The rotating blades in an electric razor are under a great deal of strain. Throughout their life, they make millions of quick rotations, while also being subject to wear from cutting hair. This takes place in a corrosive environment due to water vapour in the bathroom and moisture from the user’s skin.
At the same time, the blades and other components in electric razors are expected to become smaller every year. That is the reason why Philips a few years ago launched a European project on simulation of the properties of stainless steel.
“Philips produces millions of small stainless steel components with micrometre precision. We’re always looking to improve our products to maintain our world-leading position within electric razors,” says Professor Jan Post, head of Philips Corporation’s department for strategic partnerships in consumer products.
“Thanks to complex numerical simulations of surface properties of stainless steel, we’ve managed to cut years off our development time,” says Jan Post about the collaboration project, where DTU researchers have participated in the development of the simulations and have also developed a new surface treatment process.
Maintaining Europe’s position
The project ‘PressPerfect’ (Prediction of Stainless Steel Performance After Forming and Finishing) is coordinated by Philips and supported by EU’s Research Fund for Coal and Steel. The objective of the EU funding is for the project to contribute to maintaining Europe’s leading position within high-precision steel components, which is threatened by lower-cost countries.
“We were already using steel of the highest quality on the market, but we asked Professor Marcel Somers from DTU Mechanical Engineering and other European partners to help us further improve the quality. We know the DTU group from previous collaboration. It’s one of the leading groups in Europe working with stainless steel in general and particularly with the effects of heat treatment,” says Jan Post.
Stainless steel production is seeing a change, in that companies are abandoning the electrochemical processes which have dominated for several years in favour of purely mechanical processes.
“Electrochemical processes have proven to lead to more waste during production and higher energy consumption. However, switching to purely mechanical processes requires a good understanding of how the mechanical processes and the subsequent finish affect the materials’ strength, internal stress, the ability to withstand corrosion, and other properties,” explains Professor Marcel Somers.
Iron in steel can change structure
The ‘Materials Design and Surface Engineering’ research group headed by Marcel Somers is interested in how the surface properties of stainless steel change when the materials are designed.
To understand the problem, it is necessary to know that the iron crystals in steel can take on different crystal structures.
For electric razor components, it is clearly preferable that the crystals have an austenite structure. Austenite is characterized by being flexible and yet strong. In addition, its anti-corrosive properties are particularly good (A in the figure below).
However, the mechanical deformation which occurs when designing the components means that the iron crystal structure changes locally. Instead of austenite, you now get martensite, which provides more rigid and stronger steel (B in the figure below).
“You would think that added strength was an advantage, but if it occurs too early during the processing, it actually poses a problem. With stronger steel, more force is required—and thereby higher energy consumption—to give it the right shape during the later stages,” explains Professor Marcel Somers.
New process preserves corrosion properties
Even worse than the increased energy consumption, however, is that the ability to withstand corrosion is reduced when austenite is transformed into martensite.
“Unfortunately, austenite in itself is not sufficiently strong to withstand the constant wear when the blades cut hair. Therefore, the steel is hardened by adding nitrogen to the surface at a low temperature. However, our studies show that the local transformation of austenite into martensite during the design of the hardened steel leads to premature development of nitrogen compounds that bind chromium with the result that the corrosion protection is lost,” explains Professor Marcel Somers.
The DTU researchers have therefore developed a new process, in which the austenite is stabilized by dissolving a small amount of nitrogen at a high temperature before initiating the design work. The new process can, for example, be carried out at by the steel supplier (C in the figure).
“The process prevents the formation of martensite due to mechanical deformation. The surface treatment can then be performed at a low temperature without nitrogen compounds being formed,” says Marcel Somers. “As an extra bonus, even better corrosion resistance than in the original steel is achieved.” (D in the figure).
Model saves development time
Another result of the research group’s work in the project is a numerical model capable of predicting the profiles with regard to the composition and internal stress of stainless steel which has undergone surface hardening.
“Together with the other tools developed in the project, the model means that we can cut off years of our development time,” says Jan Post, Philips. “We aim to be an innovative company and use many types of simulations to predict the properties of the products. Most of the processes we use consist of several steps, including thermal influence, which means that it’s very difficult to predict how the material will behave—especially when it comes to the types of high-quality stainless steel being studied in the project.”
The PressPerfect project was officially completed in 2016, but the activities in the area continue at DTU Mechanical Engineering, says Professor Marcel Somers:
“The project has been a natural part of our efforts in relation to surface treatment of stainless steel. We are working on a number of projects, many of which are in collaboration with companies.”