FASTENER HEAT TREATMENT
Fastener Heat treatment
Heat treatment of fasteners and fixings is a critical part of the manufacturing process. The control of temperature, time and atmosphere determine the quality, capability and reliability of the fastener. Fastener finished properties have to comply with a targeted strength range; If after heat treatment the material properties are not correct, the fastener will fail in its application.
Annealing is a heat treatment process to soften steel; the process is used in the treatment of forging wire and bar before cold forging to ensure the material is soft enough to forge. Many cold forgers have annealing plants within their factories ensuring they have control.
Annealed steel is used to produce fasteners that need to be soft and ductile to perform correctly in their chosen application. Soft fasteners include bucket and tunnel bolts and solid rivets. Bucket bolts fail when the bucket is overloaded and avoid damaging the bucket. Tunnel bolts bend along with the tunnel movement, keeping the tunnel together. Solid rivets require the second head to be formed as part of the assembly process, and the material needs to be soft enough to achieve his.
Annealing consists of heating the fasteners in a temperature controlled atmosphere followed by slow cooling.
Hardening & Tempering
Carbon and alloy steel fasteners, bolts class 8.8 (USA grade 5) and above, and nuts class 8 (USA grade 5) and above; will all require heat treatment to achieve the mechanical properties specified by the standards authorities. British inch size fasteners grade R and above also require heat treatment.
The type of heat treatment is known as hardening and tempering. Hardening occurs when specific steels are heated to a temperature that changes the structure of the steel; this is followed by quenching (rapid cooling) by submerging the steel in oil. The temperature required for structural transformation is above 850˚C but may fluctuate depending upon the amount of carbon and alloying elements in the steel. The atmosphere of the furnace is controlled to reduce the amount of oxidation in the steel.
After hardening and quenching, the steel is hard and brittle and requires a process called Tempering to reduce both the hardness and brittleness. Heating to around 450˚C lowers the hardness, reduces the brittleness, and gives the fastener toughness. Tempering is the final process, the fastener has now achieved the desired properties. High tensile finished fasteners have a Black finish that is the result of hardening and tempering.
In high volume, fasteners are transported through a series of conveyor linked heat treatment furnaces.
Solution treatment and aging
Solution treatment and precipitation hardening also called age hardening is an extensive heat treatment process to achieve hardening of some alloy steels and non-ferrous materials such as aluminium alloys, nickel-based alloys, magnesium alloys, titanium alloys and some stainless steels. The heat treatment process causes an improvement in the yield strength of the alloy. The initial solution heat treatment softens the material, this is followed by elevating temperature for hours to allow precipitation to take place, aging, which gradually increases the hardness of the material. It is a very common treatment with high-temperature alloys and aerospace alloys.
Solution treating is performed at temperatures up to 1340⁰C in a vacuum, followed by rapid gas fan cooling to room temperature. Many materials have specific cooling rates that must be achieved to ensure the proper metallurgical microstructure is achieved in the final product.
Precipitation age hardening is performed at temperatures up to 1140°C in a vacuum, inert atmosphere or air for hold times ranging up to 40 hours or more depending on the exact material and metallurgical microstructure specified. Multiple steps with decreased temperatures at each step can also be necessary to achieve the final desired results.
Link to a more in-depth article