How you manage your mining chain can make or break your longwall operation. The 34mm AFC chain used to be the norm in longwall mines, but now 42mm chain or larger is common. As mines extend their faces and install wider AFC conveyors to increase production, they are running 48mm or 50mm chain, with 52mm and larger mining chains on the horizon.
Running chain in these sizes can give you extended life – in ideal conditions, exceeding 11 million tons in the 48mm sizes to as much as 20Mt in the 50mm broadband-style before the chain is taken out of commission.
The cost of chain goes up significantly, but is worth it when you can mine multiple panels without experiencing chain breaks and the resulting costly production stoppages. Even with the best quality AFC or BSL chain, breaks can occur and more often than not the breakage is due to mismanagement or improper monitoring.
Environmental conditions cause stress corrosion cracking (SCC) which can present major challenges, but these problems can often be solved by specifying chain made from corrosion-resistant alloys. The bottom line? Buying quality is worth it to keep your mine operating without stoppages.
A single production stoppage due to chain failure will cost many times more than the savings you realize by buying strictly on price. That is why the purchasing process must consider input from the technical and operating staff regarding the specific problems they face. This will lead to a decision to buy the best quality chain available to use in your specific conditions. Spending more on chain requires that you spend the time and money necessary to properly manage your investment. Doing so will pay dividends and extend the life of your chain dramatically.
Before you can manage mining chain it is good to have some knowledge of the following factors that affect the quality and characteristics of chain.
Tenacity. The ability to resist fracture on application of a tensile or stretching force.
Softness. The characteristic of permanently yielding to pressure without fracture.
Hardness. The ability to resist wear or abrasion; resistance to penetration.
Brittleness. The tendency of a metal to fracture on impact or bending.
Toughness. The ability to resist fracture by bending, torsion or impact; toughness is the opposite of brittleness.
Fatigue. The diminished resistance to fracture caused by the continuous application of varying stresses.
Elongation. Extension of length.
Hardening. A process that increases hardness and tensile strength, but increases brittleness and decreases toughness and ductility.
Tempering. A process that reduces brittleness and increases the toughness of hardened steels.
Annealing. The process of softening by relieving internal stresses.
Normalizing. A similar process to annealing which differs only in the rate of cooling to produce a fine equal-axis grain structure.
DIN. The German Institute for Standardization system used globally to classify products.
Martensitic cracking. Changes in the steel structure that can occur in corrosive environments combined with internal stresses resulting in stress cracks on the surface of the chain which can lead to SCC and possible chain failure.
Heat treatment significantly affects a chain’s final properties, including strength, brittleness, internal stresses, wear resistance, and machinability.
Heat treating is a fine art and varies from manufacturer to manufacturer. The goal is to obtain a chain with the best properties possible for any given application.
Differentially hardened chain takes heat treating to the next level. This is one of the more sophisticated techniques utilized by Parsons Chain, which results in the crown of the chain link being harder to resist wear while the legs of the links are softer, tougher and more ductile in service.
The German “Werkstoff” metallurgical numbering system is an accepted way to measure differential hardening – for example, Werkstoff 1.6758, hardened at 900C, tempered at 200C, differentially tempered at 600C.
Hardness is measured by the symbol HB,Brinell hardness number, or by the symbol HV, Vickers hardness number. The Vickers hardness scale is truly proportional, so a material of 800HV is eight times as hard as one having a hardness of 100HV. The Vickers hardness number therefore provides a rational scale of hardness from the softest to the hardest material.
For low hardness values, up to about 300, Vickers and Brinell hardness results are about the same, but for higher values the Brinell results are lower due to distortion of the ball indenter.
The Charpy Impact Test is a measure of the brittleness of a material as can be obtained from an impact test. The chain link is notched at the weld point on the link and placed in the path of a swinging pendulum, the energy required to fracture the specimen being measured by the reduction in the swing of the pendulum.
Chain manufacturers generally save a few meters of each production run so they can do full destructive testing. Elongation at test force and total elongation at fracture are also graphed during this destructive test.
Test results and certificates are normally supplied with the chain so the customer will know the critical properties of what they have purchased.
Round-link and compact-link mining chain is normally shipped in 50m matched pairs. Matching is established under controlled stretching to specified Â± industry limits, usually set by DIN. Parsons Chain matches its chain pairs to closer tolerances than required by the DIN standard.
The optimum chain
A general objective of combining all these characteristics is to create the optimum chain – a chain which will deliver the best possible performance to the customer considering the following parameters:
- High tensile strength
- High resistance to inner link wear
- High resistance to sprocket damage
- Improved toughness
- Increased fatigue life
- Resistance to martensitic cracking
- Resistance to stress corrosion.
There is no one perfect solution, only various compromises. A high yield point will tend to result in high residual stress. If associated with high hardness to increase wear resistance, it will also tend to decrease toughness and resistance to stress corrosion.
Mark Newton is managing director of Ellton Longwall in Australia and is the exclusive distributor of Parsons Mining Chain in Australia and the USA. George Roman is the US sales manager for Ellton Longwall and Parsons Chain.