Obr.3 Porovnání odolnosti proti opotřebení při srovnatelné tvrdosti - PDF

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Výroba oceli Hutnické listy č. 3/2009 Increasing of Material Durability for Drop Hot Forging Zvyšování trvanlivosti materiálu pro zápustkové kování za tepla Doc. Ing. Libor Čamek,Ph.D., VŠB-TU Ostrava,

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Výroba oceli Hutnické listy č. 3/2009 Increasing of Material Durability for Drop Hot Forging Zvyšování trvanlivosti materiálu pro zápustkové kování za tepla Doc. Ing. Libor Čamek,Ph.D., VŠB-TU Ostrava, 17. listopadu 15, Ostrava-Poruba, Ing. Ladislav Jelen,CSc., Výzkum a vývoj technické aplikace s.r.o. Pohraniční 693/31, Ostrava The paper concentrates on the research of technological - metallurgical processes of X37CrMoV5-1 (ČSN ) steel grade type produced in the medium frequency induction furnace. Ways of achieving higher material durability are determined by the modification possibilities of tool steel chemical composition, mainly by modifying carbon, molybdenum and tungsten content. Other possibilities how to increase abrasion resistance of the basic metal matrix with the given chemical composition can be found in making use of the different ways of heat processing of a forged piece by methods, increasing resistance to abrasive action of the flowing metal in the die cavity. The accepted ways of solving the durability problem must predict lower economic costs compared with the standard material of X37CrMoV5-1 steel grade type. Příspěvek je zaměřen na výzkum technologicko metalurgických postupů jakosti ocelí typu X37CrMoV5-1 (ČSN ) vyráběných na středofrekvenční indukční peci. Způsoby dosažení vyšších trvanlivostí materiálu jsou dány možnostmi modifikací chemického složení nástrojové oceli úpravou především obsahem uhlíku, molybdenu a wolframu. Další možností zvýšení odolnosti proti otěru základní kovové matrice při daném chemickém složení jsou ve využití rezervy v tepelném zpracování výkovku metodami, zvyšující odolnost proti abrazivnímu působení tekoucího kovu v dutině zápustky. Přijaté způsoby řešení trvanlivosti musí predikovat nižší ekonomickou náročnost proti standardnímu materiálu jakosti oceli typu X37CrMoV Chemical Composition The main hypothesis for the targeted modification of the chemical composition tool steel, which a forming tool (a die) is made of, is a thorough knowledge of the main mechanism of tool damage during its employment in production process. This is later decisive for what feature of the tool steel used is necessary to improve purposefully so that the demanded resistance increase is achieved in comparison to the still prevalent way of a die cavity degradation. Wear analysis on the die body designed for hot work showed that the prevailing damage had been caused by spillage of metal flowing through the most used areas of the die cavity at high speed [1]. In Fig.1 the wear damage is shown. The chemical composition modification has been carried out so that just this feature is enhanced. The empirical concept has included production of three options of X38CrMoV5-1grade modifications. The task was to find such steel chemical composition which, would finally advance the forging die material durability beyond the reached limit of this material. The benefit of tungsten alloying was increasing resistance to abrasive wear at reduced carbide-forming molybdenum content, and intensify abrasive wear resistance al together. Basic elements modification schemes are shown in Tab. 1. Obr. 1 Poškození dutiny zápustky Fig. 1 Die cavity damage Tab.1 Návrh modifikací chemického složení vybraných prvků experimentálních taveb Tab.1 Scheme of selected elements chemical composition modifications of experimental heats Element [weight %] no. 1 no. 2 no. 3 C Mo W Hutnické listy č. 3/2009 Výroba oceli Individual heats (no. 1, no. 2 and no. 3 modification) were processed in an electrical induction medium frequency furnace (EIPS) with Al based lining. The charge basis was returnable scrap from X38CrMoV5-1 steel grade type production, additional alloying was done with the help of clean additives. Each heat was cast into two pieces of K 208 type moulds [2]. The ingots produced were spheroidized in the 820 C / 4h / furnace, cooling speed 20 C/h mode. 2. Manufacture of Forgings from Heats with Modified Chemical Composition From each modified heat there was one ingot selected which was forged into round billet, D = 40 mm. The forging dimension was chosen so that forging of sixth degree was reached at minimum. The forging degree is understood to be a contractual value by which the transformation of a coarse casting structure in to a formed structure of proper characteristics is assessed. Forging degree (PK) for forgings the biggest cross section of which is equal or less than the mean cross section of an ingot is calculated by the equation (1) [3]. PK n n A.P.K = (1) Where A - upsetting equivalent, the value of which is considered ; P - degree of ingot upsetting; K - degree of ingot elongation; n - number of upsetting operations; Si K = S Where v S p P = (2) Sv S p - upsetted ingot cross-section surface; S j - ingot mean cross-section surface; S v - forging largest cross-section surface. The actual forging was performed on a double-stand pneumatic 2t forging hammer in combination with a single-stand pneumatic 1.5t forging hammer: - Ingots pre-heating up to C about 6 hours; - Dwell time on the temperature about 2 hours; - Charging over into the furnace with 900 C temperature, heating up to 1,100 C about 2 hours; - Forging of ingots on double-stand 2t forging hammer to a square about 60mm with bevels, cutting to 5 sections; - Forging of 60mm square with bevels to the final dimension D 40mm on a single-stand 1.5t forging hammer; - Gradual cooling and spheroidizing. An ultrasonic inspection followed on randomly chosen bars which were turned on a lathe for this purpose. The other bars remained unmachined, in a spheroidized state. After the ultrasonic inspection, the rough machined bars were heat treated in a mode consisting of quenching and tempering. The experimental material prepared in this way was used to manufacture specimens for mechanical properties tests and abrasive wear resistance tests. The results obtained will serve for further chemical composition optimization of X38CrMoV5-1 steel grade type ensuring higher abrasion resistance. 3. Performing Wear and Notch Toughness Tests Abrasion resistance tests and notch toughness tests were performed on samples treated to 53-HRC hardness values. Abrasion resistance samples were of D 10 x 60 mm dimensions and notch toughness was tested on samples 10 x 10 x 55 mm, with U notch with 2mm radius. Abrasive wear resistance was tested on a tribometer, whose diagram is shown in Fig.2. It is a testing device for tests of resistance to abrasive environment influence. The required forging degree size is chosen according to the sort of material, ingot weight and forging ingot structure character. During forging, casting structure is gradually deteriorated, coarse dendrites are disintegrated and elongated together with nonmetallic inclusions and liquation products in the main deformation direction. The characteristic fibrousness emerges, mainly in the ingot core part, from where it gradually expands to the surface. The fibrousness emerges in the ingot core after double to triple elongation degree, whereas in the ingot whole cross-section it appears only after tenfold elongation degree. Such new fibrousness is not eliminable anymore. It is only possible to change direction by further forging suitable, and with regard to a forged component strain. It is natural that mechanical values are considerably higher in the direction along fibres than fibres. Obr. 2 Schématický nákres přístroje s abrazivním plátnem Fig. 2 Schematic draft of the device with abrasive cloth Caption: 1 fixing plate for abrasive cloth; 2 abrasive cloth; 3 sample; 4 clamping head; 5 weight; 6 screw; 7 end switch. Testing was always carried out on 4 pcs of one status samples and individual values are average, ψ a shows abrasive wear resistance value. The results achieved including notch toughness values are shown in Fig. 3 and Fig.4. 19 Výroba oceli Hutnické listy č. 3/2009 Porovnání odolnosti proti opotřebení při srovnatelné tvrdosti 53 HRC 2 1,95 4. Economic Analysis of Ingot Steel Costs for Particular s in Relation to X37CrMoV5-1 Steel Type Odolnost proti opotřebení Ψa 1,9 1,85 1,8 1,75 1,7 1, HOTVAR Ψa The methodology of individual costing calculations is significantly diversified and it is not determined only by entry raw materials prices (the way and a calendar time of raw materials purchase) and/or overhead costs. Individual modifications production costs analysis results from 2008 second quarter demand. Obr.3 Porovnání odolnosti proti opotřebení při srovnatelné tvrdosti 53 HRC Fig. 3 Abrasive resistance comparison at comparable hardness 53 HRC Treated to 53 HRC hardness, the developed modifications of X38CrMoV5-1 type, shown in Fig.3, show distinctively higher abrasion resistance than the basic non modified X38CrMoV5-1 grade. Already the first modification of basic elements of the indicative chemical composition (Tab.1) almost achieves Hotvar grade (made by ) abrasive wear resistance and features markedly better abrasion resistance in comparison with the basic X38CrMoV5-1 grade. The highest abrasion resistance is shown by the third modification which contains the highest amount of carbide-forming elements [4]. Another very important tool steel material characteristic is notch toughness which also determines the forming die material resistance to a tool integrity failure. The decisive feature for a high quality tool steel grade is a well-balanced rate of hardness limit, indirectly determining forming tools abrasive wear resistance, and notch toughness. Substantial enhancement of one feature always diminishes the values of the other. Then, in Fig. 4 it is obvious that, in all modifications made, notch toughness value in higher than in selected standard Hotvar grade, even with treating to HRC hardness ,3 19, Hotvar mod.1 mod.2 mod.3 HRC KCU2 J/cm2 Obr. 4 Porovnání hodnot vrubové houževnatosti Fig. 4 Notch toughness values comparison 19 Měrné náklady 1 t ingotové oceli (CZK) t.č. 7 t.č. 8 t.č.9 Chemické složení oceli Obr. 5 Měrné náklady pro výrobu jednotlivých modifikací ocelí Fig. 5 Specific costs for steel particular modifications production The achieved results of abrasion resistance and notch toughness of no.1, no.2 and no.3 modifications of X37CrMoV5-1 steel grade type have indicated the optimal chemical composition of no.1 modification. No.2 and no.3 modifications have reached the highest values but, for the purpose of practical application, we can currently them as uneconomic when considering total costs. Fig.5 shows specific cost calculation values for 1t of steel production in individual modifications (marked as t. č. 7, t. č. 8 and t. č. 9) of X37CrMoV5-1 steel grade type with tungsten and molybdenum within specific costs compared to ČSN standard grade. According to initial specific cost of grade standard, the price growth of no. 1 modification heat 20 % ( t. č. 7 ), no. 2 modification 48 % ( t. č. 8 ) and no. 9 modification 66 % (t. č. 9). The review of achieved material durability values of dies applied in TANDEM FT TA 3/091 project offers a number of findings with considerably extensive variances, i.e. the number of forged pieces the tested die at values reaching a deviation up to 50% showed in Tab.2. Each evaluation is always very sensitive in relation to a tool applied in operation and to ways of obtained results assessment by an expert worker while adhering to all technological procedures principles during manufacture and others. It follows from the results obtained (Tab. 2) that the final heat treatment resulting in higher hardness values affects the die operating life where abrasive wear is the dominant mechanism. When using material marked , the number of forged pieces was the highest but a massive fracture occurred on the die. An 20 Hutnické listy č. 3/2009 Výroba oceli Tab. 2 Přehled použitých materiálu a počet vykovaných kusů Tab. 2 Review of materials applied and number of pieces forged Material Heat Producer Forging Fibre direction in a die ČSN,W.N.r Hotvar Die no. Forged pcs Hardened Who Specified Reality Date of forging 47HRC - 12/10/ HRC - 19/9/ HRC 56HRC HRC 13/9/ HRC 56HRC 56HRC 7/4/ HRC 54HRC 54HRC 29/4/ HRC 53HRC 53HRC 20/9/2008 uncontrollable failure of material integrity occurred which is undesirable on pressing tools. Regarding this point of view, Hotvar grade material creates a certain standard with the aim to achieve a comparable operating life at substantially lower cost for pressing tools manufacture. It follows from the results obtained that a trial heat manufacture for fully operational testing with no.1 modification chemical composition of X37CrMoV5-1 steel grade type. Due to should be recommended obtained good results of abrasion resistance and notch toughness values, in comparison to Hotvar steel grade, the initial chemical composition for the experimental heat in 1.7t medium frequency electrical induction furnace has been specified. The scheme of some elements chemical composition in original no.7 heat no.1 modification is shown in Tab. 3 and it is more specified in carbon content sphere, up to the level ranging between %, in order to achieve the required hardness values. The heat will be cast into V2A EX type ingot. After the follow-up processing the material will be used for forging dies manufacture with a full operational verification. 6. Results Achieved and Conclusion The conducted study has proved that hot working tool steels durability enhancement is not only a matter of metal matrix hardening with the aid of abrasive resistance increasing elements but it is also closely connected to a heat treatment mode which has the principal effect on the hardness level and on the reached notch toughness values. Their balanced ratio, when knowing the prevailing mechanism of forming tools damage, is decisive for achieving a significant prolongation of tools operating life and cost reduction. The influence of forming degree expressed by forging degree, and the way of deformation process control has been evaluated as well. Individual factors effect Tab. 3 Návrh chemického složení modifikace č. 1 tavby č. 7 Tab. 3 Chart of no.7 heat no.1 modification chemical composition Element [weight %] C Cr V Mo W was monitored during tool steel operational employment as a die for a front axle pivot manufacture. The preliminary results indicate that these effects influence the final result and they are an integral part of the whole technological process. However, the effect of chemical composition modification and heat treatment optimization has decisive importance for achieving the right abrasion resistance and notch toughness ratio. By this, they become the limiting components for forming tools operating life with abrasive wear as a dominant factor. This study was realized with financial support of TANDEM FT-TA3/091 project. Literature [1] Bilík, J., Životnosť zápustiek, (Working Life of Dies), Kovárenství, April 2006, No. 27, pp [2] ČAMEK, L., JELEN, L.: Vývoj modifikací středně legovaných ocelí s prodlouženou životností vyráběných na indukční peci (Development of medium alloyed steel modifications with prolonged operating life made in an induction furnace). XIV Międzynarodowej Konferencji Odlewników Polskich, Czeskich i Słowackich WSPÓŁPRACA 2008, Zamek w Krasiczynie k/przemyśla, Poland, April CD. [3] BLAŠČÍK, F., POLÁK, K.: Teória tvárenia (Theory of forming). Bratislava: SNTL, [4] ČAMEK, L., JELEN, L.: Metalurgicko strukturní souvislosti při zvyšování trvanlivosti materiálu pro práci za tepla (Metallurgical structural connections while increasing the durability of the material for heat work). Spolupráce 2009, Luhačovice. (Czech Republic). /in print/ Recenze: Doc. Ing. Miroslav Greger, CSc. 21
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