I hope this gives you a start in your research; 

The answer to this partly depends on whether you are interested in the stress analysis or the damage calculations.  

Before being called NH this was 

ASME BPVC Code Case Nuclear Components N-47.  

The version of N-47 which I have access to is 1st July 1986

Before N-47 there was Code Case 1592

Another document worth getting hold of is:

Criteria for Design of Elevated Temperature Class ! Components in Section III of the ASME B&PV Code, November 1974. 

The Chairman of the committee that wrote this report was (Bob) R I Jetter. Bob's committee was responsible for both N-47 and NH so he is the person to talk to.  

If you can contact Bob I am sure he would be able to give a more complete answer to your question  

In addition, if you can get hold of a copy there is a lot in ORNL-5073, "Time-Dependent Fatigue of Structural Alloys A General Assessment (1975).

The authors were

L F Coffin Jr.

A E Carden

S S Manson

L K Severud

W L Greenstreet

You will find a lot of references if you follow the links

https://www.osti.gov/scitech/

https://scholar.google.co.uk/scholar?q=author%3A%22ri+jetter%22&btnG=&hl=en&as_sdt=0%2C5

https://www.researchgate.net/researcher/2057175704_R_I_Jetter

Hi Michael William.

Thank you so much for your reply and the information you gave me. It will most likely help me now when I know where to start searching. I have not been able to find a copy of ORNL-5073, "Time-Dependent Fatigue of Structural Alloys A General Assessment” (1975) yet but I will continue to search for it.

Time and cycle fraction summation method is used in the ASME BPVC section III subsection NH, and as explained in the first post by Mr Michael William Spindler it is taken from code case N-47 rules (and is also available in the French RCC-MR) which was for the design and construction of high temperature nuclear plant components. This approach is based on the superposition of the Miner Palmgren linear fatigue damage accumulation model and linear creep damage accumulation model, proposed by Robinson. The linear interaction of these two damage components was firstly proposed by Taira (Taira, S., Lifetime of Structures Subjected to Varying Load and Temperature, Creep in Structures. Academic Press, New York, 1962), so this method is also called Taira method.

Thank you, Naveen Bhatt. I will see if I can find the paper from Tiara, S. I really appreciate the information.