News Forwarded from Kunststoffe: The Danger of Cracks on Screws

22/07/2021

http://www.kunststoffe.de/en/10597133

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“Hardy, we received your screws. Are you sure your people have done the inspection? Look at the crack, I think any people will see this clearly. How can we install such a screw on our machine? How fast can you get us a new screw? We need it now. Please let me know. ” As a screw manufacturer, EJS often receives such complaints.

Mistakes in the production?
When the first 125 hard-facing screw was ready for inspection, the employees at E.J.S Industry Co., LTD, Ningbo/China (EJS), noticed that there were cracks on the screws. How could that happen?

Was it caused by bad temperature control, or bad bimetallic alloy powder, or bad base steel, or process management? To find out why, a lot of tests were scheduled and a lot of labels were sticked on the product for later identification. After a lot of trial and error, EJS came to know WHO was the mother of cracks and their relationship.

Hardfacing screws are normally processed by a PTA welding machine. The energetic plasma arc has a high temperature more than 1000℃ to melt the alloy powder, which is to be welded into the flight one by one until it’s completely finished. During this process, the thermal expansion and cold contraction happens inevitably and repeatedly due to the big difference between 1000℃ and the ambient temperature(5℃ to 40℃).

Nothing to worry about
Since 2015, EJS had a special furnace to keep the hardfacing screws warm for certain hours after PTA welding, which do help us to reduce the numbers of cracks. Unfortunately some cracks are still there, big or small, more or less. Is there any way to get rid of them?
As a quality-oriented company, EJS tries everything to avoid cracks. However, it is not possible to completely exclude the possibility of cracks forming. The company consulted senior engineers, checked with experts, analyzed together with experienced production managers and sought advice from long-term customers in different countries. Finally EJS accepted the fact that cracks are inevitable for some alloys on big sizes, because no cracks means soft flights. There are some alloys which you cannot avoid micro cracks. They always exists.

Moreover EJS started to know that
When the cracks are intense, AND the flights always stay - it is acceptable.
When the cracks are long, AND the flights always stay - it is acceptable.
When the cracks are wide, AND the flights always stay - it is acceptable.
When the cracks are regular in one direction - it is acceptable.
If the crack causes the peal-off - it is NOT acceptable.


Using these simple questions, customers can check whether their screws are OK or not.
In case of doubt, the experienced EJS staff will be happy to help.