Evaluation of Additively Manufactured Ultrahard Steels

Recent advances in both alloy development and additive manufacturing have enabled the production of ultrahigh-strength steels in near net shape parts.

An image of the quasi-static compression testing setup used in this research to evaluate the strength of the alloys produced by VBN Components using additive manufacturing. (Image: Army Research Laboratory)

Ultrahigh-strength steels are traditionally defined as those steels with a minimum yield strength of approximately 1380 MPa. Notable examples of steels in this category include AISI 4130, AISI 4140, and AISI 4340. In many cases, maximizing the performance of these alloys requires a rather complex approach that involves a series of tempering, annealing, or stress-relieving treatments. As a result, they are produced using a variety of traditional processing methods such as casting, rolling, extrusion, or forging. These traditional methods — combined with the ultrahigh strength of the steels — often meant that the production of complex, near-net shape parts of high quality was quite difficult. In addition, these production methods often entailed repetitive treatments or long production cycles, both of which resulted in elevated production costs.

Additive manufacturing (AM, also known as 3D printing) has recently been recognized as a manufacturing method that enables the production of near-net shape parts. In these methods, a complex part is iteratively built in a layer-by-layer process that involves powder deposition followed by selective melting/sintering of the powder to form the part. With the continued development of processing lasers, it is now possible to form fully dense components from a wide range of metals powders, including refractory alloys, steels, and other high-temperature alloys.

Furthermore, parts produced by 3D printing often require less finishing work, which results in faster production cycles and potentially significant cost savings. Furthermore, the metal powder not incorporated into the part is often reclaimed and, after a cleaning protocol that involves sieving and removal of agglomerates, can be used in the next build operation. This ability to “reuse” the metal powder also offers appreciable cost savings relative to the scrap waste produced by the casting and machining of larger metal blocks to final parts.

Although there is great potential associated with the AM of ultrahigh-strength steel components, there are also significant challenges that must be addressed, typically on an alloy-by-alloy basis. For example, many of these steels have highly complex compositions (e.g., multiple alloying elements) that are intended to form a desired range of strengthening precipitates or phases. Due to their broad compositional range, it can be quite challenging to understand, predict, and/or control the interaction between the laser energy and metal powder during the printing process.

As a result, it often requires a prolonged investigation and process optimization routine to achieve the desired microstructure in the final, printed component. Other factors that must also be addressed to achieve high-quality parts are the removal of property dependence on build direction, high residual stresses, and undesired microstructural features such as pores and/or cracks. From even this brief consideration, it is clear that the production of high-quality parts using AM is not a trivial process, but it typically involves many hours of part building and evaluation to develop the desired process parameters.

An effort was undertaken to acquire and evaluate a series of ultrahigh strength steels produced by VBN Components (Uppsala, Sweden) using AM. In particular, VBN has successfully developed a series of ultrahard steels that have demonstrated extreme wear and thermal resistance that could potentially offer improved performance and component lifetime. The superior performance of these alloys was achieved through a combination of compositional designs as well as optimization of annealing treatments. Furthermore, VBN has developed proprietary AM-based processing routines that enable the production of near-net shape components from the entire range of alloys.

Various sample types produced from three different alloys were procured from VBN Components in the fully annealed and surface ground conditions (e.g., ready to test conditions). Analytical microscopy indicated that all three alloys contained an appreciable amount of various micronsized carbides. The presence of these carbides resulted in the steels having ultrahigh strengths and overall hardness but at the cost of a reduced level of tensile ductility. This observation was further confirmed in the wear and tensile results.

Based upon the various results, it was determined that the alloys lacked the required toughness (ductility) for use in high strain-rate and/or impact-related applications. Instead, the alloys showed significant promise in wear resistance applications. Indeed, Alloy Vibenite 280 was found to have similar overall properties (mechanical, wear resistance, and corrosion) as alloy AISI 52100 and thus could be a potential substitute for this alloy, with the noted benefit of being produced via current AM methods. This ability could enable the potential redesign of existing parts to save weight or improve performance by leveraging production features possible in AM methods that are not available in conventional metal-forming operations.

This work was performed by Vincent Hammond and a team of other researchers for the Army Research Laboratory (Aberdeen, MD). For more information, download the Technical Support Package (free white paper) below. ADTTSP-05244

This Brief includes a Technical Support Package (TSP).
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Evaluation of Additively Manufactured Ultrahard Steels

(reference ADTTSP-05244) is currently available for download from the TSP library.

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