THE EFFECT OF VANADIUM CONTENT ON MECHANICAL PROPERTIES AND STRUCTURE OF SELF-TEMPERED STEEL X160CrMo12-1
mr. Aleksandar TODIĆ1, dr. Dejan ČIKARA1, dr. Tomislav TODIĆ1, dr. Branko PEJOVIĆ1, dr. Bogdan ĆIRKOVIĆ1 mr. Ivica ČAMAGIĆ1
1Faculty of Technical Sciences, University of Priština, Kosovska Mitrovica, street Kneza Miloša no. 7, 38220 Kosovska Mitrovica Serbia
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Resume:The objective of this research was to examine the influence of vanadium on structure, hardness and strength of self-tempered steels. The tests wereperformedonsteeltype X160CrMo12-1. It is obvious that vanadium affects on the process of solidification of these alloys on such a way that narrows temperature interval of crystallization whereby from melt form V6C5 carbides which blocking further growth of austenite dentrite and thus helps obtaining of small-grained structure. Vanadium as an alloying element moves liquidus and solidus to higher temperatures, forms V6C5 carbides, partially is distributed between phases present in steel, carbide (Cr,Fe)7C3 and austenite. The existenceof vanadium allows forming (Cr, Fe)23C6 carbides and his deposition intoausteniteduringcooling process, in localareasaroundfinecarbideparticlestransformintomartensite. This means that vanadium reduces the amount of retained austenite and thereby improves the hardenability of steel.

Key words: vanadium, impact toughness, hardness, microstructure

1Name of the author, title, company, address and e-mail.

2Name of the author, title, company, address and e-mail.

  1. INTRODUCTION

Self-hardeningsteels belong to the group of wear resistant steels which, makes them usable in a wide area of application.The basic characteristic of these steels is high hardness, and strength, due to its high carbon content, and relatively small impact toughness.Research carried out on one type of these steels was aimed to investigate the effect of some alloying elements on mechanical properties of materials (strength). For our investigations we used high-alloyed, high carbon steel in which chromium, molybdenum and vanadium were the alloying elements. Research has aimed toimprove thecharacteristics ofthese steels, through increased resistanceto abrasiveandimpact-fatigue wear. The main aim of this research was to obtainappropriate structure of the metallic matrix of steel and increase the tensile strength.Such a compromise can be achieved by appropriate heat treatment, to obtain a martensitic structure with lower content of retained austenite.

  1. EFFECT OF CHROMIUM AND MOLYBDENUM ON THE STRUCTURE AND PROPERTIES OF STEEL

Chromium is an essential alloying element in steel with increased hardness and wear resistance. Chromiumis a carbide maker element thatreactswithcarbonandformshard, wear resistantcarbides. Besides that, chromium preventsthe transformation ofausteniteintopearliteduringcooling, and affectsthe structure ofthe metallic matrix of steel,closingγ-areain thephasediagram. The beststructure, regardinga combination oftoughnessandhardness, is the structure with carbides (Cr, Fe)7C3, which are formed in steel that contains more than 8% of chromium. Chromiumdoes notincrease hardenability but,incombinationwithhighercarbon content, has a beneficial effect on depth of hardened layer. The important parameter for hardening is the ratio Cr / C and, the higher the ratio, the higher the hardenability. Molybdenumpreventsthe formation ofpearlite, and the transformation of austenite moves in bainitic and martensitic area. Forthis reason, molybdenum insmall contents increases hardenability. In thiswayheprovidesobtaininghard andsolid martensiticsteel matrixthat holdsalloycarbides. Molybdenumbuildsinterstitialphase Mo2C having a hardnessapproximately1800HV, and the increased content of molybdenum forms certain amount of this phase in the structure of steel.

  1. INFLUENCE OF VANADIUM

By addingvanadium to the high alloychromiumsteels, structurebecomesfiner. Structure refining by adding of vanadium is explained by theinfluence ofvanadiumon thecrystallization process. In addition, vanadium changes the morphologyof proeutectic (Cr, Fe)7C3carbides. Withincreasing of the vanadium content, the radialdistribution ofcarbidesbecomesdominant, but the shareof longorientedlamellasandplates does not decrease [2]. Presence ofvanadiuminsmallpercentagehas a positiveeffectonhigh-alloy Cr-Mo steels. Itaffectsthe process ofsolidificationof thesealloysby narrowing of the temperatureinterval ofcrystallization. Besides that, during growth of primaryaustenitefromthe melt, V6C5carbides areformed in the steel structure. They blockfurthergrowth of the austenitedendritesandsohelp toobtain the fine grainstructure.In highchromiumsteelswith the content of 12% Cr, 1.4−2.0% C andover2.5% vanadium, the vanadiumcarbide, VCtype,with aBCClatticeis formed [7]. VC-carbides have theglobularshapeandare very oftenassociatedwitheutecticM7C3carbides. VC carbides can also appear in the form of rods, which grow radial from the nucleus, to form spherical eutectic cells together with the austenite grains. Higher content of vanadium enhances the formation of (Cr, Fe)23C6 carbides and their precipitation in the austenite grains, during the cooling process. Austenite, in local areas, around these fine carbide particles, transformed into martensite. In other words, vanadium reduces the amount of retained austenite and thus improves the hardenability of steel [3] [4] [5].

  1. DESCRIPTION OF EXPERIMENT

Researchare basedon self-hardening steel, with the chemicalcomposition of12.5% Cr, 1.2% Mo, while the content of carbonchangein therange1.4% to2.2%, andvanadiumin therangefrom0.5% to3%.

Test sampleswerecast in theform of standard test samplesfortesting the hardnessand tensilestrength. Melting of steel,is carried outin themiddle frequency,inductionfurnace ABB type ITMK-500. We used wooden patterns for mold making, and moldsforcastingare made by standard CO2 and Shall Molding process. After cleaning, the castingswereheat treated by hardening with subsequent tempering at a temperature of250oC and400oC. For all samples period of tempering was 1hour. Thistype ofheattreatmentis characteristic for high-alloy Cr-Mo steels. Foreach single carbon content vanadiumcontent is changed in the range from0.5% to3%. In this work, representative samples with carbon content of 1.6% were considered. Their chemical compositions are shown in the Table 1.

Table 1. Chemicalcompositionof steel samples

Number / Group of the samples / Chemical composition
C (%) / Cr (%) / Mo (%) / S (%) / V (%)
1 / I / 1,542 / 11,831 / 1,115 / 0,03 / 0,554
2 / II / 1,536 / 11,562 / 1,111 / 0,03 / 1,053
3 / III / 1,521 / 11,311 / 1,094 / 0,032 / 1,976
4 / IV / 1,624 / 10,076 / 1,062 / 0,026 / 2,992

Surfaceof castandheattreatedsamples wasrough, andfor this reason they were machined to the standarddimensions. Processing of samplesintendedforhardness testingwas carried outonmechanicalgrinderfor flat grinding. In order to eliminate anychange in the microstructureof the samples, they are cooled by emulsion during grinding.Samples for tensile test are processed with ceramic tiles for processing of the hard metal surface, type SANDVIK CNGA, on universal lathe, and also with emulsion cooling.Dimensions of specimens for tensile tests are made ​​according to the standard SRPS ISO 10002-1 (EU 18:1979). The tests were performed on the universal testing machine with maximum force of 200 KN.Hardness testing was performed on samples, dimensions 10x10x50, by a Rockwell-C method on the Otto Wolpert-Werke hardness testing machine.Testing of microstructureof steel sampleswascarried outwith the light microscope Olympus GX41, equipped with adigitalcameraandsoftwareforimage processing.

  1. RESEARCH RESULTS

The research resultsof heat treated samples, tempered at a temperature of250oCand400oC, withdifferentcontents ofvanadiumare shownin Table 2.

5.1.Influence ofvanadiumon hardnessandtensilestrength

Measurement of hardnesswas carried out insixpoints in thesample, and average value is taken as authoritative. From the presented data it is evident that, with increasing of the vanadium content in the alloy, hardness decreases respectively. However, the reductionislawand thehardnessremainsat a relativelyhighlevel. This decreaseof hardnessclearlyshowstheincrease inimpact toughness.Tensile testwasperformedonthreesamplesfrom each batch, and average value is taken as authoritative. Table2 showstheresults oftensilestrengthand hardness. For the illustration, characteristic diagram of tensilestrength alternation in elongation is presented on the Figure 1.Analysisof the datashowsthatvanadiumdoes notaffectsignificantlyontensile strengthanditremainsapproximatelyconstant. In general, it could be concluded that vanadium, in that content of carbon, has no significant effect on tensile strength.

5.2.Influence ofvanadiumonstructure

The samplesareheattreated by hardening and subsequent lowtemperature tempering. To complete the austenitisation of the structure, the samples are previously homogenized by the annealing on 1000°C. Hardening (quenching) was performed by cooling the cold air stream, and the rate of cooling was greater than critical.On the Figures 2 and 3 are shown the microstructure of samples after heat treatment.

Table 2. Hardnessand tensilestrengthof the samples

Chemicalcomposition / Hardness [HRC] / Tensile strength
[N/mm2]
Content
C [%] / Content of
V [%] / Tempered
at 250oC / Tempered
at 400oC / Tempered
at 250oC / Tempered
at 400oC
1,6 / 0,5 / 56,5 / 56,0 / 528,2 / 533,3
1,0 / 56,0 / 55,4 / 631,2 / 587,0
2,0 / 56,2 / 54,8 / 676,7 / 673,6
3,0 / 55,5 / 53,6 / 645,1 / 641,5
Picture 1. Diagram offorce change and elongation
Picture 2. Microstructure ofalloys, group I / Picture 3. Microstructure ofalloys, group II

In general, the structures of the samples consists of martensitic metal matrix in which are clearly visible islandsof retained austeniteanddispersedcarbides, type (Cr,Fe)7C3, deployed mainly as a network on the boundaries of metal grains. Carbidephase,formed duringsolidification ofeutectic,has a very oriented growth. The proeutecticaustenitehas been transformedintomartensite

and it can be seenasa darkfieldbetweencarbideneedles.During quenching process, primary austenite crystals and austenite from eutectic are transformed into martensite. Inthe process of tempering, oversaturated martensitesolid solutionhas been transformedintocubicmartensite, andcarbideshave remaineddeployedin the form of networkupon the grain boundariesof metalmatrix. In the Figures2and3 are shownthemicrostructure oftested samples.The pictures showsthatwithincreasing of the vanadium contentup to2%, the distribution of carbidesremains the same, but their size and stereologic shape are changed. Namely, the vanadium affects on the refining of structure componentsboth, the metal matrix and the carbide network. The increasing of vanadium content in the alloy, also reduces the dispersion of carbide particles, and increases the number of finer carbides in the structure, shown on Figures 2 and 3. Vanadiumisa typicalcarbide-maker so that built carbide V6C5with the present carbon, whosecontentin thestructuregrowswith increaseof vanadium contentin thealloy.In alloys that contain 3% V, during cooling of austenite is partly transformed into beneit, whichremainsunchangedduring low-temperature tempering, neither in form nor in size. Afterheattreatment, instructurearepresentsmallamounts ofcubicmartensiteandretained austenite. Martensiteisdistributedmainlyalongthe grain boundaryof theeutecticcarbide. Distribution of carbides is changed, carbide network is no more clearly expressed, and besides carbide V6C5, in structure appears very hard carbide VC in a significant percentage.Changes in the volume fraction, size and morphology of the phases present in the microstructure of Fe-C-Cr-V alloy, indicate that with the increase of vanadium content, alloy composition approaches the eutectic composition in four-component Fe-C-Cr-V system, causing a decrease of temperature interval of solidification.

  1. POSSIBILITY OF APPLICATION

Adding3%vanadiuminsteels, quality X160CrMo12-1, forms the alloythathaveverygoodcombination ofhardnessand tensilestrengthand therebymayhavewidearea of application. Therefore,thissteelwith addition of3% V canbesuccessfullyusedfor makingpartsand componentsthatareexposed toabrasion, corrosion-abrasion, and impact-fatigue wear or combinedtype ofwear. Assortment of thesepartsare: constructionandminingmachinery parts(excavators teethandteeth covers), parts of grindersand mills for stone, ore, coalandminerals (balls, hammers, impact plates, millliningsandseparationgrids), wear resistant partsinprocess plants(mill rodsforabrasivematerials, blades ofwheel-abrators, mud pumps bodies , moldsfor coalandsteel scrapbriquetting, tanks caterpillars etc.) [1] [6].

  1. CONCLUSION

In thiswork were considered the effect ofvanadiumonhardness, tensile strengthandmicrostructure ofsteelwith1.6% of carbon,12%chromiumand1.3% molybdenum. With increasingvanadium content,structurebecomesfiner, which affects on themechanicalproperties ofsteel, i.e. on thehardnessand tensile strengthas well ason theimpact toughness. In the testedsamples content ofvanadium was increased; in first series 0.5% V, was added, in second 1%V, in third 2%V and in fourth 3%V. It is evident that higher vanadium content of 3% does not leadtoimprovedproperties ofthissteel, andfor thisreason,researchwith a greaterpercentage ofvanadiumare notincludedin thiswork.Discussion of the resultsgivenin paragraphs5.1and5.2indicatesthatan increased amount of vanadium has positive influence oncharacteristics ofsteelanditsmicrostructure. Testing of samples group, with 0.5% vanadium have shown great hardness.With the increase of vanadium content up to 2.0%, the hardness slightly decreases, andtensilestrengthremainsapproximatelyconstant. The presence of hard carbides type (Cr,Fe)7C3, V6C5 and VC, their content, favourable distribution and morphology, provides good abrasive wear resistance, even in cases when they are in contact with extremely abrasive materials such as silica, feldspar and others.

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