REVISTA MEXICANA DE FISICA S 55 (1) 68–71 MAYO 2009

Microstructural aspects of the brittleness phenomenain steels induced by mill-annealing

J. Garcıa and J. HallenESIQIE-IPN, Unidad Profesional Adolfo Lopez Mateos,

P.O. Box 75-874, 07738, Mexico D.F., Mexico.

R. Esparza and R. Perez∗Instituto de Ciencias Fısicas, Universidad Nacional Autonoma de Mexico,

P.O. Box 48-3, 62251, Cuernavaca Mor., Mexico,Phone: +52 (55) 5622-7797, Fax: +52 (55) 5622 7835.

Recibido el 20 de agosto de 2008; aceptado el 8 de diciembre de 2008

AISI 4130 steel is a material used frequently to manufacture machinery parts and the automobile industry. So, this material is susceptibleto tempering treatment and can be achieve different annealing temperatures to obtain with this several grades of toughness. In this work amicrostructural analysis by Scanning Electron Microscopy to observe the fracture behavior was carried out. The toughness was quantifieusing an instrumented pendulum with standard Charpy specimens-probes. The specimens were annealed at 850◦C and quenched in oil.Subsequently the specimens were mill-annealed at 400◦C for periods of 1, 1.5 and 2 hours. A hardness decrease was achieved, however, thetoughness increase and later abruptly decrease with the different annealing times. The specimens were chemically etched using Nital (3%)showing the tempered martensite microstructure and martensite rounded by cementite. However, an over-etched zone was observed aroundthe inclusions of iron sulfide which could be associated with the stress field

Keywords:AISI 4130 steel; scanning electron microscopy; Charpy tests; toughness; fracture analysis.

El acero AISI 4130 es un material que frecuentemente se utiliza para la fabricacion de piezas de maquinaria y en la industria automotriz,ademas, es susceptible a temple y se pueden aplicar diferentes temperaturas de revenido alcanzando con esto varios grados de tenacidad.En este trabajo se realizo un estudio microestructural por Microscopıa Electronica de Barrido para observar el comportamiento a la fracturaen un acero AISI 4130. La tenacidad a la fractura se cuantifi o empleando una prueba de impacto instrumentada con probetas tipo Charpy.El estudio se llevo a cabo en probetas templadas en aceite a 850◦C y revenidas a 400◦C por 1, 1.5 y 2 hrs, con el fi de observar la zonaen la cual este acero presenta fragilidad por revenido. Se encontro una disminucion de la dureza y los valores de la tenacidad aumentany disminuyen abruptamente con el tiempo de revenido. El ataque quımico (Nital 3%) revelo la morfologıa de una martensita templada yrevenida ademas se encontraron inclusiones de sulfuro de hiero, donde alrededor de estas se observo una zona de sobre-ataque, la cual puedeestar relacionada con el campo de esfuerzo provocado por la interaccion de las dislocaciones.

Descriptores:Acero AISI 4130; microscopıa electronica de barrido; ensayo Charpy; tenacidad; analisis de fractura.

PACS: 81.05.Bx; 81.40.Ef; 61.72.Ff; 81.70.Bt

1. Introduction

The steels known as chromium-molybdenum such as the4140, 4340 and 4130 have been widely used in high pressuresteel containers, machinery and automotive parts, etc. [1].For practical purposes, these steels usually experienced an-nealed processes which commonly induce brittleness andhardness. To avoid these deleterious effects, these steelsare usually exposed to mill-annealing at low temperaturesin comparison with the critical temperature and they werecooling down using air, water or oil [2]. AISI 4130 is anultrahigh strength steel with medium carbon content and isnormally used in a quenched and tempered condition [3-4].However, in many applications a suitable combination ofstrength and toughness is needed. Because of low ductilityof the steel, the improvement of ductility can be achieved bydifferent methods such as modifie heat-treatments, thermo-mechanical processes and modificatio of chemical compo-sition [5-7]. The enhancement of steel brittleness can be re-lated with a reduction of the fracture energy in Charpy impact

test [8-10]. In this investigation, to get an insight on the steelbrittleness behavior, Charpy impact tests have been carriedout. Specimens of the 4130 steel have been used and themill-annealing temperature was 400◦C. The annealing peri-ods were 1, 1.5 and 2 hours. To carry out the Charpy test,a commercial pendulum (SATEL model IF-ID3) has beencompletely instrumented obtaining graphs of the load recordsversus time. A microstructural study of the steel specimenshas also been carried out. Particular attention is focus to theattack pattern regions obtained in the neighborhood of theinclusions. Scanning electron microscopy images of the mi-crostructure and the fracture regions were obtained.

2. Experimental procedures

Slabs of AISI 4130 steels have been used in the experimentaltests. The composition of this steel is given in Table I. Thesespecimens have followed an annealing process which con-sists in 30 minutes annealing at 850◦C and quenched in oil.Subsequently the specimens were mill-annealed at 400◦C for

MICROSTRUCTURAL ASPECTS OF THE BRITTLENESS PHENOMENA IN STEELS INDUCED BY MILL-ANNEALING 69

periods of 1, 1.5 and 2 hours. Charpy impact tests werecarried out using an instrumented pendulum, SATEC witha maximum energy capacity of 162 Jouls. The digital andcompacted instrumentation of this apparatus was carried outin the laboratory. Three different tests were carried for eachspecimen. For these experiments, standard probes with “V”notched machined regions following the ASTME 23-86 stan-dards were used. The size of the standard probes was 60 mmof length and 3.15 mm of square transversal section. Themicrostructural characterization was carried out using scan-ning electron microscopy (SEM). The steel specimens forobservations in SEM were initially chemically etched usingNital (3%).

3. Results and discussion

a) Microstructural characterization

When steel is annealing and subsequently quenched in somefluid the structure well-known martensite is produced; thesesteels are usually hard and brittle, depending of the uses thatwill be dedicated. Figure 1 shows a SEM micrograph ofthe AISI 4130 steel annealing at 850◦C and quenched in oil.The heat treatments produced a fully tempered martensite mi-crostructure with lath morphology. The hardness achieved bythis heat treatment was 60 HRC.

The mill-annealed time influenc on the mechanical prop-erties and the microstructure of the steels, therefore, increas-ing the mill-annealed time of the AISI 4130 steel, differ-ent changes on the microstructure are achieved. The hard-ness after the mill-annealed process decreases from 60 to 40HRC. Figure 2 shows SEM micrographs of the AISI 4130steel mill-annealed at 400◦C for periods of 1, 1.5 and 2hours. At 1 hour (Fig. 2a) found big lath martensite structurerounded by cementite; however at 1.5 hours (Fig. 2b) onlyfound martensite structure with a completely different mor-phology, a fully tempered martensite. Some transformationsdue to mill-annealed time are observed, as can be observed inFig. 2c, where the martensite structure is completely roundedby cementite, in addition to the cementite precipitate insidethe martensite structure.

The steel specimens were chemically etched using Nital(3%) for SEM observations. This chemical etching origi-nated on some parts an over-etched, mainly placed at inclu-sions zone (Iron sulfid inclusions). Figure 3 shows a coupleof SEMmicrographs of some inclusions zone over-etched. Inthese figure can be observed as the over-etched could be re-lated with stress fiel of the inclusion or with a very punctualetching localized only around/enclosed the inclusions.

TABLE I. Nominal composition of the AISI 4130 Steel.

% C % Mn % P % S % Si % Cr % Al % Mo

0.31 0.97 0.0021 0.013 0.29 0.92 0.63 0.17

FIGURE 1. Micrograph of the AISI 4130 steel annealing at 850◦Cand quenched in oil. The heat treatments produced a temperedmartensite microstructure.

FIGURE 2. Micrograph of the AISI 4130 steel mill-annealed at400◦C for periods of 1, 1.5 and 2 hours. At 1 and 2 hours foundmartensite rounded by cementite, however at 1.5 hours the marten-site change its shape.

FIGURE 3. SEM micrographs showing an over-etched of some in-clusion zones.

b) Charpy tests

A commercial pendulum SATEC model IF-ID3 was instru-mented. This new instrumentation includes the use of strain

Rev. Mex. Fıs. S55 (1) (2009) 68–71

70 J. GARCIA, J. HALLEN, R. ESPARZA, AND R. PEREZ

gages in the pendulum base and the hammer. Electronic at-tachments which include complying devices, strain gages, filtered and sign gain equipments. The data acquisition was car-ried out with a personal computer. The strain gages and finaCharpy tests were calibrated using different materials (com-mercial brass, AISI 1018 steel, and AISI 316 stainless steel).This instrument has been used for the Charpy impact tests ofthe specimens.

The impact curves of the AISI 4130 steel under the con-ditions of annealing treatment discussed above are shown inFig. 4. It can observe that the impact curves do not follow anascending behavior with the mill-annealing time, obtaining amaximum at 1.5 h and then decrease until almost obtainingthe same value for 2 and 1 h. The toughness behavior is im-precise in relation with the mill-annealing temperature (seeTable II). This behavior is due to structure produced, tem-pered martensite rounded by cementite. This is an explana-tion of because the brittleness in this type of steels. Previousworks of Blanchette [11] have shown this behavior, whichidentify with descending potential, the moment at fracturestarted, which coincides with the maximum load. The totalfracture energy value, ET , is associated with the area underthe load-time (P − t) curve up to maximum load, PM . How-ever, ET includes contributions other than that caused by thedeflectio of the specimen. Therefore, a compliance energycorrection is needed to determine the true initiation energy,

TABLE II. Toughness, initiation and propagation energies of thedifferent specimens.

Time Toughness Initiation Propagation

Annealing ( J ) Energy Energy

( 400◦C ) ( J ) ( J )

1 h 12 8.5 3.51.5 h 27 19 82 h 16 11 5

FIGURE 4. Impact curves of the AISI 4130 Steel mill-annealed at400◦C for periods of 1, 1.5 and 2 hours.

FIGURE 5. SEM images from initiation and propagation fracturesurfaces of the specimens mill-annealed at 400◦C for periods of:a) 1 h, b) 1.5 h and c) 2 h.

EM [12]. When the fracture is linear elastic (fracture beforegeneral yield), the value of EM can be calculated directlywith the area under the load-time in the linear elastic range.Assuming initiation occurs at maximum load, the propaga-tion energy, EP , is:

Ep = ET − EM (1)

Figure 5 shows the SEMmicrographs of the initiation andpropagation fracture surfaces of the mill-annealed steel, thefracture morphology consist of irregular fla zones and ductilezones associated with the presence of pores, which increasethe quantity and size according increase the mill-annealedtime. In this period, the mechanical properties of the ma-terial change due to the steel structure produced by the heattreatments. At 1 h (Fig. 5a), the mechanical behavior of thematerial consists in a brittle to ductile transition, showing dif-ferent crack initiation, nucleation and propagation zones, thisconditions could increase the fracture time. At 1.5 h (Fig. 5b),the fracture behavior of the material is ductile; the crack ini-tiation, nucleation and propagation zones are well definedAt 2 h (Fig. 5c), the mechanical behavior of the material issemi-brittle, mixing it with ductile conditions. This is due tothe mill-annealing brittleness phenomena. Such brittlenessphenomena show some changes on the microstructure, the

Rev. Mex. Fıs. S55 (1) (2009) 68–71

MICROSTRUCTURAL ASPECTS OF THE BRITTLENESS PHENOMENA IN STEELS INDUCED BY MILL-ANNEALING 71

cementite enclose the tempered martensite and also, the ce-mentite precipitate inside the martensite. This is the reasonbecause the surface microstructure shows semi-brittle frac-ture behavior in Fig. 5c. On firs one region (initiation), thefracture process is fast due to the semi-brittle conditions. Onthe other region (propagation), predominates the ductile frac-ture behavior, also it does not show some cracks very pro-nounced like specimen of Fig. 5b. These behaviors affectthe impact curve shape (Fig. 4), which shows two points ofmaximum load, the firs one is associates to the semi-brittlebehavior and the second one is associates to the ductile be-havior.

4. Conclusions

The structures of the AISI 4130 steel annealing at 850◦C andquenched in oil, subsequently mill-annealed at 400◦C for pe-

riods of 1, 1.5 and 2 hours, showed some changes relate tomill-annealed time. At 1 and 2 hours the martensite structureis enclosed by cementite structure. However, at 1.5 hoursonly martensite structure is observed. These structures arethe reason of the fracture behavior of the steel at the differentannealing times. Also, the initiation and propagation energiesare greater to mill-annealed time of 1.5 hours than the otherscases, therefore these mill-annealing conditions are favorableto improved the toughness of the steel.

Acknowledgment

The authors gratefully thank A. Aguilar for the technical as-sistance during this work.

∗. Corresponding author; e-mail: ramiro@fis.unam.m1. ASMMetals Handbook Vol. 1., Properties and Selection: Irons,

Steels and High-Performance Alloys. Tenth Edition, 1990.ASM International. p. 264.

2. J.E. Bringas, Editor, Handbook of comparative world steelstandards, Third Edition (ASTM International, 2004) p. 20.

3. A.R. Mirak and M. Nili-Ahmadabadi, Mater. Sci. Tech.20(2004) 897.

4. K.M. Rajan, P.U. Deshpande, and K. Narasimhan, J. Mater.Process. Tech.125(2002) 503.

5. G.Y. Lai et al., Metall. Trans. A5 (1974) 1663.

6. Y. Tomita, Mater. Sci. Tech.6 (1990) 843.

7. Y. Tomita, Metall. Trans.19a(1989) 2513.

8. A. Needleman and V. Tvergaard, Int. J. Fracture101(2000) 73.

9. R. Mucalek et al., Strength Mater.40 (2008) 142.

10. J. Heerens et al., Int. J. Pres. Ves. Pip.82 (2005) 649.

11. Y. Blanchette, J.P. Bailon, and J.I. Dickson, Eng. Fract Mech.33 (1989) 643.

12. H.J. Saxton et al., Load Point Compliance of the Charpy Im-pact Specimen, Instrumented Impact Testing, STP 563, ASTM,(1974) p. 30

Rev. Mex. Fıs. S55 (1) (2009) 68–71