Hadfield steel is also known as prison steel, which results from one of its applications. Hadfield steel (Table 1) is widely known for its high abrasion resistance and hardenability while maintaining good plastic properties. High manganese steel (manganese), is used in the production of parts for machinery and equipment which requires very high resistance to abrasion in impacts causing so-called cold deformation: jaws crushers for stones, bolts and sleeves for track chains, balls and plates for mills , grates, tram rails, excavator shovels, etc. Hadfield’s high-wear steel in supersaturated state is a non-magnetic steel.
I hesitated for a long time how to write this article. The extensive introduction emphasizes how important the properties of the workpiece are and how advanced the research is to explain the causes of the properties of Hadfield steel. I decided to explain in a brief way why Hadfield steel is a hard-to-machine material.
A prerequisite for the high abrasion resistance of Hadfield steel is the occurrence of considerable pressure along with the cold work.
|Chemical composition of Hadfield steel X120Mn12 in % by DIN|
Hadfield steel is austenitic manganese steel (eg X120Mn12, 1.3401, X120Mn13), which contains 1 ÷ 1.3% C and 11 ÷ 14% Mn. The ratio of carbon to manganese in this steel should be 1 to 10. This is due to the fact that only the correct carbon content can ensure the stability of the austenitic structure. Hadfield steel is characterized by a low limit of elasticity and plasticity and low hardness (210 HB) with high strength Rm = 1050 MPa and very good plastic properties [4, 5].
Hadfield steel was tested under various load conditions: compression / tension, high pressure torsion (HPT), high speed compression load, rolling, cyclic deformation and impact loads.
The main feature of Hadfield’s steel is the very high strengthening factor under the influence of cold work. Steel hardness of 190 ÷ 220 HB as a result of cold work increases even more than twice to over 500 HB. Strong hardening results from the formation of so-called microblasts in manganese austenite. This is a very high abrasion resistance at high pressures and makes this steel a hard-to-cut material .
Literature analysis indicates the differentiation of hypotheses describing the mechanisms of Hadfield steel strengthening under the influence of pressure.
In the context of machining, Hadfield steel is considered to be difficult to machine. This steel strengthens under the influence of external pressures. This is due to the fact that the austenitic structure has many directions of easy slip. Cold work also causes grain refinement and partial martensitic transformation of austenite [7, 10]. Manganese as an alloy additive contributes to lowering the temperature of the eutectoidal transformation and to a faster rate of cooling. As a result, the increase in strength is caused by the fact that pearlitic transformation occurs at lower temperatures, also during cooling in the air .
In a martensitic transformation, for which the transformation temperature for steel is below 200 ° C, time does not matter. A necessary condition is proper cooling of austenite. Important in this case is the so-called critical cooling rate, i.e. the rate protecting against the occurrence of diffusion changes, which occur at higher temperatures than the martensitic transformation, preventing its transformation into martensite .
Hardening of Hadfield steel under pressure, low thermal conductivity, good plastic properties are the main factors causing its difficult machinability.
When machining difficult-to-machine materials, the tool plays an important role (figure 1), including the technology of its implementation, including the coatings used. Even the method of coating tools (PVD or CVD) is even important. Figure 1 shows two Ø12 monolithic tungsten carbide end mills (powder metallurgy) of two different manufacturers (SANDVIK Coromant and MITSUBISHI MATERIALS). SANDVIK Coromant cutter coated with AlCrN by PVD method. The MITSUBISHI MATERIALS Milling Cutter is a spindle cutter with a two-stage corner radius with a short 6-blade cutting part.
Both cutters are dedicated to machining hardened steels. Why did I choose such tools? The plan assumed using the speed of entering the blade into the material protecting against the occurrence of Hadfield steel strengthening. Milling with a multi-edged monolithic end mill using the machining parameters typical for HSM (High Speed Machining) machining was intended to enable machining of this steel.
The machining was done on a vertical 5-axis milling center GF Machining Solutions MIKRON P 500 UD (fig. 2). This machine is dedicated for performance processing (HPM), but it enables machining with selected machining parameters for Hadfield steel machining.
Due to the limited way of fixing the workpiece (plate size 200x100x10) in the chuck (fig. 3), the available machining surface was limited. The limitation resulted from the lack of a more appropriate vise matching the available pallet as part of the pallet system used.
Taking into consideration the blank (X120Mn12 steel plate with dimensions 200x100x10) and possible technological equipment (fig. 3), the following machinings are planned:
- facing with dimensions 60×100 mm;
- machining of a blind slot with a width of 15 mm and a length of 60 mm;
- machining of a blind slot with a width of 18 mm and a length of 60 mm;
- machining of a through hole with a diameter of Ø18.
Figure 4 shows the effect of machining. Table 2 contains the machining parameters used. It was decided to maintain a constant cutting speed vc = 229 m/min.
The choice of machining parameters was based on the recommended catalog data for the selected tool – SANDVIK Coromant 2N342 1200 PC.
|Table 2.||Facing:||Slot#1:||Slot #2:||Hole:|
|Number of cuts:||2||2||7|
|entrance angle [°]||1°||0,5°|
|machining time[min]||1 min 10 sek||ok 4 min||39 sek|
Figure 5 shows the chips obtained during machining, which indicate that the parameters were correctly selected and Hadfield steel could be milled. The analysis of the wear of the cutter blade (fig. 6) does not indicate significant wear which further confirmed the correctness of the selected machining parameters. It should be emphasized, however, that these were preliminary machinings aimed at conducting the first tests. Only further machining will allow developing reliable recommendations for the selection of machining parameters for this steel.
Hadfield steel is considered to be hard to machine, but modern cutting methods, which include high-speed machining, allow machining with a small and acceptable tool wear. However, drilling with a twist drill may not be possible due to the smaller number of blades. The machining of holes would require, in many cases, the use of large-size machine tools or very conscious definition of structural requirements for structural elements – e.g. Hadfield steel housing.
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- Jabłońska M., B., Struktura i właściwości austenitycznej stali wysokomanganowej umacnianej wskutek mechanicznego bliźniakowania w procesach dynamicznej deformacji. Monografia, WPŚl, Gliwice, 2016
- Faryna Marek, for students, www.imim.pl