Formation of microstructure in rail steel grinding balls depending on quenching medium parameters

Introduction

In recent years, there has been a noticeable trend towards the intensive development of domestic ball rolling production. Several modern ball rolling mills have been commissioned [1; 2], and significant efforts are underway at various operational mills to enhance the technological processes for grinding ball production [3 – 6]. This phenomenon is attributed to the growing demand for grinding balls with superior performance characteristics, specifically hardness, wear resistance, and resistance to shock loads. This demand arises from the desire to extend the service life of grinding balls used in the metallurgical, mining, and cement industries. Prolonging the service life significantly reduces the cost of the final product and enhances its quality [7 – 9]. Cost reduction is achieved through a decrease in the specific consumption of balls, while quality improvement results from minimizing the entry of broken ball particles into the crushed materials [10].

An analysis of materials from both domestic and foreign researchers indicates that the enhancement of hardness and impact resistance in balls is primarily achieved through the optimization of the chemical composition of steels employed in ball production [11 – 13]. Additionally, improvements in heat treatment modes play a crucial role in achieving these properties [14 – 16]. It is noteworthy that, aside from the mentioned characteristics, the impact resistance of grinding balls is significantly affected by the quality of their macrostructure [17; 18].

The steels used for the production of grinding balls can be categorized into two main groups based on their chemical composition [19; 20]:

– specialized ball steel;

– steels initially designed for the production of other types of rolled products (either carbon or alloyed).

It’s worth noting that within the second group of steels, a substantial portion consists of rejected rail steel blanks [21 – 23].

The technologies for the heat treatment of grinding balls can be categorized into three main organizational options:

1) hardening followed by self-tempering of the balls in the air;

2) hardening followed by low tempering;

3) “interrupted hardening” (hardening in several stages), followed by low tempering.

The second and third options for the heat treatment of balls are deemed more preferable, as they facilitate the alleviation of quenching stresses [24; 25]. However, implementing the third option is more challenging.

Irrespective of the chosen heat treatment option for grinding balls, the development of their high-quality quenching microstructure is largely influenced by the cooling capacity of the quenching medium employed. Polymers emerge as the most promising type of quenching medium, as their cooling ability can be effectively regulated across a wide range by adjusting water dilution at different concentrations.

In summary, it can be affirmed that investigations into the processes governing the formation of the quenching microstructure in grinding balls made of rail steel using polymer quenching media are currently of significant scientific and practical interest.

Materials and methods

The research focused on grinding balls that had not undergone heat treatment, selected from the mill line after rolling but before hardening, sourced from the current production of JSC Guryev Metallurgical Plant and made from rejected rail steel grade K76F.

The research was carried out in two stages:

1 – examination of the cooling effectiveness of PCM and Thermovit polymer quenching media on the Kompaton facility, with variations in polymer concentration and temperature;

2 – investigation of the microstructure of grinding balls after quenching using PCM and Thermovit polymer quenching media, with variations in their concentration and temperature.

The temperature of the cooling medium varied in the range of 20 – 40 °C with increments of 10 °C, and the concentration of each studied polymer was set at 2 and 4 %.

The Kompaton facility used in the research is equipped with a digital thermometer featuring a temperature sensor. The temperature was recorded at specified intervals in automatic mode, and the TC Soft program was employed for data processing, enabling the construction of cooling curves.

To assess the cooling effectiveness of polymer quenching media, grinding balls were heated in a laboratory furnace to the quenching temperature and subsequently cooled in a tank filled with the respective quenching medium. The hardening temperature was maintained 30 °C higher than the Aс₃ point, taking into account the actual chemical composition factoring in the actual chemical composition of the samples, as determined by X-ray spectral analysis using the Shimadzu XRF-1800 spectrometer. The actual heating temperature of the samples for quenching fell within the range of 790 – 802 °C, with a low tempering temperature ranging between 195 – 215 °С.

Microstructure and hardness studies of the balls were conducted on samples that underwent heat treatment. For each ball, one portion underwent quenching, while the other underwent quenching followed by low tempering. The microstructure analysis utilized an OLYMPUS GX-51 optical metallographic microscope, and hardness was determined using a TK-2M hardness tester.

Results and discussion

The analysis of the obtained cooling curves for grinding balls made of K76F rail steel suggests that, with both PCM and Thermovit polymer quenching media, the cooling rate remains practically identical at solution temperatures of 20 and 30 °C, regardless of their concentrations (2 or 4 %). However, a noticeable decrease in the cooling rate is observed when the polymer solution temperature is increased to 40 °C (Figs. 1 and 2). Notably, the most pronounced decrease in the cooling rate is observed with the PCM polymer at a concentration of 2 %.

Examination of the microstructure of grinding balls after a complete heat treatment cycle (quenching + low tempering) reveals that the most optimal microstructure, comprising martensite + carbides with some residual austenite, is achieved under specific quenching medium parameters:

1) at a PCM concentration of 2 % and a solution temperature of 20 and 30 °C (Fig. 3, а, b);

2) at a PCM concentration of 4 % and a polymer temperature of 40 °C (Fig. 3, c);

3) at a Thermovit polymer concentration of 4 % and a temperature of 20 °C (Fig. 3, d).

Simultaneously, the highest hardness, aligning with hardness group IV as per the state standard GOST 7524–2015 (refer to the Table), is exhibited by balls hardened using the first set of hardening medium parameters (PCM concentration 2 %, solution temperature 20 and 30 °C). Grinding balls hardened by employing the parameters of the quenching medium according to the second and third options only meet the criteria for hardness group II, as per GOST 7524–2015 (PCM concentration 4 %, temperature 40 °C; Thermovit concentration 4 %, temperature 20 °С).

When a 4 % PCM polymer solution is utilized for hardening balls at temperatures of 20 and 30 °C, a microstructure in the form of troostomartensite + carbides + residual austenite is formed (Fig. 4). Although the hardness of such balls falls within hardness group II according to GOST 7524–2015 (refer to the Table), they exhibit potentially higher impact strength owing to the properties of the troostomartensite phase.

However, quenching balls according to other combinations of quenching medium parameters results in a defective microstructure. In addition to martensite, there is the presence of quenching troostite in various forms: acicular, spheroidal, and in the form of a network (Fig. 5). The existence of troostite in the structure indicates a lower cooling rate, signifying inadequate cooling capacity of the quenching medium. Regardless of the type of troostite, its negative impact on the hardness of the grinding balls is evident (refer to the table). Notably, the more pronounced negative influence is naturally exerted by the spheroidal troostite and the form of a grid.

In general, it is noteworthy that the quality of the microstructure in balls hardened with PCM solution is significantly superior compared to those hardened with Thermovit polymer solution. For instance, balls hardened in a PCM solution with a 2 % concentration at a temperature of 40 °C exhibit only acicular troostite in the structure (Fig. 5, a); balls hardened with Thermovit polymer at a similar concentration and temperature display spheroidal troostite and troostite in the form of a grid (Fig. 5, c). Moreover, the hardness of balls hardened using PCM polymer with the specified concentration and temperature, both on the surface and in the core, is, on average, 6 – 7 HRC higher than the hardness of balls hardened in Thermovit polymer medium (refer to the Table).

In conclusion, it can be inferred that altering the parameters of the polymer quenching medium enables the variation of performance characteristics in grinding balls made of rail steel, thus determining potential applications. For example:

• quenching balls in a PCM polymer solution with a 2 % concentration at a temperature of 20 – 30 °C ensures the production of balls with high hardness (hardness group IV as per GOST 7524–2015).

• employing the same polymer solution for quenching with a 4 % concentration and a temperature of 20 – 30 °C creates the possibility of producing balls with lower hardness but potentially high impact resistance.

However, caution is advised as certain combinations of PCM and Thermovit polymer concentrations and temperatures may lead to a high risk of obtaining a defective microstructure.

Conclusions

Based on laboratory experimental studies, cooling curves were constructed for hardening of the balls made of K76F rail steel in solutions of PCM and Thermovit polymers, each with a concentration of 2 and 4 % and temperature ranging from of 20 to 40 °C. The experimental research in the laboratory has elucidated the patterns governing the formation of the microstructure in grinding balls from the specified steel when utilizing PCM and Thermovit polymer quenching media with varying heat treatment parameters.

Notably, the use of PCM solution for ball hardening ensures a significantly higher quality of the microstructure and hardness of the balls compared to utilization of Thermovit polymer. As a result, recommendations have been developed for optimal combinations of concentration and temperature of PCM polymer. These recommendations aim to ensure the production of balls with increased hardness as well as the production of balls with lower hardness but with a heightened level of impact resistance.