Poster international tooling conference torino 2006
INTRODUCTION Recently, the automotive industry has been incorporated a new generation of steels, named as high strength steels , in some lightweight structural components. However, the forming operations of such steels require high pressure, that produce the accelerated wear of forming tools, even leading to their catastrophic and premature failure. As a consequence, the hardness-toughness relationship in cold work steels must be optimised through an accurate microstructural design . In this sense, the mechanical properties of carbides, hardness and fracture toughness, play an important role in the mechanical response of these steels (wear and fracture resistance). However there is a lack of knowledge in the mechanical behavior of carbides in tool steels, mainly due to the experimental difficulties associated with their measure. Nevertheless, in this work it is shown that nanoindentation is a suitable technique for the evaluation of the mechanical properties of micrometric-sized carbides. Hardness, elastic modulus and fracture toughness of primary carbides, exhibiting different chemical composition, of three commercial cold work tool steels have been evaluated by means of the nanoindentation technique. Results are discussed in terms of carbide morphology and chemical composition. EXPERIMENTAL The study is focused on the primary carbides of three commercial tool steels : a conventional high chromium-high carbon tool steel, with denomination 1.2379 or AISI SAE D2, and two steels developed by ROVALMA S.A., named WOV and UNIVERSAL . Table 1 details the heat treatment schedule along with the obtained hardness, in Rockwell C scale (HRC), for each steel. Microstructure was evaluated by means of scanning electron microscopy (SEM). The chemical composition of carbides was roughly evaluated by Energy Dispersive X-Ray Microanalysis (EDAX). Hardness (H), elastic modulus (E) and fracture toughness (K C ) of carbides were measured by means of nanoindentation using a NanoIndenter XP (MTS) system fitted with a Berkovich diamond tip using the Continuous Stiffness Measurement (CSM) operation mode. Indentations were carried out a maximum load of 400 mN. Table 1 .- Heat treatment and hardness of analysed steels. Fracture toughness was measured by means of the “ Indentation Microfracture ” (IM) method. This method evaluates K C from the length of cracks emanating from the indenter impression corners. The cracks were generated using a Berkovich tip at indentation load range from 200 to 500 mN. Figure 1 shows the most common type of cracks encountered in brittle materials: (a) radial cracks, and (b) Palmqvist type cracks which are commonly generated when a Berkovich indenter is used. Several expressions are available to determine K C . It has been shown that for Berkovich tips best results are obtained using a modified version of the expression proposed by Laugier : P is the indentation load, E the Young modulus, H the hardness, a is the half-diagonal of the indentation impression, l is the crack length, c = a+l , and x V is a constant determined as 0.016. RESULTS Table 2 .- Mechanical properties and chemical composition of carbides. MICROMECHANICAL CHARACTERIZATION OF COLD WORK TOOL STEELS D. Casellas*, J. Caro *, J. M. Prado*, I. Valls** *CTM Technological Centre. Av. Bases de Manresa, 1. E-08242 MANRESA (SPAIN) ** ROVALMA S.A . Apol·lo 51, P. I. Can Parellada. E-08228 TERRASSA (SPAIN) Steel Austenizing (quench in oil) Tempering HRC 1.2379 1050 ºC for 30 minutes 400 ºC 2h (x2) 57.0 0.5 UNIVERSAL 1060 ºC for 35 minutes 540 ºC 2h (x3) 61.3 0.1 WOV 1250 ºC for 5 minutes 550 ºC 2h (x3) 66.2 0.3 Figure 1 .- (a) Radial cracks emanating from a Vickers indenter impression, (b) Palmqvist cracks emanating from a Vickers indenter impression, (c) cracks emanating from a Berkovich indenter impression. Steel Carbide H (GPa) E (GPa) K C (MPa m 1/2 ) Composition (in weight) 1.2379 Cr 7 C 3 18.2 2.4 294 17 2.3 0.8 (perpendicular to the larger edge) < 1.0 (parallel to the larger edge) 40-45% Fe, 45-50% Cr, 5-7% V, 2-3% Mo UNIVERSAL Soft 20.3 1.2 272 18 2.7 0.5 30-50% Fe, 20-40% Cr, 10-20% V, 3-5% Mo, 2-4% W UNIVERSAL Hard 26.1 0.9 316 20 3.7 0.5 50-70% V, 8-12% Cr, 5-10% Mo, 5-10% W, 2-6% Fe WOV Soft 18.1 1.3 318 15 3.3 0.5 50-60% W, 20-30% Fe, 3-5% Cr, 2-3% Mo WOV Hard 24.7 1.2 338 16 2.2 0.4 50-70% V, 20-50% W, 3-5% Cr,1-2% Mo Evolution of K C as function of the ratio between V and Mo+W content of hard carbides in WOV and UNIVERSAL tool steels. High contents in V produce high K C , without decreasing H. Evolution of K C as function of hardness. Hard carbides of UNIVERSAL tool steel show the highest values of K C and H. CONCLUSIONS Nanoindentation is a powerful technique for the characterization of the mechanical properties (H, E, K C ) of micrometric sized carbides of tool steels. Quantification of these parameters can be used as guideline for the designing of tool steels with microstructures that optimize both wear behavior and fracture resistance, aimed at developing high performance tool steels. 1.2379 TOOL STEEL Young modulus as function of hardness of primary carbides and the metallic matrix In 1.2379 tool steel there is only one type on primary carbide, which have been previously identified as Cr 7 C 3 . Fracture toughness measurements show a marked dependence on the carbide orientation. Primary carbides are elongated as a consequence of the forging steps during fabrication. Results show that cracks propagate preferentially in the direction parallel to the larger edge of the carbide. Two sets of K C values are observed: one smaller than 1 MPa m 1/2 (corresponding to cracks that propagate parallel to the larger edge of the carbide) and other between 1.3 and 4.5 MPa m 1/2 . It indicates that fracture resistance in Cr 7 C 3 present in tool steels is highly anisotropic . Fracture toughness measurements on 1.2379 tool steel This commercial tool steel is formed by two types of primary carbides, named as hard and soft carbides, which show different morphologies and chemical compositions. Table 2 summarizes the mechanical properties and chemical compositions of these carbides. Hard carbides show an irregular and rounded morphology, whereas soft carbides show a flatten and polygonal morphology . Chemical composition obtained from EDAX analyses reveal the presence of V, Cr, Fe, Mo and W for both types of carbides. Hard carbides show a rich vanadium composition, whereas soft carbides have a high content of tungsten. WOV TOOL STEEL “ SOFT” CARBIDES “ HARD” CARBIDES 7 m 10 m 7 m 10 m Young modulus as function of hardness for both types of carbides and the metallic matrix Fracture toughness measurements on WOV tool steel. UNIVERSAL TOOL STEEL 9 m 8 m 8 m 10 m “ SOFT” CARBIDES “ HARD” CARBIDES The UNIVERSAL tool steel is formed by two types of primary carbides, named as hard and soft carbides (see Table 2). In this case, hard carbides show a high vanadium content, whereas soft carbides are mainly composed of Fe and Cr. Young modulus as function of hardness for both types of carbides and the metallic matrix Fracture toughness measurements on UNIVERSAL tool steel.