2. Metal Fracture during Machining
2.1. Different Techniques of Mechanical Machining
- Act as stress raisers in the shear plane, which cause a crack formation. This, in turn, leads to embrittled chips that are easily broken. In addition, the length of the contact zone between the chip and the cutting tool is reduced. Thus, is advantageous for the tool wear resistance.
- Are active in the metal flow zone (see Figure 7, where fn is the feed direction) and contributes to shearing of the metal. However, an appropriate balance of inclusions is necessary to avoid an increased tool wear rate.
- Form a diffusion barrier, isolating the rake face from diffusion induced chemical tool wear at high temperatures.
- Act as lubricant which protects the flank face of a cutting tool from an abrasive wear.
2.2. Behavior of Non-Metallic Inclusions in the Cutting Zone
|Coefficient of Thermal Expansion||Group 1: αNMI < αsteel||Group 2: αNMI ~ αsteel||Group 3: αNMI > αsteel|
|Stress in NMI||No effect||Stress in steel matrix around NMI|
|Stress in steel matrix around NMI||No effect||Formation of cavity and pores around NMI|
|Operation||Si-Deoxidation||Al-Deoxidation||Ca-Treatment (Modification of Oxides)||Addition of S||Ca-Treatment (Modification of Sulfides)||REM-Addition (Modification of Sulfides)||Addition of Zr, Ti, V, Nb or B|
|Non-metallic inclusions (NMI)||SiO2, SiO2-MnO-...||Al2O3, Al2O3-MgO||CaO, CaO-Al2O3, CaO-Al2O3-... CaO-SiO2-…||MnS, (Mn,Fe)S||Ca(O,S), CaS, (Ca,Mn)S||REM-Ox, REM(O,S)x, REM-Sx||ZrN, Zr(N,C), TiN, Ti(N,C), BN, B(N,C), BC, etc.|
|Formation of NMI in steel||Partially in liquid steel. Partially during solidification of melt due to high content of soluble O (~60–80 ppm)||In liquid steel||In liquid steel||During solidification of melt (large size sulfides—mostly in final solidified zones)||In liquid steel||In liquid steel||Mostly during solidification of melt. Partially after solidification of melt|
|Size of NMI in cast metal||1–8 µm||0.1–8 µm||1–25 µm||0.5–30 µm||1–5 µm||1–3 µm||0.01–7 µm|
|Condition/ Morphology *||Liquid or solid/SP and RE||Solid/Mostly RE and IR||Liquid or solid/Mostly SP||Solid/Mostly RE and IR||Solid/Mostly SP||Solid/SP and RE/IR||Solid/Mostly RE and IR|
|Distribution in steel||Mostly homogeneous||Mostly homogeneous||Mostly homogeneous||Mostly in final solidified zones, S inhomogeneity||Mostly homogeneous||Mostly homogeneous||Mostly on grain boundaries|
|Cluster formation||No||Very easy. Size of clusters 10–1000 µm||No||Dendrite or coral shape sulfides (Type II and IV), 10–100 µm||No||REM-oxides. Size of clusters 10–300 µm||TiN-“clusters”. Size of clusters 5–30 µm|
|Hardness of NMI (kg/mm2)||Middle/Low (~1600)||High (~3000)||Middle/Low (850–1200)||Low||Middle/Low||High||High|
|Deformability of NMI||Low at T < 900 °C High at T > 1000 °C||No at T < 1300 °C Low at T ≥ 1500 °C||No at T < 1200 °C High at T ≥ 1300 °C||Very high at T < 1000 °C||No at T < 1200 °C Low at T ≥ 1300 °C||Very low||Very low|
|Thermal expansion, α (×10−6 1/°C)||Very low (0.5–5.0)||Low (8.0–8.6)||Low/middle (for CaO-Al2O3 5.0–10.0)||MnS—high (18.1)||CaS—high (~14.7)||REM-Ox—middle (11.2–13.4) REM-Sx—middle (12.3–13.2)||TiN—low (~9.4)|
|Non-Metallic Inclusions (NMI)||SiO2, SiO2-MnO-...||Al2O3, Al2O3-MgO||CaO, CaO-Al2O3, CaO-Al2O3-..., CaO-SiO2-…||MnS, (Mn,Fe)S||Ca(O,S), CaS, (Ca,Mn)S||REM-Ox, REM(O,S)x, REM-Sx||ZrN, Zr(N,C), TiN, Ti(N,C), VN, V(N,C), BN, B(N,C), BC|
|Effect of NMI on the mechanical properties of steel.||No or some anisotropy of mechanical properties of steel due to low elongation of silicate inclusions during deformation.||No anisotropy of mechanical properties of steel.|
|Effect of NMI on the machinability of steel.|
3. Non-Metallic Inclusions in Different Steels and Their Link to Machinability Tests
|Ref.||Year||Steel Grade a||Inclusion Characteristics||Machinability Parameter b||Main Result|
|||1995||“Clean”, carbon||(Mn,Ca)S, elongated, (CaO-Al2O3), globular||TL||Ca-treatment improves machinability|
|||1995||“Clean”, carbon, M‑steel||(Mn,Ca)S, elongated, (CaOAl2O3), globular||TL, TW||Ca-treatment improves machinability|
|||1981||Ca-treated, carbon, M‑steel||CaO-Al2O3, globular, CaO-Al2O3-SiO2, anorthite, globular||TL, TW||Ca-treatment improves machinability|
|||2007||Ca-treated, medium carbon steel, 0.35%–0.40% C, 0.02%–0.04% S||Al2O3-MgO, regular, CaO-Al2O3, 12CaO-7Al2O3, globular||TW||Ca-treatment improves machinability|
|||1993||SS 2541, Q & T||MnS, elongated, (Mn,Ca)S, globular, (CaO-Al2O3)-(Mn,Ca)S and CaO‑Al2O3-SiO2, globular||TL, TW||Decreased flank wear progression due to Ca‑treatment|
|||2013||42CrMo, Q&T, 0.42% C, 0.0067% S||BN, globular, 5–20 µm||TW, CC||BN improved the machinability (drilling)|
|||1999||AISI 4140, Q&T, 0.0017%–0.0030% Ca, 0.4% C||MnS, (Ca,Mn)S, globular||TL, CF||Reduced torque and adhesion due to Ca-treatment|
|||1993||SS2541, ~0.35% C, 0.035% S 825B BB, 1% C, 0.011% S||MnS, (Ca,Mn)S, (CaO-Al2O3)-MnS, AlCaMnS||TW, CF||The protective (Mn,Ca)S layer reduced the crater wear|
|||1984||SS 2506, CH, S, Ca ~0.2% C, 0.04%–0.09% S, 0.0003%–0.0054% Ca||MnS, elongated, (Mn,Ca)S~elongated, (CaO-Al2O3)-(Mn,Ca)S and (CaO‑Al2O3-SiO2)-(Mn,Ca)S, globular||TL, TW||S and Ca-treatment improves machinability|
|||1986||SS 2506, CH, Ca additions 0.04%–0.09% S||MnS, elongated, (Ca,Mn)S, (CaO-Al2O3)-(Mn,Ca)S, globular||TL, TW||Ca-treatment improves machinability|
|||2001||40 CrMnMo8 Carbon 0.4% C, 0.008%–0.067% S||MnS, elongated, 20–100 µm, oxides, globular, 10 µm||TL, TW, CC||S addition increased the machinability by 40%|
|||2001||AISI 4340 ~0.4% C, 0.012%–0.034% S, 0–50 ppm O, 0–25 ppm Ca||(CaO-Al2O3)-(Mn,Ca)S, globular, 2–10 µm||TW, CF, CC||Ca-treatment indicates ridge formation after hard part turning|
|||1984||Structural steel||S, Se, Pb, Ca||TL||Additions of S, Se, Pb, Ca improved the machinability|
|||1975||Free mach, 0.3% S||MnS, elongated||TL, TW, CF||S additions improved the machinability|
|||1975||Free mach., 0.1% S||MnS, elongated, Al2O3, globular||TL, TW||S additions improve machinability|
|||2006||Free mach., 0.6% C, 0.3% S||MnS, elongated, 5–40 µm MnFe(Al,Si)S||CF, CC, SR||Cold deformation may improve machinability|
|||2012||Free mach., ~0.08% C, ~0.4% S||MnS, elongated, 10–20 µm (MnO-Al2O3)-MnS, globular, 15 µm (MnO-SiO2)-MnS, elongated, 20 µm||TW, CC, SR||Increased oxygen content improved the machinability|
|||1997||Free mach., 0.4% C, 0.1% S||(Mn,Ca)S, MnS, elongated, <10 µm, (RE,Ca)2S3-(Mn,Ca)S, Re2S3-MnS, globular, <10 µm||TW||Ca and RE additions increased the machinability of free‑cutting steels|
|||1996||Free mach., stainless steel, 0.04%–0.08% C, <0.1% S, <0.01% Ca||CaO-Al2O3-SiO2-MnS, MnS, Gehlenite, Anorthite||TL, TW, CF||Ca and S additions increased the machinability of stainless steel|
|||1990||Stainless steel, 316 L 0.020%–0.027% C, 0.022%–0.025% S. 0.0002%–0.0045% Ca||MnS, (Mn,Ca)S, Gehlenite: Ca2Al[AlSiO7] + MnS Anorthite + MnS, elongated phases||TW, CF, CC||Anorthite inclusions are favorable for machining of 316L stainless steel|
|||2010||Super-duplex stainless steel, 0.017%–0.021% C, 0.005%–0.034% S. REM additions||REM-O, Oxy-sulfides, (Mn,Cr)S, globular, 2–10 µm||TL, TW||S and REM additions increased the tool life but the corrosion resistance was decreased|
|||2011||Austenitic stainess steel, 0.10%–0.11% C, 0.02%–0.11% S. Cu, Bi, Ti additions||MnS, Ti4C2S2, CuO, Bi, globular||TW, CF, CC||S, Bi, Cu and Ti additives improved the machinability|
4. Control and Correction of Non-Metallic Inclusions for Improving the Machinability of Steel
4.1. Increasing the S Content of Steel
|AISI Steel Grade||C||S||Mn|
- Type I: globular, when the oxygen solubility is high and the sulfur solubility is relatively low. Such inclusions are formed by a monotectic reaction in rimmed and semi-killed steels (when aluminum in the steel is less than 0.001 wt.%).
- Type II: formed in the interdendritic spaces of austenite with a fan-like morphology. In addition, most commonly formed at grain boundaries of steel. These are formed in aluminum killed steels, without an excess amount of aluminum, as the aluminum content is about 0.007% in the steel.
- Type III: angular inclusions are formed as isolated particles in the interdendritic spaces, when excess aluminum is used for deoxidation resulting in about 0.038 wt.% aluminum in the steel.
4.2. Modification of Sulfide Inclusions by Addition of Ca, REM or Zr
- change the composition and properties (physical and chemical) of sulfides;
- change the sulfide morphology (globalization);
- decrease the size of the modified sulfides;
- obtain a homogeneous distribution of precipitated sulfides in the solidified steel.
4.2.1. Calcium Treatment
4.2.2. Rare-Earth-Metals (REM) Treatment
4.3. Modification of Oxide Inclusions by Addition of Ca
- to form the globular CaO-SiO2-... or CaO-Al2O3-... inclusions;
- 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;
- to avoid a formation of Al2O3 and Al2O3-MgO clusters in the liquid steel and clogging problems during casting;
- 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.).
|Steel||Ca-Addition||Main Type of Oxide||Main Type of Sulfide|
|1||No (Ref.)||Alumina, Al2O3||MnS|
|2||Yes||Gehlenite, Ca2Al[AlSiO7]||MnS + (Mn,Ca)S|
|Inclusion||Inclusion Stoichiometry||Hardness (kg/mm2)||Melting Temperature, Tm (°C)|
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