This information "the titanium content shall be not less than four times the carbon content and not more than 0.7%"is written in the SA312-304
what i want to know is the reason for limiting max Titanium 0.7% content.
Because you need to strike a balance with keeping carbon in solution to provide elevated temperature creep strength and avoid loosing carbon in carbides.
Since titanium acts as a ferrite stabilizer (i.e. - increases the Cr Equivalent), this max may be there to prevent delta ferrite during welding. Also since these alloys are often used for prolonged times at elevated temperatures, and Ti may promote sigma formation, it could also be related to maintaining corrosion resistance and/or preventing embrittlement due to sigma phase formation.
These upper limits are related to phase balance and sigma formation.
They were important back when people were trying to stabilize straight grade 304.
Today most 321 is down around 0.15% C, min Ti levels are way more than is needed.
Modern 321 is very soft.
Typo, 0.015% C, and so very little Ti is needed.
In 321 the Ti isn't here to impact the phase structure, it is just there to form TiC so that when you weld you will not sensitize (form CrC).
There is no reason to add more than this level of Ti, but some reasons not to.
Ed, you touched on sigma phase - Ti being a ferrite stabilizer would tend to promote this form of degradation.
"If you don't have time to do the job right the first time, when are you going to find time to repair it?"
For concast where the cooling is fast, the austenitc alloy is designed to have small amount of ferrite on solidification to improve hot workability / prevent coner cracking. Too much Ti would break that balance, and could lead to Chi or Sigma ferrite in the finish parts which is undesirable for corrosion and toughness.
Also, Ti loves N, it is easy to form TiN particles which may not undesirable in some situations. If it is strip product, the surface can easily form heat TiN tint even under bright anneal.
