Introduction

Medium-carbon steels are similar to low-carbon steels except that the carbon ranges from 0.30 to 0.60% and the manganese from 0.60 to 1.65% [1]. Increasing the carbon content to approximately 0.5% with an accompanying increase in manganese allows medium carbon steels to be used in the quenched and tempered condition. The uses of medium carbon steels include shafts, axles, gears, crankshafts, couplings and forgings. Steels in the 0.40 to 0.60% C range are also used for rails, railway wheels and rail axles.EN18 (AISI 4140) EN19 (AISI 4142) EN24 (AISI 4340) are medium carbon low alloy steels under High strength Low alloy [HSLA] categories. EN 19 is nickel free steel and EN 24(low nickel) steels. The mechanical strength of medium carbon low alloy steels can be improved by forging and heat treatment. The primary source of information for heat treatment of steels is ASM Metals Handbook Vol-4 [2]. Various other textbooks and standards are available for determining the temperatures of homogenization, quench media, tempering temperatures and procedures thereof.

Materials and Methods

Chemical composition Test

The chemical composition test of medium carbon low alloy steel samples for this investigation is given in (Table 1).

Table 1: Chemical Compositions of selected Steels

Heat Treatment

Heat treatment of steels is based on three diagrams (i) the Iron Carbon Diagram for the specific composition, (ii) The Time-Temperature-Transformation (TTT) diagram and (iii) the Continuous Cooling Transformation (CCT) Diagram. These are well explained in ASM Metals Handbook and various other handbooks [3].

The prepared tensile test samples and other samples were heated to different tempering temperatures and soaked for 45 minutes using muffle furnace. Taken Test samples were quickly taken out of the furnace after each of the heat treatment temperatures. Surface morphologies of the heat-treated samples the hardness and tensile test were carried out

Mechanical Test

Tensile Testing

The tensile strengths of the steels under various heat-treated conditions were determined using a Hounsfield Tensometer (shown in Figure 2.1), according to ASTM E8 standard.

Hardness Test

Vickers pyramid method was used for the determination of the hardness of the heat-treated samples. Each of the test specimens was flatten after heating and then mounted on the anvil. The specimens were brought in contact with the pyramid indenter and allowed to rest for a dwell time. The hardness of the specimen is indicated by the penetration of the indenter on the test specimen and average values are recorded after repeating the test for each of the test specimens.

Results and Discussion

Mechanical tests were carried out as a part of quality confirmation of forging and heat treatment process as per customer specifications. Tension, hardness was carried out for each batch of components.

The measured values of ultimate strength, yield strength, % elongation and % reduction in area, and BHN for selected steel tempered at different temperatures are tabulated in Table 2 to 4.

Table 2: Mechanical Properties of EN19 steel for various Tempering Temperatures

Table 3: Mechanical Properties of EN24 steel

Table 4: Mechanical Properties of EN25 steel forVarious Tem-pering Temperatures

1. Ultimate strength and yield strength of the EN19 steel decrease with increase in tempering temperature.
2. % Elongation and % reduction in area increase with increasing tempering temperature.
3. Hardness decreases with increasing tempering temperature.

The following observations can be made from the Figure 3.1 to 3.6.



Figure 3.1: Variation in UTS & YS with Tempering Temperature for EN19

% Elongation and % reduction in area increase with increasing tempering temperature.



Figure 3.2: Variation in %EL and %RA with Tempering Temperature for EN19

Hardness decreases with increasing tempering temperature



Figure 3.3: Variation in BHN with Tempering Temperature for EN19

Ultimate tensile strength and yield strength of the EN24 steel decrease with increase in tempering temperature.



Figure 3.4: Variation in UTS & YS with Tempering Temperature for EN24

% Elongation and % reduction in area increase with increasing tempering temperature.



Figure 3.5: Variation in UTS and YS with Tempering Temperature for EN24

Hardness decreases with increasing tempering temperature



Figure 3.6: Variation in %El and %RA with Tempering Temperature for EN24

4. Ultimate tensile strength and yield strength of the EN24 steel decrease with increase in tempering temperature.
5. % Elongation and % reduction in area increase with increasing tempering temperature.
6. Hardness decreases with increasing tempering temperature

The behavior of EN24 almost duplicates that of EN19.

Since the industrial test values are compared with the values specified by customers and not with standard values, no correlation is attempted for the mechanical properties of EN25 steel.