Effects of Heat Treatment on the Properties of Mild Steel Using Different Quenchants

This study was conducted in order to improve the mechanical properties o f mild steel materials used as bolts and studs in coupling agricultural machinery fo llowing their frequent failure in service. Heat treatment at 900C for four hours was done and six specimens of each were then quenched with different media used as the major source of strength enhancement. The universal testing machine (UTM) was utilized for the various mechanical tests. The results of the tests showed positive changes in the strength properties of the mild steel material, in terms of h igh tensile strength, toughness, ductility and hardness. Water quenched specimen has the highest tensile strength (497.76N/mm), hardness value (138.27), toughness (168.38) and bending at yield (749.49N/mm). It also recorded the lowest ductility of 28.36% when compared with ductility values for other quenchants. These desirable qualit ies are needed for durability in service, especially for rugged agricultural operations like tillage. Water proved to be the best quenchant for ach ieving these desirable qualit ies among the quenching media used.


Introduction
The subject of mechanical testing of materials is an important aspect of engineering practice. Today, mo re attention is being given to the interpretation of test results in terms of service performance, as well as giving reliab le indications of the ability of the material to perform certain types of duty. Mechanical tests are also emp loyed in investigational work in o rder to obtain data for use in design to ascertain whether the material meets the specifications for its intended use [1]. For this purpose, the tests should provide the information accurately, rapid ly and economically.
Heat treat ment, an operation, or co mb ination of operations, involving the heating and cooling of a metal or an alloy in the solid state, is another method of obtaining certain desirable conditions in metals. [2] reported the effect of heat-treatment on the properties of case-hardening steel with 0.17% carbon, 0.24% silicon, 0.72% manganese and 0.04% sulphur and phospho rus. The resu lt sho wed an in crease in Brin ell hardness, yield point and ultimate tensile stress but a slight decrease in percentage elongation wh en the steel was quenched in water after being normalized at 920℃, reheated and quenched in water fro m 760℃.Hardness testing is often used to evaluate heat t reat ments and can be used as an approximation for tensile strength [3].
In agricultural production operations, some general-purpose machine elements are used extensively on tractors as well as on tillage imp lements. Typical of such elements are fasteners like bolts and studs used to couple implement gangs unto the implement frame.These elements during field operations are subjected to fatigue loading and tend to fail mainly by shear and bending. Bolts and studs are supposed to be made fro m mild steel (up to 0.25% carbon) with characteristic toughness and ductility. These elements, however, tend to fracture with repeated or frequent bending cycles. According to [4], the performance and reliability of an engineering structure or assembly is a function of the quality of the various components. Failure of any component of an equipment may have serious consequences on the functionality of the equip ment or machine.So me co mmon causes of failure include design deficiencies, material selection, material imperfections, ambiguous manufacturing processes and fatigue [3].
The available studs and bolts found in the local markets in Nigeria and often used for coupling on tractor imp lements are of the inferior grade that fail too frequently. Their specifications are often not known which makes it impossible to predict their suitability for the intended task. The producers of these studs probably machine them out fro m any cheap grade of iron or steel material without consideration for their intended applications and service life.Quenching is usually used to obtain a required micro structure, hardness, strength or toughness, while minimizing residual stresses, distortion and the possibility of cracking.Quenching effectiveness is dependent upon steel composition, type of quenchant or quenchant use conditions [5].
Studies have shown that low carbon steel such as mild steel can be strengthened through heat treatment, wh ile quenching after heat treatment imp roves the mechanical properties of the steel material [6,7,8,9]. For most metallic materials, the high-cycle resistances are dominated by the strength and ductility, respectively [10]. These desirable attributes can only be achieved through appropriate heat treatment and quenching. Several quenchants have been used for steel hardening among which are water, air, oil, brine and superquench. Each has its merits and demerits. Superquench according to [11] is a heavy brine solution with g reater percentage of water by weight. Brine has some beneficial effects such as fast quenching rates than water and reduction in non-uniform heat transfer during quenching. However, it has a majo r disadvantage of being corrosive both to the quenched material and the equip ment used [8]. This g ives rise to more frequent equipment shut downs and the associated higher maintenance costs. Accordingly, brine was not used in this study.
Most research efforts on the effects of heat treatment on the mechanical properties of steel materials were carried out in more developed countries where standards are maintained [12,4,8,13,14]. This calls for more concerted efforts in the developing countries like Nigeria where substandard materials saturate the markets.

Objectives of the Study
The objectives of this study were to: (i) investigate the effects of heat treatment on the mechanical properties of locally available mild steel studs using different quenchants, and (ii) reco mmend the most appropriate quenchant from the standpoint of strength and economy.

Methods
Thirty samples of the stud (M20) were obtained fro m the local market fo r the purpose of this investigation. Fifteen (15) Standard test specimens were machined, each for tensile and bend tests, respectively. The stud sample and test specimens are shown in figures 1 and 2.Twenty-four specimens were case hardened by pack carburizing method, while the remain ing six samp les were not subjected to any heat treatment to serve as control.The method involved placing the samples in a steel bo x along with 80% wood and 20% bariu m carbonate. The wood acted as the carburizing material wh ile the bariu m carbonate was the energizer for promoting rapid action on the steel. The specimens were placed in an electric furnace and heated at 900℃ for four hours. Six specimens each was then quenched using four different media v iz.: air, furnace, o il and water.
As a result of prolonged heating at a high temperature in the carburizing operation, both the core and the case were expected to exhib it overheated structure, which would be unsatisfactory in service, especially under severe conditions. The specimens were then further heat-treated to refine the core, and also to refine and harden the case. Co re refining was accomplished by heating just above the upper critical point for the core (870℃), soaking for 30 minutes and then quenched using the quenchants described above. The refining and hardening of the case was produced by quenching the specimens fro m 760℃.
After the heat treatments, 15 specimens each were then subjected to uniaxial tensile and 3-point flexural (bend) tests, respectively using the Testometric Universal Testing Machine (Ax model M500 -50KN) with 500 kgf load cell at a constant cross-head speed.A Personal Computer was used to record the output.Brinell hardness test which is usually associated with decarburization on the measured surface hardness [15]was evaluated using the empirical relationship between tensile strength and Brinell Hardness Number(BHN) for steels [16] as: Where k is a constant ranging from 3.4 -3.9 fo r steels.An average value of 3.6 for k was used for the evaluation of BHN in this study.

Results
The follo wing mechanical properties were obtained fro m the tests: (i) Tensile Strength (T.S): is the load required to fracture unit area of the metal.
(iii) Percentage Elongation (E.L) in gauge length: is a measure of the ductility of the material.
(iv) Percentage reduction in cross-sectional area (R a ) measured at the point of fracture is also related to ductility. Very ductile materials are considerably reduced in crosssection before they break.
(v) Young's Modulus of Elasticity (E): is the stiffness, rig idity, or springiness of a material. It is the slope of the linear portion of the stress/strain graph for the material. It is also the ratio between the stress applied and the elastic strain it produces.
(vi) Toughness (Er): is the energy absorbed by the material before it fractures. This is the area under the forceextension or stress -strain curve up to rupture point (N.mm).
(vii) Brinell Hardness Number (BHN): indicates the surface hardness of the material.
The stress-strain curves for the tensile tests are shown in figures3 to 5, whilefigures 6 to 8 depict the load-deflection curves for the bending tests. The summary data of the tensile and bend tests are presented in table 1.

Discussions
Fro m the stress-strain curves (figures 3-5), it can be inferred that the control (specimen 1) did not yield until it attained the tensile strength.Specimen 5 (water quenched) followed a similar t rend to that of specimen 1, except that it yielded at a higher tensile strength.Thus, the heat treated and water quenched specimen exhib ited superior mechanical properties over the original untreated specimen in terms of higher tensile and fatigue strengths.This result is consistent with the report of [17] that one of the major ad justable parameters in heat treat ment that affect the mechanical properties of materials, including the yield strength and hardness is the quenching media.
The results of the treatment effects on mild steel shown in table 1 indicate that specimen 5 (water quenched) proved superior to other specimens quenched in air, furnace and oil, respectively with respect to high strength properties, Brinell hardness, toughness and yield points. This result is in agreement with the findings of [12] and [7] that hardness and toughness exhib it mirro r behaviour. In contrast, the furnace and air-cooled treatments proved better than other treatments in ductility. This is as a result of their h igher values of elongation (peak strain) and percentage reduction in area. Among the treat ments, oil quenching proved to be the poorest med ia in terms of ductility. The high cost of oil will also discourage its use as a quenching media coupled with its poor performance.
The stress and strain at peak for both the control and the water quenched specimen were the same for the corresponding stress and strain at yield (figures 3 and 4). However, the water quenched treatment gave higher values of stress and strain at yield than the original specimen ( figure  5).This imp lies that the water-quenched samples are better in terms of ductility than the original specimen.The achievement of the difficult co mb ination of strength and ductility gives the water-quenched sample an edge over all other treatments. Earlier works [10,11] indicate that for most metals, the high-cycle resistances are dominated by strength and ductility. The results of this study shows that water quenched samples are better than the other quenchants in terms of strength and ductility. Therefore, water remains the most co mmon quenchant. More importantly, water is inexpensive, easy to use and has minimal safe handling or disposal considerations [8].
In terms of strength and hardness, the order of performance is as follo ws:Water-cooled > Control >0il cooled > Air cooled > Furnace cooled. Figures 6 to 8 and table 1 also show that the water quenched specimen gave the highest values of bending strength and deflection at break; thus, confirming the consistency of this method of cooling in improving the mechanical properties of mild steel as was the case in the tensile tests.These results are in agreement with the findings fro m similar studies reported by [2].When cost of quenching is put into consideration, it becomes obvious to conclude that using water, as a quenching media is more cost effective.

Conclusions
Tests were carried out to determine the effects of heat treatment on the mechanical properties of locally availab le mild steel studs used for agricultural operations using different quenchants. The findings fro m the study showed that water quenched specimens proved superior to other specimens quenched in air, furnace and oil, respectively with respect to high strength properties, Brinell hardness, toughness and yield point. Water being relat ively mo re readily available than other quenchants except air, easy to use and safe to handle is preferred. It is therefore concluded that fro m the standpoint of strength and economy that water quenching should be used for heat-treating mild steel components for coupling agricu ltural machinery.