Effect of Intermediate Quenching and Tempering on the Mechanical Behaviour of Low Carbon Steel

This research has assessed the impact of intercritical annealing using the intermediate quenching technique and tempering on the mechanical properties of low carbon steel. The procedure involved austenitizing at 850OC for 1hour followed by quenching in water and thereafter annealed at 730OC, 750OC and 770OC (i.e. α + γ region) for 30 minutes and then quenched in water again. Some of the as-quenched samples were tempered at 320OC for 1hour and cooled in still air. Tensile, hardness and impact tests as well as microstructure characterization were conducted for samples from all the heat treatment schedules. It was observed from the results that martensite volume fraction increases almost linearly as a function of temperature. Ductility and impact decreased with increase in temperature. Tempering deteriorated the assessed mechanical properties. Hence, for a steel of this composition, intermediate quenching should not be followed by tempering.


INTRODUCTION
The microstructure of steels can be altered by changing processing parameters, which ultimately affects their mechanical properties. These process parameters which can be altered include the base steel composition, mode of manufacture, type of heat treatment and parameters of heat treatment such as temperature, soaking time, heating and cooling rates, cooling media etc. Intermediate quenching is one of the various types of heat treatment for developing dual phase (DP) steels. Over the recent years, DP steels have been widely used in the automotive industries because of good compromise between its high strength and reasonable ductility which enhances performance and crash safety as well as high fuel economy due to weight reduction as a result of the improve strength. This weight reduction also impacts positively on the environment because of drastic reduction in emissions to the environment. Apart from the automotive industry, DP steels have found applications in oil and gas industries, building and structural industries, earth moving equipment (yellow goods) etc. Furthermore, very tall structures in the form of skyscrapers are becoming common these days due high demand on land. Hence, the urgent need to provide common materials with ultra-high strength at reasonable cost cannot be over emphasized. This will help to militate against the frequent occurrence of collapsed buildings in the country. Again, the high demand for large diameter and high strength pipes for the conveyance of crude oil and petroleum productions requires materials with excellent formability, high strength and good weldability. Hence, this research is intended to investigate the influence of intermediate quenching and tempering on the mechanical behaviour of low carbon steel. Intermediate quenching uses martensite microstructure as the starting or initial microstructure for the intercritical annealing heat treatment process. Bagetal have worked on intermediate quenching, step quenching and direct quenching using high strength low alloy (HSLA) steel. Ikpeseni etal had worked on step quenching and direct quenching using low carbon steel [17,18]. A good number of researchers have worked on the effect of processing parameters on the properties of dual phase steels with encouraging results [1 -20]. [1 -3] examined the effect of cooling rates; the effect of alloying element on mechanical properties was investigated by [4 -12]; while [13,16,17] worked on the effect of the temperature. Furthermore, [14,15,17] examined strain or deformation effect while [15, 18 -21] examined the impact of microstructure on mechanical properties of the investigated steels.

II. MATERIALS AND METHOD 2.1
Materials The carbon steel used for this research was supplied by universal steel Lagos, Nigeria. Its chemical composition shown in Table 1was determined as documented in [17].  Sample preparations Standard samples for tensile test, impact test, hardness test and microscopic examination were prepared from the asreceived 16mm diameter rod. All the samples were prepared following standard procedures.

2.2.2
Heat Treatments All the samples were first of all normalized in a muffle furnace at 850 0 C for 1hr in order to cancel the effects of previous mechanical, thermal or thermo-mechanical treatments. After normalizing some of the samples were left as control, while others were subjected to the intermediate quenching (an intercritical annealing) heat treatment. This involved austenitizing at 850 0 C for 1hr and quenching in water to produce martensite which was used as the starting or initial microstructure for the intercritical annealing. Thereafter, all the samples were intercritically annealed at various temperatures of 730 0 C, 750 0 C and 770 0 C (i.e. in α + γ region) for thirty minutes each, followed by quenching in water. Then some set of these sample were tempered at 320 0 C for 1hr, while the others are left in their intermediate quenched state.

2.2.3
Mechanical Properties Testing Tensile test: an instron Universal tensile testing machine was used to conduct the tensile test. The sample were tested at a cross head speed of 20mm/min and were all tested to fracture at room temperature (25 -27 0 C). All the tensile properties data were captured by the interfacing computer system. Impact test: The charpy impact tester (Avery) was used to determine the absorbed energy and thereafter the impact strength (toughness) of the heat treated samples were evaluated. Again, all the samples were tested to fracture at room temperature. Thereafter the fractured surfaces were examined under the scanning electron microscope (SEM) in order to ascertain the mode of fracture. Hardness test: The hardness property of samples from all the heat treatment schedules were examined using the Vickers hardness testing machine (LM 7 700AT Leco) with a dwell time of 10 -15S. The hardness values are digitally displayed on the machine screen.

2.2.4
Microstructure Characterization Nikon Eclipse (me 600) was used to examine the microstructures developed after the various heat treatment schedules. This was preceded by sample preparation using standard procedures. A combination of sylvert cloth and 0.2µ diamond paste was used to polish the samples while 2% NITAL was used as etchant. The standard grid point count technique was used to determine martensite volume fracture (MVF) as contained in Russ and Dehoff (1999) [22].  Fig. 1b, d). Martensite volume fraction is noticed to increase as a function of intercritical annealing temperature. This is so because austenite nucleation at different sites mentioned above continued and increases at higher temperatures. Hence the microstructures became greatly enriched with more martensite on quenching. It was clearly shown (qualitatively) that martensite grain size remained fairly the same. Pinning down of the grain boundaries by precipitated carbide particles in prior boundaries of austenite must have been responsible for this. Honeycombe and Bhadeshia [23] stated that these are usually present in grain boundaries; as such an interaction occurs between the grain boundary and the particles. They explained that when there is replacement of a short length grain boundaries by particles, the interfacial energy help to maintain a stable configuration such that any attempt for the grain boundary to move away or separate from the particles, there is an increase in local energy; as a result the particle exerts a drag on the boundary.  [19,20,24] As intercritical annealing temperature increases, the amount of austenite increased which transformed to martensite upon quenching. The simulated (fitted) linear curve for the treatment schedule is shown in equation (1)

International Journal of Advanced Engineering Research and Science (IJAERS)
[  Fig. 1c, while for IQ750T and IQ770T, the ferrite phase became well defined and coarse with plenty of carbide especially IQ770T as shown in Fig. 1e and g. Figures 3 -6 present the results of the mechanical properties which show the relationships between the mechanical properties and intercritical annealing temperatures.  /dx.doi.org/10.22161/ijaers.4.8.1  ISSN: 2349-6495(P) | 2456-1908(O) www.ijaers.com Page | 6  Fig. 6. The hardness of intermediate quenched samples increases steadily with rise in temperature within the investigation limit. The rise in hardness value is attributed to the increasing martensite volume fraction as intercritical annealing temperature increases (Fig. 1). The ultimate tensile strength of samples given the intermediate quenching intercritical heat treatment, increased steadily with increasing temperatureas shown in Fig. 3 (IQ series). The decrease in strain at fracture (i.e. ductility or total elongation) and impact strength could be traced to nucleation, growth and recrystallization of ferrite and austenite from the initial martensite structure, which upon quenching the nucleated austenite transforms to martensite. Consequently, strain at fracture and impact toughness of samples given this same treatment decreased as temperature increased ( Fig. 4 and 5). Continuous yielding was observed for all the samples given this particular treatment, which is a common characteristic of regular dual phase steels. Austenite to martensite transformation involves volume expansion which introduces  /dx.doi.org/10.22161/ijaers.4.8.1  ISSN: 2349-6495(P) | 2456-1908(O) www.ijaers.com Page | 7 residual stress on the surrounding ferrite as a result of the strain produced during the transformation [25,26,27]. Davis (1979) [28] and Rigsbeeet al (1979) [29] Fig.6 shows that tempering the as-quenched intermediately quenched sample at 320 O C for 1hr decreases hardness (IQT series). However, the hardness increased with temperature and reached a peak at 750 O C. The increased hardness with increase in martensite volume fraction can be attributed to the precipitation of carbide on temperingsee Fig. 1e and g. The decrease in value of hardness observed with tempered intermediate quenched (IQT) samples could be as a result of the coarsening of soft ferrite phase as shown in Fig. 1e and g respectively. This emanated from precipitation of more ferrite from martensite on tempering. All the other properties equally deteriorated on tempering at 320 O C for 1hour. Conventional stressstrain curve with discontinuous yielding was observed for IQ730T (i.e. sample intermediately quenched with 33% martensite volume fraction and tempered at 320 O C for an hour), while the others exhibited continuous yielding which is typical of conventional dual phase steel. Figure 7 shows the fractured surfaces of the failed impact test samples upon testing. Figures 7b, c and d present the fractured surfaces of IQ730, IQ730T and IQ750 samples respectively. They revealed predominantly dimple fibrous surface which is typical of materials with good combination of high strength, ductility and impact toughness. It showed that IQ730T has majorly dimple fibrous fractured surface with dislocations cutting across circular obstacles. On the other hand, Fig. 7e, f and g display the fractured surfaces of IQ750T, IQ770 and IQ770T respectively, which revealed majorly pure or quasi cleavage fracture, no wonder the low impact strength exhibited by these samples (Table 2).

International Journal of Advanced Engineering Research and Science (IJAERS)
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