ss 304 or 302

Date 06-28-2007 04:09

304 will be more expensive while 302 (because it contains sulfur and phosphorous) is cheaper to machine. However, those elements that make steel more machiniable also make it more difficult to weld by causing porosity and cracks. If you end up using 302 check weld quality by metallurgical cross-sections.

Girish

By ramakant_v (*)

Date 06-28-2007 16:46

hello,

I have different reason for selection of the material however the recommendation is the same as suggested by Mr. Girish. if we see the ASTM composition of SS304 and SS302 then the difference is not in allowed max limits of Sulphur or Phosphorous. The code has given same limits for these two alloys. The basic difference is in % of C, Cr and Ni

1. %C: 302...0.15%
304...0.08%

2. %Cr: 302...17-19%
304...18-20%

3. % Ni: 302...8-10%
304...8-10.5%

4. %P & S same in both case.

So if we see the difference, it is basically in % of C with slight variation in % Ni & Cr (i.e. on lower side). If you are going to weld the material, there will be more chances of sensitization in case of 302 where as in case of 304 it will definitely be on lower side. As such, as far as sensitization is concerned, you should go for 304. However, the cost of 302 is low because of higher % of C (because of less processing of molten material during its melting) and of course because of slightly lower amount of alloying elements.

On the other hand, the mechanical strength of SS302 is on higher side because of higher C%. But, why don't you try for SS304L material which has max. C 0.03%. this is most suitable for the jobs where corrosive fluids are to be handled, and welding is involved. As it seems that you are going to give fillet welds, i would strongly suggest you to use 304L. if you go for 304, there will be 100% chance that the material will still be sensitized during the welding process and when you apply pickling solutions to clean it after welding or it is subjected to corrosive medium in its service, it will fail because of IGC. Hence, to achieve better equipment life go for SS304L in stead of 304

RK

By sabir

Date 07-15-2007 04:24

Hi Mr. RK...

Nice to see your comments....

I'm a new lad to this industry and most of the words are quite unfamiliar to me...
It's really appreciable if you could spare some time for my doubts

Well, what exactly is this "sensitized" means and what is IGC.

Could you please suggest me some web sites or documents which can help me to study more about Pipe material selection.

Thank you,

Regards

Sabir

By prasad

Date 07-15-2007 16:00 Edited 07-18-2007 14:49

Chromiun depletion in the grain boundary area can be a serious condition with regards to corrosion resistance and is referred to as sensitisation .
Austenitic steel in the sensitised condition is susceptible to intergranular corrosion ( IGC ) by any reageant that is able to dissolve or attack the depleted area . Chromium depletion happens when carbon in the steel reacts with Chromium to form chromium carbide.

This removes chromium from the system and the alloy can lose its corrosion resistance in this region. Carbide precipitation happens when the alloy is heated in or slowly cooled through the range 425 - 900 degrees centigrade.The carbides can form around grain boundaries and provide a line of weakness in the steel.

Sensitization can be minimised by using low carbon steels or stabilised steels like SS321 or SS347 which contain elements like titanium or colombium which have a closer affinity for carbon than chromium.