There is a clear trend towards the increased use of PM steels in highly stressed applications such as gears, where high fatigue performance is required. Density and microstructure are the key parameters for the fatigue performance of PM steels, and in depth knowledge in these areas is an essential requirement if PM components are to compete successfully with wrought steel in high-performance applications.

A few years ago Höganäs AB embarked on an unprecedented project to map the microstructure and fatigue performance of its materials in the minutest detail. The result is an unrivalled knowledge bank based on the work of the Fatigue Research Team, a group of four that works exclusively on fatigue-related research.

A close-up view of microstructure reveals a lot about a metal’s fatigue life.

Sigurd Berg, Manager Product Development at Höganäs says: "We are at the forefront of knowledge on fatigue properties. We now have comprehensive knowledge on our materials' fatigue performance in density areas up to 7.3 g/cm³."

"We can rank our materials from the starting point of structure," he continues. "This means that from the requirements of an application we can assist customers in rapidly selecting the right material and process route for a specific component."

"Our knowledge, particularly the fatigue performance of our high-performance materials, is of great practical value and is underpinning efforts to expand the PM market into the huge potential market for highly loaded gears in applications such as vehicle transmission and power train gears."

Anders Bergmark, a member of the team, says the project has included a close study of nickel's ability to enhance fatigue limits. "Traditionally, improved fatigue properties have been achieved by adding a little nickel to the mixture in order to raise the fatigue limit. However, powder mixes with small additions of nickel cannot fully utilise the nickel's performance, since the nickel particles are present as isolated islands of austenite surrounded by a border of martensite. It is not until a continuous network of martensite is formed that the fatigue properties really improve."

"Nickel and copper additions of around 4 per cent and 2 per cent respectively efficiently create this continuous network. But, at these high nickel levels, particle size segregation can be a severe problem, disturbing the continuity of the network. The remedy is to use diffusion-bonded grades, notably Distaloy AE and Distaloy HP, which exhibit a microstructure with a continuous network of martensite, and consequently have good fatigue properties." Anders Bergmark presented the findings of the Fatigue Research Team in a paper entitled Microstructure Enhancement for Fatigue Improvement.

Steel alloy in close-up.

Another recent deposit in the knowledge bank is a paper by another member of the Fatigue Research Team, Ola Bergman. His presentation: Chromium-Alloyed PM Steels with Excellent Fatigue Properties Obtained by Different Process Routes examines the fatigue properties of the pre-alloyed water-atomised chromium materials, Astaloy CrL and Astaloy CrM.

"We can show that these materials have the required properties for applications where high fatigue performance is essential," he says.

"The bending fatigue limits at a sintered density of 7.1 g/cm³ is around 260 MPa for Astaloy CrL + 0.8 per cent graphite and around 290 MPa for Astaloy CrM + 0.45 per cent graphite after conventional sintering at 1120ºC in a nitrogen-based atmosphere with a low dew point.

"Even better results are achieved with high-temperature sintering, which increases the fatigue limit of Astaloy CrL to 310 MPa. A combination of conventional sintering and rapid cooling improves the fatigue limit of Astaloy CrM to 380 MPa."

The explanation for this consider able rise in fatigue performance lies in the microstructure shift from mainly bainitic to mainly martensitic structure, which occurs as a higher cooling rate is applied.

"What is significant here," emphasises Ola Bergman," is that you can optimise microstructure and heighten characteristics with high-temperature sintering or rapid cooling in a cost-effective single-stage process without secondary operations."

Monica Carlsson in the Höganäs fatigue laboratory.

Words: Minett Media
Illustrations: Höganäs AB

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