What is the effect of increasing Al in steels? 2

[sayee1]

Can someone explain what is the effect of Al addition to steel. Of course I know that Al is added as a deoxidiser but I woulkd like to know specifically with respect to grain size and also in that case can someone explain why the restriction on Al in pipeline steels especially? Thanks and regards
Sayee Prasad R
Ph: 0097143968906
Mob: 00971507682668

 

[mantill]

With reference to pipeline steels. Al deoxidizes steel by forming Al2O3 particles. These are very hard refractory particles which do not melt at steel forming temperatures and end up as inclusions in the steel.

When it comes to forming the pipe the inclusions then act as both stress raisers and cracks. This gives problems with during rolling with the formation of cavities (read as cracks) around the particles, cracking during welding welding, etc. On top of this the cavities that form around the inclusions can give rise to the formation of hydrogen cracking.

All in all, for pipeline applications it is best to keep the amount of Al as low as possible.

 

[NickE]

In the low carbon steels IE: 1006, 1008 and other forming steels the deoxidation caused by adding aluminium in the melt allows for better forming properties. Also the aging of the steel can be delayed.

typical properties for a Aluminium Killed Drawing Quality steel (2" gauge length) are:

Yeild strength: 25-32ksi
Tensile strength: 40-60 ksi
Elongation%: 42%+
R-value: 2.0+
N-value: .20+

I'm not familar with the pipe making industry so I'm not shure of the non-metallic inclusions problems related with aluminium killed steels. In the steel stamping industry you wnat a small amount of aluminium IE: .02-.06wt%, that way you know that the steel was properly killed.


Nikc

 

[sayee1]

And what about the effect of Al on the grain size? Thanks and regards
Sayee Prasad R
Ph: 0097143968906
Mob: 00971507682668

 

[mantill]

Al killing generally leads to smaller grain sizes owing to the effect of nucleation of more grains in the melt and the effect of Al particles pinning the grain boundaries helping to prevent grain growth.

However, the strength benfits of the reduced grain size are offset by the lower fracture properties. In pipeline steel you want achieve smaller grain sizes from the use of other mechnisms, e.g. precipitation hardening in micro alloyed steels or from controlled cooling.

Of course these advanced manufacturing methods make the pipeline steel more expensive.