African Fusion March 2015

Microstructural study of turbine rotors

AFRICAN FUSION

March 2015

steel and different interlayers were investigated – B1 and B2

weldments with interlayers of 2.5Cr1Mo steel (Union I) and

2.5Cr1MoVNb steel (P24) respectively. The Interlayers were

depositedon the 28CrMoNiVbasematerial. The temperatureof

postweld heat treatment (PWHT) of the variant B2was slightly

higher than that of the B1 variant.

Microstructures as well as mechanical properties of both

variants are similar. Themicrostructure of theweldments cor-

responded to tempered bainite that can be described based

on LM and SEM observations as carbides spread more or less

homogeneously in a ferritic matrix. The density of the carbide

particles was different in individual zones of the weld joint, ie,

it was higher in the base materials than in the weldmetal and

interlayers. The size of precipitates was higher in 28CrMoNiV

base material than in 27NiCrMoV base material. The higher

temperature of tempering in the case of 28CrMoNiV steel was

the cause. No significant coarsening of the structure in the

heat affected zones (HAZ) was observed in either of the base

materials.

Microstructures in of B2 variant were more tempered in

compliance with the rather higher temperature of PWHT in

comparison to the B1 variant. Nevertheless, precipitates in

the P24 interlayer of the B2 weld joint were finer and more

regularly spread than in the Union I interlayer of the B1 weld

joint. In addition, carbon enrichment at the interface of the

interlayer and the weld metal was observed in the B1 variant.

This fact revealed itself in the significantly higher density of

precipitates in the interlayer near the fusion line (Figure 5).

The microstructure of the B2 weld joint seemed to be better

than the B1 weld joint. The crossweld hardness profile (Fig-

ure 6) showed more balanced hardness values in the case of

the B2 variant.

Mechanical tests revealed that the interlayer was the

weakest zone of both theweld variants. Fracture during tensile

tests occurred in central parts of the interlayer. Both B1 and B2

variants satisfiedmechanical requirements. Due to its slightly

better mechanical properties and more favourable micro-

structure, the B2 weld joint was selected for real production.

Dissimilar weld joint C

Themicrostructure of dissimilar weld joint Cof 27NiCrMoV and

COST F steels with NiCrMo2.5 weld metal and P24 interlayers

is very heterogeneous. After PWHT the microstructure of the

COST F base material corresponded to temperedmartensite,

while other parts of the weldment consisted mainly of tem-

pered bainite with possible small amount of martensite in the

27NiCrMoV base material. Seven main zones were observed

in the cross section of the weldment – two base materials

(Figures 7 and 8), three heat affected zones, the interlayer

(Figure 9) and the weld metal (Figure 10). Bands of coarse-

grained and fine-grained structures could be distinguished

within the interlayer and in the weld metal. The structure of

the COST F basematerial unaffected bywelding is significantly

coarser than that of 27NiCrMoV basematerial. Near the fusion

line at the interface between the high-alloyed COST F base

material and the low alloyed P24, carbon enriched bands

were observed. As could be expected, carbon enrichment oc-

curred in the basematerial, which resulted in a local increase

in hardness (Figure 11).

The crossweldhardness profile is shown in Figure 12. Mean

hardnesses of the COST F base material and the weld metal

were the same – 265 HV. Hardness of 220 HV and 300 HV were

Figure 5: LM micrographs of Weld B1 showing a carbon enriched interface between

the weld metal and the interlayer.

Figure 6: The crossweld hardness profile of Weld B after the PWHT: a) B1 variant, b)

B2 variant.

Figure 7: LM micrograph showing the

Bainitic structure in Weld C’s 27NiCrMoV

base material.

Figure 8: LM micrograph of Weld C

showing a tempered martensite of the

COST F base material.

Figure 9: LM micrographs of Weld C showing the Bainitic structures of the

interlayer: a) coarse-grained and b) fine-grained microstructures.

Figure 10: LM micrograph of Weld C

showing the Bainitic structure of the

weld metal.

Figure 11: LM micrograph of the COST F/

P24 base material for Weld C showing

carbon enrichment at the fusion line.

mechanical values for this weld joint were satisfied.

Dissimilar weld joint B

Two variants of the weld joint of 27NiCrMoV and 28CrMoNiV

steelswith the sameweldmetal on thebasisof 2.5Cr0.4Mo2.5Ni