Edited by Jessica Shapiro
Even the most quality-obsessed gear manufacturers sometimes see corrosion, discoloration, and contamination on their gears after heat treatment or during bouts of heat and humidity. The problems range from aesthetic annoyances to surface structural degradation.
These problems can be compounded because many companies contract with outside heat-treatment vendors. When parts come back in-house and head directly to the next processing step, gear grinding, cylindrical grinding, and other machining operations can increase damage from corrosion or contamination and make correcting it difficult or even impossible.
Luckily, a few changes to early processing steps can prevent issues from blossoming into costly rework.
Adding a media-blasting step immediately after heat treating can ensure corrosion and discoloration don’t persist on parts during downstream processing. Media-blasting forms range from those that gently remove contamination to ones that change surface stress states with peening action or remove enough material to change part dimensions.
For postheat-treatment blasting, stick to gentle, cosmetic cleaning. The goal is a uniform and clean surface on all areas that are not ground, including gear root diameters, where appropriate. Hand blasting gives operators the control needed to prevent part damage. High-volume blasting techniques like tumble blasting are not recommended.
Some heat treaters can do their own hand blasting and gear manufacturers need only add the cleaning step to the routing. However, if a gear manufacturer lacks confidence that a vendor has the expertise, equipment, or attention to detail to perform adequate blasting without part damage, the step can also be done in-house.
In some shops, hand-blasting units have been phased out because of the extra time and labor required to individually clean each part. Excessive dust and grit escaping into the work environment is another concern.
However, the expense associated with returns and rework might justify reinstating hand blasting as an aggressive step to prevent corrosion, discoloration, and contamination from reaching later processing steps. This is true for many parts, not just those which have been heat treated.
Another way to keep heat-treated parts clean is to use cleaner heat-treating processes. Vacuum heat treating is one such process that may work for some parts.
Because the high temperatures used in heat treating can speed oxidation if done in an oxygen-rich environment like room air, removing the air results in parts that are cleaner and have a better appearance. Vacuum heat treatment doesn’t change a gear’s metallurgical properties.
Clean parts make it easier for outside parts inspectors to discern if a part meets specifications, so vacuum heat-treated parts are less likely to be returned for rework.
Most parts, whether they see heat treatment or not, go through some kind of final cleaning process before shipping. Ultrasonic cleaners are one technology. Many gear manufacturers also prefer parts washers that use soap and hot water.
Although these systems remove excess grease, dirt and grit, and process fluids, they can leave water spots and soap contamination that customers might object to. In these cases, gear manufacturers can change final cleaning solutions and use additives that cut down on staining and spotting. Solvents and degreasing agents are some alternatives, although they may not be as environmentally friendly.
On the lookout
Gear manufacturers that emphasize quality at every stage of processing can prevent costly rework and damage to their reputation. Employees should keep their eyes out for surface corrosion, discoloration, contamination, or pitting.
Sending gears with surface issues through corrective procedures before they get any further in the production process may seem like a costly delay, but it minimizes overall cost. This is because it is very difficult to address these issues after grinding.