Influence of long-term heating on the fatigue properties of low-alloyed cast steel STN 42 2707

In this paper the authors publish own experimenta l results of examination of low-alloyed cast steel STN 42 2707 fatigue properties (nearthreshold regime of fatigue crack propagatio n for rates from da/dN =5.6x10 m.cycleto da/dN =10 m.cycle and Kath at da/dN =10 12 m.cycle) initial state and after long-term heating (T = 40 0 °C, t = 4000 hours ≈ 167 days) obtained at high-frequency loading (f ≈ 20 kHz, T = 20 ± 5 °C, R = -1). The long-term heating of mate ri l caused decrease of K ath and increase of fatigue crack growth rate in the n ear-threshold regime; then worse of low-alloyed cast steel STN 42 2707 fatigue properties. Key words– cast steel STN 42 2707, long-term heating, fatig ue properties, Kath, da/dN = f(Ka)


Introduction
Fatigue is a predominating fracture mode of components and constructions.Therefore, its prevention is a fundamental design criterion (BOKŮVKA O. et al. 2002, ULEWICZ R., MAZUR M. 2013).Lifetime of components or constructions (number of cycles N) contains number of cycles necessary for fatigue crack nucleation N i and number of cycles for crack propagation N s .Components or constructions in terms of their resistance to fatigue failure can be evaluated by two methods; that is according to total lifetime (number of cycles N, where N = N i + N s ) or according to fatigue crack propagation with respect to laws of fracture me-chanics (according to number of cycles necessary for crack propagation N s ) (TRŠKO L. et al. 2013, SKOČOVSKÝ P. et al. 2015).Evaluation of components and constructions resistance according to fatigue crack propagation describes well the real state, because the most of operation fractures is caused by fatigue crack growth from defects of different character.Amplitude of stress intensity factor K a is the determining characteristic for describing fatigue crack propagation.In this case for evaluation of material fatigue properties is used experimentally obtained fatigue crack growth curve that is dependenton the rate of fatigue crack growth da/dN on the amplitude of stress intensity factor K a and where the K ath is basic threshold value of amplitude of stress intensity factor K a .In this curve there can be observed the near-threshold regime (da/dN< 10 -9 m.cycle -1 , mid-growth rate regime (da/dN = 10 -9 m.cycle -1 ÷ 10 -6 m.cycle -1 ) and high-growth rate regime (da/dN> 10 -6 m.cycle -1  In this paper the authors publish own experimental resultsof examination of low-alloyed cast steel STN 42 2707 fatigue properties (near-threshold regime incl.K ath ), initial state and after long-term heating, obtained at high-frequency loading (f = 20 kHz, T = 20 ± 5 °C, R = -1).

Experimental part
The experimental work, quantitative chemical analysis, optical light microscopy, tensile tests and fatigue tests were carried out on two different states (initial and after long-term heating) of low-alloyed cast steel STN 42 2707.Chemical analysis was performed by the emission spectrometry on an ICP (JY 385) emission spectrometer using a fast recording system Image.A light metallographic microscope AXIO Imager A1m was used for metallography analysis.Tensile tests were carried out on a ZWICK Z050 testing machine at ambient temperature of T = 20 ± 5 °C, the loading range in interval F = 0 ÷ 20 kN and the strain rate ɛ m = 10 -3 s -1 .Round cross-section specimens were used; the shape and dimensions of the test specimens met the requirements of EN 10002-1 standard (5 specimens were used).Fatigue tests were carried out at high-frequency sinusoidal cyclic tension-compression loading (f = 20 kHz, T = 20 ± 5 °C, R = -1, cooled by  The shape and dimensions of specimens used in the fatigue crack growth tests are given in Fig. 1.The applied amplitude of stress intensity factor K a was determined using the following equation: in which σ a is stress amplitude (MPa), a is the half crack length (m) and W is the specimen width (m), m.cycle -1 to da/dN= 10 -12 m.cycle -1 and the value of K ath was determined at da/dN= 10 -12 m.cycle -1 ; four specimens of initial state of low-alloyed cast iron and the four specimens after long-term heating (T = 400 °C, t = 4000 hours (≈ 167 days) in the furnace with protective atmosphere) were used.

Results and discussions
The results of quantitative chemical analysis (chemical composition), tensile tests (yield point R p 0,2 tensile strength R m , elongation A 5 , reduction of area Z, toughness KCV +23 ), optical light microscopy (microstructure) and high-frequency fatigue tests (da/dN = f(K a ), K ath )) are shown in Table 1, Table 2, Table 3 and on the Fig. 2 and Fig. 3.The chemical composition and mechanical properties of tested material, Table 1, 2 correspond with STN 42 2707 standard.The microstructure (Fig. 2) was created by ferrite and perlite and is characteristic for cast steel.
The obtained results about fatigue resistance of low-alloyed cast steel STN 42 2707 are shown in Table 3 and on the Fig. 3.The basic amplitudes of stress intensity factor K ath were K ath = 5.14 MPa.m 1/2 (initial state A) and K ath = 4.46 MPa.m 1/2 (state after long-term heating, B); decrease (B) against (A) state is cca 13.2 %.We can calculate the critical size of crack a c (permissible size of defect) with using equation ( 2), where K ath is basic threshold value of amplitude of stress intensity factor (MPa.m 1/2 ) and σ s the safe stress (MPa); σ s = 80 MPa and is valid in the power industry for water pumps design.The critical size of defect is a c = 1.31 mm (for initial state, A) and a c = 0.98 mm (for state after long-term heating, B); decrease of this value for heat threated state against value for initial state is cca 25.With regard to the results,it can be assumed that ageing is the reason of fatigue resistance decrease.This effect is observed very often in the mild steels.The aging is the reason for changes of material strength and deformation properties.The tensile strength, yield point, hardness increase but, on the other hand, the elongation and toughness decrease.The size of plastic zone on the crack tip decrease.With regard to this fact, the retardation effect against fatigue crack growth decrease and therefore decrease the value of K ath (PÍŠEK F. et al. 1975).

Conclusions
With reference to the experimental results carried out at high-frequency loading (f = 20 kHz, T = 20 ± 5 °C, R = -1) the following can be summed up: • in the low-alloyed cast steel STN 42 2707 was recorded negative influence of long-term heating (T = 400 °C, t = 167 days) on her fatigue resistance in the near-threshold regime; • the basic threshold value amplitude of stress intensity factor K ath due to long-term heating decrease cca 13.2 % (initial state K ath = 5.14 MPa.m 1 ; state after long-term heating, K ath = 4.46 MPa.m 1/2 ); • the critical defect size decreasecca 25.2 % (initial state, a c = 1.31 mm ; state after long-term heating, a c = 0.98 mm); • in the mid-growth regime of da/dN = f(K a ) curve was not recorded differences in fatigue behavior; • these facts must be taken in consideration with reference to reliability and safety when designing the structural elements.

Fig. 1 .
Fig.1.The shape and dimensions of the fatigue crack growth test specimenSource: own study

Table 2 .
Mechanical properties of low-alloyed cast steel STN 42 2707 Source: own studyFig.2Microstructure of low-alloyed cast steel STN 42 2707, etch.3 % Nital Source: own study Table 3. Basic threshold values of amplitude of stress intensity factor K ath ,, low-alloyed cast steel STN 42 2707, initial state (A) and after long-term heating (B), high frequency fatigue loading (f = 20 kHz, T = 20 ± 5 °C, R = -1)Source: own study 1/2(3)where R m is tensilestrength[MPa].This equation is valid in the region from R m ≈ 360 MPa to R m ≈ 1040 MPa, after R m ≈ 1040 MPa the K ath is constant, K ath = 3.21 MPa.m 1/2 .With using equation (3) the value of K ath = 6.17MPa.m 1/2 .The experimentally determined value of K ath in the case of low-alloyed cast steel STN 42 2707 is lower.The obtained results mentioned above confirm the conclusions about a considerable influence of different factors(e.g.structure, temperature, mechanical properties incl.toughness) on the fatigue resistance of structural materials in the near-threshold regime (TRŠKO L. et al. 2013, RITCHIE R. O. 1981,BOKŮVKA O. et al. 2015).The results (Fig.3) after da/dN = 10 -10 m.cycle -1 are for the both state (initial and state after long-term heating), is practically the same.It corresponds with works (KUNZ L. 2003, RITCHIE R. O. 1981,BOKŮVKA O. et.al. 2015, TRŠKO L. et al. 2016) which stated small influence of structure, temperature, mechanical properties incl.toughness, σ m , and enviroment on the dependence of rate of fatigue crack growth da/dN vs. amplitude of stress intensity factor K a in the midgrowth rate regime.