“I first tried to fabricate a test piece of the target hardness.”
In die-quenching, a steel sheet is hot heated at high temperatures then rapidly pressed and cooled (quenched) at the same time in a mold to obtain a very hard form of steel crystalline structure. What you need to do is prepare a suitable test piece to know what relation hardness and magnetic property have. For example, a test piece with local parts heated at different temperatures would reproduce properties of acceptable and rejectable parts. Those exposed to different cooling rates are desirable to reproduce accidental line processing conditions, e.g., poor quenching, that can take place when the temperature decreases slower due to insufficient contact with the mold.
That's easier said than done. The production lines are not designed to produce nonconforming parts. In other words, they cannot provide specimens ideally constructed for this specific purpose. Seo worked with Toyota Central R&D Labs, one of the leading automotive research institutes to have test pieces that would offer what he needed.
He also spent a lot of time and effort to create a coercivity testing machine. Prof. Kamada's lab used commercially available computers and monitors and manually wound coils to create testing environments. But what Seo needed was a test package that would ensure a high level of testing performance. He even visited test machine manufacturers without appointments to ask for cooperation. Gradually, the new tester specifications started to take form.
Once the trials with test pieces was completed, he moved on to testing actual parts. There are 50 to 60 types of parts to be tested, all in different geometries, thickness, materials, and surface treatments (e.g., plated or not). One by one, Seo measured hardness and coercivity and examined metallographic microstructure on an electron microscope to see whether they could be nondestructively tested. He experienced days of endless trial and error. The efforts of collecting piles of data were well rewarded later on when the project phase shifted to a production run.
September 2018 marked the launch of non-destructive hardness testing on some production parts. Yet, there was another challenge that would not be easily conquered—customer approval. The new test method needed to be approved by auto manufacturers.
The quality of all past die-quenched parts had been assured through destructive testing. Before shifting to non-destructive hardness testing in production, the data would have to demonstrate that the quality of parts tested nondestructively would be assured and then be approved by the car manufacturers. The raw data Seo collected day after day helped.
As of the interview in March 2019, the non-destructive hardness testing is approved for only several types of products, but still the inspection hours and process have been noticeably reduced. The technique reduced inspection costs by 90% and the time they took by 85%, now requiring only 3 minutes for one test cycle per part which used to take 20 minutes before the new test method was incorporated. (Test cycle time per part is given by multiplying 15 sec by 12 locations.) A proper set of equipment would automate most of the inspections.
Leveraging the new method to other products and sites requires factors irrelevant to quenching parameters to be accounted for, e.g., part geometry, disturbances caused during surface treatment, and noise levels of the facility. It may be a long way away, but the non-destructive hardness testing will immensely benefit everyone if rolled out across the group in the future.
Non-Destructive Hardness Testing
▲ Non-destructive hardness testing allows to measure a material’s hardness with a probe in contact with the material.
About the project, Seo said, "I was given a lot of opportunities to meet leading researchers in both academia and industry, those whom I would have never had a chance to talk to. They were nice and willing to help. I am so grateful for that.”
“My boss was absolutely supportive. He always allowed me to go for what I wanted to try. There were hard times, but I had more pleasure devoting myself to development.”
He added, “The project gave me a feeling that I can do something to help people at manufacturing sites by, specifically, reducing the workload and time needed for testing. It was worth doing. I decided to do it right. I was happy to know that even a newcomer like me was able to do something helpful.”
The technique benefits the industry in another way. It brings lighter, safer vehicles.
The non-destructive hardness testing method contributes to increased quality and reliability of die-quenched automotive parts, and the reduced cost will lower the barrier to incorporating these parts and, consequently, will improve overall car safety. The story behind the technology may sound distant and unrelated to drivers, but it's about the safety of all those who drive cars.