You’ve no doubt heard it said that a chain is no stronger than its weakest link. How do we know the links are strong enough? The key is heat treatment.



You’ve no doubt heard it said that a chain is no stronger than its weakest link. This is not comforting when you are crossing the manufacturing plant floor with a large, heavy object suspended from a crane using a chain. How do we know the links are strong enough? The key is heat treatment.

Like the bolts discussed in last July’s installment of NYK, conventional welded-steel chain is graded for certain strength levels and a variety of end uses. As defined by the Welded Steel Chain Specifications – established by the National Association of Chain Manufacturers (NACM) – there are nine different “grades” of chain. These are differentiated by their end use and/or material. For instance, the stainless, passing link, coil and machine chains have differences in design or material that make them unique.

The remaining five chains are graded. Grade 30 is a general-purpose, carbon-steel chain, and grade 43 is a widely used carbon-steel chain with a minimum breaking force of about three times that of grade 30. Grade 70 is a high-quality, high-strength, carbon-steel chain used for load securement and is about 50% stronger than grade 43. Grade 80 is the first one capable of being used for overhead lifting applications. It is a premium-quality, high-strength, alloy-steel chain that is heat treated and tempered. Also heat treated, grade 100 is the highest-strength alloy chain. Only alloy-steel chain should be used for overhead lifting.

What makes grades 80 and 100 better than the rest? A significant difference is that they are manufactured from alloy versus carbon steel. The most significant difference, however, is their heat treatment to optimize the mechanical properties. One of the improved property requirements is elongation, which is required to be “not less than 20%” at normal temperatures. Normal operating temperatures for chains are -40°F-400°F (-40°C-204°C). Specifications allow for the use of grades 80 and 100 chains at elevated temperatures with a commensurate reduction in the working load limit.

Standard grades of chain can be enhanced to adapt them for specific applications. For example, a complete line is manufactured for marine applications. In the most demanding marine applications – such as Tuna-Net and Scallop Chain – only heat-treated chain is used. Tuna-Net chain is also continuously hot-dip galvanized to provide a deep, strong protective coating.

In addition to welded-steel chain, another type – steel roller chain – is commonly used. Hans Renold is credited with inventing roller chain in 1880, although da Vinci’s sketches – from the 16th century – show a similar design. Roller chain is most commonly used for transmission of mechanical power on cycles and in industrial and agricultural equipment. Roller chains are used in a wide range of applications with the most demanding requiring precise heat-treatment parameters.

An example of this is a complete line of roller chains for use in the citrus industry. Called “Citrus Chains,” they are made from a special alloy to withstand the corrosive environments found in the citrus industry. Coupled with an optimized heat treatment for extra strength and wear resistance, these chains are ideally suited for the operating conditions of this industry. Another application is the “Oil Field Roller Chains.” Also made from special alloy steel with the latest heat-treatment techniques, these chains can be simplex (single row), duplex or triplex.

Similar to welded-steel, roller chain is also standardized. Introduced in 1925, BS 228 was the original standard. ISO 606 is now the drive-chain standard adopted globally. In addition to the standards, software has also been developed to simplify the selection of the right roller chain for the application. Based on factors such as speed, load and the power ratings of the user’s application, the software selects the most suitable chain from 11 different product types and 400 chain variations.

Now you know what it takes to make a good chain. Without proper heat treatment, it’s just another weak link. IH