Mechanism of Anisotropic Lamellar Fractures

HUNG-CHI CHAO

Texture banding is common in sheet steels, especial ly in highly alloyed steels. Aniso- tropic plastic flow during tensi le stra in ing is known to cause cold-rol led and annealed fe r r i t i c - s ta in less -s tee l sheet to exhibit " r idg ing" or " rop ing" in the plane of the sheet even though there is no banding due to grain size, carbides, or inclusions. It has been ob- served that groups of grains having a common crystal lographic or ientat ion can appear in r ibbon- l ike form and can result in anisotropic banding. In ferr i t ic sta in less steel, such bands frequently consist of the cube-on-face texture component imbedded as layers in a cube-on-corner matr ix. In the present study, it was demonstrated that in tensi le defor- mation, such " texture" banding resul ts in anisotropic plastic flow, causing thin sheet to ridge on the surface, and thick sheet to develo p cracks that propagate in planes paral le l to the sheet surface. In this study, room- and low-temperature tension tests were conducted at low and high stra in rates on samples of thin and thick AISI Type 430 fer r i - t i c - s ta in less -s tee l sheet, and room-temperature impact tests were conducted on the thick sheet. The resul ts showed that, at low temperature or high stra in rates, the banded cube-on-face texture component imbedded in other matr ix or ientations, such as the cube- on-corner texture, or a purely cube-on-face textured matrix, tends to cause lamel lar f racture of the sheet. This was especial ly true in thick sections. The proposed mecha- n ism is a general ized crysta l -p last ic i ty phenomenon that is applicable to any mater ia l in which texture-or ientat ion banding or c luster ing effects occur and the mechanism may be one of severa l possible causes of "sp l i t t ing" which is somet imes observed in high- strength a l loy-steel plates, such as l ine-pipe plates, when fractured dynamical ly.

TEXTURE banding is common in sheet steels, es- pecial ly in highly alloyed steels. ''~ Anisotropic plas- tic flow during tensi le stra in ing is known to cause cold-rol led and annealed sheet of ferr i t ic stain less steel to exhibit " r idg ing" or " rop ing" (a corrugated surface) in the plane of the sheet,' even in the absence of banding from grain size, carbides, or inclusions. It has been observed that anisotropic banding may appear in layered or r ibbon- l ike form ',~ when the cube-on face (CF) texture component is imbedded as layers in a cube-on-corner (CC) matrix, Fig. l(a). 2,3 In a re- cent discussion on the r idging phenomenon, 4 it was suggested that this type of anisotropic banding may cause interracial spl itt ing or delaminat ion in the plane of the sheet when stra ined at low temperature or at high speed.

In the present investigation, an anisotropic texture analys is and metal lographic study was performed on light- and heavy-gage AISI Type 430 s ta in less -s tee l sheet to demonstrate that the anisotopic plastic flow which causes thin sheet to ridge on the surface may also cause cracks to init iate and to propagate para l le l to the sheet surface, resul t ing in lamel lar f racture of plate product. It is bel ieved that this mechanism con- tr ibutes to the "sp l i t t ing" which is somet imes ob- served in a l loy-steel plates.

mm) sheet, and the other from cold-rol led and an- nealed 0.020- in.-thick (0.5 mm) sheet. The composi- tion of each sample is shown in Table I.

Room-temperature tension tests were used to s t ra in both sheets, and tension tests at - 320~ (- 196~ were used to stra in the thicker sheet. Both tests were con- ducted at a stra in rate of 0.167 s- ' . Also, a high stra in rate of 200 s-' was used to break some samples of thin sheet a t - 100~ (-73~ The low-temperature tests were used to lower the ducti l ity of the steel and thus to ensure that lamel lar f racture would occur if the

MATERIALS AND EXPERIMENTAL WORK

The two Type 430 s ta in less -s tee l sheet samples used in this investigation were produced from two dif- ferent commerc ia l heats. One sample was obtained from hot-rol led and annealed 0.120 in. - thick (3.05

HUNG-CHI CHAO is Research Consultant, United States Steel Cor- Fig. 1--Schematic diagram depicting anisotropic plastic- poration, Monroeville, PA 15146. flow of ribbon-like cube-on-face texture component imbedded

Manuscript submitted July 22, 1977. within cube-on-corner matrix under external force.

ISSN 0360-2133 / 78 / 0410-0509500.75/0 METALLURGICAL TRANSACTIONS A 9 1978 AMERICAN SOCIETY FOR METALS AND VOLUME 9A, APRIL 1978 509

THE METALLURGICAL SOCIETY OF AIME

Table I, Chemical Composition of AISI Type 430 Stainless Steel Investigated-Pct

Thin Sheet, Thick Sheet, 0.02 in. (0.5 mm) 0.120 in. (3.05 ram)

C 0.071 0.069 Mn 0.43 0.46 P 0.024 0.025 S 0.0O4 0.003 Si 0.42 0.47 Cu 0.05 ND Ni 0.17 ND Cr 17.10 16.40 Mo

q

Rol l ing D i rec t ion Fig. 4--Micrographs of midthickness of 0.020 in. thick (0.5 mm) sample. Electrolytic polish. Vilel la's-reagent etch, 1 min. (a) magnified 5.4 times; (b) magnified 385 times.

*l I*

Rol l ing D i rec t ion Fig. 5--Micrographs of midthickness of 0.120 in. thick (3.05 ram) sample. Electrolytic polish. Vilella's-reagent etch, 1 rain. (a) magnified 22 times; (b) magnified 385 times.

separat ing the (100) bands f rom the surrounding (111) matr ix , as welt as inclusion and carb ide bands of weak interface, lying para l le l to the sheet sur faces, wil l be subjected to a separat ion force because of the d i f fer - ing s t ra in behaviors of the two textures . There is s t rong res i s tance to thhnning by the (111) matr ix , whereas thinning is the predominant flow mode for the (100) bands. The banding of these textures and carb ides, and unavoidable inclusions along the ro l l ing d i rect ion produced d i rect iona l ly weak inter faces be- tween the bands. The sever i ty of lamel la r f rac tures wil l vary with both the d i rect ion of the weak inter faces and the d i f ferences in s t ra in rat ios between textures .

Force for Lamel la r F racture

For thin sheets s t ra ined at a temperature above the br i t t le t rans i t ion temperature , r idging wil l appear on the sheet sur face 1,2 because of the continuity requ i re - ment imposed by this an isotropic p last ic flow. The r idging occurs because the r ig id i ty in the th ickness d i rect ion of thin sheet is very min imal and because

ducti le mater ia l can withstand st ra in ing without f rac - tur ing. For thick mater ia l , such as thick sheet or plate, under impact or other modes of h igh- rate s t ra in - ing in the plane of the sheet or plate, or for thin sheet st ra ined at low temperature , the r ig id i ty imposed by the thickness or by the low duct i l i ty at low tempera- ture prevents the appearance of r idging on the sur face. However, the tensi le force in the th ickness d i rect ion induced by the plast ic flow of the ( l l l ) and (100) com- ponents is wel l beyond the capabi l i ty of the mater ia l to maintain continuity, Thus, spl i t t ing or lamel la r f racture resu l ts , par t i cu la r ly at the re la t ive ly weak inter faces of banded regions in the ro l l ing plane along the rott ing d i rect ion resul t ing f rom the segregat ion of inclusions, carb ides or elongated gra ins .

Cleavage Fracture

Lamel la r f racture can occur when high normal s t ra ins and shr inkage forces f racture along the in ter - faces of banded regions, but lamel la r f racture due to cleavage is par t i cu la r ly prevalent when mater ia l has

METALLURGICAL TRANSACT1ONSA VOLUME 9A, APRIL 1978-511

Fig. 6--SEM micrographs of fracture surface of 0.020 in. thick (0.5 ram) sample tested at room temperature. (a) mag- nified 58.2 times; (b) magnified 1455 times.

low ductil ity, as at very low temperatures, especial ly since the (100) plane is a favorable cleavage plane for body-centered-cubic steels. It is known that be- cause of elast ic energy conservation, the dislocation mechanism for nucleating cleavage cracks takes the form

[ i11 101 + [111]/10i - a[0011 100t

for two intersect ing {110} slip bands in bcc mater ia l , s Because al l {110} planes in the {100} orientations are 45 deg 0r/4 rad) to the sheet plane containing the load- ing direction, this or ientat ion tends to initiate cracks.

With this c leavage-nucleat ion mode, a CF band, as schematical ly shown in Fig. l(b), should be able to

512-VOLUME 9A, APRIL 1978

Fig. 7--SEM micrographs of fracture surface of 0.120 in. thick (3.05 mm) sample tested at -320~ (-196~ Arrows indicate lamellar fractures. (a) magnified 57.6 times; (b) magnified 1440 times.

split into layers if the CF band is thick enough. Also, as shown in Fig. 10, severe splitt ing or lamel lar f racture should occur if the CF texture is the pre- dominant or ientat ion through the thickness of the ma- ter ia l , because the thinning tendency is extremely high; no other orientation has such a high thinning tendency. Splitting of this type can be avoided if the CF or ientat ion and its banding is prevented.

Splitting Phenomenon

In studying the fracture behavior of mater ia ls , we general ly pay most attention to the effects of inclusions and other phases. The present investigation i l lus-

METALLURGICAL TRANSACTIONSA

Transverse Longitudinal Longitudinal Impact Impact Tensile

Fig. 9--Micrographs of fracture surface of impact and tensile specimens of 0.120 ino thick sheet {3.05 ram) tested at room temperature. Magnified 2.7 times.

Fig. 8--SEM micrographs of fracture surface of 0.020 in. thick (0.5 ram) sample tested at 200 in.fin./s at -100~ (-73~ Arrows indicate lamellar fractures. (a) magnified 112 times; (b) magnified 558 times.

trates that the crystal lographic texture of the matr ix is also an important var iable, part icu lar ly with re - spect to anisotropic tamel la r - f rac ture behavior. For example, f inish hot rol l ing of l ine-pipe steels at low temperatures caused the development of elongated duplex grains with an unfavorable (100) texture that was observed to promote splitt ing f ractures . 9

In a recent paper, 1~ Picker ing reported that the com- monly observed CF texture in high-strength low-alloy steels is [100](011), which causes delaminat ion para l - lel to the plane of the sheet. However, some investiga- tors discounted the crysta l lographic- texture effect as the cause of splitt ing and lamel lar f racture of high- strength low-alloy mater ia l . 9,n-~3 Without consider ing the or ientat ion effect, however, it is hard to explain why splitt ing also occurred in an inc lus ion- f ree s im- ple i ron-carbon system without alloying. 9 Even low-

Fig. 10--Amount of induced thickness and width strains as re- lated to longitudinal strain of (100) and (111) orientations.

metal loid iron plates without any addition of alloy will also show f issures in the fracture surface paral le l to the sheet plane. 9 The fact that mass ive cleavage f rac- ture or quasicleavage fracture is associated with the spl itt ing surface paral le l to the sheet plane strongly suggests that the mechanism of anisotropic spl itt ing outlined in the present work plays a dominant role. Since anisotropic f racture is related to microscopic or ientat ion banding and since this type of or ientat ion var iat ion cannot be detected by conventional macro- scopic X- ray analysis, such as pole f igures, crysta l - life or ientat ion distr ibut ion function analysis , 14 or in- tegrated intensity values, it is not surpr is ing that the texture effect was discounted as the cause of splitt ing. However, the pole f igures and inverse pole data 9'n documented by previous invest igators do show high (100) or ientation along with (111) intensit ies for steels prone to spl itt ing but not for steels which are not prone to spl itt ing. 9

Microstructure Effect

It should be noted that alloy part i t ioning due to sol i - dif ication and subsequent thermomechanica l process- ing can also resul t in the formation of segregated bands, 3 and that the banded a l loy- r ich and al loy-poor layers could provide additional lamel la r - f rac ture sites because such bands further promote banded crystal lo- graphic s t ructures as well as form layers of different

METALLURGICAL TRANSACTIONSA VOLUME 9A,APRIL 1978-513

s t rengths and s t i f fnesses . That s t ronger c rys ta l lo - g raph ic band ing is p romoted by a l loy ing is ev idenced by the increases in mass ive c leavage- type sp l i t t ing wi th la rger th rough- th ickness shr inkage in the h igh ly a l loyed spec imens than in the low-meta l lo id spec i - mens . 9 Fur thermore , mic ros t ructura l var ia t ions , such as low- temperature -aged carb ide prec ip i ta tes hav ing a (100) hab i t p lane in fe r r i te , wi l l se rve to enhance sp l i t - t ing. A l te rnat ive ly , the h igh- temperature -aged carb ide prec ip i ta tes a re e i ther at the gra in boundar ies 13 or on (110)(111) 1~ hab i t p lanes in the fe r r i te phase . The gra in -boundary prec ip i ta tes as we l l as inc lus ions wi l l weaken the gra in boundar ies . The (110)(111) prec ip i - ta tes located jus t at the h igh ly s t ressed s l ip bands of the (100) texture then serve to promote c leavage f rac - tu re .

The proposed mechan ism for lamel la r f rac ture is based on a genera l i zed c rys ta l -p las t i c i ty phenomenon that is app l i cab le to any mater ia l in wh ich texture or o r ientat ion band ing or c lus ter ing e f fec ts occur . A l l commerc ia l l y p roduced mater ia l s wi th h igh ly an iso - t rop ic c rys ta l s t ruc tures -s tee l , nonfer rous meta ls , o r p las t i cs -have a tendency toward or ientat ion band- ing and a l l a re potent ia l l y sub jec t to an isot rop ic lamel - la r f rac ture under cer ta in cond i t ions of s t ra in ing . Th is p roposed mechan ism is jus t one of many fa i lu re modes ; i t does not ru le out o ther modes of lamel la r f rac ture such as severe carb ide or inc lus ion band ing . The mechan ism proposed here suggests that the banded an isot ropy of p rone- to - th inn ing or ientat ions produce h igh normal shr inkage s t ra ins and fo rces , wh ich cause the weak in ter faces between banded re - g ions to be prone to lamel la r f rac ture .

SUMMARY

For fe r r i t i c - s ta in less -s tee l sheet , the banded cube- on- face texture component imbedded in o ther matr ix o r ientat ions , such as the cube-on-corner texture or a pure ly cube-on- face textured matr ix , tends to cause lamel la r f rac ture of the sheet at low temperatures or a t h igh s t ra in ra tes , o r both, espec ia l l y in th ick sec - t ions . Th is phenomenon is re la ted to the r idg ing ob- served when th in sheets of fe r r i t ie s ta in less s tee ls

a re s t ra ined at room temperature . The mechan ism proposed to exp la in th i s behav ior

is a genera l i zed c rys ta l -p las t i c i ty phenomenon that is app l i cab le to any mater ia l in which texture -or ienta - t ion band ing or c lus ter ing e f fec ts occur .

ACKNOWLEDGMENT S

The author acknowledges h i s indebtedness to K. G. Br i ckner , M. Kotyk, W. J . Murphy , and L . F . Por ter for the i r c r i t i ca l d i scuss ions and he lp fu l ass i s tance in the preparat ion of th i s paper . Spec ia l thanks is due to W. E . Spang, D. A. Gomr ick , and J . M. Hef fe rnan fo r obta in ing many of the micrographs in th i s invest i - gat ion .

REFERENCES

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514-VOLUME 9A, APRIL 1978 METALLURGICAL TRANSACTIONS A