Longwall mining chains are among the most critical yet cost-intensive consumable components in underground coal extraction. As a result, many overseas clients — particularly for smaller chain sizes such as 14×50 mm, 18×64 mm, 19×64 mm, and 22×86 mm — are actively exploring lower-cost steel alternatives to the premium alloys traditionally specified. While these economic grades can meet the minimum breaking force requirements, their performance under the combined assault of abrasion, corrosion, cyclic fatigue, and impact loading often falls significantly short. This paper provides a comparative engineering assessment of four candidate steels — 20Mn2, 25MnV, SAE8620H, and 23MnNiMoCr54 — across material characteristics, manufacturing and heat treatment requirements, inspection protocols, and in-service behaviour in demanding underground conditions.
Material Characteristics and Metallurgical Fundamentals
20Mn2 is a low-alloy manganese steel widely used in China for chain manufacturing. It offers moderate hardenability and, with appropriate heat treatment such as medium-temperature carbonitriding and rare-earth accelerated carburizing, can achieve C-grade strength levels per GB/T 12718. However, its relatively low alloy content limits its ability to resist corrosion and maintain toughness under severe service conditions.
25MnV builds upon the manganese base with the addition of vanadium, a powerful grain-refining element that enhances strength and toughness through precipitation strengthening and refined austenite grain size. This steel is specifically designated for high-strength round link chains used in coal mine scraper conveyors and coal ploughs. With strict control of chemical composition, inclusions, and surface quality, 25MnV offers improved hardenability over 20Mn2.
SAE8620H (equivalent to 20CrNiMoH in Chinese standards) is a nickel-chromium-molybdenum case-hardening steel. Its defining characteristic is its ability to develop a "hard case, tough core" through carburizing, quenching, and low-temperature tempering — achieving surface hardness of HRC 58–62, core tensile strength ≥835 MPa, and impact toughness ≥27 J at -20°C. The addition of molybdenum shifts the TTT curve rightward, enhancing hardenability. It exhibits excellent weldability, low cold-cracking tendency, and good machinability. However, it is important to note that SAE8620H is fundamentally a carburizing steel: without a prior carburizing treatment, its as-rolled hardness is relatively low (maximum Brinell 28), and direct induction hardening would produce only modest surface hardness in the HRC 40–50 range.
23MnNiMoCr54 (material number 1.6758 per DIN 17115) represents the premium end of the spectrum. This high-alloy steel is deliberately designed for C-grade mining chains, incorporating manganese, nickel, molybdenum, chromium, and aluminium to achieve solid-solution strengthening, grain-boundary strengthening, and second-phase particle strengthening. It delivers outstanding comprehensive mechanical properties: tensile strength ≥1200 MPa, yield strength ≥1000 MPa, and impact energy ≥60 J at 0°C. The controlled levels of sulphur and phosphorus (each ≤0.020%) significantly enhance fatigue life, while the nickel and chromium additions confer superior corrosion resistance and toughness even in aggressive mine-water environments.
In-Service Performance in Challenging Underground Conditions
The true differentiator among these steels emerges not in the factory, but underground — where chains face four simultaneous degradation mechanisms:
1. Abrasive wear: The constant friction between chain links, sprockets, and conveyed coal/rock causes progressive loss of section. While 20Mn2 can achieve adequate surface hardness through carbonitriding, the relatively shallow hardened layer and lower base hardness lead to more rapid wear once the case is worn through. 25MnV, with its vanadium-enhanced hardness and finer grain structure, offers improved wear resistance. SAE8620H, when properly carburized, can develop a deep, hard case with excellent wear resistance, but the softer core may become exposed once the carburized layer is worn away. 23MnNiMoCr54, by virtue of its higher alloy content and ability to achieve both high surface hardness and deep hardness penetration through its inherently high hardenability, delivers substantially superior wear life.
2. Corrosion and stress corrosion cracking: Underground mine water, often acidic (pH <7) with high chloride content, attacks chain surfaces. For 20Mn2 and 25MnV, which lack significant corrosion-resistant alloying elements, pitting corrosion creates stress raisers that accelerate fatigue crack initiation. The nickel and chromium in SAE8620H and particularly in 23MnNiMoCr54 provide a degree of inherent corrosion resistance, reducing the formation of corrosion pits and the risk of stress corrosion cracking. This is critical, as existing cracks grow rapidly in aggressive environments.
3. Fatigue under cyclic loading: Longwall chains experience millions of stress cycles from start-stop operations, sprocket engagement, and load fluctuations. 20Mn2, while capable of meeting static breaking load requirements, exhibits lower fatigue limits. Advanced processing — such as geometrical optimisation and anti-corrosion coating — can improve 20Mn2 chain fatigue life by up to 38% over baseline, but this still represents a compromise. 25MnV offers superior fatigue performance due to its finer grain structure. SAE8620H, with its tough core and hard case, provides good fatigue resistance for carburized components. 23MnNiMoCr54, however, is specifically engineered for fatigue-critical applications; its combination of high strength, high toughness, and corrosion resistance delivers fatigue lives substantially exceeding those of the economic grades. Field data indicate that under frequent start-stop conditions, even high-grade chains like 23MnNiMoCr54 can experience a 20–40% reduction in fatigue life — underscoring how much more severely the economic grades would be affected.
4. Impact loading: Sudden shock loads from conveyor jams or uneven material flow impose instantaneous high stresses. Here, the toughness and ductility of the steel are paramount. 23MnNiMoCr54's optimised combination of strength and toughness (impact energy ≥60 J) provides a substantial margin against brittle fracture. 25MnV offers a reasonable balance, while 20Mn2, with its lower alloy content and toughness, is more susceptible to brittle failure under severe impact.
Recommendations and Conclusions
For smaller round link chain sizes (14×50 mm through 22×86 mm), the selection of steel grade should be governed by the specific underground conditions rather than by first cost alone:
- 20Mn2 should be considered only for applications with mild to moderate wear, dry conditions, and low impact frequency. Where corrosion is minimal and start-stop cycles are limited, 20Mn2 can provide an acceptable economic solution. However, life expectancy will be substantially shorter than higher-grade alternatives, and the total cost of ownership — including more frequent replacements and associated downtime — must be carefully evaluated.
- 25MnV offers a modest upgrade over 20Mn2. Its vanadium refinement provides better wear resistance and toughness, making it suitable for moderate-duty applications where some improvement in life is desired without the full premium of high-alloy steels. It represents a pragmatic middle ground for many overseas clients.
- SAE8620H presents a specialised case. While it can achieve excellent surface hardness through carburizing, the additional processing steps add cost and complexity. It is best suited to applications where **carburized surface properties are specifically required** and where the manufacturing infrastructure supports the full carburizing cycle. For direct quench-and-temper applications without carburizing, SAE8620H is unlikely to deliver the required wear resistance.
- 23MnNiMoCr54 remains the preferred choice for demanding longwall operations — where corrosive mine water is present, start-stop cycles are frequent, impact loads are severe, and extended chain life is essential to minimise costly downtime. The higher initial material cost is typically offset by longer service intervals, reduced maintenance, and improved operational reliability. Where total cost of ownership rather than procurement cost is the deciding metric, 23MnNiMoCr54 invariably proves the most economical choice over the full life cycle.
In summary, while economic steels such as 20Mn2 and 25MnV can satisfy the static breaking force requirements for smaller mining chains, they represent a calculated trade-off: lower upfront cost in exchange for reduced resistance to the combined degradation mechanisms — wear, corrosion, fatigue, and impact — that define the harsh underground environment. For clients prioritising short-term capital expenditure over long-term operational cost, these grades may appear attractive. However, for those seeking reliable, extended service life under challenging longwall conditions, investment in 23MnNiMoCr54 — or, where carburizing capability exists, properly processed SAE8620H — is strongly recommended as the technically sound and economically prudent choice.
Post time: Aug-02-2026
