Composite Mechanism of Scale Adhesiveness Kompozitni mehanizem oprijemljivosti škaje
B. Kosec, L. Kosec, FNT, Odsek za metalurgijo in materiale, Ljubljana
In the scale vvhich is formed on the surface of alloys during the annealing process, metallic and oxidic phases are mtervvoven in various ways, vvhich are characterized by the shape, portions, and size of both phases. Duetile scale component enables certain deformation of the scale, and it hinders propagation of cracks in the brittle oxidic phase. Key vvords: scale, composite material, crack, propagation, separation, adhesiveness.
I/ škaji, ki nastane med žarjenjem na površini zlitin, se kovinska in oksidna faza prepletata na različne načine, kijih karakterizira oblika, delež in velikost obeh faz. Duktilna sestavina škaje omogoča določeno deformacijo škaje in preprečuje širjenje razpok, nastalih v krhki oksidni fazi.
Ključne besede: škaja, kompozit, razpoka, napredovanje, ločitev, oprijemljivost.
Scale is product of the high temperature oxidation of metals and allovs. Structure of scale depends on the chemical composi-tion of allov. temperature atmosphere and on the time of annealing. The scale vvhich adheres to metal during working and ser-vice reduces in most cases the quality of ihe surface of product. Therefore it should be rentoved in single stages of technological process. The most simple ways of scale removal are mechanical forces vvhich appear due to temperature changes or in vvorking.
Scale and metal differ in their physical properties, among others, also in ali mechanical properties and in thermal expan-sion. Great differencess in thermal expansion during the temperature changes cause stresses vvhich practically separate both con-stituents. or thev fraetured only oxide. Scale adhesiveness depends on the microstructure, geometry of constituents, and the boundarv vvith the metallic matrix.
Due to properties and the way hovv scale constituents are in-tervvoven. and depending on its properties, the scale can be treated as a composite material.
Composite materials have different properties in comparison to the properties of constituents. One of essential characteristics
Fig. 2: Scale region with pronounced composite structure, vvith long, wide and overlapping metallic lamellae vvhich successfully stop the propagation of cracks (200 x) Slika 2: Del škaje /, izrazito kompozitno zgradbo, dolgimi, širokimi in prekrivanimi lamelami kovine, ki dobro zaustavljajo razpoke (200 x)
Fig. 1: Well defined simple boundary betvveen scale and parent metal in carbon steel (200 x) Slika 1: Dobro definirana enostavna meja med škajo in kovino v ogljikovem jeklu (200 x)
Fig. 3: Weak regions in the scale on the boundarv betvveen composite and oxide part
Slika 3: Šibka mesta v škaji na meji. ki loči oksidni del od kompozitnega
Fig. 4: Composite scale with a great amount of metal phase, vvhich is not ahle to stop the crack propagation (100x) Slika 4: Kompozitna škaja z veliko kovinske komponente, ki nisposobna ustavljati razpoke (I0()x)
of composite materials is crack arrest. In composite materials vvith ductile matrix (e.g. metallic or of polymers) cracks appear usually in rigid constituents of the armature (most frequently in fibres). vvhile in composite materials vvith ceramic matrix and ductile fibres the situation is reversed.
In the scale the nonmetallic constituents are alvvavs more brittle than the metallic ones, and thev have also more defects vvhich appear alreadv during the grovvth. Portion, way of being intervvoven, and geometrv of both constituents determine the ahilitv for stopping crack propagation and thus the obstinacv vv ith vvhich the scale resists to separation from the metal. In the čase s. illustrated in Figs 2, 5 and 6. the microstructure has such portions of metallic constituent, and such combination of both phascs. that separation on the boundarv vvith pure metal cannot be expected, and scale can be removed onlv by additional ma-chining of the surface. On pure metals the scale has usually a vvell defined boundarv vvith the metal. The oxide metal boundarv is the vveak point for ideal fracture and thus good separation of scale from metal (Fig. 1). In allovs the metallic and o.xidic con-
Fig.5,6: Weak directions for crack propagation in the compositescale vvith variouslv big metal "fibers", and along boundaries rich vvith oxide
of alloving element (Cr) (200x) SI. 5,6: Šibki mesti na meji dveh kompozitnih con z različnovelikimi "vlakni" kovine in vzdolž mej. bogatih z oksidom
legirnega elementa (Cr) (200x)
Fig. 7: Scheme of microstructural composition oi" some scales vvith composite structure Slika 7: Shema mikrostrukturne zgradbe nekaterih škaj s kompozitno zgradbo (3a.b,4)
Fig. 8: Variations in mechamcal characteristics of oxyde and metal in the scale
Slika 8: Razlike v mehanskih lastnostih oksida in kovine
Fig. 9: Arrest of crack propagation in oxide on the metallic fibres Slika 9: Ustavljanje širjenja razpok v oksidu na vlaknih kovine
stituents are most frequently intervvoven. the boundary betvveen scale and metal is not even vvhich highly renders the separation of both phases more difficult (Fig. 2).
In steels composed of elements vv ith thermodynamic properties different from those of iron, the scale of heterogeneous composition is formed. In the lovver part of scale. metal and oxide particles are intervvoven. This part behaves under mechanical loading identically to composite materials. The stresses vvhich appear due to temperature variations or other loads can cause eracks in the oxide. Their propagation can be stopped by suitably distributed metal in the scale, and thus the fracture of scale is pre-
geometrv of metallic phase
SI. 10: Širjenje razpok v kompozitni škaji / neugodno geometrijo kovinske faze.
vented. The vveak point in sueh scales is the surface betvveen the composite zone and the upper scale layer being vvithout metal (Fig. 3).
Some heterogeneous lovver scale parts have infavourable shape of metallic phase to stop the crack propagation in oxide. In sueh a scale crack propagates betvveeen metallic grains, and it can even cut some thin grains (FTg. 4).
In the oxide grains of parent metal there are also oxides of al-loying elements, being either dispersed or predominantly pre-cipitated in certain direetions or in form of a net vvhich corre-sponds to metal grain boundaries before the oxidation. These direetions are mechanicallv vveak points in the scale and cracks can propagate along them to parent metali matrix (Figs. 5, 6).
Some patterns hovv the boundary scale metal region is formed, are presented in Fig. 7.
Intervvoven mineral and metal constituents give to scale ali the characteristics of composite materials vvith the usuallv pre-dominant oxidic phase also in the respect of microstructure vvhile in the boundary vvith metal often metallic phase in the scale is prevailing (Figs. 7. 9 and 10).
Rigid mineral components render rigidness and compression strength to oxide, bul they are very sensitive to various flavvs vvhich appear during the grovvth of sueh oxide. Metallic matrix of suitable geometry is able to stop cracks, and it inereases the adhesiveness of scale (Fig. 9).
If a scale vvhich vvill easily separate from metal is to be obtained. il must be composed mainly of mineral constituents.
Metallic particles being intervvoven in the scale, especially if they are also connect vvith parent metali, can only increase the scale adherence.