^{1}

^{*}

^{1}

^{1}

^{1}

Theoretical analysis of the effects of velocity of impact using suitable heat transfer equations expressed in forms of finite difference method was developed and used to determine their effects on the characteristic cooling parameters during quenching process. Various velocities of impact obtained by varying the heights of specimen drops were also used to experimentally determine their effects on characteristic cooling parameters and mechanical properties of medium carbon steel using water as the quenching medium. At height of drop of 0.5 m, 1.0 m, 1.5 m, and 2.0 m, the tensile strength of the material is 410.4, 496.12, 530.56, and 560.40 N/mm
^{2} respectively. The corresponding hardness values are 42.4, 45.2, 46.2, 50.5 HRC respectively. It is found that as the velocity of impact increases, maximum cooling rate increases. Hardness and ultimate tensile strength also increase. There are good agreements between theoretical and experimentally determined values of critical cooling parameters of water during quenching operations.

Quenching of steel involves heating a part above upper critical temperature to austenitizing temperature and holding at this temperature for a specified soaking time followed by intense cooling in a suitable quenching medium to transform it into the hard structure―martensite. This is generally achieved by cooling at a sufficiently fast rate to avoid the formation of soft constituent (ferrite and pearlite) in the steel [

Quenching occurs in three stages. The first stage of cooling is characterized by the formation of a vapour film around the component. This is a period of relatively slow cooling during which heat transfer occurs by radiation and conduction through the vapor blanket. Upon further cooling, the nucleate boiling stage begins during which the vapour film collapses and cool quenchant comes into contact with the hot metal surface resulting in violent boiling and high extraction rates. Maximum part distortion also occurs during this stage due to difference in heat extraction from the part surfaces. As cooling continues, the surface temperature is below the boiling point of the quenching fluid and the metal surface is completely wetted by the fluid. At this point, the cooling rate is low and determined by the rate of convection and the viscosity of the quenching fluid. Heat transfer rates in this region are affected by various process variables, such as agitation, quenchant viscosity and bath temperature. The duration of the vapour phase and the temperature at which the maximum cooling rate occurs have a critical influence on the ability of the steel to harden fully [

Quench hardening is a vital part of production based on steel; it is also one of the major causes of rejected components and production losses due to cracks and distortion in the material. Therefore the technical challenge of quenching is to select the quenching medium and process that will minimize the various stresses that develop within the part to reduce cracking and distortion, while at the same time providing heat transfer rates sufficient to yield the desired as-quenched properties [

Achieving desired mechanical properties and minimizing the possibility of occurrence of quenching cracks are the key indicators of successful hardening process. Apart from the hardenability of the alloy, the geometry of the part, and the quenchant used, the effectiveness of quenching depends on a number of other external factors such as temperature, agitation and volume of the quenchant [

Agitation or forced circulation of the quenchant during quenching generally enhances heat transfer at all stages of cooling [

Modeling and simulation of quenching process help to establish a predictive theory that enables us to predict the final properties and structure (shape) of the component caused by the manufacturing process. It is also a way to reduce cost and time in product development [

In the present research work, the effects of velocity of impact on the theoretical and experimentally determined cooling parameters of carbon steel using water as a quenchant are presented and compared. Effects of specimen’s velocity of impact on the mechanical properties such as hardness and ultimate tensile strength of medium carbon steel specimens are also given.

To formulate the governing equations for the heat transfer during quenching process, some assumptions are necessary. The following assumptions are made.

1) The material is homogenous and isotropic.

2) Velocity of impact is considered to coincide with the velocity of specimen length mid-center.

3) Velocity is assumed to be terminal velocity in the liquid and taken to be constant throughout the fall in the liquid.

4) Because of the small size of the specimen compared to the large volume of the liquid, the heat transfer between the liquid and the container is small and neglected.

5) Heat losses through convection to the atmosphere from the container are very small and also neglected.

Heat Transfer Equations within the Zones during Quenching ProcessFor symmetrical specimen (e.g. rods and pipes), the heat transfer equation for one dimensional analysis where there is no internal heat generation and where the length l, of the specimen is much greater than the radius r (i.e.

where

1) Zone 1: Heat transfer in the air;

2) Zone 2: Heat transfer in the quenchant; and

3) Zone 3: Heat transfer at the point of sudden stop of fall in the quenchant.

Based on Equation (1), the governing partial differential equations [

1) Heat transfer in the air

Using Equation (1) for symmetrical objects as in cylindrical specimens, we have:

For quenching operation, where effect of velocity of drop

Energy balance between the specimen and ambient air.

Heat convected from the surface of the specimen into the atmosphere

2) Heat transfer in the quenchant

Conduction heat transfer equation within the quenchant

where V_{t} = terminal velocity in the quenchant

At the center of the specimen i.e. r = 0,

Energy balance between the specimen and the quenchant

3) Heat transfer at the point of sudden stop in the quenchant, V_{s} = 0, i.e. z = H_{0}

Equation (2) reduces to

Energy balance between the specimen and the liquid:

The solution to the governing equations and energy balances (1)-(7) expressed in finite difference forms are as given below.

Finite Difference Forms of Equation Representing Temperature Distribution in Each Zone1) Heat transfer equation in the air:

Substituting Equations (8)-(10) into Equation (2) we have:

Multiply through by

Equation (12) can be rearranged to give:

Using

For stability criterion, the coefficient of

Similarly, equation representing the energy balances between the material and ambient air, using Equation (7) is given by:

For stability

2) Heat transfer in the quenchant:

Finite difference discretization of the governing energy equation in this zone we have for each term;

Substituting Equations (19)-(21) into Equation (4) we have:

For stability criterion, the coefficient of

Similarly, equation representing the energy balances between the material and the quenchant using Equation (7) i.e.

Substituting, we have:

For stability

3) Heat transfer at the point of sudden stop, V_{s} = 0, i.e. z = H_{0 }

For stability,

The energy balance between the specimen and quenchant:

For stability

Matlab approach was used to obtain the theoretical data used to obtain temp versus time curves for various heights of drop.

The experimental investigation was conducted using a standard probe made of a cylindrical medium carbon steel specimen of 12.5 mm diameter × 60 mm long. The composition of the carbon steel used is shown in

Element | C | Si | Mn | P | S | Cr | Fe |
---|---|---|---|---|---|---|---|

% Composition | 0.357 | 0.16 | 0.75 | 0.032 | 0.041 | 0.1 | 98.2 |

The hardness values determinations were carried out on test-pieces cut from the middle portion of the quenched specimens. The hardness impressions were taken transversely in two perpendicular directions along the cross- section of the quenched specimens by Rockwell hardness testing machine (Leco LM 700 AT) under applied load of 490.3 MN and dwell time of 10 sec. using a’ C’ scale (HRC). Hardness values were taken at different points to obtain an average value. Multiple repeat tests were performed on each specimen and the average taken as a measure of the hardness of the specimen.

The tensile tests were conducted on a computerized Instron Universal Testing machine (Model 3369) by using machined cylindrical tensile test specimens of 5.0 mm in diameter and gauge length of 20 mm. The maximum strength, percentage reduction in area and percentage elongation were determined from the Load-extension curves. The impact test was carried out to measure the toughness of the quenched specimens according to ASTM D256.

Optical micrographs of quenched specimens were carried out on sectioned surface using laboratory microscope, grinded with silicon carbide papers of different sizes. They were subsequently polished using a cloth impregnated with alumina until a mirror surface was obtained. Grinded surface were cleaned with water and ethanol. Etching with 2% Nital was done and microstructures observed using a high powered optical microscope.

Height of drop (m) | Critical cooling parameters | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|

Experimental | Theory | |||||||||||

t_{AB } (s) | CR_{max} (˚C/s) | CR_{700} (˚C/s) | CR_{300} (˚C/s) | t_{300} (s) | T_{CRmax} (˚C) | t_{AB} (s) | CR_{max} (˚C/s) | CR_{700} (˚C/s) | CR_{300} (˚C/s) | t_{300} (s) | T_{CRmax} (˚C) | |

0 | 60 | 36 | 37 | 4 | 100 | 690 | 40 | 42 | 41 | 21 | 58 | 640 |

1 | 80 | 40 | 40 | 6 | 110 | 720 | 80 | 55 | 53 | 5 | 100 | 740 |

1.5 | 20 | 44 | 36 | 12 | 50 | 650 | 80 | 57 | 57 | 10 | 120 | 690 |

2.0 | 45 | 56 | 55 | 16 | 60 | 750 | 70 | 61 | 58 | 19 | 80 | 640 |

Cooling curves of steel samples dropped from different heights show the three stages of quenching mechanism. However, with increasing height of drop, the film boiling stage time reduces. Maximum cooling rates of, 40, 44, 56 m/s with corresponding height of drop of 1 m, 1.5 m, and 2 m was obtained (

In contrast to the ductile behavior of the as-received carbon steel samples, specimens dropped from various heights into the quenching medium exhibited brittle fracture, as indicated by the negligible percentage elongation and reduction. Higher Izod impact energy values of 10.5 J was obtained for samples dropped from 1.0 m height than sample dropped from 2.0 m height. The decrease in impact energy value as the hardness increase is in agreement with earlier research of [

Height of Drop (m) | Centre Hardness (HRC) | Average Hardness (HRC) | Tensile Strength (N/mm) | Percentage Elongation (%) | Percentage Reduction (%) | Izod Impact Value (J) |
---|---|---|---|---|---|---|

0 | 40.0 | 42.4 | 410.4 | 4 | 4 | 12 |

1.0 | 44.0 | 45.2 | 496.12 | 2 | 2.5 | 10.5 |

1.5 | 45.0 | 46.2 | 530.56 | 2 | 2.5 | 10.0 |

2.0 | 45.9 | 50.5 | 560.40 | Nil | Nil | 8.3 |

As received | 36.2 | 36.8 | 510 | 21 | 38.0 | 34.0 |

The variation in grain structures obtained for the steel specimens dropped from different heights is found to be more pronounced at the quenched specimen edges than at the centre, as evidenced by higher hardness values at the edges than at the centre (see

The microstructure of the as-received sample is composed of mixture of ferrite (white) and pearlite (dark) (see

From the results obtained, it can be concluded that:

1) It has been established that cooling rates exhibited significant dependence on velocity of impact with the general trend showing increased cooling as velocity of impact increased.

2) It is found that as the velocity of impact increases, maximum cooling rate increases and the hardness and ultimate tensile strength also increase.

3) The microstructure of the specimen dropped from height of 2 m showed predominantly martensitic structure while the specimen dropped from 0.5 m revealed a mixture of bainite and pearlite structures.

4) Experimental and theoretical values are different, especially during nucleate boiling stage; however, the level of agreement is reasonable as they follow the same pattern. Both investigations revealed that increasing the velocity of impact increased CR_{max}, CR_{700} and CR_{300}.

5) The proposed model can be used to simulate the quenching process of steel materials by using the material properties as the input data.

Joseph BabalolaAgboola,Oladiran AbubakreKamardeen,EdekiMudiare,Michael BolajiAdeyemi, (2015) Effect of Velocity of Impact on Mechanical Properties and Microstructure of Medium Carbon Steel during Quenching Operations. Engineering,07,434-445. doi: 10.4236/eng.2015.77039

_{ }