The purpose of this work consists in using the method of incremental permeability to find the coercive force of the boiler tubes material and in correlating its value with the microstructure of the tubes and their residence time inside the boiler.

We have studied tube samples originating from a boiler that used lignite on black oil support as fuel, had 30 t/h steam rate, 15 bars steam pressure and temperature of 250°C.

The tubes were of ribbed type for bundle mounting, with o. d. of 61 mm, wall thickness of 6 mm, manufactured of K47 steel with the chemical composition: 0.21 wt. %C; 0.75 wt. % Mn; 0.37 wt. %Si; 0.043 wt. %P; 0.046 wt. % S.

The measurements were performed on both the witness sample and tube samples assayed from the boiler after 10, 000; 20, 000 and 30, 000 hours of operation respectively.

We performed measurements of electric resistivity, initial permeability (µ_i), maximum magnetic permeability (µ_max), remanent flux density (B_R). The magnetic measurements were done by using the hysteresis loop method. The coercive force was determined by the incremental permeability method at the frequency of 100 Hz.

The metallographic analysis of the samples was accomplished according to the standard procedure and initial attack.

The examination of the experimental data revealed that both the remanent flux density and the electric resistivity of the tube samples are only slightly influenced by the boiler tubes working duration and by the structural alterations occurring in the tube material [2]. The measurements of the initial and maximum permeabilities, through giving revealing results concerning the boiler tubes condition, are not adequate in this case. The parameter that is sensitive to tubes structural modifications and can be measured by a nondestructive procedure is the coercive force determined by means of the incremental permeability method.

The incremental permeability method can be used for the nondestructive examination of the boiler's tubes. For the tubes manufactured of K47 steel, having the o. d. of 61 mm and wall thickness of 6 mm, the emergency value of the coercive force established by incremental permeability method can be considered as45 Oe, above this value structural alterations occurring which can lead to blistering. REFERENCES

1. C. Jiles, "Review of magnetic Methods for Nondestructive Evaluation", Proceedings of the 6th Annual Conference on Properties and Application of Magnetic Materials, Chicago, May 20-21 (1987). 2. F. Borik, R. D. Chapman, Trans of the ASMI. III, 53, pp. 447- 463, 1967