Sub-Critical Annealing

Sub-critical annealing is a relatively low temperature heat treating process used to reduce the hardness and increase the ductility of casting.  The sub-critical annealing process results in casting with improved machinability and a reduced likelihood of cracking during cold working operations.

Capability

ThermTech has multiple furnaces capable of meeting your sub-critical annealing projects.  The equipment used for the sub-critical annealing process meet the requirements of AMS 2750 Rev E class 3 with temperature uniformity of +/-15F.  The used of heat treating equipment with tight temperature tolerances insure your parts consistent with minimal part to part variation.  ThermTech has furnaces ranging from 12” X 12” X 12” to 72”W X 48”H X 156”L to meet you annealing needs in a timely manner.

Process

The sub-critical annealing process is a relatively low temperature heat treat process with temperatures ranging from 1275F to 1450F depending on the material.  The parts are heated to the specified temperature and held at that temperature for sufficient time to insure the parts are uniform in temperature throughout the largest cross section.  Sub-critical anneals typically require cooling in the furnace at a rate slow enough to meet the specified hardness and mechanical properties.  Typically the lower the required hardness, the longer the furnace cool.  
The process is usually performed in furnaces where the combustion gases are directly introduced into the heating zone.  Thus, oxidation in the form of rust typically occurs during the process. ThermTech has the capability to mechanically remove the surface oxidation using a variety of blast media.  Where cleanliness is critical, ThermTech does has the capability of running the parts in vacuum to insure the surface is free from heavy oxidation.  Please contact our sales group at sales@thermtech.net for recommendations on the most cost effective heat treat option for your parts.
Because the sub-critical annealing process requires relatively low temperatures, distortion is typically not an issue.  However, pre-heat treat operations that introduce stress into the part, can result in movement as the stresses are removed during the sub-critical anneal.

Recommendations

Specify the applicable hardness and mechanical requirements on your incoming paperwork.  ThermTech’s metallurgists and process development group use this information to develop a custom process that will insure your product meets your requirements. Anticipate some post heat treat surface oxidation.  If cleanliness is a concern, please request blasting.  Blasting is a valued added operation and will not be performed unless requested on the incoming purchase order.  If the work has a straightness requirement, please request straightening and provide the specific tolerance on the incoming purchase order.
Because of the requirement to furnace cool most sub-critically annealed product, turnaround times are typically 2-3 days.  Faster turnaround is possible when requested.  Please contact our customer service group at csr@thermtech.net to discuss your turnaround requirements.