Heat treatment can improve the properties of steel produced by 3D printing

A study conducted at the National Synchrotron Light Laboratory in Campinas, Brazil, shows that it is possible to enhance the ductility of additively manufactured maraging steel, an ultrahigh strength material used in the aerospace industry.

By Chloé Pinheiro  |  Agência FAPESP – Additive manufacturing (referred to as three-dimensional (3D) printing) of steel is seen as a promising alternative for the aerospace industry because it can create customized parts with complex shapes. However, practical applications are challenging, as steel produced with 3D printing technology has a different microstructure from that obtained via conventional manufacturing routes and this microstructure can impair the material’s properties.

In an article published in the journal Additive Manufacturing, Brazilian researchers have shown that heat treatment can be used to manipulate the properties of a type of steel produced by additive manufacturing. The study was supported by FAPESP and conducted at Brazil’s National Synchrotron Light Laboratory (LNLS) in Campinas, São Paulo State. The LNLS is run by the National Center for Research in Energy and Materials (CNPEM).

The type of steel investigated was maraging 300, an ultrahigh strength steel alloy obtained by aging martensite, which in turn is formed by cooling austenite (“mar-” refers to martensite, and “-aging” refers to a heat treatment called age hardening). In addition to its mechanical strength, maraging 300 steel is ductile, so that it can be bent or stretched into wire without breaking, for example. Ductility is an important property in materials subject to heavy loads and stress cycles, such as aircraft engines and landing gear.

“We found out from the literature that when maraging steel is produced by additive manufacturing, the desired strength is achieved but ductility is impaired,” Julian Arnaldo Avila Diaz told Agência FAPESP. Diaz is a professor of aeronautical and telecommunications engineering at São Paulo State University (UNESP) in São João da Boa Vista, São Paulo State.

During the conventional age hardening process, in which time (up to four hours) and temperature (in the range of 500 °C) are key factors, the different elements of maraging steel’s microstructure regroup to enhance its mechanical strength and ductility.

In additive manufacturing, however, conventional aging leads to the nonuniform precipitation of these elements in the alloy, and the desired microstructural effects do not occur.

Therefore, researchers have set out to change the way the elements regrouped using different temperatures from the usual ones to increase the level of austenite in the martensitic matrix. Austenite is more ductile than martensite.

“We looked for a temperature range and exposure time in which part of the martensite would dissolve enough to form austenite and remain stable instead of returning to its original form,” Diaz said.

Synchrotron light

Maraging steel samples were produced by the 3D printing technique called selective laser melting and were homogenized at 820 °C. These samples were then tempered (heat treated in the biphasic region) at 610 °C, 650 °C and 690 °C for approximately 30 minutes.

In the first two cases, the researchers observed a gradual but significant transformation of martensite into austenite with high thermal stability, which is the ideal scenario to promote ductility. However, at 690 °C, there was an excessive formation of the austenite phase and an undesirable conversion of the material into martensite during cooling.

The amounts of austenite and martensite were measured experimentally and compared with thermodynamic simulations. The study was conducted at the LNLS’s XTMS experimental station associated with the XRD1 X-ray diffraction beamline, which can analyze highly specific portions of materials at the microscopic level and transmit information in real time regarding the behavior of the sample.

“Thanks to the synchrotron, for the first time, we observed all phases of the process in this type of steel in real time. Previously, only static images were available in the literature,” Diaz said.

Another advantage was being able to define exactly the factors that produce the transformation-induced plasticity (TRIP) effect at the different temperature levels tested. TRIP is crucial to the achievement of maximum ductility.

For Diaz, synchrotron light is an essential tool in metallurgical research and can lead to new materials and parts for all segments of the Brazilian manufacturing industry. “In this case, we’re studying the additive manufacturing of steel, but all metal manufacturing processes can be analyzed in situ in the synchrotron,” he said.

Future possibilities

Generally, the research lays the foundation for a new type of high strength, high ductility steel produced by 3D printing. “We succeeded in obtaining a matrix that may not be as outstandingly strong as conventional maraging steel but combines adequate strength with high ductility,” Diaz said.

The next steps will involve more crystallographic analysis at different temperatures, after which the material will be submitted to mechanical tests to confirm in practice the group’s hypothesis that ductility can be improved.

The additive manufacturing of steel is currently used only for prototypes, precisely because of the unpredictability of its microstructure. With the results of this research as well as future studies, the use of this manufacturing process is expected to be more viable in critical industries. “On these foundations, we can create technologies that will change people’s lives in various ways,” Diaz said.

The article “Austenite reversion kinetics and stability during tempering of an additively manufactured maraging 300 steel” by F.F. Conde, J.D. Escobar, J.P. Oliveira, A.L. Jardini, W.W. Bose Filho and J.A. Avila can be retrieved from www.sciencedirect.com/science/article/pii/S2214860418308030?via%3Dihub.