Steels with 18 to 19 mass% Cr and Mn each were studied in the as-cast condition containing 0.85 mass% C þ N and in the elektro-slag-remelted and hot worked condition containing 0.96 mass% C þ N after final solution annealing. The latter was also tested after 20% prestraining. The results of tensile tests were compared to those of rotating bending and push/pull loading. The higher C þ N content raised the 0.2% proof strength to about 600 MPa of which 70% were retained as fatigue limit of rotating bending at 10 7 cycles and a failure probability of 50%. Prestraining further improved this limit but lowered it in relation to the proof strength. The structural components of cold work hardening under unidirectional loading and cyclic loading were similar (planar slip, dislocation, twins and e-martensite) except for precipitates in the latter. Nitrides appeared in the austenite and carbides in the e-plates.

Interstitial atoms are most effective in strengthening austenitic steels. In stainless grades, chromium strongly reduces the solubility limit of carbon. High-nitrogen contents require costly pressure or powder metallurgy to dissolve N in the melt. The combination of both elements comes with a high-interstitial solubility at normal pressure of air. Sand casting with 18 mass% Cr and Mn each and 0.85 mass% (C þ N) were industrially produced. The investigation revealed: proof strength R p0.2 ¼ 457 [MPa], true fracture strength R ¼ 1714 [MPa], fracture elongation A ¼ 44%, notch impact toughness KV ¼ 290 J combined with a DBTT of À948C, an impact wear resistance comparable to Hadfield steel X120Mn12 but combined with a good corrosion resistance. Deep freezing and cold working does not effect the low relative magnetic permeability. This unique combination of properties offers advantages in application.

Heat-resistant ferritic steels containing Laves phase precipitates were designed supported by thermodynamic modeling. High-temperature compression tests at 1173.15 K (900°C) and a detailed characterization of the microstructural evolution during annealing at 1173.15 K (900°C) were carried out to investigate the effect of Laves phase formation on the high-temperature strength. Due to the addition of W/Mo and/or Nb, the high-temperature strength of the newly designed alloys is significantly higher than that of the reference steels. However, the high-temperature strength of all investigated steels decreases slightly as the annealing time is increased up to 1440 hours. To determine the influence of Laves phase formation and coarsening on the high-temperature strength during long-term annealing, the precipitates were extracted from the ferritic matrix in different annealing states. The phases in the powder residue were determined by XRD, and the chemical composition of the Laves phase in dependence of the annealing time was analyzed by EDS measurements.

Downsizing trends in the design of internal combustion engines require ferritic steels with greater strength at elevated temperatures. One method of improving the hightemperature strength is precipitation hardening with intermetallic phases such as the Laves phase. Thermodynamic calculations show, that the elements Nb and Si contribute to the Laves phase formation strongly. In this work, the influence of intermetallic precipitates on the mechanical properties of three different ferritic Fe-Cr stainless steels was investigated and compared to a reference material. The three main hardening mechanisms -precipitation-hardening, grain refinement, and solid-solution strengthening -were studied with appropriate alloy compositions and thermo mechanical treatment. Investigations were performed with uniaxial compression tests of samples aged isothermally at 9008C for up to 1440 h. It is shown that, the solid solution effect of Mo and W increases the high-temperature strength about 40%, also after long-term annealing. The contribution of the Laves phase precipitates on the high-temperature strength is rather small due to their rapid coarsening.[ Ã ] N. Nabiran, W. Theisen

Three newly designed heat‐resistant ferritic alloys containing the intermetallic Laves phase were investigated with respect to an annealing dwell time of up to 1440 h at 900°C and were compared with commercially available steels. A detailed characterization of the microstructure evolution in dependence of the annealing dwell time was performed. In order to estimate the influence of Laves phase formation and coarsening on the strength, ductility and toughness, the results of the microstructural analysis were correlated with tensile tests at room temperature and with Charpy‐V impact tests. Precipitates of the Laves phase were observed in the recrystallized state with a mean particle diameter about 0.25 μm. The Laves phase in all investigated alloys showed rapid growth and coarsening with increasing annealing time. In spite of this behavior, the strength and ductility of the newly designed alloys were conserved, even after annealing for 1440 h. However, the toughness decreased with coarsening of the Laves phase, which is expressed by a shift of the ductile‐to‐brittle transition temperature to a higher temperature. Overall, it was shown that the influence of grain growth on the mechanical properties is more significant than the presence of the Laves phase. Precipitation of Laves phase lowers the mobility of the grain boundaries so that grain growth can be avoided.