As I wrote in the article Hadfield Steel, prison steel, milling, the aforementioned steel grade is difficult to machine. This is due to the specific austenitic properties of high manganese steels (Hadfield steels). These steels have very high tensile strength (from 350 to 900 MPa), which increases even to 2000 MPa (2 GPa). The increase in the tensile strength limit is strongly associated with the process of strengthening and hardening of Hadfieds steel subjected to deformation.

During the tensile test, it was observed that after stretching to the break point, the material did not “narrow”. At the weakest point, the diameter decreased but the sample did not break. The deforming material causes its strengthening, including a significant increase in hardness. Typical elongation can be from 18 to 65%, depending on both the exact composition of the alloy and the previous heat treatment. Alloys with a manganese content of 12-30% are resistant to brittle cold, sometimes to temperatures in the range of -197 ° F (-127 ° C).

For comparison, structural steels have a yield strength of 185 to 345 MPa, depending on the steel grade, and elongation is 9 to 32%.

Chip formation

When discussing machining, the concepts of machine tool machining space and cutting zones are often used. The cutting zone is the space where the tool interacts with the workpiece. In this area, the cutting layer becomes a chip. The cutting zone has a complex state of stress and deformation, which depends primarily on the cutting conditions.

As a result of the cutting tool blade’s impact on the cutting layer, its elastic and then plastic deformations are formed along the shear zone. The transition of the shear zone turns the machined layer into a chip that has a shear oriented structure. Chip is formed after crossing the yield point.

Greater deformations in the cutting zone are in the so-called the secondary shear zone, which is located on the seizure zone. This zone is associated with the impact of the chip on the rake surface (high normal forces, high temperature). Providing for [3] the connection of the chip to the tool surface in the seizure zone is very strong. The pressing material above causes shearing of the chip material (secondary shear zone).

Strengthening as a result of pressure (during machining) of austenitic high-manganese steels is characterized by an increase in yield strength, which makes machining of these steels very difficult.

Cutting speed – minimum value

During the considerations, the thesis was put forward that when milling with a multi-edge tool, with the right cutting speed, subsequent cutter blades will sink into the workpiece quickly enough that changes in the microstructure of Hadfield steel will not happen and thus the steel will not be able to strengthen preventing machining. It was decided to make a series of machining grooves with a decreasing cutting speed (table 1; figure 1).

Fig.1. Grooves on one of the Hadfield steel plates. Burrs have already been removed.

GF Machining Solutions MILL P 500 UD CNC milling machining center was used for processing. Tool SANDVIK Coromant 2N342 1200 PC – cutter with a diameter of Ø12 and five blades. No cooling method was used in the treatment. Figures 2 and 3 show burrs and flashes when the cutter exits the workpiece.

Fig. 2. Burrs formed when milling grooves in a Hadfield steel plate.

Fig. 3. An example of outflow of workpiece material when the cutter exits the material.

At the machining planning stage, it was assumed that the so-called minimum cutting speed below which machining will not be possible. However, the assumption turned out to be wrong. The machining was started for v_c = 229 m/min – cutting speed that was used in the initial trial machining of Hadfield steel. Gradually, the cutting speed value was reduced to 19 m/min. However, v_c = 29 m/min was adopted as the final one due to incomplete data from additional measurements for v_c = 19 m/min. Other selected machining parameters are:

• working feed per tooth fz mm/tooth, which was constant for all machining and was 0.061 mm/tooth;
• main propulsion speed n ranged from 6074 to 769 rpm;
• thickness of cut layer ap = 1.5 mm.

The workpiece was a Hadfield steel plate (X120Mn12) with dimensions of 200x100x10 (figure 4).

Fig.4. The workpiece was a Hadfield steel plate (X120Mn12) with dimensions of 200x100x10 (figure 4).

Estimating the minimum cutting speed was assumed in the judgment because it is not possible to precisely define such cutting speed. Strengthening Hadfield steel preventing further machining means a significant load on the main drive and linear drives. Load detection and reference load limits defined in the CNC control cause machining to stop. It is possible to specify the minimum cutting speed at which machining is still possible.

Conclusions

The minimum cutting speed turned out to be impossible to determine for the selected machining parameters and machine tool. Milling was possible at each set cutting speed. Sample preparation for crystallographic tests is underway. Roughness measurements (figure 5) have shown that there is a cutting speed range that provides less roughness. This conclusion is concurrent with [4].

Fig.5. The value of the roughness of the machined surface depending on the cutting speed.

Sources

1. Schwartz M., Encyclopedia of Materials, Parts and Finishes, page 392, CRC Press 2012
2. Šalak A., Selecká M., Manganese in Powder Metallurgy Steels, page 274, Cambridge International Science Publishing 2012
3. Jemielniak K., Obróbka skrawaniem, OWPW 1998
4. Zaleski K., Matuszak J., Badania porównawcze wpływu parametrów technologicznych frezowania wybranych stopów tytanu na moment skrawania i chropowatość obrobionej powierzchni,  ZESZYTY NAUKOWE POLITECHNIKI RZESZOWSKIEJ 295, Mechanika 89, RUTMech, t. XXXIV, z. 89 (4/17), październik-grudzień 2017, s. 563-572
5. AlfaTech – stale specjalne