Development of New Tool Steels for Forging Dies

Transcription

1 Journal of Materials Science and Engineering B 1 (2011) Formerly part of Journal of Materials Science and Engineering, ISSN D DAVID PUBLISHING Pavel Šuchmann 1, Jiří Krejčík 2 and Ludvík Martínek 3 1. COMTES FHT a.s., Průmyslová 995, Dobřany, Czech Republic 2. SVÚM a.s., areál VÚ, Podnikatelská 565, Praha 9, Běchovice, Czech Republic 3. ŽĎAS a. s., Strojírenská 6, 1 71 Žďár nad Sázavou, Czech Republic Received: March 30, 2011 / Accepted: April 10, 2011 / Published: October 25, Abstract: Forging dies used for close dies forging of small and middle-sized steel parts are required to possess an optimum combination of strength, toughness, resistance to tempering and other properties. Typical alloying elements include chromium, vanadium, molybdenum, or tungsten. This alloying strategy can be found e.g. in the steels , and others. The present paper describes new alloying concepts of steels for forging dies based on the steel. By increasing the carbon content, and in some cases increasing the tungsten level and adding niobium, using a sufficient metallurgical procedure and subsequent special process for ingot forging, the hardness, toughness and wear resistance of the steel have been enhanced significantly. The lifetime of dies made from the improved steels is about 50%-100% longer than that of dies made from conventional steel. Key words: Tool steel, metallurgy, forging die, lifetime. 1. Introduction Manufacturers of small closed-die forgings have been for a long time greatly interested in affordable tool steels with a good strength-toughness ratio and with a precisely specified metallurgical processing procedure. Such steels are fit for rather versatile applications involving most types of forging dies. The steel which is one of the most widely used materials in forging plants across Europe is a typical representative of such tool material. However, it is often difficult to find such a supplier of this steel on the Czech market which guarantees the required chemical composition and homogeneous microstructure with very low inclusion content, uniform carbide distribution and other parameters which are decisive for end-use properties of forging dies. In recent years, companies ZDAS a.s. and in cooperation with forge VIVA ZLIN Comp., and research institutes SVÚM a.s. and COMTES FHT a.s. have been working intensively on optimizing the Corresponding author: Pavel Šuchmann, research fields: tool steels, forging, heat treatment, wear resistance. pavel.suchmann@comtesfht.cz. production of steel with the aim of achieving best possible properties while keeping competitive price. Another goal of the proposed research was to define internal technical standards for metallurgical quality of tool steels to be applied at ŽĎAS a.s. in order to update widely used but dated Poldi standards [1]. Besides the development of metallurgical processing of this steel, several chemical composition variants have been proposed for making dies with high hardness and wear-resistance requirements. 2. Manufacturing of Experimental Materials 2.1 Quality Requirements On the basis of long-term requirements of tool steel buyers, the following fundamental quality specifications for tool steel forgings have been identified: sulphur level of no more than wt%, very low content of phosphorus and other undesirable residual elements; non-metallic inclusion content (according to ASTM E45-97) should not exceed values listed in Table 1;

2 637 Table 1 Highest acceptable amount of non-metallic inclusions in the tool steel acc. to ASTM E Inclusions Type Fine Coarse A (sulphides) B (aluminates) C (silicates) D (globular oxides) prior austenite grain size of G = 8 or finer (according to ASTM E 112). In addition to these fundamental criteria, microsegregation, carbide distribution and other standard microstructure parameters were evaluated according to NADCA Method of Production Experimental ingots were processed in vacuum (VD process). One of the ingots was arc-remelted in VAR equipment prior to forging. The above quality specifications have also been met by the material produced by the VD process without remelting. As expected, the vacuum remelting improved the quality parameters (resulting in zero inclusion content). However, due to high cost of this process and unavailability of the VAR equipment in the ŽĎAS company, remelted ingots ceased to be used. In order to achieve optimum amount of forging reduction, all the above materials were deformed along three axes (involving both drawing out and upsetting) with the forging reduction of at least Modification of the Chemical Composition of Steel With regard to special parameters of the trial die used for tool steel testing (see section 5), several chemical composition variants have been proposed (Table 2), leading primarily to higher hardness and wear resistance. In the variant no. 1 of the chemical composition, an addition of niobium was used. Niobium s ability to form carbides is often used both in structural [2, 3] and tool steels [4, 5]. In the variant no. 2, carbon and vanadium levels have been increased in addition to alloying with niobium. The purpose was to increase hardness and hardenability of the material. The variant 3 included higher carbon content and additions of tungsten and vanadium in comparison with the standard composition. 4. Analysis of Properties of Experimental Material Specimens taken from experimental melts were heat treated to hardnesses of (all specimens) and and (only those with modified chemical compositions). Subsequently, impact toughness tests were carried out for longitudinal and transverse directions (relative to the axis of the bar forged from the initial ingot) and abrasive wear resistance tests. Their results are listed in Table 3, showing that modifying the chemical composition significantly improved the material s hardenability (hardness above is not achievable in the conventional steel) and wear resistance. However, all modified variants have been found to have lower impact toughness, in particular in the direction perpendicular to the axis of the forged workpiece. In order to eliminate this, the ingot forging process will be optimized further. In addition to mechanical tests, an evaluation of microstructure according to specification described in 2.1 was carried out. All experimental materials satisfied all testing criteria. Minute inclusion content Table 2 Chemical composition of tool steels investigated. Material Chemical composition (wt%) C Si Cr Mn Mo V Nb W standard Variant 1 (Nb) Variant 2 (Nb, C, V) Variant 3 (W, V, C)

3 638 Table 3 Mechanical properties of specimens made from experimental alloys (tests at room temper). Steel type Hardness Impact toughness KCU (J/cm 2 ) standard Variant 1 (Nb) Variant 2 (Nb, C, V) Variant 3 (W, C) II Resistance to wear ψ was achieved. The prior austenite grain size in all tested specimens was between G 8 and 10. An example of microstructure of standard grade produced in the ŽĎAS company with visible prior austenite grain boundaries is shown in Fig. 1. Tempering curves were obtained for the variants with modified chemical compositions. They are shown in Figs Fig. 3 Annealing curves-variant 2 (Nb, C, V) Fig. 1 Microstructure of a tool steel with visible prior austenite grain boundaries (G = 8) Fig. 2 Annealing curves-variant 1 (Nb) Fig. 4 Annealing curves-variant 3 (W, C). 5. Testing of Forging Dies The forging die shown in Fig. 5 is used in Kovárna VIVA Zlín for manufacturing small forgings (with the weight of about 1 kg). With regard to production of large series (up to 100 thousand pieces yearly over several years), the die is considered a suitable candidate for testing of new tool materials. Prior to this testing initiative, the die used to be made from standard

4 639 Its life was almost 100% longer than that of the initial die. All chemical composition variants tested have proven to be beneficial in practice and can be considered usable for manufacturing forging dies. 6. Conclusions Fig. 5 Forging die used for field tests and its worn surface steel quenched and tempered to and its life was equal to 4,500 strokes. The nature of its wear (Fig. 5, bottom) indicates that during its service, abrasive wear and local plastic deformation rather than cracking are the chief phenomena taking place in the die. For this reason, the die material was proposed to be treated to higher hardness. A series of tests was performed on dies from standard steel and from the above described modified materials conventionally treated to a hardness above 50. Forging conditions were kept stable in the course of testing. The life of dies was monitored. Results of selected tests are shown in Table 4. They indicate that higher hardness led to a significant improvement in the die life (by about 70%) even in the variants with no modification of chemical composition. Using steel with modified chemical composition and equal hardness extended the life of the die even more. The above results indicate that the development of a high-quality hot work tool steel with chemical composition based on that of was successful. The newly implemented quality standards containing requirements on both micropurity and grain size make the quality management of the steel producer more efficient. Furthermore, test pieces from steels with three different modified chemical compositions were made. The modifications led to higher hardenability and abrasive wear resistance. All investigated materials have been field-tested as forging die materials in Kovárna VIVA Zlín. Their utilization greatly extended the life of forging dies. Testing in the forging plant has also shown that in some cases the dies can be quenched and tempered to a hardness significantly higher than 50 without causing in-service cracking. With regard to favourable results of the tests, all examined variants of the steel can be regarded as usable hot work tool steels. Since 2009, the above variants have been protected by utility designs. Acknowledgments The presented results have been achieved with support of Ministry of Industry and Trade of the Czech Republic within the project TANDEM FT-TA 3/091. Table 4 Results of field tests of forging dies. Die material Relative position of workpiece axis and Hardness Life die parting plane (No. of strokes) perpendicular perpendicular parallel Variant 1 (Nb) perpendicular Variant 2 (Nb, C, V) perpendicular Variant 3 (W, V, C) perpendicular Variant 3 (W, V, C) parallel

5 640 References [1] E. Přibil, A. Engst, V. Eichler, J. Průcha, B. Esterka, J. Krejčík, Tool Steel Poldi and Their Use, I. Part Prague, [2] E.V. Pereloma, I.B. Timokhina, P.D. Hodgson, Transformation behaviour in thermomechanical processed C-Mn-Si steel with and without Nb, Materials Science and Engineering A (1999) [3] D. Hauserová, H. Jirková, B. Mašek, Investigation of phase transformations in high-strength low-alloyed steel, in: Proceedings of the 20th International DAAAM Symposium Intelligent Manufacturing and Automation: Theory, Practice and Education 20 (1) (2009) , [4] L.A. Dobrzanski, A. Zarychta, M. Ligarski, High-speed steels with addition of niobium or titanium, Journal of Materials Processing Technology 63 (1997) [5] P. Novak, D. Vojtech, J. Serak, Pulsed-plasma nitriding of a niobium-alloyed PM tool steel, Materials Science and Engineering A 393 (2005)