Dropping from .15 to .08 C, the lower carbon grade will have less tendency to crack.
410 is a martensitic stainless and can be welded in the annealed, hardened and tempered conditions. Regardless of the prior condition, welding produces a hardened martensitic zone adjacent to the weld. The hardness in this zone depends primarily on the carbon content of the base metal. As hardness increases, toughness decreases, and the zone becomes more susceptible to cracking. Preheating and control of the interpass temperature are the most effective means of avoiding cracking. Postweld heat treatment is required to obtain optimum properties.
Check your parameters against these:
410 filler
300F minimum preheat (400 would be a good target)
600F maximum interpass
1200Fmin anneal (heat and allow to air cool)

309 filler
300 F Min Preheat
350 F min interpass
I do not recommend the anneal with 309 filler because you will cause a brittle and less corrosion resistant phase to form in the austenitic 309 weld deposit.

Ron,
A few years back we had to weld some 316 sockolets to CA6NM (410) pump casings on an oil refinery in Queensland, Australia.
This was prior to me knowing about this forum and I spent a fortune ringing all over the world ( Sandvik etc) for advice.
I do not have a copy of the WPS but I managed to find some notes I kept.
I don't know if they are relevant to the job you are doing but may be of some benefit.
WT of pump casing 12mm
25mm Sockolet
GTAW
1 Using thermo-couples administer pre-heat of 150°C
2 Using thermo-couples maintain interpass temp of 150°C min and 250°C max during welding.
3 Weld 4-5mm thick "butter" layer of 310 filler metal to an area twice as large as diameter of sockolet.
4 UT "butter" layer to ensure sound fusion between filler metal and parent metal.
5 Stress relieve for required period at 560 - 595°C as soon as welding is completed.
6 After PWHT and after cooling a hole is to be drilled in the pump casing and the 316 sockolet is welded to the 310 "butter" layer with 310 filler metal.
Hope this helps,
Regards,
Shane

Ron,

Shane's advice provides a well thought out approach. I finally took a moment to check ASME VIII, Table UHA-32 since you did mention P6 and P7 material groups.

I note that for P7 there is a requirement to postweld heat treat at 1350F and a special note requireing controlled cooling of 100F per hour max down to 1200F. From 1200F the cooling rate shall be sufficiently rapid to prevent embrittlement.

ASM allows that after annealing either subcirtical at 1250-1400F or full at 1525-1625 the part should be furnace cooled to 1100F and then allowed to air cool.

My point in discussing these temeperature ranges is that Shane's procedure stayed below the anneal/transition temperature and provides some stress relief although it does not meet requirments for pressure vessel fabrication it's probably fine for a drain provided the connection is not hard piped and/or subjected to significant vibration.

On the other side of the coin, the welding will invariably provide a 1350F temperature range somewhere within the HAZ and therefore, a brittle phase. Wheter stress relief is required or not boils down the the service loading the weld you are making will be subjected to as well as any requirements for weld qualification specified by your customer (or potential liability).

I'm still hopeing for a reply to an even crazier attempt to weld 420, which has no P number.

Good luck,
Allen

Gentlemen;
Thanks for the replys. They have been very helpful.

Ron B