MICROSTRUCTURAL DEGRADATION OF BOILER HEADERS STEELS UNDER LONG TERM EXPOSURE TO HIGH TEMPERATURE

Abstract

This paper describes the process of degradation of the structure of the inlet header of the 
intermediate reheater (MPr) 2 input header at the boiler of Unit 7 of the Thermal Power Plant 
Kakanj, determined on the basis of tests of replicas and hardness at characteristic places of the 
header. Headers, with minor delays, have been in continuous operation since 1988. Hardness was 
tested at replica pickup sites, on the basis of which the correlation between structure degradation 
and hardness drop resulting from exploitation during the long-term high-temperature operation 
was estimated. 
The change in hardness is certainly one of the most reliable indicators of the structure degradation 
degree, which is directly related to the service life of the installed components. The subject of the 
test was the inlet header of the intermediate heater made of low alloy steel 15Mo3, consisting of 
ferrite-pearlite microstructure, and having calculated values for the allowed hardness in the range 
from 133 to 176 HB. The structure degradation assessment was performed according to VGB-S-
517 by comparing the microstructure of the replicates with the corresponding standards, and the 
hardness was tested according to ASTM A 956/1996-method A. 
The examination has shown that in the exploitation life achieved so far, there has been a partial or 
complete degradation of the structure, which is reflected in the initial and delayed coagulation of 
the perlite lamellae (Grades 2a and 3a). The hardness at the test sites is at or below the lower than 
the prescribed 133HB. The tests confirmed that there is a good correlation between the degree of 
degradation of the structure and the hardness, which can be used in assessing the condition and 
planning the extent of testing on critical components. 
1. INTRODUCTION 
As part of the regular tests of critical components of the