ESTIMATION OF FRACTURE TOUGHNESS OF THE OXIDE SCALES. Jozef HRABOVSKÝ, Petr LOŠÁK, Jaroslav HORSKÝ

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1 ESTIMATION OF FRACTURE TOUGHNESS OF THE OXIDE SCALES Jozef HRABOVSKÝ, Petr LOŠÁK, Jaroslav HORSKÝ Heat Transfer and Fluid Flow Laboratory, Faulty of Mehanial Engineering, VUT Brno, Tehniká 2, Brno, Czeh Republi, Abstrat Oxide sale growth and its removal from steel surfae during heat treatment of steel is a omplex and exating proess requiring a lot of information and analyses. Therefore many methods and approahes were developed to solve it. In order to improve the final quality of the surfaes after desaling it is neessary to pay attention to all aspets related to desaling. The basi aspets inlude sale growth, properties and behavior, parameters affeting its removal. This paper is foused on the study of frature toughness of oxide sales. Study of frature toughness is fundamental for understanding the behavior of oxide sales. For the analysis of frature toughness Small Punh Testing was used. Small Punh Tests are generally used in energy industry, when it is neessary to determine the mehanial properties of the material that has been exposed to environmental influenes or load without signifiant damage of the devie. The typial result of the Small Punh tests is fore vs. displaement urve. Out of this urve it is also possible to evaluate the frature toughness using several methods. Experimental measurements of mehanial properties of oxide sales were based on two types of steel. The first type represents the strutural steel, the seond type of steel is haraterized by higher silion ontent. For both materials the speimens with oxide sales layer were prepared. The Small Punh Tests were arried out and fore vs. displaement urve was obtained. Out of these urves the frature toughness of eah material was evaluated. Keywords: oxide sales, Small Punh Test, frature toughness, fore-displaement urve 1. INTRODUCTION A measurement of frature toughness is desribed in all known national and international standards from ASTM to ISO. All standards desribed the type of experimental devies, shape of the experimental speimens and measurements proess of frature harateristis. The basi frature harateristis are K IC, J IC, CTOD (rak tip opening displaement). A frature toughness experiments are foused on the measurement of the resistane of a material to rak extension. Experimental measurements of frature toughness bring a single value of frature toughness (K IC, J IC, CTOD) or a resistane urve. All frature harateristis are depited against rak extension [1]. In this paper small punh tests (SPT) were used for estimation of the frature toughness harateristis of the speimens with oxide sale layers. The SPT method is onsidered as non-destrutive test. The speimen for this test is typial by small dimensions and it is possible to obtain the speimen without signifiant destrution of the examined omponent. The SPT an be applied in two experimental ases. The first ase is haraterized by punhing the speimen at defletion rate. The typial results of this first ase are fore displaement urves. This ase of SPT is equivalent to standard tensile tests. Test at onstant defletion rate is referred to as SPT-CDR [2]. The seond experimental ase is defined by punhing at onstant fore CF-SPT. The results of this test are defletion in time. This type of experiments is used as the reep test [2]. This paper is foused on the study of frature toughness of two types of material with defined oxide sale layer based on the small punh tests results. An integral part of all steel prodution is the formation of oxide sales in a variety of morphologial and hemial onditions. All modifiations have been sorted into three basi groups, depending on the phase of manufaturing proess. In the first phase primary sales are formed, in the seond phase seondary sales grow and in the final phase of manufaturing proess tertiary

2 sales grow. Heat treatment of steel leads to growth of oxide sales in the three types of hemial ompounds: wüstit (FeO), magnetite (Fe 3 O 4 ) and hematite (Fe 2 O 3 ). These hemial ompounds of iron and oxygen are present in different amounts in all types of sales (primary, seondary and tertiary). The growth of eah layer has followed the LeChatelier priniple. The inner layer is wüstit, middle layer onsists of magnetite and top layer is reated from hematite [3]. 2. EXPERIMENTAL APPROACH Experimental approah that has been used in this paper an be sorted into three basi groups. The first group is harateristi by preparation proess of speimens and its use. The seond group is foused on SP testing. The last group is devoted to the evaluation and analysis of the results. 2.1 Experimental proedures and material seletion The fundamental goal of experimental proedures was to identify and study mehanial behavior of the base material with oxide sale layer. Two types of steel were used to experimental measurements. The first type of steel was strutural steel (type A). The seond type of steel was haraterized by higher ontent of silion (type B). The reason for hoosing these two different types of steel is its harateristi. The strutural steel is haraterized by a rapid growth of oxide sales and its relatively simple removing. The silion steel is typial by slower growth of oxide sale layers but also by reating stronger bounds with base material whih ompliates its removing. The hemial omposition of material A and material B is presented in Tab. 1. Tab. 1 Chemial omposition of the speimens Speimen C (%) Mn (%) Si (%) P (%) S (%) Cr (%) Cu (%) Sn (%) A B The speimens used for SPT measurements were prepared in several phases. In the first phase the speimens of eah material were inserted into the eletrial furnae at temperature 1000 C during defined time interval. The atmosphere inside furnae was not ontrolled. For material type A five speimens were prepared. The speimen type A was omposed from base material and oxide sale layers. For material B similar speimens as for material type A were prepared. Thus prepared speimens were used for SPT measurements. The SPT measurements were arried out at elevated temperature (600 C 880 C) for all prepared speimens. Temperature during the tests was maintained onstant within ± 0.5 C. Defletion rate at measurement was set up to mm/s. This defletion rate orresponds with movement of the upper frame of measuring apparatus. SPT measurements were arried out on the experimental apparatus onstrution by IPM ASCR Brno (see Fig. 1). As the results of SPT measurements the fore-displaement urves were obtained for all prepared speimens at defined temperatures. The measured urves for eah speimen are depited in Fig. 2. Shape of the speimens after SPT measurements are presented in Fig. 3.

Fig. 1 SP experimental apparatus outside view (left), inside view (right) Fig. 2 Results of the SP measurements 600 C 700 C 800 C 874 C 600 C 700 C 800 C 870 C Fig.

RESULTS EVALUATION In this paper analysis of the frature harateristis obtained from the measured fore displaement urves is based on the empirial equations presented in literature.

3 Fig. 1 SP experimental apparatus outside view (left), inside view (right) Fig. 2 Results of the SP measurements 600 C 700 C 800 C 874 C 600 C 700 C 800 C 870 C Fig. 3 Shape of the studied speimens after SP tests 3. RESULTS EVALUATION In this paper analysis of the frature harateristis obtained from the measured fore displaement urves is based on the empirial equations presented in literature. The aim of this analysis was to estimate the basi frature harateristis of the measured speimens. Evaluation of the frature harateristis was arried out in next steps. In the first step frature energy from area under measured fore displaement urves was evaluated. Evaluation of the area under measured urves an be arried out based on the several

assumptions. As the first assumption the whole area under fore displaement urve is onsidered that defines the fore needed to final frature of the speimen [2].

4 assumptions. As the first assumption the whole area under fore displaement urve is onsidered that defines the fore needed to final frature of the speimen [2]. Another possibility of the frature energy evaluation is onsideration the area under the urve to the value 0.8*F m [2]. This value approximately orresponds with point where the rapture of the speimen ours. The last evaluation possibility of the frature energy is to onsider the area under fore displaement urve to the value F m [2]. This value is reahed at maximum fore during SPT measuring. All desribed possibilities of the frature energy evaluation are depited in Fig. 4. To evaluate the frature energy of the speimens type A and type B the third mentioned method was used. For all speimens the evaluation of the frature energy from the measured urves was performed in MATLAB. The evaluation method onsisted of alulating the definite integral of the measured urve with lower boundary orresponding to beginning of the measurement and upper boundary orresponding with maximum fore F m. The alulations of the definite integrals whih orrespond with the frature surfae of all desribed possibilities are shown in Fig. 4. Evaluated frature energy for all measured speimens depending on the temperature is presented in Fig. 5. Fig. 4 Evaluation methods of the frature energy In the next step of the frature harateristis analysis existing theories desribing the speimens rapture proess at SPT measurements were used. A lot of authors foused on the issue of frature harateristis evaluation based on the SPT measurements. The most famous authors whih were also referened in this paper are X. Mao at al. or J. H. Bulloh. Aording to the basi theory during SPT in a speimen of biaxial stress and strain our [4], [5]. Mao at al. presented proess of the frature strain (ɛ qf ) from fore displaement urves evaluation aording to equation (1) [4]. J. H. Bulloh presented several empirial equations for the studied materials, he defined the ritial biaxial strain (ɛ ) based on the frature energy evaluated from fore displaement urve. One of this is presented below equation (2) [5]. The results of the ritial strain values aording to defined equations for all measured speimens are presented in Fig. 6. In this paper linear relationships between frature energy and frature strain were also evaluated as performed by J. H. Bulloh (see Fig. 6). qf 0.12( / t0) 1.72 (1) 2 (3) J kj / m IC 0.54ESP (2) J kJ / m (4) IC J IC kj / m 2 (5) The final step in the frature harateristi analysis based on the SPT measurements was to evaluate the frature toughness. The frature toughness an be defined as material resistane to rak initialization and growth. The frature toughness an be defined by single value of the parameter K IC, ᵹ or J IC. Authors dealing with the evaluation of frature toughness from SPT measurements defined the onversion between results of the measured data and frature toughness value J IC. The basi differene between J IC and K IC is that the parameter J IC overs the elasti plasti behavior around the rak and K IC overs only elasti behavior. Mao at al. [4] presented the equation for estimation of the frature toughness J IC based on the ritial strain ɛ, as desribed in previous setion (equation (3)). Other authors Suzuki et al. equation (4) [6] or Lee at al. equation (5) [6] defined his empirial equations of frature toughness evaluation from SPT

5 measurements. The empirial equations presented by the referened authors were applied to the measured speimens. The evaluated results are depited in Fig. 7. Fig. 5 Evaluation of the frature energy E SPT Fig. 6 Evaluation of the ritial strain vs. frature energy Fig. 7 Evaluation of the frature toughness J IC

6 Fig. 8 Evaluation of the frature toughness vs. frature energy 4. CONCLUSION Evaluations of the frature toughness and frature harateristis were based on experimental results obtained from small punh tests and empirial equations from literature. Signifiant frature harateristis as ritial strain (rapture of the speimen ours) and frature toughness J IC were evaluated for studied types of materials. In the first step the frature energies obtained from integration of the fore displaement urves were evaluated. The values of the frature energy depending on the temperature with exponential trend line with 90% reliability were presented. Based on the frature energy the ritial strain and frature toughness were evaluated. Evaluation of the ritial strain and frature toughness depending on the frature energy onfirmed linear relationship. The linear relationship was presented by referened authors. Frature toughness J IC depending on the temperature was evaluated aording three empirial equations defined by three referened authors. This evaluation is ompared and presented. The omparison of the results shows that for material A the frature toughness evaluating aording to Mao at al. and Lee et al. reahed negative values, whih is not realisti. These results onfirm that it is not possible to use empirial equations without their verifiation. To hek the validity of applied equations for studied materials numerial analysis for orrelation of the parameters of the used equations should be arried out, or experimental measurements of other frature harateristis should be performed. ACKNOWLEDGEMENT The paper presented has been supported by projet CZ. 1.07/2.3.00/ and projet CZ.1.07/2.3.00/ of Brno University of Tehnology. REFERENCES [1] ANDERSON, T. L. Frature Mehanis - Fundamentals and Appliations, Seond Editions, CRC Press In. 1995, ISBN [2] MILIČKA, K., DOBEŠ, F. Small punh testing of P91 steel, Int. J. Pressure Vessels Piping, Volume 83, Issue 9, 2006, Pages [3] BLAZEVIC, D. T. Hot strip mill operations, Volume V, Sales, Sun Lakes, Arizona, USA, 2005 [4] KAMEDA, J., MAO, X. Small-punh and TEM-dis testing tehniques and their appliation to haraterization of radiation damage, Journal of Material Siene, 27, 1992, Pages [5] BULLOCH, J. H. A study onerning material frature toughness and some small punh test data for low alloy steel, Engineering Failure Analysis, 11, 2004, Pages [6] KAISHU GUAN, LI HUA, QIONGOI WANG, XIAOUHI ZOU, MING SONG. Assessment of toughness in long term servie CrMo low alloy steel by frature toughness and small punh test, Nulear Engineering and Design, 241, 2011, Pages

Arch. Metall. Mater. 62 (2017), 1,

Arch. Metall. Mater. 62 (2017), 1, Arh. Metall. Mater. 6 (017), 1, 167-17 DOI: 10.1515/amm-017-003 P.G. KOSSAKOWSKI* # EXPERIMENTAL DETERMINATION OF THE VOID VOLUME FRACTION FOR S35JR STEEL AT FAILURE IN THE RANGE OF HIGH STRESS TRIAXIALITIES