Mechanical properties and cyclic fracture behavior of differently heat-treated steel

M. Leonavičius*, E. Stupak**, G. Petraitis*** *Vilnius Gediminas Technical University, Saulėtekio 11, 10223 Vilnius, Lithuania, E-mail: mindaugas.leonavicius@vgtu.lt **Vilnius Gediminas Technical University, Saulėtekio 11, 10223 Vilnius, Lithuania, E-mail: eugenius.stupak@vgtu.lt ***Vilnius Gediminas Technical University, Saulėtekio 11, 10223 Vilnius, Lithuania, E-mail: gediminas.petraitis@vgtu.lt


Introduction
For mining equipment structural elements such as gears and bolts the specific requirements are applied.Gears must be of sufficient strength with high surface hardness (preferred 280-320 HRB) in order to resist wear but remain ductile in deeper layers, have a good weldability and be easily processed and be fairly light.While for the threads the large localised concentration zones arise.Therefore in this case it is important that the threshold stress intensity factor range was as higher.When there is a need of providing the elements with those properties the heat treatment shall be applied.Medium carbon low alloyed steel grade AISI 4130 meet those requirements due to controllable heat-treatment processes and good mechanical processability and is widely used for this industry [1][2][3][4][5][6][7].
The research includes the determination of influence of hardening with high tempering and normalisation on steel's AISI 4130 static, dynamic and cyclic properties.

Material and heat treatment
For the experiment two kinds of steel grade 4130 was used, produced in different foundries.Chemical composition is presented in Table .Table Chemical composition  Note that chemical composition slightly differs.Manganese amount was 0.4 ... 0.6% in St-1 and 0.7 ... 0.9% in St-2 respectively.The larger amount of manganese determines finer ferrite grains and shape [3,4], and improve the mechanical and cyclic strength properties.
Heat treatment of the both steels was different.St-1hardening with high tempering (austenitized at 870 o C and quenched in oil, then tempered at 650 o C to receive required hardness).Hardening with high tempering is applied in order to obtain the required hardness at element's working parts, but also enables to maintain a high strength necessary for gears.During hardening the steel is heated up to austenitizing structure then is cooled in oil.During tempering it is heated up to 650°C temperature, hold at it and slowly cooled in the air.Tempering temperature was chosen in order to obtain the desired hardness in the range of 280-320 HRC.Microstructure of the steel consisted of ferrite and pearlite (white and dark areas), and is shown in Fig. 1, a.
The heat treatment of St-2 was normalization (austenitizing at 870 o C, temperature 30-50 o C higher than austenitizing temperature and air cooling to room).It was attempt by normalization to obtain fine grained structure of steel whose hardness and strength should be slightly higher than after annealing.Normalized steel's structure consisted of fine grained pearlite and ferrite, and is shown in

Mechanical and dynamic properties
St-1 static mechanical properties were determined by testing round tensile specimens (d = 8 mm), made of CT specimens after their cyclic tests.Determined mechanical properties are as in Table 2. St-1 hadn't the yield point, therefore the 0.2% offset yield strength was determined.Hardness BHN = 288 ... 299slightly ranges and satisfy the values required for gears.Large dispersal of the values is explained by the fact that the workpiece for specimens was cut off from rolled steel beam, for which due to large dimensions it was difficult to ensure uniform heat transfer during heat treatment procedure.Larger determined values were of specimens of superficial layer, the smaller ones are of specimens cut off in around near the workpieces centre.
The Charpy notch type (10 x10) specimens for impact tests were made of workpieces.

Resistance to cyclic loading
Resistance to cyclic loading was determined according to ASTM E 647-93.In CT specimens, by applying cyclic loading, the crack is grown up, which is periodically stopped up to defined crack propagation rates.By the above referred methodises the crack is stopped until crack propagation rate decreases up to 10 -10 m/cycle [8,9].In order to apply the obtained results for larger operational longevity in this work the threshold was fixed at crack propagation rate less than 10 -11 m/cycle.
Compact tension specimens (CT) for determination of stress intensity factor K were produced from both steels.CT specimen is presented in Fig. 5. St-1 specimens were made of specially prepared plate, and the cutting scheme of St-2 CT specimens is presented in Fig. 2. The notch for each specimen was made in different direction.The stress intensity factor ΔK = K max -K min was calculated by ASTM E 647-00 formula: where     According to the data [7] and Fig. 6 the dependencies of threshold stress intensity factor range on ultimate stress R m and yield limit R el (R 0.2 ) ratio (Fig. 7, a); and hardness (Fig. 7, b) are constructed for both steels.From the data it is clear that ΔK th increases due to increased plasticity.Values of ΔK th for steel St-2 increases for decreasing hardness in comparison with steel St-1.
After the threshold stress intensity factor determination tests the CT specimens were tested for the fracture toughness.If the stress-strain state at the vicinity of crack tip becomes critical, the sudden fracture arises.The critical stress-strain state is described by parameter K IC (fracture toughness), which quantitatively evaluates the material's property to resist brittle fracture.Fracture toughness is obtained according to the standard ASTM E 399-83 [8].The curves of fracture toughness determination test are presented in Fig. 8. Curve "1" is of St-1 specimens.This curve is II standard type crack opening diagram [9], with the force F max between lines 0-a and 0-b and F max = F Q .Curve "2" is of St-2 specimen is a I type diagram, where the forcevalue is as F 5 = F Q .Value of critical stress intensity factor K C is determined using formulas ( 1) and ( 2).It can be used as parameter of fracture toughness K IC if four additional conditions are fulfilled; if conditions are not satisfied then instead of K IC the K C is determined.In all specimens of St-1 crack size is out of the range of 0.45W ≤ a ≤ 0.55W, so the values of K C were determined and had spread in the range from 51.3 up to 74.9 MPa•√m.For St-2 specimens requirement of F max /F Q ≤ 1.1 was not satisfied, and also the K C were determined and ranged from 75.6 up to 86.2 MPa•√m.
In order to obtain better understanding of fatigue crack propagation fractured views were examined in optic microscopy (Fig. 9) and scanning electron microscopy (SEM) as presented in Fig. 10 and Fig. 11.
Fractured micrographs of CT specimens are presented in Fig. 9. Tonques initiated near to the few cracks origins are observed in the influence zone of the notch.Further the tonques coalescence into the main crack after its length reached approximately 3 mm.In fatigue zone the tunneling effect was observed caused by stress-strain state.Zone of static break for selected St-1 specimen is as brittleto-ductile shape, for other specimens also.For St-2 this zone is mainly brittle, but in the part between fatigue zone and static zone ductile fracture signs and tunneling effects are observed.The flutes seen on the CT fractures (Fig. 9) are obtained due to the load decreasing procedure used for threshold determination.Magnified fractures of St-1 specimen are presented in Fig. 10.Round-and oval shaped dimples are observed in fatigue zone perpendicular to crack propagation front.It is a crystals of pearlite or its groups.In the view presented in Fig. 10, a one could observe inclusions of heterogeneities which consist of nitrides, oxides, carbides and sulphides.In these places dominate flakesround shaped or elliptical internal voids, which are as silver colour flakes in the break as presented in Fig. 10, b.It makes influence for crack propagation and change size and direction of fatigue striations.
Views of St-2 specimen 4 th (Fig. 9, b) fatigue zone are presented in Fig. 11, where brittle-to-ductile fracture mechanism is observed.Fatigue striations, broken crystals (caused due to granular and intergranular fracture), failure facets, differently sized and shaped dimples are observed.Larger and deeper material destructions, wider dimples are caused by non-metallic inclusions between grains of different size.Fractured views of tension specimens, CT and Charpy specimens (macro-and micromechanisms of failure of specimens cut of the CCM roll surface with crack-like defects [10,11]) are compared with material microstructure, described by different sized and shaped crystals of ferrite and pearlite, various inhomogeneities effecting static, dynamic and cyclic fracture.Insignificant change of chemical content, heattreatment allowed to obtain materials usable for structural elements with different functionality.

Conclusions
1. Low carbon steel AISI 4130 used for evaluation of suitability for parts of mining equipment was prepared in order to determine an influence of thermal treatment on mechanical, dynamic properties; and cyclic resistance.The analysis revealed that above mentioned properties are spread in a wide range.

Fig. 7
Fig. 7 Dependence of threshold stress intensity factor range on plasticity (a) and hardness (b)

Table 2
2. Static mechanical properties show that steel's St-2 tensile strength R m = 640 ... 732 MPa is less in comparison with the steel's St-1 tensile strength R m = 938 ... 942 MPa.The yield stress is also less but the plasticity is higher.3. Determined values of resistance KCV to impact loading obtained by Charpy specimens: for steel St-1 are 28.1 … 32.0 J; and for St-2 are 44.9 ... 78.2 J, spread is caused by inhomogeneous microstructure of material.4. Steel's St-2 threshold at low crack propagation rates da/dN < 10 -11 m/cycle is ΔK th = 9.3 ... 10.6 MPa•√m and is approximately 20 % larger than it is of St-1 ΔK th = 7.1 ... 8.8 MPa•√m.