Residual Stress Determination of Duplex and Austenite Steels Machined using Different Tool Geometry

J. Čapek¹, K. Kolařík¹, Z. Pitrmuc², L. Beránek², N. Ganev¹

¹Department of Solid State Engineering, Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague

²Department of Machining, Process Planning and Metrology, Faculty of Mechanical Engineering, Czech Technical University in Prague

capekjir@fjfi.cvut.cz

Duplex stainless steels have high corrosion resistance in many environments, where the standard austenite steel is consumed, and where its properties significantly exceed austenite steel. Duplex steels combine properties of both phases and moreover, due to two-phase microstructure, some properties are better than high-alloyed austenite steel, e.g. abrasion resistance [1]. Thereby, smaller amount of material from duplex steel is necessary to manufacture function components. Austenite and duplex steels are susceptible to mechanical reinforcement, i.e. local changes in mechanical properties of surface layers. Local changes, e.g. hardness, can lead to tools vibration during machining of the final component, which results in additional material inhomogeneity and blunting tool [2].

Realising that austenite steel has face centred cubic (fcc) lattice with close-packing structure of atoms, the primary slip system is <110>{111}. The number of slip systems is 12, which is the sufficient amount to plastic deformation. Moving dislocations form so called stair-rod dislocations which have small stacking fault energy, i.e. high energy is necessary to have for intersect or cross slip of these dislocations [3]. Therefore, the austenite steels are prone to work-hardening, which cause mechanical modification and inhomogeneity on the machined surface, and leads to e.g. unstable chip formation. On the contrary, the ferrite crystallizes in a body centred cubic lattice (bcc). The direction slip in bcc materials is always <111>. Since in the bcc lattice is not close-packing structure of atoms, more slip planes assert during the deformation, mostly planes {110} and {211}.

The tested samples of tube shape of 100/86 mm in diameter were made of AISI 304 (austenite) and AISI 318LN (duplex) type of stainless steel. The samples were annealed in air laboratory furnace for 5 hours at 420°C in order to reduce bulk macroscopic residual stresses. For machining of the surfaces, four types of side rake angle were used (-6°; -2°; +7° and +12°), namely F3M, SF, NF, and PP chip breakers of Iscar Cutting Tools.

Cutting conditions were as followed: feed rate 0.14 mm/rev, cutting speed 140 m/min, and depth of cut 2 mm. Direction of feed rate was parallel to axis of the sample (tube) A and perpendicular to tangential direction T. According to the principles of design of experiments (DOE) method, three 1cm tube segments were machined using the same cutting conditions.

Using MnKα and CrKα radiation, X'Pert PRO MPD diffractometer was used to measure lattice deformations in austenite and ferrite, respectively. Diffraction angles 2θ^hkl were determined from the peaks of the diffraction lines Kα₁ of planes {311} and {211} of austenite and ferrite, respectively.

In Figs. 1a-c, there are influences of surface macroscopic residual stresses <σ_A>; <σ_T>, MPa on the side rake angle, °. These residual stresses were averaged from three values of RS of tube segments machined the same side rake angle.

a) Austenite steel.
b) Duplex steel – austenite phase.	c) Duplex steel – ferrite phase.
Figure 1. Axial and tangential residual stresses <σA>,  <σT>  as a function of side rake angle.

Generally, the increasing of the side rake angle in the positive direction leads to a lowering of cutting force and temperature in the cutting zone [4]. For prediction of RS dependence on the side rake angle, the yield strength ratio Rm/Rp_0.2 of the given material is necessary to take into account. Generally, the temperature influence causes the tensile RS and contrarily, the plastic deformation leads to compressive RS. The type of the RS and their value deeply depend on the mechanical and thermal properties of the machined material [4, 5].

For austenite steel, higher compressive (axial direction) and smaller tensile (tangential direction) RS were determined with increasing of the side rake angle, see Fig. 1a. On the other hand, for ferrite steel, the greater force causes that the plastic deformation influence is predominant and higher compressive or smaller tensile RS may be determined with increasing of the side rake angle. Furthermore, for duplex steel, which is consisted of both phases, it is possible to presume that the dependence of RS on the side rake angle is generally not monotonic for both the phases because of their mutual influence during plastic deformation, see Figs. 1b-c.

1. R. Dakhlaoui, C. Braham, A. Baczmański, Mater. Sci. Eng.: A, 70.1, (2007), 6-17.

2. J. Čapek, K. Kolařík, L. Beránek, A. Molotovník, N. Ganev, in The 5th Student Scientific Conference on Solid State Physics, edited by ČVUT Praha, 2005, pp. 11-15.

3. J. J. Moverare, M. Oden, Mater. Sci. Eng.: A, 337.1, (2002), 25-38.

4. F. Neckář, I. Kvasnička, Vybrané statě z úběru materiálu. Praha: ČVUT. 1991.

5. T. Leppert, R. L. Peng, Produc. Eng., 6.4-5, (2012), 367-374.

This work was supported by the Grant Agency of the Czech Technical University in Prague, grant No. SGS16/245/OHK4/3T/14.