Austenitic stainless steel E309L is frequently used for repairs on 13 %Cr-4 %Ni martensitic stainless steel blade runners. However, very high residual stresses are found at the surface of the welds, thus reducing their fatigue resistance. The use of hammer peening to increase the fatigue life of E309L welded on UNS-S41500 base metal plates is investigated. The welds used in this research are designed to replicate those found in an in situ blade runner repair. Residual stress measurements using the contour method have confirmed the high stresses caused by FCAW welding of E309L, finding stresses near the ultimate tensile strength of the material. Hammer peening of the surface beads effectively creates high compressive stresses at the weld’s surface up to 3 mm depth. High compressive stresses were also found at the surface of welds using 410 NiMo filler metal, the metal used for blade runner fabrication. The stresses are however not as uniformly distributed as in the hammer peened E309L. This type of filler metal requires a post welding temper which greatly reduces the residual stresses.
At all stress levels of alternate bending fatigue testing, the hammer peened specimen showed great improvement over non hammer penned samples. At 330 MPa, peened spécimens showed an average life increase of 585%. Even after peening, the fatigue life of E309L is much lower than the UNS-S41500 base metal. It could however be greater than the fatigue life of CA6NM steel also used for blade runners. For fatigue testing, the welds surfaces were milled and hammer peened to represent a blade runner repair. Those processes were done in varying sequences to see the potential effect on residual stresses, hardness and fatigue life. It was shown that peening the weld surface once after having milled the weld flush with the base metal allows for the best fatigue results. Not milling the surface beforehand still increases the fatigue life over non peened samples. Peening the surface multiple times reduces the near-surface compressive stresses and offers diminishing returns on fatigue life. It is therefore not recommended. Hammer peening increases hardness up to about 4 mm under the surface, regardless of the sequence. Hammer peened UNS-S41500 samples have shown that this material could benefit from peening.
Observations while testing have indicated that hammer peening seems to increase total fatigue life by slowing down crack propagation and not by inhibiting crack initiation.
| Date | 26 May 2016 |
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| Original language | French |
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| Awarding Institution | - École de technologie supérieure
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| Supervisor | Henri Champliaud (Supervisor), Zhaoheng Liu (Co-supervisor) & Jacques Lanteigne (Co-supervisor) |
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Curtat, J.-L. (Author),
Champliaud (Supervisor),
Liu (Co-supervisor) & Lanteigne (Co-supervisor),
26 May 2016Student thesis: Master's thesis › Master in Engineering: Mechanical Engineering