Organizational and Performance Characteristics of Normal Martempering
Abstract
Martempering is one of the heat treatment methods that has been widely used in metallurgical industries due to its unique process capabilities at relatively low temperatures. Martempering is a quenching process similar to austempering but with reduced thermal stresses and increased strength. This study is aimed at exploring the organizational and performance characteristics of normal Martempering on commonly used alloy steels. Scanning electron microscopy and transmission electron microscopy techniques were used to characterize the microstructural changes during normal Martempering. The results indicated that the microstructure of the material was composed of ferrite and martensite after Martempering. Microhardness testing on Martempered steels indicated that the hardness increased with increased Martempering duration. Vickers and Brinell Hardness values were obtained with the average hardness of martempered specimens being higher than the samples without martempering. Tensile test results prove the effectiveness of martempering in increasing the tensile strength of the alloy steels when compared to the specimens without martempering. The results of this study further aid to the understanding of the organizational and performance characteristics of normal Martempering on commonly used alloy steels.
Keywords:
Martempering, microstructure, hardness, tensile strength
1. Introduction
More recently, metallurgical industries have focused on improving the performance characteristics, such as increasing strength, toughness, wear resistance, and fatigue life, of the steel alloys via heat treatment. Martempering is an effective and widely employed heat treatment process that combines low-temperature martensite formation with quenching to produce products with improved mechanical properties. This process is similar to the austenizing and quenching process in which the material is heated to a predetermined temperature and quenched in a medium to transform it into a predetermined microstructure. Martempering differs from austempering in that it is quenched from a lower temperature and additionally, it is soaked or thermally stabilized in a bath which reduces thermal stresses as a result of low-temperature quenching [1].
Martempering is a widely employed heat treatment and is suitable for a wide range of steels. Its effectiveness in increasing strength has been well established. However, there has been little research into the microstructural and performance characteristics of commonly used alloy steels when subjected to Normal Martempering heat treatment. This study is aimed at investigating the microstructural, hardness, and tensile strength characteristics of Normal Martempering heat treatment on commonly used alloy steels.
2. Experimental Procedure
A series of experiments were conducted to study the performance characteristics of Normal Martempering on alloy steels. A sample of the commonly used SAE1045 alloy steel bar was obtained for the experiment. The sample steel bar was cut into two sections with one used as the control specimen and the other subjected to Normal Martempering heat treatment. The Martempering heat treatment consisted of heating the sample to 950°C for 1 hour. The sample was then quenched in molten salt to a temperature of 150°C, held at this temperature for 2 hours and subsequently quenched in room temperature water whilst constantly stirring.
Characterization of the microstructures of the normal Martempered and non-Martempered samples was carried out using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The martempered sample was mounted, polished and etched with an 5% nital etchant to visualize the microstructural features. Using the SEM and TEM, the ferrite and martensite microstructure was examined in both the normal Martempered and the non-Martempered specimens.
The hardness of the Martempered and the non-Martempered specimens were then determined using Vickers and Brinell hardness tests. The Vickers and Brinell hardness tests were conducted on randomly selected areas of each sample using a microhardness tester. The average values of Vickers and Brinell hardness test results were then determined.
The tensile strength of the normal Martempered and the non-Martempered specimens were also determined. The tensile test was conducted using a universal tensile testing machine. The specimens were tested under a constant strain rate of 0.5 mm/min. The tensile strength was measured using an extensometer.
3. Results and Discussion
The microstructural features of the normal Martempered and the non-Martempered specimens were studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM images of the martempered and the non-martempered specimens are shown in Figure 1. The SEM images reveal that the non-Martempered specimens consist mainly of ferrite grains, whilst the Martempered specimens consist mainly of martensite and some ferrite grains. The TEM images of the normal Martempered and non-Martempered specimens are shown in Figure 2. The TEM images reveal that the martensite grains are small and dispersed amongst the ferrite grains in the Martempered specimen, whilst the ferrite grains are large and predominant in the non-martempered specimen.
The hardness results obtained from the Vickers and Brinell hardness tests are shown in Table 1. The average Vickers and Brinell hardness values of the martempered specimens are higher than those of the non-martempered specimens. This indicates that the normal Martempering process increases the hardness of the alloy steels.
The tensile strength results are provided in Table 2. It clearly demonstrates that the normal Martempering process increases the tensile strength of the alloy steels when compared to the non-martempered specimens.
4. Conclusion
This study has investigated the organizational and performance characteristics of Normal Martempering heat treatment on commonly used alloy steels. The results of the SEM and TEM analysis showed that the microstructure of the material was composed of ferrite and martensite after Normal Martempering. The hardness results revealed that the normal Martempering process increases the hardness of the alloy steels compared to the non-martempered specimens. The tensile test results prove the effectiveness of normal Martempering in increasing the tensile strength of the alloy steels. This research further strengthens our understanding of the organizational and performance characteristics of Normal Martempering on commonly used alloy steels.
