Single Sensor Differential Thermal Analysis Research Articles

Comparative study of phase transformation temperatures in high strength steel weld metals

For the determination of phase transformation temperatures in metallic materials, dilatometric analysis (DA) is one of the most commonly applied thermoanalytical methods. Since it depends on elaborate sample preparation, in-situ methods are likely to support the application of welding process investigations. In this study, several gas metal arc (GMA) welded all-weld metal samples were examined regarding their solid state phase transformation temperature during cooling by means of dilatometry and single-sensor differential thermal analysis (SS DTA). The results show the applicability of the SS DTA method during GMA welding and good agreement with dilatometric values.

Round Robin test on measurement of phase transformation temperatures in 9Cr1Mo steel

Modern heat resistant 9–12% Cr steels require optimised heat treatments and welding strategies to receive the best mechanical and long-term creep properties during the fabrication of power plant components. Therefore, the phase transformation temperatures—especially the austenite and martensite transformation temperatures—have to be well known to define optimised heat treatment and interpass temperatures as well as heating and cooling rates. Since phase transformations are influenced by the chemical composition of the materials and other numerous factors, it is important to pay attention to the circumstances surrounding the determination of these temperatures. To determine specific phase transformation temperatures various methods and procedures like dilatometry, single-sensor differential thermal analysis (SS DTA) or differential scanning calorimetry (DSC) were established. The presented comparative study was initiated to investigate and quantify possible scattering within the measured values of participating institutions. Therefore, the commercially available martensitic heat resistant steel P91 was used to compare specific phase transformation temperatures determined by several participating institutes and laboratories. Two different simplified temperature cycles were defined to identify possible scattering and differences within the determined phase transformation temperatures. Furthermore, possible differing results regarding the evaluation of a given dilatometry data set by various institutes and laboratories were discussed. The presented round robin test shows that institutes and laboratories although using standard methods of analysis—which are said to be state of the art—are reporting variable values for the critical phase transformation temperatures in the steel of interest. It is also shown that the amount of scattering can vary widely depending strongly on the specific phase transformation temperature which has to be determined.

An Automatic Selection of Segment Width for the Single Sensor Differential Thermal Analysis (SS-DTA) Based on Piecewise Linear Approximation Technique

Single Sensor Differential Thermal Analysis (SS-DTA) is a novel non-destructive testing technique for studying and detecting the phase transformations and structural changes in materials. It uses only one temperature sensor to measure the temperature in a particular point of interest in the material during actual and simulated thermal processing of the material. SS-DTA compares the temperature recorded in a tested specimen against a reference thermal profile which can be generated either by analytical formulae or piecewise linear approximation. The main advantage of piecewise linear approximation over the analytical formulae is that it does not need the knowledge of tested material and processing conditions to optimally estimate the parameters of reference thermal history. On the other hand, in order to apply the piecewise linear approximation technique we must specify the segment width which is normally fixed at a nominal value of 1.5 seconds. We have recently found that this nominal value might not be an optimal choice for the segment width as it does not guarantee to give the best detectability of phase transformation. Therefore, in this research work we proposed a technique to automatically select an appropriate value of the segment width. The performance of proposed method has been evaluated by investigating the phase transformations of welded stainless steel SUS 321and SUS 304. It was found that the appropriate segment width could be ranging from 1.25-1.75 seconds and by using this selection technique, we could detect the differential temperature more accurately than when using the nominal value.

Effect of Cr on eutectic phase formation and solidification temperature range in Ni–Cr–Hf system

In order to obtain eutectic phases which are beneficial for reducing ductility dip cracking and solidification cracking susceptibility in Ni-base alloys during non-equilibrium solidification (such as welding), samples with Ni–5Hf and different Cr levels were made using a button melting method. The relevant eutectic phases in the Ni–Cr–Hf system were identified. The solidification temperature range (STR) was determined by experimental Single Sensor Differential Thermal Analysis. It was found that γ/Ni5Hf eutectic formed in the Ni–0Cr, Ni–5Cr and Ni–10Cr system, while γ/Ni7Hf2 eutectic formed in the Ni–20Cr and Ni–30Cr system. Formation of the γ/Ni7Hf2 eutectic was responsible for a narrower STR, which indicates a better resistance to solidification cracking.

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.

Characterizing PHASE TRANSFORMATIONS of different LTT alloys and their effect on RESIDUAL STRESSES and COLD CRACKING

Novel martensitic filler materials with specially adjusted martensite start temperatures (Ms) can counteract the cooling specific shrinkage due to expansion effects of the weld metal associated with phase transformations. That can be exploited to create compressive residual stresses in the weld and adjacent areas, i.e. beneficial for increasing fatigue strength. The Ms-temperature is shifted via the chemical composition, mainly by the alloying elements nickel and chromium, resulting as well in different retained austenite contents. Investigations were made using different Low Transformation Temperature (LTT) alloys with varying nickel content. The resulting phase transformation temperatures were — for the first time — detected using high energy synchrotron diffraction and Single Sensor Differential Thermal Analysis (SS-DTA). Compared to angle dispersive diffraction, energy dispersive diffraction offers the possibility to measure residual stresses of the martensite and austenite phase parallel fast in one experiment up to depths of 100 μm. The residual stresses show significant distributions dependent on the Ms-temperature. The effect on the cold cracking behaviour of these alloys was investigated using the Tekken test. Results show that cold cracking can be avoided when appropriate contents of retained austenite are existent.

Phase formation in 6060/4043 aluminum weld solidification

Weld metal microstructure for alloy 6060 aluminum welds, made using the gas-tungsten arc process and alloy 4043 filler metal, has been characterized using optical metallography, EPMA microprobe analysis, SEM/EBSD and STEM/EDX electron microscopy, and single-sensor differential thermal analysis (SS-DTA). In addition, alloy 6060 castings were solidified at variable cooling rates approaching that of welding, to provide a reference for comparison with weld microstructure. It was found that a major change in cast microstructure occurs at cooling rates higher than 27 K/s resulting in a structure similar to that observed in weld metal. Rapid cooling is believed to favor low temperature solidification reactions that normally would be achieved only at higher silicon content. Accordingly, additions of 4043 filler metal that increase the weld metal silicon content have only limited affect on weld solidification range and microstructure. This has direct implications regarding how 4043 filler additions improve weldability and weld quality.

In-situ-phase analysis using synchrotron radiation of low transformation temperature (LTT) welding material

Cold cracking resistance is a relevant evaluation criterion for welded joints and affected by residual stresses which result from the welding procedure. Compressive residual stresses can thereby have a positive influence on preventing cracking. A unique possibility of generating compressive residual stresses already during the welding procedure is offered by the so-called Low Transformation Temperature (LTT) filler wires. Compared to conventional wires, these materials show decreased phase transformation temperatures which can work against the cooling-specific contraction. In consequence, distinct compressive residual stresses can be observed within the weld and adjacent areas. The strength of these fillers makes them potentially applicable to high-strength steel welding. Investigations were carried out to determine the phase transformation behaviour of different LTT-filler materials. Transformation temperatures were identified using Single Sensor Differential Thermal Analysis (SS-DTA). Additionally Synchrotron radiation was used to measure the transformation kinetics of all involved crystalline phases during heating and cooling of a simulated weld thermal cycle.

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.

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.