African Fusion March 2015

Microstructural study of turbine rotors

Also delivered at last year’s IIW International Assembly and Conference, this paper details procedure development for welding turbine rotors for steampower plant using creep resistant rotor steels and the hot wire GTAW welding process. Microstructural study of trial weld joints for steam turbine rotors Dagmar Jandová and Josef Kasl: Research and Testing Institute, Czech Republic

T rial weld joints were made using a special station for welding large forgings of up to 135 tonnes in weight for use in the production of combined rotors for steam power plants. The hot wire GTAW process was applied for joining various low and high-alloyed creep resistant steels (28CrMoV 4-9, 27NiCrMoV 15-6, COST F andCOST FB2). Different filler metals together with the application of interlayers were used for individual weldments and the best variants were se- lected on the basis of mechanical testing andmicrostructural investigations. Weld joint designed for creep conditions underwent creep testing at temperatures up to 650 ° C. Fracture surfaces of ruptured cross-weld specimens were analysed using a scan- ning electron microscope, and microstructures of individual zones of weldments were investigated using methods of light scanning and transmission electron microscopy. The critical zones were determined from a point of view of creep failure in relation to temperature and stress conditions. Introduction Large rotors for fossil fired power plants are produced as one piece from a huge ingot. Their weight is very high and it is dif- ficult to forge such big components and subsequently carry out heat treatment. In addition, different material properties are desired for high-pressure, intermediate and low-pressure parts of these rotors, which are exposed to different steam conditions. All these disadvantages can be solved by adopt- ing combined rotor designs, which consist of several parts made of different steels. Individual pieces can be heat treated separately, joined together by welding and then heat treated in special furnaces separated by walls. Using this technology, the properties of the rotor gradually change between the low

pressure and the high pressure parts. The weld joints, however, are usually susceptible to pre- mature failure. Therefore, the selection of the base materials andconsumables, aswell as theweldingprocess andpostweld heat treatments have to be optimised based on the results of proper weld joint testing. The paper deals with investigations involving several trial weld joints, which are designed for production of combined rotors for fossil fuel power plants using a special station for automaticwelding of large forgings up to 135 tonnes inweight. Creep resistant low alloyed as well as new high chromium steels for applications at ultra-supercritical conditions, which were developed within the frame of the European COST pro- grammes [1], were used for the production of the trial welds. Welding procedures and experimental materials Several similar and dissimilar weld jointswere produced using the sameweldingmethod – hot wire gas tungsten arc welding (GTAW) into a narrow gap. Low alloyed and creep-resistant high chromium steels were used as the base materials. Two discs of the basematerials with a diameter of about 600mm, a length of 200mmand a wall thickness of 120mmwere joined together using different consumables. Welding was carried out in the PC position – around the vertical longitudinal axis of the discs. A list of the trial weld joints and materials used for their production is given in Table 1. An interlayer was de- posited on one of the base materials of B and C weld joints using the above mentioned welding method and special heat treatments were carried out before the final joints were completed. Macrostructures of individual weld joints are shown in Figure 1.

Weld joint

Base material BM1

Base material BM2

Weld metal NiCrMo 2,5-IG NiCrMo 2,5-IG

Interlayer

Similar weld A

27NiCrMoV 15-6 27NiCrMoV 15-6

27NiCrMoV 15-6 28CrMoNiV 4-9

-

Dissimilar weld B1

Union I CrMo 910 (2.5Cr1Mo)

Dissimilar weld B2 Dissimilar weld C

27NiCrMoV 15-6 27NiCrMoV 15-6

28CrMoNiV 4-9

NiCrMo 2,5-IG NiCrMo 2,5-IG

P24-IG (2.5Cr1MoVNb) P24-IG (2.5Cr1MoVNb)

X14CrMoVNbN 10-1 (COST F) X13CrMoCoVNbN 9-1 (COST FB2) X13CrMoCoVNbN 9-1 (COST FB2)

Dissimilar weld D1

X14CrMoVNbN 10-1 (COST F) X14CrMoVNbN 10-1 (COST F)

Thermanit MTS 3

-

Dissimilar weld D2

Thermanit MTS 616

-

Table 1: List of weld joints investigated.

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March 2015

AFRICAN FUSION