Timo Juuti

Precipitation of Si and its Influence on Mechanical Properties of Type 441 Stainless Steel

In this study the precipitation of silicon in Type 441 steel (18%Cr-0.4%Nb-0.5%Si) was investigated and its influence on strength properties were determined. To simulate high-temperature service conditions, heat treatments with various ageing times up to 120 hours and temperatures up to 800 °C were performed. Following the aging treatments, micro-and macro-hardness and tensile properties were measured. Microstructure and precipitation were analyzed using scanning electron microscopy and energy-dispersive spectroscopy. Predictions for equilibrium pericipitation were calculated using the Factsage software. According to observations, coarse titanium nitrides (TiN) and niobium carbides (NbC, Fe3Nb3C) were present in all specimens including non-aged ones. These precipitates did not coarsen during ageing, which implies that their growth already occurred in the sheet production process. However, silicon started to precipitate in the course of prolonged annealing. Si contributed to the formation of a secondary phase resembling the Laves-phase (FeNbSi) on grain boundaries. Hardness and yield strength were found to decrease with prolonged ageing at high temperatures. Factors affecting the silicon precipitation are discussed.

Static Strain Ageing in Some Austenitic Stainless Steels

The yield strength in austenitic stainless steels can be improved by cold rolling. Recently, it has been realized that a considerable further increase can be achieved through static strain ageing (SSA). The effect of SSA in four austenitic stainless steel grades was studied. The test materials were formerly cold rolled to three different reductions of 15%, 30% and 40%. Subsequently, the steels were aged at temperature range between 160 and 400 °C with ageing times from 15 to 15000 seconds. Owing to SSA, increments over 200 MPa in yield strength were observed, while elongation decreased only slightly or even improved by 1 to 2%-units. The influence of ´-martensite on the strength increase was apparent. The maximum strength increase with relatively small drop of elongation was achieved in the steels cold rolled to 30% reduction while approximately 50% of ´- martensite was formed. However, a small increase in the yield strength was detected even in steels cold rolled to 15% reduction and containing 0 to 2% of ´-martensite only. Therefore, SSA seems also to take place in the austenite phase. To clarify the reason for improvement of the ductility in the instance of strengthening, work hardening rates were determined and found to differ considerably between aged and non-aged structures. The activation energy of the SSA process determined was found to be almost equal to the activation energy of carbon and nitrogen diffusion in the austenite phase. A mechanism resembling the Suzuki effect was suggested as the main mechanism of the SSA process.

The Effect of Niobium Carbides and Laves Phase on the Yielding Behaviour of a Stabilized Ferritic Stainless Steel

High-chromium ferritic stainless steels have been developed for applications such as exhaust systems that require good formability. To improve formability, continuous yielding is preferred. However, in high-chromium ferritic stainless steels an upper yield point is often present as a result of free interstitials and Cottrell atmospheres. The upper yield point can be removed by temper rolling but it would be better to avoid it via a suitable heat treatment. This paper describes how this can be done in the case of a ferritic stainless steel containing 0.011%C, 0.012%N, 18%Cr, 2,1%Mo, 0.33%Nb, 0.15Ti%. Despite the presence of Nb and Ti, which should bind the free carbon and nitrogen as carbides and nitrides, an upper yield point was still observed. Previously it has been suspected that this is due to an intermetallic Laves phase present in this steel depleting the Nb in the matrix so that some carbon remains free. A series of short-term annealing experiments showed that the upper yield point diminishes, when the annealing temperature increases above 550 °C, finally disappearing after a heat treatment at 750 °C. On the basis of Thermo-Calc calculations and EDS analyses, free interstitials in the matrix could be related to depletion of MX or insufficient time to reach the equilibrium state.

Influence of Cooling Rate on Free Interstitial Concentration in Type 430 Ferritic Stainless Steel

In ferritic steels, the amount of free C and N should be as low as possible to avoid the formation of Cottrell atmospheres and their associated discontinuous yielding and Lüders bands during forming. During the post-annealing cooling of ferritic stainless steel, carbides and nitrides of the type MX and M23C6 precipitate. The volume fraction of the precipitates is determined by chemical composition, microstructure and the cooling path. In some cases, precipitation might not be sufficient to remove all free interstitials from the matrix, in which case, the process parameters or composition of the steel should be reconsidered. Here, thermodynamic and kinetic calculations using Thermo-calc and TC Prisma software have been made to investigate the precipitation of C and N as a function of total interstitial content and cooling rate. According to the calculations, decreasing the cooling rate would result in a more efficient precipitation and hence, less free C and N in the matrix, but the amount is not sufficient to remove the upper yield point. Furthermore, changing the C and N content of the steel was found to have insignificant influence. However, the free C and N could possible be bound through a more complex cooling.