How are round steel chains produced?
Round steel chains are bent from a wire section and welded in the middle. The welding bead is then deburred. After welding, the chains are hardened by heating them to over 1,000 °C. This changes the structure of the material. This structure, which is responsible for greater hardness and strength, should be maintained. To achieve this, the chains are quickly quenched to room temperature.
What is tempering?
The chains are now extremely hard. Depending on the application, it is then annealed again, i.e. heated to over 300 °C. This process is referred to as tempering. Although it reduces the high degree of hardness, it increases the toughness and also improves many other properties of the chains.
How do you recognise the grade?
In principle, however, you cannot see the grade of a chain from the outside. For this reason, chains are already given a quality stamp during production, which clearly defines the grade. In the case of lifting chains, this can be the stamp "(H1) 8" for Grade 8 or "(H1) 10" for Grade 10. The H stands for "high-strength" and is awarded by the German Social Accident Insurance (DGUV). The number after the H identifies the manufacturer of the chains. Because RUD was always the first to have chains tested by the DGUV, the 1 always stands for RUD.
For Grade 12, the responsible German Social Accident Insurance has issued a completely new stamp "(D)" for certain reasons. This is why RUD was the first manufacturer of round steel chains to receive approval for Grade 120 with the stamp (D1) 12 in 2007. These chains are called ICE chains at RUD.
How do you determine the grade?
If this pre-calibrated chain is subjected to a tensile load (F), it may only break after reaching the so-called minimum breaking strength value. In the event of Grade 80, 8 mm chains, this can be the case at F = 80,000 N (80 KN ≈ 8,000 kp [kg]). To determine the grade, you need another value: The area of the chains (both wire diameters); this is also called the loaded cross-section.
By the way: since the characteristics of a hoist chain differ considerably from those of a lifting chain, hoist chains are stamped with letters rather than numbers to indicate their grade. This should prevent dangerous mix-ups.