According to the publication Welding of Stainless Steels and Other Joining Methods, which a product of the Nickel Development Institute (NiDI) and the American Iron and Steel Institute (AISI), "Carbide precipitation, for instance, will not occur during laser welding because of the high speed and low heat input".

Based on your question, I would say that cooling rate is a secondary variable to consider.  You should probably be addressing the speed at which you are welding, and the amount of heat being generated in the weld metal during the process.  Can you adjust weld parameters (beam power, beam diameter, etc.) to lower the heat input into the joint?

The next thing to consider is whether or not TiCN formation is really the problem.  Slow cooling of ferritic steels, including stainless steels, through the temperature range of 565-400 C (1050-750 F), produces brittleness in the steel (known as 475 C embrittlement).  This embrittlement is usually associated with ferrite grain boundary embrittlement by P, ans since TiCN will also be along the grain boundary, you may be experiencing this phenomenon.  Ideas to consider for reducing this tendency include:

1. more aggressive cooling rate - forced air, water quenching (spray), etc.
2. Specify a lower P and C content - maybe limit P to 0.015 (instead of 0.045 max), and limit C content to 0.04 instead of 0.08.
3. Shield the weld metal from the atmosphere to limit O & N uptake in the weld.

oimar

I concur with TVP.  A smilar phenomenon occurs in 300 series S.S. called sesitization when slow cooled or held too long in the temp range 1500-800deg F.  Cr-C's precipitate to grain boundaries which seriously impairs corrosion resistance.  So yes it would seem that cooling rate is very likely pertinent but as TVP says, the welding process should be analyzed first as being suspect because there should not be enough input heat capcity to cause the cooling rate to be substantively retarded.

To add a twist to TVP's No.2 comment:  Are you just melting parent metal and if so do you have the option of adding to the weld deposit an alloy wire of composition that will dilute the melt to achieve the chemistry that TVP suggests?

Leonard

Jesus is the WAY

In a properly produced 409 all the carbon and nitrogen will be tied up by titanium in the form of titanium carbo-nitides. These precipitate both above and below the solidus temperature. If [Ti]x[N]> 0.0025% then the amount over that level precipitates in the molten metal.
 These precipitates exist at all times in 409 and are not injurious or embrittling. If you have weld zone embrittlement, you should look elsewhere for its source. It is possible to form martensite in 409 welds if Mn, Ni or Cu levels get too high.
 409 is generally a very ductile and weldable grade. Contact the steel mill that produced the steel and explain the problem to them. This is not something that should be happening with 409.