Can different sections of a model have different ambient temperatures?

The scenario:
An existing 2” 150 psi steam line with a valve (closed) at the end of the line. The line is to be extended by adding a couple of hundred feet of new pipe downstream of the valve. The fabrication will be done with the existing line still in the hot position. Design pressure = 160#
Design Temp = 500º F (superheated)

The problem:
Trying to figure out how to model the new line for the case when the whole system is cooled to ambient. For Operating case 1 I will have T1=70º for the existing piping and 500º for the new piping. For Operating case 2, I wanted to use T2=70º for the new, T2= -360º. This, I figured, should give me the stresses when the existing line contracts back to it's original installed position and pulls the new piping with it. The problem is that –360º F is outside the allowed range for the material. (A53 Gr.B)
One way that could work is to have the ambient temperature set at 500º for the existing piping and 70º for the new piping. If T2=70º it should model the contraction on the existing piping.

Can anyone offer suggestions?

Dave,you might consider using the thermal expansion coefficient (alpha*dT) in the T2 field. CAESAR takes this number and multiplies by the element length to calculate the thermal growth (negative or positive).
This may require that you adjust the default "alpha tolerance" in the Config/Setup dialog. See Technical Reference Manual pages 2-4 and 3-9 for more info.
An alternate solution may be to edit the materials database (although I don't think this is the preferred method).

Thanks for the tip!
I will try T2= -0.0029227 in/in for the existing piping and T2=70º for the new piping.
(-.0029227 = 6.797e-6 in/in/ºF x -430ºF)

To model a hot tap first find out the displacement of the connection point from its installation position to its hot position. This distance is essentially a cut short or cold pull in the new line. Ideally, the new line will be installed with a length to fit up to the hot position and it will be "short" by this distance. So the new element connecting to the existing (hot) line will have the same X,Y,Z length as the distance calculated above. Call out this element as material #18 "Cut Short". Then enter the remaining elements of the new system conected here using the lengths that you will use to install the piping to fit right. Don't forget to reset the material. Set up three thermal sets: for the existing system T1=hot, T2=hot, T3=ambient; for the new system T1=ambient, T2=hot, T3=ambient. Set up three operating cases: L1)...+T1+CS, L2)...+T2+CS, L3)...+T3+CS. L1 is the intial fit up to the hot line and this run will tell you whether you modeled everything right as, except for deadweight sag, there should be no load in the new line. L2 is everything hot and L3 is everything cold. The deadweight run should also include CS.

Give it a try...