If you want to accurately determine the effect of temperature on the strength of stainless steel, you can conduct tensile testing at different temperatures. This can be done using a testing machine that is capable of controlling the temperature of the sample during the test.

It's recommended to use a temperature range that is relevant to your application. For example, if you're designing a heat treatment fixture, the temperature range should be similar to the temperatures that the fixture will be exposed to during use.

During the tensile test, you can measure the load and displacement of the sample, and from that, you can calculate the yield strength, tensile strength, and Young's modulus of the stainless steel at different temperatures.

It's recommended to use samples with a similar size, shape and microstructure to be able to compare the results of the test.

You can also use Finite Element Analysis (FEA) to predict the strength of stainless steel at various temperatures. This will require you to model the specimen geometry, load, and boundary conditions and then assign a material model that considers the effect of temperature on the material properties.

You can also consult literature such as ASTM standards or material data sheets to find the temperature-dependent properties of the material.

It's also important to consider other factors that may affect the strength of stainless steel at high temperatures, such as the presence of residual stress, microstructure, and the effect of creep, which is the gradual deformation of the material under a constant load over time.

To optimize the design of the fixture for better strength and reduced weight, you can consider using lightweight materials, such as aluminum or titanium, for the heat treatment fixture.

These materials have a high strength-to-weight ratio and are suitable for high-temperature applications. Additionally, you can also use advanced manufacturing techniques such as additive manufacturing (3D printing) to reduce the weight of the fixture while still maintaining the required strength.

First I assume that both FCC and BCC phases are available for stainless stell you have mentioned.

The things you need to optimize for a Heat Treatment are Holding Time, Holding Temperature, Cooling (Quenching) rate, t (Tempering) time and Temperature. For Stainless steel, composition (e.g. Ni-Cr-Fe ratio) and presence of C and carbide/nitride forming elements (V, Mo) that also provide corrosion resistance, and alloying elements that might lead to temper embrittlement (P, Sn, Sb...) are of utmost importance. Unless you specify composition, nothing can be ascertained about HT schedule. However, target is to avoid temper embrittlement and Grain Boundary carbide precipitation Temperature range by any means (it might be difficult to predict for arbitraty compositions) is a good starting point. carbide coarsening should be avoided as well as their dissolution up to some certain extant to be controlled by changing the holding (soaking) temperature and time. try to find avg. soaking time per cross-section area/ thickness for stainless stell,m from which you would optimize time and temperature.

As for the quenching stage, whether you require to use without tempering (the SS would be very hard but siginificantly brittle and corrosion resistance might be impaired significantly) or not. The degree of martensite formation at surface and at interior (a.k.a case depth) would depend on cooling rate and amount of alloying elements liberated from carbide at soaking stage. I suggest you to go with Tempering. You need now optimizing tempering T and t- for a better controlled balance of strength and ductility required for application. longer t (time) or higher T (temperature) would make the article softer

But, If BCC phase is not accessible even by drastic cooling, the heat treatment then would not be able to make grain size any smaller (unless you heat the SS to the point of melting and cool rapidly), and cold working would be only option to elevate strength in expanse of ductility. If precipitation strengthening is possible, then the steps are same as what mentioned before, just precipitates of controlled size from supersaturated solution rather than controlled reversion of metastable phase partially to stable state would be done.

In all cases, you can design a factorial experiment, and determine strength, ductility and toughness optimization schemes. For stength vs ductility, you might have to use material property chart approach (and material index) for optimization as used by M. F. Ashby.

And remember,

• stength is not the only parameter to consider
• You are dealingf with soaking and tempering temperatures- these two are not the same
• Time and rate of temperature decrement with time are equally as crucial as temperatures themselves

Section 8 in the Design Manual for Structural Stainless Steel gives the reduction factors for the yield strength and tensile strength from room temperature to 1000 degrees centigrade.

http://www.steel-stainless.org/designmanual