Foundry technologies and materials for cast cutting tools.

Caus, Alexander ; Beznak, Matej ; Caplovic, Lubomir 等

1. INTRODUCTION

Cutting tools that are made from wrought HSS are shaped to the required geometries by machining. When machining a great amount of tool material is removed in the form of chips to the wastes and the production time is long. Production of cutting tools by foundry technologies results in dramatic improvements in material saving and production efficiency (Gelin & Chaus, 2007). Since the application of precision casting of cutting tools allows casting near-net-shape work-pieces of tools this reduces the volume of roughing operation and large depth of cut, and consequently, the material consumption. Table 1 and 2 show the material consumption and processing time for the cutting tools produced from wrought HSS bars using machining, and by casting (Caus, 2008).

Foundry technology provides more effective tool material utilisation because multiple using of a tool scrape is possible when melting charge as well as the use of both wrought wastes and chip briquettes of HSS. It is necessary to emphasise that utilisation of high alloy powder wastes produced upon grinding of hard metals is also possible at melting.

On the other hand, absence of special casting alloys for cutting tool production inhibits broad application of chipper cast tools in industry. As a rule, HSS of conventional chemical composition are usually used as a material for cast cutting tool but their casting and mechanical properties, primarily impact toughness do not suit perfectly. Therefore, in order to exhibit good all-round performance the impact toughness enhancement of as-cast HSS is obligatorily needed.

2. FOUNDRY TECHNOLOGIES FOR CAST CUTTING TOOLS PRODUCTION

To date different foundry technologies can be used for metal cutting tools production. Among them the most attractive technology is casting in metal moulds. The reason is that due to high cooling rate of the melt upon solidification in a metal or graphite mould the high density of castings as well as the structure refinement can be achieved (Fig. 1) that leads to enhanced mechanical properties of HSS. In addition the casting into metal mould offers other advantages, which are as follows:

--Use of sand mixtures is not obligatory that simplify the very foundry technology and accelerates production cycle;

--Investments into both technology and equipment is low;

--Implementation of technology is quick and easy;

--Technology provides high stability and accuracy of both dimension and shape of castings;

--High surface quality allows reducing the allowance for machining;

--Technology improves hygienic and ecological conditions of casting process.

The main advantage of lost-wax casting is extremely high quality of casting surface, which allows producing near-net-shape work-pieces practically without allowance for rough machining (Fig. 2) that benefits to improvements in material saving and production efficiency.

Electro slag remelting (ESR) is also attractive for production of cast tools primarily with large cross sections. Dropping mechanism of remelting of consumable electrode, at which the material in small portions (drops) is conveyed from the consumable electrode through a slag pool to forming ingot, provides the high density and lower level of structural heterogeneity in HSS (Chaus et al., 1997) even though the diameter of ingot is larger than 100 mm. Beside this, ESR uses highly reactive slags thus reduces the amount of sulphides and other types of inclusions present in HSS.

[FIGURE 1 OMITTED]

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3. MODIFIED AS-CAST HIGH-SPEED STEELS

Different methods of structure and properties improvement of cast HSS can be commercially used. In terms of efficiency and the production cost of all methods used for as-cast HSS quality improvement, it appears that modification of the melt is more simple and effective one. However, the effect of modifying additions in HSS has been studied insufficiently. As a consequence, a restricted number of additions are used for as-cast HSS compared to common cast alloys.

In order to evaluate the surface activity of the elements in molten iron the calculations, using such criteria as melting temperature, surface energy, specific heat of sublimation, entropy in the standard condition, statistic generalized moment, total electron potential barrier for iron and addition have been carried out. According to the received activity series built up from the derived values on surface activities, Bi leads the list of the most surface-active elements followed by Ca, Sr, Sn, Sb, Cd, Mg, and so on. It was shown that Ti, Zr, Hf, Nb, Ta and B belong to so-called inoculating additions (Chaus, 2005).

Experimental verification of effects of different additions in as-cast M2 and T30 type HSS have been carried out. Calculated amounts of additions were as follows, mass%: for Zr and Bi 0.05-0.1-0.3-0.6; Nb, Ti, Ge, Ni, SiMM, Y, Cd 0.1-0.3-0.6; B 0.05-0.1-0.3-0.6-1.0; FeCe 0.1-0.3-0.6-1.2; Si and Al 0.4-0.81.2. Relationships between the structural parameters and mechanical properties have been established (Chaus & Rudnickii, 1989). The comparison of the structure of non-modified and modified as-cast HSS is shown in Fig. 3. On the basis of the received results a comprehensive range of new-patented as-cast HSS with the wide choice of improved structures and properties for cast cutting tool applications have been designed (Gelin & Chaus, 2007).

4. SPECIAL AS-CAST HSS

After casting the structure of the HSS of 11M5F type contains small volume fraction of the eutectic carbides (Fig. 4), which total amount does not exceed 1-5% (Chaus, 1998). As a consequence, the impact toughness of this steel is more than a factor of 1.8 higher than that of M2 HSS. Another feature of the 11M5F steel is the presence of carbides with the secondary origin, which dissolute at lower austenitising temperature and thus provide higher saturation of a solid solution with alloying elements. As a consequence, the 11M5F steel has hardness and a heat resistance no worse than those of the M2 steel.

For carburised cast cutting tools a new-patented M2-based HSS with ferritic matrix has been developed (Chaus & Latyshev, 1999). Composition of the steel is as follows, wt. %: 0.80-0.88 C; 3.8-4.4 Cr; 4.9-6.5 W; 4.5-5.5 Mo; 2.6-3.8 V; 0.4-1.8 Ti; 0.8-2.4 Nb. Varying the chemical composition of the steel it is possible to achieve different level of carbide dissolution during austenitising and, as consequent, different level of carbide heterogeneity in the diffusion layer after heat treatment (Fig. 4). The mechanical properties of the carburised HSS after heat treatment were as follows (Chaus et al., 2001): impact toughness 20 J/[cm.sup.2], hardness 68-69 HRC, heat resistance 675[degrees]C and wear resistance (rate of wear) 36.4 mg/h which is higher than that of the M2 HSS, 76.2 mg/h.

[FIGURE 3 OMITTED]

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The results of durability tests performed in turning proved excellent durability of the carburised cast cutting tool in comparison with conventional one (Chaus & Latyshev, 1999).

5. CONCLUSION

Modification of as-cast HSS with inoculants and surface-active elements provides favourable structural changes that benefits in improved mechanical properties of as-cast HSS and finally in enhanced cutting performance of these steels. Employment of HSS with the chemical composition specially designed for cast tools is also very effective from viewpoint of the impact toughness and durability of cast tools.

6. ACKNOWLEDGMENT

The financial support of grants from the Ministry of Education of the Slovak Republic VEGA 1/4109/07 and VEGA 1/3191/06 is gratefully acknowledged.

7. REFERENCES

Caus, A. S. (2008). Advanced Materials and Technologies of Production of Cast Cutting Tools. AlumniPress, ISBN 978-80-8096-060-5, Trnava

Chaus, A. S. (2005). Application of Bismuth for Solidification Structure Refinement and Properties Enhancement in As-cast High-Speed Steels. ISIJ International. Vol. 45, No. 9, (2005) 1297-1306, ISSN 0915-1559

Chaus, A. S. (2001). Heat Treatment of As-cast Carburised High-Speed Steel. Metal Science and Heat Treatment. Vol. 43, Nos. 5-6, (2001) 220-223, ISSN 0026-0673

Chaus, A. S. & Latyshev, I. V. (1999). Effect of V, Ti and Nb on the structure and properties of cast tungstenmolybdenum HSS. Physics of Metals and Metallography. Vol. 88, Iss. 5, (1999) 152-156, ISSN 0031-918X

Chaus, A. S. (1998). On the Prospects of the Use of Low-Alloy Tungsten-Free HSS 11M5F for Cast Tools. Metal Science and Heat Treatment. Vol. 40, Nos. 7-8, (1998) 319-325, ISSN 0026-0673

Chaus, A. S. et. al. (1997). Structural Inheritance and Special Features of Fracture of HSS. Metal Science and Heat Treatment. Vol. 39, Nos. 1-2, (1997) 53-56, ISSN 00260673

Chaus, A. S. & Rudnickii, F. I. (1989). Effect of Modification on the Structure and Properties of Cast W-Mo HSS. Metal Science and Heat Treatment. Vol. 31, Nos. 1-2, (1989) 121-128, ISSN 0026-0673

Gelin, F. D. &Chaus, A. S. (2007). Metallic materials, Vyshejshaja shkola, ISBN 978-985-06-1362-2, Minsk

Tab 1. Material consumption upon production of tools from
wrought HSS bars by cutting, and by casting.

Tool Mass of work- Saving of HSS
 piece [kg]

 Wrought Cast [kg] [%]

Slotting tool 2.153 0.750 1.403 65.1
Set of inserted blades 2.748 1.682 1.066 65.3
Side milling cutters 1.040 0.440 0.600 57.7
Shell mill 1.360 0.410 0.950 68.7

Tab 2. Processing time upon production of tools
from wrought HSS bars by cutting, and by casting

 Processing
 time [min]
 upon production Time
Tool of tool by: saving [%]

 Casting Machining [%]

Slotting tool 393.4 360.7 8.3
Set of inserted blades 47.2 38.07 19.3
Side milling cutters 135.0 90.1 33.9
Shell mill 105. 0 67.45 35.7