Mechanical properties of the intercritically annealed 0.15wt%C - 0.32 wt% Mn steel and quenched in SAE engine oil at room temperature were investigated. The steel samples prepared for quenching and those prepared for quenching and then tempering were intercritically heat treated at 750, 760, 770, 780 and 790EC for 1hr in a laboratory muffle furnace and quenched to room temperature in SAE 40 engine oil. The steel samples prepared for quenching and then tempering were further subjected to a low temperature tempering at 150EC for 1 hr and air cooled to room temperature. The results revealed that strength and hardness values increased (from 512.29N/mm2 at 750EC to 674.62N/mm2 at 790ECfor strength and from 197BHN at 750ECto 241BHN at 790EC for hardness) with intercritical annealing temperatures but ductility and notch impact toughness decreased (from 12.18% at 750EC to 7.42% at 790EC for ductility and from 9.08 J/cm2 at 750EC to 5.55 J/cm2 at 790EC for notch impact toughness) with intercritical annealing temperatures. Tempered steel has yield strength values which increased with intercritical annealing temperatures from 180 N/mm2 at 750EC to 258 N/mm2 at 790EC. From the values it was observed that tempering gave rise to a decrease in tensile strength (from 674.62 N/mm2 at 790EC  to 637.65 N/mm2 at 790EC ) and hardness (from 241 BHN at 790EC  to 190 BHN at 790EC  ) and an increase in ductility ( from 7.97% at 750EC   to 12.18% at 750EC  ) and notch impact toughness (from 9.08 J/mm2 at 750EC  to 76.76 J/mm2 at 750EC ) of the as quenched steel. Tempered steels from the values presented better compromise between strength, hardness, ductility and notch impact toughness for automobile and other structural applications

Daniel-Mkpume, CC

Obikwelu, DON

Offor, PO

English

AJOL - African Journals Online

 32 MECHANICAL PROPERTIES OF THE INTERCRITICALLY ANNEALED 0.15Wt%C- 0.32 Wt%Mn STEEL AND QUENCHED IN SAE ENGINE OIL AT ROOM TEMPERATURE  P. O. OFFOR, C.C. DANIEL, D. O. N. OBIKWELU Metallurgical and Materials Engineering Department, University of Nigeria, Nsukka. peterjoyoffor@yahoo.com  ABSTRACT Mechanical properties of the intercritically annealed 0.15wt%C - 0.32 wt% Mn steel and quenched in SAE engine oil at room temperature were investigated. The steel samples prepared for quenching and those prepared for quenching and then tempering were intercritically heat treated at 750, 760, 770, 780 and 790C for 1hr in a laboratory muffle furnace and quenched to room temperature in SAE 40 engine oil. The steel samples prepared for quenching and then tempering were further subjected to a low temperature tempering at 150C for 1 hr and air cooled to room temperature. The results revealed that strength and hardness values increased (from 512.29N/mm2 at 750C to 674.62N/mm2 at 790C for strength and from 197BHN at 750C to 241BHN at 790C for hardness) with intercritical annealing temperatures but ductility and notch impact toughness decreased (from 12.18% at 750C to 7.42% at 790C for ductility and from 9.08 J/cm2 at 750C to 5.55 J/cm2 at 790C for notch impact toughness) with intercritical annealing temperatures. Tempered steel has yield strength values which increased with intercritical annealing temperatures from 180 N/mm2 at 750C to 258 N/mm2 at 790C. From the values it was observed that tempering gave rise to a decrease in tensile strength (from 674.62 N/mm2 at 790C  to 637.65 N/mm2 at 790C ) and hardness (from 241 BHN at 790C  to 190 BHN at 790C  ) and an increase in ductility ( from 7.97% at 750C   to 12.18% at 750C  ) and notch impact toughness (from 9.08 J/mm2  at 750C  to 76.76 J/mm2  at 750C ) of the as quenched steel. Tempered steels from the values presented better compromise between strength, hardness, ductility and notch impact toughness  for automobile and other structural applications.  Key words: Quenching, tempering, mechanical properties, intercritical annealing temperatures.  Symbol notation:  σy = yield strength, σ t = tensile strength,  = ductility,  H = Hardness, BHN = Brinnel hardness number, an = notch impact toughness, qs = quenched sample, ts = tempered sample.   1.0 INTRODUCTION  Steel is the world’s most used material. It is a very versatile material with a wide range of attractive properties which can be produced at a very competitive production cost [1, 2]. The optimization of alloying contents in the iron – carbon alloy system combined with different mechanical and heat treatments lead to immense opportunities for parameter variations and these are continuously being developed [ 1-3].  The development of new structural materials with good strength, impact and plasticity, which allow for weight reduction of cars, is still an open task for manufacturers. Up till now 63% of car body weight is made of steel structures [4].   Dual – phase steel (DP) is one of the family members of high strength low alloy steels which has a very good combination of strength and ductility.  They are characterized by the microstructure consisting of the dispersion of hard martensite particles in a soft ferrite matrix. These steels are very important for the automobile industry and are important in many other application fields, MECHANICAL PROPERTIES OF THE INTERCRITICALLY ANNEALED 0.15Wt%C- 0.32 Wt%Mn STEEL …            33   NIGERIAN JOURNAL OF TECHNOLOGY, VOL. 29 NO 2, JUNE 2010  especially in areas related to structural applications [5-12]. The dual phase steels are produced by reheating low carbon steel into the a intercrititical [ + ] phase field, and cooling at a rate so as to produce the desired microstructure containing martensite phase. They can also be obtained directly from the hot rolled mill, by the control of composition and processing [5].  Dual phase steels have relatively high tensile strength, continuous yielding and low 0.2% offset yield strength and usually higher uniform and total elongation than other high strength steels. This combination of high tensile strength and good ductility is obviously a major point in favour of dual phase steels compared to other high strength steel [5].       The improvement of the mechanical properties of the low Carbon steel leading to increased strength was the goal of some heat treatment technologies developed in the last twenty years [13]. Their potential as superior strength and formability substitutes for current automotive steels, was recognised and has provided an incentive for their rapid development and acceptance [14]. This successful application has preceded the  acquisition of a complete understanding of the detailed relationships between their process route, microstructure, and mechanical properties, although some research has been carried out in order to optimize the variables in the strength/formability balance [6, 15, 16]. However, the microstructural evaluations and mechanical properties of tempered dual phase steels is also of interest but has received little attention [1].  2.0 OBJECTIVES OF THE STUDY The objectives of this work were to investigate the influence of various intercritical annealing temperatures and tempering on the mechanical properties of 0.15 wt% C - 0.32 wt % Mn steel. So the goal of this work was the finding of the intercritical annealing temperature range from which quenching produces  better mechanical properties when combined with low temperature tempering.  3.0 MATERIALS AND METHODS  3.1 Materials: For the experimental work, hot-rolled steel rod of 16mm (5/8 inch) was used. The chemical analysis of the steel is shown in table 1.  Table 1: Chemical analysis of the steel used (wt %) with its critical temperatures (calculated). C Mn Cr Si Al P Ac1  AC3 0.15 0.32 0.03 0.14 0.04 0.004 728 810 The mechanical properties of the original as hot-rolled steel used is given in table 2.  Table 2: Mechanical properties of steel used.  y(N/mm2) t(N/mm2) δ(%) H(BHN) an(J/cm) 276.06 385.29 37.63 177 78.08  3.2 Methods The samples used for the experimental work were machined from the hot-rolled steel rod. The steel samples prepared for quenching and those prepared for quenching and then tempering were intercritically heat treated at 750, 760, 770, 780 and 790C for 1hr in a laboratory muffle furnace and quenched to room temperature in SAE 40 engine oil. The steel samples prepared for quenching and then  tempering were further subjected to a low temperature tempering at 150C for 1 hr and air cooled to room temperature to improve ductility and notch impact toughness . Estimation of the critical temperatures AC 1 and AC3 was made using empirical equations  developed by Andrews [17-19].  After the heat treatment of the test samples, tensile tests were carried out at room temperature using 10 ton universal testing machine. Brinnel hardness testing method was used to determine hardness while the Charpy 34            P. O. OFFOR, C.C. DANIEL, D. O. N. OBIKWELU   NIGERIAN JOURNAL OF TECHNOLOGY, VOL. 29 NO 2, JUNE 2010 impact testing machine was used for the determination of the notch impact toughness.    4.0 RESULTS AND DISCUSSIONS The results of measurements made are tabulated in tables 2 and 3.  Table 3:  Mechanical properties of heat treated steel samples. TC y   qs N/mm2  y   ts N/mm2  t    qs N/mm2  t    ts N/mm2   qs % ts % H qs BHN H ts BHN an  qs J/cm2  an ts J/cm2  750 - 180.18 512.29 449.13 7.95 12.18 197 148 9.08  76.76 760 - 191.43 541.47 503.91 6.85 11.01 213 154 8.16  73.05 770 - 213.27 596.84 558.22 5.64 10.17 221 166 7.94 70.04 780 - 232.82 631.96 614.48 4.86 8.84 232 177  6.27  66.01 790 - 258.57 674.62 637.65 4.22 7.42 241 190  5.55  61.94   Figure 1. Strength and ductility – temperature relationships of heat treated steel samples.  Figure 1 shows the strength and ductility – temperature relationships of the steel samples prepared for quenching and those prepared for quenching and then tempering   intercritically heat treated at 750, 760, 770, 780 and 790C for 1hr in a laboratory muffle furnace and quenched to room temperature in SAE 40 engine oil. The steel samples prepared for quenching and then tempering were further subjected to a low temperature tempering at 150C for 1 hr and air cooled to room temperature. Table 3 and figure 1 reveals clearly that intercritical annealing temperature and low temperature tempering had a significant effect on the strength and ductility values. Steel samples quenched in oil had higher tensile strength values (from 512.24N/mm2 at 750C to 674.62N/mm2 at 790C) than those subjected to low temperature tempering after oil quenching (From 449.13N/mm2 at 750C to 637.65N/mm2 at 790C). Conversely, steel MECHANICAL PROPERTIES OF THE INTERCRITICALLY ANNEALED 0.15Wt%C- 0.32 Wt%Mn STEEL …            35   NIGERIAN JOURNAL OF TECHNOLOGY, VOL. 29 NO 2, JUNE 2010  samples tempered after oil quenching had higher ductility values (from 7.42% at 790C to 12.18% at 750C) than that quenched in oil but not tempered (from 4.22% at 790C to 7.95% at 750C). Also, the results show that higher intercritical annealing temperatures gave higher tensile strength values (from 512.29N/mm2 at 750C to 674.62N/mm2 at 790C) but lower ductility values (from 12.18% at 750C to 7.42% at 790C). The higher the annealing temperature selected in the intercritical temperature region the more austenite forms and transforms to martensite, but the less carbon content in this martensite [20]. The prediction of mechanical  properties of dual phase steels based on mixture-role calculation [13] should take into consideration not only the volume fraction of martensite but also the variation of its properties due to variation of i ts carbon content by varying the annealing temperature. It is pertinent to note that steel samples tempered after oil quenching had yield strength values unlike those quenched but were not subjected to low temperature tempering. It is also of note that the yield strength values increased with intercritical annealing temperatures (from 180.18N/mm2 at 750C to 258.57N/mm2 at 790C). The large influence of the intercritical annealing temperature on the volume fraction of new ferrite can also be explained in terms of the austenite hardenability. A reduction in the intercritical annealing temperature will result in a smaller fraction of intercritical austenite, with higher carbon content and therefore higher hardenability. In consequence, this will lead to a slower epitaxial ferrite reaction. As the epitaxial ferrite reaction will be diffusion controlled, a higher carbon concentration will require more time to deplete the transformation front diffusionally [21].  Intercritical heat treatment is the way to enhance low alloys (carbon less than 0.2%) steels to dual phase microstructure with superior strength – ductility combination. This thermal treatment involved heating the specimens in intercritical temperature range to obtain ferrite and austenite followed by quenching to obtain dual – phase (martensite – plus – ferrite) structure [22].   Figure 2. Hardness and notch impact toughness – temperature relationships of heat treated steel samples.   Figure 2 shows the hardness and notch impact toughness versus temperature relationships of the steel samples prepared for  quenching and those prepared for quenching and then tempering were intercritically heat treated at 750, 760, 770, 780 and 790o C for 1hr in a laboratory muffle furnace and quenched to room temperature in SAE 40 engine oil. The steel samples prepared for quenching and then tempering were further subjected to a low temperature tempering at 150o C for 1 hr and air cooled to room temperature. Table 3 and figure 2 indicates that intercritical annealing temperature and 36            P. O. OFFOR, C.C. DANIEL, D. O. N. OBIKWELU   NIGERIAN JOURNAL OF TECHNOLOGY, VOL. 29 NO 2, JUNE 2010 low temperature tempering had a significant  effect on the hardness and notch impact toughness values. Steel samples quenched in oil had higher hardness values (from 197 BHN at 750C to 241 BHN at 790C) than that subjected to tempering after oil quenching (from 148 BHN at 750C to 190 BHN at 790C). Contrarily, steel samples tempered after oil quenching had higher notch impact toughness values (from 61.91 J/cm2 at 790C to 76.76 J/cm2 at 750C) than that quenched in oil but not tempered (from 5.55 J/cm2 at 790C to 9.08 J/cm2 at 750C). Also, the results show that higher intercritical annealing temperatures gave higher hardness values (from 197 BHN at 750C to 241BHN at 790C) but lower notch impact toughness values (from 9.08 J/cm2 at 750C to 5.55 J/cm2 at 790C). By tempering process, the properties of quench steel could be modified to decrease hardness and increase ductility and impact strength gradually [1].  An increase in austenitizing temperature resulted in coarsening of the grain structure, increased dissolution of carbides, increased as quenched and tempered hardness capability, and decreased impact toughness [23].     5.0  CONCLUSIONS  Strength and hardness values increased with intercritical annealing temperatures   Ductility and notch impact toughness decreased with intercritical annealing temperatures   Tempered steel has yield strength values which increased with intercritical annealing temperatures. As quenched steel has  no yield strength values   Tempering gave rise to a decrease in tensile strength and hardness and an increase in ductility and notch impact toughness of the as quenched steel   Tempered steels presented the better compromise between strength, hardness ductility and notch impact toughness     REFERENCES   1. Bello, K.A., Hassan, S.B. and Abdulwahab, M., “Effects of Tempering on the microstructure and mechanical properties of low carbon, low alloy martensitic steel”, Journal of Applied sciences Research, 3 (12): 2007, 1719-1723. 2. Sinha, A.K., Ferrous Physical Metallurgy, Butterworth, London, 1989, pp: 56. 3. Bello, K.A., “Effect of Intercritical and Tempering heat treatment on the microstructure and mechanical properties of a high martensite dual – phase steel”, MSC, ABU, Zaria, 2007.  4. Lis, J., Lis, A.K. and Kolan, C., “Processing dual-phase microstructure”, 13th International scientific conference on Achievements in Mechanical and Materials Engineering”, Gliwice -Wisla, Poland, 2005, pp: 395 – 398. 5. Khamedi, R., Fallahi, A. and Zoghi, H., The Influence of Morphology and volume during tensile loading of dual-phase steels, International journal of recent trends in Engineering, vol.1, No 5,2009 pp. 30-34. 6. Bag, A., Ray, K.K. and Dwarakadasa, E.S., Influence of Mertensite content and morphology on tensile and impact properties of high – martensite dual – phase steels, Metalt and Mater. Trans. Vol. 30 (A), 1999, pp. 1193-1202 7. Balliger, N.K. and Gladman, T.,  Work hardening of dual phase steels, Metall Sci, vol 15, No. 3, 1981, pp. 95-108. 8. Fallahi, A., The effect of heat treatment on fatigue and tensile properties of Dual -phase steel, Amirkabi J. of Sci. and Tech., Vol.2, No.6, 1987, pp. 141-148. 9. Fallahi, A., PhD thesis, Sheffield University, 1990. 10. Sun, Sh. And Pugh, M., Properties of thermomechanically processed dual-phase steels containing fibrous martensite, Mater. Sci and Eng. A, Vol-333, pp 398-308, 2002. MECHANICAL PROPERTIES OF THE INTERCRITICALLY ANNEALED 0.15Wt%C- 0.32 Wt%Mn STEEL …            37   NIGERIAN JOURNAL OF TECHNOLOGY, VOL. 29 NO 2, JUNE 2010  11. Speich, G.R. and Miller, R.L., Mechanical properties of ferrite. Martensite steels. Structure and properties of dual-phase steels, Edited by Kot, R.A. and Morris, J.W., TMS-AIME, New York, , 1989, pp. 145-182. 12. Fallahi, A., Microstructure propert ies correlation of dual phase steels produce through rolling process, J. of Mater. Sci. and Tech. vol. 18, No 5, 2002, pp. 451-454. 13. Dobranszky, J., Quality improvement of low carbon reinforcing steel by Rapid Heat treatment, J. heat treating, Vol. 9, No. 1, 1991, pp. 63-67.  14. Bhadeshia, H. K. D.H. and Edmonds, D. V.,Analysis of mechanical properties and microstructure of high – silicon dual – phase steel, Journal of metal science, vol.10, 1980, pp. 41 – 42. 15.  Honey Combe, “microstructure and properties”, Adward Arnold, 1982. 16. Hulka, K., Dual – phase and trip steels, ASM, Metal park, Ohio, 2002, pp. 1 – 4. 17.   Offor, P.O. The Effect of Diverse Heat Treatment on the Mechanical Properties of Structural Steels, M.Eng. Thesis, ESUT Enugu, 1993.  18.   Leslie, W.C., the Physical Metallurgy of Steels. McGraw-Hill, International: New York, 1983, 219 -257.                  19.  Birgani, E.N. and Pouranvari, M., Effect of martensite volume fraction on the work hardening behavior of dual-phase steels, Hradec nad Moravici, 19-215. 2008, pp. 107. 20.  Speich, G. R., Tempering of Ferrite – Martensite Steels, p. 279-303 in Fundamentals of Dual-Phase    Steels, Conf. Proc. The Metal. Soc. Of AIME 1981, Ed. R.A. Kot and B.L. Bramfitt, Warrendale, Pa. 1981. 21 Garcia –Gonzalez, J.E., Fundamental study of the austenite formation and decomposition in high strength low –Si, Al added Nb –Mo trip steels, PhD thesis, University of Pittsburgh, 2005.  22 Htun, M.S., Kyaw, S.T. and Lwin, K.T., Effect of heat treatment on microstructures and mechanical properties of spring steel, Jour. Of Met., Mater and Min, vol. 18 No 2, 2008, pp. 191-197. 23 Arain (Ayooq), A., Heat treatment and toughness behavior of tool steels (D2 and H13) for cutting blades, M. AS. thesis, University of Toronto, 1999.  

Mechanical Properties of the Intercritically Annealed 0.15wt%C- 0.32 Wt%Mn Steel and Quenched in SAE Engine Oil at Room Temperature

Effects of various quenching media on the mechanical properties of intercritically annealed 0.15wt%C – 0.43wt%Mn were studied. Prequenching of a hot rolled low carbon steel was previously done from 900oC (within the full austenitic range) using SAE 40 engine oil as quenchant. Sets of steel samples made from the previously quenched steel samples were intercritically heat treated from 750oC to 810oC at intervals of 10oC for 1 hr in a laboratory muffle furnace and quenched in SAE 40 engine oil, water and brine quenchants respectively. The effects of quenching media used and the intercritical annealing temperatures on tensile, hardness, ductility and notch impact toughness properties are discussed. The quenching media increased the strength and hardness properties but decreased the ductility and notch impact properties of the original hot-rolled steel. Steel quenched in brine had the highest strength (708.02N/mm2 at 810EC) and hardness values (233 BHN at 810EC) followed by those quenched in water (666.73 N/mm2 at 810EC and 226 BHN at 810EC respectively) while those quenched in oil had the least values (618.56 N/mm2 at 810EC and 215 BHN at 810EC respectively). Steel quenched in oil had highest ductility and notch impact toughness values (24.07% at 750EC and 22.8 J/cm2 at 750EC respectively), followed by those quenched in water (20.33% at 750EC and 18.14 J/cm2 at 750EC  respectively) while those quenched in brine had the least values (16.49% at 750EC and 13.96 J/cm2 at 750EC respectively). Higher intercritical annealing temperatures gave higher strength and hardness values (from 445.94 N/mm2 at 750EC to 708 N/mm2 at 810EC and from 165 BHN at 750EC to 233BHN at 810EC respectively), but lower ductility and notch impact toughness values (from 10.71% at 810EC to 16.49% at 750EC and from 7.38J/cm2 at 810EC to 13.96J/cm2 at 750EC)

 76 EFFECTS OF VARIOUS QUENCHING MEDIA ON THE MECHANICAL PROPERTIES OF INTERCRITICALLY ANNEALED 0.15Wt%C – 0.43Wt%Mn STEEL  P. O. Offor, C.C. Daniel, D. O. N. Obikwelu  Metallurgical and Materials Engineering Department, University of Nigeria, Nsukka. peterjoyoffor@yahoo.com   ABSTRACT  Effects of various quenching media on the mechanical properties of intercritically annealed 0.15wt%C – 0.43wt%Mn were studied. Prequenching of a hot rolled low carbon steel was previously done from 900oC (within the full austenitic range) using SAE 4 0 engine oil as quenchant. Sets of steel samples made from the previously quenched steel samples were intercritically heat treated from 750oC to 810oC at intervals of 10oC for 1 hr in a laboratory muffle furnace and quenched in SAE 40 engine oil, water and brine quenchants respectively. The effects of quenching media used and the intercritical annealing temperatures on tensile, hardness, ductility and notch impact toughness properties are discussed. The quenching media increased the strength and hardness properties but decreased the ductility and notch impact properties of the original hot-rolled steel. Steel quenched in brine had the highest strength (708.02N/mm2 at 810C) and hardness values (233 BHN at 810C) followed by those quenched in water (666.73 N/mm2 at 810C and 226 BHN at 810C respectively) while those quenched in oil had the least values (618.56 N/mm2 at 810C and 215 BHN at 810C respectively). Steel quenched in oil had highest ductility and notch impact toughness values (24.07% at 750C and 22.8 J/cm2 at 750C respectively), followed by those quenched in water (20.33% at 750C and 18.14 J/cm2 at 750C  respectively) while those quenched in brine had the least  values (16.49% at 750C and 13.96 J/cm2 at 750C respectively). Higher intercritical annealing temperatures gave higher strength and hardness values (from 445.94 N/mm2 at 750C to 708 N/mm2 at 810C and from 165 BHN at 750C to 233BHN at 810C respectively), but lower ductility and notch impact toughness values (from 10.71% at 810C to 16.49% at 750C and from 7.38J/cm2 at 810C to 13.96J/cm2 at 750C).        Keywords: intercritical, mechanical properties, temperature, quenching media.  Symbol notation:  σy  = yield strength, σ t = tensile strength,  = ductility,  H = Hardness, BHN = Brinnel hardness number, an = notch impact toughness.   1.0 INTRODUCTION  Applications of steels for engineering components require a complete understanding of material properties and design requirements. Through the last few decades a category of steels known as high strength steels have undergone constant research [1, 2]. As a result, quenched and tempered microalloyed steels are most likely candidate materials for the next generation of high – strength steel sheets. For a given alloy content, quenched and tempered microalloyed steel exhibit good combination of strength and toughness [1-6].  Traditionally, quenched and tempered steel sheets are employed in automotive industry in the areas of structural members, power transmission and impact resistance systems. With the advent of dual – phase (DP) heat treatment, the possibility of introducing dual – phase treated sheets is becoming attractive proposition in those areas. Dual – EFFECTS OF VARIOUS QUENCHING MEDIA ON THE MECHANICAL PROPERTIES OF …            77   NIGERIAN JOURNAL OF TECHNOLOGY, VOL. 29 NO 2, JUNE 2010 phase microalloyed steel consists of martensitic islands in a ductile ferrite matrix [1, 7, 8]. Their potential as superior strength and formability substitutes for current automotive steels was recognized and has provided an incentive for their rapid development and acceptance in this role [9].  It is necessary for an engineer engaged in testing work to have a general understanding of the common methods of testing properties of the metals. Mechanical properties are the most important requirements of the metals from the engineering point of view in selecting them for design purposes. Mechanical properties and microstructures of metals describe their behaviour under mechanical and physical usage [10 – 13]. The required mechanical properties can be altered by method of manufacturing process and heat treatment [14].  2.0 OBJECTIVES OF THE STUDY   The objectives of the present work were to investigate the effects of quenching media and intercritical annealing temperatures on the mechanical properties of prequenched 0.15wt% C - 0.43wt%Mn steel.   3.0  MATERIALS AND METHODS   3.1  Materials  The experimental material  was a prequenched low carbon steel rod with a diameter of 16mm. The chemical composition (wt %) and the Critical temperatures of the steel materials are shown in table 1.  Table 1. The chemical composition (wt %) of the steel used, with its critical temperatures (calculated)  C  Mn  Si  Cr  Ti  P  AC1 AC3 0.15 0.43 0.37 0.17 0.02 0.001 732 839  The mechanical properties of the steel in its original state and after prequenching from 900oC are shown in table 2. Table 2. The mechanical properties of the steel in its original state and after prequenching from 900oC  Mechanical  properties  σy  σt    H  an - N/mm2 N/mm2 % BHN J/cm2 Original  242.13 426.28 36.58 141 72.64 After prequench  - 570.00 13.56 225 24.32  3.2  M ethods  Test  samples  were  p repared  fro m previo usly p requenched hea t - t rea ted  s tee l ,  grouped into  se ts  and  c lear ly  marked .  The prequenched  hea t - t rea ted  s tee l  was  produced  by hea t ing the  or igina l  as ho t -ro l led  16 mm diameter  s tee l  rods to  900C in a  labora tory muff le  furnace  and quenched to  roo m tempera ture  in  SAE 40  engine  o i l .  Each  se t  o f  s tee l  samples  was  inte rcr i t ica l ly hea t  t rea ted  fro m 750C to 810C at  inte rva l s  o f  10C for  1  ho ur  in  a  labora tory muff le  furnace  and  quenched  to  roo m te mpera ture  in  a  p repared  quenchant .  Three quenchants  were used  SAE 40 engine  o i l ,  water  and  br ine .  The  cr i t ica l  tempera tures  AC 1  and AC 3 ,  which  def ine  the  fe r r i te  + aust eni te  regio n,  were  ca lcula ted us ing  empir ica l  equat ions  developed b y Andrews [ 15 ,  16 ] .  Propert ies  invest iga ted  were  the  yie ld  and tens i le  s t rengths ,  no tch impact  toughness  and  hard ness  proper t ies .  A 10  ton universa l  tes t ing mach ine  was used  for  tens i le  tes t ing,  Cha rp y imp act  tes t ing  machine  fo r  no tch impact  toughness  tes t ing and  the  Br in nel  hardness  method  for  hardness  tes t ing.   78               P. O. OFFOR, C.C. DANIEL, D. O. N. OBIKWELU   NIGERIAN JOURNAL OF TECHNOLOGY, VOL. 29 NO 2, JUNE 2010 4.0   RESULTS AND DISCUSSIONS  The  resul t s  o f  the  var ious  measurements  a re  tabula ted  in  tab les  2 ,  3  and  4 .  Table 3. Mechanical properties of sets of steel samples quenched in oil, water and brine after intercritical annealing heat treatment   T σy oil N/mm2 σy water N/mm2 σy brine N/mm2 σt oil N/mm2 σt water N/mm2 σt brine N/mm2 δ oil % δ water % δ brine % 750C - - - 410.74 438.17 445.94 24.07 20.33 16.49 760C - - - 435.79 461.32 470.01 23.82 19.88 15.25 770C - - - 466.83 500.04 512.31 22.48 18.28 14.69 780C - - - 502.16 533.59 555.08 21.4 17.13 13.74 790C - - - 535.91 580.72 610.49 20.64 16.98 12.08 800C - - - 568.2 613.61 647.88 19.76 15.37 11.87 810C - - - 618.56 666.73 708.02 18.04 14.22 10.71  Table 4. Mechanical properties of sets of steel samples quenched in oil, water and brine after intercritical annealing heat treatment.  T H oil BHN H water BHN H brine BHN an oil J/cm2 an water J/cm2 an brine J/cm2 750C 150 157 165 23.41 18.14 13.96 760C 160 165 174 22.81 17.85 12.04 770C 170 175 183 21.43 16.33 11.36 780C 180 185 198 20.76 14.36 9.75 790C 190 200 210 19.16 13.14 9.09 800C 204 215 222 17.23 11.67 8.47 810C 215 226 233 16.33 10.04 7.38   Figure 1. Strength and ductility – temperature relationships of sets  of steel samples quenched in oil, water and brine.  Tables 2, 3 and 4 show that quenching in different media eliminated the yielding point , increased the tensile strength and hardness properties but decreased the ductility and σt oil σt water σt brine EFFECTS OF VARIOUS QUENCHING MEDIA ON THE MECHANICAL PROPERTIES OF …            79   NIGERIAN JOURNAL OF TECHNOLOGY, VOL. 29 NO 2, JUNE 2010 notch impact toughness properties of the original hot-rolled steel sample.  Figure (1) shows the strength and ductility – temperature relationship of the three groups of steel samples intercritically heat treated from 750C to 810C at intervals of 10C quenched in oil, water and brine.  Table (3) and figure (1) clearly show that intercritical annealing temperatures and the quenching media had significant effect on the strength and ductility properties.   Sets of steel samples quenched in brine had the highest strength values (708.02 N/mm2 at 810C) followed by those quenched in water (666.73N/mm2 at 810C) while those quenched in oil had the least strength values (618.56N/mm2 at 810C). Reversely, sets of steel samples quenched in oil had the highest ductility values (24.07% at 750C) followed by those quenched in water  (20.33% at 750C) while those quenched in brine had the least ductility values (16.49% at 750C). Generally, higher intercritical annealing temperature gave higher strength (from 445.94 N/mm2 at 750C to 708.02N/mm2 at 810C) but lower ductility values (from 10.71% at 810C to 16.49% at 750C). The mechanical properties improvement especially strength is (in compliance with Hall – Petch equation [13]) a function of the grain size of the transient phase (austenite), which depends on the heating and transforming rate and a function of the quality of the final structure, which depends on the cooling rate [17].   Figure 2. Hardness and notch impact toughnesss – temperature relationships of sets of steel samples quenched in oil, water and brine.  Figure (2) shows the hardness and notch impact toughness temperature relationships of the three groups of steel samples intercritically heat treated from 750C to 810C at intervals of 10C and quenched appropriately in oil, water and brine.  Table (4) and figure (2) clearly show that  intercritical annealing temperatures and the quenching media had significant effect on the hardness and notch impact toughness properties. Sets of steel samples quenched in  brine had the highest hardness values (233 BHN at 810C) followed by those quenched in water (226 BHN at 810C) while those quenched in oil had the least hardness values (215 BHN at 810C). Reversely, sets of steel samples quenched in oil had the highest notch impact toughness values (23.41 J/cm2 at 750C) followed by those quenched in water, (18.14 J/cm2 at 750C) while those quenched an oil an water an brine 80               P. O. OFFOR, C.C. DANIEL, D. O. N. OBIKWELU   NIGERIAN JOURNAL OF TECHNOLOGY, VOL. 29 NO 2, JUNE 2010 in brine had the least notch impact toughness  values (13.96 J/cm2 at 750oC). A close observation showed that higher intercritical annealing temperatures gave higher hardness  values (from 174 BHN at 750C to 233 BHN at 810C) but lower notch impact toughness  values (from 7.38 J/cm2 at 810C to 13.96 J/cm2 at 750C).  As the hardness decreased with increasing temperature, the toughness increased with corresponding increasing temperature in the corresponding quenching media thus indicating a mirror behavior by both properties. This is in line with an earlier report in literature [18] that hardness and toughness exhibit mirror behaviour, if the hardness curve is decreasing – increasing the toughness curve shows an increasing – decreasing pattern [18]  The contributions of hardening mechanisms in the martensitic structure according to [19, 20] include the solid solution hardening, the precipitation hardening, the primary austenitic grain size hardening and the martensite morphology hardening. The dominant hardening effect of martensite in dual phase steels is the carbon concentration in martensite [20].   5.0 CONCLUSIONS  The quenching heat treatments increased the strength and hardness values but decreased the ductility and notch impact values of the original hot-rolled steel.   The yield points of the quenched steel were eliminated.  Steel quenched in brine had the highest strength and hardness, values followed by those quenched in water while those quenched in oil had the least values.   Steel quenched in oil had highest ductility and notch impact toughness values, followed by those quenched in water while those quenched in brine had the least.   Higher intercritical annealing temperatures gave higher strength and hardness values but lower ductility and notch impact toughness values.         REFERENCES: 1. 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Effects of Various Quenching Media on the Mechanical Properties of Intercritically Annealed 0.15wt%C – 0.43wt%Mn Steel

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Mechanical Properties of the Intercritically Annealed 0.15wt%C- 0.32 Wt%Mn Steel and Quenched in SAE Engine Oil at Room Temperature

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