Why is service temperature such an important factor to consider in safety-critical lifting applications? Two possible changes in the steel, depending on the temperature, can take place when lifting products are subjected to heat.
1. As temperature increases, the chain strength will decrease.
Example:
On grade 8 chain, the following guidelines are given to compensate for this effect:
·For temperatures up to 200⁰C, no reduction in working load limit is required.
·For temperatures > 200⁰C up to 300⁰C, a 10 % reduction in working load limit is recommended.
· For temperatures > 300⁰C up to 400⁰C, a 25 % reduction in working load limit is recommended.
In all three scenarios, no permanent change in properties (and subsequently the working load limit) has taken place.
"As soon as the chain has cooled down to room temperature, the original working load limit will still be applicable."
This is because the original temper temperature during the heat treatment of the steel of 400⁰C was not exceeded. The minimum temper temperature of 400⁰C is a requirement in the national and international chain and component specifications for example such as EN 818 Part 2, SANS 189and EN1677 Part 4.
2. As temperature increases, the chain strength will decrease.
When the temper temperature exceeds 400⁰C, a permanent change on a microstructural level in the steel will take place. Also, a permanent decrease in hardness will result. This is caused by the steel being tempered back more compared to the original heat treatment.
For this reason, grade 8 products that have been exposed to temperatures exceeding 400⁰C should be taken out of service and replaced with new chains.
In case of uncertainty, McKinnon Chain can be approached for advice regarding products that are subjected to high temperatures in service.