ECCC TEST PROGRAMME AND DATA ASSESSMENT ON GTD111 CREEP RUPTURE, STRAIN AND DUCTILITY

GTD111, a creep resistant Ni-based superalloy developed by GE, is widely used in land-based gas turbine first stage blades. However, there is little published information on its creep properties and microstructure. The European Creep Collaborative Committee (ECCC) Working Group 3C consequently selected GTD111 as a model material for testing and complementary data assessment. The aim of this paper is to present the results from the ECCC test program and data assessment, and to compare equiaxed (EA) and directionally solidified (DS) material performance. Testing and metallographic laboratories from six European nations collaborated to produce strain monitored creep rupture data on four EA and DS materials out to beyond 10,000 hours within a wide range of temperatures, 850-950°C, and stresses, 293-99 MPa. Available (generally short term) results from other sources were also included in the compiled, small but viable, 51-test data set. Assessment was carried out by three different assessors using different tools and adopting different prediction models. Conventional ECCC post-assessment techniques and novel “back-fitting” methods were used to identify a preferred model. It was shown that assessing all the EA and DS data together can lead to non-conservative predictions for EA materials, but separating the two classes creates small data subsets which cannot be modelled effectively. As a pragmatic compromise, the DS data and those EA data which also showed good ductility were included in a final "ductile GTD111" assessment. The resulting creep rupture material models and rupture strength predictions are presented up to 3 times the longest test duration. It was then shown that the performance of lower ductility EA materials can also be predicted effectively with the "ductile" model by truncating the rupture time at the measured fracture strain. For this exercise, a creep strain model based on rupture and time to strain data was fitted. In parallel, microstructural examination was performed to characterize the damage modes involved in the low ductility failures. It was thereby shown that the creep rupture strength shortfall of an EA material compared to its DS equivalent is not a constant factor, but is primarily governed by the reduced creep ductility. Hence, the shortfall varies between different EA casts, and tends to become greater in the longer term.JRC.F.4-Innovative Technologies for Nuclear Reactor Safet

Zn-Al-Mg coatings : thermodynamic analysis and microstructure related properties

Demands for highly corrosion resistant coated steel are growing. As a result, Zn-Al-Mg coatings were developed. The possibilities of these coatings were investigated and the thermodynamics of the Zn-rich corner of the Zn-Al-Mg system were modelled. Different Zn-Al-Mg-coatings were produced and the microstructure was studied. Simulations of the solidification microstructures were carried out. The properties of the different coatings, like corrosion resistance and formability, were investigated. The thermodynamic model fairly accurately predicted the liquidus and transformation temperatures for low amounts of Al ( 0.2wt%Mg), a ternary Zn-hcp/Al-fcc/MgZn2 eutectic appeared. Cyclic corrosion tests and bending tests showed that the addition of Mg greatly enhanced the corrosion resistance, but decreased the cracking resistance of the coatings

Experimental study and microstructure simulation of Zn-Al-Mg coatings

The high corrosion resistance afforded by Zn + 5wt.% Al coatings is further improved by low level additions of Mg in the galvanizing bath. The effect of Mg additions on the microstructure and on the properties of the coatings has been investigated. Phase field modelling was used to simulate the influence of Mg on the eutectic microstructure. Experimental results on the effect of Mg on the ductility and on the corrosion resistance of the coating are also reported

Corrosion testing of a heat treated 316 L functional part produced by selective laser melting

Selective Laser Melting (SLM) shows a big potential among metal additive manufacturing (AM) technologies. However, the large thermal gradients and the local melting and solidification processes of SLM result in the presence of a significant amount of residual stresses in the as built parts. These internal stresses will not only affect mechanical properties, but also increase the risk of Stress Corrosion Cracking (SCC). A twister used in an air extraction pump of a condenser to create a swirl in the water, was chosen as a candidate component to be produced by SLM in 316 L stainless steel. Since the main expected damage mechanism of this component in service is corrosion, corrosion tests were carried out on an as-built twister as well as on heat treated components. It was shown that a low temperature heat treatment at 450°C had only a limited effect on the residual stress reduction and concomitant corrosion properties, while the internal stresses were significantly reduced when a high temperature heat treatment at 950°C was applied. Furthermore, a specific stress corrosion sensitivity test proved to be a useful tool to evaluate the internal stress distribution in a specific component.status: publishe