I question your spectrometer results. Alloy 21-2N (UNS S63012) is a common alloy for this application, and is close to your composition:

C 0.50-0.60
Mn 7-10
Cr 19.5-21.5
Ni 1.5-2.75

TQ CoryPad.

Actually there are a lot of result but some one told me to select only this value...therefore i post that result only. What is the important material to highlight if we do not know the standards?

I do not understand your question. Everything is important - chemical composition of many elements, microstructure, processing methods, etc.

did the valve burn at the sealing face?
Did the head break off.
Some other failure altogether?

ok...http://www.eng-tips.com/viewthread.cfm?qid=371870
to make it clear below are the detail:
C=0.78 ; Si=0.243; Mn=8.850; Cr=18.32; Mo=0.152; Ni=2.3; Cu=0.155; v=0.07; w=0.081; P=0.0236; S=0.0124; Al=0.002; Co=0.042; Nb=0.0532;Ti=0.0129; Pb=0.038; Sn=0.0109; Mg=0.0137; As=0.0024; Ca=0.0011; Ce=0.0144; Sb=0.0161; Te=0.0066; B=0.0041; Zn=0.0373; La=0.0017; Fe=68.7

There is no burn sign on the sealing face. However, the valve is cracked from the edge to center.

So you have many certified reference standards for your spectometer that fall in this range of chemistry?
What kind of engine was this in?
How old? Made where?
You also have to be careful doing chemistries because many valves are plated or coated.
There have been dozens of proprietary alloys used over the years.
The problem with your chemistry is that the C is too high, and the Mn and/or Ni too low for the alloy to be austenitic.
At the same time the Mn is too high for a ferritic stainless.

8440: Medium to heavy duty steel alloy with a higher chromium content to enhance high temperature strength.
Sil 1: Heavy duty steel alloy with an 8.5% chromium content for excellent high temperature performance. Used for many factory high performance intake valves.
Sil XB: A ferritic stainless steel alloy, with 20% chromium content and 1.3% nickel. Used in heavy duty intake valves.
422: A stainless steel alloy specified for high performance intake valve usage. This specialty type of stainless is designed with the temperature ranges and characteristics of intake valves usage in mind, and is different from the more common exhaust valve stainless materials.
21-2N: Austenitic stainless steel with 21% chromium and 2% NICKEL. As the most popular exhaust valve material, it has excellent performance characteristics at elevated temperatures. With additional heat treating and manufacturing processes, this material can be upgraded to become the ultimate choice for exhaust usage. With different processes, it makes for a good quality, inexpensive intake valve as well.
21-4N: An Austenitic stainless steel similar to 21-2N, except for a greater nickel content (4%), used as an alternate to 21-2N.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

Is this just one valve that ever failed? What was the analysis on the valves that have survived?

" We had one bar of another totally inferior alloy that had gotten mixed in with the run on the Nimonic 80's."

The story of a totally unexpected valve failure ( breakage, not cracking) is aboout half way down the page here -
http://www.vv.corvair.org/Library/benzinger.htm
Toward the end of the paragraph that starts "The turbocharger is not only interesting to me... "