Modification of oxide inclusions by addition of Ca

Modification of oxide inclusions by addition of Ca

Another direction for an improvement of mechanical machinability of steels is Ca-treatment for modification of oxide-based non-metallic inclusions (such as SiO2, Al2O3, Al2O3-MgO, etc.). The Ca-treatment can improve the characteristics of the formed calcium-based oxide inclusions (e.g., composition, morphology, size, and physical and chemical properties) as well as the lubrication effect between the cutting tool and steel piece. In this case, the main advantages of a Ca-treatment for oxide-based inclusions in the liquid steel can be summarized as follows:

(i) to form the globular CaO-SiO2-... or CaO-Al2O3-... inclusions;
(ii) to avoid the presence of SiO2 oxides, which have a high deformability at T > 1000 °C and which can increase the anisotropy of mechanical properties of steel after deformation;
(iii) to avoid a formation of Al2O3 and Al2O3-MgO clusters in the liquid steel and clogging problems during casting;
(iv) an application of relatively soft CaO-SiO2-… and CaO-Al2O3-… inclusions as natural lubricants for cutting tools during mechanical machining for improvements of the surface quality of machined steels and to increase the tool life (reducing the tool wear etc.).

Aluminum and silicon deoxidized steel grades have compositions of oxide inclusions within Zone I and Zone II of the ternary phase diagram, as is shown in Figure 1. Hard inclusions such as Al2O3, SiO2 and 3Al2O3·SiO2 will fracture during rolling and form hard fragments. This is detrimental for final mechanical properties and for the cutting tools during machining of such steel grades. In addition, Al2O3-based inclusions often cause nozzle clogging during casting. Calcium addition results in inclusion compositions moving in the direction of the arrows, towards Zones III and IV, respectively. Inclusions of Zones III and IV are softer and have lower melting temperatures (1400–1500 °C), a spherical shape, and better machinability properties. Thus, calcium aluminates form instead of Al2O3 inclusions in Al-deoxidized steels. In Si-deoxidized steels, mullite (Al6Si2O13) transforms into gehlenite (Ca2Al[AlSiO7]) or anorthite (CaAl2Si2O8).

Figure 1: Compositions of different oxide inclusions precipitated in aluminum (a) and silicon (b) deoxidized steel grades.

Bletton et al. studied the effect of Ca-additions on the composition of oxide inclusions and on the machinability of AISI 316L stainless steel after continuous casting and hot rolling. The main types of oxide and sulfide inclusions in experimental trials of AISI 316L steel are listed in Table 1. The typical compositions of oxide inclusions observed in experimental trials are shown in the CaO-SiO2-Al2O3 ternary phase diagram shown in Figure 2. The comparative flank and crater wear progressions obtained during machining of these steels are shown in Figure 23a,b. A cemented carbide cutting tool was used for a conventional turning test using the cutting speed 180 m/min, the feed rate 0.25 mm/rev and the depth of cut 1.5 mm in dry machining. It can be seen that the flank wear (FW) of the tool is approximately similar for all steels, during the initial 10 min of machining. However, the FW values for the Ca-treated steels decreased significantly at a machining time larger than 10 min. For instance, the FW of the Ca-treated steels was 39% (Steel 3) and 15% (Steel 2) smaller in comparison to the reference steel (Steel 1) after 25 min of machining.

Table 1: Types of main inclusions in experimental trials of AISI 316L stainless steel.

Figure 2: Typical compositions of oxide inclusions observed in experimental trials of AISI 316L stainless steel.

Reference: Niclas Anmark, Andrey Karasev and Pär Göran Jönsson, The Effect of Different Non-Metallic Inclusions on the Machinability of Steels, Materials 2015, 8, 751-783.