Zum Wechselverformungsverhalten von vergütetem 42 CrMo 4 bei zweistufiger Zug-Druck-Beanspruchung

Cyclic deformation behaviour of quenched and tempered 42 CrMo 4 (AISI 4140)at two-block push-pull-loading The behaviour of steels in the course of two- and multi-block cyclic loading has been investigated up to now almost exclusively regarding the fatigue life. According to this, only a few papers exist, dealing with the cyclic deformation behaviour at two- and multi-block-push-pull-loading. Therefore, in stress- and total strain-controlled experiments with a single change of the amplitude (two-block-experiments) and multiple changes between two blocks of different lengths and amplitudes (multi-block-experiments) the cyclic deformation processes have been investigated for the quenched and tempered steel grade 42 CrMo 4 (equivalent to AISI 4140). Using the data of stress- and strain-Wöhler-curves determined in usual fatigue tests, damages defined according to Miner's rule were adjoined to the blocks. The Miner-damages at failure observed in the two-black-experiments with changes from high to low amplitudes were smaller than one and at inverse changes of amplitudes larger that one. In contrast to this, in multi-block-experiments no universally valid correlations were observed between the Miner-damages at failure and the test-parameters applied. At all tests cyclic work-softening was observed as in single-step-experiments. However, work-softening processes at high amplitude loadings yield to much larger plastic strain amplitudes after changing to smaller amplitudes than in single-step tests. Contrarily, in multi-block-tests work-softening at higher amplitude loadings reduces with decreasing block-length and increasing portion of the blocks with the smaller amplitude. This is attributed to effects of static strain-ageing. Total-strain-controlled two-block cyclic deformation experiments yield to similar effects as in stress-controlled tests. However, the behaviour at high strain-amplitudes was influenced by distinct work-softening in the first cycles and by self-unloading due to the applied strain-control, which promotes quasi-stabilization-effects.     

Zum thermischen Abbau von Kugelstrahl-Eigenspannungen bei vergütetem 42 CrMo 4

Thermal relaxation of shot-peening-induced residual stress in quenched and tempered steel 42 CrMo 4 Thermal relaxation of shot-peening-induced macro and micro residual stress in a quenched and tempered state of the 42 CrMo 4 steel is investigated with X-rays and is described by the Avrami-approach. The parameters of this relationship can now be determined more exactly using a new, iterative method. By means of the parameters determined, the relaxation of macro and micro residual stress is evaluated and compared. The influence of annealing time and temperature on the dependence of macro residual stress on distance to surface is modelled using the Avrami-approach. The transient relaxation of macro residual stress during heating up to the temperature required can be determined by applying the stress-transient-method which will be presented here. It turns out that not the elapsed heating time but rather the relaxation of residual stress during heating up determines the further relaxation of residual stress.     

Randschichtzustände von normalisiertem und vergütetem 42 CrMo 4 nach konsekutiven Kugelstrahl- und Festwalzbehandlungen

Near surface materials states of normalized as well as quenched and tempered SAE 4140 after consecutive shot peening and rolling treatments The near surface states of normalized as well as quenched and tempered SAE 4140 after consecutive shot peening and rolling treatments are presented and discussed. Especially in the normalized state separately shot peened or rolled specimens reveal characteristic differences in the near surface dislocation structures and in the thickness of the subsurface areas influenced by the mechanical treatments. In case of the consecutive application of shot peening and rolling, a sequence effect occurs in such a way that the finally applied surface treatment determines the microstructure of the very surface and the subsurface materials layers.     

Inhomogene Verformungserscheinungen bei Schwingbeanspruchung des Vergütungsstahls 42 CrMo 4

Inhomogeneous Deformation Processes During Cyclic Loading of Quenched and Tempered Steel SAE 4140 The cyclic deformation behavior of a normalized steel SAE 4140 in the loading range σ_a < R_eS is characterized by softening and hardening phenomena. Cyclic softening processes are combined with inhomogeneous plastic deformations and the propagation of a fatigue Lüdersband. The following hardening process occurs homogeneously. At stress amplitudes σ_a < R_p0.2 the quenched and tempered SAE 4140 worksoftens continuously until crack initiation. Plastic deformation processes are extremely inhomogeneous and are concentrated on small fatigue zones. The inhomogeneity increases with decreasing tempering temperatures und stress amplitudes.     

Ermüdungsverhalten von normalisiertem 42CrMo4 unter Torsionsbeanspruchung bei Raumtemperatur

Room Temperature Fatigue Behaviour of a Normalized Steel SAE 4140 in Torsion Cyclic deformation behaviour of a normalized steel SAE 4140 in shear strain-controlled torsion is characterized by cyclic softening and cyclic hardening. If mean shear stresses are superimposed to an alternating shear stress, cycle-dependent creep occurs, and the number of cycles to failure decreases. In shear strain-controlled torsional loading, mean stresses are observed to relax nearly to zero within a few cycles. Fatigue life is not influenced by mean shear strains.     

Langzeit- und Dauerschwingfestigkeit des Vergütungsstahls 25CrMo4 bei mehrachsiger Beanspruchung durch drei schwingende Lastspannungen

High-Cycle and Long-Life Fatigue of 25CrMo4 under Multiaxial Load Conditions by three Alternating Stresses Statistically verified experimental results from high-cycle and long-life fatigue tests (HCF and LLF) with altogether 537 unnotched solid cylindrical and thin-walled hollow specimen are demonstrating the fatigue behaviour (S-N-characteristics, scatterband) of 25CrMo4 under uniaxial loading with superimposed static stresses (consideration of the mean stress effect) and under biaxial loadings in variation of phase differences between the three combined normal and torsional stresses σ_x, σ_y, τ_xy The fatigue strength is commonly decreasing with life time in the high-cycle regime until reaching the fatigue endurance limit in the transition range to infinite life. The “ductility level” τ_w/σ_w and the “mean stress sensibility” p = p (σ_w, σ_zSch, R_m) are relatively independent of the intensity by stress amplitudes and fatigue life to failure. In comparision with the specific case of biaxial combined loading with synchroneous amplitudes, the fatigue resistance characteristics are detrimentally influenced by out-of-phase normal stresses σ_x, σ_y; a phase difference of 180° between the normal stress amplitudes is the most critical state of combination, especially in the lower cycle regime caused by a greater slope coefficient (probability of survival P_s = 50%). On the contrary is there in the high-cycle regime as well as in the long-life range no significant influence to the fatigue strength by biaxial load conditions of simultaneously normal stresses with out-of-phase torsional stress τ_xy     

Ebene Wellenausbreitung im Halbraum bei Zufallserregung und Kopplung zwischen Spannungs- und Temperaturfeld

Zusammenfassung Die ebene Wellenausbreitung im homogenen, isotropen Halbraum, die von zeitlich zufälligen Störungen auf der Oberfläche des Halbraumes herrührt, wird mit Hilfe der Korrelationstheorie untersucht. Der Einfluß der Kopplung der thermoelastischen Grundgleichungen auf das örtliche und zeitliche Verhalten der Korrelationsfunktionen wird mit den Voraussetzungen der linearisierten Theorie beschrieben. Von Temperatur und Spannung werden die Varianzen numerisch berechnet.

---

Summary Propagation of plane waves in a homogeneous, isotropic semi-space due to time-random disturbances on the surface of the semi-space is investigated in the sense of correlation theory. The influence of coupling in the basic equations of thermoelasticity on the behavior with respect to space and time of the correlation functions is described. Linearized theory is assumed. Variances of temperature and stress are computed.

---

---

Mit 11 Textabbildungen

---

Die vorliegende Arbeit wurde von der Regierung der USA unter Contract 61 (052)-645 unterstützt.     

Das Verhalten von galvanisch verzinktem unlegiertem Vergütungsstahl bei schwingender und stoßartiger Beanspruchung

The Behaviour of Zinc Coated Unalloyed Quenched and Subsequently Tempered Steel with Cyclic and Impact Stress With many experiments was shown the influence of hydrogen embrittlement on zinc coated specimens of high strength quenched and subsequently tempered steel with cyclic and impact stress. The influence of hydrogen depends on the structure of the material and on the storing time between the end of zinc coating and the start of loading. Therefore zinc coated pieces of high strength quenched and subsequently tempered steel should be stored after galvanizing. Then there will be no hydrogen embrittlement in cyclic or impact stress.     

Bestimmung der Streckgrenze und der Hall‐Petch‐Konstanten des Vergütungsstahles 42CrMo4 unterschiedlichen Gefüges mittels Eindruckprüfungen

Determination of yield stress and Hall‐Petch coefficients of the tempering steel 42CrMo4 with different microstructure by means of indentation testing Yield stress of steels can be determined by tensile tests or by indentation testing. Indentation tests offer the advantage of measuring the yield stress of certain parts that otherwise could not be used to withdraw tensile samples. Furthermore, measurements in parts with a varying yield stress e.g. surface hardened steels can be carried out. This work investigates several methods of indentation tests to measure the yield stress of the tempering steel 42CrMo4. The yield stress of different microstructures is compared to results of tensile tests. Using the indentation test according to GOST 22762‐77 the Hall‐Petch coefficients of microstructures consisting of perlite and ferrite are determined. Thus the influence of grain size and distance of perlite lamellas on the yield stress can be taken into account.     

Zur zyklischen Plastizität von anisotropem Aluminium

Cyclic plasticity of anisotropic aluminium A phenomenological material model for anisotropic plastic flow is expanded in order to take into account the effects of cyclic stress‐strain behaviour. The proposed model uses a two‐surface workhardening model and has been confirmed by uniaxial and multiaxial tests for the aluminium alloy AlMgSi1.     

Die Rißzähigkeit von Vergütungsstählen

The Crack Toughness of Heat Treatable Steels. A survey on K_Ic- measurements of heat treatable steels is presented. The influences of factors like the heat-treatment, test-temperature, micro-structure and the previous thermal and mechanical history of the starting material on the K_Ic-values are described. K_Ic-values at room-temperature of typical steels are tabulated.     

Werkstoffmechanische Beanspruchungs- und Lebensdaueranalyse zweistufig schwingbeanspruchter Biegeproben aus 42CrMo4

Strees-strain- and fatigue life analysis on specimens of 42CrMo4 under two-step bending Fatigue test results from unnotched specimens of 42CrMo4 under constant amplitude- and two-step-bending were analyzed using both the Local Strain Approach (LSA) and the Nominal Stress Concept (NSC). These analyses were carried out in order to compare the predictive accuracy of both concepts. As a first approximation it was assumed that only the materials ultimate tensile strength or the experimentally attained constant amplitude life curve at R = 0 respectively is known. Within the scope of LSA two damage parameters were applied. The parameter P_J considering mean stresses and load sequence effects and the parameter P_SWTconsidering mean stresses only. For the above mentioned assumptions the predictions of the LSA using the parameter P_SWT are slightly better than those obtained by the NSC. A pronounced improvement can be attained using the parameter P_J. However, compared to the predictions by the LSA in combination with P_SWTand cyclic material properties, the fatigue life calculated by the NSC using experimental constant amplitude loading curves are closer to the experimental results. Based on these findings, which can be interpreted as trends valid for other cases as well, the frequent use of the parameter P_J within the framework of the LSA is recommended. If available, experimentally attained constant amplitude loading curves should be used to adjust the P-Woehler curves.     

Messungen der Rißschließeffekte an Al 8090-T6 und 42CrMo4

Two different materials, a quenched and tempered steel with German designation 42CrMo4 and an Al-Li 8090-T6, were used to investigate the influence of crack closure on the lower part of the loading cycle. The usually measured closure parameter K_op was determined, too. But, the primary target of the present investigation was the lowest K_min-value experienced by the crack front in the tension-tension fatigue regime. This lowest K_min was denoted as K_min. In addition, the modification of ΔK to a lower value ΔK_red due to crack closure during the lower part of a fatigue cycle has been determined. It was found that for 42CrMo4 the influence of crack closure on K_min or ΔK is negligible, while for the Al-Li 8090-T6 closure can substantially alter K_min and ΔK. The two materials investigated can – in respect to their closure effects – be considered as the two boundary conditions enclosing the range of structural materials.     

Lokale Last- und Eigenspannungsverteilungen an rißbehafteten Oberflächen nach Korrosionsermüdung von vergütetem Ck 45

Distributions of Loading and Residual Stresses in Cracked Surfaces of Corrosion Fatigued Quenched and Tempered Steel Ck 45 Corrosion fatigue in the active state leads to a great number of short cracks and corrosion pits in the surface of metallic materials. This paper deals with the resulting distributions of loading and residual stresses in the case of bending fatigue. The material investigated was the quenched and tempered CBN-ground steel Ck 45. Results presented clearly indicate that stress distributions around cracks and corrosion pits are very inhomogeneous. Corrosion pits lead to a relaxation of manufacturing induced residual stresses. In the loaded state as a consequence of notch effects, maximum stresses are observed, which explains the importance of corrosion pits as crack initiation sites.     

Anwendung und Prüfung von Kitten im Säureschutzbau

Application and Testing of Mortars for acid-proff Construction Acid-proof ceramic materials are used to protect the surface of chemical equipment, tanks and sections of plants and buildings. Bricks and tiles are bedded in resin-based or silicate mortars. The field of application of those mortars is described. Especially the following guideline gives rules for a uniform determination of mechanical and other physical data of mortars for acid-proof construction.