Castings Dimensions Influence on the Alloyed Layer Thickness

The paper presents the results of simulation of alloy layer formation process on the model casting. The first aim of this study was to determine the influence of the location of the heat center on alloy layer’s thickness with the use of computer simulation. The second aim of this study was to predict the thickness of the layer. For changes of technological parameters, the distribution of temperature in the model casting and temperature changes in the characteristic points of the casting were found for established changes of technological parameters. Numerical calculations were performed using programs NovaFlow&Solid. The process of obtaining the alloy layer with good quality and proper thickness depends on: pouring temperature, time of premould hold at the temperature above 1300 o C. The obtained results of simulation were loaded to authorial program Preforma 1.1 in order to determine the predicted thickness of the alloy casting.


Introduction
Nowadays, layer steel casting have become the most interesting subject because of great industry demand for the parts of machines resistant to abrasive wear [1][2][3][4][5][6][7][8]. The steel casts need to be subject of heat treatment or chemical constitution modification in order to gain high resistance to abrasive wear [9]. It is not economical. The foundry technology of surface alloy layers forming on the steel cast satisfies the needs of contemporary industry: high hardness, strength, resistance to abrasive wear and concurrently high plasticity of the core. The process of forming such layers is possible thanks to foundry technology of forming the element with required properties only for chosen parts instead of all cast [10]. Specially prepared pad is fixed on the chosen surfaces of the mould cavity and poured with the liquid metal [11,12].
The technology of surface composite layer forming process on the chosen surfaces of the cast also guarantees the following properties [13][14][15]: • the hardness much higher than the hardness of the basic cast alloy, • the abrasion resistance much higher than the abrasion resistance of the basic cast, • optimal thickness of the surface composite layer depending on the work conditions and the thickness of the cast face, • the possibility of the heat treatment avoidance -usually one -stage of full annealing or normalization instead of twostage. The process of creating a surface layer depends on many physical and chemical factors. Properties mainly depends on the self-colling conditions and the reaction on the surface of the metal / pad (that is the kind of liquid cast steel impact on the pad during pouring and self-colling process) [13][14][15].

The aim of the study
The main aim of this work was to determine the effect of the location of the heat center on the layer's thickness. There were also the attempts of the prediction of the layer's thickness. Tests were conducted with the use of computer programs. There have been changes in the size of the casting, without changing the module casting.

The range of studies
To achieve the aims of the work, the following scope of research was taken: 1.
working out the constructing assumptions of a model casting 2.
simulation of the process of creating alloy layer for the following assumptions: a) pouring temperature changes at three levels: casting material -low-carbon cast steel (Table 1) c) material pad: • high-carbon ferrochromium FeCr800 (

Casting model design assumption
The shape of the casting was designed so that the construction of the pad and form was not troublesome and time-consuming. Location of the pad and shape of ingate were selected to minimize the erosion of the metal stream (Fig. 1).
The basic model was the cubicoid of dimensions 80x80x100mm (model no 2) and 80x80x60mm (model no 1) - Fig. 1. The dimensions (a1, a2) were reduced or increased to change the position of the heat center ( Fig. 2). Dimensions of the cast were changed in such a way that modules of examined casts responded to base cast.
There are presented the changes of dimension with the same module in Table 3.

The results of the simulation.
The results of temperature (at the measurement point) for different cuboids and different pouring temperatures are given in Table 5. Three models of cast, where the pad holding time above 1300˚C was the longest, were noticed by the analysis of the obtained rezults: The graphs of self -cooling curves for particular virtual measurement points which allow to determine the heating time of pad above the temperature 1300 0 C are presented on Fig. 4 -9 . You can specify: • Hold time at a temperature of 1300 0 C, • maximum temperature in the pad. The obtained results (for the three cubicoid castings) were loaded to the program Preforma 1.1 [4]. in order to determine the thickness of alloy layer. The results are shown in Table 5.

Conclusions
It is possible to change the geometry of the casting by changing the location of the heating center in relation to the alloy promould, and to increase the thickness of the layer by the proper choice of pouring temperature.
The greatest thickness of layers was obtained for casting with the dimensions 70x70x150mm and pouring temperature 1600 0 C.
The thickness of alloy surface layer can be predicted with the use of programs NovaFlow&Solid and Preforma 1.1 (without performing costly trial castings).
The program NovaFlow&Solid calculates the time of pad holding at the temperature above 1300 0 C. Program Preforma 1.1 calculates the thickness of the alloy casting with the use of obtained data (Table 5).