African Fusion March 2015

Figure 1: Macrostructures of trial weld joints.

Testing according towelding standards EN288-2,3was car- ried out. Mechanical properties, microstructure and results of fractographic analyses were used for assessment of individual weldments. Mechanical properties satisfied requirements. Special attentionwas paid to critical zones of weld joints from a point of view of crack propagation. Microstructural analyses of individual weldments were carried out using light (LM), scanning (SEM) and transmission electron microscopy (TEM) in a range that was necessary for optimisation of the production process. Crossweld hardness profiles of individual weld joints were compared with results of microstructural analyses. Hardness wasmeasured near the face of eachweld joint, in the centre and across the root. Hard- ness profiles in the Figures shown are all taken at the central parts of weldments. Results Similar weld joint A The base material of 27NiCrMoV 15-6 steel underwent special heat treatment in order to obtain mechanical properties near those of the weld metal. The temperature of final annealing was as high as possible, but lower than that which could cause recrystallisation with an associated undesirable drop in strength properties. The appropriate temperature was selected on the basis of mechanical properties and detailed microstructural analysis using TEM. The microstructure of the base material condition before welding corresponded to a tempered mixture of bainite and martensite. Relatively coarse particles of cementite and finer Cr 7 C 3 carbides were observed in bainitic regions (Figure 2a) while only very thin precipitates occurred in tempered martensitic laths. A higher dislocationdensitywas characteris- tic formartensite (Figure 2b). If the annealing temperaturewas kept high, then small crystallised grains appeared (Figure 3). The goal of TEM analysis was to find an optimal annealing temperature in order to avoid recrystallisation. The crossweld hardness profile after optimised heat treatment is shown in Figure 4. Hardness of the weld metal was lower (from 230 to 241 HV10) than that of the base ma- terial (from 270 to 285 HV10). The local maxima in the heat affected zones near the fusion line did not exceed the value of 314 HV10. In compliance with the hardness profile, tensile tests ruptured in the weld metal, even though the desired

Figure 2: TEM micrographs of Weld A base material: a) bainitic structure, b) bainite with tempered martensite in the central parts.

Figure 3: TEM micrograph of Weld A showing small recrystallised grains.

Figure 4: The crossweld hardness profile of Weld A after PWHT.

25

March 2015

AFRICAN FUSION