Boian T. Alexandrov

Development of rapid heating and cooling (flash processing) process to produce advanced high strength steel microstructures

Abstract Flash processing of an AISI8620 steel sheet, which involves rapid heating and cooling with an overall process duration of <10 s, produced a steel microstructure with a high tensile strength and good ductility similar to that of advanced high strength steels. Flash processed steel [ultimate tensile strength (UTS): 1694 MPa, elongation: 7·1%], showed at least 7% higher UTS and 30% greater elongation than published results on martensitic advanced high strength steel (UTS: 1585 MPa, elongation: 5·1%). The underlying microstructure was characterised with optical, scanning electron, transmission electron microscopy as well as hardness mapping. A complex distribution of bainitic and martensite microstructures with carbides was observed. A mechanism for the above microstructure evolution is proposed.

Single Sensor Differential Thermal Analysis of Phase Transformations and Structural Changes During Welding and Postweld Heat Treatment

The technique of Single Sensor Differential Thermal Analysis (SS DTA) has been subjected to series of verification experiments. Its accuracy and sensitivity to various phase transformations and structural changes has been evaluated by comparison to the classic differential thermal analysis (DTA) and to dilatometric analysis (DA). The reliability of thermocouple and SS DTA measurements in the typical ranges of heat treatment and weld heating rates has been estimated utilizing the endothermic effect of the ferromagnetic to paramagnetic transition and the Curie temperature as a reference point. The sensitivity of SS DTA to various phase transformations and structural changes has been demonstrated by in-situ applications during fusion and solid-state welding, casting, heat treatment and post weld heat treatment (PWHT). Its application range includes construction of continuous cooling transformation (CCT) diagrams, development and testing of processing procedures, weldability studies, development of welding consumables and new alloys.

Weldability Studies of High-Cr, Ni-Base filler metals for power generation applications

The solidification behaviour and weld solidification cracking susceptibility of high-Cr, Ni-base filler metals that are widely used, or proposed for use, in the nuclear power industry have been investigated. Two heats of ERNiCrFe-13 (filler metal 52MSS), one heat of ERNiCrFe-7A (filler metal 52M), and one heat of a modified ERNiCr-3 (filler metal 82 with higher Cr content, designated here as filler metal 52i) have been tested using both the Transvarestraint test and the Cast Pin Tear test (CPTT). The solidification behaviour in these alloys has been studied by a newly developed procedure that accurately replicates the solidification process in fusion welds of Ni-base alloys and is based on the patented technique for Single Sensor Differential Thermal Analysis (SS DTA™). Results of the solidification studies showed that filler metal 52i has the widest solidification range, followed by the two heats of filler metal 52MSS, and filler metal 52M. The filler metal 52i also has the widest eutectic temperature range. The interdendritic eutectic constituent formed in weld metal of this filler metal and filler metal 52MSS is enriched in Nb and results from the eutectic reaction of γ + L → γ + NbC at the end of solidification. Both the CPTT and the Transvarestraint test provided the same ranking of solidification cracking susceptibility among these filler metals. Both heats of 52MSS and the heat of 52i were found to be more susceptible to solidification cracking than filler metal 52M. The slightly higher resistance to solidification cracking of filler metal 52i relative to the 52MSS filler metals is attributed to crack “healing” during the final stages of solidification. This is the result of the higher fraction of eutectic liquid of filler metal 52i, as confirmed by metallographic studies. The results of this study confirm the higher solidification cracking susceptibility of high-Cr, Ni-base filler metals that contain higher Nb levels to counteract ductility-dip cracking, relative to filler metals that are Nb-free. This study has also shown that the CPTT can be used as an alternative, and reliable, tool for ranking the solidification cracking susceptibility of high-Cr, Ni-base filler metals proposed for use in nuclear power plants and other applications.

Weld Solidification Cracking in Solid-Solution Strengthened Ni-Base Filler Metals

The weld solidification cracking susceptibility of several solid-solution strengthened Ni-base filler metals was evaluated using the transverse Varestraint test. The alloys tested included Inconel 617, Inconel 625, Hastelloy X, Hastelloy W, and Haynes 230W.* Susceptibility was quantified by determining the solidification cracking temperature range (SCTR) which is a direct measurement of the range over which cracking occurs. This temperature range was then compared to the equilibrium solidification temperature range derived from Calphad-based ThermoCalc™ calculations, Scheil-Gulliver solidification simulations, and in-situ measurements using the single sensor differential thermal analysis (SS-DTA) technique.

In-Situ Weld Metal Continuous Cooling Transformation Diagrams

A new methodology for studying of weld metal phase transformations is presented that is based on a single sensor differential thermal analysis technique. This methodology is applied for in-situ measurement of phase transformations and construction of continuous cooling transformation diagrams for as-solidifying E11018-MR and E6010/C-Mn steel weld metals. Valuable information is obtained about the evolution of microstructure in these weld metals. The ranges of welding conditions which provide an optimal combination of microstructural constituents with respect to weldability and mechanical properties are determined. This paper demonstrates the application potential of the new methodology in studying weld metal phase transformations, development of welding consumables and development/testing of welding procedures.

Cold Cracking in Weldments of Steel S 690 QT

The cold cracking process in shielded metal arc and gas metal arc welding of steel S 690 QT is investigated by Tekken test. The conditions for cold cracking are varied by changing the specimens’ thickness, heat input, initial weld metal hydrogen concentration and preheating temperature. The kinetics of initiation and propagation of cracking is monitored by acoustic emission equipment. It is found out that the crack’s propagation path shifts from weld metal to fusion line and HAZ with decreasing initial weld metal hydrogen concentration and increasing hardness of HAZ. In the investigated range of cold cracking conditions the magnitude of cracking is mainly controlled by the preheating temperature and heat input. The acoustic emission signal provides valuable information about the factors controlling the kinetics of crack initiation and propagation and the intensity of cold cracking. The initial hydrogen concentration in weld metal controls the incubation period’s duration and affects the cracking intensity at higher heat inputs. The preheating temperature does not influence the incubation period, but significantly affects the kinetics of crack propagation. The heat input has a complex influence on the cracking kinetics and intensity. This is related to the contradictive effects of heat input on the initial hydrogen concentration per unit length of weld metal and on the behaviour of hydrogen during cooling. The obtained results provide a basis for combined experimental — modelling investigations on the cold cracking phenomenon in weldments of higher strength structural steels, aiming at quantitative evaluation of the influence of the main controlling factors.

Evaluation of Weld Solidification Cracking in Ni-Base Superalloys Using the Cast Pin Tear Test

A second-generation cast pin tear test (CPTT) that is capable of ranking the weldability of Ni-base superalloys has been developed at the Ohio State University. The CPTT utilizes an optimized testing procedure and apparatus design that provide controllable and repeatable testing conditions, and yield reproducible and reliable test results.

Hot Cracking Study of High Chromium Nickel-Base Weld Filler Metal 52MSS (ERNiCrFe-13) for Nuclear Applications

High chromium nickel-base weld filler metals 52 (ERNiCrFe-7) and 52M (ERNiCrFe-7A) have in recent years replaced filler metal 82 (ERNiCr-3) for new fabrication and for repair applications in commercial nuclear power plants. Filler metals 52 and 52M are selected because they have excellent resistance to primary water stress corrosion cracking (PWSCC). Unfortunately, filler metals 52 and 52M exhibit a higher susceptibility to ductility-dip cracking (DDC) compared to filler metal 82. Filler metal 52MSS (ERNiCrFe-13) is a new high chromium nickel-base alloy with Nb and Mo added to improve resistance to ductility-dip cracking. Increasing Nb has in previous research been shown to widen the solidification temperature range in nickel-base alloys. A wider solidification temperature range can potentially increase susceptibility to hot cracking. This study investigated the solidification behavior and hot cracking susceptibility of three heats of 52MSS and compared the results to a heat of filler metal 52M and a heat of filler metal 52i. The solidification behavior and hot cracking susceptibility were investigated by an optimized Transvarestraint test and by a next generation Cast Pin Tear Test (CPTT). The solidification temperature range and eutectic transformations were measured by a patented Single Sensor Differential Thermal Analysis (SS-DTA) technique. This study showed that filler metal 52MSS was slightly more susceptible to hot cracking than 52M and 52i. This study also demonstrated that the next generation CPTT and SS-DTA technique are effective methods for evaluating the solidification behavior and hot cracking susceptibility of high chromium nickel-base weld filler metals.Copyright

Low heat input gas metal arc welding for dissimilar metal weld overlays part I: the heat-affected zone

Dissimilar metal weld overlays of nickel base alloys on low alloy steel components are commonly used in the oil and gas, petro-chemical, and power generation industries to provide corrosion and oxidation resistance in a wide range of service environments and temperatures. Traditionally, weld overlays are produced using cold or hot wire gas tungsten arc welding (GTAW). Potential advantages of cold metal transfer (CMT) welding, a low heat input gas metal arc welding process, over the conventional GTAW in production of weld overlays were evaluated. Metallurgical characterization was performed on CMT overlays of Alloy 625 filler metal on Grade 11 and Grade 22 steels. Significant grain refinement was found in the high temperature HAZ compared to the traditional coarse-grained HAZ in arc welding. Evidences of incomplete carbide dissolution, limited carbon diffusion, and incomplete transformation to austenite were also found. These phenomena were related to high heating and cooling rates and short dwell times of the high-temperature HAZ in austenitic state. Tempering effects in the steel HAZ were identified, showing a potential for development of CMT temperbead procedures. Based on the results of this study, the steel HAZ regions in CMT overlays were classified as high-temperature HAZ and intercritical HAZ.

Solidification Behavior and Weldability of Dissimilar Welds Between a Cr-Free, Ni-Cu Welding Consumable and Type 304L Austenitic Stainless Steel

The solidification behavior of a Cr-free welding consumable based on the Ni-Cu system was evaluated in conjunction with Type 304L stainless steel. The weld metal microstructure evolution was evaluated with optical and secondary electron microscopy, energy dispersive spectroscopy, X-ray diffraction, button melting, and thermodynamic (CALPHAD-based) modeling. Solidification partitioning patterns showed that higher dilutions of the filler metal by Type 304L increased segregation of Ti, Cu, and Si to interdendritic regions. Button melting experiments showed a widening of the solidification temperature range with increasing dilution because of the expansion of the austenite solidification range and formation of Ti(C,N) via a eutectic reaction. The model predictions showed good correlation with button melting experiments and were used to evaluate the nature of the Ti(C,N) precipitation reaction. Solidification cracking susceptibility of the weld metal was shown to increase with dilution of 304L stainless steel based on testing conducted with the cast pin tear test. The increase in cracking susceptibility is associated with expansion of the solidification temperature range and the presence of eutectic liquid at the end of solidification that wets solidification grain boundaries.