الفهرس 
IntroductionOnly 14 pages are availabe for public view
 |  | from | 146 |  |  |
| | | from | 146 | | |
Abstract
Superior alloys ar, cobalt- and nickel- base alloys that contain chromium for resistance of oxidation and hot corrosion and other elements for strength at elevated temperatures.
Generally superior alloys exhibit good resistance to corrosion, creep, fatigue, thermal shock, impact, cavitation and erosion. In addition they must have good fracture properties, forming characteristics and weldability. Conventional superior alloys can be strengthened by either solid solutionstrengthening or precipitation hardening.
Iron-base superior alloys are nominally defined as those alloys that have iron as major constituent and are hardened by carbides or intermetallic precipitates.
Alloy 800 is one of the heat and corrosion resisting iron-base alloys. It was developed in 1949 by the International Nickel Company Inc. The excellent oxidation resistance and fabrication characteristics demonstrated by this alloy caused it to be chosen as a construction material for high temperature components operating under extremely arduous conditions. It is extensively used in petrochemical processing, gas transfer piping, superheater, steam and nuclear power equipment.
Alloy 800 is characterized by a ductile, fully austenitic structure. It imparts high strength combined with good resistance to oxidation, carburization and corrosion in many aggressive industrial environments at elevated temperatures.
Regarding its fabrication, the most important factor to be considered is its weldability . As all austenitic struc-can be minimized by correct welding procedure and appropriate welding conditions.
In order to thoroughly investigate this problem, a mean for quantitative measurement of hot cracking tendency is required. Also a need for a crack index to quantity hot cracking susceptibility has to be considered.
In this work the full design and construction of a variable restraint (varestraint) testing machine has been carried out . Cracking susceptibility of alloy 800 was then studied in detai. Two important crack indices were found out, namely; threshold strain and the critical strain rate to time.
In addition , simple and rapid hot cracking tests were selected to be applied in industrial field . Also weld-repair conditions for alloy 800 were investigated.
Finally , based on experimental results, a proposed mechanism of hot cracking phenomenon was given.