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)

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. Microstructurewas 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 (KC) 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.

SteelAustenizing

(quench in oil)Tempering HRC

1.23791050 ºC for 30

minutes400 ºC 2h

(x2)57.0 ± 0.5

UNIVERSAL1060 ºC for 35

minutes 540 ºC 2h

(x3)61.3 ± 0.1

WOV1250 ºC for 5

minutes550 ºC 2h

(x3)66.2 ± 0.3

Table 1.- Heat treatment and hardness of analysed steels.

Fracture toughness was measured by means of the “Indentation Microfracture” (IM) method. This method evaluates KC 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 KC. It has been shown that for Berkovich tips best results are obtained using a modified version of the expression proposed by Laugier:

lc

a a

l

a

(a) (b) (c)

lc

a a

l

alc

a a

lc

a a

l

a

(a) (b) (c)

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.

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 xV is a constant determined as 0.016.

2/3

3/22/1

c

P

H

E

l

axK VC

=

RESULTS

Steel Carbide H (GPa) E (GPa)KC (MPa m1/2) Composition

(in weight)

1.2379 Cr7C3 18.2 ± 2.4 294 ± 172.3 ± 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 ± 182.7 ± 0.5 30-50% Fe, 20-40% Cr,

10-20% V, 3-5% Mo, 2-4% W

UNIVERSAL Hard 26.1 ± 0.9 316 ± 203.7 ± 0.5 50-70% V, 8-12% Cr, 5-10%

Mo, 5-10% W, 2-6% Fe

WOV Soft 18.1 ± 1.3 318 ± 153.3 ± 0.5 50-60% W, 20-30% Fe,

3-5% Cr, 2-3% Mo

WOV Hard 24.7 ± 1.2 338 ± 162.2 ± 0.4 50-70% V, 20-50% W,

3-5% Cr,1-2% Mo

Table 2.- Mechanical properties and chemical composition of carbides.Evolution of KC as function of the ratio between V and Mo+W content of hard carbides in WOV and UNIVERSAL tool steels. High contents inV produce high KC, without decreasing H.

14 16 18 20 22 24 26 28 300

1

2

3

4

5WOV UNIVERSAL 1.2379

hard carbides hard carbides soft carbides soft carbides

KC(M

Pa

m1/

2 )

H (GPa)

Evolution of KC as function of hardness. Hard carbides ofUNIVERSAL tool steel show the highest values of KC and H.

CONCLUSIONS

Nanoindentation is a powerful technique for the characterization ofthe mechanical properties (H, E, KC) of micrometric sized carbidesof tool steels. Quantification of these parameters can be used asguideline for the designing of tool steels with microstructures that

optimize both wear behavior and fracture resistance, aimed atdeveloping 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 Cr7C3. 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 KC values are observed: one smaller than 1 MPa m1/2 (corresponding to cracks that propagate parallel to the larger edge of the carbide) and other between 1.3 and 4.5 MPa m1/2. It indicates that fracture resistance in Cr7C3 present in tool steels is highly anisotropic.

Fracture toughness measurements on 1.2379 tool steel

5 10 15 20 25 30200

250

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400

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(GP

a)

Hardness (GPa)

1,2379 steel Carbides Matrix

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 µm 10 µm

7 µm 10 µm

Young modulus as function of hardness for both types of carbides and the metallic matrix

Fracture toughness measurements on WOV tool steel.

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Hardness (GPa)

WOV steel Matrix Soft carbides Hard Carbides

UNIVERSAL TOOL STEEL

9 µm 8 µm

8 µm 10 µm

“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.

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Hardness (GPa)

UNIVERSAL steelMatrixSoft CarbidesHard Carbides