Horst Cerjak

Design of improved heat resistant materials by use of computational thermodynamics

Abstract Ferritic-martensitic steels are widely used for high temperature applications. Compared with austenitic materials they own a better thermal conductivity, lower thermal expansion coefficient, lower costs and a better resistance against stress corrosion cracking. In this contribution the modelling of the microstructure based on thermodynamic calculations has been used for the description and further alloy design for this steel group. The results are phase diagrams showing the precipitates to be expected as well as driving forces, phase compositions and diffusion coefficients as base for further modelling activities. A calculation model based on the determination of the (calculated) transformation temperatures A e1 and A e3 and the diffusion coefficients is proposed, which can help to predict the microstructural stability and the creep strength from the chemical composition. Based on this model, test melts for some new alloys has been produced. First creep tests of these alloys confirm the presented approach.

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

In this work, three joints of a X11CrMoWVNb9-1- (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.

Correlation of creep strength and microstructural evolution of a boron alloyed 9Cr3W3CoVNb steel in as-received and welded condition

Abstract The creep strength of martensitic 9% chromium steels is strongly coupled with the formation and interaction of different precipitate populations like MX (Nb,V)(C,N), M23C6, laves phase and modified Z-phase. Changes in microstructure can be correlated to a change in creep strength. Within this work, a boron-nitrogen balanced 9Cr3W3CoVNb steel is investigated in normalised and tempered (as-received), creep exposed and welded condition. Microstructural characterisation of as-received and creep exposed samples have been carried out by elemental mapping techniques in transmission electron microscopy. The creep strength of the base material has been evaluated at 650°C up to a current testing duration of 24,000 hours. Comparison of creep rupture strength after 20,000 hours shows a benefit in strength of the new grade of about 25 MPa compared to conventional steel grade P92 and 45 MPa to grade P91. The results of the microstructural investigation show only very little changes of the initial microstructure after 10,000 h of creep exposure. The investigations imply a higher microstructural stability of the new steel, compared to other 9% chromium steel grades. In welded condition, the steel shows unique formation of its heat-affected zone. Although the steel experiences several phase transformations during the weld thermal cycle, the original base material grain structure is almost completely retained in the heat-affected zone. The formation of a homogeneous grain refined region is suppressed and, therefore, less susceptibility to Type IV cracking is expected. Creep tests of crossweld specimens at 650°C and various stress levels show similar creep strength to grade P91 base material.

Computer Simulation of the Precipitate Evolution during Industrial Heat Treatment of Complex Alloys

Precipitates are the key ingredient for the strength of heat treatable alloys. To optimize the mechanical properties of alloys it is important to know the response of precipitates to thermomechanical treatments. In the past, application of computer models to describe the evolution of precipitates in the course of these processes has proven difficult due to the complexity of the problem. In this work, a new model based on a mean-field representation of precipitates in a multicomponent matrix is applied to heat treatments of steels. Example simulations are presented for a 9- 12% Cr ferritic/martensitic heat resistant steel for power plant application and a complex tool steel with both carbides and intermetallic phases using the software MatCalc. The predictions of the model are verified on experimental results and the potential application to industrial heat treatment simulation is discussed.

Characterisation of Cracks in High Strength Steel Weldments

After the big accident in the Cleuson-Dixence hydropower plant in December 2000 several investigations were performed to clarify questions concerning properties and behaviour of high strength steel S890 and its weldments. This contribution summarises the efforts undertaken to answer the question which mechanism caused the catastrophic failure that was originated in the weld deposit of a longitudinal weldment. Original material was taken from the shaft near the zone of the accident and was investigated. By extensive non-destructive testing four plates were decided to be removed by flame cutting and then retested in the laboratory. From these results some specimens were taken for further investigations using light microscope as well as scanning electron microscope. It was found that the investigated cracks were caused by the hydrogen induced cold cracking mechanism after welding, which is also called delayed cracking. By applying a proper pre and post weld heat treatment these cracks can be avoided, and by a diligent use of non-destructive testing methods the required quality of the welding can be assured.

Development, Experiences and Qualification of Steel Grades for Hydro Power Conduits

In this paper the development and experiences made with the application of high strength steels for penstocks and steel lined shafts for stored and pumping stored hydropower plants is discussed. The evaluation of failure cases showed that high strength steels have to be treated very carefully to assure safe fabrication and service conditions. The results out of those experiences have been introduced in quali cation programs which leaded to measures to be applied to allow safe design, manufacturing and service behaviour under static as well as dynamic loading conditions of the conduits.

LONG-TERM CREEP BEHAVIOUR OF E911 HEAT RESISTANT 9% CR STEEL WELDMENTS FABRICATED WITH FILLER METALS OF DIFFERENT CREEP STRENGTH

In this work an X11CrMoWVNb9-1-1 (E911) pipe with an outside diameter of 355 mm and a wall thickness of 43 mm was welded with three different filler metals by GTAW and SMAW process. The used filler metals differed in creep strength level, compared to E911 grade pipe base material creep strength. 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. Uni-axial creep tests at 600°C (873 K) and stresses ranging from 70 to 130 MPa were performed using cross-weld samples of all three welds. Fractured samples were investigated by optical microscopy, electron microscopy and hardness testing. The results showed that the use of undermatching weld metal of P91-type led to premature fracture in the weld metal at higher stress levels. At lower stresses the fracture location was shifted into the fine-grained heat affected zone (HAZ) and samples failed by characteristic Type IV failure mode. The use of matching (E911 type) and overmatching (P92 type) filler material increased the time to rupture only at high stress levels. The fracture mode for all samples at lower stress levels was identified as characteristic Type IV failure.Copyright

The Influence of Peak Temperature and Deformation on Welding CCT Diagram of Eutectoid Carbon Steel

The welding continuous cooling transformation (WCCT) behavior of eutectoid carbon steel was investigated in different peak temperatures and in the undeformed and deformed conditions. The corresponding WCCT and welding continuous cooling compression transformation (WCCCT) diagrams were constructed by means of dilatometric and metallographic analyses in addition to hardness measurements. It was found that the higher austenitizing temperature slightly accelerates pearlitic transformation, i.e., it shifts the WCCT diagram to shorter times. Furthermore, heavy hot deformation of austenite could strongly promote the formation of pearlite, that is, the WCCCT diagram moved toward the top left corner compared to the WCCT diagram, while martensite start temperature was lowered slightly, which is a characteristic of a displacive transformation mechanism.

Evolution Of Precipitate Structure in the heat-affected zone of a 9 wt. % Cr Martensitic Steel during welding and post-weld heat treatment

Martensitic 9–12 wt. % Cr steels have been favoured grades for high temperature components in thermal power generation industry. The excellent creep properties of these steel grades can be related to their optimized martensitic microstructure containing finely dispersed precipitates. In the present work, the influence of weld thermal cycle and subsequent post-weld heat treatment on precipitate evolution of a martensitic 9 wt. % Cr steel is investigated. The microstructure at different stages of the thermal cycle is characterized by optical microscopy, scanning electron microscopy and transmission electron microscopy. The precipitates are identified by analytical transmission microscopy, namely energy-filtered TEM, energy dispersive X-ray diffraction and electron energy loss spectroscopy. The experimental findings are compared to kinetic simulations of the precipitate evolution during the complete thermal cycle using the software MatCalc. Results of kinetic simulations are in good agreement with experimental data. Optimization of heat treatment procedure as well as further application of microstructure modelling is discussed.

On the Influence of Hot Straining of Austenite in Solid-State Welding of High Carbon Steel

The present work deals with the experimental determination of the influence of hot straining of austenite on its decomposition in high carbon steel. The results show that the transformation of austenite to pearlite is accelerated by deformation, whereas the martensite-start temperature (Ms) is lowered as a result of mechanical stabilization. Using FE analysis and a recent theory of mechanical stabilization, the change in Ms due to the heterogeneous compressive plastic deformation of austenite is analyzed.