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Carrier gas hot extraction (CGHE) is a commonly applied technique for determination of hydrogen in weld joints using a thermal conductivity detector (TCD) for hydrogen measurement. The CGHE is based on the accelerated hydrogen effusion due to thermal activation at elevated temperatures. The ISO 3690 standard suggests different specimen geometries as well as necessary minimum extraction time vs. temperature. They have the biggest influence on precise hydrogen determination. The present study summarizes the results and experience of numerous test runs with different specimen temperatures, geometries (ISO 3690 type B and small cylindrical samples), and factors that additionally influence hydrogen determination. They are namely specimen surface (polished/as-welded), limited TCD sensitivity vs. specimen volume, temperature measurement vs. effects of PI-furnace controller, as well as errors due to insufficient data assessment. Summarized, the temperature is the driving force of the CGHE. Two different methods are suggested to increase the heating rate up to the desired extraction temperature without changing the experimental equipment. Suggestions are made to improve the reliability of hydrogen determination depended on the hydrogen signal stability during extraction accompanied by Evaluation of the recorded data. Generally, independent temperature measurement with dummy specimens is useful for further data analysis, especially if this data is used for calculation of trapping kinetics by thermal desorption analysis (TDA).
In situ analysis of hydrogen behaviour in stainless steels by high energy synchrotron radiation
(2011)
Hydrogen generally causes lattice distortions and phase transformations when introduced into a metal matrix. The present contribution provides a report of first in situ investigations of hydrogen effects on the lattice of the austenite and the martensite phase in a supermartensitic stainless steel using hard synchrotron radiation for respective energy dispersion diffraction measurements. Lattice distortions, such as the planar spacing expansion, have been monitored during the complete hydrogen effusion process over 24 h and are directly correlated to the dissolved hydrogen in the metal matrix. Among other results, it turned out that hydrogen at the introduced concentration level causes a reversible lattice expansion and after hydrogen effusion the lattice nearly regained its original dimensions. Hydrogen much more significantly affects the austenite phase with respect to lattice expansion than the martensite phase, but no phase transformations occur during the hydrogen desorption process.
Numerous national and international activities of regulation and standardization of cold cracking tests are currently being undertaken. Most of these test procedures operate as crack-no-crack tests and are focussed on the determination of technological parameters, as for instance the welding conditions and the heat input as well as pre- and postheating temperature and times. Such parameters predominantly influence the cooling conditions of the weld, which significantly affect the three well known local parameters on hydrogen cracking microstructure, hydrogen concentration and mechanical load. As one of the test procedures, the CTS-Test is currently standardized in the draft standard prEN ISO17642 which has been prepared on the European level by CEN/TC 121/WG 13 (destructive tests on welds). As compared to other cold cracking tests, quite extensive specimen cooling procedures are prescribed for the CTS-Test.
As a contribution to the ongoing discussion the present document highlights the effect of different cooling procedures after welding propagated for the modified CTS-Test. The major part of the work includes a comparison of experimentally determined cooling curves at specific weld locations with respective thermal finite element analyses. As a result, it turned out that the prescribed water cooling after welding does not alter the microstructure relevant cooling time Δt(8/5) and the cold cracking relevant cooling time Δt(3/1) as compared to simple air cooling. With increasing preheating temperature the effect of water cooling becomes more pronounced on the cooling time Δt(3/1), but still does not affect the cooling time Δt(8/5). The effects of rapid cooling of the CTS specimens after welding have thus to be reconsidered, in particular, with respect to its transferability to component welds and to additional non-realistic quenching effects.
