High Manganese Austenitic Steels: Part Two

---

Introduction to High Manganese Hadfield Steel

The main advantages of high manganese inclusion can include increased ductility, very high toughness, and much improved tribological characteristics in the material's resistance to abrasion.

The mechanical properties of austenitic manganese steels are directly related to the carbon and manganese content. Typically, manufacturers aim for mid-range content to balance acceptable properties and economic considerations.

Microstructural Characteristics and Heat Treatment Effects

Cast high-manganese steel containing 1.2% C and 12% Mn is a material with high resistance to abrasion, provided that it is used under conditions of high dynamic loads. In its as-cast condition, this steel is characterized 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 microstructure is unfavorable due to the presence of (Fe,Mn)xCy carbides spread along the grain boundaries. Combined with the effect of non-metallic inclusions, these carbides significantly 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 of heat treatment (which involves solutioning Hadfield steel castings and cooling them in water) is to produce a purely austenitic microstructure free from 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

Experimental Results on Heat Treatment Parameters

In the research by G. Tęcza and S. Sobula, samples were treated at three different temperatures: 1100°C, 1150°C, and 1200°C, with durations of 40 minutes and 80 minutes. Additional samples were solution treated at 1150°C with heat treatment extended to 4 hours (in 30-minute increments). Metallographic sections were prepared from these samples to examine the chemical composition of visible carbides and measure the microhardness of both matrix and precipitates.

Their findings revealed that solution treatment of cast high-chromium Hadfield steel at 1100°C, 1150°C, and 1200°C for 40 minutes does not produce a purely austenitic structure. Metallographic cross-sections showed 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

Influence of Composition on Mechanical Properties

The mechanical properties of austenitic manganese steels vary with carbon and manganese content. Industry practice tends to favor the midpoint carbon range with 12-13% Mn, as the lower levels of the composition range are associated with somewhat inferior tensile properties, while the upper extremes offer 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