2.4 Heat Treatment & Metallurgy in Lifting Equipment
Key Takeaways
- Strength, ductility, toughness and hardness are different properties that must be balanced by the product design and heat treatment.
- Tempering reduces the brittleness and residual stress of hardened steel, with exact cycles determined by alloy and process specification.
- Welding, arc strikes, fire and uncontrolled heating require withdrawal and authorised assessment rather than a universal temperature assumption.
- Acid cleaning, plating and galvanising can introduce hydrogen or heat effects and must not be performed as uncontrolled site repairs.
- Temperature and coating limits apply to the exact chain, fitting and assembly, not to all alloy steel as one category.
Heat Treatment and Metallurgy in Lifting Equipment
Material choice and heat treatment create the combination of strength, toughness, ductility, wear resistance and environmental performance required by a lifting component. Grade, colour and appearance alone do not prove those properties; the finished product's standard, marking and certificate do.
Mechanical Properties
Strength is resistance to applied stress. Ductility is the ability to deform plastically before fracture. Toughness is the ability to absorb energy before fracture, especially in the presence of a notch. Hardness resists indentation and wear but does not by itself prove toughness or WLL.
A lifting component needs balance. Very hard steel can be brittle; very soft steel can deform at low load. Product standards therefore specify material, heat treatment, tests and minimum properties as a system.
Carbon and alloy steels dominate many chains, hooks, shackles and links. Stainless and non-ferrous materials serve particular corrosion, spark or weight needs, but their exact grade and product design govern capacity. “Stainless” is not one composition and is not immune to chloride attack.
Heat-treatment Processes
Annealing generally heats and cools material to soften it, reduce stresses or prepare it for working. Normalising uses controlled heating and air cooling to refine structure in suitable steels. Hardening forms a harder microstructure through austenitising and sufficiently rapid cooling. Tempering reheats hardened steel below its critical transformation range to reduce brittleness and develop the required strength-toughness balance.
Exact temperatures depend on composition, section size and process specification. It is incorrect to assign one “above 850°C” austenitising temperature or one 400-650°C tempering range to every lifting alloy. Quenched-and-tempered Grade 8 or Grade 10 components are produced through validated manufacturer cycles and testing.
A quiz calculation should therefore distinguish process purpose from production recipe: tempering reduces the brittleness and residual stress of as-quenched martensitic steel; it does not melt the steel or add carbon.
Unauthorised Heat and Welding
Welding, flame straightening, torch cutting, grinding burns and arc strikes can create a heat-affected zone, local hardening or softening, cracks, residual stress and loss of section. The precise mechanism depends on alloy, heat input, cooling and prior treatment; an arc strike does not always create an identical microstructure, but it is a serious unapproved modification.
Do not weld or heat a rated alloy-chain component unless the manufacturer or an authorised engineering procedure expressly permits it and specifies re-verification. Visible heat tint, scale, distortion or fire exposure requires withdrawal and competent assessment.
Temperature limits and reductions are product-specific. Some Grade 8 chain systems publish use ranges and reductions up to a defined temperature; Grade 10 systems may use different limits. Do not apply a universal 400°C discard threshold to every steel component. After a fire reaching about 450°C, remove a Grade 8 sling from service and follow the manufacturer's assessment or disposal criteria—cooling to room temperature does not itself restore verified capacity.
Hydrogen Embrittlement and Coatings
Acid cleaning, electroplating, corrosion and cathodic processes can introduce hydrogen into high-strength steel. Trapped hydrogen may promote delayed brittle cracking under tensile stress. Hot-dip galvanising also exposes components to heat and process chemicals.
Users must not acid-pickle, electroplate or galvanise lifting chain as an uncontrolled site repair. It is equally unsafe to prescribe a universal “bake at 200°C for 4-8 hours” as a user remedy. Any coating process for high-strength lifting components must be designed and controlled by the manufacturer or an authorised specialist, including material-specific hydrogen-relief steps and final verification.
Inspection Clues
Look for:
- arc strikes, weld beads or spatter;
- grinding marks, blueing, scale or local heat tint;
- flaking or unapproved coating;
- corrosion pits and cracks;
- bent, stretched or softened contact areas; and
- missing grade or traceability marks.
Withdraw affected equipment and record the exposure. Hardness testing, metallography or NDT may help an authorised investigation, but a single spot hardness reading cannot recreate the product's original certification.
Selection and Service
Before using metal lifting equipment in a furnace, foundry, freezer, chemical plant or offshore environment, verify material, coating, temperature-time curve, WLL reduction, corrosion compatibility and inspection plan. Include fittings and terminations: a chain may tolerate an environment that its tag, pin, ferrule or lubricant does not.
The Foundation rule is: heat treatment is a controlled manufacturing property; field heating is a change requiring authority and evidence.
What is the purpose of tempering after hardening suitable alloy steel?
Why is unauthorised welding or arc striking on rated alloy lifting equipment unacceptable?
A Grade 8 chain sling is exposed to a fire around 450°C. What is the correct action?