Temperature and speed influence on plastic deformation strength of high speed steel.

Catana, Dorin ; Catana, Dorina

1. INTRODUCTION

The current economic juncture requires reduced production costs, requirement that is present in cutting tools manufacture domain. At this moment mass production of cutting tools is obtained by cutting processing and the future is to obtain these cutting tools from laminated semi-products.

Because material microstructure is influencing mechanical and technological properties, being dependant of cutting tools exploitation properties, the search for new solutions has begun, considering that such material microstructure can determine superior mechanical and technological properties.

A negative aspect met in cutting tools is big consumption of expensive high alloy steels that are practically turned into chips. Because most cutting tools are made from molten semi-product, a large microstructure was noticed within cutting tools and this fact can determine an unwanted behaviour.

Tools cutting structure is not suitable for cutting; the molten ingots are processed by plastic deformation in order to finish the structure and to obtain laminated semi-products that will be used to remake other cutting tools.

Due to the addition agents within the steel used in the making of cutting tools, hot forming is required. Considering that high alloy steel deformation strength has been raised, sometimes fine crevices can appear during plastic deformation which are difficult to be seen due to their presence within the material and this fact can affect the quality of the tools.

Another technological process which begins to often use in the plastic deformation of the tool steels is extrusion. Besides the decrease of the consumption of material, extrusion offers significant decreases of the cost through the elimination of chip removal processes for the punching parts, due to the high accuracy obtained.

That is why plastic deformation parameters have to be monitored very carefully in order to eliminate as much as possible of these aspects. Mostly used plastic deformation methods are: precision forging, extrusion and isothermal forging. In figure 1 are presented steel types from database of plastic deformation simulation program.

[FIGURE 1 OMITTED]

This program will be used to design cutting tools obtained by plastic deformation. After steel type was selected if push "Preview flow stress" button, another window will be opened by displaying different values of material properties.

From all steel types, the user of this software does not find the Rp5--SR equivalent to (M2--AISI/SAE) steel and for this reason the simulation of plastic deformation can not be applied to this type of material. Based on further determined equation for specific material values, simulation process can be applied. Determined equation that shows plastic deformation resistance depending on temperature and speed was determined based on experimental values.

2. THEORETICAL CONSIDERATION

High speed steel is dedicated to achieve highly effective cutting tools for materials with high hardness value. Steel cutting tools are: drills, taps, cutters and mills. Because cutting tools have a complicate geometry, semi-finished product can be made by plastic deformation, technological processes will increase mechanical characteristics and reduce high speed steel consumption (which is expensive). High speed steel consumption is reduced because material quantity removed by cutting device will be smaller.

The Rp5 high speed steel has following chemical composition (main elements):

C--0.84 - 0.94%;

Mn--maximum 0.40%;

Si--maximum 0.45%;

Cr--3.80 - 4.50%;

Mo--4.70 - 5.20%

W--6 - 6.70%;

V--1.70 - 2%.

Technological and mechanical characteristics are: forging or rolling 1050 - 900[degrees]C, soften annealing 770 - 820[degrees]C (240 - 300 HB), quenching 1190 - 1230[degrees]C and drawing 540 - 560[degrees]C (minimum 64 HRC).

As a result of hot forming tests, the evolution of deformation strength depending on temperature was established for Rp5 steel mark.

The tests have been made at 900 - 1200[degrees]C temperature range (with an increment of 50[degrees]C) and were in compliance only for various deformation speeds (66.5 - 77 - 87.5 l/s). Average deformation speed is related with tools movement deformation speed and semi-finished product height (Adrian & Badea, 1983):

[epsilon] = [v.sub.i]/[h.sub.0] [??] [v.sub.m] = [square root of gH/2 x [h.sup.2.sub.0]] [1/s] (1)

where:

[v.sub.m]--average speed deformation;

H--free fall height;

[h.sub.0]--semi-finished height.

For each deformation speed was established the equation that allowed to determine strength deformation equation depending on temperature. These equations are presented below (Catana, 2005):

[R.sub.d] (t) = a x [e.sub.b-t], (2)

where:

[R.sub.d]--plastic deformation strength;

a, b--equation coefficients;

t--deformation temperature.

In order to have a complete image of deformation strength evolution, a larger range of deformation speed was selected for hot forming area.

Using previous determined equations it is possible to establish the equation capable to determine deformation strength depending on two parameters: temperature and deformation speed.

To determine equation coefficients, first we have to set its shape. The basis of measurement values consists in [R.sub.d](t) graphic at different deformation speeds, followed by amorphous materials (using appropriate scale) that have been brought up to a correct shape (Stetiu & Oprean, 1988).

Considering equation (2), the result is an exponential equation of two variables (Catana, 2007). The equation is presented below:

[R.sub.d](v,t) = a * [e.sup.bt+cv], (3)

where:

[R.sub.d]--plastic deformation strength;

t--deformation temperature;

v--relative deformation speed;

a, b, c--equation coefficients.

Using equation coefficients established to constant relative deformation speed, it is possible to calculate equation coefficients (3). Following the calculus, the equation of deformation strength depending on temperature and relative deformation speed has next form:

[R.sub.d] (v, t) = A * eBt+Cv (4)

Using the [PROGR.sub.d] software in Visual Basic it was determined deformation strength for different temperature and speed values (Eftimie et al., 1998).

[FIGURE 2 OMITTED]

Because relative deformation speed has values between 66.5 and 87.5 1/s this means that is possible to use equipments such as mechanical press or hammer.

3. CONCLUSION

Making cutting tools using plastic deformation started to be used widely. This development was possible due to following advantages:

--up to 60 % high steel alloy saving;

--chip removal is no longer required;

--better microstructure compared with traditional technologies

--cutting tools hardness is increased;

--cutting tools mechanical features are improved.

Determination of deformation strength, for a large range of relative deformation speeds and for an adequate temperature range, allowed an easy selection of deformation parameters (figure 2). With determined values for deformation strength it is possible to use simulation software for plastic deformation of semi-finished product.

The optimization of deformation process, besides material saving, will lead to a homogeneous and fine microstructure which will increase mechanical and technological properties, for the next cutting tools generation.

4. REFERENCES

Adrian, M. & Badea, S. (1983). Plastic deformation process fundamentals, Technique Publishing, Bucharest

Catana, D. (2005). Theoretic contributions for the plastic deformation simulation process, Proceedings of International Scientific Conference "Modern Technologies, Quality, Restructuring" TMCR 2005, pp. 333-337, ISBN 9975-9875-4-5, Technical University of Moldova, 05-2005, University of Moldova, Chisinau

Catana, D. (2007). Dependence between deformation speeds and high alloy steel plastic deformation strength, Bulletin of the Polytechnic Institute of Jassy, Vol. LIII, No. 4, 05.2007, pp. 63-66, ISSN 1453-1690

Eftimie, L.; Dinescu, I. & Catana, D. (1998). Materials Engineering, Lux Libris Publishing, ISBN 973-9240-55-0, Brasov

Stetiu, C. & Oprean, C. (1988). Geometrical measurements in machines building, Didactics-Pedagogic Publishing, Bucharest