fracture toughness determination of heat treated aisi d2

ISIJ International, Vol. 51 (2011), No. 2, pp. 305–312

1. Introduction

AISI D2 cold-work tool steel is used extensively for op-erations when high levels of properties like hardness,strength and wear resistance are required. However, im-proving these parameters made by tempering procedureleads to considerable affect on the fracture toughness (KIC)and microstructure. Because of the high alloying content,the AISI D2 steel has high strength but, relatively lowtoughness. Strength, wear resistance and impact toughnessof AISI D2 steel have been studied considerably.1,2) How-ever, not so much work has been reported on the fracturetoughness of AISI D2 steel due to the experimental difficul-ties.

The material property associated with the ability to carryloads or resist deformation in the presence of a crack is de-fined as the fracture toughness. It is a comprehensive mate-rial property determined by the fracture mechanism, mi-crostructure, etc., of the material. To determine fracturetoughness of metallic materials a large number of standardsare currently available. These standards use load-displace-ment plot to estimate the fracture resistance of the materi-als.

In terms of real-time monitoring of the fracture process,the use of acoustic emission technique (AE) has proven tobe an excellent diagnostic tool. To exploit this potential ofAE, extensive work has been done using AE studies to un-derstand fracture processes.3–8) Moreover, some investiga-tors9–12) have carried out conventional fracture toughness

tests in liaison with AE monitoring; however, calculatingKIC value using AE technique in current studies is few innumber. These numerable investigations have used acousticemission cumulative counts (AECC) characteristic to deter-mine the critical load value (PQ). This method estimates the(PQ) value easily, but in some cases,9–11) considerable differ-ences have been reported between calculated (PQ) valuesusing ASTM standard method and AE technique.

In this study, acoustic emission analyses have been madesynergistically with standard fracture toughness tests (asper the guidelines of ASTM E399) in order to determinethe critical load value (PQ). The estimation of (PQ) valuesof the selected steel is carried out using cross examinationof AE parameters (AECC, acoustic emission counts rate(AECR) and acoustic emission energy rate (AEER)) varia-tion, versus time, against the recorded data of ‘load versustime’ as obtained from the fracture toughness tests.

The selected steel (AISI D2) was austenitized (1 010°C)and tempered (0–575°C) then, studied on the basis of itstoughness variation with tempering temperature. Plainstrain fracture toughness (KIC) and fracture mechanisms ofAISI D2 cold-work tool steel was investigated using ASTMstandard and AE techniques. So two major purposes of thisinvestigation are as below:(1) Studying the effect of tempering procedure on fracture

mechanism and fracture toughness of AISI D2 steel,and

(2) Developing new techniques to determine values usingacoustic emission characteristics.

Fracture Toughness Determination of Heat Treated AISI D2 ToolSteel Using AE Technique

Cevat Teymuri SINDI, Mehdi Ahmadi NAJAFABADI and Seyed Ali EBRAHIMIAN

Department of Mechanical Engineering , Amirkabir University of Technology (Tehran Polytechnic), No. 424, Hafez Avenue,Tehran 158754413 Iran. E-mail: [email protected]

(Received on April 26, 2010; accepted on October 7, 2010)

In this study, acoustic emission behavior of a tool steel during fracture toughness tests was investigated.Selected steel (AISI D2 cold-work tool steel) was heat treated at 5 different conditions (austenitized at1 010°C and tempered at 0, 300, 450, 525 and 575°C) and its properties were characterized using standardmetallographic examinations, hardness and tensile tests. Compact specimen testing according to ASTMstandard E 399 and acoustic emission technique were used to determine the fracture toughness value (KIC)and fracture process. Determination of fracture toughness using AE technique was carried out according toclassical and two modified methods. Results showed that: (a) by increasing tempering temperature, plainstrain fracture toughness (KIC) values and the fraction of ductile fibrous fracture increase except at 525°Cwhen tempering process leads to secondary hardening, (b) fracture toughness values determined, using AEtechnique is lower than standard ASTM E399, (c) estimation of KIC values, using modified methods (AECRand AEER), is more accurate in comparison with the classical AE method and (d) the effect of temperingtemperature on AE characteristics during fracture toughness test is similar to its effect on hardness.

KEY WORDS: acoustic emission; fracture toughness (KIC); AISI D2; tool steel; heat treatment.

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2. Experimental Procedure

2.1. Material

The selected steel was AISI D2 High Carbon HighChromium Cold-Work tool steel. The chemical composi-tion of the investigated steel was analyzed and the resultsare given in Table 1.

2.2. Heat Treatment

After preparing starting material in the form of 50�50mm, fracture toughness test specimens were machined andsubjected to the following heat treatment:(a) Cyclic annealing (heating to 900°C and keeping for

2 h, cooling very slowly to 775°C and keeping for 6 hfollowing with air cool).

(b) Austenitizing at 1 010°C for 15 min followed by aircooling.

(c) Double tempering of the solution treated specimens at300°C, 450°C, 525°C and 575°C for 1 h each time,followed by air cooling.

It should be noted that tempering temperature at secondtimes was 50°C lower than the first time. Heat treated speci-mens, according to their tempering temperature, are de-noted as A, B, C, D and E (Table 2).

2.3. Fracture Toughness Test

To determine fracture toughness value (KIC), compacttension [C(T)] specimens were used. Specimens were 36�34 mm in size and 8 mm in thickness. The size and geome-try of the test specimens are shown in Fig. 1.

On the basis of ASTM standard E399,13) it was demon-strated that plain-strain condition can be achieved if the fol-lowing relation holds true:

..........................(2-1)

Where b is the thickness of the specimens, KQ is the condi-tional fracture toughness value and sys is the yield strengthof the material.

The crack starter slot was cut after preparation and heattreating, following with pre-cracking the specimens toachieve a/W�0.5. Then, specimens were loaded at the rateof 0.2 mm/min to carry out fracture toughness tests. All

tests were performed on a servo-electric Instron machine(model 8500) and load-CMOD (Crack Mouth Opening Dis-placement) data for each of the specimens was recorded forsubsequent analysis to estimate their fracture toughness val-ues. These tests were carried out following the guidelinessuggested in ASTM standard E399-01.13) Fracture tough-ness tests were carried out at ambient temperature (about25°C).

2.4. AE Monitoring

During the fracture toughness tests, AE signals wererecorded synergistically using a PCI-2 AE System. Threepiezoelectric (PZT) sensors with a bandwidth from 100 to300 kHz and 40 dB pre-amplification, threshold level of thefilter 0.20 V and 3 ms of events interval (other characteris-tics) were used to monitor the AE signals. The first sensorwas coupled to the specimen and two others were coupledto machine jaws for detecting machine noisesusing petroleum jelly. Because that AE signals generatedduring crack growth are propagated as transverse waves onthe surface of material, the sensor is placed on the plan ofspecimens parallel with loading direction. Attached posi-tion of the AE sensor on the samples has been shown inFig. 2.

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ISIJ International, Vol. 51 (2011), No. 2

306© 2011 ISIJ

Table 1. Chemical composition of the investigated AISI D2 steel (in wt%).

Table 2. Denoting heat treated specimens based on temperingtemperature.

Fig. 1. Geometry of the C(T) fracture toughness test specimens.

Fig. 2. Attached position of the AE sensor on the samples.

2.5. Microstructure and Fractography Investigation

Microstructure of the specimens with various heat treat-ment conditions, were investigated using optical mi-croscopy (OM). For OM observation, specimens were firstmechanically polished, then etched with 4% Nital reagent.

After fracture toughness tests, fractography examinationwas carried out to determine fracture mechanism of thespecimens. The fractography analysis was carried out onfractured surfaces near the central plane of the fracturedsurface of each tested specimen using scanning electron mi-croscopy (SEM).

3. Results and Discussion

3.1. Calculation of KIC Value Using ASTM StandardE399

Plain strain fracture toughness (KIC) tests were made asper the guidelines of ASTM standard E399.13) The load-CMOD plots of specimen A2 and the procedure used fordetermination of the critical load (PQ) have been shown inFig. 3. As it has been presented, after plotting tangent lineto the initial linear part of the recorded load-CMOD plot(Fig. 3(a)), the secant line through the origin of the testrecord with slope of 0.95 the line plotted in Fig. 3(a) havebeen drown (Fig. 3(b)). The critical load (PQ) is correspon-dent to the point that has maximum value on the record

which precedes the secant point made by recorded plot anddrown line.

Now, the conditional value of fracture toughness (KQ) iscalculated using the following expressions13):

..................(3-1)

Where f(a/W) is:

.....................(3-2)

Yield strength (sys), critical load (PQ), Pmax/PQ ratio andconditional fracture toughness (KQ) values are listed inTable 3. To determine sys values at various tempering con-ditions, tension tests were made as per the guidelines ofASTM standard E8.14)

Considering the estimated KQ and the yield strength val-ues of the investigated steel in different tempering condi-tions, the minimum thickness required for valid KIC meas-urement derived from Eq. (3-2), have been shown in Table3. Comparison of the estimated minimum specimen thick-ness with the thickness of tested specimens, indicate that allestimated KQ values are valid. Also estimated Pmax/PQ ratio,that according to Ref. 13) must be (�1.15), shows that forall cases except sample C3, KIC values are valid.

The variation of plain strain fracture toughness (KIC)value versus tempering temperature has been shown in Fig.4.

Results showed that the specimens not tempered havelowest KIC values. Increase in tempering temperature leadsto the transformation of tempered martensite and conse-quently plain strain fracture toughness (KIC) values in-crease. At 525°C, we have a little decrease in fracturetoughness irregularly. This reversal in the usual relationshipis known as secondary hardening, and is caused by trans-formation of retained austenite during tempering at thehigher temperatures.2) As it can be seen from Fig. 4, athigher tempering temperatures, the (KIC) values increaseagain.

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307 © 2011 ISIJ

Table 3. The results of fracture toughness measurement of theD2 steel according to ASTM standard E 399.

Fig. 3. Load-CMOD plot of specimen A2: (a) tangent line and(b) the secant line with slope of 0.95 the tangent line.

3.2. AE Characteristics and Revelation of Crack Initi-ation

Typical plots of normalized AE cumulative energy andAE amplitude versus time for AISI D2 steel in differentheat treatment conditions have been shown in Figs.5(a)–5(d).

The point of crack initiation in the investigated steel ischaracterized by the first sudden rise in cumulative AE en-ergy along with high decibel signals. This point of crackinitiation for all specimens has been marked as ‘c’ in Fig. 5.As we see from Fig. 5, except in specimen tempered at

575°C, the unstable crack propagation is associated with al-most 100 dB signals. This can be explained by consideringthe nature of fracture in D2 steel tempered at low tempera-tures. As it has been demonstrated, usually brittle fractureproduces high amplitude and high energy AE signals incomparison with ductile fracture. It is probably because ofhigh elastic energy realizing during brittle type of fracture.

As Fig. 5(c) shows, for specimen tempered at 525°C,there are multiple crack jumps occured during tension load-ing. It is might because of transformed layers of retainedaustenite that creates an inhomogeneous structure resultingin different AE behavior of AISI D2 tempered at this tem-perature.

3.3. Determination of Fracture Toughness ValuesUsing AE Technique

During fracture toughness tests, as the loading increases,different mechanisms control the fracture procedure. Be-cause of this, various regions on AE event plots can be dis-tinguished. Analysis of AE plots monitored during fracturetoughness tests of AISI D2 tool steel, as a material havingrelatively low toughness, indicates that we can find 3 spe-cific regions. These regions are as follows:

Region 1: When the specimen is still in elastic state,some weak AE peaks occur. These weak peaks may becaused by decohesion or cracking of inclusions. Recordeddata in this region do not have so much worth in fracturemechanics and are ignored during data analysis.

Region 2: When the load reaches to a certain level, somestrong AE peaks are shown. Because of high strength of


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Fig. 4. The variation of plain strain fracture toughness (KIC)value with tempering temperature.

Fig. 5. Typical plots of AE cumulative energy (AECE) and AE amplitude with time during fracture toughness tests ofAISI D2 specimens tempered at: (a) 300°C, (b) 450°C, (c) 525°C and (d) 575°C.

AISI D2 tool steel and accordingly low plastic deformationat the crack tip of the specimens, these relatively high am-plitude signals are probably related to the initiation andpropagation of micro cracks. First increases in AE ampli-tude and cumulative counts usually happen during propaga-tion of these micro cracks. This region is separated from re-gion1 according to the considerable growth in AE countsvalue, determined easily by visual inspection.

Region 3: The last region in AE-time plots is concernedwith the quick crack propagation along with high decibelsignals that lead to a complete fracture. Occurring unstablecrack propagation, AE counts reach their maximum valuein this region.

Above regions are shown on AE-Time plot of the speci-men A2 in Fig. 6.

Estimation fracture toughness (KIC) by means of AEtechnique, is based on determining critical load value (PQ)using AE characteristics. In previous works4,5,15,16) almostall investigators have used the point corresponding to thefirst jump in AE cumulative counts plot as the critical load(PQ). Notwithstanding the advantages of this method, be-cause of that this first jump is probably corresponded topop-in fracture especially in brittle materials, in somecases, there are sizable differences between fracture tough-ness values calculated using this method in comparisonwith the standard ASTM E 399 results. The points of criti-cal load according to this technique for specimens C2 and

E3, are demonstrated in Fig. 7 and Fig. 8 respectively.In this study, to creation a more accurate method to cal-

culate the critical load (PQ) from acoustic emission activi-ties, two techniques are used:

a) Acoustic Emissions Count Rate (AECR), andb) Acoustic Emissions Energy Rate (AEER).In AECR technique, the load corresponding to the point

having the maximum value of AE Counts Rate Vs time, be-fore sudden drop of load, is considered as the critical load(PQ, AECR). Applying this method to investigate fracturetoughness value of the specimen D2 is shown in Fig. 9. Asit can be seen, the point marked as PQ, AECR indicates thecritical load value using AECR technique.

On the other hand, AEER technique uses the load corre-sponding to the point having maximum value of AE EnergyRate Vs time, before sudden drop of load, as the critical load(PQ, AECR). The point marked in Fig. 10 determines the criti-cal load (PQ, AECR) value in order to fracture toughness esti-mation of the specimen D2 using AEER technique. For ex-plicit comparing of all discussed techniques, points deter-mining critical load values of specimens B2 and C2 esti-mated using above methods are shown for two specimens inFig. 11 and Fig. 12.

As it can be seen from Fig. 11 and Fig. 12, AECR andAEER methods are more accurate than the technique basedon AE cumulative counts. Results of investigated average


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Fig. 6. Different regions on AE counts plot determine fracturemechanisms.

Fig. 7. Estimation of critical load (PQ) according to conventionalAE technique for specimen C2.

Fig. 8. Estimation of critical load (PQ) according to conventionalAE technique for specimen E3.

Fig. 9. Determination of critical load (PQ, AECR) for specimen D2using AECR technique.

fracture toughness (KIC) values using standard ASTM E399, conventional AE, AECR and AEER methods for fiveheat treatment conditions are listed in Table 4.

Figure 13 presents a comparison of (KIC) values given inTable 4. As Fig. 13 shows, AEER method is more accuratethan AECR and these two methods are very accurate incomparison with conventional AE method. Also it can befound that by increasing fracture toughness value, the dif-ference between KIC values determined using ASTM E399and conventional AE method increases. However, accuracyof AECR and AEER methods is independent of fracturetoughness values and has excellent adaption with ASTME399. Moreover, like previous reported investigations, all

AE based techniques determine KIC values lower thanASTM E399 method.

Generally, acoustic emissions observed during fracturetoughness tests are attributed to both plastic deformation atthe crack tip and crack growth. Since, in this study, AE ac-tivities were predominantly related to the initial crackgrowth under plane strain conditions, therefore, the pro-posed methods estimate the critical load for high strengthmaterials with relatively low ductility. Furthermore howeverin this study AECR and AEER techniques were used to de-termine fracture toughness values, because acoustic emis-sion is sensitive to almost types of deformations and trans-formations, also these techniques are suitable for investigat-ing other material properties such as yield stress and so on.

3.4. Effect of Tempering Temperature on AE Charac-teristics

The effects of tempering temperature on the cumulativecounts, cumulative energy, average amplitude and peak fre-quency for three subsets of high decibel signals (N�200,500 and 1 000) are demonstrated in Fig. 14.

As Fig. 14 demonstrates, by increasing the temperingtemperature, average AE counts, energy, amplitude andpeak frequency decrease till temperature 525°C. However,at 525°C, average value of investigated AE parameters, es-


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Fig. 11. Comparison of critical load values determined byASTM and AE techniques for specimen B2.

Fig. 10. Determination of critical load (PQ, AECR) for specimenD2 using AEER technique.

Table 4. The results of fracture toughness (KIC) measurementaccording to standard ASTM E 399, conventionalAE, AECR and AEER methods.

Fig. 13. Comparison of (KIC) values determined according tostandard ASTM E 399, conventional AE, AECR andAEER methods.

Fig. 12. Comparison of critical load values determined byASTM and AE techniques for specimen C2.

pecially for N�200, increases strongly. This irregular be-havior can be justified with respect to the effects of temper-ing procedure on microstructure. Not tempered specimenshave predominantly martensitic structure that leads to brit-tle fracture and subsequently high amplitude, energy, andcounts values. Tempering till approximately 450°C, reducesfraction of brittle fracture and therefore reduces the value ofAE parameters. At about 525°C we have second hardening

caused by transformation of retained austenite that leads toa considerable increase on the value of AE parameters. Asit is expected, increasing tempering temperature above525°C makes ductile fracture type and so, once again, de-crease is observed in AE parameters.

AE events related to the fracture of more brittle compo-nents in the microstructure usually have higher average fre-quency than those of less brittle ones. Thus, decrease in the


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Fig. 14. The effects of tempering temperature on AE characteristics: (a) Average counts, (b) Average energy, (c) Averageamplitude and (d) Average Peak Frequency.

Fig. 15. SEM fractograph of: (a) A1 showing the highest fraction of brittle cleavage fracture, (b) C3 showing increase ofdimples on surfaces, (c) D1 showing cleavage fracture on the secondary carbides and (d) E2 showing predomi-nantly ductile failure.

peak frequency for specimens tempered at temperatureshigher than 525°C can be related to the changes in the mi-crostructure fracture behavior.

3.5. Fractography

Analyzing fracture surfaces of tested specimens showedthat fracture mechanism of all specimens is a mixture ofbrittle cleavage on the primary carbides and ductile fibrousfracture. Figure 15 shows fractured surfaces of samplestempered at different temperatures. As can be found, high-est fraction of brittle cleavage fracture occurs when no tem-pering is applied (Fig. 15(a)).

Tempering investigated steel leads to an increase of dim-ples on surfaces along with improving (KIC) value (Fig.15(b)). In specimens tempered at 525°C, because of sec-ondary hardening, we see cleavage type of fracture on thesecondary carbides resulting in decrease of toughness value(Fig. 15(c)). Increasing tempering temperature, the fracto-graph of tested specimens reveals that the mechanism offailure is predominantly ductile with dimple formations onfracture surfaces (Fig. 15(d)).

4. Conclusions

From the study of AE characteristics of fracture tough-ness tests and fractography of AISI D2 tool steel, the fol-lowing conclusions can be drawn:

(1) Fracture toughness tests showed that not temperedspecimens have the lowest KIC values. Increasing temperingtemperature leads to the transformation of temperedmartensite and consequently plain strain fracture toughness(KIC) values increase. However at 525°C, there is a little de-crease in fracture toughness caused by transformation of re-tained austenite during tempering at the higher tempera-tures known as secondary hardening.

(2) Increasing the load during fracture toughness test,because of different fracture mechanisms, three regions onAE counts plot are observed. At first region, the specimenis still in elastic state and only some weak AE peaks occur.Reaching the load to a certain level, the second regionlooks with some strong AE peaks related to the initiationand propagation of micro cracks. The third region is con-cerned with quick crack propagation along with high deci-bel signals that lead to a complete fracture.

(3) Two techniques used to create more accuratemethod than conventional AE, were AECR and AEER.Critical loads determined using AECR and AEER tech-niques were corresponding to the point that has the maxi-mum value of AE Counts Rate and AE Energy Rate, re-spectively. AEER technique is more accurate than AECRand these two methods are very accurate in comparisonwith conventional AE method.

(4) Tempering process, affecting mechanical proper-ties, has an obvious effect on AE characteristics duringfracture toughness tests. Increasing tempering temperature,average value of counts, energy, amplitude and peak fre-quency decrease except at temperature 525°C.

(5) The fracture mechanism of tempered AISI D2 steelis a mixture of ductile fibrous and brittle cleavage. Increas-ing tempering temperature, (KIC) values and fraction ofductile fibrous fracture increase except at 525°C when tem-pering process leads to the secondary hardening.

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fracture toughness determination of heat treated aisi d2

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