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Research Papers

# The Impact of Weld Metal Creep Strength on the Overall Creep Strength of 9% Cr Steel Weldments

[+] Author and Article Information
Peter Mayr1

Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139; Institute for Materials Science and Welding, Graz University of Technology, Kopernikusgasse 24, 8010 Graz, Austriapeter.mayr@tugraz.at

Stefan Mitsche

Institute for Electron Microscopy and Fine Structure Research, Graz University of Technology, Steyrergasse 17, 8010 Graz, Austria

Horst Cerjak

Institute for Materials Science and Welding, Graz University of Technology, Kopernikusgasse 24, 8010 Graz, Austria

Samuel M. Allen

Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139

1

Corresponding author.

J. Eng. Mater. Technol 133(2), 021011 (Mar 07, 2011) (7 pages) doi:10.1115/1.4003112 History: Received July 06, 2010; Revised November 17, 2010; Published March 07, 2011; Online March 07, 2011

## Abstract

In this work, three joints of a X11CrMoWVNb9-1-1 (P911) pipe were welded with three filler metals by conventional arc welding. The filler metals varied in creep strength level, so that one overmatched, one undermatched, and one matched the creep strength of the P911 grade pipe base material. The long-term objective of this work was to study the influence of weld metal creep strength on the overall creep behavior of the welded joints and their failure mechanism. Uniaxial creep tests at $600°C$ and stresses ranging from 70 MPa to 150 MPa were performed on the cross-weld samples of all three welds. A total creep testing time of more than 470,000 h was accumulated. The longest running sample achieved a time-to-rupture of more than 45,000 h. Creep testing revealed that the use of undermatching weld metal led to a premature fracture in the weld metal at higher stress levels. Compared with undermatching weld metal, the use of matching and overmatching filler materials increased the time-to-rupture at high stress levels by 75% and 33% at lowest stress levels. At typical component stresses below 100 MPa, all samples failed in the grain-refined heat-affected zone by characteristic type IV failure. For investigations of the failure modes, cross sections of fractured samples were investigated by optical light microscopy, scanning electron microscopy, and electron backscatter diffraction. The mechanism of weld metal creep failures and type IV creep failures is discussed in detail.

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## Figures

Figure 1

Schematic of the welding setup of four P911 pipe sections using three different filler metals differing in creep strength

Figure 2

Weld groove geometry (unit: mm) and applied welding sequence for all three butt welds (13)

Figure 3

Schematic of the specimen location within the butt-welded joint (left) and parametric drawing of a creep specimen with ridges for integral strain measurement. Reference length L0 and diameters are adjusted according to the applied force and the desired stress level.

Figure 4

Creep rupture strength of P911 cross-weld specimens at 600°C compared with the mean P911 base material creep strength (11) and its ±20% scatter band (top). Accumulated creep strain as a function of time for low, medium, and highly stressed specimens of the three welds (bottom).

Figure 5

Creep fracture (a) in the weld metal of an undermatching specimen tested at 120 MPa at 600°C (873 K) and (b) in the heat-affected zone of the equivalent matching specimen. The white arrow in (b) marks a region in the left HAZ where damage has progressed to severe cracking.

Figure 6

Optical micrographs of a weld metal failure in the undermatching weld specimen tested at 600°C (873 K) at 120 MPa (UM120) for 8869 h revealing fracture and secondary cracking along the columnar grains of solidified weld metal

Figure 7

Optical micrograph of a weld metal failure in the undermatching weld specimen tested at 600°C (873 K) at 130 MPa (UM130) for 8982 h, showing (a) fracture in the refined weld metal by creep cavitation preferably along prior austenite grain boundaries (b) perpendicular to the loading direction.

Figure 8

(a) SEM image of void distribution in a matching series specimen (M120) fractured after 13,945 h at 120 MPa. (b) SEM image at higher magnification of the fine-grained region in specimen M120.

Figure 9

Inverse pole figure of an area close to the failed region in specimen M120. Clusters of black pixels indicate the locations of voids.

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