18Ni300 Alloy Powder Stainless Steel Nickel In 3D Priting

Written By：Tresa M. Pollock

Dr. Tresa M. Pollock is the Alcoa Distinguished Professor of Materials at the University of California, Santa Barbara, and a member of the U.S. National Academy of Engineering. She earned her B.S. in Metallurgical Engineering from Purdue University and her Ph.D. in Materials Science and Engineering from MIT.

Her research spans alloy design, 3-D materials characterization, processing and performance of structural materials in extreme environments, and ultrafast laser interactions. Dr. Pollock has been recognized with numerous honors—including fellowships in professional societies like TMS and ASM, and leadership roles such as past president of The Minerals, Metals & Materials Society.

In the world of high-performance metal materials, maraging steel is undoubtedly the bright star. With its unique ultra-high strength, excellent toughness and good workability, it occupies an irreplaceable position in many cutting-edge fields. In this type of steel, 18Ni300 maraging steel, as shown in its number, nickel content of up to 18%, is one of the high-profile. It is showing more and more extensive application prospects in aerospace, precision mold manufacturing and high-performance mechanical parts with extremely stringent performance requirements. The appearance of 18Ni300 has solved the bottleneck of traditional high strength steel in toughness and fatigue performance to a certain extent.

This paper will analyze the unique advantages of 18Ni300 alloy powder. We will explore how its exquisite chemical composition gives it extraordinary mechanical properties, as well as the cutting-edge applications and surfaces brought by 18Ni300 alloy powder in the current high-profile field of additive manufacturing.

why choose 18Ni300:

Among the many alloys, why do we favor 18Ni300? 18Ni300 alloy powder, especially its unique position in the stainless steel nickel-based alloy system, is an example of the perfect combination of performance and application.

Ultra-high strength and toughness: It has a perfect balance between yield strength, tensile strength and fracture toughness. In the field of traditional steel, you often need to make a trade-off between strength and toughness. But 18Ni300, like the all-round player, provides extreme strength far beyond traditional steel while still maintaining excellent fracture toughness, which is the perfect material for designing key components that need to withstand high stress and high impact loads.

Excellent dimensional stability: In the field of precision manufacturing, the dimensional stability of materials is one of the key factors that determine the success or failure of products. The 18Ni300 performed exceptionally well in this regard. Its lower martensite transformation temperature means that

Good machinability: Don’t look at its final strength so high, in the annealed state, 18Ni300 is actually relatively “docile”, with good machining performance. This means that before the final hardening process, we can cut, drill and other operations, greatly improving manufacturing flexibility and efficiency. What worth mentioning is that when it is in powder form, it is very adaptable to additive manufacturing processes (such as SLM, EBM). I personally think that the combination of powder metallurgy and additive manufacturing is an important direction for the future development of 18Ni300. It provides infinite possibilities for us to break through the geometric limitations of traditional manufacturing and realize the integrated molding of complex structures.

Application areas: Because of these excellent characteristics, the application areas of 18Ni300 are more and more extensive, and they are all “high-precision” areas. For example, the aerospace industry’s landing gear components and various structural parts, its high strength and high toughness to ensure flight safety. High-performance molds, especially die-casting molds and plastic molds, 18Ni300 can withstand high temperature and high pressure, significantly extending mold life. In the field of racing, lightweight and high strength is the eternal pursuit, 18Ni300 alloy components can significantly improve vehicle performance. It can even be seen in medical instruments, such as surgical tools or implants. All in all, as long as it is the strength, toughness, dimensional stability of the occasion, 18Ni300 is a priority option.

What does 18Ni300 mean?

“18”: The content of nickel (Ni) in this alloy is about 18% by weight. Nickel is not only a key element in the formation of martensite, but also a decisive factor in subsequent age hardening. Without enough nickel, the properties of this alloy are out of the question. I always like to think of it as the “skeleton” of the alloy, supporting the entire performance system.

“Ni”: this is more intuitive, clearly pointed out that this is the nickel-based alloy, or at least nickel content is very significant alloy. In the world of alloys, the identification of elements is crucial, allowing us to quickly lock in the broad categories and potential properties of materials.

“300”: This number represents the typical tensile strength rating of this alloy after a series of heat treatments, usually in KSI (kilopounds per square inch), which is about 300 KSI, converted to the more familiar unit, It is about 2070 MPa. This is its most important mechanical performance index. When engineers tell me they need to a high-strength material, I first think of these alloys with high-strength digital markings.

18Ni300 chemical composition:

The main alloying elements and their role:

• Nickel (Ni, ~ 17.0-19.0%): Nickel is the “soul” element of maraging steel. It not only stabilizes the austenite, reduces the martensitic transformation point, and ensures that we get a soft martensitic matrix, which is essential for subsequent age hardening. And its presence significantly improves the toughness of the alloy, you know, high strength without toughness, that’s not what we want. Even better, nickel is also the basis for the formation of many key precipitates.
• Cobalt (Co, ~ 8.5-9.5%): Cobalt is a bit of an accelerator to me “. It can increase the martensite transformation temperature, which means that it is easier to form martensite during cooling. At the same time, cobalt can also subtly reduce the solubility of nickel in iron, which sounds a bit counter-intuitive, but in fact, it can accelerate the formation of precipitated phases during the aging process, thereby improving hardness faster and more effectively.
• Molybdenum (Mo, ~ 4.6-5.2%): Molybdenum plays multiple roles in 18Ni300. First, it can enhance the strength of the matrix by solid solution strengthening. Secondly, it helps to refine the grain and inhibit the grain boundary migration, which is very beneficial to improve the comprehensive mechanical properties of the material. Of course, more importantly, molybdenum will form intermetallic compounds with nickel, such as Ni3Mo, which precipitate during the aging process and further contribute to the hardness improvement of the alloy.
• Titanium (Ti, ~ 0.6-0.8%): When it comes to age hardening, we can never ignore titanium! In my opinion, titanium is the key to the 18Ni300 “hardness explosion. It forms intermetallic compounds such as Ni3Ti with nickel, and these nanoscale precipitates are dispersed in the martensite matrix to form strong dislocation pinning points, thereby significantly improving the yield strength and hardness of the material. It can be said that without titanium, there would be no 18Ni300 iconic high strength.
• Aluminum (Al, ~ 0.05-0.15%): Aluminum in this formula, although the amount is not much, but the effect is not small. It is first and foremost an effective deoxidizer that helps us control the oxygen content in the smelting process. Second, aluminum can also refine grains to some extent. Of course, it may also be involved in some complex precipitation strengthening mechanisms.

Trace element control:

As a materials scientist, I have an almost paranoid demand for the “purity” of materials. We strictly control the content of harmful impurities such as carbon (C), sulfur (S) and phosphorus (P), which are like the “black sheep” in the alloy “. Excessive carbon content will affect the weldability and toughness; sulfur and phosphorus are easy to form low melting point compounds at grain boundaries, resulting in material embrittlement. Therefore, keeping their content at a very low level is essential to ensure the excellent performance of 18Ni300.

To make it more intuitive, I ‘ve compiled a table showing the typical chemical composition range of 18Ni300.

Table: Typical chemical composition range of 18Ni300 (weight percentage)

Element	Content(%)
Ni	17.0-19.0
Co	8.5-9.5
Mo	4.6-5.2
Ti	0.6-0.8
Al	0.05-0.15
C	<0.03
S	<0.01
P	<0.01
Fe	Balance

Application and Challenges of 18Ni300 Alloy Powder in Additive Manufacturing

Synergistic Effect of Powder Metallurgy and Additive Manufacturing

The reason why 18Ni300 can be perfectly combined with additive manufacturing is that its powder form is the core. Additive manufacturing, or 3D printing as we often say, is essentially “building blocks”, stacking materials layer by layer. How can we achieve those complex geometries without the powder form? I often emphasize with my students that the flowability, bulk density and uniformity of the powder-seemingly trivial parameters-actually directly determine the quality and efficiency of our final print. Especially for the high-performance alloy 18Ni300, the high purity and perfect sphericity of the powder are fundamental to ensuring the performance of the print.

At present, there are two main types of additive manufacturing technologies that we use most commonly. The first is laser powder bed melting (L-PBF), which uses a high-energy laser to selectively melt on a dense powder layer, which is particularly suitable for manufacturing complex parts that require extremely high precision. The other one is electron beam melting (EBM), which works in a vacuum environment and melts powder with an electron beam. EBM is more energy efficient and can effectively reduce residual stress, but the disadvantage is that the surface roughness may be slightly inferior.

Regardless of the technology, the requirements for powder quality are almost stringent. The particle size distribution of the powder is usually between 15-45 microns or 20-63 microns, and this range must be strictly controlled. There are also sphericity, satellite powder content, oxygen content, and powder surface morphology, which directly affect the stability of the additive manufacturing process and the density and mechanical properties of the final part.

Effect of Additive Manufacturing Process on Properties of 18Ni300

The beauty of additive manufacturing is its “speed”. But this speed also brings unique challenges.

rapid solidification effect

During additive manufacturing, materials undergo a extreme cycle of rapid heating and rapid cooling. For 18Ni300, this means that its microstructure and phase transition process will be very different from the traditional process. Rapid cooling causes the grains to become extremely fine, forming what we call an ultra-fine martensitic structure. Sometimes, this rapid cooling may even inhibit the formation of certain aging precipitates, or change their morphology.

From a performance point of view, fine-grained strengthening usually improves the initial strength of the material, which sounds great. However, we also need to see the challenge. The crystal orientation and tissue unevenness caused by rapid solidification may lead to anisotropy in the printed product, which needs to be seriously considered for our subsequent design and application.

residual stress and cracking

residual stress, which is a long-term problem in the field of additive manufacturing. High temperature gradients, rapid cooling of the material, and uneven shrinkage are all causes of residual stresses. I have seen too many parts warped and deformed due to residual stress, and even macro or micro cracks. This is really a headache, after all, no one wants to see the hard printed parts fall short.

In order to solve this problem, we usually adopt some strategies, such as preheating the substrate, optimizing the scanning strategy (chessboard scanning is a common method), adjusting the power and speed of the laser or electron beam, of course, the later heat treatment, especially stress relief annealing, It is also essential.

Density and defect control

The density of parts, I think this is the cornerstone of the success of additive manufacturing. If the density is not high, it is impossible to talk about other properties, especially fatigue performance. We must start from the source, that is, control the quality of the powder-fluidity, bulk density, particle size distribution and purity, which are the basis for obtaining high-density prints.

Next is the optimization of process parameters, such as laser power, scanning speed, line spacing, layer thickness, which directly affect the energy density, must ensure that the powder can be fully melted and form a stable molten pool. At the same time, the scanning strategy is also critical to avoid over-burning or under-burning and to optimize the remelting area. Common defects such as incomplete fusion, porosity and inclusions, we must try our best to avoid them.

The importance of post-treatment processes

Additive manufactured parts often require a series of post-processing. Of these, two processes are crucial.

Hot Isostatic Pressing (HIP)

Hot isostatic pressing, referred to as HIP, is the “killer” to eliminate internal pores “. Under the environment of high temperature and high pressure, the material will undergo plastic deformation and diffusion creep, so that the isolated internal pores are closed. I often compare it to giving the part a deep “massage” to make it denser inside.

After HIP treatment, the density of the material will be significantly improved, and the mechanical properties will also be a qualitative leap, including yield strength, tensile strength, elongation and fatigue life. My experience is that the performance of HIP-treated additive manufacturing 18Ni300 parts can be very close to that of traditional forgings. Our commonly used parameters are usually high temperature (such as 1150-1200°C), high pressure (such as 100-150 MPa), and holding for 2-4 hours.

Heat treatment

Heat treatment, especially solution treatment and aging treatment (solution-quenching-aging), is a key step to induce 18Ni300 precipitation hardening and optimize mechanical properties.

The first is the solution treatment. Its purpose is to allow the elements of the alloy, especially titanium, molybdenum, cobalt, fully dissolved into the martensitic matrix. This not only eliminates internal stress, but also allows the organization to become more uniform.

Next is the aging treatment. This is a critical moment for the 18Ni300 performance explosion. At a moderate temperature (usually 480-520°C) for a period of time, we will see that nickel, titanium, and molybdenum-rich intermetallic compounds (such as Ni3Ti, Ni3Mo) are uniformly precipitated in the martensite matrix to form nanoscale dispersion strengthening phase. These small precipitates are like countless “steel nails”, which greatly improve the hardness and strength of the material.

The typical heat treatment parameters suggested are: solution treatment at 815-830°C for 1 hour, followed by air cooling or water quenching; aging treatment at 480-520°C for 3-6 hours, followed by air cooling. But here is a small reminder that due to its unique microstructure, the optimal heat treatment parameters of the additive manufacturing 18Ni300 may be different from traditional castings and forgings, so we still need to do some optimization work.

In summary, 18Ni300 maraging steel powder has shown broad application prospects in aerospace, precision molds and high-performance machinery manufacturing due to its ultra-high strength, excellent toughness and good dimensional stability. Especially in additive manufacturing (3D printing), 18Ni300 alloy powder provides a new solution for complex structure design and lightweight manufacturing with its excellent powder characteristics and adaptability. Through reasonable control of powder quality, optimization of process parameters, and combined with hot isostatic pressing and heat treatment and other post-processing means, the comprehensive performance of 18Ni300 parts has been comparable to or even better than traditional forgings. It is foreseeable that with the continuous maturity of powder metallurgy and additive manufacturing technology, 18Ni300 maraging steel will become an important supporting material for high-end manufacturing and a key force to promote the development of advanced manufacturing technology in the future.