John Hald

Behaviour of Z phase in 9–12%Cr steels

Abstract The literature on the behaviour of modified Z phase Cr(V,Nb)N in creep resistant martensitic 9–12%Cr steels is briefly reviewed. Ten different 9–12%Cr steels were investigated after prolonged exposure at 600–660°C; the modified Z phase was found in all of them. In steels with high Cr content (11–12%), Z phase precipitates much faster than in 9%Cr steels. Precipitation of Z phase is associated with dissolution of MX carbonitrides, and causes a breakdown in long term creep strength in 9–12%Cr steels. High Cr steels show creep instabilities accompanied with Z phase precipitation, whereas low Cr steels show good long term creep stability. A niobium free CrVN variant of the modified Z phase was observed for the first time during the course of this work. The solution temperature of the Cr(V,Nb)N and CrVN modified Z phases was found to be close to 800°C for 11–12%Cr steels, much lower than the 1200–1250°C solution temperature of the unmodified CrNbN Z phase. Above the solution temperature the modified Z phase is replaced by MX particles.

Analysis of creep in a welded ‘P91’ pressure vessel

Abstract The present work considers the uniaxial and multiaxial creep behaviour of the tempered martensite 9%Cr 1%Mo steel ‘P91’ and the creep behaviour of welds in a P91 pressure vessel. The microstructure of a base metal/weld metal transition in a thick section pipe was analysed by means of optical microscopy and hardness measurements. Special emphasis was given to three microstructural states: the base metal (BM), the weld metal (WM) and the intercritical heat affected zone material (IC-HAZ). A significant difference between these three microstructures was their subgrain size, which was measured in the transmission electron microscope and was found to be smallest for the weld metal and largest for the intercritical heat affected zone material. The uniaxial creep behaviour of the three material states was analysed and it was shown that the creep strength increased with decreasing subgrain size. The elastic modulus of P91 was measured and the uniaxial creep behaviour of the three material states was characterized and represented by (i) the Norton law and (ii) in terms of the Robinson model. A welded pressure vessel was creep tested and hoop and axial strains were measured for three welds in the vessel. A creep stress analysis of the welded pressure vessel was performed based on (i) Nortons law and (ii) the Robinson model concentrating on the accumulated hoop and axial strains in the welds. Measured and calculated axial and hoop strains were found to be in good agreement.

Quantification of Laves phase particle size in 9CrW steel

Abstract Tungsten-alloyed martensic 9Cr steels are applied for streamlines of advanced power plants because of their superior creep performance. Tungsten, as the main new alloying element, induces precipitation of intermetallic Laves phase during long-term exposure at service temperatures around 600 °C. The growth and coarsening of Laves phase was investigated for the martensitic 9CrW steel P92 after aging and after creep testing at 600 or 650 °C for times up to 59,000 h. For measurement of the size of Laves phase particles, field emission gun scanning electron microscopy (FEGSEM) was used along with image analysis. This technique allows discrimination of Laves phase particles from M23C6 carbides. The measured particle sizes were statistically evaluated and compared with values obtained using energy-filtered transmission electron microscopy (EFTEM).

On the crystal structure of Z-phase Cr(V,Nb)N

TheZ-phase Cr(V,Nb)N particles in various 9 to 12 pct Cr creep-resistant steels were investigated with electron diffraction, energy dispersive spectroscopy (EDS), and electron energy loss spectroscopy (EELS). In addition to the well-known tetragonal crystal structure forZ phase, a cubic crystal structure was identified for Cr(V,Nb)N and CrVN particles, but not for CrNbN. The tetragonal and cubic crystal structures were observed to coexist within the same particles, and the orientation relationship between the two lattices was determined. Understanding and controlling the nucleation of Cr(V,Nb)N particles could be of crucial importance to enable improvement of the long-term creep stability of 9 to 12 pct Cr martensitic steels.

Creep strength and ductility of 9 to 12% chromium steels

Abstract The present paper focuses in on long-term creep properties of parent material of the new 9-12%Cr creep resistant steels, P91, E911 and P92 developed for use in advanced ultrasupercritical power plants. These steels have been at the center of activities in the ECCC Working Group 3A (WG3A) “Ferritic Steels”, which covers creep data development and analysis for parent materials and welds of all ferritic creep resistant steels ranging from low alloy steels up to 12%Cr steels. The opinions stated in the paper represent the views of the author rather than the whole ECCC WG3A group.

Development of gold based solder candidates for flip chip assembly

Flip chip technology is now rapidly replacing the traditional wire bonding interconnection technology in the first level packaging applications due to the miniaturization drive in the microelectronics industry. Flip chip assembly currently involves the use of high lead containing solders for interconnecting the chip to a carrier in certain applications due to the unique properties of lead. Despite of all the beneficial attributes of lead, its potential environmental impact when the products are discarded to land fills has resulted in various legislatives to eliminate lead from the electronic products based on its notorious legacy as a major health hazard across the spectrum of human generations and cultures. Flip chip assembly is also now increasingly being used for the high-performance (H-P) systems. These H-P systems perform mission-critical operations and are expected to experience virtually no downtime due to system failures. Thus, reliability of the solder joint is a major critical issue. This reliability is directly influenced by both the phases in the bulk solder and also the intermetallic compounds formed between the solder and the solder wettable layer of the under-bump metallization during both the wetting reaction or/and the solid state ageing. In the present work, an attempt has been made to develop new solder alloys for flip chip assembly using the CALPHAD approach based on gold, the safest element among all the elements being considered for this application. Specifically, efforts have been made to predict the phases in the bulk solder of the promising solder candidates and also the intermetallic compounds formation, using the CALPHAD approach.

Thermodynamic and kinetic modelling: creep resistant materials

Abstract The use of thermodynamic and kinetic modelling of microstructure evolution in materials exposed to high temperatures in power plants is demonstrated with two examples. Precipitate stability in martensitic 9–12%Cr steels is modelled including equilibrium phase stability, growth of Laves phase particles and coarsening of MX, M23C6 and Laves phase particles. The modelling provided new insight into the long term stability of new steels. Modelling of the detrimental precipitation of Z phase Cr(V,Nb)N is described, which points to new approaches in alloy development for higher temperatures. Predictions of interdiffusion between a MCrAlY coating and an IN738 bulk alloy by multicomponent diffusion calculations provide a highly versatile tool for life assessment of service exposed gas turbine components as well as for the development of improved coatings.

Interdiffusion Between Ni-Based Superalloy and MCrAlY Coating

Interdiffusion at the interface between a Co-36.5Ni-17.5Cr-8Al-0.5Y, MCrAlY coating and the underlying IN738 superalloy was studied in a large matrix of specimens isothermally heat treated for up to 12,000 hours at temperatures 875°C, 925°C or 950°C. Modelled results using the finite difference software DICTRA was compared with experimental average composition profiles measured across the interface using a new experimental approach.

In Situ Techniques for the Investigation of the Kinetics of Austenitization of Supermartensitic Stainless Steel

The austenitization and inter-critical annealing of X4CrNiMo16-5-1 (1.4418) supermartensitic stainless steel were investigated in-situ with synchrotron X-ray diffraction (XRD), dilatometry and differential scanning calorimetry (DSC) under isochronal heating conditions. Austenitization occurred in two stages: the austenitization started at approx. 600 °C, decelerated at approx. 700 °C at 60 to 75 v.% of transformed austenite, and first resumed after heating for approx. 100 °C. This plateau in the transformation curve was more dominant for faster heating rates. Inter-critical annealing at 675 and 700 °C revealed, that austenite can to a certain extent be stabilized to room-temperature. There was good agreement for the transformation curves yielded by dilatometry and XRD. Some deviation occurred due to the different applied heating principles, different temperature monitoring and the impact of surface martensite formation on the XRD measurement. The applicable temperature range for DSC as well as the close proximity of the Ac1- and the Curie-temperature limited the usage of the technique in the present case.

A Feasibility Study of Lead Free Solders for Level 1 Packaging Applications

An attempt has been made to determine the lead free ternary combinations that satisfied the solidification requirement for a solder used in level 1 packaging applications, using the CALPHAD approach. The segregation profiles of the promising candidates were analyzed after scrutinizing the equilibrium calculations by Scheil solidification simulations and optimization. A feasibility study has been carried out for the replacement of high-lead-containing solders with the focus on surface tension, natural radius of curvature, oxidation resistance, intermetallic compound formation, and environmental oriented issues.