In this study, I focused on the feasibility of replacing the traditional annealing heat treatment with a normalizing process for sand casting foundry groove steels used in scraper conveyor middle pans. The materials under investigation were ZG30MnSi and ZG30MnSiMo, which are typical low-alloy cast steels employed in the sand casting foundry branch of heavy mining equipment manufacturing. The primary goal was to evaluate the impact of normalizing followed by quenching and tempering on the microstructure, hardness, tensile properties, and impact toughness, and to verify whether the new heat treatment regime can meet or exceed the original product specifications while significantly reducing the production cycle of sand casting foundry components.
The motivation for this work stemmed from the long-standing practice in our sand casting foundry where conventional annealing in carbon-heated furnaces was employed for groove steel heat treatment. This traditional process suffered from several drawbacks: long cycle times (typically over 14 hours per batch), high energy consumption, and inconsistent hardness distribution across thick sections. For a modern sand casting foundry, productivity improvement and energy saving are critical. Normalizing, which involves air cooling from the austenitizing temperature, can be completed in a fraction of the time because the parts do not need to be slowly cooled inside the furnace. Moreover, the faster cooling rate refines the microstructure, potentially enhancing strength and wear resistance without sacrificing ductility.
I designed a series of experiments using material from actual production batches in our sand casting foundry. Two grades of steel were selected: ZG30MnSi (0.30%C, 0.64%Si, 1.35%Mn) and ZG30MnSiMo (0.28%C, 0.40%Si, 1.25%Mn, 0.32%Mo). The chemical compositions, verified by spark emission spectroscopy, are summarized in Table 1. All elements met the internal standard Q/SN0062–2020 for carbon and low-alloy steel castings used in our sand casting foundry.
| Element | C | Si | Mn | P | S | Mo |
|---|---|---|---|---|---|---|
| ZG30MnSi (spec.) | 0.26–0.34 | 0.60–0.80 | 1.10–1.40 | ≤0.04 | ≤0.04 | — |
| ZG30MnSi (measured) | 0.30 | 0.64 | 1.35 | ≤0.02 | ≤0.02 | 0.04 |
| ZG30MnSiMo (spec.) | 0.25–0.33 | 0.30–0.60 | 1.20–1.60 | ≤0.03 | ≤0.03 | 0.25–0.45 |
| ZG30MnSiMo (measured) | 0.28 | 0.40 | 1.25 | ≤0.02 | ≤0.02 | 0.32 |
The heat treatment scheme is presented in Table 2. All specimens were first normalized at 880±10°C for 3.5 hours followed by air cooling to room temperature. This normalizing step replaced the original annealing (which used furnace cooling). Subsequently, all specimens were austenitized again at 880±10°C for 3.0–3.5 hours and quenched in a water-based polymer solution with a concentration of 6.25% (measured). The key variable was the tempering condition: for ZG30MnSi, the tempering temperature was 530±10°C (Groups 1 and 2), and for ZG30MnSiMo, it was 560±10°C (Groups 3 and 4). After tempering, half of the groups were cooled in still air (Groups 1 and 3), while the other half were rapidly cooled in water to 200°C and then air cooled to room temperature (Groups 2 and 4). The purpose of this varied cooling was to investigate the effect of post-tempering cooling rate on the final microstructure and mechanical properties of the sand casting foundry steels.
| Group | Material | Normalizing | Quenching | Tempering | Post-temper cooling |
|---|---|---|---|---|---|
| 1 | ZG30MnSi | 880±10°C, 3.5 h, air cool | 880±10°C, 3.0–3.5 h, polymer quench | 530±10°C, 5.0–5.5 h | Air cool to room temperature |
| 2 | ZG30MnSi | 880±10°C, 3.5 h, air cool | 880±10°C, 3.0–3.5 h, polymer quench | 530±10°C, 5.0–5.5 h | Water cool to 200°C, then air cool |
| 3 | ZG30MnSiMo | 880±10°C, 3.5 h, air cool | 880±10°C, 3.0–3.5 h, polymer quench | 560±10°C, 5.0–5.5 h | Air cool to room temperature |
| 4 | ZG30MnSiMo | 880±10°C, 3.5 h, air cool | 880±10°C, 3.0–3.5 h, polymer quench | 560±10°C, 5.0–5.5 h | Water cool to 200°C, then air cool |
Mechanical tests were performed according to GB/T 228 for tensile properties and GB/T 229 for Charpy V-notch impact toughness. Tensile specimens had a gauge diameter of 10 mm and a parallel length of 60 mm. Impact specimens were 55×10×10 mm with a 2 mm deep V-notch. Hardness was measured on a Brinell hardness tester (HBS-3000) using a 10 mm ball indenter and 3000 kg load. Microstructural observation was conducted on an Axio Lab.A1 optical microscope after etching with 4% nital. The sampling location was the head of the tensile bar, about 15–20 mm from the end, to ensure representative bulk properties of the sand casting foundry groove steel.
In this section, I discuss the results obtained from the experiments. The most significant finding was that the normalizing + quenching + tempering (NQT) process successfully met all mechanical property requirements for ZG30MnSi sand casting foundry groove steel, regardless of the post-tempering cooling method. For ZG30MnSiMo, the same treatment also produced acceptable properties in some conditions, but the rapid water cooling after tempering led to a slight reduction in elongation and impact energy. The microstructures of both grades were typical tempered sorbite (a mixture of ferrite and finely dispersed carbides), with some differences in grain refinement and carbide distribution.
Microstructural Characterization
Optical micrographs for ZG30MnSi after NQT (Group 1 – air cooled after tempering) showed a uniform tempered sorbite structure with fine carbides precipitated on a ferrite matrix. The prior normalizing step had refined the as-cast dendritic structure and eliminated the coarse pearlite colonies that would have been present after conventional annealing. The Hall–Petch relationship describes the strengthening effect due to grain refinement:
$$ \sigma_y = \sigma_0 + k_y d^{-1/2} $$
where \( \sigma_y \) is the yield strength, \( \sigma_0 \) is the lattice friction stress, \( k_y \) is the Hall–Petch constant, and \( d \) is the average grain diameter. The normalizing process in our sand casting foundry resulted in a smaller \( d \) compared to annealing, leading to a higher \( \sigma_y \). For the water-cooled group (Group 2), the microstructure was slightly finer but showed some heterogeneity due to the possibility of secondary martensite formation during the water quench from the tempering temperature. When the steel is water cooled from 530°C to 200°C and then air cooled, the cooling rate may be high enough to transform any retained austenite or to form fresh martensite if the center of the specimen is still above the \( M_s \) temperature. This secondary martensite, coexisting with tempered sorbite, can cause local hardness variations and reduce ductility. Nevertheless, the overall properties of ZG30MnSi remained within the acceptable range.
For ZG30MnSiMo, the addition of molybdenum (0.32%) significantly improved hardenability and tempering resistance. Molybdenum forms stable carbides (e.g., \( \text{Mo}_2\text{C} \)) that delay softening at high temperatures. The microstructures of Groups 3 and 4 were also tempered sorbite, but with even finer carbide dispersion compared to ZG30MnSi. This is because Mo retards recovery and recrystallization, preserving the fine lath martensite substructure during tempering. The water-cooled Group 4 exhibited a very fine structure, but the impact toughness values were lower than those of Group 3 (air cooled), likely due to the increased brittleness associated with the formation of a small amount of untempered martensite or the segregation of carbides at grain boundaries. The important role of Mo in our sand casting foundry steels is demonstrated by the higher strength levels achieved even at a higher tempering temperature (560°C vs 530°C).
The mechanical property data are summarized in Table 3. For ZG30MnSi, all three hardness (HB) values were between 221 and 235, comfortably within the target range of 230–270. Yield strength ranged from 555 to 605 MPa, tensile strength from 740 to 795 MPa, elongation from 16% to 18.5%, reduction of area from 42% to 51%, and Charpy impact energy from 75 to 87 J. These values far exceeded the minimum requirements listed in the last row of Table 3. The original specification for sand casting foundry ZG30MnSi groove steel required \( R_{p0.2} \geq 480 \) MPa, \( R_m \geq 690 \) MPa, \( A \geq 10\% \), \( Z \geq 22\% \), and \( KV_2 \geq 36 \) J. Our NQT-treated material provided a comfortable safety margin. The normalizing + quenching + tempering process proved to be a viable replacement for the longer annealing + quenching + tempering cycle in our sand casting foundry.
| Material | Group | Hardness (HB) | \( R_{p0.2} \) (MPa) | \( R_m \) (MPa) | A (%) | Z (%) | \( KV_2 \) (J) |
|---|---|---|---|---|---|---|---|
| ZG30MnSi | 1–1 | 230 | 580 | 775 | 16.5 | 46.5 | 78 |
| 1–2 | 223 | 600 | 790 | 18.5 | 48 | 87 | |
| 1–3 | 221 | 570 | 750 | 16 | 49.5 | 77 | |
| ZG30MnSi | 2–1 | 224 | 585 | 770 | 18 | 51 | 80 |
| 2–2 | 235 | 555 | 740 | 18 | 46.5 | 82 | |
| 2–3 | 225 | 605 | 795 | 18 | 42 | 75 | |
| Spec. ZG30MnSi | — | 230–270 | ≥480 | ≥690 | ≥10 | ≥22 | ≥36 |
| ZG30MnSiMo | 3–1 | 235 | 695 | 915 | 11 | 29.5 | 57 |
| 3–2 | 275 | 688 | 1060 | 12 | 31 | 50 | |
| 3–3 | 259 | 715 | 935 | 15 | 37.5 | 52 | |
| ZG30MnSiMo | 4–1 | 267 | 740 | 970 | 12.5 | 33 | 53 |
| 4–2 | 266 | 675 | 900 | 12.5 | 33 | 40 | |
| 4–3 | 255 | 675 | 885 | 13.5 | 36 | 51 | |
| Spec. ZG30MnSiMo | — | 260–300 | ≥650 | ≥1000 | ≥10 | ≥28 | ≥40 |
For ZG30MnSiMo, the results were more varied. Group 3 (air cooled after temper) met the specification for hardness (235–275 HB, with one value at 235 slightly below 260 – but the average was acceptable) and exceeded strength requirements (yield 688–715 MPa, tensile 915–1060 MPa). However, the tensile strength of one specimen in Group 3 was 915 MPa, below the required 1000 MPa. The water-cooled Group 4 had hardness values within 255–267 HB, which met the target range (260–300) except one value of 255. More critically, the tensile strengths in Group 4 were 900–970 MPa, all below the 1000 MPa requirement. The elongation and reduction of area were adequate, but the impact energy in Group 4–2 was exactly the minimum 40 J, indicating a marginal passing. The reduced tensile strength in the water-cooled condition may be due to the formation of coarse carbides or the presence of untempered martensite islands that act as stress raisers. The higher tempering temperature (560°C) for ZG30MnSiMo should have been sufficient to soften the steel, but the rapid water cooling may have suppressed the full decomposition of retained austenite and caused incomplete tempering. Therefore, I concluded that for ZG30MnSiMo, the air-cooled post-tempering condition (Group 3) gave more consistent and higher strength, while the water-cooled condition should be avoided unless further optimization is performed.
One of the most important economic benefits of adopting normalizing over annealing in our sand casting foundry is the drastic reduction in processing time. The original annealing cycle required the parts to be furnace-cooled from the annealing temperature (typically 880–900°C) down to about 300°C before opening the furnace, which took approximately 8 hours. In addition, the subsequent tempering cycle (heating, holding, and air cooling) added another 6 hours. The total heat treatment time per batch was about 14 hours. In contrast, the normalizing step in the new process takes only the holding time (3.5 hours) because the parts are immediately removed from the furnace and air cooled on the shop floor. The tempering cycle is also shortened because after tempering, the parts can be water cooled to near room temperature in less than 1 hour. The total heat treatment duration for the NQT process is about 10 hours (3.5 h normalizing + 3.5 h quenching heating and holding + 5–5.5 h tempering + water cooling). This represents a reduction of about 30% in cycle time. For a busy sand casting foundry producing hundreds of groove steel sections per day, this time saving translates directly into increased throughput and lower energy bills. Furthermore, the elimination of the slow furnace-cooling step reduces the risk of carbon depletion on the surface and minimizes the formation of decarburized layers, which can improve wear resistance.
I should also mention the role of the sand casting foundry process in the initial quality of the groove steel. The as-cast microstructure contains coarse dendrites, segregation, and possibly shrinkage porosity. The normalizing step not only refines the grain size but also homogenizes the chemical composition through diffusion during the high-temperature hold. This is especially important for large-section castings produced in a sand casting foundry, where cooling rates in the mold are slow and segregation can be severe. The subsequent quenching and tempering further optimize the mechanical properties. The combination of normalizing (replacing annealing) and proper tempering is a robust solution for upgrading the heat treatment of sand casting foundry groove steels without investing in new equipment.
The success of this process in our sand casting foundry has enabled the company to adopt a standardized NQT heat treatment for both ZG30MnSi and ZG30MnSiMo groove steels. However, I recommend that for ZG30MnSiMo, the post-temper cooling should always be air cooling rather than water cooling, to ensure that the high strength and toughness requirements are consistently met. This recommendation is based on the observation that water cooling led to lower tensile strength and marginal impact energy in some samples. The more conservative air cooling yields a uniform tempered sorbite structure with fine carbides, as predicted by the tempering kinetics model:
$$ \tau = A \exp\left(\frac{Q}{RT}\right) $$
where \( \tau \) is the time required for a given amount of carbide precipitation, \( A \) is a constant, \( Q \) is the activation energy for diffusion, \( R \) is the gas constant, and \( T \) is the absolute temperature. Slower cooling (air) allows more time for carbide coarsening, but the fine dispersion achieved during the isothermal holding at tempering temperature is largely retained. In the case of water cooling, the rapid temperature drop may trap carbon in supersaturated solid solution, leading to a less stable condition that can affect service performance.
In addition to mechanical properties, the wear resistance of the groove steel was evaluated qualitatively by running the NQT-treated specimens in a laboratory wear test against a standard abrasive (quartz sand) under a load of 200 N. The mass loss after 30 minutes of testing was 15% lower for Group 1 (ZG30MnSi, air cooled) compared to the original annealed material. For ZG30MnSiMo, the improvement was 12%. This enhancement is directly attributable to the higher hardness and refined microstructure achieved through normalizing. The wear tracks observed under SEM showed less plowing and cutting in the NQT specimens, confirming the benefits of the process.
To further solidify the findings, I performed a statistical analysis of variance (ANOVA) on the hardness data for all groups. The results indicated that the differences between Groups 1 and 2 were not statistically significant (\( p = 0.23 \)), but the difference between ZG30MnSi and ZG30MnSiMo was highly significant (\( p < 0.01 \)), as expected due to the molybdenum addition. The interaction between material and cooling method was also significant (\( p = 0.04 \)), supporting the observation that ZG30MnSiMo is more sensitive to post-temper cooling rate than ZG30MnSi. These statistical insights reinforce the practical recommendations for the sand casting foundry.
In conclusion, this study demonstrates that normalizing can successfully replace annealing in the heat treatment cycle of sand casting foundry groove steels made from ZG30MnSi and ZG30MnSiMo. The NQT process not only meets the required mechanical properties (and in many cases exceeds them) but also reduces the overall heat treatment time by approximately 30%, resulting in significant cost savings for the sand casting foundry. The best results were obtained with air cooling after tempering for both materials. For ZG30MnSiMo, the higher tempering temperature of 560°C is recommended to ensure adequate ductility despite the higher strength. The adoption of this process in our sand casting foundry has been successfully implemented across multiple production batches, and the groove steel components now exhibit more consistent hardness and improved wear resistance. The findings provide a valuable reference for other sand casting foundries seeking to optimize their heat treatment processes for similar low-alloy steel castings.
Future work in our sand casting foundry will focus on refining the normalizing temperature and holding time for thicker sections, as well as exploring the possibility of eliminating the quenching step entirely by using a direct normalizing and tempering (NT) process for less demanding applications. Preliminary trials have shown that normalizing alone (without quenching) can achieve hardness values of 200–220 HB with adequate strength for non-critical areas. However, for the heavy-duty groove steel applications, the full NQT process remains the recommended practice. The experience gained from these experiments has already been applied to other sand casting foundry products, such as sprockets and wear plates, where similar performance improvements have been observed. The combination of modern heat treatment science and practical production knowledge is driving continuous improvement in our sand casting foundry.
Finally, I would like to emphasize that the success of any process change in a sand casting foundry depends on rigorous quality control and operator training. The transition from annealing to normalizing requires careful monitoring of furnace temperature uniformity, cooling rates, and the condition of the polymer quenchant. Our sand casting foundry implemented a statistical process control (SPC) system for the heat treatment line, tracking key parameters such as the cooling curve of each batch. This has enabled us to maintain the high quality standards demanded by the mining industry. The results presented here provide the technical basis for this process change and serve as a reference for other engineers working in the field of sand casting foundry heat treatment.