Abstract
The paper is dealing with the carbide reactions occuring during the PWHT (Post Welding Heat Treatment) of weld metal of conventional and enhanced chemical composition. Investigated were initial (as welded) microstructures of martensite, upper and lower bainite and the effect of heat treatment in the temperature range 650-720 °C with the tempering parameter (TP) values from 18.46 to 21.36 on the microstructure, hardness, notch toughness at -18 °C and carbides precipitation. Compared to classical chemical composition we have investigated the effect of different carbon and vanadium contents, carbon ranging from 0.10 to 0.19% and vanadium from 0.01 to 0.37%. The low carbon upper bainite welds and lower bainite welds have microstructure composed of granular bainite which contains up to 15% of M-A (Martensite-Austenite) constituent exhibiting high carbon content (over 1%) and some portion of retained aus-tenite. During PWHT the supersaturated solid solutions in welds as well as the M-A constituent is decomposing into ferrite-cementite mixture. First carbide to precipitate in each weld was cementite, M₃C. Later on this carbide was either dissolving on account of newly precipitated other carbides, or transformed in situ to other carbides like M₆C. The precipitation sequence observed in conventional composition welds fit well the published datas and can be summarised as follows:

θ →	θ + M2C + M23C6 → M23C6 + M6C
	↓
	(M7C3)

Alloying the weld by vanadium changes, however, this precipitation sequence. Instead of MO₂C carbide the precipitation of vanadium carbide, VC was observed. In welds with 0.27% V the precipitation sequence observed was as follows:

θ →	θ + MC → MC + M23C6 → MC + M23C6 + M6C
	↓
	(M7C3)

At highest vanadium content (0.37%) the only precipitate observed at highest values of tempering parameter was vanadium carbide, MC. Increasing carbon content increased the amount of precipitates but didn't affect the precipitation's sequence. Vanadium carbide can contain Cr. The presence of Cr was observed also in M₂C carbide. M₂₃C₆ can dissolve V and Mo. Precipitation of MC results in remarkable hardening. The precipitation was influenced very much by weld microseg-regation.

Key words
carbide and vanadium reactions, PWHT (Post Welding Heat Treatment), precipitation, microstructures

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