8th International Research/Expert Conference „Trends in the Development of Machinery and Associated Technology“ TMT 2004, Neum, Bosnia a Herzegovina, 15-19 September, 2004 FEM-AIDED EXPERIMENTAL DETERMINATION OF FATIGUE PROPERTIES OF THERMOMECHANICALLY PROCESSED 34CRNIMO6 STEEL Bohuslav Mašek 1+2, Uwe Mahn1, Jiří Malina1+2, Klára Dalíková2, Hana Stanková1+2, Jaroslav Drnek3, L.W. Meyer1 1 - University of Technology, Chemnitz Faculty of Mechanical Engineering, Chemnitz Germany 2 - University of West Bohemia in Pilsen Pilsen Czech Republic 3 – COMTES FHT, s.r.o. Borská 47 Pilsen Czech Republic ABSTRACT The study is aimed at investigating the impact of the notch effect on fatigue properties of thermomechanically treated very fine-grained 34CrNiMo6 steel. FEM analysis has been conducted on the equivalent and normal stresses along longitudinal axis of test specimen during rotating bending fatigue test.The experimentally determined stress concentration factor αK has been compared with results achieved by numerical modelling. The fatigue limit has been measured by means of the rotating bending test on unnotched and notched specimens. The results are shown as S-N curves. Keywords: Fatigue, Fatigue Properties, FEM Simulation, Thermomechanical Treatment. 1. INTRODUCTION The last decade has been marked with great emphasis on special thermomechanical treatment (TMT) techniques for high strength low-alloyed steels. Low concentrations of alloying elements may effectively promote grain refinement, strengthening and increasing the of ductility in steels. Forming at lowered temperatures utilizes higher formability of material, while preserving the accuracy of cold forming operations. Some manufacturing operations may thus be reduced to a certain extent or even omitted. That is why new technologies may be developed for use with common structural materials, widening the range of their application. This study focused on 34CrNiMo6 medium carbon steel. Samples have been thermomechanically treated by controlled forging on a press. Notched and unnotched fatigue test specimens have been made of these samples. The value of stress concentration αK in the notch, which has been found by analytical techniques, was compared with results of numerical simulation. Thermomechanically treated specimens have been tested by means of a rotating bending test and the fatigue limit of notched and unnotched specimens has been measured. Determination of notch-related stress concentration factor made it possible to compare results of fatigue tests on both types of specimens. 2. EXPERIMENTAL 2. 1 Experimental Material: 34CrNiMo6 Steel The experimental material (Table 1), 34CrNiMo6 steel, contains a cost-effective set of alloying agents, being used mostly as hardened material for dynamically stressed components. 30 1.035 Smax ≤ 0. which took place at specified intervals. Controlled forming entailed five drawing-out reductions by flat dies to achieve a square cross-section of 30 × 30 mm. six reductions were performed whereby the finish-forging temperature of 700°C has been reached.30 1.80 0. Microstructure of the as-forged specimen drawing-out procedure was repeated in six steps resulting in the cross section of 30 × 30 mm.Table 1.70 0.50 0. Chemical composition of 34CrNiMo6 steel with upper and lower limit Chemical composition [wt. Subsequently. Samples were soaked at 1100°C for 60 minutes in an electric resistance furnace.40 0.3 Fatigue Testing 2. where similar notches occur in actual components. Tempering at 570°C for 4 hours resulted in fine-grained bainite microstructure (Figure 1).1 Specimens for Rotating Bending Test Standard specimens for the rotating bending test of 3 mm diameter and neck-down radius of 18 mm (Figure 2). Were created this shape of specimens eliminates structural notch effects in specimen loading. Adapted rotating bending test specimen 2. upon which the specimen was oil-quenched. Practical considerations led to testing of the second specimen type: 2.035 2.40 1.3. Subsequent cooling down to 500°C by pressure air for 180 seconds led to phase transformation. The notch represents a stress raiser.15 0. the sample body was upset and the Figure 1.8 mm diameter with a notch of 1 mm radius (Figure 3). The specimen has been reheated to 1100°C with soaking time of 600 s. . which brings the test closer to practical conditions.30 ≤ 0.70 1. Figure 2.2 Thermomechanical Treatment Cylindrical samples with the following dimensions were processed by controlled forging on a hydraulic laboratory press: diameter of 53 mm and height of 46 mm. Upon obtaining the said cross-section dimensions. The axis of cylindrical samples was thus parallel with die surfaces and the samples were rotated by 90° between reductions. %] Steel C Mn Si Cr Ni Mo Pmax 34CrNiMo6 0. Standard rotating bending test specimen Figure 3.32 0.20 0. 2 Determination of True Stress in the Vicinity of the Notch Notches in mechanically loaded components raise values of stress in their vicinity as compared with the unnotched state.3. which might act as micro-notches and minimising depth of mechanical disturbance of subsurface layers. where the notch shape was machined in stages. Precise turning has been employed. Nomogram for determination of stress-concentration coefficient αK in a rotational-symmetric notched part under Figure 5. 3). . and notch depth t = 0.80 . B = 1. A small-radius tool was used for gradual removal of material.max.. This procedure has been selected in order to avoid excessive deformation of the surface and strengthening of material due to repetitive bending of specimen. The stress-concentration factor αK is expressed as the ratio of stress present in the notched specimen and the stress in the unnotched specimen: αK = σ max . σn .. Magnitude of this increase is typically described with the stress-concentration factor αK.  . rather than necking down by a tool with a tip of the notch radius.6 mm (see the drawing of the test specimen in Fig. Distribution of equivalent and tensile stresses in longitudinal section of specimen under bending load modelled by means of FEM.(1) where σmax is the maximum stress in the notch and σn is the nominal stress. 1 A(rK / t ) + B(2rK / d)(1 + 2rK / d) 2 [-].25.At the same time.(2) Unnotched reference specimen-above Specimen with radius of 18 mm Specimen with 1 mm.radius notch σy max [MPa] 216 194 320 σeqv. yet minimise formation of toolmarks.8 mm. notch radius rK = 1 mm. [MPa] 216 192 295 Figure 4. The stress-concentration factor αK may also be expressed as: αK = 1+ where the parameters of the rotationalsymmetric test specimen under bending load are as follows: A = 0... emphasis was placed on obtaining similar effects on surface to those occurring under industrial conditions. diameter of the part d = 2. 2. bending load. The stress concentration factor αK in notched test specimens is 1.25(1/ 0. The nomogram in Figure 4 is a graphical depiction of the relationship in the equation 1-2. Berlin-München-Düsseldorf. It´s results have shown that the maximum stress in notched specimens is 1.8 ) 2 = 1.8(2 / 2. S-N curves of 34CrNiMo6 steel – unnotched and notched specimens 3.000 Number of cycles to failure log N [-] 10.6 ) + 1.000 Figure 6.000.5 . REFERENCES  Betz. logarithm of the number of cycles graph represents the sought fatigue limit σc.5. as well the stress amplitude (Figure 6). Mechanik.000.5 times higher than maximum stress in unnotched specimens.5 times higher than that in unnotched specimens.3 Rotating Bending Test The testing machine was SCHENCK PUP G – type. ISBN 3-433-005125 .: Hütte des Ingenieurs Taschenbuch . which had been determined by means of analytical methods.(3) As shown above. featuring variable loading frequence between 0 and 70 Hz. The highest value of the stress concentration factor has also been found in a notched testing body..8 )(1 + 2 / 2. The presence of notch had an impact on the slope of the S-N curve and slight influence on the point of slope discontinuity. CONCLUSIONS The value of stress concentration factor αK.Physikhütte.000 1.all specimens 850 800 750 A notchless specimens B notched specimens Stress [MPa] 700 650 600 550 500 10. The fatigue limit measured by means of rotating bending fatigue test was 580 MPa in both notched and unnotched specimens..3. Verlag von Wilhelm Ernst & Sohn. 2. where the maximum stress was 1. This result has been confirmed by means of FEM simulation of the behaviour of the test specimens with three different shapes (Figure 5). the maximum value stress at the notch is αK –times higher than the nominal stress present in the unnotched specimen. 1971. Numerical modelling of behaviour of test specimens with three different shapes has been performed for comparison.5. is 1. 34CrNiMo6 . The horizontal part of the S-N curve in the stress vs. The number of cycles to fracture has been recorded.000 100.Upon substitution αK = 1+ 1 0. A.