BULETIN ŞTIINŢIFIC, Seria C, Fascicola: Mecanică, Tribologie, Tehnologia Construcţiilor de Maşini SCIENTIFIC BULLETIN, Serie C, Fascicle: Mechanics, Tribology, Machine Manufacturing Technology, ISSN 1224-3264, Volume 2016 No.XXX

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Prediction Model of Surface Quality after Plasma Cutting of Steel S355

Miroslav Mičko1, František Botko1*, Michal Hatala1, Svetlana Radchenko1, Pavol Radič1

Abstract: Almost every technological process can we currently predict somewhat. The article presents the design and evaluation of plasma arc cutting technological process. Influence of technological factors on roughness parameters Ra of the steel surface EN ISO S355 have been evaluated using planned experiments. Using factor experiment 24, the significance of the four process factors: plasma burner feed speed, plasma gas pressure, nozzle diameter, distance between nozzle mouth and material have been observed. Regression models obtained by multiple linear regressions indicates the quality level as observed factors function. The heat from plasma arc is affected the microstructure of the material too. The heat used for cutting of material affects its microstructure changes. Keywords: steel; plasma arc; factor analysis; quality 1 INTRODUCTION

Modern industry depends on the manipulation of

heavy metal and alloys. We need metals to build the tools and transportation necessary for day-to-day business. For example, we build cranes, cars, skyscrapers, robots, and suspension bridges out of precisely formed metal components. The reason is simple. Metals are extremely strong and durable, so they're the logical choice for most things that need to be especially big, especially sturdy, or both. The funny thing is that metal's strength is also a weakness. [1]

Because metal is so good at resisting damage,¬ it's very difficult to manipulate and form into specialized pieces. So how do people precisely cut and manipulate the metals needed to build some-thing as large and as strong as an airplane wing? In most cases, the answer is the plasma cutter. It may sound like something out of a sci-fi novel, but the plasma cutter is actually a common tool that has been around since World War II.

Fig. 1.The general principle of plasma cutting.

Conceptually, a plasma cutter is extremely

simple. It gets the job done by harnessing one of the most prevalent states of matter in the visible universe. In this article, we'll cut through the mystery surrounding the

plasma cutter and see how one of the most fascinating tools has shaped the world around us. Plasma is the fourth and most highly energized state of matter: solid, liquid, gas and then plasma. In fact, plasma looks and behaves like a high-temperature gas, but with an important difference - it conducts electricity. [2]

The plasma arc results from electrically heating a gas (typically air) to a high temperature. This ionizes gas atoms and enables them to conduct electricity. A fluorescent light is an example of plasma in action. A plasma arc torch spins a gas around an electrode. The gas is heated in the chamber between the electrode and torch tip, ionizing the gas and creating plasma. [3]

2 ADVANTAGES AND DISADVANTAGES OF PLASMA CUTTING

Advantages [4]: • better cut quality when cutting thick materials

(compare to flame cutting) • cheaper than laser for cutting of thin sheets • low impact of working environment • suitable for thicker sheets with medium demand

on cut edge quality and correctness • high cutting speed (depends on material

thickness) • possibility of reaching good surface roughness

for steels and anticorrosion steels • simple replacement of throat, electrode, torch • relatively wide thickness extent of cut materials Disadvantages [4]: • noise level raises with raising of current • cannot create neither little rounds of cut edge

nor strait grooves • serious cut width • different cut quality /bad side gets to waste) • harmful steams ensuing (exhausting needed)

3 CONDITIONS OF EXPERIMENT The process parameters are caused in common

interactivity in praxis with plasma cutting. For the analyzing of this model is used factors experiment. This experiment evaluates combinations of all planning factors. As experimental material was constructional

BULETIN ŞTIINŢIFIC, Seria C, Fascicola: Mecanică, Tribologie, Tehnologia Construcţiilor de Maşini SCIENTIFIC BULLETIN, Serie C, Fascicle: Mechanics, Tribology, Machine Manufacturing Technology, ISSN 1224-3264, Volume 2016 No.XXX

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steel ISO Fe510 with 6 mm thick-ness and the plasma gas was air. We made for the experiment 16 pieces of samples.

Factors: • nozzle diameter, • plasma burner feed speed, • plasma gas pressure, • distance between nozzle mouth and material. In this example we have four factors, it is thus

tetradimensional model. By two levels of different factors, where particular levels are coded as –1 and +1 it presents full factor of 2k type. Than for four factors particular attempts will be realized in 24 = 16 different relations. Such two-leveled factor experiment will be used for simple specification of factors, that statistically significantly im-pact the variability of values of variables yRa, yRz, yδ. [5]

By the exact test that besides main factors includes also interactions off all combinations of factors and levels, there can be investigated the impact of all potentially possible combinations of factors and after elimination of statistically not-signified factors it can be moved to more detailed and accurate experimental schemes. To the coded values of –1 and +1 levels, that present the level of observed area of particular factors, there have been assigned real level values of these factors. (Table 1).

Table 1. Table of encoded factors of experiment

Factors Interval marking parameter unit -1 +1

X1 d mm 1 1,2 X2 v m.min-1 0,6 0,9 X3 p MPa 0,55 0,68 X4 z mm 6,6 7,8

Fig. 2. Encoded process parameters.

The roughness was measured in two lines, one and five millimeters from upper edge. Measurement was realized by apparatus Mitutoyo. Roughness of surface is creating by anomalies of surface with small separation, which are including anomalies from the different technology of manufacturing or another effects. We suppose that these anomalies are within defined borders, for example in border of primary length. [6]

3.1. Measurement in upper edge

Signification of bearing for monitored factors to

parameter of roughness Ra in distance one millimeter from upper edge is shoved in Pareto graph (Figure 3.).

The result from Pareto graph is the most influence for the roughness Ra factor of the plasma burner feed speed. From Pareto graph it is obvious, that highest impact to middle arithmetical deviation of surface roughness has the feed factor of plasma torch.

Significant impact to the quality of machined surface has also the pressure of plasma gas. Lower impact to the quality presents the nozzle diameter. Lowest impact is presented by distance of nozzle mouth from material.

After realization of hypothesis about importance of particular equations coefficients and elimination of not-signified factors out of linear regression, final equation was acquired, that express the middle arithmetical deviation of Ra profile in relevant measured depth h = 1 mm.

Fig. 3. Pareto graph for distance one millimeter from upper edge.

BULETIN ŞTIINŢIFIC, Seria C, Fascicola: Mecanică, Tribologie, Tehnologia Construcţiilor de Maşini SCIENTIFIC BULLETIN, Serie C, Fascicle: Mechanics, Tribology, Machine Manufacturing Technology, ISSN 1224-3264, Volume 2016 No.XXX

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Signification of bearing for monitored factors to parameter of roughness Ra in distance five millimeters from upper edge is shoved in Pareto graph (Figure 4.).

Fig. 4. Pareto graph for distance five millimeter from upper edge

From Pareto graph it is obvious, that highest impact to the middle arithmetical deviation of surface roughness has feed factor of plasma torch. Significant impact has also the pressure of plasma gas.

Minimal impact level is presented by nozzle diameter and distance between nozzle mouth and material.

Fig. 5. Relief surfaces after cutting of plasma arc, thickness 10 mm

4 HEAT AFFECTED ZONE AFTER CUTTING Investigations of microhardness showed that, the

maximum value immediately at the cut surface increases appreciably about 255 HV1 in the deposit across a distance of 0,7 mm (thickness 20 mm), about 240 HV1 in the deposit across a distance of 0,5 mm (thickness 15 mm) and about 110 HV1 in the deposit across a distance of 0,4 mm (thickness 10 mm). The microhardness is connected basically to the local changes in mechanical properties of the material.

Some differences in microhardness can be observed, which results in narrow HAZ. The microstructural damage zone (heat – affected zone) is approximately 0,7 mm deep. The heat affected zone from a plasma cut is narrower and peak hardness is higher than that produced for example by flame cutting.

Austenite formation is found to be complex while heated to a temperature 741 ºC ( in between Ac1 and Ac3 temperatures ). The result of this show continued growth of austenite.

Passing the eutectoid temperature during cooling requires a radical change. Practically all the homogeneously dissolved carbon now has to go to the inhomogeneously distributed cementite - by diffusion. The austenite is quenched, i.e. rapidly cooled. The carbon stays in place - more or less- and this necessarily prevents pearlite and ferrite formation. Instead, a new lattice type is found, called "martensite". It´s volume is getting down to the core of base material. HAZ goes through the narrow zone of normalization with fine – grained structure and considerably wider zone of partial pre – crystallization. Damaged created by a plasma torch cut – microstructure was originally a banded perlite and ferite, 3% picral etch. Original magnification 63x a 600x.

Fig. 6. Heat affected zone – zoom 63x a 600x

BULETIN ŞTIINŢIFIC, Seria C, Fascicola: Mecanică, Tribologie, Tehnologia Construcţiilor de Maşini SCIENTIFIC BULLETIN, Serie C, Fascicle: Mechanics, Tribology, Machine Manufacturing Technology, ISSN 1224-3264, Volume 2016 No.XXX

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5 CONCLUSION Paper deals with definition and evaluation of

process factors and parameters of cut surface while cutting the material EN S355J0. Methods of planned experiments are used for these evaluations. Using factor experiment, importance of four factors was observed (feed rate of plasma torch, plasma gas pressure, nozzle diameter and distance between nozzle mouth and material), that influence the parameter of roughness profile Ra and Rz.

On the base of results that were analytically processed by factor analysis it can be said, that impact of process parameters during the material cutting was different in particular depths. It was found out, that most significant impact to the machined surface roughness have factors of feed rate of plasma torch and plasma gas pressure. Among other factors that are less important belongs diameter of nozzle and distance between nozzle mouth and material. From the experimental results it can be said, that for achieving higher quality of cut surface it is recommended to use higher pressures of plasma gas and appropriate feed rate of plasma cutting head

Fig. 7. Heat affected zone – zoom 63x a 600x

ACKNOWLEDGEMENTS

This paper has been elaborated in the framework of the project of the project VEGA 1/0492/16.

REFERENCES

[1] Buschow, J.: (2001). Encyklopedia of materials: Science and technology, volume 7. Elsevier, London, 2001, ISBN 0-08-043152-6

[2] Kubica, P.: (2009). Factor Analysis of Plasma Arc Cutting Technological, Diploma Thesis, Faculty of Manufacturing Technologies, Technical University of Košice with a seat in Prešov, Prešov.

[3] Mayers, A. – Ramtech: (2002). Encyklopedia of physical science and technology, volume 12. Academic Press, California, USA, ISBN 0-12-227422-9.

[4] Čuma, M. – Zajac, J.: The impact analysis of cutting fluids aerosols on working envi-ronment and contamination of reservoirs, In: Tehni ki vjesnik. Vol. 19, no.2 (2012), p. 443 446. ISSN 1330 3651.

[5] Orlovský, I. – Zajac, J. :Mathematical model of the spray dry, In: ICMaS: proceedings of the International Conference of Manufacturing Systems: 5 - 6 November, 2009, Bu-charest, Romania: Vol. 4. Bucharest: University Politehnica, 2009 P. 275 278. ISBN 1842 3183.

[6] [6] Šomšáková, Z.; Tarasovi ová, A.; Kasina, M.: Study of materials with natural reinforce-ment and thermo plastic matrix (Wood Plastic Composite), In: Internet Journal of Engi-neering and Technology for Young Scientists. Ro . 1, . 1 (2010), s. 1 6. ISSN 1338 2365.

Authors addresses

1 Faculty of Manufacturing Technologies TUKE with a seat in Prešov, Bayerova 1,08001 Prešov, Slovakia

Contact person

* Frantisek Botko, Faculty of Manufacturing Technologies TUKE with a seat in Prešov, Bayerova 1,08001 Prešov, Slovakia

HAZ Findings • All of this HAZ measurements were

between 0,4 – 0,7 mm. • HAZ varies with speed and power. The

extent of the HAZ in low steel is related to process variables, such as cut speed and power, as well as material thickness.