Solid solution hardening and strain hardening are the dominating strengthening mechanisms to achieve a high yield stress and ultimate tensile strength in Cr-Mn-N steels for drill collars. The fatigue limit can be improved more effectively by solid solution hardening than by strain hardening, but the attainable hardness due to soluble elements is restricted by other metallurgical demands. Strain hardening significantly increases the strength and the fatigue limit is improved too, but to a lesser extent. The reason for this behaviour is the introduction of internal stresses of the I, I1 and 111 kind by forging the drill collars. This results in macroscopic stresses which are varying over the cross section and in stresses of a microscopical scale which lead to an early loss of linearity in the elastic line of the stress strain curve and to the well known Bauschinger effect. Cyclic softening and hardening is accompanied by a rearrangement of the dislocation structure as revealed by transmission electron microscopy. The results indicate that a multiaxial and homogeneous cold working to produce a stable dislocation structure and to avoid large directional internal stresses is extremely important to achieve high fatigue strengths. Crack growth and crack closure measurements were performed for determination of the effective cyclic threshold stress intensity range for evaluation of the influence of the grain size on the fatigue limit. 1 NOMENCLATURE A, = elongation after fracture c = fatigue ductility exponent da/dN = fatigue crack growth rate dG = grain size tap, = plastic strain amplitude i = strain rate c; = fatigue ductility coefficient Kmai = maximum stress intensity factor AK = cyclic stress intensity factor range AK,h = threshold stress intensity factor range AK,,,, = effective threshold stress intensity factor range N = number of cycles Nf = number of cycles to failure N, = number of cycles to crack initiation R = load ratio (R = um,n/a,,,) Rp0,* = yield strength R, =tensile strength u = stress u, = stress amplitude pr = relative permeability 2 = reduction of area after fracture Au = stress range
Versuchsdurchführung unter Argonatmosphäre bei 800, 900 und 1000 °C – mechanische Kennwerte bei Raumtemperatur in Abhängigkeit von Prüftemperatur und Auslagerzeit – licht‐ und elektronenoptische Untersuchung des Mikrogefüges – Zuordnung mechanischer Kennwerte und Mikrogefüge.
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