Service Reliability Assessment of Weld Repaired CrMoV Steel Rotors

This research deals with the service evaluation of a retired CrMoV steel rotor that has been repaired using deep groove multi‐pass submerged arc welding (SAW) and gas tungsten arc welding (GTAW). Accelerated creep tests were conducted at several elevated temperatures and extrapolated to the service temperatures. The coalescence of carbides and cavity growth during creep were evaluated. For the CrMoV‐GTAW samples, it was found that the creep life depended on the location of the crossweld in the welded specimen. The service lives of the weldments were extrapolated.     

Assessment of the Effect of Microstructure on the Magnetic Behavior of Structural Carbon Steels Using an Electromagnetic Sensor

The magnetic properties of four carbon steels were evaluated using an electromagnetic sensor and correlated with their microstructures. Their composition, microstructure features (such as ferrite volume fraction, grain size, inclusions, etc), and hardness were compared with their saturated magnetic flux density, retentivity, and coercivity. The four steel rods used in this study were hot-rolled AISI 1010, AISI 1018, AISI 1045, and AISI 1045-high manganese/“stress proof.” The results show that microstructures have a notable effect on the magnetic properties of the steels. In addition, the effect of variations in cross-section area of the steel rods on the magnetic response was investigated. The steel rods diameters were systematically reduced by machining and then magnetically evaluated. Consistent relationships between metallurgical characteristics of the structural carbon steels and their magnetic properties measured with the electromagnetic sensor were obtained. In addition, the sensor was found to be able to detect changes in magnetic properties due to variations in cross-section area. These results reveal that the electromagnetic sensor has the potential to be used as a reliable nondestructive tool to detect and monitor microstructural and morphological changes occurring during the different stages of steel manufacturing or alterations caused by a degradation mechanism.     

Reaktives Diffusionslöten von Keramik an Stahl mittels Zr–Cu–Zr‐ und Zr–Ni–Cu–Zr‐Schichten für Anwendungen im Hochtemperaturbereich

Joints manufactured by transient liquid phase bonding feature comparable properties as diffusion weldements, but considerably lower process temperatures and pressures have to be applied. The liquid phase, which is hereby used, occurs due to interdiffusion between the base and/or the filler materials at a constant temperature, which lies below the melting temperature of the substrates. An essential requirement for this diffusion‐based melting is that the involved materials have low melting alloy‐constitution areas, such as eutectics. The aim of the study, presented in this contribution, is to evaluate an approach, in which an active transient liquid is created by suitable interlayers, in order to facilitate the wetting of ceramics. The potential of this attempt will be illustrated on zirconia/stainless‐steel‐joints for high temperature applications, such as solid oxide fuel cells. In such applications, the used materials have to withstand harsh conditions, e.g. high operating temperatures, oxidizing or reducing environments, which represent a demanding challenge for joining technologies, even at the latest state of research. In this study interlayers, consisting of Zirconium, as the active element, in combination with Copper and/or Nickel, have been investigated. These systems exhibit a wide range of alloy‐constitutions with low melting temperatures, which can be used for the formation of the transient liquid phase. For the application of the interlayers, physical vapor deposition as well as 75 µm‐thick Nickel‐foils have been used. The joining was carried out in high vacuum with changing holding times and temperatures. Additionally, the ratio of the thickness of the used interlayers was changed. Results of microstructural investigations, nano‐hardness measurements of the joining area as well as shear strength and fractography are presented.     

Silicon nitride-stainless steel braze joining with an active filler metal

Hot-pressed Si₃N₄ was brazed to 410-stainless steel using a Ag-Cu-Ti alloy foil in a vacuum. The occurrence of cracking due to processing was examined by systematically varying the brazing temperature and time between 840 and 900 °C and 6 and 60 min, respectively. Cracks were found in Si₃N₄ parallel to the bonding interface when the braze joints were processed at the lower temperatures (for all processing times at 840 °C and for times of 6 and 12 min at 860 °C). A reaction layer was observed to develop in the filler metal adjacent to Si₃N₄, rich in Ti and containing some Si. The thickness of this layer depended on brazing temperature and time. Microcracks were found in the reaction layer normal to the bonding interface in the joints processed at higher brazing temperatures (880 °C for 60 min and at 900 °C for 30 and 60 min). The low temperature cracks occurred, apparently, as a result of the incomplete relaxation of thermal stresses due to the presence of a hard continuous titanium strip in the filler metal; the high temperature microcracks seemed to be affected by the increase in thickness of the reaction layer and by the precipitation of intermetallic compounds. The compressive shear strength of the braze joints were evaluated and correlated with the cracking behaviour and microstructure changes in the joint. A strong braze joint was obtained when the reaction layer was relatively thin and no cracks were present in either the reaction layer or the Si₃N₄.     

Interface Interactions of Dissimilar Materials at Elevated Temperatures

This paper reviews some of the chemical interactions that occurred at the interface of ceramic/molten metal liquids. Control of interfacial reactions between dissimilar materials is an important issue in numerous technological applications, such as brazing of ceramics to metals, design of ceramic–metal composites, coatings of ceramics on metal substrates, and development of crucibles for melting of refractory metals. In ceramic/metal systems, wetting of the ceramic surface by the liquid metal is typically accompanied to some extent by interfacial reactions. The chemical incompatibility between the metal and non‐metallic materials can result in the formation of undesirable phases, due to the chemical and metallurgical reactions that take place during processing or in service. There is a need, therefore, to characterize the governing factors and reaction pathways at these interfaces. So, when the reaction products obtained during interdiffusion processing are not favorable, the diffusion pathway can be modified to control their formation.     

Kinetic and microstructural aspects of the reaction layer at ceramic/metal braze joints

The formation and stability of the reaction layer when brazing non-oxide ceramic materials were studied. Si₃N₄-Si₃N₄, SiC-SiC and Si₃N₄-stainless steel braze joints were produced and investigated. Several filler metals, most Cu- and Ag/Cu-based, containing different amounts of titanium were used to evaluate the effect of titanium on the formation and growth of the reaction layer. Some braze joints were processed using filler metals containing precious metals for high-temperature and oxidation-resistant applications. It was established that the matrix composition of titanium-bearing filler metals affects the ceramic wetting characteristics and the reaction layer kinetics. In the Si₃N₄ braze joints, the reaction layer consisted of TiN and titanium silicides. An activation energy corresponding to the diffusion of nitrogen in TiN was calculated for the growth of the reaction layer. During fabrication of the braze joints with precious-metal-containing filler metals at 1250°C, Si₃N₄ decomposed and a sound joint could not be processed. Premetallizing the Si₃N₄ with an AgCulnTi filler metal resulted in the formation of the reaction layer and permitted the fabrication of sound braze joints at 1250°C. Attempts to produce SiC braze joints with CuTi filler metals were unsuccessful owing to the decomposition of the SiC; a TiCreaction layer had developed, but this did not prevent the diffusion of copper into the ceramic substrate, nor did it slow down the decomposition of the SiC.Visiting Professor at Werkstoffwissenschaften, Aachen, Germany.     

Hydrogen induced cracking in cold worked AISI 4140 steel welds

The relative susceptibility to hydrogen induced cracking of cold deformed AISI 4140 steel welds was investigated. Controlled amounts of diffusible hydrogen were introduced into these weldments by utilizing the gas metal arc welding process with additions of hydrogen to the shielding gas. The crack behaviour was measured in terms of total crack length and distance from the fusion line. The crack propagated along the coarse grain structure of the HAZ about the fusion line. The refinement of the grain resulting from the increased cold roll reduces the crack activity. The susceptibility to hydrogen induced cracking of the cold worked samples appear to be sensitive to a critical value of residual stresses and to the orientation of the weld with respect to the rolling direction.     

Reheat cracking studies on simulated heat-affected zones of CrMoV turbine rotor steels

The reheat cracking susceptibility of the heat-affected zone (HAZ) of a CrMoV turbine rotor steel was investigated. Two base materials, one with a coarse-grain (155 μxm) and the other with a fine-grain (55 μim@#@) microstructure, were submitted to Gleeble HAZ weld simulations. Three peak temperatures were utilized: 1350,1150, and 950 °. Some samples were single cycled, and others were exposed to a double cycle. The samples that were double cycled experienced a second peak temperature 150 to 250 ° lower than the first peak temperature. The samples were then stressed in bending for different amounts and stress relieved under load to determine their reheat cracking susceptibility. All samples were metallurgically evaluated before and after the reheat cracking test. It was found that the prior-austenite grain size of the original base metal did not influence the reheat cracking susceptibility, but increases in peak temperature did. It was observed that the grain size and grain matrix microhardness that developed after the Gleeble cycles affected reheat cracking. It was found that reheat cracking did not occur when the microhardness was below 350 DPH and the prior-austenite grain size was less than about 80 μm.