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Revista Ciencias Técnicas Agropecuarias

versão On-line ISSN 2071-0054

Rev Cie Téc Agr vol.29 no.1 San José de las Lajas ene.-mar. 2020  Epub 01-Mar-2020

 

VIEW POINTS

Procedure on the selection of steels and their heat treatment technology

Dr.C. Francisco Martínez-PérezI  * 

I Universidad Tecnológica de La Habana-CUJAE, Centro de Estudios de Ingeniería de Mantenimiento, Marianao, La Habana, Cuba.

ABSTRACT

The proper selection and correct application of heat treatment technology for the manufacture of machine elements require elements, requires procedures and knowledge that today, become much more complex. In all careers of mechanical content, it is essential in undergraduate and postgraduate not only their knowledge but their correct application. It is essential to know previously the required working conditions of temperature, environment, resistance of different kinds (tensile, flexion, torsion, fatigue) but sometimes it is necessary to differentiate the properties of the core from those of the surface, which requires of the knowledge of a new science, "surface engineering". In the present work in the form of algorithmic is introduced reasoning and applying novel aspects, some of theme, developed by the author. It is intended to facilitate the essential elements to achieve a successful application that guarantee the work requirements of the element and its correct teaching.

Keywords: Education Production of Machine Elements; Engineering; Algorithmic Reasoning

INTRODUCTION

The integral response that is needed in a machine element is closely related not only to the correct selection of the material to be used, but also to the heat treatment technology that must be applied. The resistance of a machine element, in many cases requires the obtaining of martensitic structures, which provide greater hardness and resistance. In turn, many machine elements require different properties in the core than on the surface, where low wear requirements are present. Today the problem of the demands of a surface needs the application of tribology elements and surface engineering (Apraiz, 1984, 1985, 2000; Agüero, 2007).

The formation of stress gradients in adhesive wear processes implies the possibility that wear and friction are concentrated only in the surface layer with the least possible penetration of the acting forces in it. Otherwise, there is a risk that wear will occur from inside the layer and therefore, it and the damage are much greater (Martínez y Gordon, 1985; Jorochailov y Gordon, 1988; Valencia, 1988; Kudriatseva, 2005; Martínez, 2016).

In the quenching process of a steel, its chemical composition, the actual carbon content, the cooling medium (its temperature and stirring), the dimensions and shapes of the element and the need or not to differentiate properties in the core and superficial have to be taken into account. In the present work some algorithms design by the author are introduced and several tables and figures that allow achieving these purposes are presented. They were employed in industrial practice with satisfactory results.

DEVELOPMENT OF THE TOPIC

The work is not intended to deepen in the selection of the steel to be used. The existence of various publications and articles referring to that Apraiz (1984); Martínez (1988, 2000); Kudriatseva (2005); Arza (2006), makes this unnecessary. On the contrary, everything related to the selection and correct application of the heat treatment technology or technologies will be deepened, as well as in regard to surface engineering Martínez (2014), in terms of differentiating the properties of core and surface and the interrelation between both.

Volumetric Treatment and that in which it is Necessary to Differ Core and Surface Properties

The first aspect to take into account in the selection of the heat treatment technology to be used for the complete fulfilment of the work requirements will be aimed at knowing the need or not to differentiate the properties of the core and surface and ensure its proper interrelation. It must be taken into account from the beginning that, as this procedure progresses, there may be a need to change the steel initially selected. This first aspect led to an algorithmic approach can be seen in Figure 1.

FIGURE 1 Algorithmic approach in the first step of the procedure. 

In the case that is not necessary to differentiate the properties of the core and surface, the procedure of the technology to be used must include the rigorous analysis of the resistance required to guarantee the working conditions and this will include the structural characteristics of the material. This analysis must include:

  1. What kind of requests are acting? (Tensile, flexural, torsional and fatigue resistance, combined applications, impact loads, working temperature, etc.)

  2. Dimensions of the element.

  3. Existing technological conditions of manufacture, number of elements to be manufactured.

All these well-known aspects are not essential parts of the procedure that is presented. This, in turn, will provide an analysis that essentially starts from knowing how much martensite is needed in the nucleus and how this is achieved, taking into account the shape and dimensions of the piece and the characteristics of the tempering medium (this will be closely related to the concept of hardenability).

The calculation of hardenability is a perfectly defined technological problem in numerous textbooks and other publications on Heat Treatment (Caballero, 2012; Martínez, 2015). In the fundamental, it starts with the results obtained in an analysis on an equipment for the Jominy test. In this test, it is possible to establish a relationship between the distance to the tempered end in the test tube standardized for it and, using the standard parameters for the Jominy test, to know the maximum diameter in which a 50% martensitic transformation in the steel can be obtained in the steel in question. Obtaining 50% of martensitic transformation will depend on the carbon content and the presence of alloying elements. Table 1 shows the average hardness possible to reach in an area with 50% of martensite for carbon and alloy steels (Martínez, 2000).

TABLE I Average hardness in the area of 50% martensite based on carbon content and type of steel 

CARBON CONTENT (%) HRC
Carbon steels Alloying steels
0,08-0,17 - 25
0,18-0,22 25 30
0,23-0,27 30 35
0,28-0,32 35 40
0,33-0,42 40 45
0,43-0,52 45 50
0,53-0,62 50 55
≥ 0,63 55 60

Table 2 shows with greater precision the hardness that can be achieved depending on the carbon content and the content of the martensitic transformation (Martínez, 2000).

TABLE 2 Effect of carbon concentration and % martensite (M) on the average hardness of tempered steel 

Carbon content, % HRC
99% M 95% M 90% M 80% M 50% M
0,10 38,5 32,9 30,7 27,8 26,2
0,20 44,2 40,5 38,2 35,0 31,8
0,30 50,3 47,0 44,6 41,2 37,5
0,36 53,9 50,4 47,6 44,4 40,5
0,38 55,0 51,4 49,0 45,4 41,5
0,40 56,1 52,4 50,0 46,4 42,4
0,42 57,1 53,4 50,9 47,3 43,4
0,44 58,1 54,3 51,8 48,2 44,3
0,45 59,1 55,2 52,7 49,0 45,1
0,48 60,0 56,0 53,5 49,8 46,0
0,50 60,9 56,8 54,3 50,6 46,8
0,52 61,7 57,5 55,0 51,3 47,7
0,54 62,5 58,2 55,7 52,0 48,5

However, if it is needed to analyze the hardenability of any piece, the results of the Jominy test, constitute only a first step, which also needs additional considerations, and which after obtaining the result, requires the application of a series of additional steps, which conform a calculation methodology (Apraiz, 1984; Caballero, 2012; Martínez, 2015).

This paper intends to present this methodology, through a calculation algorithm, which was used successfully for the analysis of the hardenability of several elements.

This methodology in the form of an algorithm is shown in Figure 2

FIGURE 2 Algorithm for the correct calculation of the hardenability of a machine element. 

In the Jominiy test, the hardenability is found with a ratio L / D = 3 (like that of the Jominy test), in which, with different cooling means, a total volumetric hardening is achieved (at least 50% of martensite in the core). This diameter is called the ideal diameter (Di).

As it is understood, the pieces do not necessarily have this relationship (L / D = 3). To know if any piece can have total hardenability, a form factor Kf is used.

Dp= Di·Kf (1)

Where:

Dp

is the diameter or average thickness of the piece possible to obtain total tempering;

Di

Ideal diameter according to the Jominy test;

Kf

form factor.

In turn, the form factor is equal to

S/Vp= S/Vj·Kf (2)

Where:

(S / V) p

is the surface / volume ratio of the actual piece to calculate;

(S / V) j

is the surface / volume ratio from the Jominy test.

On the other hand, the different cooling means affect the value of the ideal critical diameter (Di). This is due to several causes, but fundamentally, to the severity with which the heat of the piece is transmitted to the medium. The cooling means have different hardening severities, not only between them (as can be seen in Table 3), but also depending on their temperature and the speed of circulation.

TABLE 3 Severity of the comparative tempering of different cooling media depending on the temperature of the medium in centigrade and the speed of circulation of the medium in m/s 

COOLING MEDIUM TEMPERATURE OF THE MEDIUM 0 C CIRCULATION VELOCITY m/s SEVERITY
WATER 32

  • 0

  • O,25

  • 0,51

  • 0,76

  • 1,1

  • 2,1

  • 2,7

  • 2,8

55

  • 0

  • 0,25

  • 0,51

  • 0,76

  • 0,2

  • 0,6

  • 1,5

  • 2,4

FAST OIL 60

  • 0

  • 0,25

  • 0,51

  • 0,76

  • 0,5

  • 1,0

  • 1,1

  • 1,5

From the table it can be seen that the water has a harder severity than the oil; and that as the speed of circulation increases, the severity of cooling increases, both in water and in oil.

There are different types of cooling oil. Table 3 compares water with fast oils, which double or triple their severity of tempering compared to conventional ones. It is also recommended to use oil cooling, calculating it up to 60-70 °C (Apraiz, 1984).

On the other hand, the cooling rate of the different media also depends on the temperature range in which it occurs. This can be seen in Table 4 (Martínez, 2000). In conclusion, the results of the percent of martensite to be obtained and to what depth it is reached depend not only on the steel used, but on the cooling medium, its temperature, its stirring characteristics and the shape and size of the piece treated, all factors that must be taken into account and with them influence the primary outcome.

TABLE 4 Cooling speed of different media depending on the temperature range during cooling 

COOLING MEDIUM COOLING SPEED IN 0C/s
RANGE FROM 650-500 0C RANGE FROM 300-200 0C
Water at 30 0 C 500 270
Water at 50 0 C 100 270
Water at 75 0 C 30 200
Emulsion of oil and water 200 -
Medium mineral oil 100-150 20-50
Compressed air 30 10
Air 3 1

In this way, taking into account all the proposed elements, a favorable result of the required hardenability can be obtained.

The calculation procedure in cases where the properties of the core and the surface need to be differentiated is shown below.

Treatment in which the Properties of the Nucleus (CORE) and the Surface are Needed to be Differentiated [7].

The first aspects to consider in this case are:

  1. What properties are required on the surface?

  2. What properties are required in the core?

  3. What layer depth is needed?

  4. Possible gradient of stresses generated in the core-surface.

The first two aspects are data on working conditions. In the case of axes subjected to bending, the calculation of the layer depth can be carried out according to the procedure shown.

In a coordinate system to scale, they will be located on the Y axis, dimensions in mm, first placing the radius of the axis and on the X axis in MPa, the values of different efforts will be placed. This scheme will look as it is shown in Figure 3

In other cases where contact stresses penetrate deep into the component, towards the entire surface layer or even below it (negative gradients) methods that generate thicker surface layers are needed.

FIGURE 3 Scheme for calculating the layer thickness depending on the acting efforts of bending. 

On the line that demarcates the end of the radius of the axis, the value of the acting bending stress will be and it will be designated as σfla. Then, on the same axis, the value of the flexural stress of the material in its surface layer will be placed and it will be designated as σflm. Naturally, σflm> σfln, which, together with the necessary depth of layer to obtain, will indicate, the value that must be obtained in the layer, which, in turn, will delimit, as it will be seen later, the type of surface treatment to be used.

Then, on the line X, the value of the required bending stress will be placed in the core of the material that will be designated as σfln. As it is known, bending forces applied to an axis, follow a linear distribution from the surface to the center, so σfla will be connected to the origin of coordinates that will be the center of the axis (See. Fig. 1). Then, by raising a line from σfln to point D, it will be known to what depth the layer must be obtained, to give a comprehensive response to the requested effort request, this value will be given by σfln-D.

In this way, a mathematical formula for calculating the thickness of layer e can be established based on the similarity of triangles.

σfla/r =σflaσfln/e (3)

where:

σfla

is the applied bending effort;

r

is the radius of the axis;

σfln

is the bending effort that supports the core;

e

is the layer thickness to be calculated.

Thus:

e = (σflaσfln)r/σfla (4)

It will always be convenient to increase the calculated thickness by 0.1 - 0.15 mm in order to avoid the critical point D.

The final aspect to be analyzed, from the point of view of surface engineering, is what type of layer should be used, which may also establish the need to vary the steel to be used, provided that the conditions established above are guaranteed therein. For this analysis, the gradient of efforts that may occur must be taken into account, which is not within the objectives of this work.

The wide diversity of surface engineering materials existing, allow the designer to select them, at least to a certain extent, instead of using materials that are volumetrically equal to that of their surface (ASM, 1988; Martínez, 2012).

Figure 4 shows the wide range of combination of layer depth and hardness that can be obtained on surfaces by these methods (Apraiz, 1984).

FIGURE 4 Depths and hardness typical of different forms of coatings and surface hardening. 

From Figure 4, it can be concluded that different methods offer different possibilities of combining depths and hardness of the surface layer. It is noteworthy that some methods such as chemical nickel, nickel plating, chromate phosphate and others are missing. Those methods such as surface depositions with PVD, CVD or ionic implants that produce only very thin layers and great hardness, will be useful for use in those applications with a minimum wear extension and where the stress acting on the surface decreases rapidly, during the work, so that the thin surface layer is not removed. This is associated with the elastic interaction stage being reached quickly.

CONCLUSIONS

In the present work it has been shown, through algorithmic reasoning and using various tables and figures, some elaborated by the author, a procedure for the most accurate calculation of technology selection for volumetric treatment of steel parts or those in which it is needed to differentiate the properties between the core and the surface of some steel machine elements. In the latter, the concept of surface engineering has been introduced to achieve a response, not only in terms of the strength and hardness requirements of the outermost layer of the element, but also in terms of the depth of the layer to prevent acting loads cause damage to the core of the material, due to insufficient layer thickness. A figure was provided for the calculation of the layer depth in elements subjected to bending loads.

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3The mention of trademarks of specific equipment, instruments or materials is for identification purposes, there being no promotional commitment in relation to them, neither by the authors nor by the publisher.

Received: January 11, 2019; Accepted: December 19, 2019

*Author for correspondence: Francisco Martínez Pérez, e-mail: fmartinez@ceim.cujae.edu.cu

Francisco Martínez Pérez, Profesor e Investigador Titular Universidad Tecnológica de La Habana-CUJAE, Centro de Estudios de Ingeniería de Mantenimiento, Marianao, La Habana, Cuba, Teléfono: 7 2663642, e-mail: fmartinez@ceim.cujae.edu.cu

The authors of this work declare no conflict of interests.

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