the effect of silicon on the high temperature oxidation

ISIJ International, Vol. 47 (2007), No. 9, pp. 1245–1254

1245 © 2007 ISIJ

1. Introduction

During the production of low-carbon steels from recy-cled scrap, surface cracking during hot working is com-monly observed.1) This phenomenon, known as hot short-ness, arises from the preferential oxidation of iron and sub-sequent enrichment of the residual element copper. The en-richment rate is a balance between iron oxidation rate anddiffusion of copper back into the steel, but iron oxidationrates are so rapid at high temperatures that the solubilitylimit is exceeded after times on the order of milliseconds,according to calculations by Pötschke.2) Compounding theproblem of rapid enrichment is that the new phase that sep-arates is liquid at temperatures commonly encountered dur-ing steel production (�1 100°C).3) This liquid wets andthereby embrittles austenite grain boundaries, promotingintergranular surface cracking during hot deformation.4)

These surface cracks are obviously undesirable for steelproducers.

Currently, the common ways of controlling copper con-tent in steels are to use low-residual, i.e. low-copper, scrapor to dilute the copper when the steel is liquid by addingcopper-free iron sources, e.g. direct-reduced iron, hot-bri-quetted iron, iron carbide, or pig iron. Industry could takeadvantage of significant cost savings if the hot shortnessproblem could be minimized without these expensive iron

additions. In addition, higher-residual and therefore less-ex-pensive scrap could also be used. Yalamanchili et al.5) esti-mates saving $ 1.00/ton by an increase of 0.01 wt% in cop-per tolerance. As electric arc furnace steelmaking becomesmore common and demand for quality scrap and residual-free iron increases, one would expect these estimates to rise.

Surface hot shortness can be controlled by eliminating orreducing the amount of liquid copper in contact with theunderlying steel. A potentially beneficial effect from thepresence of nickel is promotion of a phenomenon known as“occlusion”.6) This process involves metal enriched in cop-per and nickel to become entrapped in the iron oxide layer.Despite having potential to reduce hot shortness, only someaspects of occlusion have been given significant attention inthe literature and as a result the conditions needed to pro-mote occlusion have not been adequately identified. It hasbeen reported that nickel contents greater than 0.02 wt%7)

and oxidation temperatures greater than 1 200°C were nec-essary for occlusion.7,8) However, not all investigators havefound this condition on temperature to be the case.9) Thewide variety of oxidation conditions (time, temperature,and atmosphere) applied in these different studies make de-termination of consistent conclusions regarding occlusiondifficult. In addition, the studies above involve oxidationtimes greater than 30 min, most on the order of hours. Oxi-dation times during casting and hot-rolling are expected to

The Effect of Silicon on the High Temperature Oxidation Behaviorof Low-carbon Steels Containing the Residual Elements Copperand Nickel

Bryan A. WEBLER and Seetharaman SRIDHAR

Center for Iron and Steelmaking Research, Department of Materials Science and Engineering, Carnegie Mellon University,Pittsburgh, PA 15213, USA.

(Received on April 9, 2007; accepted on June 28, 2007 )

This study investigates effects of silicon on copper- and nickel-rich phases during the oxidation of iron-based alloys containing 0.3 wt% copper and 0.3 wt% copper �0.15 wt% nickel in addition to steel samples containing various amounts of copper (0.17–0.41 wt%), nickel (0.03–0.13 wt%), and silicon(0.03–0.12 wt%). Samples were exposed to air at 1 150°C for 60, 300, and 600 s. A low-carbon steel sample(0.02 wt% silicon) without copper and nickel was subjected to the same conditions for comparison.

The oxidation rates of copper- and nickel-containing steels decreased with time and were consistentlylower than the rate of the residual-free low-carbon steel. An internal oxide layer was observed only in thecopper- and nickel-containing steels. The number of internal oxides in this layer increased with oxidationtime and larger internal oxides in this layer were characterized to be rich in iron and silicon. Compared to theiron–copper–nickel alloy, steels containing copper, nickel, and silicon, had more copper-, nickel-rich materialfound as particles entrapped in the oxide.

It is proposed that the population of internal silica particles increases due to increasing oxygen contentnear the oxide/metal interface. The rise in oxygen content results from increased oxygen solubility causedby copper and nickel enrichment. These internal oxides decrease oxidation rate and assist occlusion. An in-crease by a factor of 10 in amount of occluded material was measured in material containing copper, nickeland silicon compared to copper and nickel.

KEY WORDS: high-residual containing steels; copper; nickel; silicon; internal oxidation; occlusion; surfacehot shortness.

be 1 to 5 min. Therefore, studies involving shorter oxidationtimes would give insight into the evolution of occluded ma-terial and also simulate the significant oxidation that occursduring casting and hot-rolling.

Along with promoting occlusion, nickel additions in-crease the solubility of copper in austenite, but completeprevention of a liquid copper-rich phase requires twice asmuch nickel as copper,10) making it impractical as a cost-ef-fective industrial remedy. Promotion of occlusion requiressubstantially less nickel, making it more practical for indus-try as a method to reduce hot shortness. In addition tonickel, some effects of silicon on occlusion have been pre-viously observed. Chen and Yuen9) found iron–silicon oxideparticles and suggested that they aided occlusion. Copelandand Kelley11,12) have also observed the benefits of siliconadditions (greater than 0.3 wt%). Their work attributes thebeneficial effect of silicon to both increased occlusion andalso to electrolytic cell action. The latter occurs only attemperatures above 1 205°C, where fayalite is liquid.12) Atwo-phase region in the oxide consisting of wüstite and fay-alite was also observed by Seo et al.13) but they did not dis-cuss in detail the effect of this region on occlusion.

In the present study we investigate the evolution of resid-ual-rich phases during isothermal oxidation at 1 150°C(which is a typical steel temperature after casting and dur-ing hot-rolling) in air for times less than 10 min. Low-car-bon steel samples with copper (0 to 0.41 wt%) and nickel (0to 0.13 wt%) were compared to iron-based alloy sampleswith 0.3 wt% copper and 0.3wt%copper–0.15wt%nickel.Our objectives were to establish the effect of silicon com-bined with copper and nickel on the processes causing en-richment and occlusion.

2. Materials and Methods2.1. Materials

Six low carbon steels containing various amounts ofresidual elements and three iron-based alloys (not steels)were used in this study. Their chemistries are listed inTable 1. Due to deoxidation, the measured aluminum con-tent in Steel 1 is mostly present as alumina inclusions andcan be ignored. Some of the silicon measured in Steels 2–5is also present as silica inclusions. All steels were industri-ally-produced and supplied as hot-rolled rods or plates. The

alloy samples, named Fe, FeCu, and FeCuNi, were obtainedfrom the Materials Production Center at the Ames NationalLaboratory in Ames, Iowa. The chemistries in Table 1 werecombined from the raw materials specifications. The alloyswere arc-melted in vacuum, cold-rolled, and cyclically heattreated between 850°C and 950°C in vacuum for 1 h.

Due to their copper and nickel contents, all steels wouldbe expected to exhibit varying degrees of hot shortness.Different silicon levels between Steel 1 and Steels 3–5 wereused to examine the effect of silicon in the presence of cop-per and nickel. Steel 6 was used to examine the behavior ofa steel with silicon in the absence of copper and nickel. TheFe, FeCu, and FeCuNi alloys were used to investigate thebehavior of copper and nickel without silicon.

2.2. Experimental MethodsOxidation kinetics were measured by thermogravimetric

(TG) analysis. A schematic of the apparatus used is shownin Fig. 1(a). Isothermal oxidation experiments were carriedout in a 6 cm diameter vertical tube furnace with a constanttemperature hot zone 3 cm in length (temperature range wasmeasured to be �1°C). The furnace hot zone temperaturewas measured with a K-type thermocouple before a seriesof experiments. Actual furnace temperatures were notmeasured for each experiment. The samples were attachedto a Mettler-Toledo AT261 DeltaRangeTM balance by a plat-inum chain, and sample weight was recorded every second.The results are reported as mass change per unit surfacearea.

The samples were also attached to a winch that allowedthe samples to be lowered from the top of the tube furnaceinto the hot zone. The sample was then suspended in thehot zone by the chain connected to the balance. Initially, thesamples were lowered in flowing air but there were occa-sional instances when the samples were not completely sus-pended in the hot zone until 20 or 30 s had passed and theytherefore oxidized under non-uniform conditions. Any ex-periment where this occurred was terminated. Two experi-ments with Steels 1 and 2 were successfully lowered in airand are included in the analysis. Further samples were low-ered in an argon–5% (by volume) hydrogen gas mixture toprevent oxidation during lowering. After the samples de-tached, air was allowed to flow. The high air flowrate (dis-cussed below) ensured rapid replacement of the argon–hy-

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Table 1. Chemistries of samples investigated in this study.

drogen mixture. No significant differences were observedbetween samples lowered successfully in air and those low-ered in argon–hydrogen.

The high temperature oxidation was conducted at1 150°C in air at a flowrate of 2.1 standard liters per minute(sL/min), corresponding to a linear velocity of 6 cm/s.Flowrate was measured with a variable-area flowmeter. Ve-locities higher than 4.2 cm/s supply enough oxygen so thatafter an initial period, iron cation diffusion in the oxide con-trols the rate of oxidation.14) The air used was of unspeci-fied quality but the partial pressure of water vapor was

measured to be 1.4�10�4 atm at 23°C.Figure 1(b) shows a schematic of the heat treatments

used in the TG experiments. All details for the individualoxidation experiments are found in Table 2. After the hightemperature experiments, the sample microstructures wereinvestigated in a scanning electron microscope (SEM). Topreserve the interface structure, the samples were mountedin a cold-curing epoxy resin with the following vacuum-im-pregnation procedure: (i) samples were placed in a chamberthat was subsequently evacuated, (ii) the epoxy was addedunder vacuum, and (iii) air was slowly introduced to forcethe epoxy into pores and gaps in the oxide. The sampleswere then sectioned with a low-speed diamond saw andpolished to a 1 mm diamond finish. All samples were coatedwith 2–2.5 nm of chromium or platinum to prevent charg-ing in the SEM.

The SEM used was a Philips XL-20 scanning electronmicroscope with a field emission gun source operating at25 kV and a solid-state backscattered detector installed. Theworking distance was 10 mm. Images were taken inbackscattered electron (BSE) mode. Energy dispersive x-ray spectroscopy (EDS) was used to obtain qualitative com-positional information.

From the BSE images, the following were quantified:1) Areas of separated copper-rich phase in the metal;

these areas appear brighter than the surrounding iron.EDS was used to confirm the presence of copper andnickel.

2) Areas of occluded phase; these areas appear brighterthan the surrounding oxide.

3) Length of the oxide/metal interface.4) Total oxide layer thickness and thickness of the wüstite

layer.The open source software ImageJ15) was used to measure

the above quantities. For all measurements, conversion fac-tors were established between number of pixels and actuallengths using the scale bar on the SEM micrograph. Theamount of separated or occluded material was quantified asfollows: isolate the brighter enriched and occluded areas bymanually thresholding the image and measure them using


1247 © 2007 ISIJ

Fig. 1. Schematic of (a) the TG setup and (b) the experimentaltime-temperature profile used in this study.

Table 2. Experimental conditions employed during the oxidation experiments.

ImageJ’s “Wand” function (an automatic area trace). Thesearea measurements were then normalized to the length ofthe field of view, 63.2 mm in all cases. The results are re-ported as “Amount of Separated Phase” or “Amount of Oc-cluded Phase”. This procedure amounts to taking the sepa-rated and occluded area as layers of uniform thickness overa length of 63.2 mm. The length of the oxide/metal interfacewas normalized to the length of a hypothetical flat interfacestretching between the start and end points of the originalinterface. Usually, this corresponded to the image field ofview, 63.2 mm. The measurements were averaged over oxi-dation time for all chemistries. Several measurements wererepeated and error due to reproducibility was found to benegligible.

3. Results3.1. Oxidation Behavior—TG

The graphs in Fig. 2 show examples of the oxidation be-havior measured by the TG setup for the materials in thisstudy. It was found that the oxidation kinetics obeyed a lin-ear rate law,

..............................(1)

where Dm/A is mass gain per unit area, kl is the linear rate

constant, and t is time, for the first 60–70 s. Magnifiedviews of the linear portion of the curves are shown in Figs.2(c) and 2(d). Figure 3(a) shows the measured linear rateconstants for all chemistries. The inset in Fig. 3(a) depicts atrend line fit to the linear portion of the data. Comparingwith Eq. (1), it is seen that the slope of this trend line is thelinear rate constant.

As oxidation proceeds, the shape of the curves in Fig. 2change, indicating that the oxidation kinetics become con-trolled by another process. The exact time of the transitionregion is unknown, but it appears from Fig. 2 that this re-gion, the knee of the curves, occurs between 70 s and 100 s.To eliminate any potential effects of including this transi-tion region, it was assumed to last between 70 s and 150 s.After oxidation for 150 s, solid-state diffusion through theoxide layer becomes rate-controlling and the oxidation ratebecomes parabolic,

.............................(2)

All parameters are as defined above except kp which de-notes the parabolic rate constant, a description of which canbe found in nearly any book on oxidation (e.g. Ref. 16)).The parabolic regime lasted from 150 s until the end of theexperiment. Figure 3(b) shows the parabolic rate constants

∆m

Ak t

2

2� p

∆m

Ak t

� l


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Fig. 2. Example TG measurements of sample mass change (normalized to sample surface area) with oxidation time forthe samples oxidized for 600 s. Steel samples are shown in (a). The numbers and names adjacent to each curverefer to sample chemistry listed in Table 1. The behavior of samples Fe, FeCu, and FeCuNi samples are shown in(b). The linear regimes for the samples are shown in (c) for the steel samples and (d) for samples Fe, FeCu, andFeCuNi.

Fig. 3. (a) Linear and (b) parabolic rate constants obtained by TG methods. The numbers on the x-axis refer to the chem-istry listed in Table 1. The insets (data from Steel 6) show the trend lines used to calculate the linear and parabolicrate constants from Eqs. (1) and (2).

for each steel chemistry. The constants were determined byfitting a straight line to a plot of (Dm/A)2 vs. t. An exampleof this plot, known as a “parabolic plot” is shown in theinset of Fig. 3(b). The trend line is shown in the inset andthe slope of this line is related to the parabolic rate constant(see Eq. 2). The squares of the correlation coefficients (r2

values) for the straight-line fits of all steels except Steel 6ranged between 0.96 and 0.97. The r2 value for Steel 6 was0.99. The parabolic oxidation rate is highest for the resid-ual-free steel, Steel 6. In Steels 1–5, the rate constants de-crease as the silicon content of the copper- and nickel-con-taining steels increases, i.e. kp for Steel 1 (with 0.03 wt%silicon) is greater than kp for Steels 2–5 (with 0.1 wt% sili-con).

3.2. Oxidation Behavior—Wüstite FractionFigure 4 shows example micrographs of the oxide layers

for Steels 1 and 5.These micrographs show that wüstite, magnetite, and

hematite layers can be identified in BSE-SEM images fromthe differences in contrast. Wüstite in particular is identi-fied by the magnetite precipitates that form on cooling as aresult of the eutectoid decomposition.17) The wüstite thick-ness fraction (wüstite layer thickness divided by total oxidelayer thickness) for each sample was measured from theBSE images. Examples of high (0.95) and low (0.48)wüstite thickness fractions are shown in Figs. 4(a) and 4(b),respectively. The wüstite thickness fractions for the copper-and nickel-containing steels decrease with time as shown inFig. 5. The wüstite thickness fraction of the residual-freesteel, Steel 6, remains constant at 0.95 after oxidation for300 s and 600 s. The wüstite thickness fractions of samplesFe, FeCu, and FeCuNi remain constant at 0.95 after oxida-tion for 300 s (note in Fig. 5 that the points for Fe, FeCu,FeCuNi and Steel 6 overlap at 300 s).

By considering the connection between overall oxidationrate and transport processes in the oxide layer, wüstitethickness fraction can be used as an alternative descriptionof the oxidation rates. This connection will be discussedlater in the paper.

3.3. Internal OxidationAlong with external iron oxidation, the internal oxidation

behavior is also affected by sample chemistry. Figure 6shows that little internal oxidation is observed in the resid-ual-free steel, Steel 6, while a significant number of parti-cles can be seen in the steel containing copper and nickel,Steel 1. The amount of internal oxide particles and their

depth from the interface increases with time as shown inFig. 7(a)–7(c).

While the composition of the small internal oxides couldnot be determined, some of the larger internal oxides, iden-tified by the arrows in Fig. 7, were found by EDS to consistof primarily iron and silicon. Small amounts of manganesewere also detected. This suggests the internal oxide parti-cles are silicon oxides which react with the iron or wüstiteto form fayalite. The manganese content likely results fromsmall amounts of manganese leaving the steel and dissolv-ing into the oxide.

3.4. Oxide/Metal Interface StructureFigure 8 shows micrographs of the metal/scale interface

region for several steel and alloy samples after 300 s oxida-tion. The lowest interface roughness is found in Fe (Fig.8(a)) which was pure iron and FeCu (Fig. 8(c)) which con-tained 0.3 wt% copper. A relatively similar low interfaceroughness is found for Steel 6 (Fig. 8(b)) which was theresidual-free steel that contained silicon. The simultaneouspresence of nickel and copper in FeCuNi increases theroughness as seen in Fig. 8(d) compared to only copper ad-ditions (Fig. 8(c)). The oxide/metal interface appears wavyeven in the presence of a liquid layer (Fig. 8(d)). Steel 3 hascomparable copper content as FeCuNi and slightly lessnickel but it also contains silicon and exhibits significantinternal oxidation. Comparison of Figs. 8(d) and 8(e) showsthe interface is significantly rougher in the presence of sili-


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Fig. 4. BSE-SEM micrographs showing example oxide layers in(a) Steel 1 and (b) Steel 5 oxidized for 600 s. The wüstitelayers are easily identifiable in each image by the mag-netite precipitates that form on cooling. These imagesdemonstrate examples of high (0.95 in (a)) and low (0.48in (b)) wüstite thickness fractions. The measured wüstitethickness fractions will be used as a measure of oxidationkinetics in addition to the TG data.

Fig. 5. Plot of wüstite thickness fraction vs. oxidation time. TheFe, FeCu, and FeCu Ni samples were oxidized only for300 s and all had wüstite thickness fractions of 0.95 (thepoints for these samples and for Steel 6 oxidized for 300 sall lay upon each other in the plot).

Fig. 6. Comparison of (a) Steel 6 and (b) Steel 1 oxidized for10 min showing the difference in amount of internal ox-ides. An iron–silicon–oxygen compound (presumablyfayalite) is denoted in (b). The arrows in (b) show thatfayalite is present as a film at the interface and as parti-cles further up in the oxide.

con (and therefore internal oxides). Steel 5 has a lower cop-per-to-nickel ratio than Steel 3 and the same silicon con-tent, but the interface is the most rough of all the samplesstudied (Fig. 8(f)). From these micrographs, it is apparentthat nickel additions alone lead to the development of awavy oxide/metal interface, but the interface is dramaticallymore perturbed as a result of the simultaneous presence ofcopper, nickel, and silicon.

3.5. Evolution of Enriched PhasesNoted in Figs. 8(c), 8(d), and 8(e) is the presence of a

copper-rich liquid as a bright film along the oxide/metal in-terface. In the steel samples, the separated copper-rich liq-uid is often present as pockets as shown in Fig. 8(e). Figure9(a) shows that the amount of separated copper-rich liquid

in the steel samples decreases as oxidation time increases,especially after 600 s. Figure 9(b) shows that the amount ofmaterial occluded into the oxide follows the opposite trendand increases with decreasing oxidation time.

Small copper-rich particles were found to be distributedthroughout the wüstite layer up to the wüstite/magnetite in-terface. An example of these particles is shown in Fig. 10.

The iron–silicon internal oxides mentioned above areoften observed to contain occluded material as shown inFig. 11.

To investigate the effect of silicon additions on occlu-sion, FeCuNi (which is silicon-free), was used for the pur-pose of comparison. Figure 12 shows micrographs of Fe-CuNi, Steel 3, and Steel 5 oxidized for 300 s. The amountof the occluded material is significantly higher in both Steel


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Fig. 8. BSE images of (a) Fe, (b) Steel 6, (c) FeCu, (d) FeCuNi, (e) Steel 3, (f) Steel 5 oxidized for 300 s. Also displayedare the oxide/metal interface lengths for each chemistry.

Fig. 9. Plot of (a) amount of copper-rich separated phase and (b) amount of occluded material versus oxidation time.

Fig. 7. BSE-SEM images of Steel 1 oxidized for (a) 60 s (b) 300 s and (c) 600 s. The amount of internal oxidation in-creases with oxidation time. The small arrows denote oxides that were found to contain iron, silicon, and occa-sionally manganese.

3 and Steel 5. Some regions of FeCuNi, such as that shownin Fig. 8(c) have significant amounts of copper-rich liquid.No such regions were detected in the steels of comparablechemistry (Steel 3 and Steel 5).

4. Discussion

The results presented above show that both oxidation be-havior and the microstructure near the oxide/metal interfaceare influenced by the presence of copper, nickel and silicon.Steels containing these elements have lower oxidation ratescompared to the residual-free steel sample containing sili-con but no copper and nickel. The presence of copper andnickel was found to promote internal oxidation of silicon.Interface roughness was increased in the samples contain-ing the elements copper, nickel, and silicon compared to theresidual-free sample (no copper and nickel) or the FeCuand FeCuNi samples (no silicon). Finally, the amount of en-riched material occluded in the oxide was found to behigher in the steel samples with copper, nickel and siliconthan in the FeCu and FeCuNi samples (with no silicon).These results are discussed below in the context of the fol-lowing phenomena: external iron oxidation, internal oxida-tion, interface roughness and occlusion of phases rich incopper and nickel.

The TG measurements in Fig. 2(a) showed that the oxi-dation behavior of the steel samples varied significantly.The oxidation behavior of the Fe, FeCu, and FeCuNi sam-ples (see Fig. 2(b)) is expected to differ in magnitude some-what from the behavior of steels for reasons unrelated to thepresence of residual elements.17) The causes for these dif-ferences have been documented and are not the focus ofthis study. The analysis of the TG data is confined to the

steel samples.During the initial linear stage, the rate-controlling

process for oxidation is oxygen diffusion through a gasphase boundary layer.14,18) Sample geometry affects thisboundary layer (for a discussion on the development of anddiffusion through a boundary layer see e.g., Ref. 19)), andtherefore the linear rate constants are slightly different be-tween the parallelepiped and cylindrical samples (see Table2 and Fig. 3(a)). Between samples of the same geometry,there is little variation of the linear rate constant with steelchemistry.

According to the plot in Fig. 3(b) the residual-free steel,Steel 6, had a significantly higher parabolic rate constantthan any other steel. Its value, 1.5 mg/cm2 s, is comparableto those found in the literature for residual-free low carbonsteel oxidation, e.g. 1.996 mg/cm2 s found by Sachs andTuck.20) Steel 6 also had a high r2 value for its parabolic fit,so only the kinetics of Steel 6 were well-described by a sin-gle parabolic rate constant. The presence of copper, nickel,and silicon then results in some process that slows downoxidation rate and causes Eq. (2) not to be strictly obeyed.

The thermogravimetric measurements in Fig. 2 are of thetotal measured mass change that occurred during oxidation.Analysis of this data assumes oxidation occurs uniformlyover a constant sample surface area. Upon examination ofthe samples in the SEM, the oxide layer thicknesses werefound to be different at different points on the sample. Thisis a common occurrence during oxidation of iron and steeland occurs due to oxide blistering,17) or local separation ofthe oxide from the metal. Blistering is dependent on growthstresses,21) porosity,22) oxide/metal interface adhesion,23)

and specimen geometry.16,24) The major consequence ofblistering is a change in the oxidation rate in a blistered re-


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Fig. 10. BSE SEM image of small copper-, nickel-rich particlesin the wüstite layer. These particles are commonly ob-served in all steels for all times and are the result of theiron in occluded particles oxidizing away. The particlesize is �0.5 mm.

Fig. 11. BSE SEM image of Steel 2 oxidized for 600 s. The darkareas of the image are an iron–silicon–oxygen com-pound (assumed to be fayalite) that is often seen to con-tain occluded particles.

Fig. 12. BSE-SEM images showing the significant differences in interface structure between (a) Steel 5, (b) Steel 3 and(c) FeCuNi oxidized for 300 s. The average amount of occluded material is also displayed for each chemistry.

gion, where the oxide has separated from the metal, com-pared to an adherent region. The occurrence of blisteringimplies that the oxidation rate is not uniform over the sam-ple surface and the TG data may not be an accurate repre-sentation of the oxidation behavior.

To investigate any complications from blistering on thevalidity of the TG data, the ratio of wüstite thickness tototal oxide thickness (the wüstite thickness fraction) wasused as an alternate description of oxidation behavior. Dis-tinct transition regions between blistered (wüstite thicknessfractions 0) and unblistered regions were observed in theSEM micrographs. Areas with the highest wüstite thicknessfractions were assumed to be unblistered. Analysis of boththe oxide and the underlying interface was confined to theseregions.

The use of wüstite thickness fraction as a measure of oxi-dation kinetics can be justified by experimental and theo-retical work on the relationship between oxidation ki-netics and oxide microstructure, specifically relative oxidelayer thicknesses. For isothermal oxidation of pure iron,Païdassi25) measured parabolic kinetics and wüstite thick-ness fractions of 0.95. Calculations by Garnaud and Rapp26)

(using a theory developed by Yurek et al.27)) showed thatthe thickness fraction of wüstite should be 0.96 when ironexhibits ideal parabolic oxidation behavior, i.e. Eq. (2) isobeyed and the rate constants relate to cation diffusivi-ties.16) Another consequence of Yurek’s theory27) is that theoxide thickness ratio XWüstite/XMagnetite should not changewith time when iron exhibits ideal parabolic oxidation be-havior. Païdassi’s25) experimental work also showed that thewüstite thickness fractions remained constant with time.Thus, calculations and experiments show that when iron ex-hibits parabolic oxidation kinetics the thickness fraction ofthe wüstite layer should be constant in time and equal to0.95. Slower diffusion through the wüstite layer results in alower wüstite thickness fraction and, since wüstite growthdetermines the overall rate of iron oxidation,28) a sloweroverall oxidation rate. Although the magnitude of the para-bolic rate constants differ between low carbon steels andpure iron,17) the wüstite thickness fractions in unblisteredregions are similar to those of pure iron, 0.95.17) Thus theabove relationships between oxidation rate and wüstitethickness fraction should be applicable to steels.

It should be noted that an increase in the growth rate ofmagnetite could also produce a decrease in wüstite thick-ness fraction according to the above discussion. Below, itwill be shown that all of the processes affecting ion trans-port occur near the wüstite/metal interface. No significantion dissolution is expected to occur that could change thetransport behavior in the magnetite and the growth of mag-netite layer should be unaffected. There is therefore reason-able confidence in using changes in the wüstite thicknessfraction to complement the TG measurements.

The wüstite thickness fraction measurements were usedto confirm: (1) that the oxidation rates of steels containingcopper, nickel, and silicon were considerably less than theresidual-free steel, and (2) that only the behavior of theresidual-free steel was well described by a single parabolicrate constant. Figure 5 shows that the wüstite thicknessfractions of the steels containing copper, nickel, and siliconare less than 0.95 and decrease with increasing time. Thesesteels thus have slower overall oxidation rates than theresidual-free steel and since their wüstite thickness frac-tions vary with time, their oxidation rates cannot be de-scribed by single parabolic rate constants. Similar conclu-sions were reached from considering the TG data, as dis-

cussed above. The wüstite thickness fraction data confirmsthe TG results and demonstrates that oxide blistering doesnot influence the qualitative analysis of TG-measured oxi-dation kinetics. The decreased wüstite thickness fractionalso implies that some process occurs to slow down diffu-sion in the wüstite layer. The use of wüstite thickness frac-tion thus confirms and complements the TG results by sug-gesting a potential mechanism (decrease in diffusion ratethrough wüstite) for the observed features of the TG curves.

In the wüstite thickness fraction measurements of Fig. 5and the TG measurements of Figs. 2 and 3(b), significantdifferences in behavior were observed between Steels 1 and6. Since the only difference between these two steels is thatSteel 1 contains copper and nickel, the enrichment of theseelements must affect the oxidation rate. One effect involvesthe relationship between oxide/metal interface equilibriumand the parabolic rate constant. As copper and nickel en-rich, the iron activity decreases near the oxide/metal inter-face. To maintain local equilibrium, the wüstite dissociates,increasing the oxygen activity at the oxide/metal interfaceand decreasing the net oxide growth rate. In a pure iron-copper alloy oxidized between 700°C and 1 000°C, the ef-fect of this decrease in iron activity and increase in oxygenactivity on the overall oxidation rate was found to be negli-gible if no significant concentration gradients in the ironexisted.29) The composition of the copper-rich liquid in theiron–copper–nickel system is not appreciably different fromthat of the iron–copper system30) and therefore this effect isnot expected to be significant. In the iron–copper alloymentioned above the observed decrease in oxidation ratewas attributed to slow iron diffusion through the solid cop-per-rich phase dramatically decreasing the iron content atthe oxide/metal interface.29) Similar phenomena have beenobserved in other alloys with a noble component.31) In theiron–copper–nickel system at the temperatures studied, theiron-depleted phase that forms is liquid and transport ofiron through the liquid to the oxide should not be an issue.The solid enriched material behind the liquid layer shouldconsist of approximately 80 wt% iron30) and transport ofiron through this material should also not affect the oxida-tion rate.

Figure 5 shows that the wüstite thickness fractions ofsamples Fe, FeCu, and FeCuNi have the ideal value ex-pected for pure iron, 0.95. This near-ideal oxidation behav-ior of the alloy samples suggests that there are no signifi-cant effects of iron activity as discussed above on oxidationrate due to enrichment of copper and nickel, at least for thetimes studied here. However, the decrease in Dm/A for sam-ple FeCuNi, shown in Fig. 2(b), suggests that the presenceof copper and nickel has some effect on oxidation rate,though the effect is smaller than for steels having the sameamount of copper (Steel 3) or the same copper–nickel ratio(Steel 5). This effect does not appear to influence the meas-ured wüstite thickness fractions and further work is neces-sary to investigate the mechanism by which nickel and cop-per alone can affect oxidation rate.

The effects of nickel and copper on the TG-measured ox-idation behavior are relatively small, so other consequencesof enrichment must also affect the oxidation rate. The mi-crographs in Fig. 6 show one such consequence, an increasein number of internal silica particles in the copper- andnickel-containing Steel 1 compared to Steel 6. The pres-ence of silicon in steels has long been known to affect oxi-dation rates. Evans and Chatterji32) compared iron with0.52 wt% silicon to pure iron, finding non-parabolic behav-ior and significantly less oxide formed in the silicon-con-


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taining sample. The mechanism is dependent on siliconcontent, oxidizing gas, and temperature, but is attributed tothe formation of silica or fayalite layers that impede irondiffusion in the oxide layer.32,33,34)

The steels in this study contain much less silicon than inthe studies mentioned above (see Table 1). However, the en-richment of copper and nickel can act to increase theamount of internal silicon oxides or fayalite that forms asthe steel oxidizes. As copper and nickel enrich, iron activityat the oxide/metal interface decreases. To maintain localequilibrium, wüstite at the interface dissociates, increasingthe amount of oxygen at the oxide/metal interface. Accord-ing to Fisher,6) the enriched nickel raises the oxygen solu-bility in iron and the oxygen from wüstite dissociation dis-solves into the metal, increasing its concentration near theoxide/metal interface. Böhm and Kahlweit35) showed thatthe nucleation rate of internal oxide particles at a given dis-tance into the metal is proportional to the oxide/metal inter-face oxygen content cubed. Thus, as the interfacial oxygencontent increases during enrichment, more internal oxideparticles are expected and this expectation is observed inthe present study as shown in Figs. 7(a)–7(c).

As the wüstite/iron interface advances into the metal,wüstite comes in contact with the silica particles and reactswith them to form fayalite. Since there is not enough sili-con to form a continuous silica or fayalite layer, the wüstitecontinues to grow, deforming plastically around the fayaliteand introducing voids in the oxide layer. Some of thesevoids are eliminated by creep of the oxide layer and someby dissociative transport of oxygen.36) The result is thenear-surface oxide layer of wüstite, fayalite, and voids asdepicted in Fig. 6(b). Fayalite is observed as a film near theoxide/metal interface and as particles in the oxide.

The presence of fayalite restricts iron transport either tothe oxide or in the near-interface region of the oxide, de-creasing the diffusion rate through the wüstite layer andtherefore the overall oxidation kinetics. As this interfacialregion changes with time (Fig. 7), the transport path of ironions changes with time and the consequences are reflectedin both the TG and wüstite thickness fraction measurementsas described above.

The decrease in oxidation rate due to internal oxidationof silicon is one of the potential roles silicon plays inprocesses related to hot shortness because the decreased ox-idation rate should reduce formation of the separated liquidand promote processes leading to occlusion. The internaloxide particles themselves are also important in promotingocclusion. As discussed below, the presence of silicon canpotentially explain the trends in amount of separated andoccluded material shown in Figs. 9(a) and 9(b); however,the dominant mechanism cannot yet be identified.

A decrease in oxidation rate represents a decrease in theenrichment rate of copper and nickel which affects the bal-ance between enrichment and diffusion of copper andnickel back into the steel. Simplified calculations byMelford4) demonstrated this balance between oxidation rateand back-diffusion rate. Fast oxidation rates lead to suchrapid enrichment that back-diffusion of copper and nickel isnearly negligible. As oxidation rate decreases, the amountof material diffusing away may become comparable or evensurpass the amount enriching due to oxidation. This wouldlead to results similar to those shown in Fig. 9(a), i.e. thatthe amount of separated phase follows the same trend asoxidation rate and decreases with time.

In addition to directly influencing the separated copper-rich material, silicon also affects the conditions necessary

for occlusion, resulting in a significant increase the amountof material occluded into the oxide layer. Occlusion of cop-per-rich material into the oxide layer has two requirements:development of rough oxide/metal interface and formationof internal oxides that act as lateral bridges between partsof the uneven interface (these are often observed to be ironoxides).9) The interface roughness is often attributed to thedevelopment and stabilization of perturbations in theoxide/metal interface from diffusion considerations. Analy-ses of this problem have been conducted by Wagner37) andWhittle et al.38) Formation of a liquid layer at theoxide/metal interface should result in a planar oxide/metalinterface,39) while a solid enriched layer would result in arough oxide/metal interface. The planar interface reducesthe amount of material that can be occluded into theoxide.9) As mentioned above, the decrease in oxidation ratedue to internal oxidation of silicon reduces the amount ofseparated copper-rich liquid which should promote inter-face perturbations and occlusion. The measured interfacelengths support this hypothesis, since the interface lengthsincrease in the steels containing copper, nickel, and siliconcompared to the iron–copper–nickel alloy, see Figs. 8(d),8(e) and 8(f).

Independent of its effect on oxidation rate, formation ofinternal oxides of silicon and subsequent fayalite formationcan contribute to interface roughness and occlusion. The in-terface lengths represent the interface perturbations (as dis-cussed in the previous paragraph) as well as the combina-tion of internal oxide particles, which are heterogeneouslydispersed near the oxide/metal interface, with the externaloxide layer. This combination should also lead to an in-crease in interface roughness as shown in Figs. 8(d), 8(e),and 8(f). The internal oxides themselves also assist the lat-eral bridging of perturbations. An example of how growthof these internal oxide particles contributes to occlusion isillustrated in Fig. 13. The dashed lines and arrows in Fig.13 indicate the projected growth path of the internal oxideand the oxide at the metal surface. Eventually they willcombine and that large amount of iron in the center of theimage will be occluded. As the number of internal oxidesincreases with time (see Fig. 7) more of these bridgesshould be created. This would result in an increase in theamount occluded with time as shown in Fig. 9(b).

The combination of increased interface roughness andinternal oxidation has a substantial effect on the amount ofmaterial occluded into the oxide layer. As shown in Fig. 12,


1253 © 2007 ISIJ

Fig. 13. BSE SEM image of Steel 2 oxidized for 600 s (sameimage as Fig. 12). The dashed lines and arrows show theprojected growth path of the internal iron–silicon ox-ides. The large amount of material in the center of theimage will be occluded into the oxide.

the steel samples with 0.1 wt% silicon increased the amountof material occluded by a factor 10 over the alloy samplecontaining only copper and nickel. Steels 3 and 5 were cho-sen because they have similar copper content (Steel 3) andcopper-to-nickel ratio (Steel 5) as FeCuNi. The increase inamount occluded with also decrease the amount of sepa-rated copper-rich material, contributing to the trend in Fig.9(a).

There is a complication in the measurement of occludedmaterial that may affect the trend shown in Fig. 9(b). Mea-surements of occluded particles did not take into accounttheir distance from the oxide/metal interface. It is possiblethat those particles furthest from the interface were oc-cluded during early stages of oxidation. If this were true,the measured “Amount Occluded” includes particles oc-cluded early in oxidation in addition to those occluded justas the experiment finished. Particles would be counted mul-tiple times, leading to the same trend as shown in Fig. 9(b)but the trend would be caused by the accumulation of oc-cluded particles.

To determine whether multiple counting occurs, the timenecessary for the iron in a spherical occluded particle of di-ameter 10 mm to oxidize away was calculated using rateconstants in Fig. 3 and Eqs. (1) and (2). Although it hasbeen discussed that the TG-measured behavior is not thebest representation of oxidation kinetics it is used here asan approximation and the calculations show that regard-less of the kinetic regime, the time required to oxidize theiron is less than 10 s. Once the iron has oxidized acopper/nickel-rich particle remains trapped in the oxide.Assuming the near surface region to of composition equalto the solubility of iron enriched in copper and nickel(80 wt% iron, 16.7 wt% copper, 3.3 wt% nickel for a cop-per/nickel ratio of 530)), the diameter of the occluded parti-cle should be about 20% of its original diameter, or 2 mm.The micrograph of Steel 1 in Fig. 10 shows a typical distri-bution of these particles and shows their size to be�0.5 mm. EDS measurements show the occluded particlesare usually richer in iron than assumed above, so the meas-ured/predicted size difference is reasonable. The occludedparticles included in the current measurements are shown inFigs. 11 and 12 and are clearly larger than 2 mm. Since thelarge occluded particles oxidize to negligible size in a muchshorter time than the intervals studied (60, 300, 600 s), thevalue “Amount Occluded” relates to the amount occludedsoon after the experiment was completed and the trend inFig. 9(b) can be attributed to the effects discussed above.

The current study supports the previous findings that thepresence of silicon plays a role in reducing the amount ofseparated copper-rich liquid. The results here show that in-ternal oxidation of silicon reduces the amount of liquid bydecreasing the enrichment rate of copper and nickel and byassisting occlusion. The latter effect appears to be due to acombination of increased interface roughness and increasednumber of internal oxides acting as bridges between theperturbations. The relative importance of the two effectscan not be quantified in this study. It appears that no delib-erate silicon additions beyond that used for attaining deoxi-dation equilibrium in the ladle are necessary for it to play asignificant role on the separated and occluded copper-richphases.

5. Conclusions

(1) Increased internal oxidation of silicon in low car-bon steels causes the oxidation rate to decrease during oxi-

dation times less than 600 s. The rates are also non-para-bolic. The increase in internal oxidation results from an in-crease in near-surface oxygen content due to copper andnickel enrichment. Silica and fayalite restrict iron diffusion,causing the deviations.

(2) Increased internal oxidation of silicon increases theinterface roughness compared to an iron–copper–nickelalloy. The internal oxides were also observed to act as lat-eral bridges between perturbations in the oxide/metal inter-face.

(3) The amount of separated copper-rich liquid de-creases with increasing oxidation time. The amount of oc-cluded material exhibits an opposite trend. These trends areexpected when considering the decrease in iron oxidationrate due to increased internal oxidation as well as the roleof the internal oxides in promoting occlusion of enrichedmaterial. The individual contributions of the effects cannotbe separated in this study.

AcknowledgementFinancial support from the Center for Iron and Steelmak-

ing Research (CISR) and the Pennsylvania InfrastructureTechnology Alliance (PITA) is gratefully acknowledged.

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the effect of silicon on the high temperature oxidation

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