Abstract
	This article presents hardening of weld joints in 15 128, T23, T24 and T25 grade steels during extensive exposure at elevated operating temperatures. The monitored hardening relates to the supplementary precipitation of dispersed particles. Electron microscopy identified hardening particles MX in 15 128 grade steel. The affect of tempering temperature on mechanical and, in particular, plastic properties of weld joints was demonstrated. In the case of T23 and T24 grade steels, which should be used for manufacture of so-called membrane walls,  manufacturers are making efforts to completely leave out tempering after welding. However, achieved results show that the extent of secondary hardening is clearly dependent on application of tempering after welding. Non-tempered weld joints are subject to unacceptable increase of hardness during exposure to operating temperatures. The results of this research can be applied to increase the operating reliability of power engineering equipment manufactured from 15 128, T23, T24, or T25 grade steels.