Quenching and Finish of Fasteners

You are simply worried about the color of the fastners or is there an issue with the heat treatment?

I'm confused about many things...

• When you say "after tampering" do you mean "after tempering"? or is someone tampering with your stuff?
• When you say "We are using Water based Blackening oil." I am instantly confused... Is it a water, or an oil? they typically do not mix well without an emulsifier, and then you have an emulsification, not a oil, or a water.
• Are you simply looking for a coating which can be applied after heat treatment (quenching/tempering) to make your fasteners black in color?

Please advise.

These are good questions, making sure we understand the problem to be solved.

I have another question: when you say are having the problem lately, does that mean you have not had the problem in the past? If you were getting the color and finish you want before and can't get it now, it's clear that something in your manufacturing process has changed. Have you changed the process steps, materials, suppliers or methods? If so, you need to look at what's different now compared to the time when production was successful.

Dear,

> It is tempering

>It is a Solution and then it mixed with water. We Maintain a concentration of about 6% to 8%. (It is used in our continuous furnace on tempering side). The Capacity is about 8000 liters.

>Yes, we are looking for some coating which can be applied after Quenching/tempering to make fasteners black.

>We have not made any changes in our manufacturing process. We have sent our water based blackening solution along with DM water for checking in external Lab.

You did not confirm whether this is a new problem or has been happening all along, but I will assume it is new.

You did say that you have not changed the process in any way, but since yo are getting different results, we must assume that something supplied to you by a vendor has changed or is out of spec. I think having the blackening solution tested is a good start.

I would suggest that the smaller fasteners are cooling faster in the quench and so making it possible to get the black finish you desire. The larger fasteners, which i think might be a new product for you, are more massive, holding the heat longer, and so not developing the full black finish that you wish to have. I would experiment with extending dwell time in the tank.

A design of experiments matrix can help you identify the factors that control the finish. I'd consider an L8 matrix to cover lots of variable; and choose such as supplier of steel, alloy of steel,. Size, Temperature of parts when put in bath, and time in bath, bath concentration, etc.

https://controls.engin.umich.edu/wiki/index.php/Design_of_experiments_via_taguchi_methods:_orthogonal_arrays

Milo