Abstract
This paper investigates the formation mechanisms of microcracks in thick and complex high manganese steel (ZGMn13) castings, taking a finished product from a factory as the research object. By analyzing the phase diagram of high manganese steel component segregation, metallographic microscopy, and scanning electron microscopy (SEM) observations, it was found that microcracks distribute along grain boundaries and are enriched with a large number of inclusions, carbides, and low-melting-point phosphorus eutectics around the cracks. The tensile stress during the solidification process of the casting causes microcracks to initiate at these oxides, carbides, and low-melting-point phosphorus eutectics. Therefore, the tensile stress during solidification and the presence of these inclusions are the main reasons for the formation of microcracks in thick and complex high manganese steel castings.
1. Introduction
High manganese steel exhibits rapid work hardening on the surface under large impact loads, maintaining its unique wear resistance. When cracks appear in high manganese steel castings during use, the crack propagation is relatively slow. Due to these special properties, high manganese steel has been widely used in various industries such as railways, mining, and military applications. However, the linear shrinkage value of high manganese steel (2.4% to 3%) is much larger than that of carbon steel, and its thermal conductivity is only 1/4 to 1/6 that of carbon steel. Therefore, high manganese steel castings are prone to generate large thermal stresses due to large temperature differences between different parts of the casting during solidification and heat treatment heating and cooling processes, leading to cracks, especially in complex high manganese steel castings with a thickness greater than 120 mm.
Scholars at home and abroad have conducted extensive research on the work hardening mechanism, heat treatment process, and casting process of high manganese steel. However, there is less research on microcracks in thick and complex high manganese steel leading wheel castings. High manganese steel leading wheel castings are key components in excavator electric shovels used in mining exploitation, with stringent requirements on surface quality, dimensional accuracy, and internal quality. However, their structure is complex, with significant wall thickness variations, with a maximum wall thickness of 142 mm. X-ray flaw detection of this complex and thick casting produced by a factory revealed cracks of varying sizes on both sides of the holes or arcs indicated by the arrows in the as-cast and heat-treated casting blanks. Most of the cracks detected by X-ray flaw detection were around 25 mm in length, with a few reaching 50 mm in length, significantly reducing the product yield and service life of this product. Based on previous work optimizing the casting process of high manganese steel leading wheels for mining machinery, the authors conducted further research on microcracks in this casting. The research results can provide a certain reference for eliminating microcracks in such products.
2. Experimental Materials and Methods
2.1 Experimental Materials
The experimental material was a finished ZGMn13 high manganese steel casting for mining machinery. The steel water was produced using pig iron, ferromanganese, and alloy steel as raw materials, smelted in an electric furnace, and then vacuum refined (VD). The oxygen content and oxidation tendency of alloy elements in the steel water were strictly controlled. Samples were taken from the accompanying cast test block and analyzed chemically to obtain their chemical composition, as shown in Table 1. The compositions were within the range specified in the national standard GB/T 5680-1998 “High Manganese Steel Castings”.
Table 1 Chemical Composition of ZGMn13 Casting (Mass Fraction, %)
| Element | C | Si | Mn | P | S | Mo | Fe |
|---|---|---|---|---|---|---|---|
| ZGMn13 | 0.98 | 0.59 | 13.42 | 0.036 | 0.005 | 0.92 | Balance |
| GB/T 5680-1998 | 0.90-1.30 | 0.30-0.80 | 11.0-14.0 | ≤0.040 | ≤0.070 | – | Balance |
2.2 Experimental Methods
To obtain samples containing complete microcracks, the accurate location and direction of microcracks on the surface of the casting at position 43# were displayed using PT coloring, and then samples of 20 mm × 25 mm × 20 mm containing cracks were cut at appropriate positions. Additionally, samples of 30 mm × 30 mm × 20 mm were cut at completely cracked cracks. The surfaces of the cracked samples were mechanically ground and polished, followed by chemical etching with a 4% nitric acid alcohol solution for approximately 6 seconds. Optical microscopes and Nova NanoSEM 450 field emission scanning electron microscopes were used for microstructure and crack observations. Energy spectrum analysis was conducted on the inclusions near the cracks and in the steel matrix to determine their chemical compositions.
3. Results and Analysis
High manganese steel has a relatively high linear shrinkage value. When the contraction of high manganese steel is obstructed, significant internal stress will be generated. High manganese steel has a low thermal conductivity, and thick and complex castings experience large temperature differences between different parts during cooling or heating, resulting in large thermal stresses within the casting. During crystallization, high manganese steel castings are prone to forming coarse grains and columnar grains. Brittle carbides and non-metallic inclusions present at grain boundaries weaken the bonding force of the matrix and embrittle the casting. These factors determine that if there are many inclusions, carbides, and phosphorus eutectics present at grain boundaries in the casting, they will weaken the grain boundary strength and may become crack sources under thermal stress. The crack morphology and distribution in the steel used in this experiment. The cracks in the casting are widely distributed, appearing as linear or reticulate shapes along grain boundaries under a low-power microscope, with most of them interconnected. There are many carbides and inclusions of different morphologies near and within the crack propagation paths.
3.1 Effect of Inclusions on Cracks
High manganese steel has a high alloy content and is prone to oxidation during melting and pouring. The oxidation inclusions produced during smelting are relatively large in size and easily float to the surface, while those produced during pouring are smaller in size, making it difficult for them to float and remain in the casting. During solidification, these oxide films accumulate at grain boundaries and gradually unfold, separating the metal on both sides, destroying the continuity of the metal matrix, reducing the matrix strength, and making the casting prone to cracks during machining. cracks coexist with inclusions. Analysis of the inclusions around the cracks revealed that they are mainly spherical alumina inclusions, small complex oxide inclusions containing Al, Mg, Cr, irregular TiN inclusions, and a small amount of MnS inclusions. Oxide and nitride inclusions are mostly brittle phases and are distributed at grain boundaries with low bonding strength with the matrix. When the metal liquid solidifies and shrinks, the large difference in cooling and solidification rates between different parts of thick and complex castings, combined with the difference in physical parameters between the inclusions and the casting, will inevitably lead to stress concentration around the inclusions, resulting in microcracks forming in areas where the bonding strength between the inclusions and the matrix is low. stress concentration caused by irregular inclusions is more severe, with microscopic cracking already occurring around them.
3.2 Effect of Component Segregation on Cracks
3.2.1 Effect of Carbon and Manganese Segregation on Cracks
C atoms are interstitial solid solutions in Fe-Mn alloys, causing lattice distortion and forming stress fields in the material. The combined action of stress fields and C-Mn atom clusters hinders dislocation movement and enhances material strength . Additionally, C expands the austenite phase region, enabling high manganese steel to form a single austenite phase, thereby imparting good plasticity and toughness to high manganese steel. the phase diagram obtained from the calculation and analysis of carbon and manganese segregation. The results indicate that excessively high C and Mn contents may cause the precipitation of secondary cementite in high manganese steel and promote the formation of coarse grain boundary carbides, affecting its mechanical properties.
The metallographic images of the cracked samples, revealing that a large number of chain-like, needle-like, and blocky carbides are precipitated in the high manganese steel casting and distributed reticulated along grain boundaries. Such continuous reticulated carbides significantly affect the performance of the casting. The structures of all carbides in high manganese steel castings tend towards the cementite structure of M3C, but there are significant differences in their chemical compositions, as shown in the EDS analysis results in Table 2. It can be seen that the blocky carbide with a size of around 10 μm is essentially a phosphorus eutectic product with a fine dispersed lamellar structure and irregular size orientation. The chemical composition of the eutectic, in addition to iron, includes manganese, carbon, and a small amount of molybdenum, with an Fe/Mn ratio of 2.2, indicating a higher manganese content. This morphology of carbide precipitates both at grain boundaries and within grains. The carbides , with widths less than 5 μm and lengths of around 30 μm, exhibit a distinct inline needle-like structure with Fe/Mn ratios of 3.4 to 3.6, indicating a decrease in manganese content.
The carbides with even smaller sizes in chain-like or inline structures, have Fe/Mn ratios of 4.7 to 4.8, indicating a further decrease in manganese content, with a significant amount of manganese retained in the austenite. Research has shown that the morphology, quantity, and composition differences of carbides are mainly due to variations in cooling rates in different parts of the casting. In regions with lower cooling rates, carbides manifest as thin-layer phosphorus eutectics with high manganese content, as indicated by the EDS analysis of point A, which is phosphorus eutectic. An increase in cooling rate hinders phosphorus diffusion, and separated lamellar eutectic fragment carbides continue to grow in an inline manner, reducing the amount of blocky carbides while decreasing the manganese content in the carbides and increasing the alloying degree of austenite. However, regardless of the morphology of the carbides, they are brittle phases that weaken grain boundary strength and easily become crack sources. Therefore, under the action of solidification shrinkage and phase transformation stress, microcracks are prone to form in the casting at locations rich in brittle carbides.
Fine carbides (sizes < 2 μm) tend to precipitate first at austenite grain boundaries in high manganese steel castings, primarily because the energy required for the formation of fine carbides is small, and they match well with the austenite lattice. When fine carbides grow to a certain size, their growth is hindered, and nucleation becomes more stable, resulting in the formation of thick carbides (sizes > 2 μm) at the interfaces between fine carbides and austenite grain boundaries. Once formed, thick carbides rapidly grow along grain boundaries, engulfing fine carbides in their growth direction. Due to their large size and mismatch with the austenite lattice, thick carbides have poor mutual bonding forces and are more likely to become crack sources, reducing casting performance. Furthermore, thick carbides distributed reticulated along grain boundaries make cracks more prone to propagation under stress, resulting in reticulated cracks .
Table 2 EDS Analysis Results of Different Morphologies of Precipitates
| Element | C K | P K | Mo L | Mn K | Fe K | Fe/Mn |
|---|---|---|---|---|---|---|
| Point A | 1.29 | 1.21 | 4.84 | 28.59 | 62.77 | 2.2 |
| Point B | 3.52 | – | 74.00 | 9.77 | 8.57 | 0.9 |
| Point C | 2.94 | – | 1.87 | 21.69 | 73.08 | 3.4 |
| Point D | 2.15 | – | 2.19 | 20.65 | 75.56 | 3.6 |
| Point E | 2.23 | – | 1.95 | 16.71 | 78.52 | 4.8 |
| Point F | 1.28 | – | 1.29 | 16.38 | 80.10 | 4.7 |
3.2.2 Effect of Phosphorus Segregation on Cracks
Phosphorus is an undesirable element in high manganese steel due to its low solubility in austenite, which easily segregates to grain boundaries and interdendritic regions, forming low-melting-point phosphorus eutectics, as shown in the calculated phase diagram of phosphorus segregation for the steel in this experiment. During crystallization and solidification, as well as cooling and shrinkage, phosphorus eutectics are prone to segregation, distributing at interdendritic and primary grain boundaries , leading to a decrease in intergranular bonding force, causing grain boundary embrittlement, and reducing the strength of austenite grain boundaries, thereby reducing the hot ductility of the steel. Moreover, the heat treatment temperature of high manganese steel castings is generally around 1050°C, higher than the melting points of binary phosphorus eutectics (Fe + Fe3P with a melting point of 1005°C) and ternary phosphorus eutectics (Fe + Fe3C + Fe3P with a melting point of 950°C). During heat treatment, the phosphorus eutectics in the casting melt into a liquid state, reducing their bonding force with the matrix and causing cracks to form under stress.
Furthermore, especially in thick and complex high manganese steel castings with uneven cooling rates, an increase in carbon content in austenite reduces the solubility of phosphorus, making it more prone to segregate at grain boundaries and form phosphorus eutectics during the late solidification stage. Under stress, areas with phosphorus eutectics become crack sources. Cracks induced by phosphorus eutectics , with the cracks exhibiting a reticulated distribution. The black regions in the cracks are pores, and the gray regions are primarily phosphorus eutectics. The analysis results indicate that the atomic percentage of phosphorus is 12.20%, indicating severe phosphorus segregation in the matrix. Even when the phosphorus content in the matrix is not very high, due to segregation, the phosphorus content at grain boundaries can be very high, forming phosphorus eutectics. In addition, low-melting-point phosphorus eutectics have a strong wetting ability for austenite dendrites, tending to distribute continuously rather than aggregately at grain boundaries. The continuous distribution of phosphorus eutectics poses a more severe hazard to the matrix.
4. Conclusions
(1) There are many carbides and inclusions enriched at grain boundaries in the internal structure of thick and complex high manganese steel castings, forming brittle phases that embrittle the grain boundaries and cause cracks. To avoid the negative impact of carbide networks, it is necessary to control the chemical composition and ensure that the solution treatment temperature, holding time, and cooling rate are sufficient to completely dissolve undesirable phases and prevent the formation of grain boundary carbides.
(2) The carbon and manganese contents in high manganese steel have a combined effect. An increase in the content and segregation of these elements easily forms coarse grain boundary carbides, significantly weakening the matrix strength and making it prone to forming and propagating microcracks under tensile stress, leading to a sharp decline in ductility and toughness. In practical applications, it is relatively safe to control the carbon content at 1.15% to 1.2% and the manganese content at around 13%.
(3) The presence of impurities such as phosphorus and sulfur is one of the reasons for casting cracks in high manganese austenitic steel. Phosphorus segregates at austenite grain boundaries, forming low-melting-point phosphorus eutectics, which reduce grain boundary strength and easily cause intergranular cracks during solidification. Therefore, it is necessary to strictly control the amount of phosphorus brought in by raw materials to reduce the possibility of phosphorus eutectic formation from the source. At the same time, methods such as increasing the solidification cooling rate, homogenizing solution treatment, and microalloying can also be used to reduce the adverse effects of phosphorus eutectics.