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Fastener Heat-Treating Tips (part 2)

May 9, 2012
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We continue with our discussion on the heat treatment of fasteners by offering additional tips for the heat treater.  

Tip #2: Pay Attention to Material Chemistry
Certain alloying elements have a strong influence on both the response to heat treatment and the ability of the fastener to perform its intended function. One such element is boron, which is a common addition to fastener steels in a composition range of 0.0005-0.003%. It is extremely effective as a hardening agent and impacts hardenability. It does not adversely affect formability or machinability. Boron permits the use of lower-carbon steels with improved formability and machinability.  

During the steelmaking process, failure to tie up the free nitrogen results in the formation of boron nitrides that will prevent the boron from being available for hardening. Titanium and/or aluminum are added for this purpose. It is important, therefore, that the mill carefully control the titanium/nitrogen ratio. Both titanium and aluminum tend to reduce machinability of the steel. However, the formability typically improves. Boron content in excess of 0.003% has a detrimental effect on impact strength due to grain-boundary precipitation.  

In addition, trace-element chemistry is an important consideration since these tramp elements (e.g., titanium, niobium and aluminum) may retard carburization.  

Tip #3: Control the Annealing Process
Spheroidize annealing is an important step in the cold-forming process for fasteners because it ensures that the microstructure of the steel is soft and has maximum formability. Since the fastener manufacturer does not often perform this process, be sure to specify and check on a routine basis the level of spheroidization required.  

In spheroidize annealing, the cementite layers are caused to collapse into spheroids or globules by time and temperature. This globular form of cementite tends to facilitate cold deformation in such processes as cold heading, cold rolling, forming and bending.  

Tip #4: Remove Excess Residues and Coatings Present on Parts
Phosphate coatings (e.g., zinc, manganese) present on many fasteners from the thread-rolling process have been reported[1] to cause problems in which fasteners clump or fuse together or even melt at austenitizing temperature. Unless phosphates are present in large quantities, in general they do not cause furnace issues, although there is a potential to alter the furnace atmosphere. Zinc phosphate, for example, will begin to deteriorate starting at 105°C (225°F), a temperature typically found in the entry vestibule of most furnaces.

Residue Tips

1. Inspect thread areas for residues, fines (metal particles) or the presence of abnormally high levels of lubricants.

2. Visually inspect for excessive amounts of sooting or spotty surface appearance on the fasteners, especially in the thread areas.

3. Check the operation of the first zone of a continuous furnace (e.g., temperature drop, smoking) to ensure it is handling the residue load without impacting the remainder of the furnace.

4. Inspect the entrance end of the furnace for deposits (e.g., zinc vapors leave a whitish residue).

5. Monitor the carbon dioxide (CO2) and carbon monoxide (CO) levels in the furnace atmosphere via a three-gas analyzer when phosphate-coated parts are run. Watch for spikes in CO2 levels that may indicate that the phosphate coating is decomposing and reacting with CO.  
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