]

Much appreciation to everyone who offered their insights on my question. Type 321 remains as the predominate grade of stainless that is specified for both military and commercial aerospace tubing applications. This includes updated versions of a variety of respective specifications. Contemporary production of Type 321 typically contains carbon less than or equal to 0.05% along with a nitrogen max of 0.10%. Sensitization has never been an issue in our experience with 321 SS, irrespective of either 5XC or 5X(C+N) being used to "define" the required Ti content.

Does anyone have a familiarity with the Ti requirements of 321 per DIN, JIS, etc.???

Your further comments would be welcome. Thanks!!!

 

[TVP]

Type 321 from SAE J405 is equivalent to grade SUS321TKA from JIS G3446 (tubes). This standard lists the Ti requirement as Ti > 5 x C, with no maximum. EN 10088-2 is the replacement for the various European standards (DIN, AFNOR, etc.) and its equivalent is X6CrNiTi18-10, which lists the Ti requirement as Ti = 5 x C up to 0.70 max. Only SAE J405 (and similar AMS and ASTM standards that reference these compositions) lists the requirement as Ti = 5 x (C+N) up to 0.70 max.

 

[Maddog914]

Thanks TVP. I appreciate the "international" overview.

 

[gstark]

Maddog914:

A good portion of the C-130 bleed air ducting, from the nacelles inboard, is 321CRES, insulated. Operating temps in the 600-800+F range.

In service failures of the CRES ducting from stress-corrosion cracking is driving the Air Force to convert existing C-130 pneumatic ducting to Inconel 625.

Also, the Air Force C-141 aircraft utilizes ducting fabricated from AM350 material, and it too is experiencing in-service failures from stress-corrosion cracking. As a result, a portion of flight critical ducting is being reconfigured to utilize Inconel 718 material.

 

[Maddog914]

Thanks gstark. Since we are also involved with nickel alloy tubing, this is good info.