An innovative heat treatment process at temperatures between 750 and 900 °C, i.e. above the upper limit of the miscibility gap (MG), is found to slow down the subsequent phase separation occurring in the bcc phase when a super duplex stainless steel (DSS) 2507 is further treated within the MG. This delayed phase separation postpones low temperature embrittlement of the DSS and provides unprecedented life time. The innovative heat treatment can be applied to other DSSs, and thus it could provide means to prolong the service life of DSS in general. The underlying mechanism of the delayed phase separation can be sought in the initial nanostructure, studied here through atom proble tomography and scanning transmission electron microscopy. The results lead us to suggest that the change in short-range order and solute interstitial elements are responsible for the reduced phase separation kinetic imposed by the innovative heat treatment.

In this work we investigate the development of functionally graded cemented carbides, featuring macro gradients on the millimeter scale, where Fe, or Ni is the binder phase. Two composites, WC-Ni and WC-Fe with 20 % binder by volume were produced by addition of TiC on the surface of the samples before sintering at 1475 °C for 1 h. The sintered samples were analyzed using electron microscopy and microanalysis. For the Ni-binder sample, the results show a clear WC grain size gradient, with the smallest average grain size close to the TiC layer. This sample also exhibits compositional gradients, where Ni increases while Ti and C decrease from the added TiC layer and outward. The same effect of TiC addition on WC grain growth is observed in the Fe-binder sample, however, the effect is much smaller. The addition of Ti is known to influence the morphology of WC grains in Co-binder systems, and this effect is observed here in both Ni- and Fe-binder samples. WC growth ledges areobserved on the WC facets near the applied TiC layer where Ti levels in the binder are high. This suggests that the WC grain growth inhibition mechanism imposed by Ti is similar in these alternative binders as what has previously been reported for conventional Co-binders.

Functional gradient sintering of WC-TiC-Co cemented carbides was studied to reveal the effect of powder metallurgical processing conditions on compositional and microstructural, as well as hardness and toughness, gradients. The samples were created by local addition of TiC prior to sintering. Two different starting WC powders, one sub-micron and one medium-coarse, were used. Sintering at two different temperatures of 1410°C and 1475°C was compared. The local addition of TiC created a Ti/Co gradient that affected the structural evolution during sintering. The measured Ti/Co gradient after sintering at 1410°C and 1475°C differed, and the difference was especially apparent for the sample prepared using a sub-micron powder. After sintering at 1475°C, the sample prepared from the sub-micron powder exhibited a large WC grain size gradient and elongated or plate-like WC grains where the Ti concentration was high. In contrast, the sample prepared from the medium-coarse powder only showed a WC grain size gradient and no plate-like WC grain formation. It was also observed that the WC grain surfaces had growth ledges in both samples when the Ti content was enhanced, indicating the influence of Ti on the WC grain growth mechanism.

In-situ small-angle neutron scattering (SANS) experiments, with and without an applied magnetic field of 1.5 T, were performed for two duplex stainless steels: 22Cr-5Ni and 25Cr-7Ni (wt.%) during isothermal heat treatment at 450 ∘C. The kinetics of phase separation was suppressed by the external magnetic field in both steels; however, the suppression was much more pronounced in 25Cr-7Ni, where phase separation was nearly eliminated. The difference in magnetic energy contributions from the external field in each steel explain their different degrees of phase separation. The findings are believed to have large technical implications for mitigating low-temperature embrittlement in Fe-Cr-Ni based alloys.

In duplex stainless steels (DSSs), phase separation of iron and chromium is a well-known phenomenon causing low-temperature embrittlement, which greatly limits the lifetime of components in service conditions at temperatures above 250 °C–300 °C. Hence, means of mitigating the underlying phase separation causing this embrittlement is highly interesting to extend the service life of DSSs in certain applications. In this work, we investigate the effect of intermediate heat treatments (5 minutes annealing at temperatures between 700 °C and 900 °C), performed after the conventional solution treatment, on the kinetics of phase separation super duplex stainless steel 2507. Using in situ small-angle neutron scattering at accelerated aging conditions (i.e., aging at 475 °C), we show that the application of intermediate heat treatments, which change the “initial state” of the material, can slow down development of the concentration fluctuation amplitude by up to 65 pct during aging inside the miscibility gap. This indicates great potential to delay the embrittlement process of duplex stainless steel. All intermediate heat treatments, conducted prior to aging, change the phase separation kinetics but to different extent. The 800 °C intermediate heat treatment shows the largest reduction in phase separation kinetics as compared to the reference sample. This sample also correspondingly shows the lowest hardness increase after aging. These findings show that intermediate heat treatments can be effective to reduce phase separation kinetics in duplex stainless steel and thus mitigate low-temperature embrittlement during service. The origin of the intermediate temperature treatment effect on phase separation kinetics is discussed in relation to the short-range atomic order introduced during intermediate heat treatment, prior to accelerated low-temperature aging.

The low-temperature embrittlement limits the service temperature of ferritic and duplex stainless steels. The effects of alloying elements added to Fe-Cr binary system on the low-temperature embrittlement have been reviewed critically. Prior literature on the underlying phase transformation, i.e., phase separation (PS) and changes of mechanical properties, is surveyed. The available literature indicates that the rate of PS is accelerated by Ni or Co in Fe-Cr binary system. The increased kinetics of PS also lead to an enhanced hardening rate during aging for Ni and Co alloyed Fe-Cr alloys. In low Cr (< 17 wt.%) ferritic alloys, the additions of Al or Co can reduce embrittlement because these elements contribute to lowering the driving force for PS. The influence of other alloying elements such as Mo, Cu, Mn, Nb, and Ti is inconclusive but also discussed here. Thermodynamic and kinetic calculations were performed to evaluate current CALPHAD databases and to further investigate the thermodynamic and kinetic reasons for the effect of the additional alloying elements added to Fe-Cr alloy on PS. Some indications were provided for improving physically-based predictions of low-temperature embrittlement as well as opportunities to mitigate the phenomenon by alloying.

Small-angle neutron scattering (SANS) is a valuable method for the analysis of phase decomposition in Fe-Cr alloys; however, quantification of the decomposition requires careful modeling of the scattering data considering factors such as interface character and short-range order. Here, we quantify the phase decomposition in a high-performance super duplex stainless steel in situ during accelerated aging in the early stage of decomposition by modifying a previously suggested quantitative SANS data modeling method. The proposed revised method can accurately model the SANS data and paves the way for revisiting the detailed phase decomposition kinetics in situ during aging in various Fe-Cr-based alloys.

The theory of diffusion processes in solids has achieved significant results in recent decades, but the development of methods for calculating diffusion in a multicomponent thermodynamic system is still an urgent task. Problems of diffusion in solid and liquid solutions with small deviations from the equilibrium state, or fluctuations, are of significant interest. The work develops a general methodology for calculating diffusion flows in a multicomponent thermodynamic system for small deviations from the equilibrium state. A connection has been established between the mechanical approach to the analysis of generalized systems and the phenomenological equations of nonequilibrium thermodynamics. Examples are given of the use of the developed methodology for the analysis of carbide transformations in chromium steel.

A phase field model is developed to study the discontinuous (cellular) precipitation (DP) reaction in a hypothetical A-B miscibility gap system. Unlike previous treatments, the model employs an interaction energy term which couples the composition and the phase field variable to enable the necessary grain boundary movement. The influence of factors such as interaction energy, interfacial mobility and grain boundary diffusivity on the transformation rate and overall microstructure evolution are discussed.

In this study two functionally graded cemented carbide samples with gradients in both composition and grain size have been produced and studied. The two samples are manufactured by local addition of TiC to a pressed WC-Co green body prior to sintering. The two samples differ only by the in-going WC particle size, where one sub-micron and one coarse WC particle size is used. The Ti, Co, and C concentration profiles are analysed using energy−/ and wavelength-dispersive X-ray spectroscopy; Vickers hardness profiles are also measured. Furthermore, the measured experimental concentration profiles are compared with diffusion simulations using the DICTRA software. The concentration and hardness profiles show a similar trend for both samples with decreasing Ti and C concentrations while Co concentration increases with distance from the applied TiC layer. The composition gradient affects the number of stable phases and the WC grain size. Furthermore, there are distinct differences between the samples with different initial WC particle size. The sample with an initially finer WC particle size has a shorter gamma-phase zone and the difference in WC grain size across the gradient is larger as compared to the sample with an initially coarser WC particle size. Finally, abnormal grain growth occurs in both samples but it is suppressed with increasing Ti concentration.