High Manganese Austenitic Steels: Part Two
The main advantages of high manganese inclusion can include increased ductility, very high toughness and much improved tribology characteristics in the materials resistance to abrasion.
The mechanical properties of austenitic manganese steels are directly related to the carbon and manganese content. Typically the optimum is to go for mid-range content to balance between acceptable properties and economic sense.
Cast high-manganese steel containing 1.2% C and 12% Mn is a material with high resistance to abrasion, provided, however, that it is used under the conditions of high dynamic loads. In its typical embodiment, this steel in as-cast condition is characterised by an austenitic microstructure with precipitates of alloyed cementite and the triple phosphorus eutectic of an Fe-(Fe,Mn)3C-(Fe,Mn)3P type, which appears when the phosphorus content exceeds 0.04%. It also contains nonmetallic inclusions such as oxides, sulphides and nitrides (Figures 1-2). This type of microstructure is unfavourable due to the presence of the (Fe,Mn)xCy carbides spread along the grain boundaries. Combined with the effect of non-metallic inclusions, these carbides clearly reduce the ductility of cast Hadfield steel.
Figure 1: Cast high-manganese steel Hadfield steel; austenitic matrix with precipitates of acicular alloyed cementite; Nital etching
For this reason, the aim to be achieved during heat treatment (the heat-treatment of Hadfield steel castings involves solutioning and cooling in water) is to produce a purely austenitic microstructure, i.e. free from the carbide precipitates (Figure 2).
Figure 2: Cast Hadfield steel after solution treatment in water; austenitic matrix free from the precipitates of alloyed cementite spread along the grain boundaries; Nital etching
In the paper of G. Tęcza, S. Sobula, is showed the result of the samples which were treated at three different temperatures, i.e. 1100, 1150 and 1200°C, one batch for 40 minutes, another batch for 80 minutes. Other samples were solution treated at a temperature of 1150°C with the time of heat treatment extended to 4 hours (in 30 minute steps). From thus prepared samples, the metallographic sections were prepared, the chemical composition of the visible carbides was examined, and microhardness of the matrix and precipitates was measured. Solution treatment of the cast high-chromium Hadfield steel carried out at 1100, 1150 and 1200°C for 40 minutes does not lead to a purely austenitic structure. The cross-sections of metallographic specimens show the presence of undissolved cementite present along the austenite grain boundaries. (see Figure 3).
Figure 3: Cast Hadfield steel after solution treatment in water (1150°C/240 minutes); austenitic matrix with undissolved carbides spread along the grain boundaries; nital etching
The mechanical properties of austenitic manganese steels vary with carbon and manganese content. There is a tendency to operate close to the midpoint carbon range and with 12-13%Mn since the lover level of the composition range is associated with somewhat inferior tensile properties and the upper extreme has no economic advantage.
Figure 4: Effect of carbon content on (a) yield and tensile strength, (b) ductility of the 12-14Mn Hadfield steels
Figure 5: Effect of Mn content on (a) yield and tensile strength, (b) elongation and total reduction of height of Hadfield steels