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Journal of Materials Processing Technology 209 (2009) 4292–4311

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Journal of Materials Processing Technology

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nternal ductile failure mechanisms in steel cold heading process

. Sabiha,∗, J.A. Nemesb

Department of Mechanical Engineering, McGill University, 817 Sherbrooke Ave West, Montreal, Quebec, Canada H3A 2K6Engineering Division, Pennsylvania State University-Great Valley, 30 East Swedesford Road, Malvern, PA 19355-1443, United States

r t i c l e i n f o

rticle history:eceived 22 February 2008eceived in revised form 5 November 2008ccepted 7 November 2008

eywords:uctile failurediabatic shear bandold headingoid nucleation

nclusions

a b s t r a c t

The occurrence of internal ductile failure in cold-headed products presents a major obstacle in the fastexpanding cold heading (CH) industry. This internal failure may lead to catastrophic brittle fracture undertensile loads despite the ductile nature of the material. Comprehensive testing and investigation method-ologies were used to this work to reveal the complicated interplay of process and material parameterscontributing in the initiation and propagation of internal ductile failure in six CH quality AISI steel grades.

The metallurgical and microscopic investigations showed that internal ductile failure occurs pro-gressively by void nucleation and growth mechanisms with increasing plastic strain inside the highlylocalized adiabatic shear bands (ASBs). The void nucleation occurs by decohesion at second-phase par-ticles, inclusion–matrix interfaces, grain boundaries and by particle or inclusion cracking. Therefore,the number and morphology of any inclusions and second-phase particles are key factors in materialformability.

The metallurgical investigations showed that under compressive loading conditions, the nature of themetal flow pattern promotes different rates of material flow around the inclusions and stringers whichsupports decohesion and void nucleation since the early stages of deformation. At advanced stages of
deformation, the metal flow pattern contributes to the ASB localization in supporting void growth andcoalescence along the band leading to narrow void sheets.
All tested materials in this work experienced ductile failure by void nucleation and coalescence, formingcracks along the ASBs. The ductile failure of each material was the result of the contribution of all themechanisms of void nucleation at the inclusion–matrix interface, second phase–matrix interface and atthe grain boundaries. However, the level of contribution of each mechanism in the final ductile failure

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varied depending on mat
. Introduction

The occurrence of any failure is a major limitation governing theimits of any forming process. Therefore, understanding the fail-re mechanisms and the complex interplay of process and materialarameters in the failure occurrence in metal forming operationsas attracted the attention of many researchers for more than sixecades. The knowledge of different failure mechanisms is crucialo improve product quality and design methodologies.

Barrett (1997) considered in his report about the fasteners thathe cold heading (CH) process is one of the most important multi-tage metal forming processes because of its many advantages over
achining of the same part, including high productivity for com-
lex final shapes, minimum material waste and increased tensiletrength from cold working. Chitkara and Bhutta (2001) who stud-ed the near-net shape heading of splines and solid spur gear forms

∗ Corresponding author. Tel.: +1 514 398 8546; fax: +1 514 398 7365.E-mail address: [email protected] (A. Sabih).

924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2008.11.023

properties and their microstructure.© 2008 Elsevier B.V. All rights reserved.

reported that the decision to use the CH process to manufacturecertain products depends on the complexity in the shape of thehead and the material employed and the possibility of internal andexternal failure occurrence.

Currently, the CH industry favors using faster headers, reducingthe number of manufacturing stages, and producing high-strengthfasteners without final heat treatment. To achieve these goals, mod-ified process designs are required which result in higher strains andstrain rates, which cause two familiar types of ductile defects in thecold-headed products. The first is the external oblique or longitu-dinal crack caused by the exhaustion of the material ductility. Thesecond failure was reported by Bai and Dodd (1992) in their studyfor the adiabatic shear band (ASB) phenomenon. They reported thatthe ASBs may lead to internal crack. Okamoto et al. (1973) stated intheir study of different forming processes that this type of failure
may result in splitting of the fasteners’ heads as shown in Figs. 1–3,respectively.
Many researchers have focused on surface defects. Cockroft andLatham (1968) and Lee and Kuhn (1973) studied this defect in upset-ting and presented different criteria to predict it. Nickoletopoulos

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A. Sabih, J.A. Nemes / Journal of Materials

2000) studied the surface defect in CH process and reached to aonclusion that Cockroft–Latham criterion can be used to predicthe surface defect in CH process with a reasonable accuracy. Inddition to the large number of studies that covered this type of

ig. 1. Cold-headed industrial bolt: (a) without defect, using material 1, (b) shank of the dmaterial 2), and (d) magnified detail of the ductile crack of the separated industrial bolt h

ig. 2. Sectioned bolts revealing material flow inside the bolt head after etching with Fry’efect bolt (material 1), (b) with defect bolt (material 2).

ssing Technology 209 (2009) 4292–4311 4293

failure extensively, it can be visually inspected and accounted forin the process design of the component for removal by means ofmachining or trimming. Thus far, internal ductile failure due to theASB phenomenon in CH process has not received the same atten-

efective bolt (material 2), (c) separated industrial bolt head due to an internal crackead (material 2).

s reagent (cupric chloride 36 g; 145 ml hydrochloric acid; 80 ml water): (a) without

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294 A. Sabih, J.A. Nemes / Journal of Materials

ion. The clear need for an in-depth understanding of the internaluctile failure mechanisms which limits the development of newH designs and the improvement of cold-headed product qualityotivated this work.To have a full understanding of the complicated parameters and

echanisms contributing in the initiation and propagation of thenternal ductile failure, the first part of this paper will provide aetailed introduction covering important details about the CH pro-ess and the ASB phenomenon followed by examples of ductileailure in cold-headed parts. The second part of this paper will focusn the experimental and metallographic procedures used to inves-igate the failure initiation and propagation. The last part of thisaper presents the detailed result and discussion.

.1. Cold heading process

The term heading refers to cold, warm, or hot working of metalmploying a static or dynamic impact compressive force causing theetal at one end of the rod or blank to form a head. Yoo et al. (1997)

efined the cold heading (CH) process as a forming process that iserformed without an external heat source and it involves applyingforce to the free end of a metal workpiece contained between aie and a punch, by one or several blows of the punch. The CH pro-ess is a high-rate deformation process with strain rates exceeding00 s−1. Kawashima (1992) reported that the CH process is usedo produce a large variety of components such as fasteners, studs,mall shafts, and other machine parts. The automotive, construc-ion, aerospace, railway, mining and electrical product industriesre major consumers of such parts.

Matsunaga and Shiwaku (1980) concluded in their study of man-facturing CH quality wire rods and wires that material propertiesnd process parameters significantly affect the heading results.oreover, the CH material must meet two sets of requirements.

he first set concerns the material’s ability to be formed with-ut failure while the second set is linked to the properties of thenal product. Sarruf et al. (1999) stated that in order to formithout failure, CH wire should be free from surface defects and

nternal inclusions. Nickoletopoulos et al. (2001) reported that thehemical composition of CH wires plays a significant role in the
aterial ductility and the final properties of the products. Low- andedium-carbon steels (0.06–0.30% C), are commonly used in the
H process, as they offer good ductility. Also, Nickoletopoulos et al.2001) referred that the CH material microstructure is usually com-rised of a ferrite matrix with varying amounts of lamellar pearlite.

ig. 3. Cold-headed brass bolt experienced ductile failure in service along the ASB: (a) anith permission of ASM International (www.asminternational.org).

ssing Technology 209 (2009) 4292–4311

A spheroidized microstructure provides the desired ductility forCH of complex geometries. The required final strength propertiesare usually achieved through alloy addition and/or heat treatment(quenching and tempering).

1.2. ASB phenomenon

Wright (2002) defined the ASB as a two-dimensional, narrow,nearly planar region of very large shearing that occurs in metals andalloys experiencing intense dynamic loading (as in the CH process).When the band is fully developed, the two sides of the region aredisplaced relative to each other. However, the material still retainsfull physical continuity from one side to the other. The thicknessof the most heavily sheared region might be few tens of micronsor less, and its length might extend many millimeters or centime-ters. Generally, ASBs form under impact loading at high-strain rates(higher that 102 s−1) and high strains. With increasing plastic defor-mation, the work hardening (from increases in strain and strainrate) results in an increase in the flow stress. However, most ofthe plastic work (90–95%) is converted into heat causing a localtemperature increase and a flow stress decrease due to thermal orwork softening. Thus, a competing mechanism between the workhardening and the thermal or work softening commences and con-tinues in the deformation zone. As the work hardening mechanismdominates over the work softening at the beginning, an increase inthe flow stress occurs and with continuing deformation, the ther-mal or work softening mechanism can progressively dominate overwork hardening increases, which then triggers unstable deforma-tion. Wright (2002) also reported that this instability conditionwill force the deformation to localize into a narrower band thatthrough further localization can lead to final failure. Cowie et al.(1989) showed in their study of microvoid formation during sheardeformation of ultrahigh-strength steels that this failure initiatesand develops by progressive nucleation, growth and coalescenceof microvoids and microcracks. A sharp drop in the load-carryingcapability for the deformed zone will associate with microvoid for-mation causing what is known as the microvoid softening. Once thefailure starts to develop, the combined work softening effect of thethermal and microvoid softening mechanisms will compete with
the work hardening effect within the band.
Zukas (1990) reported that there are two types of ASBs, namelythe deformed adiabatic shear bands (DASBs) and the transformedadiabatic shear bands (TASBs). DASBs occur in materials that donot undergo phase transformation, or when the local temperature

d (b) sectioned failed bolt, (c) crack surface in a failed bolt (Reitz, 2005) Reprinted

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ithin the band is not high enough to cause phase transformation.he damage and fracture process in DASBs involves a number ofetallurgical events involving different steps between void nucle-

tion and crack propagation. The initial phase of damage coincidesith void nucleation at inclusion edges or at grain boundaries due

o the interaction of local stresses and dislocations phenomena. Theamage in the material is then driven by the accumulated plastictrain and affected by the stress triaxiality. Hambli (2001) reachedn important conclusion that deformation under compressive (neg-tive) stress triaxiality increases material formability in comparisonith deformation under tensile loading state, i.e. positive stress tri-

xiality. Zukas (1990) declared that the competition between thetress triaxiality levels reached in the specimen and the high-plastictrains will determine the failure site.

Daridon et al. (2004) reported that the localization of plasticeformation in narrow shear bands is a major damage mechanismhat leads to ductile failure during high-strain rate deformation. Leet al. (2004) reported that internal cracks occur within the shear

Fig. 4. As-received testing materials microstructure: (a) 1008 steel, (b) 101


band, and become the sites of eventual failure. Venugopal et al.(1997) concluded that this can take place when the material is sub-sequently subjected to dynamic impact loading during forming orin service.

TASBs occur in materials that undergo phase transformations,and are often found in high-strength steel in locations where thecritical temperature for the transformation of ferrite to austenite(Ac3) is exceeded. After high-strain rate forming, the rapid heat dis-sipation from the ASB to the surrounding matrix material resultsin a fast decrease in temperature below the Ms limit. This sud-den change in temperature leads to austenite transformation intomartensite, yielding the TASBs. TASBs may fracture in a brittle man-ner in directions normal to the local tensile stress. If the matrix issufficiently ductile, brittle fracture will be limited to the band, but
sometimes the fracture may extend to the matrix. Rogers (1983)stated that even if the TASB does not fracture, it remains a brit-tle fracture path in the middle of the ductile matrix and mightlead to catastrophic failure. Moreover, Klepaczko et al. (1988) have
8 steel, (c) 1038 steel, (d) 1541 steel, (e) 8640 steel, and (f) DP steel.

4 Processing Technology 209 (2009) 4292–4311

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Table 1Chemical analysis of the testing materials (wt%).

AISI steel grade C Mn P S Si Cu Ni Cr

1008 0.09 0.4 0.018 0.012 0.01 Nil 0.01 0.021018 0.16 0.69 0.006 0.006 0.22 0.04 0.08 0.081038 0.38 0.82 0.007 0.002 0.19 0.04 0.08 0.081541 0.37 1.41 0.011 0.018 0.22 0.16 0.08 0.098640 0.40 0.91 0.009 0.005 0.25 0.14 0.42 0.43DP 0.088 1.69 0.009 0.011 0.62 0.04 0.07 0.07

AISI steel grade Mo Sn Al N B V Co

1008 0.002 0.002 0.038 0.0046 – – 0.0041018 0.001 0.002 0.002 0.0026 0.0001 0.005 –1038 0.001 0.003 0.003 0.0044 0.0001 0.005 –

Method A requires a survey of a polished surface square areaof 160 mm2 at 100× magnification parallel to the longitudinal axisof the wire. The severity level of the worst fields and the seriestype of four inclusion types (A, B, C, and D) should be reported forevery specimen depending on Tables 2 and 3. Table 2 presents the


ndicated that shear bands either with or without phase transfor-ations, are considered as a site of fracture initiation.

.3. Examples of internal ductile failure in cold-headed products

The following paragraphs present cold-headed products failedy internal ductile fracture along the ASBs. Fig. 1 shows in detailndustrial bolts were made of two different 1018 steels from twoifferent CH steel providers (material 1 and material 2) using theame CH sequence. The industrial bolt made of material 1 (Fig. 1(a))id not exhibit internal cracks while the second bolt made of mate-ial 2 failed by a crack resulting from the head splitting when aow-tensile load was applied. Fig. 1(b)–(d) shows the details of therack inside the head of the bolt made of material 2. The met-llographic study of the sectioned bolts made of the material 2Fig. 2(b)) confirmed the nucleation, growth and coalescence of

icrovoids leading to microcracks.Reitz (2005) investigated the failure of brass bolts after 12 years

n service. The microscopic investigation revealed that the boltsailed by macrocracks along the highly deformed band inside theolt’s head (Fig. 3).

In general, the ductile crack occurred inside a highly localizedegion of the ASB. The occurrence of ductile failure in material 1sing the same CH multistage design in the part in Fig. 1 raises

mportant questions about the ASB phenomenon and how it leads touch a catastrophic failure. The ductile failure that took place insideolt heads made of material 1 presents clear evidence that the cur-ent rules of thumb are neither effective nor adequate in fulfillinghe needs of the new design developments in the rapidly expand-ng CH industry. They do not provide enough guidance to fulfill theeeds of new design developments in the CH industry. Therefore,he CH industry is in need of systematic and theoretical studies tonderstand the failure mechanisms and the different parametersontributing to failure initiation and propagation in cold-headedroducts.

These examples of failed cold-headed products reveal the appar-nt lack of knowledge about the internal ductile failure mechanismnd the role of the ASB phenomenon in the presence of these defectsn cold-headed products

. Experimental

.1. Material selection and characterization

The CH quality AISI steel grades chosen for this research are:008, 1018, 1038, 1541, 8640 and dual phase (DP) steel which coverow- and medium-carbon content steels and also provide differentH quality steels ranging from best to poorest CH formability steels.he 1018, 1038, 1541, and 8640 steels were received in rod formrom Ivaco Rolling Mills (Ontario, Canada), while the 1008 steel wasrovided from the undeformed part of the bolts made of material.

In addition, DP steel was also included in the test materials as itossesses unique properties which show great potential in the CH

ndustry. Pierson (1986) reported that the DP steel is intended toe cold headed directly from the “green rod” and reaches the finalroperties of the cold-headed products without heat treatment.

Optical microscopy revealed a microstructure consisting of fer-ite matrix and lamellar pearlite for all steel grades except the DPteel which consisted predominately of ferrite grains and a marten-
itic second phase. The as-received microstructure of all the testingaterials is shown in Fig. 4. Table 1 lists the chemical composition
f the selected material.Quasi-static tensile tests were performed on specimens

achined to ASTM-E8 standard at an approximate strain rate

1541 0.021 0.010 0.004 0.0058 0.0000 0.002 –8640 0.217 0.010 0.005 0.0063 0.0000 0.032 –DP 0.458 0.005 0.003 0.0035 – 0.007 0.033

of 0.002 s−1, the force and the axial displacement values wererecorded. The engineering stress–engineering strain curves wereconstructed from the load–elongation measurements. These curveswere used to calculate the true stress–true strain curves (Fig. 5) forall testing materials.

2.2. Testing material cleanliness

Cleanliness is a measure of the inclusion content and inclusiontypes that are deleterious to processing performance and final prod-uct properties.

Dodd and Bai (1987) indicate that the inclusions refer to non-metallic particles that are held mechanically in the material matrix.Such particles include oxides, sulfides or silicates. The presence ofthese inclusions strongly influences the ductile failure mechanism.Thus control of cleanliness and second-phase particle shape caninfluence material ductility.

The inclusion rating process for this study was performed usingautomatic image analysis inclusion ratings in accordance withmicroscopic method A (worst fields) of E45 of the ASTM standard.Worst field is a class of rating in which the specimen is rated foreach type of inclusion by assigning the value of the highest severityrating observed of the inclusion type anywhere on the specimensurface.

Fig. 5. The tensile quasi-static true-stress vs. true strain curves for the testing mate-rials.

A. Sabih, J.A. Nemes / Journal of Materials Processing Technology 209 (2009) 4292–4311 4297

Table 2Total inclusion length or number for minimum severity level numbers (method A) (ASTM E45-97).

Severity level Total length in one field at100×, min. (mm) Number of inclusions in one field

Type A Type B Type C Type D

0.5 3.7 1.7 1.8 11 12.7 7.7 7.6 41.5 26.1 18.4 17.6 92 43.6 34.3 32.0 16

Table 3Inclusion width and diameter parameters (method A) (ASTM E45-97).

Inclusion type Thin series (T) Heavy series (H)

Width min.(�m)

Width max.(�m)

Width min.(�m)

Width max.(�m)

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Table 4Inclusion rating test results for each testing material using ASTM E45-97 methodA.a.

AISI grade A B C D

1008steel

None None None D1None None None D1

1018steel

None None C1 D1None None None D1None None None D1

1038steel

None None C1 D1A1 None None D1

1541steel

A1 None C2 D1A1 None None D1A1 None None D1

8640steel

A1 None None D1A1 None C2 D1HNone None None D1H

DPsteel

None None C1 D1None None C2 D1None None C1 D1

ype A 2 4 >4 12ype B 2 9 >9 15ype C 2 5 >5 12ype D 2 8 >8 13

inimum severity level number for each inclusion type and Table 3rovides the inclusion dimensions for the thin (T) and heavy (H)eries according to method A.

This microscopic method places inclusions into severalomposition-related categories: sulfides, oxides, and silicates. Theypical chemical types of inclusions in Type A are sulfide stringers,hile Type C are silicate stringers. Type B are oxide stringers while

ype D is a globular oxide. Method A requires length measurementf Type A inclusions, the total stringer length of Type B and C inclu-
ions, and the number of Type D inclusions. Type D globular oxideay not exceed an aspect ratio of 5:1.The inclusion rating test results for the six testing materials are
isplayed in Table 4 and the morphologies of the inclusion typesre shown in Figs. 6–8.

A1 None None D1

a Cleanliness tests were performed by Ivaco Rolling Mills, L’Orignal, Ontario.

Fig. 6. Inclusion type A1 in the testing materials.

4298 A. Sabih, J.A. Nemes / Journal of Materials Processing Technology 209 (2009) 4292–4311

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Zhang and Thomas (2002) reported that the inclusion size isery important because large inclusions are the most harmful to theechanical properties of CH materials. Sometimes a catastrophic

efect is caused by a single large inclusion in an entire steel heat.hus, producing clean steel requires not only controlling the meannclusion content in the steel but also avoiding inclusions largerhan the critical size.

.3. Tests

The drop weight compression test (DWCT), as described byickoletopoulos (2000), facilitates the testing of the CHP and

s capable of generating ASBs during upset testing. The DWCTachine, as illustrated in Fig. 9, consists of a tower enabling inter-

hangeable weight plates to be dropped from heights up to 2.4 m.
he die set configuration (Fig. 10) rests on a central column that iselded to the base of the DWCT machine. Specimens for CH wereachined with a tolerance of 0.02 mm from as-rolled rod material
o a cylindrical configuration of 5.3 mm in diameter with aspectatios of 1.6 and 1.8. A series of tests is performed by varying the

the testing materials.

weights until internal fracture is found at different locations insidethe ASB.

A guided cylindrical-pocket die set was designed for the DWCTmachine. A sleeve guides the dies and reduces die movement duringtesting. Air vents in the sleeve prevent pressure build-up. An impactplate is used on top of the upper die to prevent direct impact withthe die. Fig. 10 presents a schematic assembly of a test specimenwhich is placed between the upper and lower dies. This assemblyrests atop the force sensor inside a cylindrical pocket.

2.4. Sample preparation

The cold-headed samples were sectioned transverse to the head-ing direction using a high-concentration diamond wafering bladeon a slow speed (200 rpm) cut off saw that was operated with an
oil coolant. Sectioned specimens were mounted in Bakelite andprepared for metallographic examination using automated tech-niques for grinding and polishing. Specifically, the specimens wereground with successive papers of SiC from 280 grit to 600, fol-lowed by rough polishing using a 9 �m diamond suspension with


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n alcohol based lubricant on a silk cloth and final polishing with 3nd 1 �m on a nylon and porous pad, respectively. After ultrasonicleaning, the specimens were etched by immersion in a 2% nitalolution.

Microstructural examination was performed using optical lighticroscopy with polarizing capabilities and image analysis soft-are to examine the overall characteristics of the ASBs and the

oids and cracks initiated in the interior or along the ASBs. High-esolution imaging for examining the topography of the voidsnd cracks in the ASBs was performed using scanning elec-ron microscopy operated in secondary electron (SE) mode andquipped with an X-ray detector (EDAX) for elemental analysis ofecondary phases. Mapping the topography of the voids and cracksn the ASBs was performed using an Olympus OLS1200 laser scan-
ing confocal microscope (LSCM) equipped with an argon laser488 nm) and objective specifications of up to 100× with a 6× opti-al zoom. As the image produced by scanning the x- and y-axesith a spot laser beam in the LSCM originates from a shallow focusepth, for the DP steel specimens a series of precisely focused opti-
the testing materials.

cal images were acquired along the z-axis of the microscope. Thesuccessive images were then overlapped using specialized recon-struction software to obtain a three-dimensional extended focusimage of the cross-sectional view that contained both height andintensity information. Specifically, the topographic features in theextended focus image of the ASB structure enabled the characteri-zation of the size of the width and depth of the voids and cracks.

3. Results and discussion

In general, microscopic examination and FE analysis of the DWCTspecimens at various regions of each specimen during differentdeformation levels showed that the deformation concentrates inthe ASBs (Sabih et al., 2005, 2006a,b). The microstructure of the
DASBs is laminar consisting of highly elongated grain layers of fer-rite and pearlite or second-phase particles (Fig. 11). Voids and cracksfound along the ASBs inside DWCT specimens (Fig. 12) coincidedwith the cracking that occurs inside the head of the industrial boltshown in Fig. 1. This finding agrees with that of Qiang and Bassim

4300 A. Sabih, J.A. Nemes / Journal of Materials Proce

Fig. 9. Schematic of the DWCT machine.

Fig. 10. DWCT specimen inside the die set arrangement.

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ig. 11. The microstructure of the DASB (1541 steel, aspect ratio of 1.6, drop heightf 2.4 m and drop weight 29.5 kg).

2003) who studied the effect of process parameters on the forma-ion of the ASBs in metals and observed the initiation and growth ofracks along the ASBs. Similar observation was reported in the workf Nemat-Nasser et al. (2001) who reported in their studies of ASB

ig. 12. Microscopic views showing the internal cracks and/or voids along the ASBDP steel, aspect ratio = 1.8, dropped weight = 33.8 kg, height = 2.4 m).


phenomenon and the associated failure that these bands usuallyprecede the failure mechanism for most metals and alloys.

Ductile fracture occurs progressively by void nucleation andgrowth mechanisms with increasing plastic strain. Different mech-anisms of ductile failure have been reviewed in many works: Doddand Bai (1987), Dodd and Atkins (1983) and Garrison and Moody(1987). All these reviews consider the most important factor affect-ing the material formability is the number and morphology ofinclusions and second-phase particles.

Dodd and Atkins (1983) found in their study of flow localizationin shear deformation of void containing and void-free solids thatductile failure occurs following this sequence of events:

(1) Nucleation of voids occurs by decohesion at second-phase parti-cles or at inclusion–matrix interfaces, or by particle or inclusioncracking. Dodd and Hartley (1991) concluded that in certaincases, these voids can nucleate at grain boundaries or grainboundary triple points.

(2) An increase in the volume fraction of voids with increasingapplied plastic strain.

(3) At a critical strain, plastic deformation is localized along shearbands between the voids.

(4) Cracks occur along the bands of intense shear, forming whatRogers (1959) calls “void sheets” mechanism of crack propaga-tion.

Masheshwari et al. (1978) studied quality requirements for steelCH grades and concluded that the second-phase particles and inclu-sions, such as heavy silicate, sulphide, alumina and globular oxideinclusions, should not exceed the acceptable limits of the cold head-ing material. Thomson (1990) pointed out that these particles andinclusions act as stress raisers and can support microvoid nucle-ation, growth, and coalescence, leading to ductile fracture.

Meyers et al. (2001) reported that the localization of the ASBsis an important deformation mode that causes ductile fracture inmetals. Wei et al. (2006) concluded in their study of mechanicalbehavior and dynamic failure under uniaxial compression that thethermal and geometric softening and material flow patterns areresponsible for dynamic failure under compressive loads.

Ductile failure caused by ASBs within the DWCT specimensunder compressive loads is a complicated process which dependson the interplay of many factors. The following sections will detailthe ductile failure mechanisms, the void nucleation, coalescenceand crack initiation mechanism. Furthermore, the interplay ofinclusion types and content, material microstructure, material flowpattern and the localization phenomenon will be discussed alongwith the impact of these factors on the occurrence of ductile failure.

3.1. Void initiation mechanism

A careful reading of the literature shows that there is a debateabout the issue of void nucleation under a compressive state ofstress. Petersen et al. (1997) reported that ductile failure is unlikelyto occur under a compressive stress state. Teng et al. (2007) stud-ied the transition from adiabatic shear banding to fracture andreported that ductile fracture under high-negative stress triaxial-ity cannot occur. On the other hand, many researchers have foundthat inclusions or inter-metallic particles serve as void nucleationsites under compressive loads: Puttick (1959) showed in his studyof ductile fracture that microvoid initiation and growth is causedby the debonding of second-phase particles produced by deforma-
tion in the surrounding matrix. Cox and Low (1974) found that voidnucleation and growth occurs more readily at the larger inclusions.
Rogers (1960) explained the role of the small and large particlesand inclusion in the initiation and growth of voids leading to a duc-tile fracture. Similarly, Gurland (1972) also explained the effect of


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ig. 13. DWCT specimen of 1018 steel showing the locations of the CASB, UDZ, and LDepresent the shear direction and the small arrow shows the material flow pattern.rop weight 22.5 kg).

he inclusions on the nucleation, growth, and coalescence of micro-copic voids causing fracture of ductile solids. Horstemeyer andokhale (1999) modeled void–crack nucleation process for duc-

ile metals with second-phase particles. They concluded from theodeled compression, tension, and torsion experiments that the

oid–crack nucleation can take place under compressive loadingondition.

Nemat-Nasser et al. (2001) also reported that cracks of this kind,hich are produced by local defects, can be created by a local tensile

tress state under an overall compressive applied stress. They alsoound that the occurrence of tensile stresses in the vicinity of thenclusions and/or second-phase particles create the classical con-itions for void nucleation and growth. Thus, fracture initiation inSBs under compressive loading of the DWCTs is highly probable.

The present findings are supported by previous work by Doddnd Atkins (1983) showing that void growth is sensitive to aydrostatic stress state. Furthermore, they found that voids nucle-te at second-phase particles or inclusions within the ASB underompressive hydrostatic stresses. Under compressive loading con-itions with a local tensile stress state, both void nucleation and
rowth can occur in steels, albeit at a lower rate in comparison tohe dominating failure mechanisms of void formation and propaga-ion under tensile loading. Park and Thompson (1988) have shownhat void nucleation occurs at both sides of the tensile pole of aarticle under compressive loads. Horstemeyer and Gokhale (1999)
ig. 14. Material flow contours for various regions on a DWCT specimen (drop weight =ocated at the center of the DWCT specimen (b) the region adjacent to the UDZ, (c) the rWCT dies (25×), (e) the region deformed within the DWCT dies (100×), (f) the region in

id lines represent imaginary boundaries of the ASB and the UDZ and LDZ. Big arrowsashed arrow represents the mid of the CASB (aspect ratio = 1.8, drop height = 2.4 m,

reported the possible occurrence of void nucleation even undercompression loads as tensile states exist for compression loadings.

3.2. Effect of material flow pattern on void nucleation

Void nucleation around second-phase particles and non-metallic inclusions is highly influenced by the nature of the materialflow during deformation. Sarruf et al. (1999) reported that the metalflow pattern during the CH process supports the void nucleationmechanism at non-metallic inclusions and the surrounding mate-rial. Meyers et al. (2001) reported that the material flow patternduring shear localization is an important factor in sub-grain break-age and rotation. Tvergaard (1981) reported that this mechanism isresponsible for void nucleation due to brittle cracking or decoher-ence of inclusions.

Detailed microscopic examination of various regions of theDWCT specimens showed that the deformation is concentrated inthe localized central ASBs (CASB) with well-defined flow contoursstarting from the upper dead zone (UDZ) and the lower dead zone(LDZ), as illustrated in Fig. 13. Specifically, there is a sharp transi-
tion in the behavior of the material flow as observed by the contourlines which traverse from the dead zones to the CASB (Fig. 14(a)), tothe CASB edge at the mid-axis (Fig. 14(b) and (c)). In the deformedregion within the DWCT dies, the material flow behavior showedelongated semi-elliptical contours that increasingly concentrated
35.5 kg and height = 2.4 m) showing: (a) the dead zone region adjacent to the ASBegion near the specimen edge at the mid-axis, (d) the region deformed within thethe CASB showing the merging of two dead zones and (g) the center of the CASB.

4302 A. Sabih, J.A. Nemes / Journal of Materials Proce

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ig. 15. Excessive deformation at the region between the UDZ, the LDZ and theASB resulting in microscopic crack formation at the boundary between the top andottom dead metal zones and at the inflection point of material flow (DP steel, aspectatio of 1.6, drop height of 2.4 m and drop weight 31.6 kg).

owards the mid-axis of the specimen, as shown in Fig. 14(d) and (e).t the center of the DWCT specimen, the CASB zone was observed

o have highly deformed material with voids (Fig. 14(f) and (g)).ith further deformation, the region extended along (f), (g), (b)

nd (c), and was observed to have excessive deformation result-ng in microscopic crack formation at the boundary between theUDZ) and the (LDZ) and at the inflection point of material flowFig. 15).

An analysis of the void characteristics, using the LSCM, indi-ated that an increase in the plastic strain caused both an increasen the void density and size, as illustrated in Fig. 16. Thus, in
he CASB region, longer cracks were observed with increasingrop weight for the DWCT specimens. The occurrence of the longracks in the CASB was usually observed in two zones. The firstone was along the center of the CASB (Fig. 14(f)), and the other
ig. 16. A comparison of void density inside the ASB of two specimens (DP steel,spect ratio of 1.8 and drop height of 2.4 m) that were deformed with a drop weight ofa) 38 kg and (b) 31.5 kg. The dark heights on the surface represent the void locationsnd their inverted depth.


was found in the upper and lower regions adjacent to the CASB(Fig. 14(a)–(c)).

3.3. Void nucleation mechanism at the inclusions–matrixinterface

Siruguet and Leblond (2004a,b) concluded that the void nucle-ation, growth and coalescence mechanism is highly dependant onthe stress state under which deformation takes place. Fig. 17 dis-plays possible void nucleation mechanisms under different stressstates in order to have a better understanding of the ductile fail-ure occurring inside the ASB. When the stress triaxiality is high(tensile loads/positive), voids grow in the directions of the tensileloads (Fig. 17(b) and (c)), and the enclosed inclusions do not influ-ence their growth. Moreover, it has been reported by Pardoen andDelannay (1998) that the initially spherical voids will change toellipsoidal voids under tensile triaxiality.

Siruguet and Leblond (2004a,b) showed that under compres-sive loads from all directions, or negative stress triaxiality, voidscan be subjected to compression in one or several directions,thereby changing their shape. Also, if they are still in contactwith the enclosed inclusions in these directions, inclusions pre-vent their shrinkage, a process known as void locking illustrated inFig. 17(d). Pardoen and Delannay (1998) reported that under low-compressive stress triaxiality, the ductile matrix may flow aroundthe inclusion resulting in the failure of the interface forming smallvoids at the sides of the inclusion (Fig. 17(e)). Conversely, underhigh-compressive loads, the fracture of the inclusion is possible(Fig. 17(f)).

In the case of shear banding under compressive loads, the forma-tion of microvoids has been reported by Pirondi and Bonora (2003)and Sabih et al. (2005, 2006a). They indicated that the microvoidshave smaller dimensions than the tensile cases. They also con-cluded that voids under compressive loads do not increase theirdimensions but stretch and eventually coalesce by internal necking,known as microvoid sheeting (Fig. 17(g)).

Evidence of void and crack initiation and propagation at the non-metallic inclusions and stringers sites was put forward in the workof Vora and Polonis (1976). They found that the weakly bondedparticles to the matrix had elongated in the direction of materialflow. Once decohesion between the matrix and the inclusion orsecond-phase particle occurred, the nucleated voids were observedto grow with increasing strain from an initial spherical shape to anellipsoidal form or sheet form, as illustrated in Fig. 17(h). Boyer etal. (2002) modeled void growth under different stress states andconcluded that void volume increases under compressive loads ifthese voids are filled with inclusions.

Meyers et al. (2001) noticed that the different rates of materialflow around the inclusion supports decohesion and void nucleationmechanisms between the matrix and the inclusion or the second-phase particle as shown in Fig. 17(i).

In DWC testing, the deformation takes place under compressivestress triaxiality over almost the entire specimen. The ASB occursinside the DWCT as a result of the softening mechanisms and theshear deformation between the edges and the center of the band.The combined compressive and shearing deformation results inductile failure inside the band. The void nucleation and coalescencefollows some of the illustrated mechanisms in Figs. 17 and 18.

Fig. 18 displays some metallographic observations of the voidnucleation mechanisms around the spherical inclusions (Type D).Fig. 20(b)–(f) show the steps of the void sheeting process inside the
CASB of the 1541 steel. This process starts when a void nucleates atthe edges of the inclusion due to the material flow around it. Withmore deformation, the inclusion changes its shape from spheri-cal to ellipsoidal and the void propagates along the material flowdirection forming a void sheet (Fig. 18(e)). This void sheet occurs


Fig. 17. Illustrations of the void and crack nucleation mechanisms around the inclusions under different stress states: (a) inclusion inside the undeformed material (no void);(b) under multiaxial tensile loads, void growth in all directions; (c) under uniaxial tensile load, void nucleation in the load direction; (d) under multiaxial compressive loads,void shrinks on the inclusion (void locking); (e) under uniaxial compressive load, void nucleation at the sides of the inclusion; (f) breakage of inclusion under multiaxialc on unt cohesd

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ompressive loads; (g) void sheet nucleation and growth at the sides of the inclusihe material flow direction under combined compression and shear stresses; (i) deifferent rates of material flow around the inclusion.

n a manner similar to the void sheeting mechanism illustrated inig. 17(h).

The metallographic inspection showed that some of the smalloids change their shape to an ellipsoidal geometry, while the largenclusions were not affected by material deformation and contin-ed to remain in their spherical shape. However, it was noticed thatoid sheets initiated at the vicinity of the big inclusions and propa-ated along the material flow direction (Fig. 18(g)). These findingsre supported by the results of the ductile damage finite elementodel used by Boyer et al. (2002) to predict void growth under

ormal mean stress.Due to the different rates of material flow around the inclusions,

he big spherical inclusion rotates in the direction of the high-flowate. This results in decohesion at the interface between the inclu-ion and the surrounding matrix. These findings are supported byimilar observations made by Meyers et al. (2001), who reportedhat the material flow pattern during shear localization is an impor-
ant factor in the breakage and rotation of inclusions.
Sabih et al. (2005, 2006a) showed in their metallographic inves-igations and FE simulations that at low-plastic strains (less than 1)nd low-negative stress triaxiality, voids were observed to initiateround the elongated inclusions (Fig. 19(b)). Bandstra et al. (2004)

der compressive and shear loads; (h) fractured or elongated weak inclusion alongion between the inclusion and the matrix due to rotation of inclusions caused by

reported void nucleation at the non-metallic inclusion–matrixinterface at small strains. In this work, inclusions were alsoobserved to fracture during deformation (Fig. 19(c)). According tothe microscopical examination of fracture surfaces made by Broek(1973), this can cause stress concentrations promoting ductile frac-ture.

Type A and C (long inclusion) stringers are the favored places forvoid nucleation and growth within the ASBs. The metallographicstudy of the DWCT specimens showed that the void sheeting pro-cess took place by decohesion at the stringer–matrix interface inseveral steps. The void sheeting mechanism around the stringersis the result of the material flow and strain localization within theASB. As shown in Fig. 14, the stringers move along the material flowdirection and change their alignment by approximately 90◦. Thissharp change in the flow direction at the early stages of deformationtriggers the decohesion process at the stringer–matrix interface atthe boundaries of the CASB (Fig. 14(b) and (c)).

When these stringers join the localized CASB, the softeningmechanisms and the high-strain rate deformation within the bandsupport the void sheeting process along the stringers sides andaround the spherical inclusions (Fig. 14(f) and (g)). Fig. 20(a) dis-plays the sharp change in the stringers’ direction which supports

4304 A. Sabih, J.A. Nemes / Journal of Materials Processing Technology 209 (2009) 4292–4311

Fig. 18. Changes in the inclusion morphology and void sheet nucleation mechanism around inclusions at different deformation levels in 1541 steel: (a) spherical inclusioninside the undeformed material, (b) inclusion shape changed from a spherical to an ellipsoidal form inside the DASB, (c) elongated inclusion inside the DASB, (d) void sheetinitiated at the edges of the elongated inclusion, (e) void sheet growth inside the DASB, (f) a large number of void sheets inside the highly deformed TASB, (g) void sheetpropagating ahead of the inclusion, and (h) rotation of the inclusion.


Fig. 19. The morphology of inclusions: (a) spherical inclusion in as-received (undeformed) DP steel, (b) elongated particles with adjacent voids initiated at low-plastic strains,and (c) fractured elliptical inclusion.

Fig. 20. Void sheet mechanism around the stringers: (a and b) 1038 steel, aspect ratio of 1.8, drop height of 2.4 m and drop weight 26.2 kg, (c) 1018 steel, aspect ratio of 1.6,drop height of 2.4 m and drop weight 28.5 kg, (d) 8640 steel, aspect ratio of 1.6, drop height of 2.4 m and drop weight 29.5 kg, and (e) 1541 steel, aspect ratio of 1.6, drop heightof 2.4 m and drop weight 23.4 kg.

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he void sheets inside the DWCT of 1541 steel. Usually, the voidsucleate around the stringer, forming the void sheet inside theeformed region. With further deformation, the void sheet, which

s considered as another softening mechanism, propagates alonghe localized CASB (Fig. 20(a) and (b)).

In addition to void sheeting around the stringers within theASB, metallographic inspection revealed that these stringers arehe source of ductile failure inside the region deformed within theWCT dies (Fig. 20(a) and (c)). In this region, the material flow pat-

ern forces the stringers to take the shape of semi-elliptical contours
hat become increasingly concentrated towards the mid-axis of thepecimen: Fig. 20(a), (d) and (e). The stringers and the surroundingaterial at these locations deform under low-compressive stress
riaxiality in comparison with the center of the CASB. Under thistress state, and with further deformation, cracks initiate at the tip

ig. 21. SEM micrographs display the void nucleation at the ferrite–cementite interfaceearlite (ferrite–cementite) in the undeformed zone (b) elongated pearlite in the directiucleation at the grain boundaries, (e) void nucleation at the ferrite–cementite interface1038 steel, aspect ratio of 1.8, drop height of 2.4 m and drop weight 33 kg).


of the semi-elliptical stringers due to the stretch-like deformation.This stretching occurs because the flow of the semi-elliptical sidesis restricted, due to the friction with the die faces, while the mate-rial continues to push the tip of the semi-elliptical stringers awayfrom the specimen center.

3.4. Void nucleation mechanism at the grain boundaries and thesecond phase–matrix interface

As presented in Section 2.1, the materials tested are
ferritic–pearlitic steels, except for the DP steel which has aferrite–martensite microstructure. The pearlite grains consist ofcementite lamella in a ferrite matrix. This type of microstructuremakes the materials subject to a new source of void nucleationduring plastic deformation under compressive loading.
, at the grain boundaries and around the inclusions inside the DASB: (a) lamellaron of material flow, (c) void nucleation at the ferrite–cementite interface, (d) voidand at the grain boundaries, and (f) void nucleation around an elongated inclusion


Table 5Inclusion rating result with the minimum inclusion length or number according to the minimum severity level numbers (method A) (ASTM E45-97).

AISI steel grade A1 Min. length (mm) C1 Min. length (mm) C2 Min. length (mm) D1 Min. number

1008 None None None D1 41018 None C1 7.6 None D1 41038 A1 12.7 C1 7.6 None D1 418D

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541 A1 12.7 C1 7.6640 A1 12.7 C1 7.6P A1 12.7 C1 7.6

1H is a heavy series of Type D1 inclusion, the inclusion width range is between 8 a

Cox and Low (1974) referred in their study of ductile fracture ofISI 4340 steel that this failure is caused by void sheets composedf small voids nucleated at the matrix–cementite (second-phasearticles) interface. Gurland (1972) studied the formation of cracksnd voids at second-phase particles (carbides) under tension, com-ression and torsion. Hahn and Rosenfield (1975) reported that voidrowth and coalescence was observed the cracked inclusions andhe grain boundaries. Nieh and Nix (1980) explained void growthnd coalescence at grain boundaries. In conclusion, non-metallicnclusions, second-phase particles, and grain boundaries are con-idered to be the best candidate sites for void nucleation and ductileailure.

Tanguy et al. (2006) have also reported that the presence ofecond-phase particles, such as lamellar cementite or martensite,re a source of void nucleation. Chen et al. (2002) showed thatamellar cementite has a delayed deformation rate as compared toerrite. Valiente et al. (2005) explained that the lamellar cementitef a pearlitic grain has a much higher stiffness than ferrite.

Valiente et al. (2005) showed that due to microstructuralspects, the difference in the local flow behavior can lead to cemen-ite elongation and fragmentation. This triggers void nucleation athe discontinuities produced by the breakage of cementite and byecohesion of atomic bonds at the interface between the matrixnd inclusion or second-phase particles. The voids grow, assistedy plastic deformation, until they link up. Fragmentation of cemen-ite within ASBs has also been reported by Chen et al. (2003). Argon1976) found that voids were more easily nucleated at the larger
ementite particles than at the smaller ones.
Beyond the early stages of deformation in DWCT specimens,etallographic study by SEM microscopy showed that the voids

ucleated at the cementite–ferrite interface, the pearlite–matrixnterface and at grain boundaries (Fig. 21).

ig. 22. Ductile failure in DWCT specimens of 1008 steel: (a) a long crack along the CASB (atio of 1.6, and drop weight 26.2 kg).

C2 32 D1 4C2 32 D1, D1H 4C2 32 D1 4

�m. The minimum width of the other inclusion types is 2 �m.

Within the matrix, the cementite inside the pearlite grains faroff the shear band have a lamellar shape as shown in Fig. 21(a).Lamellar cementite inside the pearlite on the shear band side iselongated along the material flow direction (Fig. 21(b)). At thehigh-deformation zone close to the CASB, broken lamellar cemen-tite was noticed, and clear void nucleation took place at theferrite–cementite interface (Fig. 21(c)).

Voids and microcracks nucleated at ferrite–pearlite interfacesclose to the localized plastic deformation zone of the CASB(Fig. 21(d)). Closer to the highly localized plastic deformation withinthe CASB, more void nucleation and cracking along the grain bound-aries along the ASB was observed (Fig. 21(e)). These voids andmicrocracks, along with the voids around the inclusions (Fig. 21(f))contribute to ductile cracking along the ASBs.

3.5. Ductile failure characterization in the testing materials

The metallographic study of the DWCT specimens showed thatall materials experienced ductile failure by void nucleation andcoalescence, forming cracks along the CASBs. The ductile failureof each material was the result of the contribution of all themechanisms of void nucleation at the inclusion–matrix interface,second phase–matrix interface and at the grain boundaries, aspreviously discussed. However, the level of contribution of eachmechanism in the final ductile failure varied depending on mate-rial.

Ductile failure along the ASB inside the DWCT occurred under
compressive loads. The voids and cracks found in the failed zoneshave the narrow, stretched, elongated shape or the void sheet shapealong the highly localized regions of the ASB.
In general, it is possible to classify the ductile failure into twogroups. The first group consists of materials that experienced very

aspect ratio of 1.6, and drop weight 37.6 kg), (b) void nucleation in the DASB (aspect

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arge cracks, including 1008 steel and 1018 steel. The second grouponsists of the material that experienced short ductile crackinglong the ASB. This group includes 1038 steel, 1541 steel, 8640 steelnd DP steel.

The first group is composed of soft steels with low-inclusionontent. Table 5 shows that the only inclusion type in the unde-ormed 1008 steel is the spherical inclusions (Type D1), while thendeformed 1018 steel contains the long stringer or inclusion Type1 in addition to Type D1 inclusions.

Both steels experienced very long cracks which extended forore than 3 mm along the mid-axis of the CASB, as shown in

igs. 22(a) and 23(a) for 1008 steel and 1018 steel, respectively. TheEM metallographic inspection of the cracked specimens revealedhat void nucleation and coalescence took place at the boundariesf the highly elongated grain within the CASB in contribution to theicrocracks at the ferrite–cementite interface. The metallographic

nvestigation of the role of the inclusions type in the 1008 steelinclusion Type D1) showed that voids nucleated along the inclu-ions leading to shorter cracks in comparison to the crack initiatedt the grain boundaries (Fig. 22(b)).

Similar conclusions can be made with respect to the 1018 steelWCT specimens. The metallographic investigation showed that all

he void nucleation mechanisms contributed to the ductile cracksound within the highly localized deformed CASB in the DWCTpecimens made of 1018 steel (Fig. 23). Void nucleation took place

ig. 23. Ductile failure in the DWCT specimens of 1018 steel: (a) a long crack along the CAaspect ratio of 1.8, and drop weight 22.5 kg), and (c) void nucleation around the inclusion


around the inclusions (Fig. 23(b)), and at the interface betweenboth types of inclusions (Type C1 and Type D1) and the matrix.The microscopic observation revealed that at the early stages ofdeformation, void sheets and short cracks extend along the grainboundaries in addition to microcracks at the ferrite–cementiteinterface. With further deformation, the coalescence of these voidsheets and cracks form bigger cracks that extend over 3 mm alongthe center of the CASB (Fig. 23(a)). The void nucleation along theType C1 inclusion is readily apparent in Fig. 20(c).

The materials in the second group can be characterized by theoccurrence of the TASBs that are accompanied with short cracks.These cracks were noticed to be within, or at, the sides of the TASBzone and the DASB zone of the band. As in the first group, all ofthe void nucleation mechanisms contributed to the ductile cracksfound within the highly localized deformed and the transformedpart of the CASB. However, the long inclusions (Type A1, C1 and C2)played an important role in ductile failure initiation as previouslydiscussed. The presence of inclusion type C2 in 1541 steel, 8640steel and DP steel means that stringers with a total minimum lengthof 32 mm are present in the matrix. As discussed previously, this
long inclusion is the favored location of void nucleation by the deco-hesion mechanism at the inclusion–matrix interface. With furtherstraining inside the localized ASB, these voids coalesce around thestretched long inclusion inside the band. Figs. 20(a), 20(b) and 24(a)display the long cracks which initiated within the CASB of the 1038
SB (aspect ratio of 1.6, and drop weight 37.6 kg), (b) cracks at the grain boundaries(aspect ratio of 1.6, and drop weight 20.2 kg).


Fig. 24. Crack initiation along the TASB: (a) 1038 steel, aspect ratio of 1.8, drop height of 2.4 m and drop weight 33 kg, (b) 8640 steel, aspect ratio of 1.8, drop height of 2.4 mand drop weight 29.5 kg.

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ig. 25. Void coalescence in the ASB region of deformed DWCT specimen of DP st5.6 kg).

teel, and shows the severe change of the material flow pattern.his is known to cause the breakage of the long inclusion, provid-ng another means of void sheet nucleation. Fig. 24(b) shows thatoids nucleated at the interface of the long inclusion–matrix in the640 steel. Fig. 25 shows ductile failure along the ASB inside theWCT specimen of DP steel. The ductile cracking mechanisms in

he 1541 steel are shown in Fig. 18.Careful investigation of the microstructure at the side of the

ASB of the second group materials reveals that the elongated fer-ite and pearlite grains also helped in the nucleation of narrow andtretched void sheets along the localized band. Nevertheless, theole of the long inclusion clearly plays a much larger part in ductileailure.

a) 500× and (b) 1000× (aspect ratio of 1.8, drop height of 2.4 m and drop weight

4. Conclusion

The occurrence of internal ductile failure in cold-headed prod-ucts presents a major obstacle in the fast expanding CH industry.The current design rules of thumb failed in avoiding this type offailure which results in reducing the material’s capability to carryloads. This internal failure may lead to catastrophic brittle fractureunder tensile loads despite the ductile nature of the material.

The lack of specialized studies regarding the mechanisms ofinternal ductile failure in cold-headed products and the failure ofthe current CH design procedures to fulfill the needs of new devel-opments in the expanding CH industry motivated this research.A comprehensive testing and investigation methodology was

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sed to reveal the complicated interplay of process and materialarameters contributing to the initiation and propagation of the

nternal ductile failure in six CH quality AISI steel grades.The metallurgical and microscopic studies showed that inter-

al ductile failure occurs progressively by void nucleation androwth mechanisms with increasing plastic strain inside the highlyocalized CASBs. The most important factor affecting materialormability is the number and morphology of any inclusions andecond-phase particles. Moreover, the thermal and geometric soft-ning mechanisms and material flow pattern are responsible forynamic failure under compressive loads.

It was confirmed that ductile failure occurs by a void nucleationechanism which occurs by decohesion at second-phase particles

nd/or at inclusion–matrix interfaces, by particle or inclusion crack-ng, or at grain boundaries. The localization process inside the CASBsupported the void growth and coalescence along the band form-ng narrow void sheets. Under compressive loading conditions, theoid nucleation, growth and coalescence mechanism occurred dueo the local tensile stress state in the vicinity of the inclusions and/orecond-phase particles.

The metallurgical investigations showed that the nature of theetal flow pattern during the CH process, and the different rates ofaterial flow around the inclusions, support decohesion and void

ucleation mechanisms between the matrix and the inclusion orhe second-phase particle in the highly localize CASBs.

The void sheeting process starts when a void nucleates at thedge of the inclusion due to the material flow around it. With moreeformation, the inclusion changes its shape from spherical to ellip-oidal and the void propagates along the material flow directionorming a void sheet.

Under combined compressive and shearing deformation, thearge inclusions continued to retain their spherical shape. However,t was noticed that void sheets initiated in the vicinity of the largenclusions and propagated along the material flow direction. Type And C long inclusions are the favored places for void nucleation androwth within the CASBs. The void sheeting process took place byecohesion at the stringer–matrix interface in several steps. Theharp change in the flow direction at the early stages of defor-ation triggered the decohesion process at the stringer–matrix

nterface. The thermal softening and the high-strain rate deforma-ion within the localizing band support the void sheeting processlong the stringers’ sides and around the spherical inclusions. Withurther deformation, the void sheet propagates along the localizedASB.

In addition to non-metallic inclusions, the second-phase parti-les and grain boundaries are considered to be the best candidateites for void nucleation and ductile failure. The difference in theocal flow behavior, due to microstructural aspects, can lead toementite elongation and fragmentation. This triggers void nucle-tion at the discontinuities produced by the breakage of cementitend by decohesion at the interface between the matrix and inclu-ion or second-phase particles.

The microstructure of the DASBs is laminar, consisting of narrow,tretched, and highly elongated grain layers of ferrite and pearliter second-phase particles. Void sheets and cracks are found in theailed zones along the ASBs.

All materials tested experienced ductile failure by void nucle-tion and coalescence, forming cracks along the ASBs. The ductileailure of each testing material was the result of the contributionf all the mechanisms of void nucleation at the inclusion–matrixnterface, second phase–matrix interface and at the grain bound-
ries. However, the level of contribution of each mechanism in thenal ductile failure varied depending on the material.
In summary, a comprehensive experimental and metallurgicaltudy of the ASB phenomenon and the associated ductile failureere introduced in this study to uncover the complex interplay of


different material and process parameters controlling the failuremechanisms within the ASBs in cold heading process.

The ductile failure initiation under compressive loads is adebated subject of many works in the literature. However, thisdetailed study reached important findings that uncovered thecomplex interplay of the different mechanisms of ductile failureinitiation and growth under compressive loads. All these findingswere based on detailed supporting experimental and metallurgicalevidences about these mechanisms.

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