Aging behaviour of particular stainless-steels and NiFeCr alloy suitable for heat exchangers

This paper deals with the testing of three materials for special heat exchanger for short-time application. Mechanical and microstructural properties after aging at 650 and 850 °C were tested and analysed. The results will serve as an input data for the design and construction of plate heat exchanger.


Introduction
The use of stainless steels as a construction material is often the only possible solution in terms of operation (parameters, environment), lifetime, safety. The application of stainless steels is mainly concerned with the possibility to increase the parameters of the technological equipment, guaranteeing long-term service life and maximum safety. Austenitic stainless steels are especially used in non-standard environments where high resistance is required. The matrix of austenitic stainless steels is formed by austenite and according to the content of other alloying elements, ferromagnetic phases -delta ferrite and deformation martensite, carbides and precipitates of various types, sigma and chi-phase, nitrides and inclusions can be present in the structure. All of these components affect the durability of austenitic stainless steels. [1] When these steels are slowly cooled after heat treatment or welding, the Cr23C6 precipitates along the grain boundaries in the critical temperature range of approximately 420 ° C to 820 ° C for these steels. Of course, this process also occurs when the austenitic component is exposed to a high operating temperature. [2][3][4]. Various producers of austenitic stainless steels guarantee the creep resistance of particular grades in the range from 15 to 100 MPa for temperatures 750 -600 °C. [5] Other materials which have become standard for high temperature application are nickel-based alloys. These alloys generally offer high corrosion and wear resistance when exposed to high temperatures. [6] 2 Experiment description 2.1 Experimental material Three alloys -2 austenitic steels and one nickel alloy were selected for this experiment. The steel grades were austenitic stainless steel 1.4541 (AISI 321) -marked as specimen 4 in following tables and figures and 1.4571 (ASI 316Ti) -marked as specimen 7. Incoloy alloy 800 HT (1.4959) -marked as specimen 5, represented the nickel-iron-chromium alloys. Standard chemical composition of experimental alloys is listed below (see table 1).

Parameters of aging
There were applied two temperatures of aging with different holding times (dwell on the temperature) and water quenching afterwards. Particular parameters are summarised in the table 2. Only one holding time was chosen for the temperature 850 °C.

Microstructural analysis
The analysed samples were subjected to standard metallographic preparation -ie grinding and subsequent polishing. The microstructure of the sample was revealed by etching in the V2A reagent and documented using a Carl Zeiss-Observer.Z1m optical microscope. The microstructure was also analysed by SEM analysis performed on the Jeol JSM 6380 scanning electron microscope. The local chemical composition was measured using the INX x-sight EDX analyser.

Mechanical properties
The results of the mechanical tests are shown in tables 1 to 3 below. The development of mechanical properties tends to better illustrate the graphs in fig. 1 -3. Table 3. Mechanical properties of as received states and aged states obtained after testing at room temperature.
In case of testing of aged materials at room temperature, the yield strength (YS) is increasing for 1.4541 (longer holding time) and Incoloy (see fig. 1

Optical microscopy
The  The microstructure of 1.4571 in the as received consists of an austenitic matrix with the presence of delta ferrite. Thermally degraded states have an austenitic matrix with the presence of a larger proportion of delta ferrite. With increasing exposure time, grain growth occurs (see figure  5). The homogeneous microstructure of Incoloy 1.4959 consists of an austenitic matrix with carbide precipitates in grains and at the borders of austenitic grains. With increasing temperature and aging, carbides precipitate inside the grain. Grain coarsening trend with increasing time and aging temperature was not observed in this alloy (see figure 6).

Scanning electron microscopy
Aged specimens were subjected to EDX analysis for identification of intermetallic particles and secondary phases. Figure 7 shows the localities of EDX measurements of matrix (spectrum 1), delta ferrite (spectrum 2) and TiN particle (spectrum 3) in steel 1.4571. EDX analysis of Incoloy revealed the presence of complex carbides composed of Cr, Nb and Ti within the grains (see fig. 8) and chromium carbides (likely Cr23C6) on the grain boundaries (see fig. 9).

Conclusion
Three alloys -2 austenitic steels and one nickel alloy were selected for experiment dealing with microstructural and mechanical properties after aging at two temperatures 650 and 850 °C. Microstructure of both steels shows grain coarsening with increasing of temperature and holding time. Volume fraction of delta ferrite was increased after aging. The carbides precipitation occurs mainly on the grain boundaries and also within the grains in case of 1.4541 steel. Grain size of Incoloy was not affected after aging nevertheless the precipitation of chromium carbides on the grain boundaries was also observed in this material. These microstructural changes affected the mechanical properties. Generally, both YS and TS tested at room temperature were increased and the elongation decreased in case of 1.4541 steel and Incoloy. YS of 1.4571 was lower after aging at 650 °C in comparison to as received state. Results of testing at elevated temperatures showed good combination of YS, TS and elongation for 1.4541 steel and Incoloy. Also the notch toughness of these materials was decent in comparison to poor toughness of 1.4571 steel. Best performance of mechanical properties was obtained with Incoloy alloy. Results of these tests will serve mainly for the design and construction of a short-time service helium-helium heat exchanger. However the tested materials could also be applied in oil, gas and vacuum industry.