Analysis of the precipitation behaviour in a high-speed steel by means of small-angle neutron scattering
Introduction
During the various stages of the heat treatment of high-speed steels different precipitation reactions occur. The microstructure of high-speed steels in the as-quenched stage consists of blocky primary carbides in a martensitic matrix. A small volume fraction of proeutectoid and/or autotempered carbides is also likely to be present because of precipitation during quenching. Subsequent triple tempering leads to precipitation of fine secondary hardening carbides which strengthen the martensitic matrix. During tempering (aging) the proeutectoid and autotempered carbides coarsen or dissolve. Depending on steel composition and tempering temperature the secondary hardening carbides are of the types MC and/or M2C in various volume fractions [1], [2], [3]. The secondary hardening carbides precipitate as small disks or needles obeying defined orientations with respect to the martensitic matrix [4]. The effect of overaging results in the precipitation of M6C and M23C6 at martensite plate boundaries and prior austenite grain boundaries. This reaction occurs on the expense of the secondary hardening carbides [3].
Extensive studies concerning the tempering and overaging behaviour of high-speed steels have been conducted [1], [2], [3], [5]; however, the knowledge is still limited. The smallness of the carbide precipitates makes it difficult to characterise them even by direct microscopy, like transmission electron microscopy (TEM) or atom probe field ion microscopy (APFIM). Moreover, both methods are restricted to a relatively small sample volume and, therefore, it is very time consuming to obtain representative size distributions by TEM or APFIM.
As modelling of precipitation reactions as well as mechanical properties requires information on type, volume fraction and size distributions of the precipitates, complementary methods have to be applied. Small-angle neutron scattering (SANS) is a suitable technique for characterisation of precipitation reactions especially at the early stages when precipitates are too small for TEM as it was shown, e.g., for maraging steels [6]. Due to the ferromagnetic nature of the martensitic matrix, additional information with regard to the chemical composition of the precipitates can be gained from the ratio of the magnetic and nuclear small-angle scattering intensity.
In this work, SANS was used to study the precipitation reactions in the steel grade HS6-5-2 during isothermal tempering at 590 °C. In order to exclude possible influences of cooling and heating between the tempering steps only single tempering was carried out. The SANS results are compared with results derived from energy-filtered transmission electron microscopy (EFTEM).
Section snippets
Sample preparation
The nominal chemical composition of the high-speed steel grade HS6-5-2 is listed in Table 1. Samples with a diameter of 25 mm and a thickness of 1 mm were used for SANS investigations. All samples were austenitised at 1210 °C for 10 min and subsequently quenched in oil. Afterwards, single tempering was carried out at 590 °C for 10, 20, 60, 180 and 540 min.
SANS
SANS was employed to analyse the evolution of the size distribution of precipitates during tempering. Basics of the SANS technique can be found
Particle size distributions obtained from SANS
Different types of particles are expected to be the dominant source of SANS intensity in this type of steel: primary carbides that are stable at the tempering temperature, proeutectoid carbides that can dissolve or coarsen at tempering temperature and secondary hardening carbides that precipitate from the supersaturated matrix during tempering [1]. Consequently, changes in the SANS scattering curves during tempering are discussed in terms of changes in the size distribution of particles.
The
Particle size distributions
From the scattering curves shown in Fig. 2, it is obvious that different changes occur simultaneously in the material: Firstly, the intensity increases at large q, corresponding to a growth of small particles, and secondly, the intensity decreases at small q, corresponding to a reduction of the number of large particles. This behaviour is reflected in the corresponding size distributions (Fig. 3). Consequently, due to the complexity and the precipitation behaviour of the investigated steel no
Conclusions
The evolution of precipitates during tempering in high-speed steel HS6-5-2 was investigated using SANS and EFTEM. The results have been discussed assuming that during tempering at 590 °C three different populations of precipitates with different sizes are present. In the first size range (particles <1 nm) a large number of small precipitates with radii around 0.7 nm are already formed after 10 min of tempering. From the obtained A-value, it is speculated that these extremely small precipitates are
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