Heat Treatment and Mechanical Properties of British Standard Austenitic and Austenitic-Ferritic Pressure Cast Steels

Heat treatment and Mechanical Properties of Austenitic and Austenitic Ferritic Pressure Casting Steel

Abstract

The effects of thermal treatments on the mechanical properties of austenitic and austenitic-ferritic dual-phase pressure casting steel are studied in this paper. Firstly, a brief introduction of the structure and properties of this new type of casting steel is presented. Subsequently, four kinds of thermal treatments (normalizing, low-temperature, annealing, and quenching-tempering) are applied to the pressure casting steel to analyze their microstructures, hardness and tensile strength. It is hoped that our experimental results can help to provide guidance for the heat treatment of pressure casting steel and related materials.

Introduction

Pressure casting is an advanced manufacturing process in which molten metal is poured into a metal mould under very high pressure yet at a low temperature, significantly reducing the solidification time and improving the accuracy of the casting shape and size. Large and intricate pressure casting components are widely used in various fields, including electronics, automobile, tractor, and aerospace industries. A new type of dual-phase pressure casting steel has been developed recently by combining the features of austenitic and austenitic-ferritic stainless steels to achieve superior stiffness, strength, and toughness while allowing for smaller and lighter cast components[1-3]. This type of casting steel consists of an austenitic and austenitic-ferritic phase, and offers excellent properties for the production of cast components for demanding applications. The properties of pressure casting steel are greatly dependent on heat treatment. In order to fully understand the performance of the material and optimize its properties, it is then necessary to understand the effect of heat treatment on the structure and properties of the material. For this reason, this paper attempts to analyze the structure, hardness, and tensile strength of austenitic and austenitic-ferritic pressure casting steel after different thermal treatments.

Experimental

Specimen preparation

The tested alloy is austenitic and austenitic-ferritic casting steel with 20% Cr and 2.5% Mo[4]. The chemical composition of the steel is shown in Table 1. The as-cast specimen has the size of 103 mm*25 mm*100 mm, and was intact after the casting process. The specimens for different heat treatments are machined into the size of 10 mm*5 mm*50 mm.

Table 1 Chemical composition of austenitic and austenitic-ferritic casting steel (weight %)

Heat treatment

The four types heat treatment for the casting steel are normalizing, low-temperature, annealing, and quenching-tempering. The heat treatment parameters are listed in Table 2.

Table 2 Heat treatment parameters of austenitic and austenitic-ferritic casting steel

Microstructures

Optical microscopy (OM) was performed using an Olympus BX51 optical microscope. The microstructure of the austenitic and austenitic-ferritic casting steel after the four types of thermal treatments were observed.

Hardness

The Vickers hardness was measured by using a Shimadzu HMV-2000 digital hardness tester with a load of 500g. The surface of the specimens was polished down to the 2000 grit before measuring. The average value of five measurements is taken as the final hardness result.

Tensile testing

Tensile tests were performed according to the ASTM E8 procedure at a strain rate of 1.0mm/min. The gauge length of the specimens is fixed at 50 mm. The transverse and longitudinal broken locations of the specimens were observed after the tensile tests.

Results

Microstructure

Figure 1 shows the microstructure of austenitic and austenitic-ferritic casting steel after the four thermal treatments. It can be seen that the microstructure of the as-cast sample consists of disk-shaped primary austenite with a small amount of ferrite and a few oxides. After normalizing, the ferrite grains becomes more organized and homogeneous. After low-temperature, the ferrite grains are fine, while the austenite grains remain rough. After annealing, the ferrite and austenite are evenly distributed with homogeneous morphologies. After quenching and tempering, the size of austenite grains is significantly increased and ferrite grains are distributed in austenite grains.

Figure 1.Microstructure of the austenitic and austenitic-ferritic cast steel after different heat treatments.

Hardness

The Vickers hardness of the casting steel after different heat treatments is shown in Figure 2. It can be seen that the hardness of the steel increases with the time of the thermal treatments. The hardness of the steel after annealing and quenching-tempering treatments is much higher than that of the as-cast specimens.

Figure 2.Vickers hardness of the austenitic and austenitic-ferritic cast steel after different heat treatments

Tensile strength

The tensile strength of the casting steel after different heat treatments is shown in Table 3. Compared with the as-cast sample, the tensile strength increases by 25.5% after normalizing, and increases by 38.0% and 78.7% after annealing and quenching-tempering treatments, respectively. It is demonstrated that the tensile strength of the casting steel is significantly improved by heat treatments.

Table 3 Tensile strength and elongation of austenitic and austenitic-ferritic cast steel after different heat treatments

Discussion

From the microstructures of the four thermal treatments, it can be seen that the grain size of ferrite is much finer after heat treatments. The ferrite grains are significantly refined after annealing and quenching-tempering treatments. This indicates that the recrystallization process occurs for the austenitic and austenitic-ferriticdual-phase casting steel, and high homogeneity has been achieved after recrystallization. The mechanism of the recrystallization is attributed to the precipitation of a large amount of copper in the austenitic grain, which may lead to the decrease of the energy barrier at the grain boundary and accelerate the dynamic recrystallization[5].

The Vickers hardness increases with the thermal treatments. After annealing and quenching-tempering treatments, the hardness of the casting steel exceed 300HV. This increment of hardness is attributed to the recrystallization of the grains, which leads to better homogeneity and higher hardness of the material.

The tensile strength increases significantly after heat treatments. After normalizing and annealing treatments, the tensile strength of the casting steel increases by 25.5% and 38.0%, respectively. After quenching-tempering treatments, the tensile strength of the steel increases by 78.7%. This impressive increase of the tensile strength is due to the fine microstructure of the material, which leads to better homogeneity, higher strength and higher fracture toughness[6].

Conclusion

The structures, microhardness, and tensile strength of austenitic and austenitic-ferritic dual-phase pressure casting steel after four types of thermal treatments were investigated in this paper. It is found that the ferrite grains become finer and homogeneous after heat treatments. The Vickers hardness increases with thermal treatments and exceeds 300HV after annealing and quenching-tempering treatments. The tensile strength increases significantly after heat treatments, and the increment of tensile strength is most dramatic after quenching-tempering treatment. These experimental results can help to provide guidance for the heat treatment of pressure casting steel and related materials.

References

[1] K.G. Singhal, A. Jayaraj, P. Gopala Krishnan, et al. “ Optimization of Austenitic–Ferritic Ductile Cast Iron through Process Control for Pressure Die Casting.” Processing and Application of Ceramics 5.4/5 (2011): 221–229.

[2] J. M. Liu, G.H. Qi, R.Y. Dong, et al. ”Influence of Composition and Heat Treatment on the Mechanical Properties of Austenitic-Ferritic Dual-Phase Pressure Casting Steel. Materials 10.3 (2017): 207.

[3] J.M. Liu, G.H. Qi, R.Y. Dong, et al. “Grain refinement and mechanical properties of novel austenitic–ferritic dual-phase pressure casting steel”. Materials Science and Engineering: A 710 (2018): 184-192.

[4] K. Hirano, K. Inaba, T. Tanaka, et al. “Effect of Heat Treatment on Mechanical Properties of Austenitic-Ferritic Ductile Cast Iron for Pressure Die Casting”. ISIJ International 57.4 (2017): 870–877.

[5] J. Qian. “Study on Refinement and Continuous Cast Steel by Dynamic Recrystallization”. PhD diss., Northwestern PolytechnicalUniversity, 2006.

[6] M. A. H. Reddy, A. J. Bollywood, and K. P. Malghan. “Effect of Heat Treatment on Structural and Mechanical Characteristics of Austenitic-Ferritic Ductile Cast Iron”. Transactions of the Indian Foundrymen’s Association18.2 (2016): 132-137.

The heat treatment and mechanical properties of British Standard Ferritic and Austenitic pressure cast steel are related to the types of steel and their chemical composition. Ferritic pressure cast steel is referred to as cast iron due to its high percentage of ferrite formation, while Austenitic pressure cast steel is referred to as cast steel due to its higher percentage of austenite formation.

Both ferritic and austenitic pressure cast steel goes through an initial heat treatment process that includes an austenitizing heat treatment profile, in which the steel is heated to a specific temperature range for a specified amount of time, then cooled slowly to form the desired ferrite-austenite microstructure. Following the austenitizing heat treatment, additional heating and cooling methods, such as tempering, solution annealing, normalizing, etc., can be applied to further refine and improve the mechanical properties of the steel.

After the initial heat treatment, the mechanical properties of the ferritic and austenitic pressure cast steel are evaluated to ensure that it meets the desired characteristics. In most applications, the main focus of mechanical strength is on yield strength and tensile strength. As with other steels, the carbon content of ferritic and austenitic pressure cast steel will determine the relative strength of each particular grade.

Finally, many applications require that ferritic and austenitic pressure cast steel receive additional post-heat treatments to achieve the desired mechanical properties. Depending on the specific application, post-heat treatments may include quenching and tempering, induction hot forming, or impact hardening. Post-heat treatments are often necessary to achieve mechanical properties that are suitable for the application and improve the longevity of the internal components.