Residual stress measurements in laser clad repaired low pressure turbine blades for the power industry

Materials Science and Engineering A 437 (2006) 70–74

Residual stress measurements in laser clad repaired low pressure turbine blades for the power industry P. Bendeich a,∗ , N. Alam b , M. Brandt c , D. Carr a , K. Short a , R. Blevins a , C. Curfs a , O. Kirstein a , G. Atkinson a , T. Holden d , R. Rogge e a

Australian Nuclear Science and Technology Organisation„ Lucas Heights, Menai, NSW 2234, Australia b CSIRO Manufacturing Science and Technology, 32 Audley St., Woodville, SA 5011, Australia c IRIS Swinburne University of Technology, 533-545 Burwood Rd., Hawthorne, Vic. 3122, Australia d Northern Stress Technologies, 20, Pine Point Rd., Deep River, Ont., Canada K0J 1P0 e National Research Council, Neutron Program for Materials Research, Chalk River Laboratories, Chalk River, Ont., Canada KOJ 1JO Received 26 June 2005; accepted 15 April 2006

Abstract Low pressure turbine blades in power stations suffer from leading edge erosion damage due to water impingement. In an effort to extend the life of these blades, repair of the eroded regions has been proposed using laser cladding with Stellite material. However, the addition of Stellite results in residual stresses being generated in the parent metal due to contraction during cooling and differences in thermal expansion between the two materials. In this work test coupons and laser clad blades were examined for residual stresses using both the L3 diffractometer at the NRU reactor, Chalk River, Canada and the TASS strain scanner at ANSTO’s HIFAR reactor, Lucas Heights, Australia. In addition XRD results were used to measure residual stresses on the surface of the blade to complement the neutron measurements. An FEA model of a simplified weld was used to explain some of the results. Crown Copyright © 2006 Published by Elsevier B.V. All rights reserved. Keywords: Low pressure turbine blade; Laser cladding; Post-weld heat treatment; Residual stress; FEA

1. Introduction Low pressure (LP) turbine blades in power stations operate at over 3000 rpm generating almost supersonic speeds near the blade tips. As a result erosion damage of the leading edge due to impingement of condensing water results in premature retirement of these expensive blades. In an effort to extend the LP blade life a technique for in situ repair of the eroded regions is currently being developed using laser cladding with a hard wearing Co-based alloy, Stellite 6. This work is being co-sponsored by the Cooperative Research Centre for Welded Structures (CRC-WS) and eleven power stations from around Australia. The CRC-WS was established and is supported under the Australian Government’s Cooperative Research Centres Program.

∗

Corresponding author. Tel.: +61 2 9717 9102; fax: +61 2 9543 7179. E-mail address: [email protected] (P. Bendeich).

Some of the major advantages of laser cladding are that of localised heating combined with accurately controlled weld metal deposition. However, the addition of a Stellite 6 clad layer results in the generation of residual stresses in the weld region due a number of complex mechanisms. These include differences in thermal expansion between the Stellite and parent metals during cooling, thermal strains resulting from differences in heating/cooling at differing locations and the directional stiffness within the sections present. In this work residual strains were measured in test coupons in the transverse, longitudinal and normal directions with respect to the weld path. A simplified FEA model of a single bead on a coupon is used to explain the results of the strain measurements near the Stellite 6 heat effected zone (HAZ) interface. This was followed by measurements in both an “as-welded” laser-clad LP blade and a post-weld heat treated (PWHT) LP blade. Measurements in the LP blades were made in six directions, three of which closely correspond with the weld path transverse, longitudinal and normal directions with the remaining three being off axis measurements for principal stress evaluation. The X-ray

0921-5093/$ – see front matter. Crown Copyright © 2006 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.04.065

P. Bendeich et al. / Materials Science and Engineering A 437 (2006) 70–74

diffraction (XRD) method was used to complement the neutron diffraction work by providing residual stress measurements on the rear surface of the blade. The results of these experiments are being used to estimate the likely effects on the fatigue life of the blades. A number of refurbished LP turbine blades are currently under trial with no adverse effects reported to date.

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perature dependant material properties for Stellite and SS420 were used [3,4]. A single cladding bead 1.7 mm wide × 1.0 mm deep was generated at the centre of a plate 30 mm wide × 10 mm deep. A temperature of 1500 ◦ C was imposed uniformly to the cladding bead for 0.10 s then removed and the model was allowed to cool by conduction only. 2.2. LP blades

2. Materials and experimental Residual stress measurements were carried out using the neutron diffraction method for both the test coupons and LP blades. The advantage in using neutrons over XRD measurements is that strains can be measured at depths of centimeters through samples rather than being confined to the surface. Detailed descriptions of the neutron diffraction method are available in the literature [1] and will not be repeated here. Both the neutron diffraction and XRD measurements at Chalk River and ANSTO utilised monochromatic beams diffracting from the (2 1 1) crystallographic plane in the sample. This plane has been reported to have similar elastic properties to the average bulk material [1]. The value used here for the elastic modulus was 208 GPa with a Poisson’s ratio of 0.28. Measurements in the Stellite 6 cladding layer were not possible due to the high neutron absorption of this material.

Residual stress measurements were made on two LP blade tips ∼300 mm long and ∼100 mm wide on TASS (The Australian Stress Scanner) on the HIFAR reactor at Lucas Heights. The width of the cladding layer for both LP blades was ∼17 mm from the leading edge. The first LP blade was measured in the “as welded” state at a point where the parent blade material was ∼4 mm thick beneath a ∼1 mm thick Stellite 6 clad layer. The second LP blade had been post-weld heat treated (PWHT) at 625 ◦ C for 2.5 h and strains were measured at a point where the parent blade material was ∼3 mm thick beneath a ∼0.5 mm thick Stellite 6 clad layer (Fig. 2). The compositions of both blades were almost identical and were identified as a martensitic stainless steel closely matching the material specified in ASTM A 768-95 [5]. In all neutron diffraction measurements of the LP blades the gauge volume used was 1.0 mm × 1.0 mm × 5.0 mm. The transverse, normal, and axial directions referred to are in relation to

2.1. Test coupons Test coupons were prepared by cladding a ∼25 mm × 25 mm × 0.8 mm patch of Stellite 6 onto type 420 stainless steel (SS420) plate ∼10 mm thick. Coupons measuring 30 mm × 30 mm were then cut from the section of plate containing the Stellite 6 patch (Fig. 1). Residual stress measurements on the coupons were made on the L3 diffractometer at Chalk River. The measurements used a gauge volume of 0.3 mm × 0.3 mm × 20 mm with strains being measured transverse, longitudinally and normal to the welding direction. Stress free planar spacing (d0 ) measurements were made on stress free pins taken from an identical coupon that did not have a cladding layer. In order to understand the process of residual stress generation, a 2D coupled transient thermal, plane strain model was produced using the FEA package ABAQUS [2]. Generalised tem-

Fig. 1. Test coupon 30 mm × 30 mm × 10 mm. Cladding beads ∼1.0 mm wide × ∼0.8 mm high.

Fig. 2. Clad section of LP blade showing strain directions in relation to weld path.

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Fig. 3. CMM generated profile of leading edge of PWHT LP blade. Distances from leading edge indicate through thickness regions measured for residual stress (8, 13.5 and 19 mm.

the blade and weld path and are indicated in Fig. 2. Throughthickness strain measurements in the as-welded LP blade were performed at locations 8 mm (A), 13.5 mm (B), and 19 mm (C) from the leading edge. These locations correspond with the mid-clad region, the rear of the clad region and adjacent to the clad region (as labelled in Fig. 3). Measurements were only performed at A in the PWHT LP blade. Measurements in six directions were performed at each point so that the principal stresses could be determined. For the “as-welded” LP blade stress free d0 values were measured in wire cut pins ∼0.75 mm × ∼0.75 mm × 10 mm taken from below the Stellite 6 clad layer. These pins, taken at specific depths, were glued together to form a measurable “strain free” sample. No noticeable variation in d0 was observed as a function of depth. Similarly, stress free d0 measurements for the PWHT blade were taken from pins taken from a region adjacent to the clad area. 3. Results and discussion 3.1. Test coupons The strains measured in the parent metal are shown in Fig. 4. The stresses calculated from the strains in the parent metal

Fig. 5. Coupon stress measurements in parent metal. Depth under the Stellite 6 layer indicated.

(Fig. 5) within ∼0.5 mm of the Stellite 6 interface appear to be in compression. A steep stress gradient occurs towards a tensile stress peaking at ∼1.3 mm below the interface before reversing back into compression through the majority of the thickness before translating again to a strong tensile stress near the back face of the coupon. The relatively high tensile stresses near the back face of the coupons reflect the dominance of the global bending force generated by the contraction of the Stellite and HAZ at the coupon/clad interface. Examination of the test coupon results in combination with the FEA modelling results shows clearly the significance of cladding a dissimilar material, Stellite 6, onto a steel substrate at the interface region, namely large stress gradients are generated at the interface is a result of the dissimilar thermal expansion coefficients of the two materials. This is particularly marked between 600 ◦ C and room temperature [3,4] where the Stellite 6 has thermal expansion values ranging between 1.4 and 6.5 times that of the parent metal, respectively. During heating the Stellite 6 layer behaves as a soft compliant material. As heating continues the substrate softens in the HAZ relieving the strain. During cooling the Stellite 6 and HAZ are simultaneously quenched by the bulk of the substrate. As this occurs the Stellite and HAZ layers begin to contract generating a global tensile stress balanced by compressive stresses in the substrate. As the Stellite 6 has a higher coefficient of thermal expansion there is also a local stress gradient generated at the interface between the Stellite and the HAZ. This local effect was seen in the simplified FEA results (Fig. 6) and explains the dip in measured stress near the interface. Global stresses in the FEA model are not representative as only a single bead was considered, hence the differences in magnitudes of the stresses. 3.2. LP blades

Fig. 4. Coupon strain measurements in parent metal. Depth under the Stellite 6 layer indicated.

The stresses calculated from the strain measurements for the transverse direction in the “as-welded” LP blade are shown in Fig. 7. These results show the highest stresses to be located at point B, corresponding to the edge of the laser clad region. It

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Fig. 6. FEA transverse stress profile through thickness of parent metal.

Fig. 7. Transverse stress measurements through “as welded” LP blade thickness (0 mm is the back face and Stellite 6 interface is at 3.1 mm for A and B only as per Fig. 3). Distances indicated for A, B and C are from the leading edge of the blade.

is clear from the stress gradient leading to the free surface at the rear of the blade that a large global bending stress has been generated by the cladding process. While the stresses in the LP blade are much higher, the trend in the result is consistent with the results of the coupon measurements. The local interfacial stresses measured on the coupons were not evident in the LP blade measurements, presumably due to the much large gauge volume. A comparison of transverse stresses at position A between the “as-welded” and PWHT LP blades, Fig. 8, clearly indicates the

Fig. 8. Comparison of transverse stress measurements between “as-welded” and PWHT stress profiles at location A (8 mm from LP blade leading edge).

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Fig. 9. Comparison of transverse, axial, and normal stresses with the calculated principals at location B (13.5 mm from LP blade leading edge).

benefit of the heat treatment in minimising tensile stress. While it is acknowledged that the two LP blades examined here have differing clad layer and substrate thicknesses, the significance of the PWHT results is that there are not the stress gradients through the parent metal that were present in the “as-welded” test coupons and “as-welded” LP blade samples. This indicates that the PWHT was effective in minimising the stresses imparted by the cladding process and implies that the stress in the Stellite 6 layer has also been considerably reduced. Any remaining stress in the PWHT LP blade due to the cladding process will be local to the interface and entirely due to the differences in thermal expansion between the parent and cladding layer as the blade is cooled slowly from 625 ◦ C. A comparison of the transverse, axial, and normal stresses with the measured principal stresses, Fig. 9, indicates that the directions chosen for measurement on this complex shaped component closely matched those of the principal directions. 3.3. XRD measurements The XRD measurements of the rear surface of the LP blade indicated a compressive stress of between −420 and −500 MPa regardless of the location on the blade. These results were not consistent with the neutron diffraction results. The reason for this was that there were compressive surface stress due to grit blasting after the cladding process was complete. Electro pol-

Fig. 10. XRD stress surface stress measurements with successive layers of electropolishing.

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ishing of the surface showed the compressive layer to be only ∼100 ␮m deep, Fig. 10.

compressive surface stress is undoubtedly reduced or eliminated by the PWHT it could just as easily be performed after PWHT.

4. Conclusions Acknowledgement Residual stress measurements in Stellite 6 clad test coupons combined with the FEA results highlight the effects of interfacial stresses when welding dissimilar materials with differing rates of thermal expansion. The major conclusion from the LP blade measurements was that significant tensile and compressive stresses are generated in the repaired region of the blade both in the Stellite 6 cladding layer and in the parent metal. In both the Stellite 6 and parent metal tensile stresses are located on the surface which would aid crack initiation in these regions under fatigue conditions. Analysis of a PWHT LP blade shows that the heat treatment minimises the magnitude of theses stresses thereby greatly reducing the possibility of crack initiation. The XRD results indicated that grit blasting of the blade surface generates a compressive stress to a depth of ∼100 ␮m which would be expected to aid in the suppression of crack initiation due to fatigue. While the

The authors would like to thank Thomas Gnaeupel-Herold, of NIST Center for Neutron Research, for assistance in resolving issues relating to principal stress determination. References [1] M.E. Fitzpatrick, A. Lodini (Eds.), Analysis of Residual Stress by Diffraction using Neutron and Synchrotron Radiation, Taylor and Francis, New York, 2003. [2] ABAQUS/Standard User’s Manual (2003) volumes 1 and 2, and ABAQUS Theory manual, Version 6.4., 2003. [3] M.F. Rothman, High Temperature Property Data: Ferrous Alloys, ASM International, 1989, pp. 9.83–9.85. [4] A.P. Wu, J.L. Ren, Z.S. Peng, H. Murakawa, Y. Ueda, J. Mater. Process. Technol. 101 (2000) 70–75. [5] ASTM A 768-95, Vacuum-treated 12% Chromium Alloy Steel Forgings for Turbine Rotors and Shafts.