Research articlesThe effect of heat treatment parameters on the microstructure and torque response of a 13 wt% Cr steel
Introduction
Magnetomechanical sensors find a variety of applications, especially in the automotive and transportation sector [1]. In these applications, the shaft being monitored is permanently magnetized and sensing devices such as hall effect sensors or fluxgates, are used to measure the stress being applied [1], [2]. Magnetomechanical sensors require materials with a good combination of magnetic and mechanical properties [3], [4], [5], [6], [7].
In the case of torque-sensing applications, both the shape of the B-H curve and the change in voltage response of a magnetized component to an applied torque (sensitivity) are of interest. Boley et al. [3] compared the sensitivity of a high-performance low alloy steel, before and after quenching and tempering. The heat-treated material was found to have a higher sensitivity. Another publication by the same authors compared the effect of different steels having different chromium contents and heat treated as per manufacturer specification, on the B-H curve and sensitivity [8]. The authors claimed that the 12 wt% Cr alloy offered superior sensitivity and attributed this performance to the lower axial coercivity of this material. On the other hand, the circumferential coercivity of all the alloys tested was found to be similar. In general, studies investigating the magnetic response to torque, have focused mainly on comparing the B-H curves of different alloys, but have ignored the three-way correlation between heat treatment, microstructure, and torque-response.
On the other hand, various other authors have investigated the relationship between heat treatment parameters, microstructure and the B-H curve, but ignored the torque-response. Selecting the heat treatment parameters that yield the target microstructure as well as specific magnetic properties may be challenging. Heat treatments such as quenching and tempering involve parameters such as austenitising and tempering temperatures, heating and cooling rates and holding times, all of which may influence the microstructure and corresponding mechanical and magnetic properties. This has in fact been the subject of various studies, some of which are summarized below [9], [10], [11], [12], [13].
Studies on the relationship between microstructure and the B-H curve often focus on the coercivity and/or the saturation flux density. With regards to coercivity, there is general agreement that domain walls are pinned by irregularities in the structure such as grain boundaries, inclusions, precipitates and dislocations [14]. According to Neel’s theory, pinning may occur as long as the magnetic properties of the inclusions/precipitates are different from that of the matrix structure [15]. Although authors generally agree that the grain boundaries pin domain wall motion, yet there is disagreement concerning the extent to which this affects the magnetic properties. Yaji et al. [11] and Tanner et al. [12] agree that in plain carbon steels, the effect of parent austenite grain size is less evident when the percentage pearlite exceeds 17 vol%. They claim that in this case the properties are dominated by the pearlitic matrix and high coercivity is experienced. The same authors [11] claim that in hardened and tempered plain-carbon steels, the effect of the parent austenite grain size is negligible. In addition, Won Byeon & In Kwun [16] claim that a martensitic structure yields a higher coercivity than either a pearlitic or a ferritic structure.
On the other hand, precipitates, are also claimed to have an effect on the magnetic properties [9], [10], [17], [18], [19]. In steels various types of precipitates may be found, depending on the chemical composition of the material. For example, Pandey et al. [20] claimed the presence of M23C6, M7C3, M3C, and MX (where M refers to metals such as Cr and Fe, whilst X may refer to N, amongst others) in P91 steel. These [21] and other authors [22], [23] have argued that the austenitising temperature influences the quantity of undissolved carbides, which in turn also affects the prior austenite grain size and matrix hardness. Furthermore, the tempering process also leads to various carbide transformations [24].
Tavares et al. [9] studied the effect of tempering temperature on a martensitic stainless steel and claimed that the drop in the coercivity, observed when tempering between 400 °C and 500 °C, occurred prior to the martensite to ferritic transformation. The authors attribute this to the coarsening of the needle-like carbides. Kameda & Ranjan made similar observations [10]. Several authors suggest that when the carbide quantity is significant, the domain pinning is controlled by the morphology, size, type and distribution of these carbides [25], [26], [17], [18], [19]. However, one must keep in mind that during heat treatment, changes in carbide type, size, and morphology, occur concurrently with changes in grain size, hardness and others. It is therefore difficult to establish which microstructural feature or combination of these, is responsible for the observed changes in magnetic properties.
The saturation flux density is said to be controlled by the chemical composition of the material [14]. Jiles [14] & Tavares et al. [9] claim that the carbon and the chromium content in solution within the iron matrix, have a significant effect on this property. The dissolution of carbides during austenitising, the super saturation of carbon within the martensite, as well as the precipitation of new carbides during tempering change the carbon content of the matrix, thus influencing the saturation flux density. These occurrences depend on heat treatment parameters such as austenitising and tempering temperature and time.
In conclusion, to the authors’ knowledge, thus far, researchers have studied the effect of heat treatment on two of the following; microstructure, B-H curve and magnetic response to torque, but never all three properties together. This paper, aims at achieving this using a 13 wt% Cr martensitic stainless steel, a material with an attractive combination of magnetic, mechanical and corrosion properties/characteristics, rendering it suitable for a vast variety of applications involving stress.
Section snippets
Material & heat treatments
The material studied is a martensitic stainless-steel type X46Cr13 (AISI 420C). It was supplied in the annealed state having a composition shown in Table 1 and a microstructure consisting of a ferritic matrix and a distribution of M23C6 carbides, as shown in Fig. 1.
As shown in Table 2, the effect of austenitising temperature was studied using samples austenitized for 45 min at 980 °C, 1010 °C and 1040 °C. These samples were snap tempered at 150 °C for 1 h and then double tempered for 2 h at
The effect of austenitising
During austenitising, the ferritic matrix changes to austenite, which upon quenching transforms to martensite. The hardness of the latter depends on the austenitising temperature as this controls the carbide dissolution [22], [30]. In fact, as the austenitising temperature increased from 980 °C to 1040 °C the matrix hardness of the material increased from 51.8 HRc ± 0.5 HRc to 54.3 HRc ± 0.4 HRc. The increase in carbide dissolution is also evident from Fig. 5, which shows structures following
Conclusion
This study focused on X46Cr13 and investigated the effect of austenitising and tempering parameters on the microstructure, the circumferential and axial B-H curves, and the response of magnetized samples to applied torque (sensitivity) . The relationship between them was also investigated. Findings may be summarized as follows:
- 1.
The axial and circumferential coercivity of annealed samples increased following quenching and tempering.
- 2.
The major changes in B-H characteristic following tempering
CRediT authorship contribution statement
Eleanor Saliba: Conceptualization, Methodology, Investigation, Validation, Formal analysis, Writing - original draft. Maurice Grech: Conceptualization, Resources, Writing - review & editing, Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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