Background

Our USA customer purchased 98 manganese crusher hammers (Mn18) for his cement vertical crushers. After six months of service, some parts broke and failed. The customer wants us to analyze the cause of the breakage and provide optimized products.

Working condition Analysis

The crusher hammer is the core component of the hammer crusher, and due to the high-impact working conditions in the crusher, high manganese steel is the most suitable metal material for the hammer. The hammerhead in the cement clinker crusher is mainly made of Mn18-high manganese steel. During the solidification process of a high manganese steel hammer, when the sum of the internal stress generated by cooling shrinkage and the thermal stress generated by the temperature difference between the inside and outside of the casting exceeds the strength of the stress-affected area, fine cracks will occur in the casting. These fine cracks may be filled with solute elements on one hand, and on the other hand, they may cause the accumulation of inclusions, both of which will form discontinuous zones in the steel matrix. These cracks and their internal fillings cannot be eliminated during the water toughening. In the harsh and complex working conditions inside the crusher, the direction and intensity of the impact on the hammer head during its operation have a certain randomness, leading to the continuous expansion of the differences in surface work hardening characteristics and microstructure features of various parts of the hammer. In addition, the existing fine cracks continue to propagate during the fatigue impact process, eventually leading to failure accidents such as fractures or hammer breakage, which affects the overall service life cycle.

Inspection and analysis of failed high manganese crusher hammers

Ingredient testing

Different casting parts were taken for composition inspection, and the results are shown in Table 1.

In Table 1, it can be observed that there is a slight difference in composition between the central region and the surface, which is attributed to segregation during the solidification process. Chromium is one of the elements added in relatively large quantities to high manganese steel, and its role is also quite clear. After water toughening treatment, chromium mostly dissolves into the austenite phase of high manganese steel, increasing the steel's yield strength and accelerating carbide precipitation during cooling, typically resulting in a continuous reticular distribution of carbides along the grain boundaries. High manganese steel with added chromium exhibits improved wear resistance when subjected to strong impact abrasion, making it suitable for crusher hammer castings.

Titanium belongs to the category of vital reducing elements in molten steel. In high-carbon, high-nitrogen Mn18 steel, it can combine with C and N to form precipitates. If high-melting-point particles such as TiN and Ti(C, N) are formed before solidification, they can act as non-spontaneous heterogeneous nucleation sites for austenite, increasing the number of grains per unit volume and thus refining the grain size. Therefore, there has been considerable research and practical application of titanium micro alloying in high manganese steel. In the Mn18 steel described in this article, around 0.5% titanium was added during the initial design phase.

Crusher hammers fracture interface analysis

After relevant inspections, Mn18 crusher hammers were used in a cement vertical crusher. After a service period of 6 months, some of them experienced fracture failures. Fractured samples were analyzed, and the relevant results are shown in Figures 1 to 5.

• Figure 1 shows that the fracture occurred around the stud connection hole of the hammer, indicating a transverse brittle fracture.
• Figure 2 reveals that the fracture surface at different positions exhibits typical cleavage fracture characteristics, indicating poor hammer toughness. Additionally, irregular block-shaped inclusions are widely distributed across different sections, indicating a relatively high total number of such inclusions.
• As shown in Figure 3, in the normal section of the structure, the grains are relatively coarse, and blocky carbides are precipitated along the grain boundaries. However, the overall austenite grain boundaries appear normal.
• Figures 3 and 4 show that the grains in the normal section of the hammerhead are approximately at the 0.5 level. In contrast, the grains near the fracture surface are larger, with an average grain diameter of over 400 μm. Continuous reticular carbides have formed along the grain boundaries, and many acicular carbides have grown from the grain boundaries into the grains. The fracture surface exhibits clear intergranular fracture characteristics. Unlike spherical carbides, which have a face-centered cubic structure, acicular carbides have a body-centered cubic structure. The different structural characteristics lead to significant differences in their physical properties.Fine acicular carbides (length ≤20 μm) are beneficial for stabilizing the grain boundaries and optimizing the mechanical properties of high manganese steel, especially the impact toughness. However, in Figure 4, ultra-long, deep-gray carbides penetrating the grains indicate that pearlite transformation will occur inside them. This is because under certain conditions of supercooling, areas with concentrated carbide precipitation will form a mixed structure consisting of lamellar Fe3C and a small amount of ferrite (formed by the local precipitation of carbides to create a low-carbon region), known as pearlite. This mixed structure has weak bonding, resulting in lower impact absorption energy than a uniform austenite matrix. When subjected to strong external forces, both the acicular ferrite and the grain boundaries in which it is located become stress concentration zones, leading to defects such as fractures and tearing, ultimately resulting in fracture failure of the hammerhead after a certain period of service.
• Figure 5 shows that TiN and Ti(C,N) aggregate continuously and extensively in a certain area, forming discontinuous regions in the steel matrix. In this area, both strength and toughness drop sharply. During the hammerhead's working process, these types of inclusions become obvious stress concentration zones. They directly act as the origin of cracks, gradually expanding and extending, ultimately leading to widespread crack formation and causing fracture failure of the hammerhead.

 

Analysis and Discussion

 

Effect of chemical composition

Carbon is one of the most important elements in high-manganese steel. Its presence facilitates the formation of a single-phase austenite structure. A large amount of carbon dissolved in austenite also helps to increase the strength of high manganese steel. Additionally, carbides formed by carbon and alloying elements such as Cr contribute to improving the wear resistance of high manganese steel. However, excessively high carbon content will increase the tendency of carbide precipitation at grain boundaries, which, under the same conditions, is detrimental to stabilizing the grain boundaries. Therefore, it is advisable to reduce the carbon content appropriately based on Table 1.

Adding Mo to high manganese steel can reduce the precipitation of carbides in the as-cast structure and decrease the tendency to form a network of carbides on the grain boundaries of austenite. Molybdenum can also slow down the steel's precipitation rate of needle-like carbides, lowering their precipitation temperature. These effects are beneficial for improving the plasticity and strength of high manganese steel in the as-cast state. They can compensate for the shortcomings caused by the addition of chromium elements. Therefore, Mo should be added appropriately to work in conjunction with Cr, leveraging the beneficial effects of both elements.

The beneficial effects of titanium micro alloying in high manganese steel discussed in this article are confirmed. However, if the titanium content is too high while the nitrogen content remains relatively stable, TiN and Ti(C, N) start to precipitate at 1400°C. They continuously and extensively form and merge within the metal melt or in the solid-liquid two-phase zone. As the solidification continues, they constantly aggregate towards the grain boundaries, with a higher solute element concentration and relatively lower melting point phases. This aggregation exceeds the amount required for their pinning effect, decreasing the initial grain boundary binding force and even severe detachment between the steel matrix and grain boundaries. During the working process of the hammerhead, the aggregation of inclusions becomes a significant stress concentration area, directly serving as the origin of cracks, which gradually propagate and extend to cause widespread cracking, ultimately leading to hammerhead failure.

Therefore, from a compositional perspective, it is necessary to fully leverage the beneficial effects of TiN and Ti(C, N) in Mn18 castings while also controlling the harmful effects of their extensive and concentrated precipitation. The titanium content can be appropriately reduced by considering the mature application practices of low-alloy steels and medium-alloy steels.

 

The impact of process technology

To produce high manganese steel with good performance and stable service life, controlling the refinement of austenite grains and the morphology of carbides are two key control points. During high manganese steel's cooling and crystallization process, both reticular and acicular carbides typically precipitate along the grain boundaries, with the acicular carbides growing inward. However, if the casting is too large, and the time from heating to water toughening treatment is too long, and the cooling penetration during water toughening treatment is insufficient, resulting in prolonged residence in the high-temperature zone (≥500°C), it will lead to continued growth of austenite grains, continuous precipitation of grain boundary carbides, and continuous growth of acicular carbides. This ultimately leads to excessive differences in internal and external stress states. Under strong external forces, high-density dislocations and deformation twinning first appear on the surface of the hammerhead, rapidly increasing its strength and hardness. The further increase in the difference in internal and external stress leads to embrittlement of the internal grain boundaries, resulting in tearing and fracturing, manifested as fracture failure of the hammerhead.

Therefore, from a process perspective, it is first necessary to ensure that the heating temperature before the water toughening treatment of the hammerhead is reasonable and sufficient, thereby ensuring that carbides are completely or mostly dissolved into the austenite. After the hammerhead is removed from the heating furnace, it is the peak period of carbide precipitation. If the time from removal to immersion in water is long, carbides will precipitate in large quantities, and acicular carbides will be rapidly produced. With insufficient cooling strength and inadequate internal cooling transfer rate, internal aging effects occur, resulting in significant grain growth and excessive carbide precipitation during the usage process, leading to grain boundary cracking and hammerhead failure. Therefore, the casting immersion time should be greatly reduced to form stable austenite as quickly as possible, reduce the amount of carbide precipitation, avoid the occurrence of reticular carbides while increasing the water volume to maintain supercooling and accelerate internal cooling, avoiding the occurrence of excessive carbide precipitation and the presence of oversized acicular carbides under aging conditions, thereby improving the consistency of the internal and external structure, grain boundary characteristics, and performance of the hammerhead, and extending its service life.

 

Optimization measures

Based on the above analysis and discussion, the following optimization measures are formulated:

1. Reduce the C content target of high manganese crusher hammer heads to 1.25% and the Ti content target to 0.15%.
2. Add a target value of 0.5% molybdenum.
3. The heating temperature of the crusher hammer water toughening treatment was increased to 1060°C, the operation was optimized, and the time from the crusher hammer coming out of the furnace to entering the water was reduced to less than 40 seconds.
4. Optimize water quenching conditions, increase the water tank volume, or use large-flow temperature-controlled circulating water to ensure cooling intensity.

 

Implementation Effect

Microstructure

The optimized process produces Mn18 crusher hammers, and Table 2 shows the actual composition of molten steel.

Table 2. Optimized Mn18 hammer head chemical composition (wt %)
	C	Si	Mn	P	S	Cr	Mo	Al	Ti
Sample Parts	1.18	0.36	17.87	0.018	0.012	1.04	0.51	0.11	0.14

From Table 2, the ingredients reach the optimization target range.

After casting is completed, the crusher hammer is dissected, and the organization is shown in Figure 6.

Figure 6 shows that after optimizing both the composition and the process, the structure near the surface of the hammerhead becomes more uniform. The grain size is at level 2, while the grains in the core region are about level 1, showing distinct grain boundary precipitation. However, the precipitates are mainly blocky carbides, and the length of acicular carbides is mostly within 10 μm, indicating a proper reduction in carbon content. Adding Mo in combination with Cr reduces the total amount of precipitates and optimizes their morphology, which is conducive to the stability of grain boundaries. Moreover, no TiN-like blocky inclusions were observed to aggregate in sheets among the precipitates, suggesting that the adverse effects of such inclusions are within a controllable range.

After 18 months of use, this batch of crusher hammers has not experienced any fracture failures apart from normal wear on the surface ends. This indicates a significant improvement in the internal and external quality of the crusher hammers, leading to a stable extension of their service life cycle.

Conclusion

1. Cracking along the grain boundary at the cross section is the direct cause of Mn18 crusher hammer fracture, and the fundamental reason is the precipitation of grain boundary network carbides caused by an insufficient cooling rate.
2. If the Ti content is too high, a large amount of square TiN will precipitate and aggregate at the grain boundaries, which will also decrease the grain boundary bonding force and promote grain boundary cracking under the action of external forces.
3. Using Cr and Mo composite alloying can reduce the precipitation of grain boundary carbides, optimize the morphology of carbides, and significantly reduce the precipitation of oversized needle-like carbides.
4. Measures such as optimization of the water toughening process based on composition optimization are adopted to refine the grains of the Mn18 hammer, control the total amount and shape of precipitates, and ultimately extend the service time.

Based on the analysis of the fracture interface characteristics, morphology, and metallographic structure of manganese crusher hammers, it has been determined that cracking along grain boundaries, excessive Ti content, and unreasonable production processes are the reasons for the failure. By reducing Ti content, increasing Mo element, changing the production process, and other measures, the microstructure characteristics, total amount, and morphology of precipitates of Mn18 crusher hammers are optimized, and the service cycle and stability of the hammerhead are effectively improved.